Gas-Phase Chemistry of Bare V + Cation with Oxygen and Water at Room Temperature: Formation and Hydration of Vanadium Oxide Cations

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1 J. Phys. Chem. A 2001, 10, Gs-Phse Chemistry of Bre V Ction with Oxygen nd Wter t Room Temperture: Formtion nd Hydrtion of Vndium Oxide Ctions Gregory K. Koyngi nd Diethrd K. Bohme* Deprtment of Chemistry, Centre for Reserch in Mss Spectrometry, Centre for Reserch in Erth nd Spce Science, York UniVersity, Toronto, Ontrio, Cnd MJ 1P Ilon Kretzschmr, Detlef Schro1der,* nd Helmut Schwrz* Institut für Orgnische Chemie der Technischen UniVersität Berlin, Strsse des 17. Juni 1, D-1062 Berlin, Germny ReceiVed: NoVember 14, 2000; In Finl Form: Februry 20, 2001 Mss spectrometric experiments t extremely low (<10-6 mbr) nd moderte (0. mbr) pressures re used to exmine the rections of tomic vndium ction with moleculr oxygen nd wter. With O 2, rpid O-tom bstrction gives rise to the formtion of VO ction (k ) cm molecule -1 s -1 ). Interestingly, despite similr thermochemistry, the O-tom trnsfer from wter to bre V is less efficient by more thn n order of mgnitude (k ) cm molecule -1 s -1 ). Subsequent ssocitions of VO with either O 2 or H 2 O occur with very low efficiencies nd involve termoleculr stbiliztion mechnisms. The low probbility of degenerte 16 O/ 18 O exchnge between VO nd wter indictes the opertion of sizble kinetic brrier. Ab initio clcultions using density functionl theory lend further support to the interprettion of the experimentl dt nd provide the first thermochemicl informtion on VO n ctions with n > 2, s well s some hydrted species. In generl, the dipolr wter lignd is found to be much more strongly bound to the ctionic vndium complexes thn is dioxygen. Introduction Vndium oxides re mong the most importnt trnsitionmetl ctlysts pplied in chemicl technology. 1 Their uses rnge from the production of sulfuric cid, vi selective oxidtions of liphtic nd romtic hydrocrbons, to epoxidtion ctlysis. As n erly trnsition metl, vndium is very oxophilic nd thus is likely to exist in its higher vlency sttes in the presence of oxidizing gents. In turn, high-vlent vndium compounds re required t the ctlyticlly ctive sites relevnt in oxygention processes. Further, in n excess of oxidnt, peroxidic structures my be ccessed. Previous mtrix isoltion studies 2 nd nion photodetchment spectroscopy hve provided evidence for the existence of vndium peroxides, such s O 2 V(O 2 ), in idelized environments. In the condensed phse, ligted vndium peroxides (L m V(O 2 ) n ; n ) 1-4) hve wide pplictions rnging from synthetic interests to the tretment of dibetics. 4 The present study describes the chemistry initited by V in O 2 t room temperture nd the formtion of the higher oxides [V,O n ] with n e 7. To this end, Fourier trnsform mss spectrometry nd selected-ion flow tube techniques re combined with theoreticl investigtion of gseous [V,O n ] ctions by mens of density functionl theory. Rections of vndium contining ctions with wter, inter li leding to formlly hydrted oxide ions, such s the OV(OH 2 ) /V(OH) 2 couple, lso re exmined. Experimentl Methods The formtion of vndium oxide ions is chieved in rections initited by tomic V ctions in the presence of moleculr oxygen. These rections re investigted with both the Fourier trnsform ion-cyclotron resonnce (FTICR) instrument t the Technicl University in Berlin nd the inductively-coupled plsm/selected-ion flow tube mss spectrometer (ICP/SIFT) instrument in the Ion-Chemistry Lbortory t York University. FTICR. These experiments use Spectrospin CMS 47X FTICR mss spectrometer equipped with n externl ion source s described elsewhere.,6 In brief, V is generted by lser bltion of vndium trget using Nd:YAG lser operting t 1064 nm. A series of potentils nd ion lenses is used to trnsfer the ions to the ICR cell, which is positioned in the bore of 7.0 T superconducting mgnet. Mss-selected 1 V is then thermlized by nonrective collisions with pulsed-in rgon buffer gs, followed by reisoltion of the metl ion nd monitoring the rection with moleculr oxygen leked-in t constnt pressures rnging from 1 to 10-8 mbr. The experimentl second-order rte constnts re evluted ssuming the pseudo first-order kinetic pproximtion fter clibrtion of the mesured pressures nd cknowledgment of the ion guge sensitivities; the error of the bsolute rte constnts is (0%, nd the ion temperture is ssumed to be 298 K. 7 To provide solid bsis for the nlysis of consecutive rection kinetics nd for the comprison with the SIFT dt, ll experiments re conducted t three different pressures. The kinetic modeling pplied here is bsed on simple rte expressions nd itertive procedures for dt fitting using Excel ICP/SIFT. The SIFT instrument is described in detil elsewhere. 8 V ions re generted in n inductively coupled plsm (ICP) nd introduced into the SIFT through n tmosphere/vcuum interfce. The ICP ion source nd the interfce re shown schemticlly in Figure 1. 9 A rdio frequency power /jp004197t CCC: $ Americn Chemicl Society Published on Web 04/07/2001

2 4260 J. Phys. Chem. A, Vol. 10, No. 17, 2001 Koyngi et l. Figure 1. Schemtic view of the ICP/SIFT pprtus. supply nd ICP torch ssembly (stndrd concentric design) from commercil instrument (ELAN series, PE/SCIEX) hve been modified for stnd-lone opertion. Typicl prmeters re s follows: the rf power supply is set to trnsmit 120 W of power (free-running frequency, 40 MHz), nd the flows of rgon into the three torch inputs re djusted to 14 to 1 l min -1 (outer gs), 2 l min -1 (intermedite or plsm gs), nd 1.2 l min -1 (inner or nebulizer gs). The ICP/SIFT interfce consists of differentilly pumped smpling nd skimmer cones with orifice dimeters of 1.14 nd 0.88 mm, respectively, nd.0 mm shdow stop to block out UV photons. The region between the smpling cone nd skimmer is pumped by 1000 l min -1 rotry vne pump (Alctel 2060) nd the region fter the skimmer is pumped by 200 l s -1 turbo-moleculr pump (Leybold Hy.Cone 200). The existing qudrupole for regent ion selection hs been ugmented with n rf-only qudrupole prefilter 116 mm in length. This prefilter is cpcitively coupled to the resolving qudrupole with mtched set of high-voltge 100 pf cpcitors. Typiclly, the prefilters re floted t -6 V with respect to the instrument ground coupled through 20 MΩ resistors. The vndium slt solution is derived from 1000 µg ml -1 tomic spectroscopy stndrd solution of mmonium metvndte, NH 4 VO (PE pure grde, stbilized with 2% nitric cid), diluted 2:1 to 40 µg ml -1. It is peristticlly pumped (Gilson Minipuls, two chnnel) to nebulizer t 0.6 ml min -1. The nebulizer is of crossed-flow gem-tip design, sprying into Scott-type double-pss chmber. The nebulized solution is crried to the plsm region by n lumin injector. The smpled plsm ions re filtered with qudrupole mss filter (Q1 in Figure 1) nd injected into the flow tube through Venturitype interfce. Helium is used s the SIFT buffer gs t 0.47 mbr nd flow-tube temperture of 296 ( 2 K. The selected ions entering the flow tube re llowed to thermlize by collisions with He (c collisions) prior to reching the rection region further downstrem t the point of ddition of the regent gs. The extent to which thermliztion of tomic vndium ction is chieved is unknown, however, nd quenching of the excited F stte of tomic V ction by collision with rre gses ppers to be prticulrly difficult. 10 According to known electronic sttes, 11 V emerges in the D, F, F, P, nd H sttes with electronic energies (lowest microstte) of 0, 0.2, 1.1, 1.4, nd 1. ev, respectively, in popultion distribution of 60:29::1:1 t nominl plsm temperture of 00 K. However, the experiments reported here give no obvious indiction for the presence of more thn one electronic stte of V in the rection region. In the rection with O 2, the observed decy of V is liner over more thn two decdes. Rectnt nd product ions re monitored still further downstrem TABLE 1: Comprison of Clculted nd Experimentl Properties of Smll Ions nd Molecules (ll properties in ev) property clcd exptl devition (rel.) D 0(O-O) ( (0.1%) D 0(V -O) ( 0.10 b 0. (8.8%) D 0(V-O) ( 0.09 c 0.19 (2.9%) IE(V) d 0.17 (2.%) IE(VO) e (2.%) D 0(HO-H) ( (4.%) Chse, M. W., Jr.; Dvies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonld, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Dt 198, 14, Suppl. 1 (JANAF Tbles). b Clemmer, D. E.; Aristov, N.; Armentrout, P. B. J. Phys. Chem. 199, 97, 44. c Reference 21. d Reference 11. e Reference 7. by smpling with skimmer nd nlyzing the ions with second qudrupole mss filter (Q2). Ions re counted with chnneltron electron multiplier. Further, nose-cone voltge experiments re conducted to probe the bond connectivities of some of the vndium oxide ions by multicollision induced dissocition. The regents O 2 (Mtheson, UHP, 99.8% min) nd D 2 O (MSD Isotopes, 99.8 tom % D) re used s purchsed; D 2 O is dded into the rection region s % mixture in helium. Rection rte constnts re determined from the mesured semilogrithmic decy of the rectnt ion s function of the flow of the dded regent gs. Theoreticl Methods The geometries nd energetics of the vndium oxide species reported here re obtined t the density functionl level of theory using the BeckeLYP hybrid method 12 s implemented in the Gussin94 progrm pckge 1 in combintion with triple-ζ qulity vlence bsis sets (TZV) for oxygen nd vndium s developed by Ahlrichs. 14 The oxygen nd vndium bsis sets re complemented with n dditionl p nd d polriztion function, 1 respectively. The comprble vlence triple-ζ bsis from Ahlrichs, lso complemented with n dditionl polriztion function, is used for hydrogen. 1 To determine the suitbility of these bsis sets for the problems tckled here, some test clcultions hve been performed. As cn be seen from the dt summrized in Tble 1, the bond dissocition nd ioniztion energies of smll frgments such s V, VO, O 2, nd H 2 O re properly described within c. % of the experimentl vlues. The lrge devition of 8.8% for D 0 (V -O) cn be ttributed to the inpproprite description of the bre vndium ction with the BLYP pproch; for detiled discussion, see ref 16. All sttionry points re fully optimized geometries nd chrcterized s minim or first-order trnsition structures by frequency clcul-

3 Gs-Phse Chemistry of Bre V J. Phys. Chem. A, Vol. 10, No. 17, tions. In cses where trnsition structures re reported, their connection to minim is ssured by internl rection coordinte (IRC) clcultions. All energies reported include corrections for zero-point vibrtionl energy (ZPVE), unless stted otherwise. The computtions hve been performed on either IBM/RS 6000 worksttions or CRAY-YMP supercomputer. Experimentl Results The mss-spectrometric experiments use two complementry techniques. FTICR ensures strictly bimoleculr rections due to the extremely low-pressure regime pplied (<10-6 mbr). Accurte precursor ion selection nd high mss resolution llow for sfe ssignment of the ions observed to specific chemicl processes. In turn, the SIFT experiments involve typicl operting pressures in the mbr regime, thereby ensuring effective ion thermliztion nd llowing for ssocition rections vi termoleculr stbiliztion with the helium buffer gs. As demonstrted previously, the two methods cn complement ech other nd led to internlly consistent results. 7 FTICR Experiments. Under low pressure conditions of FTICR experiments, the only process observed when trpping vndium ction in moleculr oxygen is oxygen tom trnsfer ccording to rection 1. V O 2 f VO O (1) The verge of the experimentl rte constnts determined t three different pressures mounts to (2.67 ( 0.22) cm molecule -1 s -1 ; considertion of the error in the bsolute pressure mesurement leds to k 1 ) (2.7 ( 0.8) cm molecule -1 s -1 s conservtive estimte. This vlue corresponds to rection efficiency of 40% reltive to the gs-kinetic collision rte 17 of cm molecule -1 s -1. A previous ICR study of rection 1 gve rte constnt bout twice s high s our vlue. 18 However, we prefer our result s these uthors similrly overestimted the rte constnt for the Fe / N 2 O system, 19 which hs been nlyzed more recently in some detil. 7,20 Aside from the excellent greement with the SIFT dt reported below, preference for our vlue of k 1 is further supported by guided ion bem studies of Armentrout nd coworkers 21 in which n efficiency of 0% ws reported for rection 1. At longer rection times, other products re formed in smll mounts, such s VOH n (n ) 1, 2) nd VO 2 H n (n ) 0-2), which re due to bckground contminnts present in the high-vcuum system, inter li wter, nd show chrcteristic dy-to-dy vritions. If wter is delibertely dded, bre V rects lmost exclusively under formtion of VO ccording to rection Interestingly, the forml dduct ion [V,O,H 2 ] is lso observed in low but yet significnt bundnces (rection 2b, brnching rtio c. 1%). 2 V H 2 O f VO H 2 f [V,O,H 2 ] (2) (2b) Despite being considerbly exothermic (0.9 ev t 0 K), rection 2 proceeds rther slowly with k 2 ) (8. ( 2.) cm molecule -1 s -1 which corresponds to less thn one percent of the gs kinetic collision rte of cm molecule -1 s -1. Qulittively, this result is consistent with rection 2 being kineticlly controlled s predicted by Irigors et l. 16 Quntittively, however, their computed brrier height of 0.6 ev bove Figure 2. Apprent rte constnt for ssocition of VO with wter s function of wter pressure s determined by FTICR mesurements. the entrnce chnnel seems to be slightly too lrge for the rection to occur t ny notble extent t room temperture. Subsequently, the VO ion undergoes clustering with wter ccording to rection to yield [V,O 2,H 2 ]. Kinetic nlysis implies tht the minor [V,O,H 2 ] product lso ffords the [V,O 2,H 2 ] ion in the presence of wter concomitnt with loss of moleculr hydrogen (rection 4). VO H 2 O f [V,O 2,H 2 ] () [V,O,H 2 ] H 2 O f [V,O 2,H 2 ] H 2 (4) Lignd ssocition in the highly diluted gs phse is likely to involve rditive s well s termoleculr processes. 24 Not t ll unexpected for such smll system, the ssocition rection is inefficient in the pressure regime of the ICR experiments. 2 Exmintion of rection t different wter pressures llows the deconvolution of the rditive nd termoleculr contributions to the pprent bimoleculr rte constnt k,pp. The dt shown in Figure 2 led to the expression k,pp ) (9 ( ) cm molecule -1 s -1 (2. ( 1.) 10-2 cm 6 molecules -2 s -1 p(h 2 O); here, the first term expresses the rditive stbiliztion of the trnsient [V,O 2,H 2 ] species nd the second term represents termoleculr stbiliztion with wter cting s third body. Subsequently, the [V,O 2,H 2 ] product continues clustering with wter, rections nd 6. [V,O 2,H 2 ] H 2 O f [V,O,H 4 ] () [V,O,H 4 ] nh 2 O f [V,O n,h 42n ] (6) Not surprisingly, the pprent rtes of ssocition increse with the number of wter lignds, for exmple, k is bout four times lrger thn k. However, in view of the incresing complexity of the underlying kinetic schemes nd evident signls due to bckground contminnts, we refrin from further nlysis of the subsequent ssocition kinetics of these experiments. V 18 O H 2 16 O f V 16 O H 2 18 O (7) Although it is somewht confusing t first glnce, 16 O/ 18 O- isotopic lbeling turns out to be most instructive. In the rection of wter with mss-selected V 18 O, generted ccording to rection 1 using 18 O 2, degenerte isotope exchnge is observed ccording to rection 7. However, the rte constnt k 7 ) ( ( 2) 10-1 cm molecule -1 s -1 is surprisingly smll compred to hlf of the collision rte ( cm molecule -1 s -1 ) expected for thermoneutrl isotope exchnge. In fct, isotope

4 4262 J. Phys. Chem. A, Vol. 10, No. 17, 2001 Koyngi et l. Figure. Rection profile obtined with the ICP/SIFT pprtus for the oxidtion rection of V with O 2 t low dditions of oxygen. exchnge nd ssocition of wter lignd effectively compete with ech other, even in the low pressure regime of ICR experiments. This observtion suggests tht the initilly formed encounter complex 18 OV( 16 OH 2 ) cn brely interconvert to the qusi-symmetric dihydroxide ion (H 18 O)V( 16 OH), which my serve s n intermedite in 16 O/ 18 O exchnge. Vndium is quite different from the few other trnsition metls studied in this prticulr respect. For exmple, though not occurring t collision rte, 16 O/ 18 O exchnges re only modertely kineticlly hindered in the systems FeO /H 2 O, 26 FeOH /H 2 O, 26,27 Fe 2 O 2 /H 2 O, 28 nd PtO 2 /H 2 O. 29 Moreover, VO 2 undergoes efficient 16 O/ 18 O exchnge in the presence of wter. 0 Interestingly, the [V, 16 O, 18 O,H 2 ] species formed by the ssocition of V 18 O with (unlbeled) wter in nlogy to rection undergoes much more rpid depletion of the 18 O lbel in the presence of wter, rection 8; note tht for this degenerte isotope exchnge, the mximum is one third of the collision rte on sttisticl grounds. [V, 16 O, 18 O,H 2 ] H 2 16 O f [V,O 2,H 2 ] H 2 18 O (8) Kinetic modeling of the time dependencies of the vrious ion intensities gives k 8 ) ( ( ) cm molecule -1 s -1, i.e., c. 100 times fster 16 O/ 18 O exchnge thn in rection 7. Qulittively, this result suggest tht the oxygen toms in [V, 16 O, 18 O,H 2 ] re equilibrted, thus implying fcile interconversion of OV(OH 2 ) nd V(OH) 2 for the long-lived ion. This is precisely the opposite of the bove deduction. As outlined in the discussion, these seemingly contrdictory results re in fct quite instructive s fr s energetic nd structurl spects of the [V,O 2,H 2 ] system re concerned. ICP/SIFT Experiments. The results of the rection of V with oxygen using the ICP/SIFT pprtus re shown in Figures nd 4. V is observed to rect rpidly with oxygen by oxygen tom trnsfer ccording to rection 1 with k 1 ) cm molecule -1 s -1 ; for ll SIFT experiments, the error of the bsolute rte constnts mounts to 0%. In ddition, smll mount of the vndium dioxide ction VO 2 is detected under SIFT conditions. The minor VO 2 chnnel in Figure 4 cn be ttributed to the occurrence of some ssocition ccording to rection 9 with n pproximte Figure 4. Rection profiles obtined with the ICP/SIFT pprtus for the higher order oxidtion chemistry initited by V in O 2 t high dditions of oxygen. brnching rtio k 9 /k 1 of The formtion of the ssocition complex is presumed to occur by collisionl stbiliztion of the encounter complex with helium. V O 2 f VO 2 Higher-order rections re observed t lrger flows of O 2. Thus, VO continues to rect with oxygen in rection 10, but only very sluggishly, with n effective bimoleculr rte coefficient of k 10 ) cm molecule -1 s -1 under SIFT conditions. Despite the wekness of the [V,O ] signl, oxygen ddition ccording to rection 10 ppers s the min rection chnnel of VO. This first ssocition step is followed by rpid dditions of two further oxygen molecules ccording to rections 11 nd 12. Kinetic fits of the [V,O n ] ion profiles provide rte coefficients of k 11 ) nd k 12 ) cm molecule -1 s -1, respectively. Rections 9 to 12 re presumed to occur by termoleculr ssocition, with helium (nd oxygen) cting s the stbilizing third body. There is no evidence for the ddition of fourth oxygen molecule to form [V,O 9 ] ; the kinetic fit in Figure 4 provides n upper limit of e 10-1 cm molecule -1 s -1 for further rections of [V,O 7 ]. Multicollision induced dissocition experiments in He buffer indicte sequentil losses of three O 2 molecules from [V,O 7 ], i.e., the reverse of rections Similr to the ICR experiments, the rection of V with D 2 O ws investigted independently. The deuterted nlogues of both chnnels 2 nd 2b were observed with k 2 ) cm molecule -1 s -1 nd brnching rtio for chnnel 2b of 2 ( 1%. These results re consistent with the ICR mesurements (9) VO O 2 f [V,O ] (10) [V,O ] O 2 f [V,O ] (11) [V,O ] O 2 f [V,O 7 ] (12)

5 Gs-Phse Chemistry of Bre V J. Phys. Chem. A, Vol. 10, No. 17, nd the presence of high mounts of O 2 in the flow my drive rection 16b lso in the reverse direction. [V,O ] D 2 O f [V,O 4,D 2 ] f [V,O 2,D 2 ] O 2 (16) (16b) Anlysis of the rtios of product-to-rectnt ion signls s function of regent flows indictes tht rections 1 nd 14 rech sttionry sttes. If these re ssumed to correspond to chemicl equilibri, equilibrium constnts of 70 nd 6000, respectively, re obtined, which trnsform to vlues for G 298 of nd ev. Furthermore, the hydrtion rections 17 nd 18 tht re evident in Figure pproch sttionry sttes with pprent equilibrium constnts of nd , leding to G 298 (17) )-0. ev nd G 298 (18) )-0.4 ev, respectively. Note, however, tht formtion of [V,O 6,D 2 ] by rection 14 is ignored in the nlysis (see below). [V,O ] D 2 O f [V,O ] (D 2 O) (17) [V,O ] (D 2 O) D 2 O f [V,O ] (D 2 O) 2 (18) Figure. Rection profiles obtined with the ICP/SIFT pprtus for rections of [V,O n] ions with D 2OtnO 2 flow of molecules s -1 dded upstrem in the flow tube. nd suggest tht helium is not very effective in the collisionl stbiliztion of the dduct ion t 0. Torr. The ppernce of [V,O 4,H 2 ] nd [V,O 6,H 2 ] in Figure 4 is ttributed to the presence of wter s contminnt in the flow system. To further elucidte these hydrtion processes, O 2 is dded upstrem t sufficient flow (c molecules s -1 )to estblish the higher [V,O n ] ions, wheres D 2 O is delibertely dded further downstrem (Figure ). The wter chemistry ppers to be dominted by the exchnge rections of [V,O ] nd [V,O 7 ] with wter, rections 1 nd 14, followed by subsequent hydrtions with further wter molecules. [V,O ] D 2 O f [V,O 4,D 2 ] O 2 (1) [V,O 7 ] D 2 O f [V,O 6,D 2 ] O 2 (14) The ions formed cn be regrded formlly s hydrted vndium oxide ions, i.e., [V,O ] (D 2 O) n nd [V,O ] (D 2 O) n, respectively, nd re observed with n up to 6. According to the initil slopes of the decys of [V,O ] nd [V,O 7 ] shown in Figure, lower estimtes for the rte coefficients of rections 1 nd 14 cn be derived s g nd g cm molecule -1 s -1, respectively. Insted, VO ppers to rect only slowly with D 2 O ccording to rection 1, with n pprent rte constnt of cm molecule -1 s -1. VO D 2 O f [V,O 2,D 2 ] (1) Only smll mounts of [V,O 2,D 2 ] re formed t high flows of D 2 O, s would be expected if this ion rects further with D 2 O nd/or O 2 (see below). Therefore, only lower limit g cm molecule -1 s -1 cn be ssigned to the rection of [V,O ] with D 2 O becuse both the ssocition rection (16) nd the lignd-exchnge process (16b) re fesible chnnels, Multiple collision-induced dissocition of the [V,O n ] (D 2 O) m ions ffords exclusive losses of intct wter nd/or dioxygen lignds, wheres frgments due to homolytic bond clevges, such s OH rdicl losses, re not observed. Theoreticl Results There re mny possible structurl isomers for the [V,O n ] nd [V,O n,h 2 ] (n ) 1-7) species observed in the experimentl studies. For exmple, lredy system s smll s [M,O 2 ] hs three conceivble structures: n inserted dioxide with OMO connectivity, side-on complex with n intct O 2 moiety, nd n end-on coordinted O 2 lignd leding to superoxide-like MOO structure. 1 Erlier clcultions for ctionic [M,O 2 ] species with M ) Cr, Fe, nd Cu reveled tht the energetic spcings of these isomers vry long the first trnsition row. 2 Thus, the dioxide is strongly preferred for the erly trnsition metl chromium ( E ) 1.4 ev), 2b wheres this preference is reduced for iron ( E ) 0.22 ev) 2 nd even reverses for the lte trnsition metl copper. 2 Although [V,O 2 ] cn sfely be ssumed to hve dioxide structure, the isomerism of the higher MO n systems suggests n even lrger vriety of structures for the [V,O n ] (n ) -7) species, becuse s the number of oxygen toms coordinted to the vndium center increses, the formtion of O 2 nd perhps even O lignds might become energeticlly fvored. Another structurl dichotomy emerges with the forml wter complexes [V,O n ] H 2 O. In these systems, the formtion of O-H bonds with the oxygen toms might be preferred over the presence of n intct wter lignd. For exmple, it hs recently been shown tht in the [Fe,O 2,H 2 ] system the iron dihydroxide ction Fe(OH) 2 is 0.6 ev more stble thn the hydrted metl oxide ction OFe- (OH 2 ). 4 In ddition to the structurl mnifold of the [V,O n ] nd [V,O n,h 2 ] species, vrious electronic sttes need to be considered, nd n intuitive ssignment of the ground-stte multiplicity is difficult, if not impossible. 26 To provide insight into the structures of the [V,O n ] (n ) 1-7) nd [V,O n,h 2 ] (n ) 2, 4) species formed in the experiments, the structures re studied by clcultions t the BLYP level of theory. The results of these clcultions re summrized below; with respect to the bsic [V,O,H 2 ] system, we refer to the extensive theoreticl work performed by Uglde

6 4264 J. Phys. Chem. A, Vol. 10, No. 17, 2001 Koyngi et l. TABLE 2: Totl Energies (E tot ), ZPVE Corrected Energies (E corr ), Gibbs Free Energies (G 298K ), nd Entropies (S 298K ) for Selected Frgments Computed t the BLYP/TZV Level of Theory species stte E tot E corr G 298K S 298K [cl mol -1 K -1 ] O P O 2 Σ OH 2 Π O 1 A H 2O 1 A V G D V 4 F D VO Σ VO 4 Σ VOH b 4 A A Selected geometric prmeters: O 2, r clc ) 1.21 vs r exp ) Å; O, r clc ) 1.26 vs r exp ) Å nd R clc ) 118. vs 117. ; OH, r clc ) 0.98 vs r exp ) Å; H 2O, r clc ) 0.96 vs r exp ) 0.97 Å nd R clc ) 10.0 vs 104. ; VO, r ) 1. Å; 4 VO, r ) 1.9 Å; 4 VOH, r VO ) 1.7 Å, r OH ) 0.97 Å, R)164 ; 2 VOH, r VO ) 1.74 Å, r OH ) 0.97 Å, R)11. b 4VOH cn be considered s qusiliner molecule becuse the liner stte VOH ( 4 Σ - ), hving two imginry modes is only little higher in energy (E tot ) ) nd even more stble when ZPVE is included (E corr ) ). nd co-workers. 16 For the ske of clrity, the presenttion is divided into five subsections: (i) frgments, (ii) [V,O n ] with n ) 2 nd, (iii) [V,O 4 ], (iv) [V,O n ] with n ) -7, nd (v) [V,O n,h 2 ] with n ) 2 nd 4. Frgments. Tble 2 summrizes sttes, energies, nd entropies clculted for the monoxides VO nd VO, the hydroxide ction VOH, nd the bre vndium species V nd V long with the lignds O, O 2,O, OH, nd H 2 O. The ground-stte configurtions given for ll species in Tble 2 reproduce those tbulted in compiltions of Moore 11 nd Herzberg. The bond lengths of 1.9 nd 1. Å obtined for VO nd VO gree well with experimentl vlues of r VO ) 1.89 Å 6 nd r VO ) Å. 7 Further, comprison of the geometries clculted for O 2,O, OH, nd H 2 O with tbulted dt 6 shows tht the clcultions yield geometries similr to those estblished by experiments. The good greement between the experimentl nd clculted vlues suggests tht the bsis sets suffice for resonble description of the systems under investigtion. As note of cution, we shll dd, however, tht density functionl theory does not correctly reproduce the energetics of trnsition metl oxides in ll cses. 29 Specificlly, even if the thermochemicl properties of the reference frgments re described resonbly well t the chosen level of theory, the energetics of unknown systems ber certin mount of imponderbility. Benchmrking by clssicl, wve function bsed b initio methods cn lend further confidence to the results, 8 but is not pursued in the present study. [V,O n ] (n ) 2, ). As vndium monoxide hs Σ ground stte, the coupling with the P nd Σ - sttes of tomic nd moleculr oxygen, respectively, my give rise to singlet, triplet, nd quintet multiplicities in the corresponding [V,O n ] ions (n ) 2, ). For [V,O 2 ], ll three isomers discussed bove for the [M,O 2 ] systems re locted s minim (Tble ). The inserted dioxide species, designted s 1 VO 2, is found to be the energeticlly lowest isomer with C 2V -symmetric 1 A 1 ground stte; 9 C s -symmetric A excited stte with two different VO bond lengths is 1.9 ev higher in energy. The side-on, V(O 2 ), nd end-on, V(OO), complexes exhibit B 1 nd Σ - ground sttes nd re 1.98 nd 2.78 ev, respectively, higher in energy thn 1 VO 2. This finding is in good greement with erlier ground-stte ssignments nd the strong metl-oxide bonds formed by erly trnsition metls. The ground-stte geometry (Figure 6) of 1 VO 2 is chrcterized by two equl V-O bonds tht re slightly longer thn tht clculted for free VO (1.6 vs 1. Å) nd n OVO ngle of 106. While the side-on V(O 2 ) complex hs long V-O bonds (1.8 Å), 10% elongtion of the O-O moiety (compred to free O 2 ) points to more thn mere electrosttic interction of the O 2 lignd with the V center. In contrst, the O-O bond length in the end-on V(OO) complex is lmost undisturbed, suggesting predominting electrosttic interction, which is in greement with the lower stbility of the end-on complex nd the oxophilicity of vndium. The preference for triplet sttes in the O 2 -ligted V ctions cn be rtionlized esily by the different bonding schemes. In 1 VO 2, perfect piring of the four d electrons of V ( D) with the four electrons provided by the two O toms ( P) leds to singlet, wheres end-on nd sideon combintions of the two unpired electrons of triplet O 2 ( Σ - ) with V in perfect-piring fshion led to triplet sttes. The lower hlf of Figure 6 displys the computed geometries of the [V,O ] isomers. The presence of three O toms lso llows for the formtion of new oxygen lignd, i.e., ozone, suggesting the possible existence of V(O ) complexes. As cn be seen in the lower hlf of Tble, ll isomers prefer triplet ground sttes. The energies given for the quintet species re derived from single-point clcultions, i.e., verticl excittion t the minimum geometries of the ground sttes (here, triplets) to the quintet surfce. Becuse the single-point clcultions result in energies much higher thn those of the singlet nd triplet species, no further geometry optimiztions of the quintet geometries seem necessry. The isomer with the lowest energy demnd is the C s -symmetric OV(O 2 ) complex, which cn be visulized s VO ction with side-on coordinted O 2 lignd. This description of OV(O 2 ) is consistent with the shortening of r OO by 0.04 Å concomitnt with elongtion of r VO by 0.08 Å in comprison with V(O 2 ) (see bove). Interestingly, OV(O 2 ) hs triplet ground stte rther thn the perfect-piring singlet configurtion intuitively expected for n oxovndium(v) peroxide ction. The computed geometry of the singlet is in ccordnce with this description, i.e., shortened V-O bonds long with n elongted O-O distnce. However, 1 OV(O 2 ) is locted 0.9 ev bove the triplet ground stte. The next higher isomer (E rel ) 0.89 ev) is the plnr OV(OO) complex ( A ) with the O 2 lignd in n end-on coordintion. The trioxide structure VO with A ground stte is locted t E rel ) 1.11 ev nd hs one short nd two longer V-O bonds (r VO ) 1.6 nd 1.72 Å). The longer bonds point to formtion of two V-O single bonds, leving free electron on ech of the two oxygen toms. The ltter nicely rtionlizes the preference of the [V,O ] system to form structure with n O 2 moiety by forming bond between two singly bonded oxygen toms. The fourth isomer investigted is the ozone complex V(O ) t E rel ) 2.74 ev in which ll three oxygen toms re in the sme hemisphere of the V center. On the singlet surfce, only sddle point (ν ) i421 cm -1 ) could be locted for 1 V(O ), nd IRC clcultions revel this structure to be trnsition structure for the degenerte rerrngement of the singlet, i.e., O tom exchnge between 1 OV(O 2 ) nd 1 OV(OO). [V,O 4 ]. Six conceivble structurl isomers re expected for species with the formul [V,O 4 ] on the bsis of the possible ddition of n O 2 molecule to the three isomers found for the [V,O 2 ] system or single O tom to the four isomers found

7 Gs-Phse Chemistry of Bre V J. Phys. Chem. A, Vol. 10, No. 17, TABLE : Totl Energies (E tot ), ZPVE Corrected Energies (E corr ), Gibbs Free Energies (G 298K ), Entropies (S 298K ), nd Reltive Energies (E rel ) of [V,O n ] Species with n ) 2 nd Computed t the BLYP/TZV Level of Theory (lowest lying isomers in bold) species stte E tot E corr G 298K S 298K [cl mol -1 K -1 ] VO 2 1 A A A V(O 2) 1 A B B V(OO) 1 Σ Σ A OV(O 2) 1 A A sp b c OV(OO) 1 A A sp b c VO 1 E A sp b c V(O ) 1 A d A sp b c 0 K vlues with respect to the lowest [V,O n] isomer of ech n. b Energy obtined upon verticl excittion from the energeticlly lowest spin stte to the quintet surfce. c ZPVE neglected. d Trnsition stucture t this level of theory. E rel [ev] Figure 6. Geometries computed for severl isomers of [V,O 2] nd [V,O ] ; bond lengths in Ångstrøm nd ngles in degree. for the [V,O ] system. Five of these re locted using the theoreticl pproch chosen here. Consistent with neutrl nd nionic [V,O 4 ] species, 2, the most stble isomer bers dioxide structure with n end-on coordinted O 2 lignd (Tble 4). The long distnce between the O 2 lignd nd the VO 2 core nd the undisturbed O-O bond indictes the presence of wek electrosttic interction between VO 2 nd the O 2 lignd (Figure 7). The lowest electronic stte for the O 2 V(OO) complex is C 1 symmetric triplet stte with 1 A stte 1.6 ev higher in energy. The preference of the O 2 V(OO) complex for triplet multiplicity is esily rtionlized by the 1 A 1 nd Σ - ground sttes of VO 2 nd O 2, respectively. The next higher isomer ( E rel ) 1.1 ev) is the OV(O ) complex composed of VO ion ligted by n ozone lignd. Here, the 1 A stte is more stble thn the A stte by 0.29 ev, indicting tht on the singlet surfce the ozone lignd is stbilized by the dditionl oxygen lignd on the V center. The A ground stte of the [V,O 4 ] isomer with two side-on coordinted oxygen moieties is found t E rel ) 1.6 ev, with the A nd 1 A sttes t 1.8 nd 2.04 ev, respectively. The two oxygen moieties re slightly rotted with respect to ech other on ll three spin surfces, leding to C 1 symmetries for the V(O 2 ) 2 complexes. The V-O nd O-O bond lengths of V(O 2 ) 2 re similr to those of the side-on V(O 2 ) complexes (see bove). Combintion of end-on nd sideon coordintion of O 2 units to V s present in the V(O 2 )(OO) species leds to A stte t E rel ) 2.1 ev. The lst isomer investigted is the tetroxide VO 4. For this geometry, sttionry points re found only on the triplet nd quintet potentil energy surfces t energies of.18 ( A) nd. ev ( A ), respectively, bove the O 2 V(OO) ground stte. All clcultions on the singlet surfce lwys converge to 1 O 2 V(O 2 ). The ltter finding cn be rtionlized by tking the electrons involved in the bonding into considertion. V ( D) hs four d electrons, nd ech O tom hs two unpired electrons. Assuming equl bonding for ll four O lignds, the number of vlence electrons on the V center is used up by formtion of four V-O single bonds, nd ech of the oxygen lignds is left over with one unpired electron. To chieve singlet multiplicity in this system, two of the electrons hve to hve opposite spin, resulting in n highly energetic open-shell singlet s is emphsized by the 6.9 ev needed for the verticl excittion from the triplet to the singlet surfce. However, the proximity of the oxygen toms fvors the formtion of O-O bonds, leding to the much more stble 1 O 2 V(OO). [V,O n ] (n ) -7). From the results presented in the previous subsections, we cn conclude tht with n incresing number of oxygen toms coordinted to the V center there is cler preference for formtion of O 2 nd O lignds. Accord-

8 4266 J. Phys. Chem. A, Vol. 10, No. 17, 2001 Koyngi et l. TABLE 4: Totl Energies (E tot ), ZPVE Corrected Energies (E corr ), Gibbs Free Energies (G 298K ), Entropies (S 298K ), nd Reltive Energies (E rel ) of [V,O 4 ] Species Computed t the BLYP/TZV Level of Theory (lowest lying isomers in bold) species stte E tot E corr G 298K S 298K [cl mol -1 K -1 ] O 2V(O 2) 1 A A sp OV(O ) 1 A A sp b c V(O 2) 2 1 A A A V(O 2)(OO) 1 A A sp b c VO 4 1 sp d c A A K vlues with respect to the lowest [V,O 4]. b Energy obtined upon verticl excittion from the energeticlly lowest spin stte to the quintet surfce. c ZPVE neglected. d Single point energy on the singlet surfce. Upon optimiztion, the VO 4 geometry converges to the VO 2(O 2) isomer. E rel [ev] Figure 7. Geometries computed for severl isomers of [V,O 4] ; bond lengths in Ångstrøm nd ngles in degree. ingly, only the [V,O ] isomers shown in Figure 8 re considered. 40 The energeticlly lowest isomer is the OV(O 2 ) 2 complex with A ground stte nd n 1 A excited stte (Tble ). According to its geometry, this complex cn be understood svo ction ligted by two equl O 2 units. The next higher isomer is C 1 symmetric quintet stte found t E rel ) 0. ev, which my be described s VO ction coordinted by sideon nd n end-on O 2 lignd. The corresponding triplet stte hs similr structure nd is locted t E rel ) 0.80 ev; not surprisingly, the singlet 1 OV(O 2 )(OO) complex is much higher in energy. The lst isomer locted is the V(O 2 )(O ) complex, which cn be described s V ion coordinted by O 2 nd O moieties. V(O 2 )(O ) exhibits 1 A ground stte t E rel ) 2.79 ev with A stte very close in energy ( E rel ) 0.06 ev). Figure 8. Geometries computed for selected isomers of [V,O ], [V,O 6], nd [V,O 7] ; bond lengths in Ångstrøm nd ngles in degree. Although [V,O 6 ] is not observed in the experiments, it is included in the theoreticl tretment for the ske of completeness. Not surprisingly, the most stble species cn be described s vndyl ction 1 VO 2 coordinted to two end-on O 2 lignds in high-spin fshion. The corresponding triplet O 2 V(OO) 2 ( A) is 0.4 ev bove the ground stte, wheres ll other species studied re much higher in energy. Addition of n O 2 lignd to the OV(O 2 ) 2 complexes leds to the OV(O 2 ) isomers shown in Figure 8. Vndium is no longer ble to coordinte ll O 2 lignds in side-on fshion. Therefore, one of the O 2 lignds binds only with one oxygen in n end-on fshion. These findings re in good greement with hex-coordintion preferred by

9 Gs-Phse Chemistry of Bre V J. Phys. Chem. A, Vol. 10, No. 17, TABLE : Totl Energies (E tot ), ZPVE Corrected Energies (E corr ), Gibbs Free Energies (G 298K ), Entropies (S 298K ), nd Reltive Energies (E Rel ) of [V,O n ] Species with n ) -7 Computed t the BLYP/TZV Level of Theory (lowest lying isomers in bold) species stte E tot E corr G 298K S 298K [cl mol -1 K -1 ] OV(O 2) 2 1 A A sp b c OV(O 2)(OO) 1 A A A V(O 2)(O ) 1 A A sp b c V(O 2) 1 A A A V(O 2) 2(OO) 1 sp b c A A O 2V(OO) 2 1 sp b c A A OV(O 2) 1 A A A K vlues with respect to the lowest [V,O n] isomer of ech n. b Energy obtined upon verticl excittion from the energeticlly lowest spin stte to the quintet surfce. c ZPVE neglected. TABLE 6: Totl Energies (E tot ), ZPVE Corrected Energies (E corr ), Gibbs Free Energies (G 298K ), Entropies (S 298K ), nd Reltive Energies (E rel ) of [V,O n,h 2 ] Species with n ) 2 nd 4 Computed t the BLYP/TZV Level of Theory (lowest lying isomers in bold) species stte E tot E corr G 298K S 298K [cl mol -1 K -1 ] VO H 2O 1 A OV(OH 2) 1 A A sp b c TS 1 A A sp b c V(OH) 2 1 Σ Σ sp b c 4 VOH 2 OH c OV(OH 2)(O 2) 1 A A sp b c V(OH) 2(O 2) 1 A A sp b c 0 K vlues with respect to the lowest isomer of ech group. b Energy obtined upon verticl excittion from the energeticlly lowest spin stte to the quintet surfce. c ZPVE neglected. E rel [ev] E rel [ev] the simplest vndyl complex O 2 V(OH 2 ) 4 formed upon dissolving V 2 O in cidic solutions. 41 [V,O n,h 2 ] (n ) 2, 4). In the experiments, [V,O n,h 2 ] ctions re formed either by ddition of wter to [V,O n-1 ] ions or by O 2 /wter exchnge of [V,O n1 ] species. It is interesting to sk whether these ions correspond to hydrted vndium oxides or if they cn undergo interconversion to the corresponding dihydroxide ions. To evlute the ltter, we hve chosen the smllest system possible consisting of VO nd H 2 O. A A ground stte is found for OV(OH 2 ) (E rel ) 0.1 ev) with the 1 A excited stte t E rel ) 1. ev (Tble 6). As cn be seen in Figure 9, the geometry of the wter lignd is lmost undisturbed compred to free H 2 O, indicting tht the interction is predominntly electrosttic. The dihydroxide species V(OH) 2 corresponds to the globl minimum with Σ - ground stte nd 1 Σ excited stte ( E rel ) 1.1 ev), both with liner geometries nd in good greement with erlier clcultions of Ricc nd Buschlicher. 42 In nlogy to the corresponding [Fe,O 2,H 2 ] system, 26b,4 the vndium dihydroxide ction is found to be 0. ev more stble thn the wter complex of VO. On both spin surfces, the trnsition structures (TSs) for the interconversion of both isomers re chrcterized by single imginry frequencies of i1812 nd i1928 cm -1 for 1 TS nd TS, respectively, which re ssigned to 1,-H shifts from the wter to the oxygen lignd. Energeticlly, singlet nd triplet surfces re lmost like except tht the singlet surfce is bout 1 ev higher in energy throughout. Figure 10 nd the lower hlf of Tble 6 summrize the results for the [V,O 4,H 2 ] system. Addition of n O 2 unit to the [V,O 2,H 2 ] system does not induce drmtic chnges in the overll picture but results in closer energetic spcing of the two isomers nd their different spin multiplicities. The dihy-

10 4268 J. Phys. Chem. A, Vol. 10, No. 17, 2001 Koyngi et l. Figure 9. Geometries computed for OV(OH 2), V(OH) 2, nd the ssocited trnsition structures; bond lengths in Ångstrøm nd ngles in degree. Figure 10. Geometries computed for the [V,O 4,H 2] system; bond lengths in Ångstrøm nd ngles in degree. droxide complex, V(OH) 2 (O 2 ), is gin found most stble with A ground stte nd close-lying 1 A excited stte (E rel ) 0. ev). The liner geometries re disturbed by the dditionl O 2 lignd, leding to bending of the two OH lignds towrd ech other. In contrst, minute chnges in geometry re observed between OV(OH 2 ) nd the corresponding OV(OH 2 )(O 2 ) complex. The only notble difference is the lignment of the wter lignd reltive to the V-O bond, i.e., perpendiculr in OV(OH 2 ) nd prllel in OV(OH 2 )(O 2 ). The ground stte of the OV(OH 2 )(O 2 ) complex is A(E rel ) 0.2 ev) with the 1 A stte only 0. ev higher in energy. Discussion Let us begin with comprehensive discussion of the clculted energetics, wherein we ssume tht only the lowest-lying isomers nd/or sttes re relevnt with respect to the experiments (Tble 7). As fr s the [V,O n ] ctions re concerned, the dt suggest tht dioxygen complexes rther thn low-spin coupled peroxides re formed upon ssocition of moleculr oxygen to VO in the gs phse. The computed sequentil bond energies for the first two dioxygen lignds show only smll decrese from D 0 (OV -O 2 ) ) 1.27 ev to D 0 (OV(O 2 ) -O 2 ) ) 1.17 ev, wheres the third dioxygen lignd in [V,O 7 ] is more wekly bound, D 0 (OV(O 2 ) 2 -O 2 ) ) 0.62 ev. The corresponding rte constnts mesured experimentlly for the consecutive ssocitions of VO with O 2 re k 10 ) cm molecule -1 s -1, k 11 ) cm molecule -1 s -1, nd k 12 ) 2.0 TABLE 7: Bond Dissocition Energies (D 0 t0k)for Ground-Stte Vndium Species (in kcl mol -1 ) Clculted t the BLYP/TZp Level of Theory nd Some Literture Dt Given for Comprison species stte bond D 0 other vlues VO Σ - V -O b VO 2 1 A 1 OV -O c /90.2 d OV(O 2) A OV -(O 2) 29. O 2V(O 2) A O 2V -(O 2) 18.1 OV(O 2) 2 A (O 2)OV -(O 2) 27.1 O 2V(O 2) 2 A O 2V(O 2) -(O 2) 14. OV(O 2) A (O 2) 2OV -(O 2) 14. VOH 4 A V -OH e /98.6 f OV(OH 2) A OV -OH V(OH) 2 Σ - HOV -OH f OV(OH 2)(O 2) A (H 2O)OV -O (O 2)OV -OH V(OH) 2(O 2) A O 2-V(OH) If desired, the vlues cn be converted into hets of formtion t 0 or 298 K using H f,0k(vo ) ) (198.8 ( ) kcl mol -1 nd H f,298k(vo ) ) (197.4 ( ) kcl mol -1 s bsolute nchors in combintion with literture thermochemistry. b Experimentl D 0, see ref 21. c Experimentl D 0, see: Sievers, M. R.; Armentrout, P. B. J. Chem. Phys. 199, 102, 74. d Clculted D 0, see ref for discussion. e Tken from: Armentrout, P. B.; Kickel, B. L. In Orgnometllic Ion Chemistry; Freiser, B. S., Ed.; Kluwer: Dordrecht, 1996; p 1. f Best theoreticl estimte tken from ref cm molecule -1 s -1. The remrkble increse by more thn n order of mgnitude from k 10 to k 11 cn be ttributed to the increse of the lifetime of the corresponding ssocition complexes with incresing moleculr size. Thus, lthough the binding energies re similr, ssocition of VO with O 2 involves six degrees of freedom, wheres twice s mny re vilble for [V,O ]. An incresed lifetime of the encounter complex fvors termoleculr stbiliztion with the helium buffer gs, nd this ccounts for the significnt increse of the pprent rte constnt. For the third ssocition, two effects compete with ech other: wheres the density of sttes increses further, the binding energy shrinks to bout hlf of tht for the two first oxygen lignds. Compenstion leds to n intermedite vlue for k 12. Hence, the experimentl results re qulittively consistent with the computtionlly predicted OV(O 2 ) m structures of the [V,O n ] ions (with m ) 1- nd n ) 2m 1, respectively). Description of the [V,O n ] ions s dioxygen complexes is further supported by the observed lignd-switching rection with wter. In greement with the experimentl findings, the computtions predict D 0 (OV -OH 2 ) ) 1.8 ev nd D 0 (OV(O 2 ) -OH 2 ) ) 2.1 ev for bonding of the wter lignd in the hydrted ions, which re much lrger thn the binding energies of the corresponding dioxygen complexes (see bove). This sitution is complemented with the observtion tht the [V,O ] nd [V,O 7 ] species undergo exchnge rections in competition with ssocition to yield the corresponding hydrted ions, which then continue to cluster with wter. In mrked contrst, ssocition of the ditomic VO ction with either O 2 or H 2 O occurs very slowly, quite obviously due to the low number of degrees of freedom vilble. The behvior of [V,O ] deserves more detiled considertion. In the presence of only oxygen in the helium flow (Figures nd 4), the loss of [V,O ] is rte-limited by its production from VO, giving rise to qusi-sttionry ion bundnce in the percent rnge. As outlined bove, this behvior cn be explined by the different rtes for formtion (k 10 ) cm molecule -1 s -1 ) nd subsequent ssocition (k 11 ) cm molecule -1 s -1 ) of this ion. Theory predicts OV(O 2 ) to be the most stble [V,O ] isomer, with binding chrcteristics of n dioxygen complex nd computed binding

11 Gs-Phse Chemistry of Bre V J. Phys. Chem. A, Vol. 10, No. 17, energy of D 0 (OV -O 2 ) ) 1.27 ev, which is substntilly smller thn D 0 (OV -OH 2 ) ) 1.8 ev. Thus, occurrence of the exchnge rection 16b is predicted by theory. However, Figure implies tht [V,O ] does not rect rpidly with wter, nd specificlly the nticipted lignd-exchnge product [V,O 2,D 2 ] is observed only in trce mounts. Accordingly, both the ssocition ccording to rection 16 s well s the ligndexchnge rection 16b hve very smll rte constnts, if occurring t ll, under SIFT conditions. OV(O 2 ) 1 H 2 O f OV(OH 2 ) O 2 (19) To be more specific, the lowest-lying sttes re considered, i.e., rection 19. BLYP predicts lignd exchnge to be energeticlly fvorble by 0.6 ev t 0 K, nd the predicted G 298 (19) ) ev is even slightly lrger. Thus, theory suggests tht rection 16b is indeed expected to be significnt product chnnel. Although inefficient ssocition my be ccepted, the filure of rection 19 is unexpected on the bsis of the computed binding energies of VO for wter nd oxygen lignds, respectively. There re severl possible resons for this behvior. (i) Lignd-exchnge ccording to rection 19 my be ssocited with considerble brrier tht prevents the process from occurring t therml energies. However, the structurl chrcteristics of the triplet ground sttes of OV(O 2 ) nd OV(O 2 )- (H 2 O) give no indictions for n origin of such brrier. (ii) Becuse of n inblnced description of correltion energy in ctionic trnsition metl oxides, 29 the DFT clcultions my substntilly underestimte the stbility of the singlet coupled peroxide 1 OV(O 2 ), which is computed to be c. 0.6 ev bove OV(O 2 ) t the level of theory pplied. If the singlet species is indeed lower lying, its contribution to the [V,O ] chnnel could well give rise to kinetic hindrnce of rection 19, considering the required chnge in spin multiplicity nd the significntly different structure of 1 OV(O 2 ) compred to OV(O 2 ). (iii) Another explntion of the discrepncies is tht, for some reson, binding to the dipolr wter lignd is exggerted by the DFT pproch in comprison to the dioxygen lignd. (iv) Wheres the previous points doubt the relibility of the clcultions, the interprettion of the experimentl dt is not unmbiguous in this cse. Thus, nlysis of wht obviously is termoleculr process by n pprent rte constnt in terms of bimoleculr process is not necessrily correct. Further, rpid clustering with wter nd/or oxygen my msk the [V,O 2,D 2 ] chnnel. Obviously, it is desirble to further probe the chemistry of the [V,O ] ion. Experimentlly, however, this is mde rther difficult by the unfvorble rtio of k 10 nd k 11 which limits the mount of [V,O ] formed to bout percent of the overll ion current. In ddition, more elborted theoreticl study of [V,O ] using sophisticted wve function bsed methods is indicted. Let us lso comment upon the equilibrium constnts derived from the SIFT experiments with O 2 nd H 2 O (Figure ). Consecutive ssocition of the [V,O n ] ions with O 2 nd wter s well s the competing lignd-exchnge processes give rise to rther complex kinetic sitution. In such cse, qusisttionry sttes rther thn equilibri might be estblished. For exmple, the SIFT dt used to derive the equilibrium constnts for rections 1 nd 17, i.e., involve the [V,O ] precursor for both rections. Moreover, the puttive products my not be formed exclusively from [V,O ] but lso vi ssocition of [V,O ] with wter, lignd exchnge of [V,O 7 ] with wter, etc. Therefore, it is uncertin whether true equilibri re relly estblished in these experiments. This skepticism is supported by the theoreticl results. Thus, the DFT clcultions predict Figure 11. Schemtic triplet potentil energy surfce of the [V,O 2,H 2] system. rection 1 to be much more exothermic ( G 298 )-1.07 ev) thn implied by K eq (1) ) 70 which suggests G 298 )-0.11 ev ccording to the SIFT dt. Finlly, let us ddress the nture of the [V,O n,h 2 ] species formed in the different experiments. While theory predicts the dihydroxide structure V(OH) 2 to be considerbly more stble thn the hydrted oxide ion OV(OH 2 ), the brrier ssocited with intrmoleculr 1,-hydrogen migrtion is substntil (Figure 11). In fct, even the lowest-lying TS is close to the symptote of the rectnts, i.e., only 0.07 ev below VO 1 H 2 O t 0 K. This result cn be correlted with the slow occurrence of 16 O/ 18 O exchnge in rection 7. As chnges in spin multiplicities do not seem to ply role, the experimentl dt cn be used to roughly estimte the brrier height. As n upper limit, we my ssume tht the rtio of the mesured rte constnt k 7 to the respective collision rte obeys n Arrhenius formlism, i.e., E rel ( TS) ) RT ln(k 7 /k c,7 ), where E rel ( TS) is the energy of TS reltive to the VO 1 H 2 O symptote, k 7 ) ( ( 2) 10-1 cm molecule -1 s -1, k c,7 ) cm molecule -1 s -1, nd T is ssumed to be 298 K. This pproch suggests brrier of less thn 0.2 ev bove the entrnce chnnel. On the other hnd, E rel ( TS) cnnot be much below the entrnce chnnel, s much more rpid exchnge is otherwise expected. For exmple, 16 O/ 18 O exchnge occurs with rte constnt of c cm molecule -1 s -1 in the FeO /H 2 O system, where the relevnt TS is situted c. 0.6 ev below the isolted rectnts. 26 Accordingly, the predicted brrier height of ev in the [V,O 2,H 2 ] system is nicely consistent with the experimentl findings. This line of resoning suggests tht the long-lived [V,O 2,H 2 ] species formed by either rditive nd/ or collisionl stbiliztion correspond to OV(OH 2 ) rther thn the more stble V(OH) 2 isomer. Likewise, genertion of OV(OH 2 ) ppers likely in the SIFT experiments considering the high mount of buffer gs present in these experiments. Multiple collisions occurring in the flow tube my, however, led to the formtion of the more stble dihydroxide species vi intermoleculr mechnisms. Specificlly, the energy demnding intrmoleculr 1,-hydrogen migrtion my be circumvented by ctive prticiption of nother wter molecule vi proton-shuttle mechnism, 4 ccording to rection 20. OV(OH 2 ) H 2 O f V(OH) 2 H 2 O (20) Direct evidence in support of this suggestion is the more rpid 16 O/ 18 O exchnge observed for the long-lived [V, 16 O, 18 O,H 2 ] ion; i.e., precisely tht result which gives rise to the seemingly contrdicting interprettions of the lbeling experiments mentioned bove. If, however, we ssume lmost exclusive formtion of OV(OH 2 ) in strictly bimoleculr collisions, but fcile

12 4270 J. Phys. Chem. A, Vol. 10, No. 17, 2001 Koyngi et l. formtion of the more stble V(OH) 2 ion upon interction with nother wter molecule, ll experimentl nd theoreticl results re in hrmony. Following these rguments implies, however, tht the kinetic nlysis of the ssocition of VO with wter ccording to rection in terms of rditive nd termoleculr stbiliztion mechnisms must be considered s rough pproximtion only, 44 becuse it neglects the existence of two low-lying [V,O 2,H 2 ] isomers nd the ctlyzing effect of wter in rection 20. Conclusions Atomic vndium ction is rpidly oxidized to VO in the presence of moleculr oxygen. The monoxide ction, however, rects inefficiently with oxygen, wter, nd severl other neutrl substrtes. 18b The only rections observed in the SIFT experiments re consecutive ssocitions with dioxygen to yield first [V,O ], then [V,O ], nd then [V,O 7 ] s the finl product t room temperture; under ICR conditions, no consecutive rections of VO with O 2 re observed t ll. Density functionl theory describes these [V,O n ] ions s species hving VO core for odd vlues of n nd hving VO 2 core for even vlues of n, which re surrounded by mostly electrostticlly bound dioxygen molecules in their respective ground sttes. Despite these seemingly simple chrcteristics of the V /O 2 rections, the kinetic schemes required for the detiled nlysis re in prt rther complex. In the presence of wter, exchnge of the loosely bounded dioxygen lignds towrd H 2 O s well s consecutive ion hydrtion leding to lrger clusters is observed. Wheres lignd exchnge nd ssocition seems not to be hindered for [V,O ] nd [V,O 7 ], nd probbly even [V,O ], the VO ction rects quite inefficiently with wter. Interestingly, even the degenerte 16 O/ 18 O exchnge between VO nd wter is very inefficient. The computtionl predictions provide very helpful tool for the interprettion of the experimentl findings. In prticulr, theory shows tht the interconversion between the hydrted oxide ion OV(OH 2 ) nd the tutomeric dihydroxide V(OH) 2 is lmost impossible in strictly bimoleculr collisions of VO nd H 2 O. However, the dt lso corroborte the suspicion tht wter my ctively prticipte in tutomeriztion rections in the gs phse. Acknowledgment. The Berlin group pprecites finncil support by the Deutsche Forschungsgemeinschft (SFB 46), the Volkswgen-Stiftung, nd the Fonds der Chemischen Industrie. Further, we thnk the Konrd-Zuse Zentrum Berlin for generous lloction of computer time. The York group recognizes the continued support by the Nturl Sciences nd Engineering Reserch Council of Cnd. The uthors pprecite helpful comments by nd coopertion with A. Irigors Bld nd J. Uglde. References nd Notes (1) Appl. Ctl. A, 1997, 17 (entire volume dels with this topic). (2) () Almond, M. J.; Atkins, R. W. J. Chem. Soc., Dlton Trns. 1994, 8. (b) Chertihin, G. V.; Bre, W. D.; Andrews, L. J. Phys. Chem. 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Atomic Energy LeVels, Ntionl Stndrd, Reference Dt Series, Ntionl Bureu of Stndrds, NSRDS-NBS, Wshington, DC, (12) () Lee, C.; Yng, W.; Prr, R. G. Phys. ReV B 1988, 7, 78. (b) Miehlich, B.; Svin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 17, 200. (c) Becke, A. D. J. Chem. Phys. 199, 98, 648. (1) Gussin 94 (Revision A.1), Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheesemn, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Rghvchri, K.; Al- Lhm, M. A.; Zkrzewski, V. G.; Ortiz, J. V.; Foresmn, J. B.; Cioslowski, J.; Stefnov, B. B.; Nnykkr, A.; Chllcombe, M.; Peng, C. Y.; Ayl, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Mrtin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Bker, J.; Stewrt, J. P.; Hed-Gordon, M.; Gonzlez, C.; Pople, J. A., Gussin Inc., Pittsburgh 199. (14) Schfer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 829. (1) Schfer, A.; Horn, H.; Ahlrichs, R. J. Chem. 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(2) In the cse of incomplete thermliztion of the V precursor, trce mounts of VOH re lso observed. Genertion of VOH from V /H 2O is clerly n endothermic process, which we ttribute to the presence of some electroniclly excited V under these circumstnces. (24) For recent guide to the literture, see: Gpeev, A.; Dunbr, R. C. J. Phys. Chem. A 2000, 104, 4084, nd references therein. (2) See lso: Kretzschmr, I.; Schröder, D.; Schwrz, H.; Rue, C.; Armentrout, P. B. J. Phys. Chem. A 1998, 102, (26) () Brönstrup, M.; Schröder, D.; Schwrz, H. Chem. Eur. J. 1999,, (b) Bärsch, S.; Schröder, H.; Schwrz, H. Chem. Eur. J. 2000, 6, (27) Blum, O.; Stöckigt, D.; Schröder, D.; Schwrz, H. Angew. Chem. 1992, 104, 67; Angew. Chem., Int. Ed. Engl. 1992, 1, 60. (28) Jckson, P.; Hrvey, J. N.; Schröder, D.; Schwrz, H. Int. J. Mss Spectrom. 2001, 204, 2. (29) For drmtic filure of DFT methods in describing [Pt,O 2], see: Brönstrup, M.; Schröder, D.; Kretzschmr, I.; Schwrz, H.; Hrvey, J. N. J. Am. Chem. Soc. 2001, 12, 142. (0) The mesured rte constnt for 16 O/ 18 O exchnge of 18 OV 16 O with wter is cm molecule -1 s -1. (1) Survey: Schröder, D.; Shik, S.; Schwrz, H. In Metl-Oxo nd Metl-Peroxo Species in Ctlytic Oxidtions in the series Structure nd Bonding, Vol. 97; Meunier, B., Ed.; Springer, Berlin, 2000, p 91. (2) () Sülzle, D.; Schwrz, H.; Moock, K. H.; Terlouw, J. K. Int. J. Mss Spectrom. Ion Processes 1991, 108, 269. (b) Schröder, D.; Fiedler, A.; Schwrz, J.; Schwrz, H. Inorg. Chem. 1994,, 094. (c) Fiedler, A.; Kretzschmr, I.; Schröder, D.; Schwrz, H. J. Am. Chem. Soc. 1996, 118, () Hrvey, J. N.; Diefenbch, M.; Schröder, D.; Schwrz, H. Int. J. Mss Spectrom. Ion Processes 1999, 182/18, 8. (4) Bärsch, S.; Schröder, D.; Schwrz, H. HelV. Chim. Act 2000, 8, 827. () Herzberg, G. Moleculr Spectr nd Moleculr Structure, reprint edition; Krieger, Mlbr; Vol. I, 1989 Vol III, (6) Lide, D. R. CRC Hndbook Chemistry nd Physics, CRC Press: Boc Rton, 1998/99. (7) Hrrington, J.; Weisshr, J. C. J. 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13 Gs-Phse Chemistry of Bre V J. Phys. Chem. A, Vol. 10, No. 17, (8) Koch, W.; Holthusen, M. C. A chemist s guide to density functionl theory; Wiley-VCH: Weinheim, (9) Although 1 VO 2 corresponds to forml closed-shell singlet, s do most other trnsition metl dioxides, the molecule is bent. For n explntion of bending, see: Siegbhn, P. E. M. J. Phys. Chem. 199, 97, (40) All structures hve been optimized in C 1 symmetry first, llowing the systems to dopt the most fvorble geometries. Upon convergence to higher symmetries, the species hve been further optimized in those symmetries. Severl strting geometries hve been investigted, which converged to one of the three geometries shown. (41) Shriver, D. F.; Atkins, P. W.; Lngford, C. H. Inorgnic Chemistry, 2nd ed.; Freemn & Co.: New York, (42) Ricc, A.; Buschlicher, C. W., Jr. J. Phys. Chem. 1997, 101, (4) Bohme, D. K. Int. J. Mss Spectrom. Ion Processes 1992, 18, 9. (44) For recent exmple using slightly more complex rte expression in ssocition kinetics, see: Schröder, D.; Brown, R.; Schwerdtfeger, P.; Schwrz, H. Int. J. Mss Spectrom. 2000, 20, 1.