Journal of the University of Chemical M. Georgiev, Technology D. Stoilova and Metallurgy, 42, 2, 2007, 211-216 METAL-WATER INTERACTINS AND HYDRGEN BND STRENGTH M. Georgiev 1, D. Stoilova 2 1 University of Chemical Technology and Metallurgy 8 Kl. hridski, 1756 Sofia, Bulgaria E-mail: Mit.georgiev@mail.bg Received 05 February2007 Accepted 20 May 2007 2 Institute of General and Inorganic Chemistry Bulgarian Academy of Sciences Akad. G. Bonchev str., bl.11, 1113, Sofia, Bulgaria E-mail:stoilova@svr.igic.bas.bg ABSTRACT The strength of the hydrogen bonds formed in some acetates,, Zn(CH 3 and, as deduced from the infrared wavenumbers of the respective uncoupled D stretching modes (matrix-isolated HD molecules) is discussed in terms of hydrogen bond lengths w, metal-water interactions (synergetic effect) and proton acceptor capabilities of the acetate oxygen atoms (competitive effect). The spectroscopic experiments reveal that water molecules bonded to Zn 2+ ions form stronger hydrogen bonds due to both the shorter Zn-H 2 bond distances and the increasing covalence of the respective bonds as compared to those coordinated to Ba 2+ ions. Four D oscillators are deduced from the infrared bands in the spectrum of, thus indicating the existence of at least two water molecules in the double salt which are bonded to both metal ions. The intramolecular -H distances are derived from the novel ν D vs. r H correlation curve [J. Mol. Struct. 404 (1997) 63]. Keywords: Metal acetate hydrates (Me = Ba, Zn); IR matrix - spectroscopy, hydrogen bond strength, synergetic effect. INTRDUCTIN The strength of the hydrogen bonds in crystal hydrates is governed by both the hydrogen bond donor strength of the hydrogen bond donors and the hydrogen bond acceptor capability of the hydrogen bond acceptors. Bonding interactions of the donors and acceptors with other entities in the structure additionally modify this strength: (i) metal-water interactions (synergetic effect) [1-4 and Refs. therein]; (ii) cooperative effect (water molecules as hydrogen bond donors and acceptors) [1,2,5 and Refs. therein]; (iii) anti-cooperative or competitive effect (different proton acceptor strength of the atoms building up one acceptor group) [1-3 and Refs. therein]; (iv) repulsive potential forces at the respective lattice sites (i.e. unit-cell volumes) [1-4], etc. In the case of synergetic effect, i.e. the bonding of water molecules to metal ions, the -H bonds of the water molecules are both weakened and polarized with the increasing strength of the respective Me H 2 bonds 211
Journal of the University of Chemical Technology and Metallurgy, 42, 2, 2007 and, hence, the acidity of the respective hydrogen atoms is increased. The synergetic effect increases with increasing charge and decreasing size of the metal ions as well as with increasing covalence of the Me-H 2 bonds. Thus, the alkali and alkali-earth metal ions exhibit a weaker synergetic effect as compared to those of the first transition metal series [1,2]. As a result of the different metal-water interactions the hydrogen bonds formed in crystal hydrates of the d-metals are stronger than those formed in crystal hydrates of alkali and alkali-earth metals, i.e. the respective ν H(D) are shifted to lower frequencies. The present paper aims at studying the influence of the metal-water interactions on the strength of the hydrogen bonds present in the metal acetates, and Zn(CH 3. An attempt is made to analyze the hydrogen bond strength in (a compound with unknown structure) and to deduce the coordination of the water molecules to the metal ions. The method of infrared matrix-spectroscopy (matrix-isolated HD molecules) is used to investigate the hydrogen bonding systems in the above acetates. observed (infrared spectra using Nujol mulls were also measured). RESULTS AND DISCUSSIN Crystal structures of and Zn(CH 3 According to the structural data crystallizes in the triclinic centric space group [8]. A part of the crystal structure is shown in Fig. 1. Due to the low symmetry of the unit - cell all species (two crystallographically different barium ions, Ba1 and Ba2 nine and eight coordinated, respectively; four crystallographically different acetate ions, and two crystallographically different water molecules, w 1 and w 2) are in C 1 site symmetry. The water molecules are EXPERIMENTAL and Zn(CH 3 C) 2 2H 2 were prepared by re-crystallization of commercial products in aqueous solutions at 30 o C according to the solubility polytherm (barium acetate monohydrate) [6] and at 25 o C (zinc acetate dihydrate). A slight excess of acetic acid was added to the solutions in order to prevent the hydrolysis processes. The crystals were filtered, washed with alcohol and dried in air. The double salt,, was obtained according to the solubility diagram of the Zn(CH 3 C) 2 - C) 2 - H 2 system at 30 o C [7]. Isotopically dilute samples containing matrix isolated HD molecules (ca 10 % D 2 ) were prepared using the same crystallization procedure in the presence of heavy water. The reagents used were p.a. quality (Merck). The infrared spectra were recorded on the Bruker model IFS 25 and IFS 113 Fourier transform interferometers (resolution < 2 cm -1 ) at ambient and liquid nitrogen temperatures using KBr discs as matrices. Ion exchange or other reactions with KBr have not been Fig. 1. Crystal structure of (dash lines hydrogen bonds) Fig. 2. Local environments of the acetate oxygen atoms in 212
M. Georgiev, D. Stoilova Table 1. Assignments of the hydrogen bonds in metal acetates. Hydrogen bonds ν H ν H ν D ν D w w H H Me H 2 RT LNT RT LNT exp cal cal cal exp (v.u.) w 1 21 3566 3566 2620 2620 3.109 3.062 2.125 0.952 2.791 1.65 w 1 41 3340 3320 2474 2452 2.750 2.769 1.816 0.971 1.60 w 2 21 3240 3240 2474 2440 2.738 2.756 1.802 0.973 2.744 1.65 w 2 41 3490 3485 2570 2570 2.823 2.943 1.999 0.957 1.60 Zn(CH 3 w 1 2 3137 3218 2352 2325 2.675 2.652 1.692 0.984 1.987 1.69 w 1 3 3073 2352 2325 2.711 2.652 1.692 0.984 1.71 3446 3405 2568 2546 2.899 1.953 0.958 3413 3390 2520 2509 2.841 1.892 0.962 3170 3014 2334 2282 2.620 1.659 0.989 2976 2334 2212 2.575 1.611 0.998 Σ Fig. 3. Crystal structure of Zn(CH 3 (dash lines hydrogen bonds). coordinated to Ba1 and the respective bond lengths have values of 2.791 and 2.744 Å (Ba- w 1 and Ba- w 2, respectively). The different local environments of the acetate ions are shown in Fig. 2. It is seen that the oxygen atoms 21 and 41 are monodentately bonded to Ba 2+ ions and as a result they exhibit the lowest Brown s bond valence sums among all oxygen atoms 1.6508 and 1.6034 v. u., respectively (BVS are calculated according to Brese and Keeffe [9]). Due to the small values of the bond valence sums the oxygen 21 and 41 act as acceptors of two protons (Table 1). Each water molecule forms two hydrogen bonds with both the oxygen atoms 21 (located 213
Journal of the University of Chemical Technology and Metallurgy, 42, 2, 2007 3566 3566 3490 3485 3320 3446 3405 3218 3413 3240 3340 BaAc 2.H 2 3240 3137 2620 2620 2570 2474 2570 2452 ZnAc 2.2H 2 2440 2352 BaZnAc 4.2H 2 3073 3170 3014 3390 2976 2568 2520 3600 3200 2800 2400 2325 2334 2282 2509 2546 Wavenumbers, cm -1 2212 Fig. 4. Infrared spectra of the metal acetates in the regions of H and D vibrations (, ambient temperature, - liquid nitrogen temperature) between the layers) and the oxygen atoms 41 (located within the layers) (hydrogen bonds are calculated from the structural data as w bonds, see Table 1). Zn(CH 3 belongs to the monoclinic space group C2/c [10]. Four acetate oxygen atoms are bidentately (chelate) bonded to the Zn 2+ ions. Two water molecules complete the coordination of the Zn 2+ ions (Zn- w bond length is 1.987 Å) (Fig. 3). Due to the C 1 site symmetry the water molecule (one crystallographical type) forms two hydrogen bonds (2.675 and 2.711 Å bond lengths, see Table 1). The bond valence sums of the oxygen atoms have close values 1.69 and 1.71 v. u. Infrared spectroscopy study Infrared spectra of the metal acetates under study in the region of the vibrations -H and -D (matrix isolated HD molecules) are shown in Fig. 4 (see also Table 1). Three bands at 2620, 2570 and 2474 cm -1 corresponding to three D oscillators are observed in the spectrum of the barium acetate monohydrate at ambient temperature. The band at 2474 cm -1 shifts to lower frequencies and transforms into two bands (2452 and 2440 cm -1 ) at liquid nitrogen temperature. The bands at the higher wavenumbers (2620 and 2570 cm -1 ) are attributed to hydrogen bonds w 1 21 and w 2 41, respectively, (bond distances 3.109 and 2.823 Å, Table 1). The band at the lowest frequency (2440 cm -1 ) is assigned to hydrogen bonds of the type w 2 21 due to both the shorter bond length (2.738 Å) and the shorter Ba- w 2 bond distance (2.744 Å, i.e. stronger synergetic effect) as compared to hydrogen bonds formed by w 1 (band at 2452 cm -1 and Ba- w 1 bond length 2.791 Å). The spectroscopic findings evidence that the water molecules w 1 are more asymmetrically hydrogen bonded than w 2 ( ν have values of 168 and 130 cm -1, respectively). The water molecule in Zn(CH 3 is expected to exhibit two bands corresponding to two different hydrogen bonds. However, as Fig. 4 shows, the water molecule in Zn(CH 3 forms hydrogen bonds of equal strength, irrespective of the different hydrogen bond lengths (band at 2352 cm -1 at ambient temperature and 2325 cm -1 at liquid nitrogen temperature) owing to both the very strong synergetic effect of the Zn 2+ ions and the close proton acceptor capabilities of the oxygen atoms (see Table 1). 214
M. Georgiev, D. Stoilova exhibits three bands in the region of the D vibrations of the matrix-isolated HD molecules (2568, 2520 and 2334 cm -1, ambient temperature) which shift to lower frequencies on cooling. Furthermore, the band at the lowest wavenumber transforms into two bands at 2282 and 2212 cm -1 (liquid nitrogen temperature, see Fig. 4). The spectroscopic experiments allow us to deduce the existence of at least four different D oscillators, i.e. the existence of at least two crystallographically different water molecules which form hydrogen bonds of quite different bond strengths. Taking into account the hydrogen bond strengths (i.e. the infrared band positions) in the simple salts, and Zn(CH 3, the bands at lower frequencies in the spectrum of are assumed to be due to hydrogen bonds formed by water molecules coordinated to Zn 2+ ions and those at higher frequencies to hydrogen bonds formed by water molecules coordinated to Ba 2+ ions. Almost all infrared bands display a positive temperature dependence ( ν/ T > 0) (see Table 1), thus indicating the formation of nearly linear hydrogen bonds (i.e. H > 140 o C) [1,3,5,11]. The intermolecular H and w bond distances calculated according to the traditional correlation curve of Mikenda [12] (i.e. ν D vs. H and w bond lengths) are presented in Table 1. Recently linear correlations between the intramolecular bond valences s H(D) and s H(D) of water molecules in condensed materials and the wavenumbers of the respective uncoupled D stretching modes of matrix-isolated HD molecules have been established [13,14]. The correlation curves allow calculations of the intramolecular -H bond lengths using infrared and Raman wavenumbers of the D vibrations in isotopically dilute samples [14] (see Table 1). The formation of hydrogen bonds of different strengths in the metal acetates reflects on the shape of the spectra in the high frequency region (3200-3600 cm -1 ) where the -H stretching modes occur. The high frequency bands in the spectrum of display small half-widths owing to the formation of comparatively weak hydrogen bonds. The bands at 3566 and 3320 cm -1 are attributed to ν as and ν s of w 1, and those at 3485 and 3240 cm -1 to ν as and ν s of w 2, respectively. The isotopic ratios ν H /ν D have values of 1.36 for the bands at 2620, 2570 and 2452 cm -1, and 1.33 for the lowest wavenumbered band at 2440 cm -1. The spectrum of Zn(CH 3 exhibits a broad band with a maximum at 3137 cm -1 ( ambient temperature) which shifts to lower frequencies at liquid nitrogen temperature (bands at 3218 and 3073 cm -1 ). The large half-width of the bands is owing to the strong interactions of the identical oscillators, i.e. to the formation of strong hydrogen bonds in the zinc compound. Two doublets are distinguished in the spectrum of the double salts two bands at 3405 and 3390 cm -1 which could be assigned to water molecules bonded to Ba 2+ ions and 3014 and 2976 cm -1 assigned to water molecules bonded to Zn 2+ ions, respectively. The isotopic ratios ν H /ν D have values of 1.34 for the high frequency bands (3405 and 3390 cm -1 ) and 1.31 and 1.35 for the lower wavenumbered bands (3014 and 2976 cm -1 ). CNCLUSINS The analysis of the infrared spectra of, Zn(CH 3 and reveals that: (i) The Zn 2+ ions exhibit strong Me-H 2 interactions (i.e. strong synergetic effect) due to the covalent character of the respective bonds, thus facilitating the formation of strong hydrogen bonds; (ii) The water molecules coordinated to Ba 2+ ions are slightly polarized and as a result comparatively weak hydrogen bonds are formed; (iii) on the basis of the spectroscopic experiments the water molecules in are assumed to be bonded to both metal ions. REFERENCES 1. H.D. Lutz, B. Engelen, Trends Appl. Spectrosc., 4, 2002, 355-385. 2. H.D. Lutz, J. Mol. Struct., 646, 2003, 227-236. 3. D. Stoilova, M. Wildner, J. Mol. Struct., 706, 2004, 57-63. 4. D. Stoilova, J. Mol. Struct., 798, 2006, 141-148. 5. D. Stoilova, Compt. Rend. Acad. Bulg. Sci., 57, 2004, 35-38. 6. J. Walker, W.A. Eyffe, J. Chem. Soc., 83, 1903, 173-175. 7. D. Stoilova, Zh. Neorg. Khim., 26, 1981, 1677-1680. 8. Ch. J. Groombridge, R. K. Harris, K. J. Packer, J. Solid State Chem., 59, 1985, 306-316. 215
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