What is a weak hydrogen bond?

Transcription

1 What is a weak hydrogen bond? Gautam R. Desiraju School of Chemistry University of yderabad yderabad , India gautam_desiraju@yahoo.com

2 What is a hydrogen bond? Under certain conditions an atom of hydrogen is attracted by rather strong forces to two atoms instead of only one, so that it may be considered to be acting as a bond between them (Pauling, 1939). D.. A

3 ther hydrogen bond definitions, X A A hydrogen bond is said to exist when (1) there is evidence of a bond, and (2) there is evidence that this bond sterically involves a hydrogen atom already bonded to another atom. Pimentel and McClellan (1960) Any cohesive interaction where carries a positive charge and A a negative charge (partial or full) and the charge on is more positive than on X Steiner and Saenger (1993)

4 Eccentricities of nomenclature D.A accepts electrons A donates electrons donates a hydrogen bond A accepts a hydrogen bond DNR ACCEPTR

5 Weak (or non-conventional) hydrogen bonds istorical survey, Glasstone. Cl 3 C =CMe 2 complex Pauling definition Sutor and the C hydrogen bond. (Nature) ki and Iwamura. π interactions (JACS) Donohue criticism of Sutor Leiserowitz review on carboxylic acids (Acta Cryst) Taylor Kennard paper (JACS)

6 The weak hydrogen bond is an interaction X A wherein a hydrogen atom forms a bond between two structural moieties X and A, of which one or even both are of moderate to low electronegativity (1999)

7 Bibliography G. R. Desiraju and T. Steiner, The Weak ydrogen Bond in Structural Chemistry and Biology, UP, Chichester, G. A. Jeffrey, An Introduction to ydrogen Bonding, UP, New York, G. R. Desiraju, Acc. Chem. Res., 24, 270,1991. G. R. Desiraju, Acc. Chem. Res., 29, 441,1996. G. R. Desiraju, Acc. Chem. Res., 35, 565,2002. G. R. Desiraju, Chem. Comm., 2996, T. Steiner, Cryst. Rev., 6, 1, 1996.

8 Different types of hydrogen bond D.. A D.. A Very strong Strong D.. A Weak

9 ydrogen bond Any cohesive interaction X A where carries a positive and A a negative (partial or full) charge and the charge on X is more negative than on (-) N π N π C s C Ni C π

10 ow to assess hydrogen bonds? Energy Spectroscopy Geometry Structure unction A complex interaction

11 Very strong Strong Weak [ ] N =C C Energy (kcal/mol) 15 to 40 4 to 15 < 4 IR, ν s >25% 5-25% <5% (X ), Å 0.05 to to 0.05 < 0.01 A, Å 1.2 to to to 3.0 Shorter than 100% ~100% 30-80% van der Waals Effect on crystal packing Pronounced Distinctive Variable

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13 Cl 3 n R n C G. R. Desiraju, J. Chem. Soc., Chem. Comm., 179, 1989

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15 Angles, X

16 IR bathochromic shifts, C G. R. Desiraju and B. N. Murty, Chem. Phys. Lett., 139, 360, 1987

17 Cooperativity R R R Anti-cooperative R R

18 C cooperativity in ethynyladamantan-2-ol r 1 r 2 = 0.025(14) Å Allen et al., J. Am. Chem. Soc., 118, 4081, 1996

19 Thermal parameters

20 Isostructurality Et Et N N N N Et Et N N N

21 Two molecules in the asymmetric unit (Z =2) Me 2 N C 3 Me 2 N

22 Electronegativity and ardness luorine C.. C Weak donor Very weak acceptor

23 C C Interactions in luorobenzenes Thalladi, Weiß et al, JACS, 120, 8702, 1998

24 C C ydrogen Bonds All C,, compounds luorobenzenes

25 Weak and reversible C C ydrogen Bridge Applications. Polymerization catalyst. Zr + Boron adducts Lancaster et al, Chem. Comm., 2148, 2003 Stereoselective polymerisation Chan et al, Angew. Chem. Int. Ed., 42, 1628, 2003 Decrease of β- transfer T. ujita et al, JACS, 124, 3327, 2002 JACS, 125, 4293, 2003

26 Weak and reversible C C ydrogen Bridge Applications. Molecular recognition. Binding in thrombin Diederich et al, Angew. Chem. Int. Ed., 42, 2507, 2003

27 ydrogen bridge (Wasserstoffbrücke) Electrostatics Charge transfer (covalency) Dispersion/repulsion (van der Waals) Polarisation A composite interaction

28 Covalent limit (40 kcal/mol) R R 2 N P() 2 2 N N C 3 Cl 5 X X [ 2 ] - very strong hydrogen bonds strong hydrogen bonds weak hydrogen bonds N 4+ N 4+ RC N 4+ Cl NMe 4+ X Electrostatic limit (15 kcal/mol) =C N =C N 4+ π (N2 ) 3 C N N s C Cl 3 C =CMe 2 C C N S C Cl π N π C=C C S C C π N Co Ph Ph C 4 π very weak interactions C C C 4 Ar C ClC van der Waals limit (0.25 kcal/mol)

29 ydrogen bond D.. A A composite interaction that spans wide ranges of geometry and energy Great chemical variations among the donor D and acceptor A groups owever, all hydrogen bonds have several features in common Notably, their effect on crystal structure and packing

30 Students Post-doctorals Collaborators University of yderabad DST, CSIR, DRD