Comparison of Cooperativity in CH···O and OH···O Hydrogen Bonds
Journal of Physical Chemistry A
American Chemical Society
The ability of one H-bond in a chain to affect others is assessed by comparing the CH···O bonds in (H2CO)n and (HFCO)n to the OH···O bonds in (H2O)n. Both sorts of interactions grow stronger, and the intermolecular distances shorter, as the number of monomers in the chain increases. The degree of cooperativity is generally proportional to the strength of the H-bond, although the CH···O bonds in (HFCO)n display a disproportionately high degree of cooperativity. The cooperativity of OH··O and CH···O bonds is similar also with respect to electron density loss from the bridging hydrogen atom, and the amount of charge transferred from the proton-acceptor molecule to the donor. The covalent CH bonds are shortened upon H-bond formation, and the associated stretching frequencies undergo a blue shift, both opposite to what is observed in OH···O systems. These properties exhibit little indication of cooperativity for CH, while the OH bond stretches and red shifts of the OH frequencies are enhanced as n increases. NMR chemical shifts of the bridging proton likewise suggest that CH···O bonds are much less cooperative than OH···O. Cooperativity is reduced in all systems as the dielectric constant of a surrounding solvent is enhanced.
Comparison of Cooperativity in CH··O and OH··O Hydrogen Bonds T. Kar, S. Scheiner J. Phys. Chem. A 2004 108 9161-9168