Inelastic neutron scattering studies of some intramolecular hydrogen bonded complexes: A new correlation of γ(OHO) vs R(OO)

Inelastic neutron scattering spectra of several complexes {potassium hydrogen maleate, nickel dimethylglyoxime, and tripotassium cis-dichloro (hydrogen disulfito) and tripotassium cis-dichloro (hydrogen disulfito) platinate (II)} with strong intra molecular hydrogen bonds have been obtained and the out-of-plane bending modes γ(OHO) of the hydrogen bond assigned. Using previously published data, primarily for intermolecularly hydrogen bonded complexes, it is shown that as the oxygen-oxgyen distance {R(OO)} decreases γ(OHO) increases. Our new data do not follow this correlation since γ(OHO) is also observed to decrease as R(OO) decreases. It is suggested that this observation is due not to a fundamental difference in the behavior of inter- and intramolecular hydrogen bonds but rather to the different range of values of R(OO) found for each type. For the intermolecular systems R(OO) is greater than ∼2.44 Å, while for the intramolecular complexes (including potassium monochloromaleate) R(OO) is less than this value. It appears that hydrogen bonds undergo a change in dynamic properties at R(OO) 2.44 Å and some geometric properties of ring systems are discussed which support this idea.