ELECTROSTATIC MODEL OF COOPERATIVE EFFECTS IN HYDROGEN-BONDED SYSTEMS - COOPERATIVE EFFECT OF BASE OF CATION ON THE A-H STRETCHING VIBRATIONAL FREQUENCY

摘要

Certain empirical regularities of cooperative effects in L...A-H...B clusters (where A-H...B is a hydrogen-bonded dimer and L is a metallic cation) are interpreted in terms of a simple electrostatic model. The potential energy surface (PES) of the intermolecular interaction is approximated by the sum of the coulombic interaction energies of the atomic effective (residual) charges, allowing for the inductive action on the neighbouring bonds and the exchange repulsion of nearest atoms. Making use of the given PES, the relative frequency shift (Delta nuu(0)) of the stretching frequency nu(S)(AH) under the action of B and L in the A-H...B dimer and L...A-H...B cluster is found. For a series of compounds with a variable base B, the quantities (Delta nuu(0))(AHB) and (Delta nuu(0))(LAHB) appear to be linearly related. This regularity, generated by the three-body interaction in the L...A-H...B cluster, has previously been established experimentally. A factor of the linear dependence of the above-mentioned quantities (the ''cooperativity factor'') is proportional to the electric field strength of the cation charge at the position of the A atom. This is in accord with the experimental data for a range of cations. One fitting parameter was used to estimate the proportionality factor. The possibility of describing cooperative effects in terms of an electrostatic model shows that these effects are not stipulated by electron delocalization in the hydrogen-bonded systems, as postulated in some investigations. (C) 1997 Elsevier Science B.V. [References: 78]