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

Certain empirical regularities of cooperative effects in L...A-H...B c lusters (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 ap proximated by the sum of the coulombic interaction energies of the ato mic effective (residual) charges, allowing for the inductive action on the neighbouring bonds and the exchange repulsion of nearest atoms. M aking use of the given PES, the relative frequency shift (Delta nu/nu( 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 nu/nu(0))(AHB ) and (Delta nu/nu(0))(LAHB) appear to be linearly related. This regul arity, generated by the three-body interaction in the L...A-H...B clus ter, has previously been established experimentally. A factor of the l inear dependence of the above-mentioned quantities (the ''cooperativit y factor'') is proportional to the electric field strength of the cati on charge at the position of the A atom. This is in accord with the ex perimental 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 the se effects are not stipulated by electron delocalization in the hydrog en-bonded systems, as postulated in some investigations. (C) 1997 Else vier Science B.V.