VIBRATIONAL ANALYSIS OF A SCHIFF-BASE BASED ON AB-INITIO MOLECULAR-ORBITAL CALCULATIONS - EFFECT OF ELECTRON CORRELATION ON THE C=N STRETCHING FORCE-CONSTANT AND THE ORIGIN OF THE SHIFT OF THE C=N STRETCHING FREQUENCY UPON PROTONATION AND HYDROGEN-BOND FORMATION

Vibrational force fields for model compounds of unprotonated and proto nated Schiff bases are obtained by nb initio molecular orbital (MO) ca lculations. The C=N stretching frequency shifts downward upon protonat ion in the calculations at the Hartree-Fock level, contrary to the exp erimental results. The effect of electron correlation on the C=N stret ching force constant and its basis-set dependence are investigated. Ca lculation at the second-order Moller-Plesset perturbation (MP2) level with the 6-31G(*) basis set (the 6-31G* basis set augmented by one se t of polarization functions on the Schiff base proton) is necessary fo r correctly calculating the change in the C=N stretching force constan t upon protonation. The effect of hydrogen bonding between the Schiff base proton and its counterion is also studied. Hydrogen bonding incre ases the frequency shift occurring upon HD exchange of the Schiff base proton (Delta(HD)), in agreement with previous studies. This increase in the value of Delta(HD) originates not only from strong mixing betw een the C=N stretch and the N-H in-plane bend in the protonated specie s but also from mixing between the C=N stretch and the N-D stretch in the deuterated species.