Force fields for neutral and protonated Schiff bases

Abstract

Abstract In order to analyse the vibrational dynamics of the imino group in its neutral and protonated forms, a normal coordinate analysis of the model aromatic Schiff base trans-N -benzylideneaniline (tBA) was undertaken. The results of the valence force field calculations for neutral tBA, based on the experimental geometry and the vibrational spectra of a series of tBA isotopomers, display a considerable mixing of imino coordinates. For the protonated species a different conformation was found by optimising the geometry using the AM1 method; in this conformation both dihedral angles are less than in the neutral tBA molecule. Although an increase in the CN stretching frequency by 45 cm 1 was observed, a decrease in the CN stretching force constant was found. The results of the calculations showed a decrease in the NPh stretching, but an increase in the imino CH and CPh stretching, as well as the imino CH in-plane deformation constants. No change in the CH out-of-plane deformation constant was found, but some interaction force constants changed their values. Protonation of tBA brings about many changes in the vibrational spectra, probably owing to the change in hybridisation and mass distribution around the nitrogen atom and to the different conformation, rather than to changes in bond lengths or counter ion effects.