AB-INITIO THEORETICAL AND MATRIX-ISOLATION EXPERIMENTAL STUDIES OF HYDROGEN-BONDING-IV - THE HBR-PYRIDINE COMPLEX

Experimental matrix isolation studies have been performed for the HBr/ pyridine/argon system at 10 K to obtain the infrared spectrum of the i solated one-to-one HBr:pyridine complex. The experimental spectrum is consistent with those for complexes with proton-shared hydrogen bonds. Details of the experimental procedure and the analysis of the experim ental spectrum are presented. Comparison of the experimental IR spectr um of the complex with the harmonic spectrum computed at MP2/631+G(d,p ) shows that both theory and experiment agree that this complex is sta bilized by a proton-shared hydrogen bond. However, discrepancies exist between computed and experimental frequency and intensity patterns. S ingle point calculations at MP2/6-31+G(d,p) for displacements along th e normal coordinate for proton stretching produced a potential energy curve which is strongly anharmonic. The anharmonic frequency for the p roton stretching mode was estimated from this curve. The ratio of the anharmonic to harmonic frequency yielded an effective force constant. The re-computed spectrum obtained with this constant is in excellent a greement with experiment. (C) 1997 Elsevier Science B.V.