Ab initio theoretical and matrix isolation experimental studies of hydrogen bonding. IV. The HBr:pyridine complex

Abstract Experimental matrix isolation studies have been performed for the HBr/pyridine/argon system at 10 K to obtain the infrared spectrum of the isolated 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 experimental spectrum are presented. Comparison of the experimental IR spectrum of the complex with the harmonic spectrum computed at MP2/6–31+G(d,p) shows that both theory and experiment agree that this complex is stabilized by a proton-shared hydrogen bond. However, discrepancies exist between computed and experimental frequency and intensity patterns. Single point calculations at MP2/6–31+G(d,p) for displacements along the normal coordinate for proton stretching produced a potential energy curve which is strongly anharmonic. The anharmonic frequency for the proton 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 agreement with experiment.