Anharmonic Vibrational Spectroscopy Calculations for Proton-Bound Amino Acid Dimers

Results of anharmonic frequency calculations carried out for GlysLysH(+) and GlyGlyH(+) are presented and compared to gas phase electrospray ionization (ESI) spectroscopy experiments. Anharmonic frequencies are obtained via correlation-corrected vibrational self-consistent field (CC-VSCF) calculations. The potential used is based on the PM3 semiempirical electronic structure method, but improved by fitting to ab initio MP2 calculations at the harmonic level. The key results are as follows: (1) Hydrogens acting as intermolecular bridges have very anharmonic stretches whose frequencies cannot be reliably predicted by the harmonic approximation. An example is the carboxylate bound NH3+ stretch. (2) The computed anharmonic vibrational frequencies are in good agreement with experiment and provides a very large improvement over harmonic frequencies especially for OH and NH stretches. For example the calculated CC-VSCF frequencies of GlysLysH(+) and GlyGlyH(+) have overall average deviations of 1.35% and 1.48% only, respectively, from experiment. (3) The harmonic OH bond stretching frequency deviates by 6.64% from experiments. The CC-VSCF calculations reduce this deviation by more than an order of magnitude to 0.56%. The anharmonicity of the OH stretch is intrinsic, rather than due to coupling with other modes. (4) Anharmonic coupling between the NH3+ stretch and several other normal modes is strong, and provide the main contribution for the anharmonicity of this mode. Properties of the potential energy surfaces of the proton-bound complexes are briefly discussed in light of the results.