Hydrogenic Stretch Spectroscopy of Glycine-Water Complexes: Anharmonic Ab Initio Classical Separable Potential Calculations

The anharmonic frequencies of O-H, C-H, and N-H stretching modes of hydrogen-bonded glycine-H₂O complexes are calculated using ab initio classical separable potential approximation. In this approach, ab initio molecular dynamic simulations are used to determine an effective classical potential for each of the normal modes of the system. The frequencies are calculated by solving the time-independent Schrödinger equation for each mode using time-averaged potentials. Three complex structures are studied, which differ in the location of the water molecule on the amino acid. Significant differences are found between the spectra of the three structures, and signatures of individual complexes are established. It is demonstrated that anharmonic effects are essential in the discrimination between different structures, while frequency differences at the harmonic level are much smaller. Intensities are also computed and found to carry information on differences between structures, but the role of anharmonicity in this is small.