Hydrogenic Stretch Spectroscopy of Glycine-Water Complexes: Anharmonic AICSP Calculations

The anharmonic frequencies of the O-H, C-H and the N-H stretching modes of hydrogen-bonded glycine-H2O complexes are calculated using the ab initio Classical Separable Potentials approximation. In this approach, ab initio molecular dynamics 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 Schrodinger equation for each mode using time-averaged potentials. Three complex structures are studied, that 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 the 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 anha...