SCALED QUANTUM-MECHANICAL CALCULATION OF THE VIBRATIONAL STRUCTURE OF THE SOLVATED GLYCINE ZWITTERION

The scaled quantum mechanical (SQM) method for calculating the vibrational structure of molecules is applied to the solvated glycine zwitterion. The ab initio force field generated by means of a molecular dynamics calculation [J.S. Alper, H. Dothe and D.F. Coker, Chem. Phys. 153 (1991) 51] was scaled using the frequencies from experimental spectra taken in aqueous solution. Theoretical frequencies, potential energy distributions, intensities, and line shapes were obtained. The theoretical frequencies and normal mode assignments were in excellent agreement with the aqueous spectra. Theoretical frequencies and normal modes calculated by scaling to frequencies obtained from the experimental alpha-crystal spectra of glycine were in significantly worse agreement with the experimental crystalline values. These results suggest that the calculation has distinguished the aqueous from the crystalline environment.