THEORETICAL PREDICTION OF VIBRATIONAL-SPECTRUM OF N-GLYCYLGLYCINE HYDROCHLORIDE - AN AB-INITIO STUDY

A complete set of force constants and their corresponding scale factor s were obtained by fitting the experimental vibrational frequencies of seven isotopomers of glycine hydrochloride (GH) to the ab initio forc e field obtained at the HF/6-31G* level for the lowest energy conform ation A recently developed fitting procedure is used for this purpose. the fitting is extremely successful in producing a force field which reproduces the frequencies within an average deviation of 9.7 cm(-1) f rom the experimentally observed fundamentals for all of the seven isot opomers. A conformational study was undertaken for glycylglycine hydro chloride (GGH) at the same level of theory. The scale factors of GH we re used to obtain the scaled ab initio force field of the minimum ener gy conformer of GGH, which in turn was used to predict the vibrational frequencies and their potential energy distribution (FED). The excell ent agreement between the experimental and predicted fundamentals offe rs a ''real'' example to the concept of building a reliable force fiel d from a smaller unit to a larger unit, i.e., of a dipeptide from its parent amino acid.