Theoretical prediction of vibrational spectrum of N-glycylglycine hydrochloride: An ab initio study

A complete set of force constants and their corresponding scale factors were obtained by fitting the experimental vibrational frequencies of seven isotopomers of glycine hydrochloride (GH) to the ab initio force field obtained at the HF/6-31G** level for the lowest energy conformation 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) from the experimentally observed fundamentals for all of the seven isotopomers. A conformational study was undertaken for glycylglycine hydrochloride (GGH) at the same level of theory. The scale factors of GH were used to obtain the scaled ab initio force field of the minimum energy conformer of GGH, which in turn was used to predict the vibrational frequencies and their potential energy distribution (FED). The excellent agreement between the experimental and predicted fundamentals offers a ''real'' example to the concept of building a reliable force field from a smaller unit to a larger unit, i.e., of a dipeptide from its parent amino acid.