Ab initio force fields of glycine dipeptide in C5 and C7 conformations

Abstract

Schafer and coworkers have previously derived, with the 4-21 Gaussian basis set ab initio optimized geometries for the glycine dipeptide, CH3CONHCH2CONHCH3, in several conformations. Using their geometries for the C5 and C7 conformations, respectively with five- and seven-membered intramolecular NH...OC hydrogen-bonded rings, we have computed the vibrational force fields and dipole moment derivatives at the 4-21 level. Scale factors for the quadratic force constants were transferred, with two changes, from the set derived by Fogarasi and Balazs from a study of small amides. The differences in the force constants, harmonic frequencies, dipole derivatives, and infrared intensities between the conformations and within each conformation are discussed and related, where possible, to differences in hydrogen bonding and structure. In particular, the NH and CO stretch and bend force constants and dipole derivatives show trends that can be correlated with the C5 and C7 hydrogen bonds. The amide modes are compared with some available data on glycine dipeptide in argon matrix, and the comparison supports the empirically-based conclusion that C5 and C7 conformations are both present in the matrix-isolated sample.