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

Abstract

We have followed up our previous ab initio calculations of the force fields and dipole-moment derivatives of glycine dipeptide in C5 and C7 conformations with similar studies of the -alanyl compound [CH3CONHCH(CH3)CONHCH3]. We have done, with the 4-21 Gaussian basis set, calculations of the three lowest-energy optimized conformations with intramolecular hydrogen bonding that were derived by Scarsdale et al.: the C5, C7eq, and C7ax structures. The quadratic force constants were scaled, as in our previous work, using the set of scale factors derived by Fogarasi and Balazs for small amides, with slight modifications. The differences in the force constants, harmonic frequencies, dipole derivatives, and infrared intensities among the conformations and within each conformation are discussed and related, where possible, to differences in hydrogen bonding and structure. The results are also compared and integrated with those on Gly dipeptide. The force constants and dipole derivatives show that the C7ax hydrogen bond is the strongest, in agreement with the NH[middle dot]O geometries, even though the SCF energy of this conformer is the highest. The NH stretch and the amide I and II modes are compared with available infrared data on Ala 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. The problem of distinguishing between the C7eq and C7ax forms is also discussed.