In-plane force constants of the peptide group: Least-squares adjustment starting from ab initio values of N-methylacetamide
Least-squares adjustment of the force constants of N-methylacetamide belonging to the A' symmetry block was carried out starting from the force constant values obtained by an ab initio SCF MO calculation with the 4-31G basis set. Fourteen diagonal force constants were adjusted by the use of 122 vibrational frequencies of 9 isotope derivatives in the liquid state observed in the region 2000-100 cm -1. Here, all the off-diagonal elements were fixed at the initial values calculated with the 4-31G basis set. The greatest difference between the observed and calculated frequencies was 30 cm -1, and the differences for 94 modes were less than 10 cm -1. Most of the force constatns of bond stretching modes were reduced by 5-10%, and those of angle deformation modes by 20-30% in the course of the least-squares adjustment. The CO stretching force constant was lowered by 30% in the adjustment, and this is understandable because the adjustment involves the effect of bringing the peptide group from a free to a hydrogen-bonded state.