Inplane force constants of the peptide group: Leastsquares adjustment starting from ab initio values of Nmethylacetamide
Abstract
Leastsquares adjustment of the force constants of Nmethylacetamide 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 431G 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 2000100 cm ^{1}. Here, all the offdiagonal elements were fixed at the initial values calculated with the 431G 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 510%, and those of angle deformation modes by 2030% in the course of the leastsquares 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 hydrogenbonded state.
 Publication:

Journal of Molecular Spectroscopy
 Pub Date:
 December 1984
 DOI:
 10.1016/00222852(84)901796
 Bibcode:
 1984JMoSp.108..206S