Infrared intensities of amide modes in N‐methylacetamide and poly(glycine I) from ab initio calculations of dipole moment derivatives of N‐methylacetamide

Abstract

The infrared intensities of the amide modes in N‐methylacetamide (NMA) and poly(glycine I) (PGI) have been studied using ab initio dipole moment derivatives obtained for the peptide group in NMA and an empirical force field refined for PGI. Good agreement is found between the calculated transition moment magnitudes and directions of the amide I and II modes and experimental intensity and dichroism data. By analyzing the separate contributions of each internal coordinate to the total intensity, we are able to understand in detail the origins of the IR intensities of the amide modes. Besides demonstrating one approach by which IR intensities can be studied in complex molecules and polymers, our results also provide a basis for using IR intensities in structural studies of peptides and polypeptides.