Structure of Liquid N-Methylacetamide: Temperature Dependence of NMR Chemical Shifts and Quadrupole Coupling Constants

Abstract

Temperature-dependent NMR chemical shifts and quadrupole coupling constants for the amide hydrogen, the nitrogen, and the carbonyl oxygen nuclei in neat, liquid N-methylacetamide are calculated by ab initio methods and compared with experimental measurements. These calculations are based on ab initio quantum cluster equilibrium (QCE) theory and standard ab initio self-consistent-field (SCF) methods at the 3-21G and 6-31G* levels for five different molecular clusters. The cluster sizes varied from the monomer up to a five-membered linear structure of N-methylacetamide. Strong cooperative effects are found in the molecular clusters and are reflected in the geometries, chemical shifts, and quadrupole coupling values for each species. The equilibrium populations of the clusters were calculated between the melting (301 K) and the boiling point (478 K). At low temperatures the linear pentamer is the dominant species. At higher temperatures these clusters are replaced by linear dimers and monomers. The calculated chemical shift values for the 1H, 17O, the 14N nuclei are in excellent agreement with experimental NMR data. The calculated quadrupole coupling constants for the amide deuteron and the nitrogen nucleus likewise agree well with the temperature behavior found in NMR relaxation time experiments.