Quantum Cluster Equilibrium Theory of Liquids:  Temperature Dependence of Hydrogen Bonding in LiquidN-Methylacetamide Studied by IR Spectra

Temperature-dependent infrared spectra for liquid N-methylacetamide are calculated by the ab initio quantum cluster equilibrium (QCE) and Gaussian-94 methods and compared with experimental measurements. The calculations are based on standard ab initio self-consistent-field (SCF) methods at the 3-21G levels for five different molecular clusters. The cluster sizes vary from the monomer up to a five-membered linear structure. Strong cooperative effects are found in the molecular clusters and are reflected in the geometries and vibrational spectra for each species. The equilibrium populations of the clusters were calculated for the entire liquid range. At low temperatures the linear pentamer is the dominant species. At higher temperatures these clusters are replaced, primarily, by linear dimers and monomers. The calculated frequencies are in excellent agreement with the temperature behavior found in FT-IR experiments.