Theoretical study of hydrogen bonding in liquid and gaseous N-methylformamide

Temperature-dependent quadrupole coupling constants (qcc), chi(Q), and asymmetry parameters, eta(Q), of neat liquid and gaseous N-methylformamide were calculated theoretically. These calculations are based on standard ab initio self-consistent-held (SCP) methods at the 6-31G* level for the six most abundant clusters which were found via the quantum cluster equilibrium (QCE) model of liquids. The cluster sizes vary from one to six molecules and include linear and cyclic structures. Using QCE theory the equilibrium populations of the different clusters were calculated for temperatures between 250 and 600 K. These calculations indicate that at low temperatures six membered rings and linear tetrameters are the dominant species. At higher temperatures the cluster populations change and these two clusters are increasingly replaced by linear trimers and dimers. In the gas phase the expected monomers as well as a substantial fraction of dimers are found, The temperature dependencies of the calculated qcc values for the nitrogen and the amide deuterium nuclei are in good agreement with results from NMR relaxation time experiments. The changes of geometry with temperature agree with experimentally measured shifts from the gas to the liquid phase. (C) 1997 American Institute of Physics.