TEMPERATURE-DEPENDENCE OF HYDROGEN-BONDING IN NEAT, LIQUID FORMAMIDE

Temperature dependent quadrupole coupling parameters (QCP), chi(Q), and quadrupole coupling asymmetry parameters, eta(Q), of neat, liquid formamide were calculated. These calculations are based on standard ab initio self-consistent field (SCF) methods at the 6-31G* level for eight different clusters and a quantum cluster equilibrium (QCE) model of liquids. The cluster sizes varied from one to six molecules and include linear and cyclic structures. The equilibrium populations of the different formamide clusters were calculated for temperatures between 270 and 400 K. These calculations indicate that at low temperatures a six-membered ring of formamide molecules involving hydrogen bonding between the trans amide deuteron and the oxygen is the dominant (95%) species. At higher temperatures the cluster populations change and the six-membered ring is gradually replaced, primarily by a linear tetramer. The temperature dependence of the calculated QCP values for nitrogen, oxygen and two of the deuterium nuclei (the carbonyl and trans deuterons) are all in good agreement with the experimentally measured NMR results. (C) 1995 American Institute of Physics.