An ab initio study of some hydrogen-bonded complexes of chloroform and bromoform: red-shifted or blue-shifted hydrogen bonds?

The properties of the hydrogen-bonded complexes of chloroform and bromoform with ammonia, water, hydrogen fluoride, phosphine, hydrogen sulphide and hydrogen chloride were studied by means of ab initio calculations. The properties of interest were the molecular structures, the interaction energies, the vibrational spectra and the nature of the orbital interactions involved in the formation of the complexes. Of particular interest was the determination of whether the various interactions exhibited red-shifted or blue-shifted hydrogen bond behaviour. The complexes optimized in three distinct structural models, singly bonded, cyclic and cage. The interaction energies, in the main, varied systematically with changes of the haloform and of the partner molecules. Changes of the CH or YH (Y = N, O, F, P, S and Cl) bond lengths correlated in most cases with the interaction energies, subject to some inconsistencies, with opposite behaviour being shown by the two sets. Similarly, the CH and YH stretching wavenumber shifts also displayed a dependence on the interaction energies, tracking with the bond length changes. Based on the diagnostic utility of these structural and spectroscopic properties, the complexes were found to be almost exclusively red-shifted, for binding through both the CH and YH groups. The orbital interactions responsible for the stability of the complexes were determined to be donation from a Y atom lone pair orbital of the partner molecule to a CH σ antibonding orbital and from a X lone pair orbital (X = Cl and Br) of the haloform to a YH σ antibonding orbital. [ABSTRACT FROM AUTHOR]