An ab initio study of the complexes of hydrogen fluoride with the chloromethanes

Ab initio calculations have been carried out at the MP2/6-31+G(d,p) level of theory to determine the equilibrium structures of complexes formed between HF and the chloromethanes, HF:CH4-nCln, for n = 1-4. The equilibrium complexes present a variety of structural types including cyclic structures stabilized by two distorted hydrogen bonds, open structures stabilized by a traditional linear hydrogen bond, dipole-dipole bifurcated and trifurcated hydrogen-bonded structures, and a van der Waals complex. Two equilibrium structures have been found on each surface except for HF:CH3Cl, where there are three. Binding energies have been computed at MP2/cc-pVTZ+//MP2/6-31+G(d,p), and binding enthalpies at 13 K range from 0.3 to 3.2 kcal/mol. The order of increasing stability of the most stable complexes on these surfaces is HF:CCl4 < HF:CHCl3 < HF:CH2Cl2 < HF:CH3Cl, which parallels the order of increasing electron density on the Cl atom. Because of the structural diversity of these complexes, the shifts of the H-F stretching frequency observed experimentally do not correlate with the stabilities of these complexes. However, when HF is a proton donor, the H-F frequency shifts do correlate with binding energies provided that complexes of the same structural type are compared. The computed results provide insight into the origin of the H-F frequency shifts and detailed structural and energetic information about these complexes which is not available experimentally.