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

Abstract

Ab initio calculations have been carried out at the MP2/6-3 l+G(d,p) level of theory to determine the equilibrium structures of complexes formed between HF and the chloromethanes, HF:CH4-,,Cln, 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, dipoledipole bifurcated and trifurcated hydrogen-bonded structures, and a van der Waals complex. Two equilibrium structures have been found on each surface except for HF:CH&l, 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:CHC13 < HF:CH2C12 < HF:CH&l, which parallels the order of increasing electron density on the C1 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.

Assessment of theoretical methods for the calculation of methyl cation affinities

“…In how far the hybrid DFT methods respond sensitively to the basis set choice was subsequently tested for B98 and B3LYP. In line with earlier systematic studies of basis set effects in PA calculations, 12,[14][15][16][17][18] poor results are obtained with basis sets lacking diffuse basis functions. Addition of further polarization functions improve the B98/6-3111G(d,p) results especially for PH 3 , and very good results are thus obtained at the B98/6-3111G(2df,p) level for both systems with DMCA 5 11.0 (NH 3 ) and 20.7 (PH 3 ).…”

Using beryllium bonds to change halogen bonds from traditional to chlorine-shared to ion-pair bonds

“…25 This basis set is derived from the Dunning aug-ccpVTZ basis set 26,27 by removing diffuse functions from H atoms. In addition, the structures of the ions (XYBeF) À and + (Cl-N-base) have also been determined.…”

Benchmark ab initio thermochemistry of the isomers of diimide, N2H2, using accurate computed structures and anharmonic force fields

“…It was previously established that such basis functions are essential for accurate computed bond angles [45] and atomization energies [33,46] of strongly polar molecules. The aug ′ -cc-pVnZ notation [47] stands for the combination of a regular cc-pVnZ basis set on hydrogen with an aug-cc-pVnZ basis set on first-row atoms. We previously found [33] that this affects computed atomization energies by less than 0.1 kcal/mol.…”