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HCl appear to be of the pseudosymmetric type II for 4-aminopyridine, 4-aminopyrimidine and 4-hydroxypyridine, while benzimidazole forms a slightly weaker complex. H-bonding of HCl with the bases 2-hydroxypyridine and purine is of the intermediate type I → II. In the case of 4-aminopyrimidine, additional bonding of the Cl atom of HCl to an amino N---H bond yields a closed complex which explains the type II behaviour. In all other cases, bonding of additional HCl molecules to the 1:1 complexes results in proton transfer towards N---H+…Cl−
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doi:10.1016/0166-1280(94)03797-O 
Copyright © 1994 Published by Elsevier Science B.V.
An ab initio study of the complexes of HCl with the chloromethanes
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Received 21 November 1993;
Abstract
Ab initio calculations have been carried out at the MP2/6-31+G(d,p) level to determine the equilibrium structures of the complexes formed between HCl and the chloromethanes, HCl:CH4 − nCln, for n = 1–4. Two equilibrium structures corresponding to global and local minima have been found in each case, except for HCl:CCl4, where only one exists. However, on the HCl:CH3Cl and HCl:CH2Cl2 surfaces, the two equilibrium structures are easily interconverted through rotation about the hydrogen-bonded Cl---C bond, and hence correspond to a single hydrogen-bonded complex. Only on the HCl:CHCl3 surface are two distinct complexes found, one stabilized primarily by a favorable antiparallel alignment of dipole moment vectors, and the other by a Cl---H … Cl hydrogen bond. The binding enthalpies of the various complexes at 13 K, computed at MP2/cc-pVTZ+//MP2/6-31+G(d,p), vary from 1.4 to 2.5 kcal mol−1. The stability of the hydrogen-bonded complexes increases in the order CCl4 < CHCl3 < CH2Cl2 < CH3Cl, which correlates with the order of increasing electron density on the chlorine atoms. The computed shifts of the H---Cl stretching frequency in these hydrogen-bonded complexes also increase in this same order. Significant differences are found between the HCl and HF complexes with the chloromethanes. In particular, distinguishable cyclic complexes and complexes with bifurcated and trifurcated hydrogen-bonded structures are not found on the HCl:CH4 − nCln surfaces, and a stable van der Waals complex is not found on the HCl:CCl4 surface.
