AN AB-INITIO STUDY OF THE COMPLEXES OF HCL WITH THE CHLOROMETHANES

Ab initio calculations have been carried out at the MP2/6-31+G(d,p) le vel to determine the equilibrium structures of the complexes formed be tween HCl and the chloromethanes, HCl:CH4-nCln, for n = 1-4. Two equil ibrium structures corresponding to global and local minima have been f ound in each case, except for HCl:CCl4, where only one exists. However , on the HCl:CH3Cl and HCl:CH2Cl2 surfaces, the two equilibrium struct ures are easily interconverted through rotation about the hydrogen-bon ded Cl-C bond, and hence correspond to a single hydrogen-bonded comple x. 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 bind ing enthalpies of the various complexes at 13 K, computed at MP2/cc-pV TZ+//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 electr on density on the chlorine atoms. The computed shifts of the H-Cl stre tching frequency in these hydrogen-bonded complexes also increase in t his same order. Significant differences are found between the HCl and HF complexes with the chloromethanes. In particular, distinguishable c yclic complexes and complexes with bifurcated and trifurcated hydrogen -bonded structures are not found on the HCl:CH4-nCln surfaces, and a s table van der Waals complex is not found on the HCl:CCl4 surface.