Conformational composition of gaseous trans-1,4-dichlorocyclohexane. Molecular structures and energy differences of the aa and ee components from gas-phase electron diffraction and ab initio calculations

Electron-diffraction patterns from gaseous trans-1,4-dichlorocyclohexane at a temperature of 105 degrees C have been analyzed with the help of results from ab initio molecular orbital calculations to yield the structures of t he ee and aa forms (equatorially and axially disposed chlorine atoms, respe ctively) of the molecule and the composition of the mixture. The model of t his complicated system was defined in terms of the structure of the ee form , tying many of the parameters of the aa form to those of the ee by paramet er differences calculated ab initio. Some of the results (r(g)/Angstrom, an gle e/deg; 2 sigma uncertainties) for the ee (aa) forms from the preferred model are (r(C-H))= 1.115(4) (1.113); r(C-1-C-2)= 1.525(6) (1.525); r(C-2-C -3)= 1.542(13) (1.535); r(C-Cl)= 1.799(3) (1.812); r(Cl ... Cl)= 6.309(11) (5.236); angle(C2C1C6) 109.9(14) (110.1); angle(CCCl) = 109.7(4) (109.8); f lap (the angle between the planes C2C1C6 and C2C3C5C6) = 51.7(19) (47.2(12) ). The mole fraction of the ee form was determined to be 0.46(6). The struc tural predictions of ab initio calculations were tested by optimizations at several levels, among them HF/6-31G*, MP2/6-311G*, QCISD/6-311 +G(2df,p), MPw1PW91/6-311G*, B3P86/6-311G* and B3P86/6-311+G(2df,p). Parameter values from each of these calculations I are in good agreement with experiment, bu t those from the HF/6-31G* are poorest. The experimental composition is mos t accurately predicted by the MP2/6-311G* and QCISD/6-311+G(2df,p) calculat ions from the conformational energy differences Delta E-theor corrected for zero-point energy and entropy differences. The composition of the system i s discussed in relation to that of monochlorocyclohexane.