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 °C have been analyzed with the help of results from ab initio molecular orbital calculations to yield the structures of the ee and aa forms (equatorially and axially disposed chlorine atoms, respectively) of the molecule and the composition of the mixture. The model of this 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 parameter differences calculated ab initio. Some of the results ( r g /Å, ∠ e /deg; 2σ 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); ∠(C 2 C 1 C 6 ) = 109.9(14) (110.1); ∠(CCCl) = 109.7(4) (109.8); flap (the angle between the planes C 2 C 1 C 6 and C 2 C 3 C 5 C 6 ) = 51.7(19) (47.2(12)). The mole fraction of the ee form was determined to be 0.46(6). The structural 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 are in good agreement with experiment, but those from the HF/6-31G* are poorest. The experimental composition is most accurately predicted by the MP2/6-311G* and QCISD/6-311+G(2df,p) calculations from the conformational energy differences Δ E theor corrected for zero-point energy and entropy differences. The composition of the system is discussed in relation to that of monochlorocyclohexane.