Molecular structures and compositions of trans-1,2-dichlorocyclohexane and trans-1,2-difluorocyclohexane in the gas phase: an electron-diffraction investigation. 
The structures and compositions of gaseous trans-1,2-dichloro- (DCCH) and trans-1,2-difluorocyclohexane (DFCH), each of which may exist with the halogen atoms in a diaxial (aa) or diequatorial (ee) conformation, have been investigated by electron diffraction.  The analysis was aided by rotational constants from microwave spectroscopy for the ee form of DFCH and by ab initio and density functional theory molecular orbital calculations for all species.  The skeletons of the molecules have similar parameter values, but for the Cl-C-C-Cl and F-C-C-F fragments there are significant differences between the corresponding C-C-X bond angles and the X-C-C-X torsion angles in the two systems.  There are also significant differences between the values of these parameters in the aa and ee forms of the same system.  The composition of DCCH at 100 degrees C was measured to be 60(4)% aa, and that of DFCH at 70 degrees C was 42(7)% aa; the uncertainties are estimated 2sigma.  From the preferred B3LYP/aug-cc-pVTZ calculations, the predicted theoretical composition is 51.2% aa for DCCH and 40.8% aa for DFCH. ( Calculations at the levels B3LYP/6-31G(d) and MP2/6-31G(d) give similar results for DCCH, but both predict more aa than ee for DFCH.)  Values (r(g)/A and angle(alpha)/degree) for some of the more important parameters of the aa/ee forms of DCCH are <C-C> = 1.525(4)/1.525(6), C-Cl = 1.806(2)/1.787(2), angleC2-C1-Cl = 107.3(3)/111.5(3), angleC1-C2-C3 = 113.9(5)/111.6(5), angleC2-C3-C4 = 111.3(12)/109.9(12), and Cl-C2-C3-Cl = 165.3(9)/-59.4(9); and for DFCH C-C = 1.525(6)/1.520(9), C-F = 1.398(2)/1.390(2), angleC2-C1-F = 106.5(6)/109.2(6), angleC1-C2-C3 = 111.4(9)/110.9(9), angleC2-C3-C4 = 113.1(10)/113.1(10), and F-C2-C3-F = 171.1(37)/-67.2(37).  The structures and compositions are discussed.