BOND LENGTHS AND QUADRATIC FORCE-FIELD FOR CUBANE

The molecular structure of pentacyclo[4.2.0.0(2,5).0(3,8).0(4,7)]octane (cubane) has been determined in the gas phase at a temperature of 77-degrees-C. Parameter values were obtained from four types of refinements: those based on electron-diffraction (ED) data alone, ED data with inclusion of a microwave (MW) rotational constant for cubane-d0 available from the literature, ED data with allowance for single-double multiple scattering (MS) effects, and the ED-MW data with allowance for MS effects. The results differ insignificantly. Bond lengths in terms of the distance types r-alpha-0 (geometrically consistent at 0 K) and r(g) (vibrationally averaged at the experimental temperature) are respectively as follows: C-H, 1.098 (6) and 1.114 (6) angstrom; C-C, 1.571 (2) and 1.573 (2) angstrom. Estimated equilibrium values are r(e)(C-H) = 1.0960 (130) and r(e)(C-C) = 1.5618 (40) angstrom. Some amplitudes of vibration are l(C-H) = 0.075 (10), l(C-C) = 0.062 (3), l(C.C) = 0.065 (4), l(C..C) = 0.072 (9); values are in angstroms with estimated 2-sigma uncertainties in parentheses. O(h) symmetry was assumed. A complete quadratic vibrational force field was also evaluated. The 32 independent force constants were fitted simultaneously to 156 observations consisting of 146 wavenumber fundamentals from eight isotopic species, 2 Coriolis constants, 1 centrifugal distortion constant, and 7 amplitudes of vibration. The values are in excellent agreement with recent ab initio results.