Bond Lengths and Quadratic Force Field for Cubane

摘要

The molecular structure of pentacyclo4.2.0.02.5.03.8.04.7 octane (cubane) hasbeen determined in the gas phase at a temperature of 77 deg 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 cubanc-do 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 ra0 (geometrically consistent at 0 K) and r, (vibrationally averaged at the experimental temperature) are respectively as follows: C-H, 1.098 (6) and 1.114 (6) A;C-C,1.571 (2) and 1.573 (2) A. Estimated equilibrium values are re(C-H) = 1.0960 (130) and re(C-C) = 1.5618 (40) A. Some amplitudes of vibration arc I(C-H) = 0.075 (10), /(C-C) = 0.062 (3). /(C.C) = 0.065 (4), /(C.C) = 0.072 (9); values are in angstroms with estimated 2 delta uncertainties in parentheses. O 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. I centrifugal distortion constant, and 7 amplitudes of vibration. The values are in excellent agreement with recent ab initio results.