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The side chain geometry and some adjacent bond lengths and angles of the ring are optimized at the STO-3G level of molecular orbital theory for the planar and orthogonal forms of benzoyl X(X = H, F, CN, CH3, OCH3). Similar calculations are reported for acetyl fluoride, acetyl cyanide, and carbonyl cyanide, for which experimental structures and reliable internal barriers are available. The calculated barriers for the benzoyl compounds suggest steric hindrance by X in the ground state as a major cause of the variation in the barrier magnitudes. Good agreement between calculated and experimental geometries for acetyl cyanide and carbonyl cyanide, as well as for the internal rotational barrier in the former, are taken to imply a reliable calculated geometry for benzoyl cyanide. A total geometry optimization for phenol agrees fairly well as for the internal rotational barrier in the ture and also with the direction and magnitude of the dipole moment. Optimization of the ring geometry does not lower the calculated internal rotation barrier.

The structure of acetyl cyanide has been determined by making joint use of the electron diffraction intensities measured in the present study and the rotational constants reported by Krisher and Wilson. The thermal average bond distances are: rg(C-H) = 1.116±0.011 A, rg(CN) = 1.167±0.010 A, rg(CO) = 1.208±0.009 A, rg(C-C) = 1.477±0.008 A and rg(C-Cmethyl) = 1.518±0.009 A. The bond angles in the zero-point average structure (rav) are: ∠(Cmethyl-CO) = 124.6±0.7°, ∠(C-C-C) = 114.2±0.9°, ∠(C-CN) = 179.2±2.2° and ∠(H-C-H) = 109.2±0.7°. The uncertainties represent the estimated limits of experimental error. The C-C single bond placed between the double and triple bonds is longer than those in vinylacetylene, acrylonitrile and propynal. Other structural parameters are also compared with those in related molecules. The infrared and Raman spectra of this molecule have been measured, and Urey-Bradley force constants have been determined.