The structure and conformational properties of oxalyl chloride, which experiences internal rotation about the C-C bond, have been reinvestigated by electron diffraction from the gas at 0,80, and 190 "C and by extensive ab initio calculations. Complete structure optimizations at a very high level (MP2/TZ2P, 166 basis functions) revealed, in addition to the anti form at LClCCCl = 180", a second stable form (gauche) with LClCCCl = 89.8" characterized by a very shallow minimum in the energy; earlier theoretical results for oxalyl chloride had been inconsistent with the existence of a second form known from experiment to be present. The electron diffraction analysis was based on dynamic models that comprised a set of pseudoconformers spaced at regular intervals around the torsional coordinate @ = LClCCCl and Boltzmann weighted according to a three-term torsional potential V(@) = I/2ClV1[ 1 -cos i ( 180 -@)I. For the more elaborate model results from the ab initio calculations were incorporated in the form of distance and angle differences among the pseudoconformers; in a second, simpler model these differences were omitted so that the structures of the pseudoconformers differed only in their torsion angles. A theoretical force field for the anti form was also evaluated ab initio, scaled to fit the observed wavenumbers, and used in each model to calculate the usual corrections for vibrational averaging. The results for the analysis of the 0 "C data for the more elaborate (preferred) model are as follows (rg/& Lddeg with 2 0 uncertainty estimates): r(C=O) = 1.184(2), r(C-C) = 1.548(8), r(C-C) = 1.749(3), LCCO = 123.8(4), LCCCl = 111.8(3), and LClCCCl,,,t,, = 76(18) where 0" corresponds to cis; results at the other temperatures are similar. Values of the potential constants, which should be temperature independent, are found in the ranges (kcaymol) 1.45 I Vi I 1.99, -0.40 d V2 d 0.03, and 0.43 I V3 I 1.05; the average values are V I = 1.59(83), V2 = -0.1 1(38), and V, = 0.74(39). The estimated mole fractions of the anti form at 0, 80, and 190 "C are 0.67, 0.62, and 0.43, from which the internal energy difference AUo = is calculated to be 0.75(50) kcal mol-' and the entropy difference ASo = $ + R In 2 --to be 1.31(148) cal mol-' K-I. The simpler model gives similar results.

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The molecular structure of gaseous p-benzoquinone has been studied by electron diffraction at a nozzle-tip temperature of 110–125°C. The molecule has D2h symmetry to within experimental error. The results for the more important distance (ra), bond angle, and rms amplitude (l) parameters are rC–H=1.089 Å (0.011), rC=O=1.225 Å (0.002), rC=C=1.344 Å (0.003), rC–C=1.481 Å (0.002), ∠C2C1C6=118.1° (0.3), ∠C3C2H=121.4° (assumed), lC–H=0.077 Å (assumed), lC=O=0.0424 Å (0.0020), lC=C=0.0446 Å (0.0024), and lC–C=0.0546 Å (0.0024). These values are in good agreement with those found in an early electron-diffraction study by Swingle, and except for the length of the carbon-carbon double bond, in excellent agreement with those found in the crystal by Trotter.

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