The absorption spectra between 22003, and 10 000~ have been measured of the conjugate acids of a number of aromatic hydrocarbons dissolved in oxygen-free, anhydrous HF and HF-BFa mixtures, using a specially constructed cell. All spectra show very intense and characteristic absorption bands in the visible or near-ultra-violet region. In order to interpret these spectra, self-consistent field molecular orbital calculations have been made for the ground and excited states. The calculations, based on a ~r-electron model, were performed on an electronic digital computer.It could be shown in a number of cases that the aromatic hydrocarbon (in the absence of oxygen) is present in the acid solution as one type of protonated species. In these cases good agreement is found between the experimental and calculated spectra. Conversely, the theoretical predictions allowed of the interpretation of other cases where the aromatic hydrocarbon, when dissolved in the acid, forms a mixture of different ions, corresponding to a more complicated spectrum.Some of the solutions, if contacted with oxygen, display fundamental changes of spectrum. For perylene these are shown to be due to the rnonovalent positive ion of the hydrocarbon.

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The molecular structure of spiropentane has been redetermined from electron diffraction by the vapour. The difference of 0.05 A found between the two nonequivalent carbon-carbon bond lengths is in agreement with the expectation based on the results of an MO calculation. IntroductionThe structure of spiropentane (C5Hs) is of interest in chemical bond theories. In 1945 Donohue et al. 1 determined this structure from electron diffraction by the vapour, the diffraction intensities being estimated visually.

KONlNKLIJKE/SHELL-LABORATORIUM, AMSTERDAMThe structure of bicyclo[2.2.1 ]hepta-2,5-diene has been investigated by means of electron diffraction of its vapour. The results are compared with those of other electron diffraction and X-ray investigations and with theoretical calculations. It appears that in this molecule the sp2-sp3 and the sp3-sp3 carbon-carbon bonds are very nearly equal. * The calculations were carried out by Dr. P. Ros of this laboratory. ** A similar calculation but now with Rc=c = 1.340 A and Rc-r = 1.533 A yielded a difference of only -0.005 between the overlap populations of C I -C~ and Ci-Cz. 9 H. Busch, A . Visfe, H . B. Gray, J. Chem. Phys. 44, 10 (1966).

Dark red acicular crystals were obtained by slow evaporation of a saturated solution in chloroform. Precession photographs were used t o determine systematic absences.Cell parameters were based on 0 and --8 values measured by diffractometer, and were refined b y least squares.

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The partial rate factors k i for H-D exchange in ~, ~-and fl, fl-dimethylnaphthalenes were measured in CF3COOD-HPO2F2-CCI~ mixtures. The values of log h i correlate well with the excess of electron charge on the carbon atoms calculated on the basis of the inductive effect of the methyl group.

The molecular structure of bicyclo[3.1.1]heptane has been investigated by means of electron diffraction of the vapour. The molecule has one plane of symmetry and an average carbon‐carbon bond length of 1.537 Å. Details of the structure are compared with those in similar molecules and differences between the carbon‐carbon bond lengths are discussed with reference to semi‐empirical predictions based on the results of MO calculations.