Vibrational spectra are reported for cubane, cubane-t/i, .rym-cubane-ti/2, Ym-cubane-í/6, and cubane-t/g-Infrared spectra are from 400 to 3600 cm-1 for CS2 and CCI4 solutions, and for a solid deposited from the vapor at ~100 K. Raman spectra are for the same solutions and for the polycrystalline solid at room temperature. Vibrational assignments have been made for all the fundamentals of all five compounds, 120 modes in all. The fortuitous crystal structure of cubane and cubane-/g was an important aid. Of the 18 fundamentals of cubane, only one or two are not certain. The spectra show almost no effect of the severe bond angle strain. Also there are no low molecular modes; the lowest for cubane is 617 cm'1.

It is suggested that electronic transitions in aromatic hydrocarbons can profitably be characterized by a quantity R, where R2 is the sum of the squares of the bond length changes accompanying the transition. R determines, via the Franck-Condon principle, the distribution of intensity within the vibrational envelope of a transition. In polycyclic aromatics, values of R can be extracted from solution spectra, if the intervals of about 1400 cm-1 which commonly separate the vibrational peaks are interpreted as defining quasi-progressions in a skeletal bond displacement vibration. Values of R so determined are compared with values computed from bond orders in excited states, using the wave-functions of Pariser. Such comparisons are made for benzene, naphthalene, azulene, and anthracene. Agreement is good. The calculated bond orders are tabulated. In an Appendix, bond angles in aromatic rings are calculated on the assumption that the observed angles minimize the σ-bond strain energy. Angles are calculated for the ground states of naphthalene and anthracene, and for two excited states of naphthalene. The excited state geometries so deduced are depicted.

A previous semi-empirical approach to the calculation of the rate of internal conversion, regarded as a tunnelling process, is reformulated on a sounder theoretical basis. Following Robinson and Frosch, tunnelling rates are correlated with Franck-Condon factors for the associated transition. The total Franck-Condon factor, S2max, is a product of three terms, associated respectively with skeletal stretching, CH stretching, and skeletal angle bending vibrations. The value of S2max may be controlled by one term only of the product, or by two or more (mixed tunnelling). Tunnelling rates should be slowed down by deuteration only when CH vibrations participate significantly; specific predictions are made here. Skeletal-angle bending vibrations are of negligible importance in internal conversion in aromatic molecules, but they are very significant in transitions between states of π,π* and n,π* type in heteroaromatics. Correlation between S2max and estimated tunnelling rates is encouraging for four aromatic molecules and certain monocyclic azines; but the failure of pyridine and pyridazine to show luminescence is still unexplained. The case of pyrazine is discussed in detail.

Excited state bond orders calculated in the previous paper (McCoy and Ross 1962) are used in a necessarily approximate attempt to predict the locations of intersections between potential energy surfaces for all the lower excited states of benzene, naphthalene, azulene, and anthracene, and thus to investigate the mechanism of degradation of electronic excitation energy. It is concluded that excited states commonly, but not invariably, intersect in such a way that there is no barrier to passage from the zero-point level of one state to some state below it. Quite often, however, tunnelling, as suggested by Robinson (1961), is necessary to effect the passage. Diagrams are presented showing the preferred routes for descent through the singlet and triplet states. The radiative properties of the compounds considered are then successfully correlated with the distances through which tunnelling needs to take place, and an approximate empirical relationship emerges in which the tunnelling rate (multiplied by 106 if there is a spin change) decreases exponentially with barrier width. The mechanism of tunnelling is briefly discussed.

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