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Electron paramagnetic resonance spectra have been obtained of tetramethylpyrazine in its lowest (ππ*) triplet state in several rigid matrices. Some unusual observations were made : (1) In a durene crystal host the in-plane zero-field principal axes are not parallel to the molecular symmetry axes. (2) In rigid glass solution the zero-field parameters are strongly dependent on solvent polarity.Both effects are discussed in terms of pseudo Jahn-Teller interaction in conjunction with a crystal-field influence.In a durene matrix the zero-field parameters are [image omitted]The relatively small value of [image omitted], as compared with that of the corresponding aromatic hydrocarbon, probably results from vibronic mixing of the lowest 3ππ* with the near-by 3nπ* state.Hyperfine structure was observed during measurements at 1·95 K ; it is in agreement with the ππ* character of the lowest triplet state and is due to coupling between the unpaired electrons and protons in the non-rotating methyl groups.

The rare syndrome of compulsive swearing and tics, named after Georges Gilles de la Tourette, has been recorded previously in the Southern Hemisphere, although these records have been overlooked in many reviews.The case history of a 17-year-old New Zealand boy is reported. In this case, there is a marked family history of schizophrenia. Some of the features of this patient are discussed.

Electron resonance spectra have been recorded of the lowest triplet states of benzene, toluene, triptycene and tribenzotriptycene. The experiments were carried out by ultra-violet irradiation of these molecules in rigid glass solutions both at 77°k and 20°k. From an analysis of the line shape of the spectra, which prove to be strongly temperature dependent, the following conclusions could be drawn.Benzene in its first triplet state has a configuration of lowest energy which is not a regular hexagon. This finding, supported by the results for toluene, was substantiated by SCF-MO calculations; it is in agreement with a recent analysis by Liehr and Moffitt. Tunnelling between equivalent conformations of benzene is thought to occur at a rate of the order of 109-1010 sec-1.The spectra of the phosphorescent states of triptycene and tribenzotriptycene are dominated by intramolecular excitation transfer between the benzene or naphthalene sub-systems. In triptycene at 77°k transfer occurs at a rate >> 1010 sec-1, so that one observes the resonance signal of a triplet exciton. In tribenzotriptycene the transfer is somewhat slower. The rate of excitation transfer decreases with temperature and at 20°k most of the excitation in tribenzotriptycene is effectively trapped in the naphthalene sub-systems.

Recently Hutchison and Mangum have observed electron spin resonance in the lowest triplet state of naphthalene. An analysis of their results is given on the basis of spin-spin interaction between the two unpaired electrons. In addition to the lines observed by these authors another transition, which corresponds to Δm = ±2 in the atomic case, must occur at low field strength. This transition has been observed in a mixed crystal as well as in rigid glass solution.

Results of further calculations on the dynamic coupling between the lower B1u and E1u electronic states of benzene via a single e2g mode are presented. In part I we gave the 'pseudo-cylindrical' solutions that result when the leading coupling term, linear in the nuclear displacements, is considered (cylindrical Born-Oppenheimer potential). Here we investigate the effect of(1) the next higher terms in the expansion of the coupling hamiltonian for the free molecule (hexagonal Born-Oppenheimer potential); (2) a crystal field anisotropy. From the scanty data on the coupling constants available a priori it would follow that the refinement introduced by (1) is small for benzene. On the contrary, the effect of even a small crystal field anisotropy is predicted to be appreciable.The results of the calculations are analysed by comparison with experimental data on the benzene crystal at low temperature. A consistent interpretation of both magnetic resonance results and the triplet ← singlet absorption spectrum is obtained.

Experiments have been performed to determine the path of entry into and exit from the phosphorescent triplet state T0 of quinoxaline in a durene host. First of all the decay of phosphorescence after flash excitation was followed at 4·2 and 1·34°k. It was found that for both perdeutero- and perhydroquinoxaline the lifetime is shortened by a factor of about three when the temperature is lowered from 4·2 to 1·34°k. At 1·34°k relaxation between the spin components (i.e. re-orientation of the triplet spin angular momentum) is slow relative to the decay, and the observed reduction in lifetime indicates that entry into and exit from T0 are through the same spin component.Similar decay experiments were then carried out at 1·34°k in a 10 kg magnetic field or in a somewhat weaker field so chosen that the effect of microwave saturation of one of the E.S.R. transitions between the components could be observed. From the results it follows that on intersystem crossing the molecules enter the manifold T0 through the top zero-field component and thus initially have their spins aligned. Decay departs almost exclusively from the same component, even in the case of perhydroquinoxaline, where at least 45 per cent of it must be radiationless. The decay route agrees with out-of-plane polarization of phosphorescence for the free molecule.Finally, spin alignment on intersystem crossing is discussed from the theoretical point of view. It appears that the phenomenon is clear-cut only in molecules such as those of the aza-aromatics, where strong spin-orbit coupling of the atomic type occurs between ππ* and nπ* states.

Intersystem-crossing in aromatic molecules is caused by spin-orbit coupling. This mechanism is expected to be highly selective for a specific spin-component of each triplet state. Experimental evidence is given for quinoxaline proving that spin polarization indeed occurs when this molecule is optically excited into its lowest triplet state. A remarkable coherence phenomenon, reminiscent of level-crossing situations in atomic spectroscopy, seems to be observed following flash excitation in a magnetic field.

Magnetic resonance spectra were recorded of glassy solutions of a number of positive aromatic ions in their triplet state, which theory predicts to be orbitally degenerate.The phenalenylium (I) and triphenylcyclopropenylium (II) cations, rather than exhibiting trigonal symmetry in their resonance spectra, are distorted. The distortion seems to result from a Jahn-Teller instability of the trigonally symmetric conformations, together with interaction with the solvent.In the triphenylmethyl cation (III) steric hindrance has a dominant influence. A symmetrical propeller type of structure has been proposed in the literature, but this model does not agree with our resonance spectra.

Electron paramagnetic resonance spectra are given of glassy solutions of triphenylene, 1, 3, 5-triphenylbenzene, coronene and naphthalene excited into their lowest triplet state by ultra-violet irradiation at liquid-nitrogen temperature. For each substance two spectra were measured: one with the r.f. magnetic field parallel to the constant field, the other with the fields at right angles.A method is suggested for analysing the spectra of those molecules having a trigonal symmetry axis and values of the zero-field splitting parameter are derived. A complete treatment for molecules of lower symmetry has not been given, but it is shown that our results on a glassy solution of naphthalene are in agreement with those obtained by Hutchison and Mangum with single crystals.

Electron resonance spectra are presented of the phosphorescent state of the mesitylene species C6H3(CD3)3 in a B-trimethylborazole host crystal. The fine structure does not correspond to trigonal symmetry of the spin distribution and it varies with temperature:[image omitted]The z axis is perpendicular to the molecular plane and the y axis along a H-C-C-CD3 diagonal to within a few degrees.Hyperfine structure is only resolved for H‖x, where one obtains a doublet with a splitting of 12-13 g. The data are consistent with a dominant interaction with a single proton bonded to a carbon atom carrying a spin density of about 0·34.The results are discussed in terms of the pseudo Jahn-Teller instability of the 3B1u state of the benzene π-electron system.

In the present paper the pseudo Jahn-Teller interaction between the four lower electronic states of benzene is considered, with special reference to the mixing of the 3B1u and 3E1u states through the e2g carbon-carbon stretching mode. First a review is given of the pioneering work of Liehr on the static problem, where the 'electronic Hamiltonian' is solved as a function of fixed nuclear coordinates. Results of a specific calculation are presented.Subsequently, the equations for the vibronic problem are set up by generalizing the methods proposed by Moffitt et al. and Longuet-Higgins et al. for the dynamic Jahn-Teller effect. These equations are then solved in the first ('pseudo-cylindrical') approximation in which only the strong coupling linear in the displacements between the B1u and E1u states is considered. The results are discussed with reference to known properties of the phosphorescent state of benzene.