Apart from any modification that conjugation may impose, the sulphoxide group itself is a chromophore producing moderately strong absorption near 220 mp. It is suggested that the transition is one of an electron of the unshared 3sp3 pair of the sulphur atom into an appropriately oriented 3d sulphur orbital, i.e., a transition between molecular orbitals of which these atomic orbitals are the main components. In a phenyl sulphoxide this transition would couple strongly with a B,,-like transition of the benzene ring because the local transition moments would be parallel; and in a parasubstituted phenyl sulphoxide it would couple strongly with the B,,-derived transition of the corresponding monosubstituted benzene for the same reason.In ovthoand mefa-substituted phenyl sulphoxides this coupling is expected to be considerably weaker, because the local transition moments are no longer parallel, and for steric reasons. The absorption spectra of a large number of aryl, diaryl, aralkyl, diaralkyl, and aralkyl aryl sulphoxides are recorded, and can be reasonably interpreted on the basis of the electronic assignment mentioned.WE have extended our study of ultraviolet spectra of organic sulphur compounds (sulphides,' sulphones,2 and thiosulphones 3, to sulphoxides. Sulphoxides R,SO, unlike the other classes of sulphur compounds mentioned, even when both radicals R are fully saturated, possess a moderately strong absorption band in the quartz ultraviolet region.4 Some Su&hoxides i~ the Near ultraviolet Region. 1387In non-polar solvents such as cyclohexane the absorption maximum is near 220 mp (log E, a. 3.30) ; in hydroxylic solvents this band is often at appreciably shorter wavelengths.** 6 We have studied the spectra of methyl phenyl, benzyl phenyl, dibenzyl, and diphenyl sulphoxides and many of their substitution products. To simplify discussion we consider first the nature of the fundamental sulphoxide transition. We cannot deduce this from the spectra, but we can make a plausible suggestion, and then show that this enables many features of the spectra to be understood.Bond lengths, bond angles, and dipole moments of sulphoxides, and the fact that sulphoxides can be optically resolved, indicate, as a first approximation, that the ground state of the valency shell of the sulphur aton can be described as an octet with 3sp3 hybridisation in the bonds and also in the unshared electron pair. To improve this description two further interactions may be taken into account. The first, conjugation of the unshared 3sp3 electron pair of sulphur with 2 x electron shell of the unsaturated group, applies only when at least one of the groups R is unsaturated at the point of its attachment to sulphur, but it may then be quite important. The other effect, conjugation between one pair of unshared 2p electrons of oxygen with a vacant 3d orbital of sulphur, is universal, but is weak in sulphoxides (considerably weaker than in sulphones) or in sulphur groups containing still more oxygen, the extreme case being the sulphate ion. This could be...

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Summar38 h single-crystal X-ray diffraction study of benzenes,ls with an integration ratio of 4:3. No other The apparent spectral equivalence p-MeC,H,-(N,S)-C,H,Me-p' has shown that the 4,sgroup has the bis-imino-sulphur structure -N = S = N-. peaks are observed. SEVERAL symmetrical Ar-(N,S) -Ax compounds have been prepared1-* and structures such as azothioarene (I), bisarylimino-sulphur (11) and 2,S-diaryl-l,Z,S-thiodiazirine (I1 I) have been suggested .lA7 ,9@Chemical evidence, partial assignment of the i.r. spectra2pp dipole moment measurements,10 and n.m.r. data11 did not allow the structure of the -(N2S)-group to be assigned unambiguously, geometric isomerism also being possible.The n.m.r. spectrum of the p,p '-ditolyl derivativeB consists of a single peak at 6 2.30 p.p.m. (CH,) (6 values from Me,Si, in CCI,) and a symmetrical resonance group centred at 6 7-18 p.p.m., which resembles an A2B2 system of p-disubstituted