Vapour phase spectra in the near-ultraviolet of some monosubstituted benzenes

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Abstract
Absorption spectra in the vapour phase and in solution of aniline, NN-dimethylaniline, anisole, benzenethiol, methyl phenyl sulphide, benzeneselenol and methyl phenyl selenide, have been investigated in the near-ultraviolet region. The various transitions observed are discussed on the basis of the frequency shift in respect to benzene transitions, intensity data, solvent effect and, where possible, vibrational analysis. It is concluded that the absorption at lower frequency is of the same type for all monosubstituted benzene derivatives and that it is related to the A1g→B2u transition in benzene. A correlation based on overlap integral C(2Pπ)–X(npπ) and ionisation potential, l(X–R) is proposed, which allows one to derive a scale of the perturbative powers of substituents. Transition exhibiting anomalous solvent effect were observed and attributed to dissociative states. The main absorption of high intensity in the middle spectral region seems to be dominated by charge-transfer structures between the substituent and the benzene ring, and thus the analogy with the benzene transition at 2000 Å breaks down for strong substituents.

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