Ultraviolet absorption spectra of arenethiols and alkyl aryl sulphides

Abstract

The u.v. absorption spectra of benzene-, toluene-, and mesitylene-thiols and the derived alkyl aryl sulphides have been determined and compared in polar and non-polar solvents. The effect of steric inhibition of resonance between the sulphur atom and aryl ring has been studied by varying the non-aryl sulphur substituent in the order H (thiols), Me, Et, Prⁱ, and Bu^t, and also by introducing methyl groups on the aryl ring. Both the short wavelength (205–230 nm) and medium wavelength (235–270 nm) bands show a marked bathochromic shift when a t-butyl group or mesityl ring is introduced into the sulphides, whilst similar structural changes cause the long wavelength (275–300 nm) bands to undergo a hypsochromic shift. A rationalisation of the spectral changes has been examined using a free electron approach and assuming a predominantly conjugative interaction between the sulphur atom and the aryl group. An increase in the polarity of the solvent causes hypsochromic shifts which are increased by steric inhibition of resonance. The solvent shifts are attributed to hydrogen bonding to the sulphur atom.