Spectroscopy and photochemistry at low temperature. Part 2.—Hydrogen bonding and photochemical addition to the benzene ring

Abstract

Perturbations in the near u.-v. absorption spectra of benzene and its derivatives are induced by CHCl₃ and other halogenated protic solutes in hydrocarbon solution, at temperatures below 200°K; they have been ascribed to H-bonding with the aromatic ring. Two stages of association have been distinguished, and ascribed to specifically oriented complexes involving one, and two molecules of the non-aromatic solute. Thermodynamic parameters for the benzene + CHCl₃ association at 77°K, have been evaluated, and the structure of the complex compared with that of pyridine and CHCl₃ at 77°K. Association modified the low-temperature photochemistry of benzene and its alkyl derivatives, and substituted polyene adducts are produced with perchloro-alkanes. With substituted ethylenes, the photochemical reaction producing octatetraenes in fluid solution, is suppressed in a rigid solvent medium or by association. The two types of reaction are compared, and possible mechanisms discussed.