ESR, Structure and Reactions of Specifically Deuterated C6— C8 n-alkane Cations in Halogenated Matrices

Abstract

Dilute frozen solutions (1-2%) of specifically labelled n-hexane, n-heptane and n-octane in three halogenated matrices (CFCl3, CF3CCl3 and CF2ClCFCl2) have been X-irradiated at 77 K in order to produce the molecular cations. ESR spectra were recorded to temperatures up to the glass transition or melting points of the matrices. Several conformers differing in the geometric structure were observed. The 120° gauche conformers of the n-alkane molecular ions are stabilized by end D-labelling. Revised assignments of their coupling constants were obtained. The extended geometry was predominant in CFCl3 with one exception. INDO MO calculations show that tilting of the CH3 group towards the -CH2- chain lowers the energy and gives better agreement with experimental values for the hyperfine coupling constants. CF3CCl3 seemed to induce the largest distortions in the geometry of the ions. The nature of a distorted conformer of hexane+ in CF3CCl3 has been determined with the aid of observed couplings and INDO MO calculations.Photobleaching causes disappearance of the cations in the CFCl3 and CF2ClCFCl2 matrices and fragmentation to produce 2-butene cations in CF3CCl3. This product was not identified when labelled compounds were employed. The mechanism is discussed.