Matrix isolation and theoretical study of the photochemical reactions of C 2H 3Br and 1,2-C 2H 2Br 2 with CrO 2Cl 2

Abstract

The matrix-isolation technique has been combined with infrared spectroscopy and theoretical calculations to characterize the products of the photochemical reactions of C 2H 3Br and 1,2-C 2H 2Br 2 with CrO 2Cl 2. For these systems, oxygen-atom transfer occurred upon visible–near ultraviolet irradiation, yielding bromoacetaldehyde and CrOCl 2 in the former case and bromoacetyl bromide and CrCl 2O in the latter. For each system, the products were formed in the same matrix cage and strongly interacted to form a distinct molecular complex. No evidence was obtained for the acetyl bromide derivative in the C 2H 3Br system, indicating the occurrence of oxygen-atom attack at the less substituted carbon of vinyl bromide, nor was any evidence obtained for the formation of a possible five-membered metallocycle. Two different modes of interaction were explored computationally: η 1 (end-on) to the oxygen atom and η 2 (side-on) to the C O bond. Theoretical calculations indicated that the η 1 complex of CH 2BrCHO–CrCl 2O was 13 kcal mol −1 more stable than the η 2 complex at the B3LYP/6-311++G(d,2p) level of theory. The binding energy of the η 1 complex was found to be 21 kcal mol −1, compared to 8 kcal mol −1 for the η 2 complex at this level of theory.