Matrix isolation and theoretical study of the photochemical reactions of C2H3Br and 1,2-C2H2Br2 with CrO2Cl2

The matrix-isolation technique has been combined with infrared spectroscopy and theoretical calculations to characterize the products of the photochemical reactions of C2H3Br and 1,2-C2H2Br2 with CrO2Cl2. For these systems, oxygen-atom transfer occurred upon visible-near ultraviolet irradiation, yielding bromoacetaldehyde and CrOCl2 in the former case and bromoacetyl bromide and CrCl2O 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 C2H3Br 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: η1 (end-on) to the oxygen atom and η2 (side-on) to the CO bond. Theoretical calculations indicated that the η1 complex of CH2BrCHO-CrCl2O was 13 kcal mol−1 more stable than the η2 complex at the B3LYP/6-311++G(d,2p) level of theory. The binding energy of the η1 complex was found to be 21 kcal mol−1, compared to 8 kcal mol−1 for the η2 complex at this level of theory.