Bimolecular reaction of CH3 + CO in solid p-H-2: Infrared absorption of acetyl radical (CH3CO) and CH3-CO complex

Abstract

We have recorded infrared spectra of acetyl radical (CH3CO) and CH3-CO complex in solid para-hydrogen (p-H-2). Upon irradiation at 248 nm of CH3C(O)Cl/p-H-2 matrices, CH3CO was identified as the major product; characteristic intense IR absorption features at 2990.3 (v(9)), 2989.1 (v(1)), 2915.6 (v(2)), 1880.5 (v(3)), 1419.9 (v(10)), 1323.2 (v(5)), 836.6 (v(7)), and 468.1 (v(8)) cm (1) were observed. When CD3C(O)Cl was used, lines of CD3CO at 2246.2 (v(9)), 2244.0 (v(1)), 1866.1 (v(3)), 1046.7 (v(5)), 1029.7 (v(4)), 1027.5 (v(10)), 889.1 (v(6)), and 723.8 (v(7)) cm (1) appeared. Previous studies characterized only three vibrational modes of CH3CO and one mode of CD3CO in solid Ar. In contrast, upon photolysis of a CH3I/CO/p-H-2 matrix with light at 248 nm and subsequent annealing at 5.1 K before re-cooling to 3.2 K, the CH3-CO complex was observed with characteristic IR features at 3165.7, 3164.5, 2150.1, 1397.6, 1396.4, and 613.0 cm(-1). The assignments are based on photolytic behavior, observed deuterium isotopic shifts, and a comparison of observed vibrational wavenumbers and relative IR intensities with those predicted with quantum-chemical calculations. This work clearly indicates that CH3CO can be readily produced from photolysis of CH3C(O)Cl because of the diminished cage effect in solid p-H-2 but not from the reaction of CH3 + CO because of the reaction barrier. Even though CH3 has nascent kinetic energy greater than 87 kJ mol(-1) and internal energy similar to 42 kJ mol(-1) upon photodissociation of CH3I at 248 nm, its energy was rapidly quenched so that it was unable to overcome the barrier height of similar to 27 kJ mol(-1) for the formation of CH3CO from the CH3 + CO reaction; a barrierless channel for formation of a CH3-CO complex was observed instead. This rapid quenching poses a limitation in production of free radicals via bimolecular reactions in p-H-2. (C) 2014 AIP Publishing LLC.