Bimolecular reaction of CH 3 + CO in solid p -H 2: Infrared absorption of acetyl radical (CH 3 CO) and CH 3 -CO complex

Prasanta Das, Yuan-Pern Lee*

*Corresponding author for this work

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Abstract

We have recorded infrared spectra of acetyl radical (CH 3 CO) and CH 3 -CO complex in solid para-hydrogen (p-H 2 ). Upon irradiation at 248 nm of CH 3 C(O)Cl/p-H 2 matrices, CH 3 CO was identified as the major product; characteristic intense IR absorption features at 2990.3 (ν 9 ), 2989.1 (ν 1 ), 2915.6 (ν 2 ), 1880.5 (ν 3 ), 1419.9 (ν 10 ), 1323.2 (ν 5 ), 836.6 (ν 7 ), and 468.1 (ν 8 ) cm - 1 were observed. When CD 3 C(O)Cl was used, lines of CD 3 CO at 2246.2 (ν 9 ), 2244.0 (ν 1 ), 1866.1 (ν 3 ), 1046.7 (ν 5 ), 1029.7 (ν 4 ), 1027.5 (ν 10 ), 889.1 (ν 6 ), and 723.8 (ν 7 ) cm - 1 appeared. Previous studies characterized only three vibrational modes of CH 3 CO and one mode of CD 3 CO in solid Ar. In contrast, upon photolysis of a CH 3 I/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 CH 3 -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 CH 3 CO can be readily produced from photolysis of CH 3 C(O)Cl because of the diminished cage effect in solid p-H 2 but not from the reaction of CH 3 + CO because of the reaction barrier. Even though CH 3 has nascent kinetic energy greater than 87 kJ mol - 1 and internal energy ∼ 42 kJ mol - 1 upon photodissociation of CH 3 I at 248 nm, its energy was rapidly quenched so that it was unable to overcome the barrier height of ∼ 27 kJ mol - 1 for the formation of CH 3 CO from the CH 3 + CO reaction; a barrierless channel for formation of a CH 3 -CO complex was observed instead. This rapid quenching poses a limitation in production of free radicals via bimolecular reactions in p-H 2 .

Original languageEnglish
Article number244303
JournalJournal of Chemical Physics
Volume140
Issue number24
DOIs
StatePublished - 1 Jan 2014

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