Communication: spectroscopic characterization of an alkyl substituted Criegee intermediate syn-CH(3)CHOO through pure rotational transitions.

An alkyl-substituted Criegee intermediate syn-CH3CHOO was detected in the gas phase through Fourier-transform microwave spectroscopy. Observed pure rotational transitions show a small splitting corresponding to the A∕E components due to the threefold methyl internal rotation. The rotational constants and the barrier height of the hindered methyl rotation were determined to be A = 17 586.5295(15) MHz, B = 7133.4799(41) MHz, C = 5229.1704(40) MHz, and V3 = 837.1(17) cm(-1). High-level ab initio calculations which reproduce the experimentally determined values well indicate that the in-plane C-H bond in the methyl moiety is trans to the C-O bond, and other two protons are directed to the terminal oxygen atom for the most stable structure of syn-CH3CHOO. The torsional barrier of the methyl top is fairly large in syn-CH3CHOO, implying a significant interaction between the terminal oxygen and the protons of the methyl moiety, which may be responsible for the high production yields of the OH radical from energized alkyl-substituted Criegee intermediates.