TRANSITION STRUCTURES FOR HYDROGEN-ATOM TRANSFERS TO OXYGEN - COMPARISONS OF INTERMOLECULAR AND INTRAMOLECULAR PROCESSES AND OPEN-SHELL AND CLOSED-SHELL SYSTEMS

The transition structures for the intramolecular hydrogen atom abstractions of the butoxy radical, the triplet and the radical cation states of butanal, and the radical cation of butanol, as well as that of the thermal retro-ene reaction of butanal, were located with ab initio molecular orbital calculations. These processes are the rate-and/or product-determining steps in common reactions like the Barton reaction, the Norrish type II photochemical reaction, and the McLafferty rearrangement of radical cations in mass spectrometry. The corresponding intermolecular hydrogen abstractions from methane by the methoxy radical, triplet formaldehyde, formaldehyde radical cation, and the methanol radical cation were located for comparison, using UHF theory and correlation theory corrections at the MP2 level. Differences in activation energy are related to the electronic differences between closed-shell, open-shell, and charged open-shell systems. Regioselective hydrogen atom transfer in the Norrish type 11 reaction is due to a combined enthalpic and entropic preference for a six-membered-ring transition structure. Our calculations indicate a strong preference for in-plane hydrogen abstraction by triplet aldehydes. © 1990, American Chemical Society. All rights reserved.