Photoelectron Spectroscopy of the Trimethylene- methane Negative Ion. The Singlet−Triplet Splitting of Trimethylenemethane

Abstract

Few reactive organic intermediates have elicited as much attention by experimentalists and theorists as trimethylenemethane (TMM, 1).1,2 The theoretical significance of this archetypal non-Kekule molecule was recognized by Moffitt and Coulson nearly 50 years ago,3 and it has since inspired numerous computational studies spanning the entire gamut of theoretical methods. TMM serves as a paradigm for basic theoretical concepts such as free valence, disjoint orbital analysis, and negative spin density in π systems, and it has been a consistently challenging subject for electronic structure calculations.4 Although trimethylenemethanes were commonly invoked as intermediates in the formation and rearrangement of methylenecyclopropanes,5 little was known experimentally about these molecules until 1966, when Dowd6 reported the EPR spectrum for 1 isolated in a glassy matrix at 88 K. Subsequent work by Dowd and co-workers verified the triplet ground state with 3-fold (D3h) symmetry that had been predicted for TMM.7,8 Many elegant spectroscopic and chemical experiments with TMM by the Pittsburgh group1a and with monocyclic derivatives by Berson and co-workers at Yale University1b have elevated our understanding of trimethylenemethanes to a high level. Practical applications of TMM derivatives now include organic ferromagnets,9 synthetic reagents,10 and even DNA-cleaving agents.11 Recently, Maier and co-workers reported lowtemperature matrix IR spectra for triplet 1 and its d2, d4, and d6 isotopomers.12,13 In contrast to the wealth of information available for the ground state of 1, very little is known experimentally about the excited singlet states. Of fundamental importance is the energy difference between the lowest energy singlet and triplet states, i.e., the “singlet-triplet splitting”. Ab initio molecular orbital and valence-bond calculations predict values of 14-20 kcal/ mol for the energy splitting between the XA′2 state and the aB1 state, 1a, which has one methylene group twisted out of the plane of the molecule.14,15 The lowest energy singlet state