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Krishna Sachdev

Other affiliations: University of Pittsburgh
Bio: Krishna Sachdev is an academic researcher from Harvard University. The author has contributed to research in topics: Trimethylenemethane & Diradical. The author has an hindex of 8, co-authored 9 publications receiving 264 citations. Previous affiliations of Krishna Sachdev include University of Pittsburgh.

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TL;DR: Arrhenius parameters for the thermal first-order geometrical isomerization of 1,2-dicyanocylopropanes(I) have been determined in naphthalene solution over the range 208.0-259.5° in both directions: where θ = 4.575T × 10−3 and k is in sec−1.

19 citations


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TL;DR: This review sets out to understand the reactivity of diradicals and how that may differ from monoradicals, and examines activation energies of prototypical radical reactions for representative organic diradICALs and diradicaloids in their two lowest spin states.
Abstract: This review sets out to understand the reactivity of diradicals and how that may differ from monoradicals. In the first part of the review, we delineate the electronic structure of a diradical with its two degenerate or nearly degenerate molecular orbitals, occupied by two electrons. A classification of diradicals based on whether or not the two SOMOs can be located on different sites of the molecule is useful in determining the ground state spin. Important is a delocalized to localized orbital transformation that interchanges "closed-shell" to "open-shell" descriptions. The resulting duality is useful in understanding the dual reactivity of singlet diradicals. In the second part of the review, we examine, with a consistent level of theory, activation energies of prototypical radical reactions (dimerization, hydrogen abstraction, and addition to ethylene) for representative organic diradicals and diradicaloids in their two lowest spin states. Differences and similarities in reactivity of diradicals vs monoradicals, based on either a localized or delocalized view, whichever is suitable, are then discussed. The last part of this review begins with an extensive, comparative, and critical survey of available measures of diradical character and ends with an analysis of the consequences of diradical character for selected diradicaloids.

184 citations

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TL;DR: In this article, the authors reported the EPR spectrum for trimethylenemethane (TMM, 1) isolated in a glassy matrix at 88 K and verified the triplet ground state with 3-fold symmetry that had been predicted for TMM.
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

158 citations