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Is the Mechanism of the [2+2] Cycloaddition of Cyclopentyne to Ethylene Concerted or Biradical? A Completely Renormalized Coupled Cluster Study†
journal contribution
posted on 2006-01-19, 00:00 authored by Armaǧan Kınal, Piotr PiecuchThe mechanism of the [2+2] cycloaddition reaction of cyclopentyne to ethylene has been studied using the
completely renormalized coupled cluster method with singles, doubles, and noniterative triples
(CR−CCSD(T)). In agreement with the experimentally observed stereochemistry, the CR−CCSD(T) method
favors the concerted pathway involving a [2+1] transition state, whereas the popular CCSD(T) method, which
is often regarded as the “gold standard” of electronic structure theory, and low-order multireference methods
support the less probable biradical mechanism. In addition, the CCSD(T) approach produces an erroneous
description of some transition states and intermediates, particularly those which have a significant biradical
character. The CR−CCSD(T) calculations indicate that the reaction is a highly exothermic (Δ
= −68
kcal/mol), predominantly concerted process with a relatively low activation barrier on the order of 13−16
kcal/mol which permits its thermal occurrence.