TY - DATA T1 - Is the Mechanism of the [2+2] Cycloaddition of Cyclopentyne to Ethylene Concerted or Biradical? A Completely Renormalized Coupled Cluster Study† PY - 2006/01/19 AU - Armaǧan Kınal AU - Piotr Piecuch UR - https://acs.figshare.com/articles/journal_contribution/Is_the_Mechanism_of_the_2_2_Cycloaddition_of_Cyclopentyne_to_Ethylene_Concerted_or_Biradical_A_Completely_Renormalized_Coupled_Cluster_Study_sup_sup_/3242941 DO - 10.1021/jp0513216.s002 L4 - https://ndownloader.figshare.com/files/5077042 KW - Ethylene Concerted KW - CCSD KW - activation barrier KW - structure theory KW - noniterative triples KW - biradical mechanism KW - transition states KW - CR KW - Completely Renormalized KW - kcal KW - biradical character KW - cluster method N2 - The 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. ER -