Diradicals singlet methylene

It has been argued128-124 that singlet methylene adds directly to the carbon double bond to form a cyclopropane ring by a three-center mechanism involving essentially simultaneous formation of two carbon-carbon bonds. We believe, however, that the detailed mechanism must include formation of a short lived diradical as the initial step for both singlet and triplet methylene. The diradical may undergo ring closure or struc-... 

As computational methods for describing excited states have been refined, additional understanding of the structures has developed. Relatively early computational studies provided some indication of the geometries associated with the butadiene excited states.The ground state has a maximum at a twist of 90° about the C(l)-C(2) bond. This structure, which can be approximately described as a singlet methylene-allyl diradical, is found at about 2.3 eV and is more stable than a structure with 90° twist at both terminal groups (3.1 eV). There is no major pyramidalization of the methylene groups in this second structure. The spectroscopic (Franck-Condon) state is about... 

Quantum-chemical calculations of the transition state of the enantiomerization of 6, that is, for the interconversion of (M)-6 and (P)-6, support the diradical 6-D [13, 18] (Scheme 6.4). Most probably, its singlet state is a few kcal mol-1 more stable than the triplet state. The zwitterions 6-Zi and 6-Z2 are excited states [1], However, if a methylene group directly attached to the allene system in 6 is replaced by an appropriate heteroatom, either 6-Za or 6-Z2 may be strongly stabilised (see Section 6.3.5). 

The origin of the difference lies in the fact that triplet carbenes are biradicals (or diradicals) and exhibit chemistry similar to that exhibited by radicals, while singlet carbenes incorporate both nucleophilic and electrophilic sites, e.g., for singlet and triplet methylene. 

Andrews and Day observed fluorescence from methylene-1-pyrazolines. They also showed that energy transfer from the lowest excited singlet state to dienes (piperylene) occurs with moderate efficiency. The decomposition products from methylene pyrazolines have been postulated to be the resonance stabilized diradicals,... 

As a starting point for the mechanistic discussion, it is useful to review the structural features of the excited states. The first singlet excited state of butadiene. Si, can be approximated as the i >2->--LUMO tt-tt transition. The lack of fluorescence from this excited state indicates that a very facile path exists for nonradiative energy transfer. The S2 state has doubly excited character and relaxes to a structure with ionic character that can rotate at the pyramidal carbon but not at the allyl fragment. The minimum energy of the Tj state corresponds to the allyl-methylene diradical with a nearly 90° twist and slight pyramidalization at the methylene carbon. In substituted systems, one or the other of the zwitterion structures (e.g., allyl cation versus allyl anion) may be favored. ... 

Abstract The singlet-triplet splittings of the di-radicals methylene, trimethylene-methane, ortha-, meta- and para-benzynes, and cyclobutane-l,2,3,4-tetrone have become test systems for the applications of various multi-reference (MR) coupled-cluster methods. We report results close to the basis set limit computed with double ionization potential (DIP) and double electron attachment (DBA) equation-of-motion coupled-cluster methods. These diradicals share the characteristics of a 2-hole 2-particle MR problem and are commonly used to assess the performance of MR methods, and yet require more careful study unto themselves as benchmarks. Here, using our CCSD(T)/6-311G(2d,2p) optimized geometries, we report DIP/DEA-CC results and single-reference (SR) CCSD, CCSD(T), ACCSD(T) and CCSDT results for comparison. 

While direct irradiation of , E-2,4-hexadiene 5 gives only , Z-2,4-hexadiene from singlet excited state, triplet-sensitized reaction gives both E,Z- and Z,Z-2,4-hexadienes. The singlet state reaction proceeds with just one terminal double bond rotation involving allyUc methylene or cyclopropane methylene diradical with just one double bond rotation, whereas the triplet excited state reaction proceeds with double double bond rotation [11]. 

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