Theoretical study on the isomerization behavior between α,β-unsaturated acyl radicals and α-ketenyl radicals

Ab initio calculations using 6-311G**, cc-pVDZ, aug-cc-pVDZ, and a (valence) double-zeta pseudopotential (DZP) basis set, with (QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional methods (BHandHLYP, B3L'YP) predict that alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals exist as isomers. At the CCSD(T)/cc-pVDZ//BHandHLY/ccpVDZ ;level of theory, energy barriers of 15.1 and 17.7-21.7 M mol(-1) are calculated for the isomerization of s-trans-propenoyl and s-trans-crotonoyl radical to ketenylmethyl and 1-ketenylethyl radical, respectively. Similar results are obtained for the reactions of s-trans isomers involving silyl, germyl, and stannyl groups with energy barriers (Delta E-double dagger) of.12.2-12.4, 13.1-13.9, and 12.9-18.2 M mol(-1) at the CCSD(T)/DZP//BHandHLYP/DZP calculation, respectively. These results suggest that (x,p-unsaturated acyl radicals and (alpha-ketenyl radicals are not canonical forms but are isomeric species that can rapidly interconvert.