Resonance energies of vinylcarbenes

The stabilization energy provided by resonance in singlet and triplet vinylcarbenes is evaluated using density functional and G3 theory, without symmetry restriction. Structural and charge change upon rotation are reviewed with singlet vinylcarbene exhibiting a larger barrier (1s-TS-1s-E), 5.49 (7.10) kcal/mol, than triplet (1t-TS-1t-E), 4.06 (4.54) kcal/mol. The changes in charge upon rotation for singlet vinylcarbene are consistent with a simple resonance picture for the ground state, whereas there is insignificant charge change for the triplet. Difficulties of the rotational analysis are avoided by evaluating the inductive stabilization difference between ethyl and vinyl in isodesmic equation 1. This inductive difference was determined by comparing the energy change for hydrogen loss for polyene series 2 (n double bonds) with polyene series 3 (n double bonds), which are combined in isodesmic equation 4. The effect of the ultimate vinyl group in equation 2 is restricted to inductive interaction by holding its pi-system so it is orthogonal to the remaining pi-system. Extrapolation of plots of the inductive difference (Delta inductive) revealed for equation 4 versus a through-bond inductive attenuation of 1.75(-2n) to n = 0 provides a Delta inductive for singlet vinylcarbene of -4.52 kcal/mol (R = 0.995) at B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) + ZPE at B3LYP/6-31G(d) and -4.16 (R = 0.993) at the G3(MP2)//B3LYP level. Correcting for Delta inductive in eq 1, the resonance energy of singlet vinylcarbene is 9.2 (8.9) kcal/mol. An analogous analysis of triplet vinylcarbene suggests that inductive effects are 0-1 kcal/mol, which leads to a resonance energy of 11-14 kcal/mol.