Electron-withdrawing groups triplet carbenes

IR bands for the both states. For example, singlet and triplet states of 2-naphthyl-(methoxycarbonyl)carbenes (NMCs, 17) show very different IR spectra (Scheme 9.4) (1590, 1625, 1640 cm for NMC and 1660 cm for NMC). Both states are observable in this case. In the singlet state, the carbomethoxy group assumes a conformation perpendicular to the naphthylcarbene plane to avoid destabilization of the empty carbenic 2p atomic orbital by the electron-withdrawing carbonyl group, while in the triplet, the methoxycarbonyl group is in the same plane to delocalize an unpaired electron. For this reason, there is a barrier between the two states and hence both of them are observable under these conditions. 

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