Dimesitylcarbene (2a) was shown to decay by undergoing dimerization and to have a half- life of 160 ms, some 5 orders of magnitude longer-lived than diphenylcarbene. Didurylcarbene (2b) was twice as long-lived as 2a, while decamethyldiphenylcarbene (2c) decayed mainly unimolecularly by abstracting H from the o-methyl group and became shorter-lived than 2b. 2, 4, 6-Tri (tert-butyl) diphenylcarbene (14) decayed unimolecularly ...