Cryogenic temperatures, triplet carbenes

The matrix-isolation technique is the method of choice for the direct spectroscopic observation of carbenes . However, efforts to generate and to observe silylcarbenes in solid matrices at cryogenic temperatures met with limited success. When (trimethylsi-lyl)diazomethane, (dimethylsilyl)diazomethane or bis(trimethylsilyl)diazomethane were irradiated in an argon matrix at <10 K, no IR spectra of the corresponding carbenes 3a-c could be obtained " . However, weak ESR signals were observed which were typical for a linear carbene with a triplet ground state . These results indicate that at least small amounts of carbenes 3a-c were present in the matrix. 

Generation of 78 by thermolysis or photolysis of a diazoalkane or diazirine precursor, however, affords the singlet carbene, whose 1,2-H shift to ethene is opposed by a barrier of only 0.678 to 1.298 kcal/mol. Consequently, even in cryogenic matrices, singlet 78 rearranges more rapidly than it intersystem crosses to the triplet, which has therefore not been detected by UV or ESR in either an Ar matrix at 8 K or a Xe matrix at 15 K." The lifetime of singlet 78 at ambient temperature has been estimated at <0.5 ns.89,98b (Note the enormous spectator substituent effect of Cl the lifetime of MeCCl is 740 ns,60 at least 1500 times longer than that of MeCH.)... 

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