Singlet state carbon-hydrogen

Carbenes with two orbitals, occupied by one electron each, have a total spin number of s = 1 the multiplicity is therefore 2s -f-1 =3, i.e., a triplet state. This is the usual ground state of a carbene. If however, the two orbitals are not degenerate, the multiplicity can become 1, i.e., a singlet state is possible. Methylene — and other carbenes — can, in principle, be linear or bent. The binding is achieved by using the 15 hydrogen Is and the 2s and 2p carbon orbitals. 

Methylcarbomethoxycarbene. Alkylcarbenes are generally useless as cyclopropane precursors since their singlet states rapidly undergo hydride shifts the prevailing reaction is insertion into carbon hydrogen bonds. For example, direct... 

Therefore, cyclization at the asymmetric carbon must be a one-step process (probably involving a singlet state nitrene, since the triplet state would be expected to abstract hydrogen). The vapor phase pyrolysis of 2-isopropyl and 2- -butyl-phenylazide yields pyrrolidines in 55 and 44 % yields, respectively. ... 

Ramalingam and coworkers have performed detailed computational studies at various levels of theory with respect to the insertion of HCIC and CbC singlet carbenes into the carbon-hydrogen bond of small alkanes and identified two insertion modes (Fig. 6.8)." Of these, the a approach has been found to be preferred over the n mode at all levels of theory because the k attack leads to the eclipsed conformation (compare transition state 143 to 144, which is a secondary saddle point). In the initial phase of the insertion process there is a net charge flow from the alkane to the carbine. Barriers for the insertion... 

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