Transition state theory singlet carbenes

Energy-resolved rate constant measurements near the threshold for diplet methylene formation from ketene have been used to provide confirmation of the fundamental hypothesis of statistical transition state theory (that rates are controlled by the number of energetically accessible vibrational states at the transition state).6 The electronic structure and aromaticity of planar singlet n2-carbenes has been studied by re-election coupling perturbation theory.7 The heats of formation of three ground-state triplet carbenes have been determined by collision-induced dissociation threshold analysis.8 The heats of formation of methylene, vinylcarbene (H2C=CHCH), and phenylcarbene were found to be 92.2 3.7, 93.3 3.4, and 102.8 33.5 kcal mol-1, respectively. 

All three levels of theory predict the ring expansion of singlet phenylcarbene ( A -la) to cycloheptatetraene (3a) to occur in two steps, via bicy-clo[4.1.0]hepta-2,4,6-triene (2a) as an intermediate. The first step is addition of the carbene carbon to an adjacent 7t bond of the ring. The second step involves a six-electron, disrotatory, electrocyclic ring opening, which is allowed by orbital symmetry67 and thus proceeds by a highly delocalized transition state. Fig. 4... 

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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