Phenylcarbene, singlet

Computational comparison of structures of the benzyl cation (A) and singlet phenylcarbene (B) indicates a much greater double-bond character for the exocyclic... 

A. Electronic Structures of the Lowest Singlet States of Phenylcarbene... 

Phenylcarbene (la). Just as in triplet methylene (CH2), in triplet phenylcarbene (3A"-la) one electron occupies the p-jr atomic orbital on the carbene carbon and one electron occupies the in-plane a hybrid orbital. However, in the lowest singlet state of CH2 and of phenylcarbene ( A -la), both electrons occupy the hybrid a orbital, because this orbital is substantially lower in energy than the p-jt AO. 

Figure 7. Orbital occupancies for the nonbonding electrons and CASSCF(8,8)/6-31G optimized geometries of the lowest triplet and singlet states of phenylcarbene (la).57 Bond lengths in angstroms, and bond angles in degrees.

Table 1. Calculated Singlet-Triplet Energy Gap (A St> kcal/mol) in Phenylcarbene (la)0...

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... 

Figure 3. Energetics of the ring expansion of singlet phenylcarbene f1 A -l o), calculated at different levels of theory.55-57...

Figure 7. Comparison of the energetics of the ring expansions of phenylcarbene ( A -la) and phenylnitrene (1A2-lb), calculated at the CASPT2(8,8)/6-31 G //CASSCF(8,8)/6-31 G level.57-61 The numbers in parentheses represent corrections for the known deficiencies of CASPT2/6-31G in computing the energies of singlet phenylnitrene61 and singlet phenylcarbene.55 The small differences in the energies in Fig. 5 are a consequence of the difference between the basis sets used in the two sets of calculations.

Scheme 6.103 Calculated relative energies of species C7H6 and transition states (TS) separating them, leading from singlet 2-bicyclo-[3.2.0]hept-3,6-dienylidene ( 511) and singlet phenylcarbene ( 512) to fulvenallene (515) via 1,2,4,6-cycloheptatetraene (5).

Phenylcarbene generated by different reaction routes always gives a similar cycloaddition pattern with butenes Xhese results indicate that an equilibrium is present between singlet and triplet phenylcarbene. 

Other differences between singlet (concerted) insertion and triplet (abstraction-recombination) carbene insertion are seen in selectivity, stereochemistry, and the kinetic deuterium isotope effect. The triplet states are more selective in C—H insertion than the singlets. For example, the triplet shows higher tertiary to primary selectivity than the singlet in the insertion reaction with 2,3-dimethylbutane. Singlet carbene is shown to insert into C—H bond with retention of configuration, while racemization is expected for triplet insertion reaction from the abstraction-recombination mechanism. For example, the ratios of diastereomeric insertion product in the reaction of phenylcarbene with roc- and mcTO-2,3-dimethylbutanes are 98.5 1.5 and 3.5 96.5, respectively. ... 

What is the cause of these dramatic changes The formation of the cyclopropane (56) is reasonably explained in terms of stereospecific addition of singlet mono-phenylcarbene (55) generated by photolysis of the diazomethane (54), while the... 

Although AEst of phenylnitrene is small in comparison to that of imidogen, it is still very large compared to phenylcarbene. Thus, while singlet and triplet phenylcarbene interconvert rapidly, singlet to triplet intersystem crossing of phen-... 

Singlet phenylcarbene is a closed-shell species, but singlet phenylnitrene has an open-shell electron configuration. This observation explains why singlet... 

Figure 22.4. Schematic depiction of the electronic structures of the lowest singlet states of phenylcarbene (la) and phenylnitrene (lb).

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. 

Use of IRMPD to dissociate precursor silanes can initiate rather complex processes. The mechanism presented in equation 116 can explain the formation of both ground-state singlet SiH2 and excited-state singlet 1D Si atoms396. This mechanism is analogous to the phenylcarbene rearrangement397 and has not been found for vibrationally unexcited... 

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