Spin-orbit coupling carbenes

Calculations predict that the lowest state of PN has an open-shell electronic configuration." " The Salem-Rowland Rule for ISC promoted by spin-orbit coupling (SOC) predicts that singlet to triplet relaxation will have its maximum rate when the singlet state is closed-shell. This is the case with diaryl carbenes where the absolute rate constants of ISC are in the order of Michl has recently pointed out the importance of donor-... 

DFT and CASSCF-PT2 computations have been carried out to smdy the oxygenation of phenylhalocarbenes in vacuum. The similar order of magnimde of rate constants between singlet and triplet carbenes is indicative of an efficient tunnelling rate associated to a large spin-orbit coupling. For reactions in solution, the solvent effect should also be considered to explain the observed rate smoothing. 

Chemical shifts and CC couplings have been calculated by Wrackmeyer by the use of the DFT method for cyclic C4H2 and C4H4 molecules containing carbene centres, and for some of their boron analogues. The author found that scalar CC spin-spin couplings involving carbene centres are frequently dominated by spin-dipole and spin-orbital interactions. 

Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron-rich, while the carbon has a chemical shift in the range expected for a multiply bonded species. The coupling constant data are difficult to rationalize, as it is not possible to predict the influence of orbital, spin-dipolar, Fermi contact, or higher-order quantum mechanical contributions to the magnitude of the coupling constants. However, classical interpretation of the NMR data indicates that the (phosphino)(silyl)carbenes have a P-C multiple bond character. 

For the OVB and VBSCF theory levels it remains to define the set of structures that comprise the wave function. If the two variably occupied orbitals on each carbene fragment [CF1(A) and XH(A)] are labeled 11, 12, rl and r2 (1 = left and r = right), then Table 1 gives the orbital distribution of all the structures that can be composed by distributing 4 electrons among the 4 orbitals. These structures can be classified as covalent, with two electrons on each carbene irrespective of spin coupling, and charge transfer (CT) in either the 1 - r or r —> 1 directions with an uneven number of electrons on each carbene... 

Spin-Spin Coupling, and ss character, 16 Spiropentadiene, 273 Split valence basis set, 24-25 State correlation diagrams, 203-208 carbene to alkene, 207 Dauben-Salern-Turro, 212-213 Norrish type I, 215-217 Norrish type II, 213-215 from orbital correlation, 203-206 rules for, 206 Stationary point, 209 Steric interaction, 47... 

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