Methylene singlet-triplet energy separation

Table 6-4 Singlet-Triplet Energy Separation in Methylene ... 

Bender, C. F Schaefer, H. F, III Franceschetti, D. R. Allen, L. C. Singlet-triplet energy separation, Walsh-Mulliken diagrams, and singlet d-polarization effects in methylene, J. Am. Chem. Soc. 1972, 94, 6888-6893. 

In the 1980 s and 1990 s, over two dozen substituted derivatives of methylene were investigated experimentally or by theoretical methods with respect to their structure and the Sq-Ti energy gap. Russon et al. (1992) have recently discussed singlet-triplet energy separations. At an early date, Hoffmann s group (Hoffmann et al., 1968 Gleiter and Hoffmann, 1968) introduced principles of the influence of... 

Table 1 Singlet-Triplet Energy Separations of Methylene with Various Theoretical Methods and DZP or 6-3IG Basis Sets...

In contrast, the B state can be described with one CSF, and the HF wavefunction is thus expected to be a reasonable approximation to the correct wavefunction. As a consequence, HF theory treats the triplet and singlet states of methylene in an unbalanced way. The triplet state is described better than the singlet state, resulting in singlet-triplet energy separations which are too large. 

Tbble 4 Singlet-Triplet Energy Separation of Simple Carbenes and Stabilization Energies vs. Methylene at B3LYP/6-31H-G ... 

The various computational results agree well with these data. For silylene the situation is opposite to that for methylene in fact, a recent experimental nuclear recoil study seems to establish for SiH2 the existence of a singlet ground state. However, in this case, the energy separation with the Bj state is not known, since, in the various spectroscopic studies reported for SiH2, no transitions involving triplet electronic states have been identified. 

Triplet methylene is 9kcal/mol lower in energy than singlet methylene [29], The triplet state is favored because separating the two nonbonding electrons into different orbitals and aligning their spins in a parallel direction minimizes their mutual electrostatic repulsion, as predicted by the extension... 

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