Methylene singlet-triplet separation

Lucchese, R. R. Schaefer, H. R, III Extensive configuration interaction studies of the methylene singlet-triplet separation, 7. Am. Chem. Soc. WLl, 99, 6765-6766. 

Roos, B. O. Siegbahn, P. M. Methylene singlet-triplet separation. An ab initio configuration interaction study, 7. Am. Chem. Soc. 1977, 99, 7716-7718. 

Keywords Methylene Singlet-triplet separation Geometry... 

Russo, N. Sicilia, E. Toscano, M. Geometries, singlet-triplet separations, dipole moments, ionization potentials, and vibrational frequencies in methylene (CH2) and halocarbenes (CHF, CF2, CCI2, CBr2, and CCI2), J. Chem. Phys. 1992, 97, 5031-5036. 

A. Baikova and R. J. Bartlett, On the singlet-triplet separation in methylene A critical comparison of single- versus two-determinant (generalized valence bond) coupled cluster theory, J. Chem. Phys. 102, 7116-7123 1995. 

ANO basis set used gives a separation in good agreement with, but smaller than, the value deduced from a combination of theory and experiment. From the convergence of the result with expansion of the ANO basis set, it is estimated that the valence limit is about 9.05 + O.lkcal/mole. The remaining discrepancy with experiment is probably mostly due to core-valence correlation effects. However, as the valence correlation treatment is nearly exact, finer effects such as Bom-Oppenheimer breakdown and relativity must also be considered. While FCI calculations have shown that a very high level of correlation treatment is required for an accurate estimate of the CV contribution to the separation, theoretical calculations indicate that CV correlation will increase the separation by at most 0.35 kcal/mole (see later discussion). Therefore, it is now possible to achieve an accuracy of considerably better than one kcal/mole in the singlet-triplet separation in methylene. 

Keywords Singlet-triplet separations Methylene DIP DBA Benzyne Multi-reference Equation-of-motion coupled-cluster... 

The importance of hyperconjugative effects on A sx was rejected on the basis of various GVB approaches and Mulliken population analyses (see Electronic Wavefunctions Analysis) for methylcarbene. It was concluded that it is not the singlet state stabilization by the methyl group but rather the triplet state destabilization which results in a smaller singlet-triplet separation with respect to methylene. The GVB Mulliken population analyses show that the hybridizations of the a orbitals are almost identical in the singlet states of methylene and methylcarbene. For the triplet states, however, Mulliken population analyses indicate a decrease in p character when... 

C. W. Bauschlicher and I. Shavitt, Accurate ab initio calculations on the singlet-triplet separation in methylene, J. Amer. Chem. Soc. 100 739 (1978). 

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 hydrocyanation of ethylene is a model for many monoenes and has been studied in some detail (47, 63). Upon addition of HCN in the presence of excess ethylene at — 40°C, the 31P-NMR resonance of (C2 H4)Ni[P(0-o-tolyl)3]2 at 141.4 ppm is quantitatively replaced by four new singlets at 129.8 (uncoordinated ligand), 118.1, 117.7, and 116.9 ppm with areas in a relative ratio of 1.00 0.14 0.80 0.06 (63). These same signals are produced when HNi[P(0-o-tolyl)3]3CN is treated with excess ethylene at — 50°C (47). These signals persist until HCN and/or ethylene is consumed. In the H-NMR spectrum at — 50°C five Ni—C2H5 protons appear as a single broad resonance at t 9.39, due to accidentally similar chemical shifts the methyl triplet may be observed separately from the methylene near 0°C, but at that temperature the rate of reductive elimination becomes appreciable. 

Irradiation of JV,A, iV-tributyl-JV-(4-methylene-7-methoxycoumarin)ammo-nium borates promotes electron transfer from the borate anion to the singlet excited state of the coumarin, and in acetonitrile separation of the radical pair is efficient. Measurements have shown that electron donors will quench the excited state of triplet methyl phenylglyoxylate in excess of three orders of magnitude more rapidly than will hydrogen donors." ... 

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