Singlet triplet separation energy

Table 2. Singlet-triplet energy separations (AEsx) in M-H monomers, M—M bond dissociation energies (BDEs), SCF calculated bond lengths for the hypothetical, linear dimers HMMH (M = B, Al, Ga, In, or Tl) ...

Table 1 Spin preference of ground state and the calculated singlet-triplet energy separation of some selected Jt-conjugated diradicals...

It is well known that double bonds have a different effect on the singlet-triplet energy separation and thus on the reactivity of the carbene than alkyl groups. The description of the electronic structure of such carbenes is rendered more difficult by the fact that several low-lying electronic states are possible. For... 

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

B. Calculations of Electronic Transition Energies. Singlet-Triplet Energy Separations AESt... 

Das, D. Whittenburg, S. L. Performance of the hybrid density functionals in the determination of the geometric structure, vibrational frequency and singlet-triplet energy separation of CHj, CHF, CF, CClj and CBr, / Mol. Struct. (THEOCHEM) 1999, 492, 175-186. 

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. 

One important conclusion from the full Cl benchmark studies of Bauschlicher, Taylor, Langhoif, and others in the 1980 s is that the MR-CISD method based on CASSCF orbitals provides potential energy surfaces which accurately parallel the full Cl surfaces.14,15 234,238-240 242 254 For example, the CASSCF MR-CISD method predicts singlet-triplet energy separations in CH2 and SiH2 within 0.01 kcal mol-1 and 0.03 kcal mol-1, respectively, of the full Cl results.236,238 The best results are obtained when no threshold is used for reference selection that is, when all CSFs in the CASSCF wavefunction are used as references. This CAS-ref MR-CISD procedure is intimately related107 to second-order configuration interaction (SOCI), which distributes electrons in... 

The chemistry of phosphinidene and phosphenium systems continues to be an active area. The electronic configurations of vinylnitrene and vinylphosphinidene have been compared in a theoretical study, which predicts that both have triplet ground states. A triplet ground state is also found for phenylphosphinidine, whose properties are very similar to those of methylphosphinidene. A theoretical consideration of factors affecting the singlet-triplet energy separation in phosphinidenes has concluded that the singlet state is favoured by substituents... 

Figure 7. Fine structure constants D, hyperfine structure constants A, activation energies of the thermal reaction and singlet triplet energy separation of the dlradlcal and dlcarbene triplet states. There is a characteristic change of all properties when the structure changes from diradicals to dlcarbenes.

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... 

Factor (a) A stable allylic structure of type 37 or 39 (a 3-center-4n -electron system) implies a large singlet-triplet energy separation, which amounts to 379 kJ/mol for the parent compound 39 but only 163 kJ/mol for 37 (R = R = H). This is due mainly to the much smaller inversion barrier at nitrogen (21 kJ/mol) than at phosphorus (146 kJ/mol), which favors Ji-bonding. In addition, 7i-bonds between phosphorus and carbon are much weaker than those for nitrogen (P=C 188 kJ/mol N=C 271 kJ/mol), which disfavors the allylic structure 37. 

Fig. 3 The estimated singlet-triplet gap (in kcal/mol) on the basis of the sum of the spin-potentials ( is + fis) plotted against the corresponding vertical singlet-triplet energy separations (A v i at the singlet geometry and AE, 2 at the triplet geometry (in kcal/mol)) for the investigated carbenes, silylenes, germylenes, and stannylenes (Reproduced with permission from J. Phys. Chem. A, 2004, 108, 490. Copyright 2004 American Chemical Society).

Of course, a proper treatment would utilize multireference methods, but these are very time consuming and limited to model stmctures only [36, 88). Furthermore, dynamic electron correlation is also very important in these reactions so that, for instance, simple CASSCF (complete active space SCF) computations are meaningless, as they mostly include static correlation. Perturbative corrections to CASSCF wavefunctions such as CASPT2 (CAS perturbation theory of second order) are necessary however, multireference perturbation methods do not improve significantly the singlet-triplet energy separations of many biradicals as compared to CASSCF results [89]. 

Borden WT, Davidson ER (1979) Singlet-triplet energy separations in some hydrocarbon diradicals. Annu Rev Phys Chem 30 125-153... 

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