Singlet-triplet gaps

Ah initio programs attempt to compute the lowest-energy state of a specified multiplicity. Thus, calculations for different spin states will give the lowest-energy state and a few of the excited states. This is most often done to determine singlet-triplet gaps in organic molecules. 

The consistent total energy makes it possible to compute singlet-triplet gaps using RHF for the singlet and the half-electron calculation for the triplet. Koopman s theorem is not followed for half-electron calculations. Also, no spin densities can be obtained. The Mulliken population analysis is usually fairly reasonable. 

Gonzalez, C., Restrepo-Cossio, A., Marquez, M., Wiberg, K. B., De Rosa, M., 1998, Ab Initio Smdy of the Solvent Effects on the Singlet-Triplet Gap of Nitrenium Ions and Carbenes , J. Phys. Chem. A, 102, 2732. 

Worthington, S. E., Cramer, C. J., 1997, Density Functional Calculations of the Influence of Substitution on Singlet-Triplet Gaps in Carbenes and Vinylidenes , J. Phys. Org. Chem., 10, 755. 

Fig. 2. The singlet-triplet reversion of SiH2, referenced to CH2, leads to the avoidance of the crossing of two potential curves. Thus, the Si=Si bond dissociation energy of Si2H4 is lowered by twice the singlet-triplet gap of SiH2, i.e. 38 kcal mol 1.

Notes ° Eq. 9, M = CR R AH, (H+) = 365.7 kcal/mol. All data refer to 298 K. b Recommended enthalpies of formation from Ref. 134b 2 kcal/mol. cTo AH, (3 CH-CH=CH2) = 93 kcal/mol was added AEst =10 kcal/mol. The calculated singlet-triplet gap of 13.3 kcal/mol (B3LYP/cc-pVTZ) from Ref. 133 was corrected by 3 kcal/mol, which is the difference between the experimental and computed (B3LYP/cc-pVTZ) AEst for methylene. 

Vargas, R., Galvan, M., and Vela, A. 1998. Singlet-triplet gaps and spin potentials. 

In summary, independently of the second carbene substituent, phosphinocarbenes have a singlet ground state, with a small singlet-triplet gap. They have a planar geometry at phosphorus and a short phosphorus-carbon bond, indicating an interaction between the phosphorus lone pair and the carbene vacant orbital. In the case of silyl- and phosphoniophosphinocar-benes, there is an additional interaction between the carbene lone pair and lowlying [Pg.179]

Figure 6. On the right, the n-system ofTMMand the electronic configuration of the ground state are shown ( 2 labels are used). The left panel presents electronic states of TMM at the ground state equilibrium D h geometry, and at the two Jan-Teller 2 distorted structures (equilibrium geometries of the 1 B and 1 i states). The corresponding adiabatic singlet-triplet gaps are also shown.

Determination of the Singlet-Triplet Gap in TMM by Electron Photodetachment Photoelectron Spectroscopy ... 

The NBMOs of the singlet state of such a biradical are called disjoint in recognition of the fact that they can be considered to be confined to different sets of atoms. The net result is that in such molecules, the singlet-triplet gap will be small, and the singlet may become the ground state. Borden and Davidson pointed out that TME (5) is the parent molecule of this non-Kekule class, which we now call disjoint. 

NH (46a-c) should have a preference of a few kilocalories per mol for the singlet, but that substitution of a sufficiently strong electron-withdrawing group for the functional hydrogen in the NH compound can reduce the singlet-triplet gap to near zero. 

TABLE 7.2. Singlet-Triplet Gaps for a Few Typical Carbenes"... 

F. Mendez and M. A. Garcia-Garibay, A Hard-Soft Acid-Base and DFT Analysis of Singlet-Triplet Gaps and the Addition of Singlet Carbenes to Alkenes, J. Org. Chem. 1999, 64, 7061. 

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