Singlet excited ground state

In the limit of infinite atom separations, or if we switch off the Coulomb repui. sion between two electrons, all four wavefunctions have the same energy. But they correspond to different eigenvalues of the electron spin operator the first combination describes the singlet electronic ground state, and the other three combinations give an approximate description of the components of the first triplet excited state. 

The previous sections have shown that cis-trans isomerization of stilbene can take place via the lowest triplet state of stilbene. The question to be considered now is whether the isomerization upon direct photolysis takes place via the singlet state, the triplet state, or a vibrationally excited ground state.a 7 81 50)... 

The photoaddition of acenaphthylene to cyclopentadiene was shown to be sensitive to the presence of heavy atoms in the solvent, as shown in Table 10.9. The data in Table 10.9 show that product (46) increases from 18% of the total product in cyclohexane to 38% of the total product in 1,2-dibromo-ethane. This suggests that the [4 + 2] cycloaddition products (47) and (48) and the [2 + 2] product (46) are produced from different excited states. Accordingly, some of the [4 + 2] product has been postulated to arise from either (a) a singlet excited state or (b) a vibrationally excited ground state... 

Thus, reaction (8.30) could specify either an excited singlet or triplet S02. The excited state may, of course, degrade by internal transfer to a vibration-ally excited ground state that is later deactivated by collision, or it may be degraded directly by collisions. Fluorescence of S02 has not been observed above 2100 A. The collisional deactivation steps known to exist in laboratory experiments are not listed here in order to minimize the writing of reaction steps. 

As an example, we investigate the spin-forbidden transition from an excited triplet (a) to a singlet electronic ground state (X). The resulting expressions can easily be extended to other cases, however. Let T denote a spin-orbit coupled state and a pure LS state. We express the unperturbed wave... 

A photon absorption leads to population of the electronic excited state of the same multiplicity, eg for the singlet S0 ground state it is the Si, S2, or S state, depending on the absorbed photon energy. When excitation leads to higher vibrational... 

The state of spin alignment in the lowest triplet state can be prepared (47) by a number of other methods in addition to its preparation by singlet-singlet absorption, e.g., by ground-state singlet+excited triplet-state absorption, excitation by electron scattering, and... 

The Curtius rearrangement also occurs in the direct photolysis of acylazides, but additional products have been isolated and have been shown to arise from typical nitrene reactions . For example, Horner et have trapped benzoyl nitrene with a variety of scavengers. Similarly to the Wolff rearrangement (vide infra) it has been shown ° that the Curtius rearrangement occius via a singlet intermediate this intermediate can be a nitrene or an excited azide molecule in which a concerted decomposition leads to isocyanate derivatives. This reaction could occur in competition with nitrene formation or, alternatively, originate from a vibrationally-excited ground state azide molecule. 

As also shown in Table 3, the energy difference between the ground Aj and first singlet excited B state is 48.0kcalmol in AsHj, decreases to 47.2kcalmol" in SbHj", but increases to 47.6 kcal mol in BiHj. This trend is a little affected by the spin-orbit interaction because of the closed-shell character of these states. 

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