Triplet-singlet transition

These hold quite well for light atoms but become less dependable with greater nuclear charge. The tenu mtercombination bands is used for spectra where the spin quantum number S changes for example, singlet-triplet transitions. They are very weak in light atoms but quite easily observed in heavy ones. 

In connection with Chichibabin s hydrocarbon it is appropriate to mention that there is little or no resonance between states of different multiplicity.4 Thus the singlet-triplet transition represented by the covalent and diradical structures of the hydrocarbon is a true equilibrium and not an example of resonance. 

An early report [110] of the occurrence of a singlets triplet transition in an apparently six-coordinate complex has recently been shown to be a fur-... 

Spin selection rule An electronic transition takes place with no change in the total electron spin - that is, AS = 0 - hence singlet <- triplet transitions are forbidden or very weakly allowed. For example, the S0 —> Ti transition in anthracene has a molar absorption coefficient, emax, some 108 times less than that corresponding to the S0 —> Si transition. 

Since the photon emitted by D is absorbed by A, the same rules will apply to radiative energy transfer as to the intensity of absorption. Because singlet-triplet transitions are spin-forbidden and singlet-triplet absorption coefficients are usually extremely small, it is not possible to build up a triplet state population by radiative energy transfer. For this... 

Let us consider in detail a recent paper by Jones, Kearns, and Wing which represents one of the most thorough investigations of singlet-triplet transitions and reveals much of the power of the phosphorescence excitation method. 

The intensity of singlet-triplet transitions can be increased by the external and internal heavy-atom effect. It has been noticed by Kearns 8) that the Tn.n - So transitions were enhanced by a factor of about 2 on passing from an ordinary low-temperature glass, such as a 2 1 1 mixture of ether, ethanol, and toluene, to a heavy-atom glass, such as a 2 2 1 1 mixture of ethyl iodide, ether, ethanol, and toluene. 

Goodman, L., Laurenzi, B. J. Probability of singlet-triplet transitions. In Advances in quantum chemistry, Vol. 4 (ed. by Per-Olov Lowdin). New York—London Academic Press 1968. 

Jones, C. R., Kearns, D. R., Wing, R. M. Investigation of singlet-triplet transitions by phosphorescence excitation spectroscopy. X. A simple i, i -unsaturated ketone. J. Chem. Phys. 58, 1370 (1973). 

Marchetti, A. P., Kearns, D. R. Investigation of singlet-triplet transitions by the phosphorescence excitation method. IV. The singlet-triplet absorption spectra of aromatic hydrocarbons. J. Am. Chem. Soc. 89, 768 (1967). 

Evans, D. F. Perturbation of singlet-triplet transitions of aromatic molecules by oxygen under pressure. J. Chem. Soc. 7957, 1351. 

Warwick. D. A., Wells, C. H. J. Perturbation of singlet-triplet transitions in aromatic carbonyl compounds. Spectrochim. Acta 24A, 589 (1968). 

Hoijtink, G. J. The influence of paramagnetic molecules on singlet-triplet transitions. Mol. Phys. 3, 67 (1960). 

Chiu, Y. N. On singlet-triplet transitions induced by exchange with paramagnetic molecules and the intermolecular coupling of spin angular momenta. J. Chem. Phys. 56, 4882 (1972). 

In order to simplify the problem we will 8issume that the state, , in which we are interested is singlet. Then, the 1-TRDM, linking to a quintuplet state vanishes. Therefore, the only cases that may arise come from singlet-singlet or singlet-triplet transitions. The cases that in principle have to be considered are ... 

Bari Sivardiere (1972) had proposed a model for spin-state transitions based on Chestnut s model for the singlet-triplet transition in radical ion salts (Chestnut, 1964), which included the displacement of the breathing mode, Q, and the elastic constant in the Hamiltonian ... 

In this picture, the starting point for the interaction is an internal transfer of one full -electron to the through a singlet-triplet transition. In this step, the molecule is prepared for bonding and the structure is distorted to a structure very similar to that of the adsorbed molecule. The thus formed -electron biradical... 

The following discussion will be based on the view that the transition responsible for the R-bands originates in the bonding electrons of the double bond. Qualitatively, it involves a charge migration (in phase) towards both atoms of this bond, the perturbation extending into the substituents (of. XV and XVI). There are two possibilities. Only one pair of electrons, e.g. the n electrons, participate. This would correspond to a singlet-triplet transition, forbidden both by spin requirements and a small transition moment. 

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