Spin allowed process

Several of the spin-allowed processes are listed below ... 

Some of the earliest studies of triplet carbenes in frozen media by epr spectroscopy revealed that these intermediates react rapidly with molecular oxygen (Trozzolo and Gibbons, 1967). This should not come as a surprise since the combination of a triplet carbene with triplet oxygen is a spin-allowed process. Indeed, recent measurements show that this reaction proceeds with a rate that is approximately at the diffusion limit. The product of this reaction (17) is the expected carbonyl oxide (Werstiuk et al., 1984 ... 

V- 6.2), although an earlier study (516) failed to delect the atmospheric band emission from the 02( A). The electronic state corresponding to the Hartley bands is probably fl2, which can dissociate by a spin-allowed process into 02( A) + 0( D) below 3100 A... 

Photochemistry of ammonia has been reviewed recently by McNcsby and Okabe (684). The photodissociation process of ammonia appears to follow the spin conservation rules [Okabe and Lenzi (762)] in that the spin-allowed process... 

The threshold energy of incident photons to produce NCN(/l n) is only 6.5 cV which is insufficient to induce the spin-allowed process... 

This process is fully allowed and may be treated as an energy transfer promotion of to higher (hot) triplet states, T2 and T3 (Ti —> T2 or Ts are spin-allowed processes). The reverse reaction, however, is spin forbidden. If the spin-symmetry rule for some reasons (e.g. spin-orbit coupling) is broken this process becomes active as well (cf. Sec. 2.4.2). 

One way of circumventing this activation energy barrier involves a free radical pathway in which a singlet molecule reacts with 302 to form two doublets (free radicals) in a spin-allowed process (Fig. 4.1, Reaction (1)). This process is, however, highly endothermic (up to 50 kcal mol-1) and is observed at moderate temperatures only with very reactive molecules that afford resonance stabilized radicals, e.g. reduced flavins (Fig. 4.1, Reaction (2)). It is no coincidence, therefore,... 

Energy transfer from triplet excited 3sens to 302 to give 02 is a spin allowed process. A representative example is the use of excited azoalkanes with 02.17... 

The thermochemical threshold tor production of O2 ( 2) in a spin-allowed process is 266 nm so one cannot a priori exclude the possibility of its formation in ozone photolysis. However I know of no evidence which would confirm the production of these higher excited states at high energies in the Hartley continuum. Looking at the characteristic red emission at 763 nm would make detection of 02( S) possible a very real experimental problem arises because deactivation of O ( D) by the trace of O2 invariably present in any O3 sample yields O2CI). This issue certainly deserves closer scrutiny. 

This reaction, however, infringes the spin conservation jrule. The nearest spin allowed process is ... 

According to Equation (5.35), energy transfer by the exchange mechanism is a short-range phenomenon since the exchange term decreases exponentially with the donor-acceptor separation / ao- Since it requires the intervention of an encounter complex (D A) it is also called the overlap or collision mechanism. The Wigner-Witmer spin-selection rules (Section 5.4.1) apply, and the spin-allowed processes... 

The Primary Process. The primary process of O3 photolysis in the Hartley bands appears to have been well established. The primary molecular product has recently been directly identified as 02( A) by detecting the infrared atmospheric band at I2,7(X) A during the 2537 A photolysis (547). Jones and Wayne (547) obtained a quantum yield of nearly unity (0.9 + 0.2) for the 02( A) production. The 02( A) produced by O photolysis was also detected by its absorption in the vacuum ultraviolet (311) (see Section V- 6.2), although an earlier study (516) failed to detect the atmospheric band emission from the O2CA). The electronic state corresponding to the Hartley bands is probably 82, which can dissociate by a spin-allowed process into 02( A)-i-0( D) below 3100 A... 

The photolysis of C3O2 at 253.7 nm in the presence of H2 has been studied by Forchioni and Willis (14) who propose that singlet C2O formed exclusively reacts with H2 in a spin-allowed process to produce singlet methylene. 

Wavelengths between 140 and 170 nm. in this wavelength Interval, production of the aeronomlcally Important metastable species 0(2 02) becomes energetically possible in a spin-allowed process and abundant evidence exists for Its formation. 

Wavelengths between 120 and 140 nm. For wavelengths shorter than 128 nm, production of 0( S) in a spin-allowed process has been confirmed by direct observation of the 557.7 nm emission corresponding to the transition 0(lSg -> 02),... 

Wavelengths between 90 and 110 nm. At wavelengths shorter than 110 nm several new primary processes are energetically possible and some have been detected. Of particular interest is the production of CO(a H) via the spin-allowed process. 

Appreciable emission has been observed in die ultraviolet dayglow of Mars in the Cameron system CO(a II - X r ) (79). Production of CO(a n) in a spin-allowed process is energetically possible at wavelengths shorter than 108.2 nm and has been detected by Lawrence (80) through observation of the subsequent emission of the Cameron system. He finds that the fractional yield of CO(a n) Increases linearly from zero at the threshold at 108 nm to a value of approximately 0.5 near the ionization limit of 90.0 nm. The wavelength dependence is illustrated in Fig. 10. 

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