Collision induced intersystem crossing

It is also possible to induce intersystem crossing from an initially formed singlet state to a lower energy triplet by forcing the singlet state to suffer collisions with an inert medium (no easy task for a species as reactive as methylene ). 

The intersystem crossing from CH2( /4,) to (3B,) is induced by inert gases (143). A theory dealing with the collision-induced singlet to triplet transition of methylene is developed by Chu and Dahler (211). 

Ashfold and Simons (47a) have recently shown that both CN(/l2ri) and B2E + ) states are formed in the vacuum ultraviolet photolysis of BrCN. At the low pressure limit CN(B3L + ) shows a vibrational population inversion at the 1236 A photolysis (a maximum at v = 2), while at higher pressures the population shows a monotonic decrease with an increase of v observed before by Melc and Okabe (692). They attribute the pressure cITcct to the collisionally induced intersystem crossing between the -42fl (F > 10) and neighboring B2E + (o > 0) levels. Because of the long radiative life of A2V state ( - 7 /(sec) (532), it is susceptible to collisions even at a pressure of 10 mtorr. 

Depopulation of S]. It is now useful to define a number of transformation channels available to an "isolated" molecule on its SVL of the state. They are fluorescence emission (F) through Si - So radiative decay, Sj-/vvW Sq, internal conversion (IC), S -AVW intersystem crossing (ISC), isomerization or decomposition (D), and time evolution of the vibrational state (TEV or vibrational energy redistribution). All of these non-radiative processes can be collision-induced, and the efficiency of collision-induced relaxation can vary with the nature of the collision partner, pressure, and other experimental parameters. 

A collision-induced process appears to be involved in product formation below 325 nm (87 kcal/mol), but the exact nature of the effect is still unknown. An internal rearrangement to HCOH, rather than a collision-induced intersystem crossing, seems likely. 

Thayer et al. (240-242) on propynal are the only reasonably complete nonradiative rate calculations done on a carbonyl. Their values for the intersystem crossing and internal conversion rates are low by factors of 10 and 80, respectively, for the vibrationless excited state when compared to the experimental values. They correctly predict the energy dependence of the decay channels, although they fail to predict the large enhancement of the intersystem crossing rate for three vibronic levels. Also, the energy dependence of the collision-induced nonradiative transitions seems to be well reproduced. 

The observation in propynal of both internal conversion and intersystem crossing on the same time scale, together with collision-induced transitions on both pathways, make this... 

The work of van der Werf et al. (246) has clarified the nonradiative pathways that depopulate Sj. Since the results of this work have been discussed in a previous section, it will not be mentioned here, except to confirm that both the aforementioned low-energy studies involve collision-induced intersystem crossing. 

The view that electronic states of different multiplicity need not be considered cannot easily be ruled out, since both deactivation of vibrationally excited carbenes and intersystem crossing between singlet and triplet states are brought about by collision with other molecules. The difficulty is not restricted to reactions in the gas phase in solution, collisional deactivation and collision-induced intersystem crossing can still be expected to compete with collisions leading to chemical reaction. However, the parallelism between the variation in stereospecificity in the gas-phase addition of methylene to the 2-butenes with the pressure of inert gas (Frey, 1959, I960 Anet et al., 1960 Bader and Generosa,... 

Nakajima and Kato used the CASSCF(8,6)/DZP approximation to examine intersystem crossing from the state of glyoxal (Structure 3.4) induced by collisions with argon atoms. They evaluated the and interaction potentials and the spin-orbit coupling matrix elements between these two states at each geometry by using the full Breit—Pauli A semi-... 

C. Collision-Induced Intersystem Crossing in Large Molecules.323... 

D. Magnetic Field Effects on Collision Induced Intersystem Crossing Rates.325... 

IV. Pressure Dependence of Collision Induced Intersystem Crossing.329... 

This review is concerned with the process of collision-induced intersystem-crossing cases in which collisions alter the spin multiplicity of the molecule. This phenomenon is closely related to collision-induced internal conversion, where the spin multiplicity is unaltered by the collision, and to collision-induced vibrational relaxation, where it is the natme of the excited vibrations that is changed by the collision process. Our discussion is centered about the spin-changing collision-induced intersystem-crossing case, but some passing references are made to the related internal conversion and vibrational relaxation. 

Fig. 1. Cross-section for collision-induced intersystem crossing in glyoxal presented in a Thayer-Yardley plot." Figure taken from Beyer and Lineberger. Symbols defined following Eq. (1.3). Note that CHjCl and CH3CN have large quadrupole moments and Oj has a magnetic moment which are not included in the Thayer-Yardley correlation.

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