Collision-induced intersystem

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. 

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,... 

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.

A previous review provides a description of the theory of electronic relaxation in polyatomic molecules with particular emphasis on the vibronic state dependence of radiationless transition rates. A sequal review considers the general question of collisional effects on electronic relaxation, while the present one covers only the special phenomenon of collision-induced intersystem crossing. It departs from the other collisional effects review in presenting only a qualitative description of the theory the full theoretical details can be obtained from the previous review and the original papers.As a review of the basic concepts of radiationless transitions theory is necessary as a prelude to a discussion of collision-induced intersystem crossing, considerable overlap exists between this section and Section II of the previous collision effects review. However, since many concepts from radiationless transition theory, such as the nature and criteria for irreversible decay, the role of the preparation of the initial state, the occurrence of intramolecular vibrational relaxation, etc. pervade the other papers on laser chemistry in these volumes, it is useful to recall the primary results of the theory of electronic relaxation in isolated molecules and its relevance to the material in the present volume as well as to this review. 

Lavolee and Tramer have observed collision-induced intersystem crossing from perturbed A H levels of CO by utilization of synchrotron excitation. These diatomic examples should provide a more quantitative test of the theoretical principles which are also applicable (with some appended summations over coupled states) to larger molecules like glyoxal. 

---