Spin-Forbidden Reaction, Intersystem Crossing

Intersystem crossing occurs when the product spin multiplicity differs from the reactant spin multiplicity. Because the spin angular momentum is not conserved in such a reaction, it is referred to as a spin-forbidden reaction. 

The two surfaces that cross at the minimum energy crossing point must have a mechanism that couple the two. In the standard nonrelativistic electronic structure theory, there is no mechanism for interaction between the two 

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Conical intersections usually appear in the Jahn-Teller form in inorganic transition metal complexes because the high symmetry of such complexes allows for this symmetry-required type of conical intersection. For example, studies of complexes of metals with carbonyls revealed that conical intersections facilitate the photodissociation of CO. It should be noted, however, that a sufficient amount of work has not been done yet in this area to reveal whether accidental conical intersections exist and what role, if any, they play in photodissociation. As a result of the larger spin-orbit coupling in transition metal systems, there exists a higher probability for spin-forbidden transitions (intersystem crossing) than in nontransition metal systems. Matsu-naga and Koseki have recently reviewed spin-forbidden reactions in this book... 

The theory underlying this effect depends critically on two selection principles the nuclear spin dependence of intersystem crossing in a radical pair, and the electron spin dependence of the rates of radical pair reactions. Combined, these selection principles cause a sorting of nuclear spin states into different products and result in characteristic nonequilibrium populations in the nuclear spin levels of geminate reaction products (whose formation is allowed for singlet pairs but spin forbidden for triplet pairs) and in complementary nonequilibrium populations in the spin levels of free-radicals ( escape ) products (whose formation is electron spin independent). The transitions between these levels will be in the direction towards restoring the normal Boltzmann population their intensities will depend on the extent of nonequilibrium population. The observed effects are... 

The only difference from the single-channel EM outlined above (Section V.A) is the substitution of k et by the sum of the spin-allowed and spin-forbidden transfer rates k et + k c, to the ground and triplet states, respectively. Like k-eh the intersystem crossing rate kKC does not depend on viscosity. Moreover, EM does not separate the two different steps of the forbidden transition spin conversion to the triplet RIP and subsequent allowed electron transfer into the triplet product [212-216]. However, as has been shown in Section XI.A, even in the case of a single channel but spin-forbidden reaction (I), one should discriminate between the spin conversion and subsequent recombination through electron transfer. The qualitative difference between the spin-allowed and... 

In another study, Mebel et al. examined the photodissociation path involving a spin-forbidden channel in methane. Their MRCI and equation of motion (EOM) CCSD calculations, including MEXP, indicated that the photodissociation pathway is likely to proceed via intersystem crossing from to repulsive Ti( A ), instead of the speculated So <— 5i internal conversion. Similar studies have been done by Hwang and Mebel on spin-forbidden reactions in N2O N2 + O ( P) and N2O N2 + O... 

Treatment of the Intersystem Crossing in the Spin-Forbidden Reaction NO (X II) -F CO... 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

Upper excited states are extremely short-lived. When the molecule is promoted to an excited singlet state beyond S1 the non-radiative deactivation by internal conversion is much faster than the spin-forbidden intersystem crossing to any triplet state. Therefore, the first excited singlet state is formed with near unit quantum yield. If an upper triplet state could be reached, it would also deactivate very rapidly to T1 and no singlet excited state would be formed. The extremely short lifetime of all upper excited states Sb(m>1) and Tb(w>1) means that luminescence emission and chemical reaction are, as a rule, not observed from such states. There are some exceptions to this rule, but there are many more mistaken reports of chemical reactions from short-lived upper excited states. Any such report... 

Exchange energy transfer from the lowest spin forbidden excited state is expected for singlet-triplet intersystem-crossing (ISC) reactions presented further in the following parts of this thesis. 

Let s summarize the key features of the process. A forbidden thermal pericyclic reaction leads to biradical character in the transition state. Spin-orbit coupling facilities intersystem crossing. The thermodynamics are such that both Ti and Sj of the product lie below the transition state for the thermal process. Put this all together and we have a remarkably efficient chemiluminescent process. 

One area of research that has not been explored very much by the techniques covered in this chapter is biological chemistry. Many enzymes carry out electron-transfer reactions, and can undergo intersystem crossing. Many of the methodologies discussed in this chapter are applicable to biological systems exhibiting spin-forbidden transitions. 

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