Spin forbidden reactions

Department of Chemistry and Biochemistry, Long Island University, 1 University Plaza, Brooklyn, New York 11201 Department of Material Sciences, College of Integrated Arts and Sciences, Osaka Prefecture University, 1-1 Gakuen-cho, 

Let us loosely define processes requiring two or more potential energy surfaces as electronically nonadiabatic processes. More specific well-known examples of electronically nonadiabatic processes include the cis-trans photoisomerization induced upon absorption of a photon by the rhodopsin molecule 

Reviews in Computational Chemistry, Volume 20 edited by Kenny B. Lipkowitz, Raima Latter, and Thomas R. Cundari ISBN 0-471-44525-8 Copyright 2004 John Wiley Sons, Inc. 

For treating either internal conversion or a spin-forbidden reaction, one can view the coupling of the two surfaces to be the off-diagonal elements of the n X n general Hamiltonian matrix, where n is the number of states included in a given problem. Equation [1] shows this matrix for a two state problem (n = 2). 

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III. Other Spin-Forbidden Reactions of Transition Metal Compounds 606... 

Rather than focusing on the short-time photochemical reactivity, our interest in the spin-forbidden reactions of iron carbonyl fragments has been mainly in the longer-time thermal chemistry of the fragments produced. This is summarized in Scheme 3. As already stated, iron tricarbonyl and tetracarbonyl are known to have triplet ground states, and for many ligands, it is assumed that Fe(CO)3L would also have a triplet ground state. Hence many of the indicated processes are spin-forbidden. 

In 2003, we presented two reviews concerning spin-forbidden reactions in transition metal chemistry (10,16). A number of other reviews in 2004 addressed closely related topics (90,91). However, since then, we are not aware of any review on this topic. We, therefore, present a short discussion of a number of recent studies of reactivity in transition metal chemistry for processes involving changes in spin-state. The aim here is not to be exhaustive, as there are now quite a lot of studies of transition metal chemistry that address the topic of spin-state changes. Instead, this is an attempt to give a flavor of the types of question that are being addressed now and the techniques used to investigate them. 

A final aspect of the review is an attempt to make some general statements and predictions concerning the rate of spin-forbidden reactions of transition metal compounds. Doing this for all such... 

Nikita Matsunaga and Shiro Koseki, Modeling of Spin Forbidden Reactions. 

The second process generating small amounts of O( D) is thought to be the spin-forbidden reaction... 

Seip M, Brauer H-D. Endoperoxide formation of helianthrene with triplet molecular oxygen. A spin-forbidden reaction. J Am Chem Soc 1992 114 4486-4490. 

The spin-forbidden reaction between HNO and 02 is quite slow [ 10 M s (147)] compared to the spin-allowed reaction between NO- and 02 [Eq. 9 ... 

Landis C. R. Morales C. M. Stahl S. S. Insights into the spin-forbidden reaction between L2Pd° and molecular oxygen. J. Am. Chem. Soc. 2004, 126, 16302-16303. 

The specificity of an oxide catalyst depends on its local electronic structure, both the electron density and the spin density. Making and breaking of bonds affects the spin of the active metal - spin-allowed reactions are kine-tically faster and thus have an advantage over spin-forbidden reactions. 

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... 

The first question that arises is how does triplet oxygen interact with reduced flavin in what is a spin-forbidden reaction We originally noted the similarity, both electronic and structural, between reduced flavin and tetrakis(dimethylamino)ethylene (33). In the presence of triplet oxygen, 33 reacts rapidly to give the corresponding urea. This... 

Spin-Forbidden Reactions in Transition Metal Chemistry... 

After describing why there is a problem, I will briefly summarize the theoretical description of spin-forbidden reactions. It will be useful at this point to draw parallels with other types of nonadiabatic chemistry, in particular, electron transfer. Then I will review some of the typical contexts in which spin-forbidden behavior occurs in transition metal systems, to try to illustrate how widespread it is. This will be followed by a presentation of strategies used for characterizing and understanding spin-forbidden reactions, based on the use of energies and... 

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