Origin of Magnetic Field Effect in the Photolysis of 7-Silanorbornadiene Derivatives in Solution Marc B. Taraban,* Alexander I. Kruppa, Olga S. Volkova, Ivan V. Ovcharenko, Ryza N. Musin, and Tatyana V. Leshina Institute of Chemical Kinetics and Combustion, NoVosibirsk-90, 630090, Russia Eric C. Korolenko † and Kazue Kitahara Tokyo Institute of Technology, Department of Applied Physics, Ohokayama 2-12-1, Meguro-ku, Tokyo 152, Japan ReceiVed: April 21, 1999; In Final Form: October 25, 1999 Theoretical modeling of the magnetic field effect detected in the photolysis of 7-silanorbornadiene derivatives has clearly demonstrated the involvement of biradical intermediates and confirmed the participation of both singlet and triplet excited states of dimethylsilylene, :SiMe 2 . Introduction Recent investigations of the photolytic decomposition of a 7,7′-dimethyl-7-silanorborandiene derivative (I) have shown that a stationary external magnetic field affects the yield of one of the main reaction products, tetraphenylnaphthalene (TPN). 1 The reaction mechanism (see Scheme 1) has been proposed on the basis of a series of investigations of the process by means of spin chemistry techniques 1,2 and laser pulse photolysis. 3 The experimentally observed magnetic effect (ME) is formed in the biradical of the disilabicyclooctadiene derivative (II S and II T , Scheme 1) possessing two silicon atoms; the external magnetic field affects the ratio of triplet (II T ) to singlet (II S ) states of the biradical, which, according to Scheme 1, are the precursors of different reaction products. The involvement of biradicals in the photolysis of 7-sila(germa)norbornadiene derivatives has been earlier hypothesized, 4,5 relying on the analysis of the end products of the photodecomposition. Nevertheless, the results of spin chemistry should be regarded as the first independent direct confirmation of the existence of biradicals obtained by means of physicochemical methods. This fact is of primary importance because of the lack of reference data on the structure and properties of short-lived biradicalss postulated intermediates in a number of organic reactions. This is especially true in regard to the reactions of organometallic compounds despite the fact that organoelement chemistry is an intensively developing field of research. 5 In particular, consider- able study is being given to photodecomposition products of 7-sila(germa)norbornadiene derivatives, since these species are the precursors of divalent silicon and germanium derivatives, which are of notable significance in organic synthesis. 5,6 In the case under study, this is a heavy carbene analogue, dimethyl- silylene, :SiMe 2 (stage 2, Scheme 1). The analysis of the magnetic field dependence of the ME 1 has allowed us to obtain data on the reactivity of singlet and triplet states of this carbene analogue. It is significant to note that, as of now, this information is also unavailable in the world literature. A previous paper 1 has provided a purely qualitative inter- pretation of the magnetic field dependence of the ME. In view of the fundamental importance 6 of the basic conclusions made therein, 1 the present work attempts to model the magnetic field dependencies of ME formed in the biradicals and to compare the calculations with the experimental results. Calculations of the magnetic field dependencies are performed in the frames of the exponential approximation of the radical pair theory. 7,8 It must be emphasized that biradical species are the most popular subjects of investigation in the area of spin chemistry. 7 However, the overwhelming majority of papers, as a rule, consider two types of model compounds: the biradicals formed through the photolysis of cyclic ketones with varying length of the hydrocarbon chain, and the compounds where donor and acceptor groups are linked by the said hydrocarbon chain. The geometry, lifetimes, and the multiplicities of the reacting state of these species are generally known in advance. As for silicon-containing biradicals examined in the present case, their properties are presently unknown. Therefore, we are virtually forced to solve the inverse problem, i.e., to gain the information on the lifetimes and the parameters of electron exchange interaction in the biradicals on the basis of comparison of the calculated magnetic field dependence with the experi- mental one. Things get worse because of the impossibility of deriving the analytical solution of a system of coupled nonlinear differential kinetic equations descriptive of the process given by Scheme 1. Hence, to construct the theoretical magnetic field dependence, we have considered only three key magnetic sensitive stages. In the present paper, the analysis of the ME magnetic field dependence is used to confirm the earlier suggested mechanism of the photolytic decomposition of I. 1 According to previous understanding, the photolysis of I includes the triplet to singlet (T f S) conversion of dimethylsilylene and the reaction of both triplet and singlet :SiMe 2 with the initial heteronorbornadiene derivative, resulting in the formation of biradical species. As it has been already stressed, up to the present the reactions of triplet dimethylsilylene have been never mentioned in the literature. † Present address: Physical Sciences, University of New Brunswick, P.O. Box 5050, Saint John, New Brunswick, Canada E2L 4L5. 1811 J. Phys. Chem. A 2000, 104, 1811-1816 10.1021/jp9913113 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/15/2000