Paramagnetic Exchange Spin-Catalysis of the Cis-Trans Isomerization of Substituted Ethylenes Olexandre Plachkevytch, Boris Minaev, ² and Hans A ° gren* Department of Physics and Measurement Technology, Linko ¨ ping UniVersity, S-58183, Linko ¨ ping, Sweden ReceiVed: NoVember 29, 1995; In Final Form: February 7, 1996 X Spin-catalysis of cis-trans isomerization reactions of substituted ethylenes by paramagnetic substances has been studied by ab initio calculations using an appropriate theoretical model: an internal rotation of the HCH group around the CdC double bond in the ethylene molecule in the presence of O 2 or NO molecules. It is shown that spin-catalysis of this reaction is caused by paramagnetic exchange interactions. An adiabatic singlet-triplet intersystem crossing was traced to a stabilization in the transition of the lowest triplet state at the ethylene region by intermolecular exchange and charge-transfer mixings. The creation of a chemical intermediate was verified for the catalysis by nitric oxide. The NO catalyst, when the N atom forms a chemical bond, leads to a more pronounced lowering of the activation energy than does the O 2 catalyst. 1. Introduction The transition states of many chemical reactions contain open electronic shells that are very sensitive to intermolecular exchange interactions. In such cases the reaction paths could be controlled by interaction with a paramagnetic substance (a catalyst). The importance of the electron spin for controlling these reaction channels in the region of an activation barrier can be explained in terms of spin-catalysis theory. 1,2 Let us consider a reaction of cis-trans isomerization of substituted ethylenes. According to a suggestion of Eyring et al., 3 this reaction could pass through the singlet (path b in Figure 1) or the triplet (path a in Figure 1) electronic states when ethylene- type molecules isomerize from the cis to the trans form. It is well-known that this reaction is catalyzed by paramagnetic molecules such as O 2 , NO, NO 2 ,S 2 , and Se 2 . 3-7 The mecha- nism of this catalysis was treated by Eyring et al. 8 from the viewpoint that a paramagnetic substance catalyzes isomerization Via the transition from the singlet (S 0 ) to the triplet (T 1 ) electronic state (path a in Figure 1) by providing “a nonhomo- geneous magnetic field which will act differently on the two magnetic dipoles arising from spin of two electrons in the double bond”. However, the probability of the triplet path in the absence of a catalyst is small because of the small spin-orbit coupling (SOC) between the corresponding singlet S 0 and triplet T 1 states. 3,9,10 It has been shown 6,7,11,12 that the singlet mech- anism is the only important thermal mechanism for the cis- trans isomerization of pure simple olefins in the gas phase in the absence of a catalyst. However, one has to suspect the triplet mechanism as responsible for the lowering of activation energy of these reactions occurring in solution and for the catalytic and photosensitized cis-trans isomerization. 3,12,13 According to Eyring et al., 3 the triplet reaction path for catalytic isomerization was supported by a magnetic interaction. This has, however, proved doubtful because the action of the nonhomogeneous magnetic field of the catalysts on the spins of two electrons in the double bond (the spin-spin interaction) is even weaker than the already mentioned small spin-orbit coupling. A qualitatively different explanation for the catalytic activity of paramagnetic substances has been put forward by McConnell. 13 He suggested that a catalyst in an electronic state with multiplicity different from one (doublet, triplet, etc.) interacts with the singlet S 0 and triplet T 1 states of the isomer and that such an interaction forms new states of the complex, e.g. doublet states 2 D and 2 D′ together with a quartet 4 Q state, if the catalyst is in a doublet state (Figure 2). Since both states 2 D and 2 D′ have the same multiplicity, an avoided crossing takes place which permits the nonadiabatic path, i.e. the S-T transition inside the isomer moiety. The same arguments are applicable for the explanation for the catalytic activity of a catalyst in a triplet state. No corresponding mechanism exists for catalysis of isomerization by substances in singlet states. 13 The intrinsic ethylene “cis-trans” isomerization, with the ethylene molecule twisting around the CdC double bond, is the simplest model for cis-trans isomerization reactions of substituted ethylenes. This reaction is analogous to the thermal isomerization of 1,2-dideuterioethylene which occurs in the gas phase. 3,4,6 It has been shown that the isomerization of this ² Permanent address: Department of Chemistry, Cherkassy Engineering and Technological Institute, 257006, Cherkassy, Ukraine. X Abstract published in AdVance ACS Abstracts, April 15, 1996. Figure 1. Possible mechanisms for the cis-trans isomerization reaction suggested by Eyring and Harman: 8 (a) reaction path Via triplet potential energy surface with S-T intersystem crossing (“triplet path”); (b) reaction path Via singlet potential energy surface without S-T intersystem crossing (“singlet path”). 8308 J. Phys. Chem. 1996, 100, 8308-8315 S0022-3654(95)03534-9 CCC: $12.00 © 1996 American Chemical Society