Bimolecular reactions, spin-orbit

Abstract Several examples are presented in order to illustrate the crucial role of high spin multiplicity electronic states on the formation and decomposition of the corresponding molecular systems. For instance, these states are good candidates where electronically excited, metastable negative ions can be found. Moreover, they are needed in order to explain fully unimolecular and bimolecular reaction pathways. During these reactions, the importance of the couplings between these states, such as vibronic and Renner-Teller, and with the states of lower spin multiplicity, such as spin-orbit, are pointed out. 

Since 1992 and the first success of combining laser techniques with the CRESU apparatus, a good number of neutral-neutral processes have been studied.These include bimolecular reactions, three-body reactions, and inelastic collisions, such as spin orbit relaxation of atoms (Al, Si and C), rotational relaxation of molecules such as NO and CO, and even for some specific cases vibrational relaxation (CH, NO and toluene). In this section we will only concentrate on reactive processes. 

The kinetics of these electronic-energy transfer processes are thus deceptively similar to those of encounter-controlled bimolecular chemical reactions, and their rates are related to viscosity in the same way as those of difhisional collision processes in which kinetic energy only is transferred. The physical mechanisms, however, must be very different. Chemical reactions that occur at every encounter require only that the reactants come into close contact and remain so for a sufficient time. Electronic energy will be transferred during an encounter only if, in addition, it is allowed by the spin-momentum rules, i.e., if there is sufficient spin-orbital coupling between D and A for the exchange of electrons to occur... 

The d phosphine and phosphite complexes of zerovalent nickel, palladium and platinum possess long-lived emissive excited states in both fluid solution and in the solid state.The lifetimes of the Ni and Pd complexes are in the 1.39-5.38 ps range, with the platinum complex Pt(PPh3)4 having the shorter excited state lifetime of 0.07 //s because of spin-orbit coupling. These long lifetimes allow for bimolecular reactions to occur, and under photochemical conditions chlorobenzene will add to the complex Pd(PPh3)3 (Ref. 75) ... 

Recently, Hoffmann and Schatz(7ij have developed a new level of treatment of spin-orbit effects in bimolecular reactions which enables a more sophisticated treatment of intersystem crossing d3mamics than in the past. In this treatment high quality electronic structure n thods are used to determine global surfaces for the reaction and spin-orbit matrix elements, and then trajectory surface hopping (TSH) methods are used to determine properties of the bimolecular collisions such as reactive cross sections and state distribution information. In an application of this theory to the O + H2 reaction, the spin-orbit matrix elements were determined as a function of position, and then TSH calculations were done within a diabatic representation to determine cross sections. Intersystem crossing effects were found to be small for O + H2 due to... 

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