Interspin Crossing and Reactivity

Abstract

The discussion of reaction mechanisms and pathways is usually based on a single potential energy surface: starting from the reactant minimum the reactive system moves through a transition state and proceeds further to intermediates and/or product minima. Although this concept represents only one aspect of chemical reactivity, it has proven to be a useful tool in rationalizing the greater part of reactions. The success of this concept and its justification, especially in organic chemistry, lies in the fact that organic species generally possess low-spin ground states, and their reactions proceed on a single energy surface (single-state reactivity). Nevertheless, the simple example of the stability of many organic compounds with regard to ground-state triplet O 2 should always remind us of the important consequences of spin states on reactivity. Reactions that involve a change in the spin state and thus occur on two or more potential energy surfaces (and are therefore nonadiabatic processes [1] ) have received much attention in recent years. A number of reactions in organic, inorganic, and organometallic chemistry, in which two states of different multiplicities determine the minimum-energy reaction pathway (two-state reactivity), have been confirmed by experimental and computational studies, and it is the aim of the present article to highlight those efforts.Gas-phase ion chemistry provides a powerful tool for the study of intermediates that are too reactive for a solutionphase characterization. Moreover, information about the intrinsic reactivity of the molecule or reactive system in question is obtained in the gas phase, unperturbed by effects of solvents or aggregation phenomena. Recently, two experimental gas-phase studies have confirmed the possibility of a spin-forbidden proton-transfer reaction, that is, a protonation during which a spin change occurs. The first reaction under consideration is the protonation of F À by HNO [Eq. (a)] [2] ).The energetics of the ground-state reaction, that is, the reaction with 1 HNO, are known from the heats of formation of the ions and molecules (Figure 1). The reaction is exothermic by 8 kcal mol À1 and leads to 3 (NO À ), since 1 (NO À ) is not a bound state. In the study of Brauman and co-workers, the nature of the intermediate complex [FHNO] À and its reactivity were addressed. [FHNO] À was synthesized by the reaction of F À with neopentyl nitrite and then treated with a variety of neutral molecules. The reaction products F -+ 3 (HNO) F -+ 1 (HNO) FH • 3 (NO -) F -• 1 (HNO) HF + 3 (NO -) HF + 1 (NO -) 18 33 18 25 8 Figure 1. Schematic potential energy diagram for reaction (a). The relative energies are given in kcal mol À1 .were monitored in a Fourier transform mass spectrometer. Reactions with alcohols and HCN give F À transfer, which indicates the structure of the complex is F À´H NO. On the other hand, reaction with NO gives N 2 O 2 , and the reactions with SF 6 , O 2 , and SO 2 lead to electron and/or HF transfer as would be expected for the triplet FH´3(NO) À . Thi...

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