Nucleophilic displacement (S N2) reactions in binary van der waals clusters induced by resonant electron capture

Abstract

The nucleophilic displacement reactions F − + CH 3Y → CH 3F + Y − (Y = Cl, Br, I) and Cl − + OCH 3Br → CH 3Cl + Br − are studied in a crossed beam experiment where binary clusters produced by supersonic beam expansion interact with a monochromatized electron beam. The nucleophile (F −, Cl −) is generated by resonant dissociative electron attachment (DA) from one component of the cluster and then undergoes an S N2 reaction with the second component (the “substrate” molecule CH 3Y). Y − ions carrying the DA resonance profile of the nucleophile are then identified as products of the corresponding S N2 reaction. Although isolated CH 3Cl has an unmeasurably low DA (Cl −) cross section (<10 −23 cm 2 [D.M. Pearl, P.D. Barrow, J. Chem. Phys. 104 (1994) 2940; D.M. Pearl et al., J. Chem. Phys. 102 (1995) 2737]), effective formation of Cl − from CH 3Cl is observed when the molecule is coupled to C 2F 6 in a binary van der Waals cluster via the respective S N2 reactions. In addition, the product ion Y − solvated by CH 3Y molecules and also ion-molecule complexes (FCH 3Y) − involved in the reaction are observed within the DA resonance of the nucleophile. The relative intensity of the naked product ion with respect to the ion-molecule complexes is a mirror of the exothermicity of the corresponding S N2 reaction. Because in DA the nuclephile can be created with a defined amount of translational energy, the potential use of the present method to derive information on the energy dependence of the reaction efficiency is discussed.