GAS-PHASE PROTON-TRANSFER AND SUBSTITUTION-REACTIONS - ENERGY-DEPENDENCE AND DISSOCIATION DYNAMICS

The energy dependence of the branching ratio for the dissociation of a proton-transfer reaction intermediate, [(NC)2CH2.Cl-], has been obser ved in the gas phase using Fourier transform ion cyclotron resonance s pectrometry. The intermediate in the near-thermoneutral proton-transfe r reaction (NC)2CH- + HCl --> (NC)2CH2 + Cl- was formed in two distinc t and widely separated energy regimes above the thresholds for either reaction channel. Direct infrared multiple photon activation of the th ermalized complex was used to access a low-energy regime (0-3 kcal mol -1 above threshold). Formation of the intermediate through the S(N)2 r eaction CN- + ClCH2CN --> [(NC)2CH2.Cl-] was used to access a high-ene rgy regime (30-33 kcal mol-1 above threshold). The experimental result s of the dissociation branching ratios were found to be qualitatively consistent with a statistical RRKM model. Implications for a general p icture for the dynamics of gas-phase S(N)2 reactions are discussed.