Time-resolved study of the symmetric SN2-reaction I−+CH3I

Abstract

Articles you may be interested inCommunication: Probing the entrance channels of the X + CH4 → HX + CH3 (X = F, Cl, Br, I) reactions via photodetachment of X−-CH4An ab initio molecular dynamics study of the S N 2 reaction Cl − +CH 3 Br→CH 3 Cl+Br − Time-resolved photoelectron spectroscopy of negative ions has been applied to study the title reaction as a model system for gas phase S N 2 reactions. Starting from the precursor cluster I 2 Ϫ •CH 3 I, the interaction of the reactants I Ϫ and CH 3 I is initiated by a pump pulse and the subsequent dynamics are observed with a delayed probe pulse used to detach the excess electron and measure their photoelectron spectra. Using two different pump photon energies, which lead to different amounts of internal energy available to the reaction complex, a number of dynamical features have been observed. For small internal excitation, the reactants only form stable, albeit vibrationally excited, I Ϫ •CH 3 I complexes. However, with increased internal excitation, complexes are formed that exhibit biexponential decay back to I Ϫ and CH 3 I reactants with time scales of 0.8 and 10 ps. Similar dynamics are expected for entrance channel complex formed in the first step of a gas phase S N 2 reaction.

Models for Intrinsic Non-RRKM Dynamics. Decomposition of the SN2 Intermediate Cl––CH3Br

“…The intermolecular and intramolecular complexes preferentially dissociate to X − + CH 3 Y and isomerize to XCH 3 -Y − , respectively. This model for the X − -CH 3 Y non-RRKM dynamics is consistent with the lifetime and/or lifetime distribution of X − -CH 3 Y dissociation following X − + CH 3 Y association [63,65,[67][68][69], the dissociation dynamics following excitation of the intermolecular and intramolecular modes of X − -CH 3 Y [39,70], and the recrossing dynamics of the [X-CH 3 …”

“…One may directly measure N(t), the population of monoenergetic reactant molecules vs. time [63], but often one measures the collision-averaged unimolecular rate constant [64]. If N(t) is exponential, the unimolecular rate constant will be independent of pressure and equal the RRKM rate constant k(E) [10].…”

Fundamental Aspects of Gas Phase Ion Chemistry Studied Using the Selected Ion Flow Tube Technique

“…Direct techniques such as transition-state spectroscopy may be used to probe the transient species trapped in this region. Recently, femtosecond time-resolved photoelectron spectroscopy has been employed 58) to monitor the dissociation dynamics of an energized S N 2 complex I ῌ ῌCH 3 I. Because of the experimental constraints 58) the complex cannot have su$-cient energy to surmount the S N 2 barrier.…”

“…Recently, femtosecond time-resolved photoelectron spectroscopy has been employed 58) to monitor the dissociation dynamics of an energized S N 2 complex I ῌ ῌCH 3 I. Because of the experimental constraints 58) the complex cannot have su$-cient energy to surmount the S N 2 barrier. In this symmetric system the S N 2 process is not distinguishable from complex dissociation, either.…”

Quantum Dynamics of Gas‐Phase SN2 Reactions

“…The first time-dependent study on an S N 2 reaction was performed by Wester et al, [42] who employed time-resolved photoelectron spectroscopy, starting from the precursor cluster I 2 À ¥¥¥CH 3 I. Using a pump-probe experiment, the authors obtained stable, vibrationally excited I À ¥¥¥CH 3 I complexes.…”