High-Pressure Mass Spectrometric Investigations of the Potential Energy Surfaces of Gas-Phase SN2 Reactions

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• 作者：Chun Li ; Philip Ross ; Jan E. Szulejko ; and Terrance B. McMahon
• 刊名：Journal of the American Chemical Society
• 出版年：1996
• 出版时间：October 2, 1996
• 年：1996
• 卷：118
• 期：39
• 页码：9360 - 9367
• 全文大小：241K
• 年卷期：v.118,no.39(October 2, 1996)
• ISSN：1520-5126

文摘

High-Pressure Mass Spectrometric (HPMS) experiments have beencarried out to probe the details of thedouble minimum potential energy surface for gas-phase S_N2reactions. The well depths and entropy changesassociatedwith the formation of entrance and exit channel electrostatic complexesfor the chloride and bromide adducts ofmethyl, ethyl, isopropyl, and tert-butyl chlorides andbromides have been determined from the temperaturedependenceof the equilibrium constants for adduct formation. In the cases of"symmetric" complexes associated with identityS_N2 reactions, there is an increase in well depth as thesize and, therefore, polarizability of the alkyl groupincreases.Concomitant with this is an increase in the magnitude of thenegative entropy change for complex formation whichis the result of an increase in the frequency of the intermolecularmode(s) of the complex arising from the increasedbond strength. The data for the unsymmetrical adducts for thenon-identity S_N2 reactions show the same patternofincreasing well depth with increasing alkyl group size with thechloride adducts of alkyl bromides being more stronglybound than the bromide adducts of the corresponding alkyl chlorides.Enthalpies and entropies associated withtransition state formation are determined from the temperaturedependence of the rate constant for the net halidedisplacement reaction. These data show that the transition statefor the reaction of chloride ion with alkyl bromidesmay lie below (CH₃Br), near(C₂H₅Br), or above(i-C₃H₇Br,t-C₄H₉Br) the energy ofseparated reactants. Thesethree situations exhibit different changes in rate constant withincreasing temperature. In addition, the lifetime ofthe transient, chemically activated intermediate formed betweenchloride ion and methyl chloride has been determinedfrom the pressure dependence of the rate constant for formation of theobservable, collisionally stabilized electrostaticadduct. The lifetime thus obtained is in excellent agreement withtrajectory calculations performed by Hase andco-workers.