Nucleophilic displacement as a function of hydration number and temperature: rate constants and product distributions for OD-(D2O)0,1,2,3 + CH3Cl at 200-500 K

Abstract

Nucleophilic displacement reactions which are exothermic do not react on every collision in the gas phase.6,7 They exhibit a negative temperature dependence, rate constants decreasing with increasing temperature,8 and reaction at 300 K is quenched dramatically by the addition of only one-three solvate molecules.9In each respect nucleophilic displacement differs from proton transfer, as contrasted in the companion paper.10 Here we report how hydration influences the rate constant and the product distribution of the nucleophilic displacement reaction OD" + CH3C1 = CH3OD + Cl' A//°= -50 kcal/mol11(1)within the temperature range 200-500 K. Such data invite interpretation using hypersurfaces calculated for hydrated reactants.12 Rate constants for reaction 1 have been measured with a selected ion flow tube (SIFT), using techniques similar to those used in the companion study.10 Because the OH"(H20) reactant and the 35C1' product have the same mass-to-charge ratio (m/e = 35), perdeuterated anions, produced from D20 in the ion source, were used throughout. Rate constants, for the process OD"(D20)" +

Temperature Dependences of the Rate Constants and Branching Ratios for the Reactions of OH-(H2O)0-4 + CH3Br

“…We have previously reported similar behavior for the reactions of OH -(H 2 O) 0-2 with CH 3 Cl. 23 As in the present study, the CH 3 Cl rate constants decrease with increasing reactant solvation, the decrease being even larger than that found here. We attribute this latter observation to the fact that the reaction of OH -with CH 3 Cl is less exothermic (and hence more sensitive to reactant stabilization) than is the reaction with CH 3 Br.…”

“…16,17,37 The amount of hydration in the products can also be compared to our previous study of OH -(H 2 O) 0-2 with CH 3 Cl. 23 In that study, we found that 15% of the product was hydrated in the n )1 reaction and 40% for the n ) 2 reaction. Both of these values are similar to the corresponding results in the present study.…”

“…23 Two factors are thought to contribute to this behavior. First, the rate constant for the collision of an ion with a polar molecule is a gradually decreasing function of temperature.…”

Thermochemistry and structures of solvated SN2 complexes and transition states in the gas phase: experiment and theory

“…(3)) using a variety of experimental techniques [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. By performing these kinds of experiments it is hoped to gain additional insight into the intrinsic contribution of the solvent molecules to the progress [25] of these reactions.…”

“…For most mono-solvated reactions only a small fraction of the leaving groups are solvated (Eq. (4)) [11][12][13][14][15][16][19][20][21][22]24,25], even though this reaction pathway is more exothermic than formation of the non-solvated leaving group (Eq. (5)).…”