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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)" +

A tandem mass spectrometer has been used to measure cross sections and product distributions for reactions of the selectively solvated anions OH-(HzO), with the neutral molecules CH3C1 (n = 0, 1, or 2) and CH3Br (n = 0 or 1) over the relative energy range 0.1-5 eV. The reactions observed include nucleophilic displacement, proton transfer, and collision-induced dissociation. With the unhydrated reactant ion OH-, the exoergic nucleophilic displacement channel dominates at low energy, and the endoergic proton transfer channel competes effectively at energies above threshold. Increasing ,hydration of the reactant ion decreases both nucleophilic displacement and proton transfer, the latter drastically, leaving collision-induced dissociation as the major process above 1 eV.

Nearresonant vibrational energy transfer in ozone. Doubleresonance measurements and calculations in the temperature range 200-300 K Forward and reverse rate coefficients have been measured in the temperature range 200-300 K for the two reactions H+ + D,-=HD + D+ and D+ + H,-=HD + H+. Equilibrium constants derived therefrom agree with theoretical van't Hoff plots calculated from statistical mechanics and confirm the temperature calibration of the SIFT apparatus used. It is suggested that these reactions can be used as kinetic thermometers to measure independently the temperature of ion-molecule reaction cells. The system provides a particularly clear example of the role of statistical factors in chemical kinetics, 1 for the forward reactions and 112 for their reverse reactions; and the system illustrates further the relationship between statistical factors in kinetics, symmetry numbers in statistical mechanics, and the corresponding thermodynamic entropy changes. Constraints upon the temperature dependence of the rate coefficients are derived from consideration of thermodynamics and collision dynamics, and the data are seen to conform to these over a limited temperature range. A further trend is suggested by the data, supporting previous observations of isotopeexchange reactions-the rate coefficients of the exoergic reactions decrease with increasing temperature-and may be described in terms of the partitioning of the system according to the number of states available to the products and the original reactants. It is suggested that this should be a general result for reactions where the exoergicity is comparable to the temperature of measurement.

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