Calculations of rate constants for ion-molecule reactions using a combined capture and centrifugal sudden approximation

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Abstract
Rate constant calculations are reported for the gas-phase reactions of HCN with H-, D- and H+ 3 ions. The technique used in the computations involves a combination of adiabatic capture and centrifugal sudden approximations. The convergence of the calculated cross sections, with respect to rotational basis functions, is improved by use of localized functions of the form exp (A cos θ), where θ is the atom-molecule orientation angle. These functions localize the hindered rotational wavefunctions about the collinear configuration, which is appropriate for the collisions of ions with molecules possessing dipole moments. Excellent agreement with experimental room temperature rate constants is obtained. The theory predicts sharply increasing rate constants as the temperature decreases, and this is related to the strong sensitivity of rotationally selected rate constants to the initial rotational state.