ICR study of nonreactive ion-molecule collision rate constants for Cl− and Cr(CO)5−

An ion cyclotron resonance line‐broadening technique has been used to determine experimental momentum transfer rate constants for systems of Cl⁻ and $\mathrm{Cr}(\mathrm{CO}{)}_5^{-}$ ions in a selected group of isotropic and anisotropic nonpolar and polar gases between 10⁻⁴ and 10⁻² torr. The nonpolar molecule systems are compared to rate constants predicted by the Langevin and Mason‐Schamp ion mobility theories, and the polar molecule systems are compared to the Langevin theory and to rate constants scaled from numerical calculations. In the polarization force dominated limit, rate constants for both Cl⁻ and $\mathrm{Cr}(\mathrm{CO}{)}_5^{-}$ in isotropic nonpolar gases give good agreement with rate constants predicted by the Langevin potential (r⁻⁴), while the anisotropic systems require corrections for the anisotropy of the neutral polarizability. $\mathrm{Cr}(\mathrm{CO}{)}_5^{-}$ in the ``light'' gases (He, Ne, H₂, Ar) shows marked deviations from the Langevin polarization limit predictions. Despite used of the hard‐sphere corrected Langevin potential only fair agreement with experiment is found in this area: The adjustable parameter γ of the Mason‐Schamp (12–6–4) potential function was calculated for each $\mathrm{Cr}(\mathrm{CO}{)}_5^{-}$ ‐neutral pair at 300°K. Most of the systems studied gave γ>0.5. In terms of the MS potential function these results suggest large r⁻⁶ contributions to the potential function. Both Cl⁻ and $\mathrm{Cr}(\mathrm{CO}{)}_5^{-}$ in the polar gases (HCN, CH₃CN, CH₃F) show significant deviations from the locked‐dipole theory suggesting that the ion‐dipole locking potential is only 50% effective. Appropriately scaled numerical calculations give satisfactory agreement with experiment and thus give support to this result.