Nascent product vibrational state distributions of ion–molecule reactions: The proton transfer reactions F−+HX→HF(v)+X−, X = Cl, Br, and I

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Abstract
Nascent vibrational state distributions are obtained for the HF products of the proton transfer reactions F−+HX→HF(v)+X−, X = Cl, Br, and I. The reactions are carried out in a flowing afterglow apparatus in which the reagents are fully thermalized (300 K). The product states are measured by low resolution infrared chemiluminescence spectra obtained with a Ge:Cu infrared detector and a circular variable filter. The nascent HF(v) distributions are as follows: for F−+HCl, N1:N2:N3 = 0.46:0.33:0.21; for F−+HBr, N1:N2:N3:N4 = 0.28:0.27:0.24:0.21; for F−+HI, N1:N2:N3:N4:N5 = 0.20:0.23:0.22:0.20:0.15. All three reactions channel the available exothermicity quite efficiently into product vibration. Product rotational state information cannot be obtained due to collisions with the He carrier gas. In spite of the deep attractive wells of the F−+HX potential energy surfaces, in all three cases the degree of vibrational excitation in the ion–molecule reaction is remarkably similar to, although distinctly smaller than, that of the corresponding neutral F+HX reactions. The results strongly suggest that these ion–molecule reactive collisions are direct encounters and that the kinematic effect of the mass combination (transfer of a light particle between two heavy particles) dominates over the influence of the shape of the potential energy surface in determining the product vibrational state distributions.