Capture of asymmetric top dipolar molecules by ions: Rate constants for capture of H2O, HDO, and D2O by arbitrary ions

The capture of rotationally state-selected and unselected asymmetric top polar molecules by ions is investigated. Analytical expressions (for all rotational states up to j = 2) of capture rate constants in the perturbed-rotor second-order limit are derived for application to low temperature conditions. Approximate analytical representations over wider temperature ranges are also given for rotationally unselected molecules. The capture of H2O, D2O, and HDO by arbitrary ions is chosen for demonstration of the approach. Capture rate constants for the about 60 reactions of H2O with ions listed in the UMIST 2006 data base for astrochemistry are calculated, compared with experimental data, and represented in the format kcap(T) ≈ c1 + c2(T/300 K)−1/2. The parameters c1 and c2 can be predicted in a very simple way. The approach allows one to identify capture-controlled mechanisms and/or to trace experimental artifacts. The approach applies equally well to the capture of symmetric top and linear dipole molecules by arbitrary ions.