HOC[TSUP]+[/TSUP] + H[TINF]2[/TINF] Isomerization Rate at 25 K: Implications for the Observed [HCO[TSUP]+[/TSUP]]/[HOC[TSUP]+[/TSUP]] Ratios in the Interstellar Medium

The recent detection of the metastable HOC + isomer toward a wide range of interstellar environments has demonstrated abundances that cannot be explained within current chemical models. The abundance of HOC + in these models relies heavily on the rate of the isomerization of HOC + by H 2 to the lowest energy isomer, HCO + . A variable temperature 22-pole ion-trap apparatus is employed to study the isomerization of HOC + by H 2 at 25 K. The observed rate coefficient for isomerization is (3.8 ± 0.5) × 10 -10 cm 3 s -1 . This indicates that there is probably no temperature dependence in this reaction below 300 K, suggesting the absence of any significant energetic barrier on the lowest adiabatic H 3 CO + potential surface. This result argues that in regions where abundant HOC + is observed, the production of this isomer must be strongly favored or that there is a concomitant rapid physicochemical loss for the isomer HCO + . Some results for forming DCO + are briefly mentioned.