| Gas phase ion-molecule reaction is a kind of very important chemical reaction, which has attracted considerable attention. Because its occurrence does not require activation energy or is exothermic, it has become the protagonist of interstellar chemistry. As the edge subject of astronomy and chemistry, the research of interstellar chemistry is significant for the origin of chemical elements, interstellar clouds as well as the evolution of stars. In this work, we study the reaction between PH+ and H2O in detail using density function theory. The result verifies the differences between the experiments and determines reaction paths. It aslo can provide the critical kinetic data and product ion branching ratios for use in the chemical modles. The main studys are as follows:The doublet potential energy surface related to the reaction between PH+ and H2O is investigated at the B3LYP/6-311G(d), CCSD(T)/6-311+G (2df,2pd) (single-point), MP2/6-311+G(2d,p) levels. We calculate the optimized geometries, energies of all the reactants, transition states, intermediates, and productions using B3LYP and MP2 theory. A total of four products are generated through two entrances. All of them have been detected in the experimental process. The main reaction paths are shown as:Path 1: R (PH++H2O)→1(H2OPH+)→3 (H2POH+)→4 (HOP++H2)→P1 (POH+ + H2) Path 2: R (PH++H2O)→2 (H2O…HP+)→5 (P…H3O+)→P2 (P + H3O+)Path 3: R (PH++H2O)→1(H2OPH+)→6 (H…H2OP+)→P3 (POH2+ + H)Path 4: R (PH++H2O)→1(H2OPH+)→3 (H2POH+)→7 (H3PO+)→P4 (HPO+ + H2) or R (PH++H2O)→1(H2OPH+)→3 (H2POH+)→7 (H3PO+)→8 (HPO+ + H2)→P4 (HPO++H2)Among the four products, P1 (HOP+ + H2) and P4 (HPO+ + H2) are exothermic products. P1 is the most competitive product. P4 is the second possible product. While P2 (P + H3O+) and P3(POH2+ + H) are endothermic. The barrier to generate P2 is smaller than to produce P3, so P2 should be more competitive than P3. In summary, the rate order should be P1 (HOP+ + H2) > P4 (HPO+ + H2) > P2 (P + H3O+) > P3 (POH2+ + H), which agrees well with David Smith's experiment. Our study may provide useful information for understanding the reaction mechanism under different experimental and interstellar conditions.
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