The role of dimers in complex forming reactions at low temperature: full dimension potential and dynamics of (H2 CO)2 +OH reaction.

The (H 2 ${{}_{2}}$ CO) 2 ${{}_{2}}$ +OH and H 2 ${{}_{2}}$ CO-OH+H 2 ${{}_{2}}$ CO reaction dynamics are studied theoretically for temperatures below 300 K. For this purpose, a full dimension potential energy surface is built, which reproduces well accurate ab initio calculations. The potential presents a submerged reaction barrier, as an example of the catalytic effect induced by the presence of the third molecule. However, quasi-classical and ring polymer molecular dynamics calculations show that the dominant channel is the dimer-exchange mechanism below 200 K, and that the reactive rate constant tends to stabilize at low temperatures, because the effective dipole of either dimer is reduced with respect to that of formaldehyde alone. The reaction complex formed at low temperatures does not live long enough to produce complete energy relaxation, as assumed in statistical theories. These results show that the reactivity of the dimers cannot explain the large rate constants measured at temperatures below 100 K.