The gas phase chemistry of the formic acid radical cation [HCOOH] [rad]+ . Mechanism for exchange of the hydrogen atoms: a quantum chemical investigation

Abstract

The CH 2O 2 + potential energy surface has been (re)explored by ab initio quantum chemical calculations executed at the SD-CI/6-31G**//UHF/4-31G level of theory. Earlier labelling experiments had indicated that prior to the loss of H the hydrogen atoms in ionized formic acid, HCOOH + , can become positionally equivalent via an unknown pathway. Using ab initio molecular orbital calculations we have located a minimum energy reaction pathway for hydrogen exchange reactions which takes place close to the dissociation level and proceeds via flexible ion/dipole complexes. Along this path three equilibrium structures of different atom connectivity are found: HCOOH + , 1, ionized formic acid; two forms of the surprisingly stable ion/dipole complex HO ·HCO +, 2, 3; and the distonic ion H 2OCO + , 4. Ion 1 is separated from ion 2 by a barrier of 142 kJ mol −1 and this barrier corresponds to a 2A′ → 2A″ surface crossing. The barrier for the proton transfer 3 → 4 is only 25 kJ mol −1 relative to 3; the transformation HCOOH + → H 2OCO + , formally a 1,2-hydrogen shift, takes place via the ion/dipole complex HO ·HCO +, H 2OC + is the reacting configuration for decarbonylation and this reaction is calculated to take place at the dissociation limit in full agreement with experiment.