Tunneling in a simple bond scission: the surprising Barrier in the H loss from HCOOH(+).
The dissociation dynamics of gas phase formic acid ions (HCOOH(+), DCOOD(+), HCOOD(+), DCOOH(+)) are investigated by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy and high level ab initio calculations. The slow rate constants for this seemingly simple H loss reaction and the large onset energy shifts due to isotopic substitution point to a substantial exit barrier through which the H or D atoms tunnel. Modeling of the HCOOH(+) experimental data using RRKM theory with tunneling through an Eckart potential are best fitted with a barrier of about 17 kJ mol(-1). High level ab initio calculations support the experimental findings with a computed barrier of 15.9 kJ mol(-1), which is associated with the substantial geometry change between the product HOCO(+) cation and the corresponding HCOOH(+) molecular ion. Because of this exit channel barrier, the formic acid ion dissociation does not provide a route for determination of the HOCO(+) heat of formation. Rather, the most accurate value comes from the calculations employing the high accuracy extrapolated ab initio thermochemistry (HEAT) scheme, which yields a Δ(f)H(o)(0K)[HOCO(+)] = 600.3 ± 1.0 kJ mol(-1) (Δ(f)H(o)(298K)[HOCO(+)] = 597.3 ± 1.0 kJ mol(-1)). The calculated proton affinity of CO(2) is thus 534.7 ± 1.0 kJ mol(-1) at 0 K and 539.3 ± 1.0 kJ mol(-1) at 298.15 K.