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 El 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 Delta H-f(0k)o[HOCO+] = 600.3 +/- 1.0 kJ mol(-1) (Delta H-f(298K)o[HOCO+] = 597.3 +/- 1.0 kJ mol(-1)). The calculated proton affinity of CO2 is thus 534.7 +/- 1.0 kJ mol(-1) at 0 K and 539.3 +/- 1.0 kJ mol(-1) at 298.15 K.