Theoretical study on ground-state proton/H-atom exchange in formic acid clusters through different H-bonded bridges

The ground-state triple proton/H-atom transfer (GSTPT/GSTHAT) reactions in HCOOH complexed cyclically with H2O, CH3OH, NH3 and mixed solvents H2O-NH3/CH3OH-NH3 were studied by quantum mechanical methods in heptane. The GSTPT/GSTHAT in HCOOH-(H2O)(2), HCOOH-(CH3OH)(2), HCOOH-(NH3)(2), HCOOH-H2O-NH3, HCOOH-NH3-H2O, HCOOH-CH3OH-NH3 and HCOOH-NH3-CH3 OH systems all occurred in an asynchronous but concerted protolysis mechanism. The formation pattern of the H-bonded chain was important to reduce the proton/H-atom transfer barrier. For the HCOOH-S-1-S-2 (S-1, S-2: H2O, CH3OH, NH3) complex, the GSTPT/GSTHAT barrier height of the HCOOH-S-1-S-2 complex, in which the H-bonded chain was formed with different solvent molecules, was lower than that of HCOOH-S-1-S-2 complex, in which the H-bonded chain was composed of same solvent molecules. H-bonded chain consisting of mixed solvent molecules can accumulate their proton-accepting abilities and then speed up proton/H-atom transfer. When the less-basic H2O or CH3OH is connected to O-H group of HCOOH directly and the PT/HAT process is started by accepting a proton/H-atom from HCOOH, the PT/HAT reaction would be pulled by the more basic NH3 along the H-bonded chain from the front. On the contrary, when the more-basic NH3 is bonded to O-H group of HCOOH directly, the less-basic H2O or CH3OH hardly pulled PT/HAT process from the front. A good correlation between the proton-accepting ability (basicity) of the H-bonded chain and the GSTPT/GSTHAT barrier height was obtained.