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/CH₃ OH-NH₃ were studied byquantum 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-S₁-S₂ (S₁, S₂: H₂O, CH₃OH, NH₃) complex, the GSTPT/GSTHAT barrier height of the HCOOH-S₁-S₂ complex, in which the H-bonded chain was formed with different solvent molecules, was lower than that of HCOOH-S₁-S₂ 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.
Volume 132, 2020
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