Matrix reorganization with intramolecular tunneling of H atom: Formic acid in Ar matrix

Abstract

The intramolecular tunneling of hydrogen atom in formic acid HCOOH at low temperatures is considered based on literature experimental data on C-O internal rotation. The energetic and geometric parameters as well as vibrational frequencies for formic acid in cis and trans configurations surrounded by 12 Ar atoms are calculated in the frame of the MP2 approach with extended basis sets. The temperature and pressure dependence of the rate constant is analyzed taking into consideration the matrix reorganization for the Debye model of lattice motion. It has been shown that the available experimental data can be explained by the suggested matrix reorganization mechanism. Theoretical expressions for the temperature dependence of the rate constant agree well with the experimental data on the cis to trans tunneling reactions in formic acid with fitting parameters attaining reasonable values. A mechanism describing pressure dependence of the rate constant for H-atom intramolecular tunneling reactions is also proposed.

Tunneling transfer of atomic particles in chemical and biological reactions: The role of intermolecular vibrations and media reorganization

“…In contrast to the previous processes, this is one in which the rate does not increase; rather, it falls with rising pressure in the range of 0-220 bar. This depen dence is in agreement with theory, since (as was seen above) the temperature dependence in these reactions is determined only by the reorganization of the medium [42], and in such reactions (as was shown in [81]) the rate constant diminishes with growing reor ganization. We should also note the great attention now being given to the effect pressure has on tunneling processes in biological systems [94][95][96].…”

“…The temperature dependence of the rate constant of this reaction was analyzed in [81,82]. A molecule of formic acid was placed in a cluster consisting of 12 Ng atoms, forming the first coordination sphere around it.…”

“…The fitting procedure used to determine the parameters is detailed in [81,82,84]. It can be seen that the values chosen for most of the parameters are logical, except for the ω D value, which is considerably lower than the characteristic Debye frequency for crystals of noble gases.…”

“…2-4. The logarithm of the ratio of the rate constants for hydrogen and deuterium at low temperatures is equal to that of the pre exponen [17,18,84]; solid lines, the results from fitting [81,82,85]. tial factors, while at higher temperatures it is described by formula (10) at α' = 0.…”

“…Dots represent experimental data[17,18,84]; solid lines, the results of fitting[81,82,85]. The insert shows the calculated temperature dependence of the kinetic (H/D) isotope effect[85].…”