Proton Transfer In Some Chemical Processes

Proton transfer (PT) is the most simple and the most basic phenomenon in the isomerization balance of chemicals and oxidation-reduction reactions. It is of key importance for various kinds of chemical processes. It is a common and important reaction in different kinds of chemical and biological processes and it exists in both intra- and intermolecules. Many theoretical and experimental researches have enabled us to understand the role that proton transfer play in the possible reaction mechanisms and in the induced isomerization processes. However, because of the complexity of the reaction systems, the researches about the possible mechanism of proton transfer is only hypothesized but not demonstrated, or the deep researches of proton transfer have not been combined to explain the mechanism of the practical reactions. Therefore, the present work concentrates on several kinds of classical chemical processes so as to explore the reaction mechanism and the properties that cannot or is difficult to obtain. The commonly used quantum chemical methods are employed herein and several general rules have been obtained.Generally, the canonical nucleic acid bases exist as the main forms in the double helix. However, the enol form tautomers of the base can also be formed by proton transfer and the tautomers may cause the nucleic acid bases mispairing, which has been proven to be one of the origins of gene mutation. The base tautomerizing processes of uracil/5-bromouracil were investigated in a microcosmic environment with both H₂O and Na⁺ (W-M environment). It was found that uracil was more stable in the W-M environment than in the microcosmic environment with only water, which suggested that the metal ions and water work cooperated to maintain the classical nucleic acid bases. However, 5-bromouracil, a chemical mutagen, was found to be less stable than uracil in the W-M environment. The systems here are more important components for the real biological environment. The results obtained here allow us to have a new insight into the structural tautomer interconversion of uracil/5-bromouracil.Though the two NH protons of biotin are placed almost symmetrically, only the 1-NH proton is selectively deprotonated and substituted by the carboxyl group. Even in vitro, the 1-NH protons are also found by various IR and NMR investigations, namely, saturation transfer method and temperature dependence measurements, to be more active than that of 3-NH. Activity differences of the two amide protons of biotin were first visited via perspective from proton transfer in the gas phase and our results reveal that for different conformation of biotin, the activity of the two amide is different: the 3-NH proton was more active than that of 1-NH for extended biotin, while less active for folded conformation, which indicated that the 1 -NH proton of biotin is not always more active than the 3-NH proton. The effects of the sulfur atom, the side chain and the solvent effects are further investigated. It is suggested that the relative proportion of the three different conformations of biotin in diverse solutions were responsible for the activity ratio differences of the two amide protons of biotin in various solutions. The method for computing the relative activity of the two amide protons of biotin in water here may be used to predict the relative activity in other solutions.Methyl rotation induced by proton transfer was found for cis-N-methylacetamide (NMA). More interestingly, the methyl rotation was found to be controlled by the microenvironment. The atom-centered density matrix propagation (ADMP) method, a recently developed ab initio molecular dynamics, was further carried out to depict the trajectories for methyl rotation of NMA. Moreover, trajectories for methyl rotation of NMA complexed with water molecules were also calculated, and water molecules at the two different sites of NMA were found to reverse or cease the rotational direction of the methyl groups of NMA. This finding that microenvironment can not only control rotational direction of methyl groups, but can also cease the rotation may be of significant importance for the control of molecular machines.Ionic liquids (ILs) are receiving an upsurge of interest in multidisciplinary areas, but the past researches focused mainly on imidazolium-based ILs. The present work concentrates on a kind of protic ionic liquid that results form proton transfer. The theoretical results here will help to understand the fundamental properties of these ILs. It is found that the H proton is not stable on the cation and tends to transfer to the anion to form neutral molecule pairs. Moreover, the neutral pairs are more stable than the ion pairs and the ion pairs tend to tautomerize to neutral pairs without barriers. It suggested that the transformation from the ion pairs to neutral pairs may be the first step for the decomposition of protoc ionic liquids. Furthermore, the ionic liquids have been illustrated to be volatilizable, the species of ionic liquids in the gas phase is still controversial. Therefore, the present work investigate a kind of protic ionic liquid, using the mass spectroscopy experiments combined with the quantum chemical calculations, so as to find which kinds of species exist in the gas phase of the protic ionic liquid. Both the MS spectra and the quantum chemical results have suggested the coexistence of the ion pairs and the neutral molecule pairs of the ionic liquid in the gas phase. The results here may contribute to the resolve of the above controversy.To sum up, the quantum chemical calculation and experiment are combined to investigate the simple proton transfer processes in several classical chemical processes. The mechanisms of the chemical processes have been analyzed and some rules on the interactions and the mechanisms are discovered. We expect it can provide a base for the future development of the research on the mechanisms for the chemical processes.