Theoretical Investigation On Chemical Reactions:Quantum Chemical Calculations And Reactive Molecular Dynamics Simulations

With the advance in experimental technology, understanding of chemical reaction has been extended from macroscopic to microscopic scale. The tremendous developments of simulation method and computational technology allow the theoretical modeling of various chemical reactions. Some new reactions and catalysts can be hence designed. In this thesis, we focus on three important kinds of chemical reactions:the Lewis-acid catalyzed [3+2] cycloaddition, the cis-trans configurational conversion of azobenzene derivatives in solution and monolayer, and the switching process between open and closed form of dithienylethene (DTE) derivative. Density functional theory and reactive molecular dynamics simulation are employed to study the reaction pathways and dynamic reaction processes. These calculations and simulations retrieve the observed reaction phenomena from different scales, and provide useful information for designing new reactions. Our results are summarized as follows:1. Study of Lewis-acid catalyzed chemoselective cycloaddition reactions of oxirane derivatives with aldehyde.The [3+2] cycloaddition of oxirane derivatives with aldehyde catalyzed by Lewis acid is a significant synthetic method to construct five-membered heterocyclic compounds. Various metallic salts are used as catalysts to lead to selective C-O or C-C bond breaking in oxiranes. Consequently, the active intermediates can cyclize with aldehyde. By using density functional theory, the role of catalyst played in reactions and the possible reaction pathways with diverse catalysts are studied systematically. According to the different chemoselectivities, these catalysts are grouped into4series, named as C-O selectivity, both, C-C selectivity, and none, respectively. It is shown that trace water in reaction system is critical to understand the perfect C-C selectivity for Ni(C104)2*6H20-catalyzed pathways, which is confirmed by the controlled experiment that originally unreactive Ni(OTf)2would be effective to activate C-C bond cleavage if mixing with water. Based on the possible reaction paths, the radius of cation in Lewis acid is considered to control the chemoselectivity. In addition, the steric hindrances from anion and coordination water in catalyst have significant influences on catalysis efficiency. A steric parameter, a, is defined as the ratio of ligand radius to effective radius of center cation to measure the steric hindrance from ligands. Ineffective catalysts always have remarkable hindrance with α>4.5. These large cations (R.M>74pm) with weak steric hindrance, a<4, tend to activate C-O bond. On the other hand, small cations (RM<70pm) with a<4.5prefer to C-C bond breaking. Knowledge of relationship between components of Lewis acid and its selectivity and efficiency may guide our design of novel reactions.2. Simulations of cis-trans isomerization processes of azobenzene derivatives in condensed phase.Azobenzene derivatives belong to an important kind of molecular switch, working as the core of various stimuli-responsive functional materials and supermolecular systems. However, the switching process of numerous azobenzene centers in condense phase, such as the self-assembled monolayer (SAM) on metal surface, is too complicated to be simulated by quantum mechanics. A reactive molecular dynamics simulation method is developed to study the dynamic cis-trans conformational changes and packing effect in the azobenzene-based SAM. Based on conventional force field, the rotational potential force field functions are constructed to depict the diabatic potential energy curves of cis and trans isomers, respectively. To trigger each reaction center independently and consider forward and backward cis-trans conversion, a switching function, dependent on N=N torsion angle, is introduced and related to a random process. The random section controls the startup and direction of the isomerization by switching the torsion function between the cis and trans atom types. Influences of different ensembles, thermostat methods, the time intervals separating each random section, and the initial probability of isomerization are systematically studied. With a coverage of5.76*10-6mol/m2, most of cis isomers transform to trans configuration in several picoseconds. The low coverage may cause surface deficiency of monolayer and lead to the prolonged isomerization time of switching process, in qualitative agreement with experiment. Furthermore, the solvent effects on the conversion rate and quantum yield of azobenzene in dilute solutions are also investigated by using reactive molecular dynamics simulation.3. Simulation of switching process of open/closed DTE derivatives.Similarly to azobenzene derivatives, DTE derivatives also worked as powerful stimuli-responsive molecular switches, because of the transformation between its open and closed forms via bond cleavage and formation. In order to simulate the conversion process between open and closed form of DTE in various solutions, two new force field functions for the two forms are constructed, respectively, coupled with a universal polynomial switching function. Simulated trajectories reproduce the solvent effect on the ring-close process qualitatively. This reactive molecular dynamics simulation method has a potential application to other reactive systems, paving the way for dynamic simulation of chemical reactions at mesoscropic scale.