Understanding Interactions Between Molecules

With the development of computer science, computational chemistry is becomingmore and more important in chemistry and other relevant areas. In this study, wemainly focus on the area of environmental and interface chemistry. By usingmulti-scale computational method, interesting topics which are difficult to study withexperiments are studied with theoretical chemistry.In the first part of the study, a potentially interstellar linear molecule, HCNS is studied.The geometry of ground state and the low-lying excited states and the adiabaticexcitation energies are calculated. The ionization energies and the electronic affinityenergies are also calculated. By analyzing anion states of HCNS, the distribution ofelectronic densities are analyzed and a stable anion state is identified. The verticalexcitation energies correspond to the spectrum of340nm,284nm, and211nm. Theseresults can be helpful in detecting HCNS molecule in experiment.For the second part of this study, An important intermediate during second aerosolformation, namely β-IEPOX, is studied with DFT method and transitional state theory.By calculating the activation Gibbs energy, the reaction rate constant, and the half lifetime of the reaction, the main path of β-IEPOX and hydroxyl radical is identified ashydrogen abstraction but not addition reaction. Further deduction contains that2-methyltetrols is mainly formed from the surface reaction of particles in theatmosphere.In the third part, we simulate the Langmuir monolayer using molecular dynamicmethod. When simulating monolayer on the surface of water, a new method isemployed by constraining surface tension at a certain value. With this method, weobtained the surface pressure-area isotherm of stearic acid-water system. Comparing with experimental value, the employed method is effective. By analyzing theconfiguration of the molecules, the ordered arrangement of molecules is observed.In the last part of our study, two synthesized molecules, namely N-stearoyldopamine(DOPA) and4-stearyl-catechol (ST) are simulated with molecule dynamics andcompared with experiments. Both of the simulations on the surface of water and onthe Au(111) surface are carried out. The hydrogen bonds of DOPA from amide groupsare identified as the main reason of a different molecular configuration in monolayer.Thus, the hydrogen bonds formed in a molecule will influence the surface structurewhen making surface membrance materials.