Small Molecule Hydrogen Bonding Theory

Hydrogen bonding play a critical role in a wide range of chemical and biological phenomena. Therefore the hydrogen bond complexes have received the more extensive theoretical study. To date, accurate experimental measurements of the radical hydrogen bonded complexes potential energy surface well depth has still been a more elusive goal due to experimental difficulties. Before embarking on a set of difficult experiment, it would be useful to have a tool that can predict them. Therefore, an accurate calculation of radical complex is of interest. The purpose of this paper is to provide theoretical prediction to assist in its experimental identification. In this paper, hydrogen bonded radical complexes have been studied using high-level ab initio methods to predict of vibrational and rotational spectrum which are extremely useful for identifying trace components in many regimes and to provide the experimentally more interesting parameters that could facilitate the experimental characterization of the complex. The important and valuable results in this thesis can be summarized as follow.1.Hydrogen bonded complex between Cl2O and OH radical Density functional theory was used to study the hydrogen bonding between the dichlorine monoxide and the HO radical, under Cs symmetry, We got two electronic structures ( 2A" and 2 A') with different symmetry and we also gottwo electronic structures ( 2B1 and 2B2 ) under C2v symmetry. The structure 1 (A) has the lowest energy of the five complexes. The energetically low-lying minima is 1 (2A'), with hydrogen bonding occurring between the hydrogen atom in the HO radical and the oxygen atom in the dichlorine monoxide. Another hydrogen bond (structures5 involving two hydrogen bonds) occurs between thechlorine atom in the dichlorine monoxide and hydrogen atom in the HO radical although it has one imaginary frequency. The optimized geometry parameters and interaction energies for various isomers were calculated at the B3LYP/6-31H+G(3df,3pd) and MP2/6-311 ++G(2df,2p) levels. The infrared spectra and the vibrational frequency shifts are also reported.2 Study of the Hydrogen Bonding Complexes of Alaninamide with WaterThe hydrogen bonding of 1 : 1 complexes formed between Alaninamide and water has been completely investigated using DFT and MP2 methods at varied basis set levels from 6-3 llg to 6-311++g(d,p). Five reasonable geometries are considered with the global minimum being a cyclic double-hydrogen bonded structure. The optimized geometric parameters and interaction energies for various isomers at different levels are estimated. The IR intensities and vibrational frequency shifts are reported. The solvent effects on the geometries of the complex have also been investigated using SCRF calculations at the B3LYP/6-31 l++g(d,p) level. The results indicate the polarity of the solvent has played an important role on the structures and the relative stabilities of different isomers.3. Reaction of CH3O2 and HO2 Study of Vibrational Mode Analysis for Reaction MechanismsAll species involved in the multi-channel reaction of CH3O2 with HO2 have been investigated using density functional theory (DFT). The molecular geometries for various species are optimized employing B3LYP method implementing 6-31 l++G(d,p) basis sets. The relative energies of all species are calculated at the same level theory. The results show that there are two kinds of channels-singlet and channels-triplet. For singlet channel, it involves four intermediates, six transition states and for triplet channel, it includes twointermediates, two transition states, respectively. The products of the reaction are four kinds and they are CH3OOH + 'O2, CH3OH + O3, CH4 + 2O2 and CH3OOH + 3O2 The vibrational mode analysis is used to elucidate the relationships of the intermediates, the transition states and the products. The extensive investigation shows that the reaction mechanism is reliable.