Theoretical Investigations On The Several Types Of Small Molecules And Radicals Containing N

The small molecules and radicals including N atom have received much attention as important as the important components of atmosphere and Titan's atmosphere. These molecules and radicals participate in many photochemistry and photophysical reactions. As a result, quantum chemical investigations on the excited states of several important free radicals containing N atom. In this thesis ab initio and density functional theory (BH&HLYP) combined with the direct dynamics methods have been used to study the reactions C₂H + RCN (R = CH₃, C₂H₅, and C₃H₇); CASPT2//CASSCF calculations were performed to investigated the XCCN (X = H, F, Cl, and Br ) radicals. The main results are summarized as followed:1. Ab initio and density functional theory direct dynamics methods have been used to take a systematic theoretical study on the following hydrogen abstraction reactions: C₂H + CH₃CN→products (R1), C₂H + CH₃CH2CN→products (R2), C₂H + CH₃CH2CH2CN→products (R3). The main object is to provide accurate results for the reaction path and the temperature dependence relation of rate constants, in order to explore the reaction mechanism of these reactions. By means of the Gaussian 98 program, at the BH&HLYP level, the geometries and frequencies of the stationary points (reactant, transition state, and product) were calculated. The minimum energy path (MEP) was calculated at the same level by intrinsic reaction coordinate (IRC) theory. Furthermore, selected points along the MEP, the force constant matrices as well as the harmonic vibrational frequencies were obtained. In order to gain more accurate information of energy, the energies of the selected points on the MEP were refined at MC-QCISD//BH&HLYP level. By POLYRATE-Version 8.4.1 program, the conventional transition state theory (TST) and canonical variational transition state theory with small-curvature tunneling correction (CVT/SCT) were applied to obtain the reaction rate constants. The main results show that : (1) The study on the reaction R1, R2, and R3 indicates that the BH&HLYP/6-311G(d,p) level of theory can give the accurate geometries and frequencies, which are in good agreement with the results by MP2/6-311G(d,p) level. The single point energy calculated at the MC-QCISD//BH&HLYP level can provide more accurate information on energy of the reaction path. This is indispensable to calculate accurately of the reaction rate constants. The CVT/SCT rate constants calculated at the MC-QCISD//BH&HLYP level are in good agreement with the experimental values. The results of this study show that the dual-level DFT direct dynamics method is appropriated for the theoretical study on the hydrogen abstraction reactions for R1, R2, and R3. (2) In the reactions C₂H + RCN(R=CH₃, C₂H₅, C₃H₇), for the hydrogen abstraction reactions, the values of classical barrier heights decrease in the order CH₃CN > C₂H₅CN > C₃H₇CN. (3) For reaction R2, theα-hydrogen abstraction channel andβ-hydrogen abstraction channel are competitive over the temperature range 104-2000K. For reaction R3, theγ-hydrogen abstraction channel is preferred at lower temperatures, while the contribution ofβ-hydrogen abstraction will become more significant with a temperature increase. The branching ratio to theα-hydrogen abstraction channel is found negligible over the whole temperature range.2. The present work investigated the ground state and low-lying excited states of cyanocarbene HCCN, its anion HCCN-, and cation HCCN+, and by using the CASPT2//CASSCF level with aug-cc-pVTZ basis set. The electronic configurations are used to explain the geometrical alteration in detail. We have presented results from a fully correlated ab initio investigation of the electronic spectrum of HCCN neutral radical by means of using the CASPT2 method, a well-established procedure for accurate calculations of electronic spectra of molecules. The five low-lying triplet valence excited states (2³A″, 3³A″, ³A′, 2³A′and 3³A′) located at 4.179, 4.395, 4.579, 4.727, and 5.056 eV with significant oscillator strength. Among the allowed transitions the energy of X³A″→2³A″is the lowest, which is the Based on the corresponding results, the PES of the HCCN radical andHCCN- anion are assigned. The vertical and adiabatic ionization energies assigned to six ionic states at the CASPT2 level. The computed first VIP and AIP are 10.652 eV and 10.513 eV, respectively.The vertical detachment energy VDE in a wide range of 2.210- 7.399 eV is assigned to a total 12 neutral states at the CASPT2 level. The ten detachment energies are also confirmed at the CASPT2 level adiabatically. For the PES of anion, the first VDE and ADE calculated are 1.568 eV and 1.768 eV, which are comparable with the experimental data of 2.003±0.014eV. The calculated VDE and ADE of the 1A′state are 2.305 eV and 2.507 eV, respectively, which agree well with the ADE values of 2.518±0.008 eV in the experiment.3. Complete active space self-consistent field method (CASSCF) calculations with ANO-S basis sets were performed for the ground and low-lying electronic excited states of halocyanocarbene XCCN and their anions, with X = F, Cl, and Br. Our results show that the XCCN have triplet X³A″neutral ground states and the singlet-triplet energy gapsΔES-T(kJ/mol)follow this order: FCCN(7.4) < ClCCN(13.4) < BrCCN(16.6). The calculated most intensive vertical excitation energies of X³A″→2³A″transitions for FCCN, ClCCN, and BrCCN at 408.3, 385.4, and 345.2 kJ/mol, respectively, are mainly attributed to theÏ€(a′)→nxy transitions. The adiabatic electron affinities for XCCN are found to be 235.7, 233.0, and 237.2 kJ/mol, respectively. Compared with the results of HCCN, the electron affinities are enhanced.