Theoretical Investigations On The Excited States And Photodissociation Mechanisms Of Several Important Small Molecules

Photochemical processes are one of the most common and important physical chemistry processes in the atmospheres of planets (or satellites) and in the interstellar space. The atmospheric phenomena in the atmospheres of planets (or satellites), such as the constitution of atmosphere, polar light, radiopaque, and climatic situation, are all related to the chemical constitutions of the atmosphere and the radiation that it undergoes. In addition, the origin of life and the laser-induced damage and denaturation of proteins in animal bodies are all based on the photochemistry. Accordingly, comprehensively understanding the microcosmic mechanisms and learning the dynamics characters of the photochemistry reactions are of great benefit for people to control the atmospheric pollution efficiently and properly, to give clues to the origin of life, and to provide possible and reliable approaches for alleviating the damages on human bodies made by ultraviolet lights.The photochemistry reaction differs greatly from the normal thermal chemistry. The thermal reaction usually takes place in the electronic ground state potential energy surface, while the primary process of photochemistry is that the molecule populates the electronic excited state from ground state by means of absorbing photons. The photophysical and photochemical processes are considerably intricate and complex, therefore, the experiment observations is often hard to give clear explanations to the microcosmic mechanisms. For this reason, turning the prospective character of theoretical chemistry to good account, that is, investigating and predicting the detailed mechanism of photochemistry theoretically, is of great significance.Our major work in this thesis is that we employ the relatively high computational level (containing complete active space self-consistent field (CASSCF) method and multiconfigurational second-order perturbation (MS-CASPT2) approach) to study the excited states properties and photodissociation mechanisms of several important molecules shown as follows: s-trans formaldazine (CH2=N?N=CH2) cyanoacetylene (H-C≡C?C≡N) dimethyldisulfide(CH3SSCH3)Our specific work contains the calculations on the vertical excitation energies, the optimizations and single-point energy corrections of the stationary points on the adiabatic potential energy surfaces, the locations of the minimum energy points of the crossing seam between different surfaces (including the conical intersections and intersystem crossing points), the analyses on the gradient difference (GD) and derivative coupling (DC) vectors at the surface crossing points,the determinations of the important consequences of the crossing points in the photochemical processes, the investigations of the internal conversions via vibronic interactions, and the calculations on the Rydberg states by adding the diffuse orbitals into the active space used in the CASSCF and CASPT2 approaches.The MOLCAS6.2, MOLCAS6.4, and GAUSSIAN03 program suites were used in our calculations. The stationary points on the potential energy surfaces (including reactant, transition state, and dissociation products) were optimized at the CASSCF level of theory. Once convergence was reached, the harmonic frequencies were analyzed to confirm the obtained geometry to be a minimum or a first-order saddle point. The minimum energy pathway (MEP) calculations were performed at the CASSCF level to confirm the right connective relationships between transition states and minima (or dissociation products),. The single-point energies were refined by means of the multiconfigurational second-order perturbation approach. The nonradiative decay processes, including the internal conversions via conical intersections and vibronic interacitons as well as the intersystem crossing through the spin-orbital couplings, from the excited states to the ground state were studied in details. The main results are summarized as follows:(1) The low-lying excited states of s-trans-formaldazine (H2C=N?N=CH2) have been investigated using the complete active space self-consistent field (CASSCF) and the multiconfigurational second-order perturbation (CASPT2) methods. The vertical excitation energies have been calculated at the CASSCF and CASPT2 levels employing the cc-pVTZ basis set. The photodissociation mechanisms starting from the S1 state have been determined. The lowest energy points along the seams of surface intersections have been located in both the Franck-Condon region and the N?N dissociation pathway in the S1 state. Once the system populates the S1 state, in the viewpoint of energy, the radiationless decay via S1/S0(3) conical intersection followed by the N-N bond fission in the ground-state is more favorable in comparison with the N-N dissociation process in the S1 state. A three-surface crossing region (S1/T1/T2), where the S1, T1, and T2 states intersect, was also found. However, the intersystem crossing process via S1/T1/T2 is not energetically competitive with the internal conversion via S1/S0(3).(2) Cyanoacetylene (H5?C4≡C3?C2≡N1) is a minor constituent of the atmosphere of Titan, the photochemistry of which plays an important role in the formation of the haze surrounding the satellite. In the present paper, the complete active space self-consistent field (CASSCF) and the multiconfigurational second-order perturbation (CASPT2) approaches have been employed to investigate the photochemical processes for cyanoacetylene in its first singlet and triplet excited states with the cc-pVTZ basis sets. The C4?H5 and C2?C3 bonds fissions in S1 yield H(2S) + CCCN(A2Π) and HCC(A2Π) + CN(X2Σ+), respectively. In T1, the corresponding dissociation products are H(2S) + CCCN(X2Σ+) and HCC(X2Σ) + CN(X2Σ+). At the CASPT2(14,13)//CASSCF(14,13) + ZPE level, the barriers for the adiabatic dissociations of the C4?H5 and C2?C3 bonds are 6.11, 6.94 eV in S1 and 5.71, 6.39 eV in T1, respectively, by taking the energy of S0 minimum as reference. Based on the calculated potential energy surfaces, the existence of a metastable excited molecule is anticipated upon 260-230 nm photoexcitation, which provides a probable approach for cyanoacetylene to polymerize. The internal conversion process via vibronic interaction followed by the C4?H5 fission in the ground state is analyzed to account for the observed diffuse character in the UV absorption spectrum below 240 nm.(3) The photophysics and photochemistry of dimethyldisulfide (DMDS) provide important clues for the photo-induced reactions between the cysteine residues in proteins. In this paper, the state-average complete active space self-consistent field (CASSCF) and multi-state extension of multiconfigurational second-order perturbation (MS-CASPT2) approaches are applied to investigate both the vertical excitation energies and the photodissociation mechanism of DMDS. The two lowest singlet excited states are predicted to be pure valence states, while the four higher states apparently have Rydberg characters. In either the S1 or S2 state, the S?S fission is the major channel compared with the S?C cleavage according to the constructed potential energy curves respectively along the S?S and S?C bonds.