Theoretical Investigation Of Non-adiabatic Process Of Diazine Molecules

Pyrazine, pyrimidine and pyridazine are diazine molecules, isomers with each other, and the female molecules of numerous important heterocyclic compounds. Therefore, they have been widely used, and the pyrimidine ring system is especially most importance, such as the bases and pyrimidine derivatives (three kinds of nucleic acid). Therefore, photophysical and photochemical properties of pyrazine, pyrimidine and pyridazine, especially the relaxation dynamics of excited states is the focus of research. Pyrazine has been reported that the first excited state lifetime is about 100 ps, but the second electronic excited state lifetime is very short, just about 22 fs, and quickly decay to the ground state through internal conversion. Radiationless decay, ultrafast internal conversion and other processes are generally explained by conical intersection theory.1. Theoretical Treatment of Ultrafast Electronic Decay in the Improved Adiabatic Approximation of pyrazinePyrazine molecule has been widely researched more than half a century of, especially in this field of spectrum and dynamics process. In this work, we have calculated the rate constant of internal conversion of the S1â†'S0 non-radiative nÏ€* transition and S2â†'S1 non-radiative ππ*â†'nÏ€* transition by using harmonic oscillator approximation and the improved adiabatic approximation for pyrazine molecule. The geometries and electronic structures of the ground and the two excited states of pyrazine are detected in detail by using several quantum chemistry methods. The geometries optimized by MP2, HF, B3LYP and CASSCF methods are quite reasonable compared with the experimental data and the previous theoretical calculations. By using the improved adiabatic approximation, the calculated lifetimes of S1â†'S0 and S2â†'S1 of pyrazine are in a reasonable agreement with experimental ones, which indicates that the present model of S2â†'S1 can provide an acceptable interpretation without considering the conical intersection. The simulated curve employed by density matrix method reproduce femtosecond time-resolved experimental one very well. Taking the ab initio data from Woywood and the experimental results of Suzuki, S2â†'S1 non-radiative ππ*â†'nÏ€* non-radiative transition is breakout within the present theoretical framework, and the results are reasonable.2. Theoretical Treatment of Internal Conversion of S1â†'S0 of pyridazineDue to the special electronic structure, diazine molecules have been attracted great concern, especially in the absorption spectrum and photochemical kinetics. The time-resolved photoelectron imaging technology has become more sophisticated in latest research by observing the time and energy photoelectron angular distribution. This technology can provide the most accurate method to calculate the internal conversion process of molecular systems. Diazine molecules have received a great deal of attention as benchmark systems for the investigation of electronic dephasing processes, but pyridazine has been studied relatively less compared with the other two triazine molecules (pyrazine and pyrimidine). We use the density functional and time-dependent density functional theory B3LYP functional, CIS, CASSCF to calculate molecular structure parameters pyridazine, vertical transition energy, oscillator strength and frequency, these results are consistent with the experimental results. By using the improved adiabatic approximation, the rate constant of internal conversion of the S1â†' S0 non-radiative nÏ€* transition is calculated. The results show pyridazine life consistent with the experimental theoretical calculations, and then prove the correctness of our theoretical approach.