Theoretical Studies On The Photodissociation Mechanism Of Several Important Molecules And Radicals

Photodissociation reaction, as one of the most important photochemistry reactions, provides significant theoretical foundation for the study of life and environment sciences. It also plays a special role in many fields, such as new materials, new energy sources, and new technique for information dispatch. In the present paper, high-level quantum chemistry methods are performed to further explore the ground and excited states properties, as well as the photodissociation reaction mechanism of some molecules or radicals, which have attracted considerable attention in the environment, atmospheric process, and interstellar chemistry. It is expected to be informative for the experiment investigations in the future. The main contributions are as follows:1. In the present study, an attempt is made to reveal the main mechanism of photodissociation on the lowest-lying Rydberg state ¹B₁ of ketene, referred to as the second singlet excited state S₂, by means of the complete active space self-consistent field (CASSCF) and second-order multiconfigurational perturbation theory methods (CASPT2). The located S₂/S₁/T₁ three-surface intersection plays an important role in the dissociation process. It is shown that the intersection permits an efficient internal conversion from S₂ to S₁ state, but prohibits the intersystem crossing from S₂ to T1 state because of the small spin-orbital coupling interaction. The main photodissociation process could be described as follows: after one photon absorption to the S₂ state, ketene preferentially relaxes to the minimum S_2C2v, and undergoes a transition state S_2TS with small potential barrier along the Cs-Ι(out-of-plane bent) symmetry, and passes through the S₂/S₁/T₁ intersection to reach S₁ surface, then arrives at the transition state S_1TS along the minimum energy path. As is well known, S₁→S₀ internal conversion around the Franck-Condon region is expected to be very efficient, and eventually the hot S0 molecule has accumulated enough energy to yield the CH₂ ((a|⁾¹A₁) and CO ( X|^<sup>1Σ⁺) products. Our conclusion supported the recent experiment findings, and also provides some necessary details for the photodissociaion mechanism. 2. Complete active space self-consistent field (CASSCF) and second-order multiconfigurational perturbation theory methods (CASPT2) have been performed to investigate the quartet excited state (a|⁾⁴A" potential energy surface of HCNN radical. Two located minima, with respective cis and trans structures, could easily dissociate to CH ((a|⁾⁴Σ^-) and N₂ ( (X|⁾¹Î£_g⁺) products with similar barrier of about 16.0 kcal/mol. In addition, four minimum energy crossing points on a surface of intersection between (a|⁾⁴A" and X (X= (X|⁾²A" and (A|⁾²A') states are located near to the minima. However, the intersystem crossing (a|⁾⁴A"→X is weak due to the vanishingly small spin-orbit interaction. It further indicates that the direct dissociation on the (a|⁾⁴A" state is more favored. This information combined with the comparison with isoelectronic HCCO provides an indirect support to recent experimental proposal of photodissociation mechanism of HCNN,i.e., internal conversion to one or both of the (X|⁾²A" and (A|⁾²A' states, instead of intersystem crossing, is involved in the photodissociation mechanism of HCNN radical. 3. DFT/B3LYP/6-311G(d) and CCSD(T)/6-311G(2d) single-point calculations are carried out for exploring the doublet potential energy surface (PES) of PC₃O, a molecule of potential interest in interstellar chemistry. A total of 29 minima connected by 65 interconversion transition states are located. The structures of the most relevant isomers and transition states are further optimized at the QCISD level followed by CCSD(T) single-point energy calculations. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d)+ZPVE level, the global minimum is the quasi-linear structure PCCCO 1 (0.0 kcal/mol) with a great kinetic stability of 47.9 kcal/mol, and the cumulenic form features largely in its resonance structures. Moreover, the chainlike isomer OPCCC 3 (64.5) and five-membered-ring species cPCCCO 19 (77.8) possess considerable kinetic stability of about 18.0 kcal/mol. All of these three isomers are very promising candidates for future experimental and astrophysical detection. Additionally, a three-membered-ring isomer CC-cCOP 10 (69.6) has slightly lower kinetic stability of around 15 kcal/mol and also may be experimentally observable. Possible formation mechanisms of the four stable isomers in interstellar space are discussed. The present research is the first attempt to study the isomerization and dissociation mechanisms of PC_nO series. The predicted spectroscopic properties, including harmonic vibrational frequencies, dipole moments and rotational constants for the relevant isomers, are expected to be informative for the identification of PC₃O in laboratory and interstellar medium.