Photodissociation dynamics of $\pi\sigma^{\ast}$ state mediated chemical reactions : structural aspects in predissociation of thiophenols and thioanisoles = $\pi\sigma^{\ast}$ 전자상태를 통한 광분해 반응 동역학 : 싸이오페놀과 싸이오애니솔 선해리 반응에서의 구조적 측면structural aspects in predissociation of thiophenols and thioanisoles

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Chemical reactions on pre-determined electronic potential energy surfaces generally are described well with adiabatic picture based on the Born-Oppenheimer approximations. On the other hand, for electronically excited states where dynamic coupling of electronic and nuclear motion is nontrivial, nonadiabatic transition becomes common and ubiquitous in many chemical and biological processes. In the endeavor to understand the nature of chemical reaction, state- and structure-based approaches have been carried out from physical chemists as nonadiabaticity is found to be extremely sensitive to the molecular structure or nuclear configuration and structure directly relates to the molecular reactivity. In this dissertation, molecular structure-based photoreactions of some intriguing molecules and experimental approach that provides the opportunity to investigate structure specific photochemistry are described. Thiophenol-(d1) $(C_6H_5SD)$ has a planar geometry at both ground and first electronically excited state and accordingly two conical intersections are formed along dissociation coordinate. Due to its ultrafast reaction feature, S-D bond rupture should be reflected in the angular distribution of fragments. Interestingly, the variation of anisotropy parameter (β), i.e. ~+0.25 at the $S_1$ origin decreases to ~-0.45 at $~600 cm^{-1}$ above the S1 zero point level, as a function of the $S_1$ internal energy is observed, which gives a broad peak in β with a bandwidth of $~200 cm^{-1}$. The peak in β is ascribed to the in-plane S-D bending mode excitation by which the nuclear configuration in the proximity of the $S_1$ (ππ*)/ $S_2$ (nπσ*) conical intersection seam is directly accessed, showing a mixed character of parallel $(S_1-S_0)$ and perpendicular $(S_2-S_0)$ transition dipole moments at the same time. As a result, a new dynamic aspect near the conical intersection formed at planar geometry has been revealed. In order to modify the molecular planarity of the photodissociation event, chemical derivatives of thiophenol (2-fluoro, chlorothiophenol) have been investigated. The molecular planarity, directed by the intramolecular hydrogen bonding, plays an important role in the nonadiabatic passage of the reactive flux at the conical intersection in the photodissociation reactions of 2-fluorothiophenol-d1 and 2-chlorothiophenol-d1. As the internal energy increases in the excited state, the intramolecular hydrogen bonding of 2-fluorothiophenol-d1 loosens. This floppiness brought into the molecular structure then modifies the dynamic path of the reactive flux, leading to the diminishment of the nonadiabatic transition probability at the conical intersection with low $\bar[X]/\bar[A]$ product branching ratios. On the contrary, for 2-chlorothiophenol-d1 having the relatively stronger intramolecular hydrogen bonding, the reactive flux seems to retain the molecular planarity even with the increase of the internal energy as manifested by the constant nonadiabatic transition probability $($\bar[X]/\bar[A])$ branching ratios) over the wide range of the $S_1$ internal energy. Consequently, the effect of the intramolecular hydrogen bonding on the molecular structure and its relation to the nonadiabatic dynamics along the tunneling path have been experimentally demonstrated. In the same vein as above thiophenol case, photochemistry of 2-fluorothioanisole, chemical derivative of thioanisole, has also been studied in terms of spectroscopic property and photodissociation reaction. The nonplanarity along φ (dihedral angle between the $S-CH_3$ bond and the ring plane) in the excited state, evidenced by strong torsional band in the resonance-enhanced two-photon ionization R2PI spectrum, considerably affects photodissociation reactions along $S-CH_3$bond elongation. No sharp increasing of $\bar[X]/\bar[A]$ branching ratio and quite low nonadiabatic transition probability are found, energies up to $~5000 cm^{-1}$ above the $S_1$ origin, which is ascribed to the absence of molecular symmetry. Notably, dynamic feature for direct $S_2$ excitation occurs from very low $S_1$ internal energy since the energetics of $S_2$ state is stabilized with molecular nonplanarity. For investigating state- and structure-specific photochemistry without averaging problem in the ground state, the electrostatic deflection experiment of molecular conforms and clusters has been demonstrated with 3-aminophenol, phenol, and their hydrogen bonded 1:1 complexes with water and ammonia. Each species are deflected according to their distinct electric dipole moment and more enhanced deflection of 1:1 complexes is found according to hydrogen bonding between solute and solvent. The modulation of a molecular beam in terms of mean velocity and rotational coldness by changing carrier gas and its pressure is effective way to increase the extent of deflection. With the rigidity of investigated species under well-controlled experimental condition, deflection profiles show broad and shifted feature to higher vertical position, consistent with the principle of deflection. The investigation of R2PI spectrum at deflected region also identifies the fact that the lowest rotational states are deflected the most. Consequently, spatial separation of molecular conformers (trans, cis-3-AP) and 1:1 hydrogen bonded complexes $(PhOH(H_2O)1$ and $PhOH(NH_3)_1)$ has been carried out under our experimental condition. On the basis of this result, the appropriate molecular systems to be applied with electrostatic deflector can be selected to investigate state- and structure-specific photochemical reactions.
Kim, Sang Kyuresearcher김상규researcher
한국과학기술원 :화학과,
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학위논문(박사) - 한국과학기술원 : 화학과, 2016.2 ,[xii, 103 p. :]


photodissociation; photoionization; specificity; conical intersection; excited state; nonadiabatic transition; reaction dynamics; product branching ratio; molecular planarity; electrostatic deflection; 광분해 반응; 원추형 교차점; 비단열적 전이 확률; 분자 평면성; 정전기적 굴절

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