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Title: Experimental and theoretical study of the photodissociation reaction of thiophenol at 243 nm: Intramolecular orbital alignment of the phenylthiyl radical

Abstract

The photoinduced hydrogen (or deuterium) detachment reaction of thiophenol (C{sub 6}H{sub 5}SH) or thiophenol-d{sub 1} (C{sub 6}H{sub 5}SD) pumped at 243 nm has been investigated using the H (D) ion velocity map imaging technique. Photodissociation products, corresponding to the two distinct and anisotropic rings observed in the H (or D) ion images, are identified as the two lowest electronic states of phenylthiyl radical (C{sub 6}H{sub 5}S{center_dot}). Ab initio calculations show that the singly occupied molecular orbital of the phenylthiyl radical is localized on the sulfur atom and it is oriented either perpendicular or parallel to the molecular plane for the ground (B{sub 1}) and the first excited state (B{sub 2}) species, respectively. The experimental energy separation between these two states is 2600{+-}200 cm{sup -1} in excellent agreement with the authors' theoretical prediction of 2674 cm{sup -1} at the CASPT2 level. The experimental anisotropy parameter ({beta}) of -1.0{+-}0.05 at the large translational energy of D from the C{sub 6}H{sub 5}SD dissociation indicates that the transition dipole moment associated with this optical transition at 243 nm is perpendicular to the dissociating S-D bond, which in turn suggests an ultrafast D+C{sub 6}H{sub 5}S{center_dot}(B{sub 1}) dissociation channel on a repulsive potential energy surface. Themore » reduced anisotropy parameter of -0.76{+-}0.04 observed at the smaller translational energy of D suggests that the D+C{sub 6}H{sub 5}S{center_dot}(B{sub 2}) channel may proceed on adiabatic reaction paths resulting from the coupling of the initially excited state to other low-lying electronic states encountered along the reaction coordinate. Detailed high level ab initio calculations adopting multireference wave functions reveal that the C{sub 6}H{sub 5}S{center_dot}(B{sub 1}) channel may be directly accessed via a {sup 1}(n{sub {pi}},{sigma}*) photoexcitation at 243 nm while the key feature of the photodissociation dynamics of the C{sub 6}H{sub 5}S{center_dot}(B{sub 2}) channel is the involvement of the {sup 3}(n{sub {pi}},{pi}*){yields}{sup 3}(n{sub {sigma}},{sigma}*) profile as well as the spin-orbit induced avoided crossing between the ground and the {sup 3}(n{sub {pi}},{sigma}*) state. The S-D bond dissociation energy of thiophenol-d{sub 1} is accurately estimated to be D{sub 0}=79.6{+-}0.3 kcal/mol. The S-H bond dissociation energy is also estimated to give D{sub 0}=76.8{+-}0.3 kcal/mol, which is smaller than previously reported ones by at least 2 kcal/mol. The C-H bond of the benzene moiety is found to give rise to the H fragment. Ring opening reactions induced by the {pi}-{pi}*/n{sub {pi}}-{pi}* transitions followed by internal conversion may be responsible for the isotropic broad translational energy distribution of fragments.« less

Authors:
; ; ;  [1]
  1. Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701 (Korea, Republic of)
Publication Date:
OSTI Identifier:
20868215
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 126; Journal Issue: 3; Other Information: DOI: 10.1063/1.2424939; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ANISOTROPY; CHEMICAL BONDS; DEUTERIUM; DIPOLE MOMENTS; DISSOCIATION; DISSOCIATION ENERGY; ENERGY SPECTRA; EXCITED STATES; GROUND STATES; HYDROGEN; MOLECULAR ORBITAL METHOD; PHOTOCHEMISTRY; PHOTOLYSIS; PHOTON-MOLECULE COLLISIONS; RADICALS; REACTION KINETICS; THIOPHENOLS; WAVE FUNCTIONS

Citation Formats

Lim, Ivan S., Lim, Jeong Sik, Lee, Yoon Sup, and Kim, Sang Kyu. Experimental and theoretical study of the photodissociation reaction of thiophenol at 243 nm: Intramolecular orbital alignment of the phenylthiyl radical. United States: N. p., 2007. Web. doi:10.1063/1.2424939.
Lim, Ivan S., Lim, Jeong Sik, Lee, Yoon Sup, & Kim, Sang Kyu. Experimental and theoretical study of the photodissociation reaction of thiophenol at 243 nm: Intramolecular orbital alignment of the phenylthiyl radical. United States. doi:10.1063/1.2424939.
Lim, Ivan S., Lim, Jeong Sik, Lee, Yoon Sup, and Kim, Sang Kyu. Sun . "Experimental and theoretical study of the photodissociation reaction of thiophenol at 243 nm: Intramolecular orbital alignment of the phenylthiyl radical". United States. doi:10.1063/1.2424939.
@article{osti_20868215,
title = {Experimental and theoretical study of the photodissociation reaction of thiophenol at 243 nm: Intramolecular orbital alignment of the phenylthiyl radical},
author = {Lim, Ivan S. and Lim, Jeong Sik and Lee, Yoon Sup and Kim, Sang Kyu},
abstractNote = {The photoinduced hydrogen (or deuterium) detachment reaction of thiophenol (C{sub 6}H{sub 5}SH) or thiophenol-d{sub 1} (C{sub 6}H{sub 5}SD) pumped at 243 nm has been investigated using the H (D) ion velocity map imaging technique. Photodissociation products, corresponding to the two distinct and anisotropic rings observed in the H (or D) ion images, are identified as the two lowest electronic states of phenylthiyl radical (C{sub 6}H{sub 5}S{center_dot}). Ab initio calculations show that the singly occupied molecular orbital of the phenylthiyl radical is localized on the sulfur atom and it is oriented either perpendicular or parallel to the molecular plane for the ground (B{sub 1}) and the first excited state (B{sub 2}) species, respectively. The experimental energy separation between these two states is 2600{+-}200 cm{sup -1} in excellent agreement with the authors' theoretical prediction of 2674 cm{sup -1} at the CASPT2 level. The experimental anisotropy parameter ({beta}) of -1.0{+-}0.05 at the large translational energy of D from the C{sub 6}H{sub 5}SD dissociation indicates that the transition dipole moment associated with this optical transition at 243 nm is perpendicular to the dissociating S-D bond, which in turn suggests an ultrafast D+C{sub 6}H{sub 5}S{center_dot}(B{sub 1}) dissociation channel on a repulsive potential energy surface. The reduced anisotropy parameter of -0.76{+-}0.04 observed at the smaller translational energy of D suggests that the D+C{sub 6}H{sub 5}S{center_dot}(B{sub 2}) channel may proceed on adiabatic reaction paths resulting from the coupling of the initially excited state to other low-lying electronic states encountered along the reaction coordinate. Detailed high level ab initio calculations adopting multireference wave functions reveal that the C{sub 6}H{sub 5}S{center_dot}(B{sub 1}) channel may be directly accessed via a {sup 1}(n{sub {pi}},{sigma}*) photoexcitation at 243 nm while the key feature of the photodissociation dynamics of the C{sub 6}H{sub 5}S{center_dot}(B{sub 2}) channel is the involvement of the {sup 3}(n{sub {pi}},{pi}*){yields}{sup 3}(n{sub {sigma}},{sigma}*) profile as well as the spin-orbit induced avoided crossing between the ground and the {sup 3}(n{sub {pi}},{sigma}*) state. The S-D bond dissociation energy of thiophenol-d{sub 1} is accurately estimated to be D{sub 0}=79.6{+-}0.3 kcal/mol. The S-H bond dissociation energy is also estimated to give D{sub 0}=76.8{+-}0.3 kcal/mol, which is smaller than previously reported ones by at least 2 kcal/mol. The C-H bond of the benzene moiety is found to give rise to the H fragment. Ring opening reactions induced by the {pi}-{pi}*/n{sub {pi}}-{pi}* transitions followed by internal conversion may be responsible for the isotropic broad translational energy distribution of fragments.},
doi = {10.1063/1.2424939},
journal = {Journal of Chemical Physics},
number = 3,
volume = 126,
place = {United States},
year = {Sun Jan 21 00:00:00 EST 2007},
month = {Sun Jan 21 00:00:00 EST 2007}
}
  • No abstract prepared.
  • The technique of velocity map imaging (VELMI) has been applied to study the photodissociation of the vinyl radical (C{sub 2}H{sub 3}) at 243.2 nm in a molecular beam. Using momentum conservation, we show that the primary product is singlet vinylidene [H{sub 2}CC({tilde X}{sup 2}A{sup {prime}})], or singlet acetylene at energies where interconversion between the H{sub 2}CC and HCCH geometries is facile. In addition, a minor contribution is seen which is assigned to triplet acetylene [C{sub 2}H{sub 2}({tilde a}{sup 3}B{sub 2})]. We argue that out-of-plane motion of the third H atom is necessary to bring the excited states, of A{sup {double_prime}}more » symmetry, to an A{sup {prime}} symmetry leading to products, and the observed tranlsational energy distribution may show evidence of the barrier to this process. The heat of formation of vinylidene is derived to be 100.3{plus_minus}4.0 kcal/mol, in agreement with literature values. From the translational energy release, we derive the T{sub 0} for triplet acetylene C{sub 2}H{sub 2}({tilde a}{sup 3}B{sub 2}) to be 28&hthinsp;900 cm{sup {minus}1}, which does not agree well with recent {ital ab initio} calculations. Possible reasons for the disagreement are discussed. {copyright} {ital 1999 American Institute of Physics.}« less
  • Single rotational levels of HF ({ital v}=3) were prepared by using overtone excitation and these molecules were then photodissociated by ultraviolet (UV) radiation at 193.3 nm. Time-of-flight spectra of the hydrogen atom fragment provided the spin{endash}orbit state distribution of the fluorine fragment. Changing the UV photolysis laser polarization confirmed an {ital A}{sup 1}{Pi}{l_arrow}{ital X}{sup 1}{Sigma}{sup +} electronic transition in the photodissociation step. Photodissociation of HF at 121.6 nm is also reported. Infrared (IR) induced alignment of the diatom was studied by monitoring the IR laser polarization dependence of the H-atom product angular distribution. Depolarization due to hyperfine interaction was studiedmore » by using the {ital R}(0) transition. Agreement with theory is excellent. {copyright} {ital 1996 American Institute of Physics.}« less
  • Ab initio CCSD(T)/CBS//B3LYP/6-311G** calculations of the potential energy surface for possible dissociation channels of the phenyl radical are combined with microcanonical Rice-Ramsperger-Kassel-Marcus calculations of reaction rate constants in order to predict statistical product branching ratios in photodissociation of c-C{sub 6}H{sub 5} at various wavelengths. The results indicate that at 248 nm the photodissociation process is dominated by the production of ortho-benzyne via direct elimination of a hydrogen atom from the phenyl radical. At 193 nm, the statistical branching ratios are computed to be 63.4%, 21.1%, and 14.4% for the o-C{sub 6}H{sub 4}+ H, l-C{sub 6}H{sub 4} ((Z)-hexa-3-ene-1,5-diyne) + H, andmore » n-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} products, respectively, in a contradiction with recent experimental measurements, which showed C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} as the major product. Although two lower energy pathways to the i-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} products are identified, they appeared to be kinetically unfavorable and the computed statistical branching ratio of i-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} does not exceed 1%. To explain the disagreement with experiment, we optimized conical intersections between the ground and the first excited electronic states of C{sub 6}H{sub 5} and, based on their structures and energies, suggested the following photodissociation mechanism at 193 nm: c-C{sub 6}H{sub 5} 1{yields} absorption of a photon {yields} electronically excited 1{yields} internal conversion to the lowest excited state {yields} conversion to the ground electronic state via conical intersections at CI-2 or CI-3{yields} non-statistical decay of the vibrationally excited radical favoring the formation of the n-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} products. This scenario can be attained if the intramolecular vibrational redistribution in the CI-2 or CI-3 structures in the ground electronic state is slower than their dissociation to n-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} driven by the dynamical preference.« less
  • The photodissociation of 1-bromo-3-iodopropane (1,3-C[sub 3]H[sub 6]BrI) at 222 nm is studied with crossed laser-molecular beam experiments. Irradiation at this wavelength excites an [ital n](Br)[r arrow][sigma]*(C--Br) transition which promotes the molecule to an approximately diabatic excited state potential energy surface which is dissociative in the carbon--bromine bond. This surface intersects an approximately diabatic surface of [ital n](I)[r arrow][sigma]*(C--I) character at extended C--Br distances; this surface is dissociative in the carbon--iodine bond. Crossings from the surface initially accessed to the intersecting surface correspond to intramolecular excitation transfer from the carbon--bromine to the carbon--iodine bond. The incidence of such transfer and hencemore » of carbon--iodine bond fission depends upon the strength of the off-diagonal potential coupling of the two diabatic states. These experiments test the dependence of the coupling and consequent energy transfer upon the separation distance of the C--Br and C--I chromophores. The data show C--Br fission dominates C--I fission by a ratio of 4:1 and determine the center-of-mass translational energy distributions and angular distributions of these processes. The measured anisotropy parameters are [beta](C--Br)=1.6[plus minus]0.4 and [beta](C--I)=0[plus minus]0.2. A third photofission process, IBr elimination, also contributes to the observed signal. The results of the study of C--Br and C--I fission are compared to previous studies on similar molecules to understand how the branching depends on the relative positioning of the C--Br and C--I chormophores. [copyright] [ital 1995][ital American] [ital Institute] [ital of] [ital Physics].« less