1(N,.pi.*)-Photochemistry of acetic acid at 200 nm: further evidence for an exit channel barrier and reaction selectivity

Hunnicutt, Sally S.; Waits, L. D.; Guest, J. A.

doi:10.1021/j100155a015

Cited by 67 publications

(66 citation statements)

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“…19, 20 The gas-phase photodecomposition of acetic acid was shown to produce mainly acetyl and hydroxyl radicals. [21][22][23][24] The present work has a 2-fold task. First, we study in detail the vibrational spectra of the cis conformer of acetic acid isolated in solid Ar, including three deuterated isotopologues (CH 3 -COOD, CD 3 COOH, and CD 3 COOD), with special emphasis on the perdeuterated form.…”

Section: Introductionmentioning

confidence: 99%

Photochemistry and Vibrational Spectroscopy of the Trans and Cis Conformers of Acetic Acid in Solid Ar

et al. 2004

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Acetic acid monomer has two stable geometries, the cis and trans conformers. The high-energy cis conformer has been recently detected experimentally for the first time [Maçôas et al. J. Am. Chem. Soc. 2003, 125, 16188]. The cis conformer can be produced in low-temperature rare-gas matrixes upon vibrational excitation of the ground-state trans conformer. Fast tunneling from cis-to trans-acetic acid takes place even at the lowest working temperatures (8 K), limiting the time available to study the high-energy form. Deuteration of the hydroxyl group reduces the tunneling rate by approximately 4 orders of magnitude, increasing accordingly the lifetime of the unstable conformer and its available concentration. In this work, we present a detailed analysis of the vibrational spectra of the cis form of four acetic acid isotopologues (CH 3 COOH, CH 3 COOD, CD 3 COOH and CD 3 COOD). Photolysis (193 nm) of the trans and cis forms of the perdeuterated compound was performed to evaluate the possible conformational dependence of photodissociation of acetic acid. However, no evidence of conformer specific photodissociation was found. The UV photolysis of the matrix-isolated acetic acid reveals very different products from the gas phase. Methanol complexed with carbon monoxide is the major product of photolysis of acetic acid isolated in Ar matrixes whereas it has never been observed as a photolysis product in the gas phase.

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Section: Introductionmentioning

confidence: 99%

Photochemistry and Vibrational Spectroscopy of the Trans and Cis Conformers of Acetic Acid in Solid Ar

et al. 2004

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“…[1][2][3][4][5][6][7][8][9][10][11][12] Recently, we investigated dynamics of photodissociation of saturated [8][9][10] and unsaturated 11,12 carboxylic acids at 193 and 248 nm, and found OH to be a primary product, formed after the C-OH bond cleavage. We observed an appreciable amount of energy being channeled into the relative translation of OH and its cofragment.…”

Section: Introductionmentioning

confidence: 99%

Dissociation dynamics of thiolactic acid at 193 nm: Detection of the nascent OH product by laser-induced fluorescence

Pushpa

Upadhyaya

Kumar

et al. 2004

The Journal of Chemical Physics

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Electronically excited thiolactic acid (2-mercaptopropionic acid), H(3)C-CH(SH)-COOH, undergoes the C-OH bond cleavage on excitation to the S(2) state at 193 nm, generating the primary product OH (v,J), which is detected by laser-induced fluorescence technique in a collisionless condition of flow system. The partitioning of the available energy between vibrational, rotational, and translational degrees of freedom of nascent photofragments is obtained from relative intensities of ro-vibronic lines in laser-induced fluorescence spectrum of OH, and their Doppler profiles. The rotational population of OH (v(")=0) is characterized by rotational temperature of 408+/-25 K. OH is produced in a vibrationally cold state, i.e., mostly in v(")=0. The average translational energy of OH (v(")=0,J(")) is found to be 21.5+/-2.0 kcal/mol, which implies 25.6 kcal/mol of energy in relative translation of photoproducts corresponding to the f(t) value of approximately 0.6. The observed high translational energy is due to the presence of a barrier in the exit channel, implying that the C-OH bond scission takes place on an electronically excited potential energy surface. The observed partitioning of the available energy between various degrees of the photofragments is theoretically modeled, and the hybrid model, with 26.0 kcal/mol of barrier in the exit channel, is found to explain the measured data quite well. The experimental results are also supported with ab initio molecular orbital calculations for both the ground and the excited electronic states. Time-dependent density functional theory is used to understand the nature of various electronic transitions connecting the lower excited states. Potential energy curves as a function of the C-OH bond length of thiolactic acid suggest distinct exit barriers in the S(1), T(1), and T(2) states. But, we could locate the transition state structure for OH formation in the S(1) state alone. Thus, although thiolactic acid is excited to the S(2) state at 193 nm, it undergoes internal conversion to S(1) where it dissociates to yield OH. In addition to the OH channel from excited electronic states, we studied theoretically all probable dissociation channels occurring on the ground electronic state of thiolactic acid.

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“…[128][129][130][131] In particular, one usually expects that most of the potential energy released along the reaction coordinate after passing the barrier is converted to relative translational energy of the photofragments, and that the translational energy distribution is rather insensitive to the total available energy. 108,[132][133][134][135] In contrast, reactions without an exit channel barrier typically have an asymmetric bell-shaped translational energy distribution with its maximum somewhat away from zero translational energy. 128,130 It can be seen from Also, from the 6S-MC-QDPT energies in Fig.…”

Section: Ivb Geometries At Avoided Crossingsmentioning

confidence: 99%

Nonadiabatic effects in C–Br bond scission in the photodissociation of bromoacetyl chloride

Valero

Truhlar

2006

The Journal of Chemical Physics

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Bromoacetyl chloride photodissociation has been interpreted as a paradigmatic example of a process in which nonadiabatic effects play a major role. In molecular beam experiments by Butler and coworkers [J. Chem. Phys. 95, 3843 (1991); J. Chem. Phys. 97, 355 (1992)], BrCH 2 C(O)Cl was prepared in its ground electronic state (S 0 ) and excited with a laser at 248 nm to its first excited singlet state (S 1 ). The two main ensuing photoreactions are the ruptures of the C-Cl bond and of the C-Br bond. A nonadiabatic model was proposed in which the C-Br scission is strongly suppressed due to nonadiabatic recrossing at the barrier formed by the avoided crossing between the S 1 and S 2 states. Recent reduced-dimensional dynamical studies lend support to this model. However, another interpretation that has been given for the experimental results is that the reduced probability of C-Br scission is a consequence of incomplete intramolecular energy redistribution. To provide further insight into this problem, we have studied the energetically lowest six singlet electronic states of bromoacetyl chloride by using an ab initio multiconfigurational perturbative electronic structure method. Stationary points (minima and saddle points) and minimum energy paths have been characterized on the S 0 and S 1 potential energy surfaces. The fourfold way diabatization method has been applied to transform five adiabatic excited electronic states to a diabatic representation. The diabatic potential energy matrix of the first five excited singlet states has been constructed along several cuts of the potential energy hypersurfaces. The thermochemistry of the photodissociation reactions and a comparison with experimental translational energy distributions strongly suggest that nonadiabatic effects dominate the C-Br scission, but that the reaction proceeds along the energetically allowed diabatic pathway to excited-state products instead of being nonadiabatically suppressed. This conclusion is also supported by the low values of the diabatic couplings on the C-Br scission reaction path. The methodology established in the present study will be used for the construction of global potential energy surfaces suitable for multidimensional dynamics simulations to test these preliminary interpretations.

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1(N,.pi.*)-Photochemistry of acetic acid at 200 nm: further evidence for an exit channel barrier and reaction selectivity

Cited by 67 publications

References 1 publication

Photochemistry and Vibrational Spectroscopy of the Trans and Cis Conformers of Acetic Acid in Solid Ar

Photochemistry and Vibrational Spectroscopy of the Trans and Cis Conformers of Acetic Acid in Solid Ar

Dissociation dynamics of thiolactic acid at 193 nm: Detection of the nascent OH product by laser-induced fluorescence

Nonadiabatic effects in C–Br bond scission in the photodissociation of bromoacetyl chloride

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