Photochemical and discharge-driven pathways to aromatic products from 1,3-butadiene

A detailed study of the photochemical and discharge-driven pathways taken by gas-phase 1,3-butadiene has been carried out. Photolysis or discharge excitation was initiated inside a short reaction tube attached to the outlet of a pulsed valve. Bath gas temperatures near 100 K were achieved in the reaction tube by the constrained expansion of the gas mixture into the tube, simulating temperatures of relevance in Titan's atmosphere. Photolysis of 1,3-butadiene was initiated at 218 nm with a laser pulse that counter-propagated the reaction tube. Discharge excitation was carried out using discharge electrodes imbedded in the reaction tube walls, enabling the study of the photochemical and discharge products under similar conditions. Products were detected using either single-photon VUV photoionization (118 nm = 10.5 eV) or resonant two-photon ionization (R(2)PI) spectroscopy in a time-of-flight mass spectrometer. Emphasis was placed on characterization of the aromatic products formed, since these may be of particular relevance to Titan's atmosphere, where benzene has been positively identified and 1,3-butadiene is projected as the principle pathway to its formation. Consistent with previous studies of the photodissociation of 1,3-butadiene, C(3)H(3) + CH(3) is the dominant primary product formed. Under the temperature-pressure conditions present in the reaction tube (T approximately 75-100 K, P = 50 mbar), C(6)H(6) is the dominant secondary photochemical product formed. A 1:1 C(4)H(6):C(4)D(6) mixture was used to prove that the C(6)H(6) product was formed by recombination of two C(3)H(3) radicals; however, a careful search for benzene revealed none, indicating that less than 1% of the C(6)H(6) formed in the reaction tube is benzene. This is consistent with expectations for these temperatures and pressures based on previous modeling of propargyl recombination. Two aromatic products were observed from the photochemistry: ethylbenzene and 3-phenylpropyne. Plausible pathways leading to these products are proposed. In the discharge, C(3)H(3) + CH(3) are also identified as significant primary neutral products and C(6)H(6) as a dominant higher-mass product. In this case, the C(6)H(6) was identified as benzene via its R2PI spectrum, appearing with intensity about 10 times larger than any other aromatic formed in the discharge. R2PI spectra of a total of about 15 aromatic products were recorded from the 1,3-butadiene discharge, among them toluene; styrene; phenylacetylene; o-, m-, and p-xylene; ethylbenzene; indane; indene; beta-methylstyrene; and naphthalene. Previously unidentified spectra in the m/z 142 and 144 mass channels were positively identified as the 1,3- and 1,4-isomers of phenylcyclopentadiene and the analogous 1-phenylcyclopentene.     

Mechanistic studies of the pyrolysis of 1,3-butadiene, 1,3-butadiene-1,1,4,4-d4, 1,2-butadiene, and 2-butyne by supersonic jet/photoionization mass spectrometry

The thermal decomposition of 1,3-butadiene, 1,3-butadiene-1,1,4,4-d(4), 1,2-butadiene, and 2-butyne at temperatures up to 1520 K was carried out by flash pyrolysis on a approximately 20 mus time scale. The reaction products were isolated by supersonic expansion and detected by single-photon (lambda = 118 nm) vacuum-ultraviolet time-of-flight mass spectrometry (VUV-TOFMS). Direct detection of CH(3) and C(3)H(3), as well as C(3)H(4), C(4)H(4), and C(4)H(5) products, provides insight into the initial steps involved in the complex pyrolysis of these C(4)H(6) species below T = 1500 K. The similar pyrolysis product distributions for the C(4)H(6) isomers on such a short time scale support the previously proposed mechanism of facile isomerization of these species. Isomerization of 1,3-butadiene to 1,2-butadiene and subsequent C-C bond fission of 1,2-butadiene to produce CH(3) and C(3)H(3) (propargyl) are most likely the primary initial radical production channel in the 1,3-butadiene pyrolysis.     

PAH formation under single collision conditions: reaction of phenyl radical and 1,3-butadiene to form 1,4-dihydronaphthalene

The crossed beam reactions of the phenyl radical (C(6)H(5), X(2)A(1)) with 1,3-butadiene (C(4)H(6), X(1)A(g)) and D6-1,3-butadiene (C(4)D(6), X(1)A(g)) as well as of the D5-phenyl radical (C(6)D(5), X(2)A(1)) with 2,3-D2-1,3-butadiene and 1,1,4,4-D4-1,3-butadiene were carried out under single collision conditions at collision energies of about 55 kJ mol(-1). Experimentally, the bicyclic 1,4-dihydronaphthalene molecule was identified as a major product of this reaction (58 ± 15%) with the 1-phenyl-1,3-butadiene contributing 34 ± 10%. The reaction is initiated by a barrierless addition of the phenyl radical to the terminal carbon atom of the 1,3-butadiene (C1/C4) to form a bound intermediate; the latter underwent hydrogen elimination from the terminal CH(2) group of the 1,3-butadiene molecule leading to 1-phenyl-trans-1,3-butadiene through a submerged barrier. The dominant product, 1,4-dihydronaphthalene, is formed via an isomerization of the adduct by ring closure and emission of the hydrogen atom from the phenyl moiety at the bridging carbon atom through a tight exit transition state located about 31 kJ mol(-1) above the separated products. The hydrogen atom was found to leave the decomposing complex almost parallel to the total angular momentum vector and perpendicularly to the rotation plane of the decomposing intermediate. The defacto barrierless formation of the 1,4-dihydronaphthalene molecule involving a single collision between a phenyl radical and 1,3-butadiene represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated counterparts in combustion and interstellar chemistry.     

Direct detection of dimethylstannylene and tetramethyldistannene in solution and the gas phase by laser flash photolysis of 1,1-dimethylstannacyclopent-3-enes

The photochemistry of 1,1-dimethyl- and 1,1,3,4-tetramethylstannacyclopent-3-ene (4a and 4b, respectively) has been studied in the gas phase and in hexane solution by steady-state and 193-nm laser flash photolysis methods. Photolysis of the two compounds results in the formation of 1,3-butadiene (from 4a) and 2,3-dimethyl-1,3-butadiene (from 4b) as the major products, suggesting that cycloreversion to yield dimethylstannylene (SnMe2) is the main photodecomposition pathway of these molecules. Indeed, the stannylene has been trapped as the Sn-H insertion product upon photolysis of 4a in hexane containing trimethylstannane. Flash photolysis of 4a in the gas phase affords a transient absorbing in the 450-520-nm range that is assigned to SnMe2 by comparison of its spectrum and reactivity to those previously reported from other precursors. Flash photolysis of 4b in hexane solution affords results consistent with the initial formation of SnMe2 (lambda(max) approximately 500 nm), which decays over approximately 10 micros to form tetramethyldistannene (5b; lambda(max) approximately 470 nm). The distannene decays over the next ca. 50 micros to form at least two other longer-lived species, which are assigned to higher SnMe2 oligomers. Time-dependent DFT calculations support the spectral assignments for SnMe2 and Sn2Me4, and calculations examining the variation in bond dissociation energy with substituent (H, Me, and Ph) in disilenes, digermenes, and distannenes rule out the possibility that dimerization of SnMe2 proceeds reversibly. Addition of methanol leads to reversible reaction with SnMe2 to form a transient absorbing at lambda(max) approximately 360 nm, which is assigned to the Lewis acid-base complex between SnMe2 and the alcohol.     

Reactions of Thienyl Containing Trisilane Under Irradiation

2-phenyl-2-furylhexamethyltrisilane and 2-phenyl-2-thienylhexamethyltrisilane (1) weresynthesized via Grignard-like reactions. The photolysis of 2-phenyl-2-furylhexamethyltrisilane inthe presence of 2,3-dimethyl-1,3-butadiene led to normal silylene-olefin addition and silylene C-Hinsertion reactions. Whereas, when 1 was photolyzed in the methanol-cyclohexene system, a radicalreaction mechanism is occurred. We suspect that the sulfur atom of the thienyl group strongly stabi-lized the silyl radical. This result was supported by both identifyling its typical radical reactions prod-ucts and ESR spectra of its quenching product with radical quencher.     

One- and two-photon laser photolysis of dicyanoacetylene

The time-resolved electronic absorption spectra of CN radical, resulting from the laser photolysis of dicyanoacetylene (DCA) at 193 and 248 nm, were analyzed. Detection of the other probable photodissociation product—C₃N radical—has not been possible. Photolysis at 193 nm produces CN both in the X²∑⁺ and in the A²Π_i manifolds, the latter—probably populated via a two-photon process—being revealed by a delayed (collision-induced) transfer to the higher vibrational levels of the ground electronic state. Photolysis at 248 nm is an efficient two-photon process. A simplified kinetic model for the decay of CN radical has been proposed and the rate constant for the CN + DCA reaction was derived. Semiempirical INDO/S CI-1 calculations of the DCA valence shell electronic transitions were performed.     

UV-absorption spectra of the radical transients generated from the 193-nm photolysis of allene, propyne, and 2-butyne

The 193-nm photochemistry of allene (H2C=C=CH2), propyne (H3C-C[triple bond]CH), and 2-butyne (H3C-C[triple bond]C-CH3) has been examined, and the UV spectral region between 220 and 350 nm has been surveyed for UV-absorption detection of transient species generated from the photolysis of these molecules. Time-resolved UV-absorption spectroscopy was used for detection of transient absorption. Gas chromatographic/mass spectroscopic (GC/MS) analysis of the photolyzed samples were employed for identification of the final photodissociation products. An emphasis of the study has been on the examination of possibilities of formation of different C3H3 isomeric radicals, that is, propargyl (H2CCCH) or propynyl (H3CCC), from the 193-nm photolysis of these molecules. Survey of the UV spectral region, following the 193-nm photolysis of dilute mixtures of allene/He resulted in detection of a strong absorption band around 230 nm and a weaker band in the 320-nm region with a relative intensity of about 8:1. The time-resolved absorption traces after the photolysis event show an instantaneous rise, followed by a simple decay. The spectral features, observed in this work, following 193-nm photolysis of allene are in good agreement with the previously reported spectrum of H2CCCH radical in the 240- and 320-nm regions and are believed to originate primarily from propargyl radicals. In comparison, the spectra obtained from the 193-nm photolysis of dilute mixtures of HCCCH3/He and CH3CCCH3/He were nearly identical, consisting of two relatively broad bands centered at about 240- and 320-nm regions with a relative intensity of about 2:1, respectively. In addition, the time-resolved absorption traces after photolysis of propyne and 2-butyne samples, both in the 240 and 320 nm regions, indicated an instant rise followed by an additional slower absorption rise. The distinct differences between the results of allene with those of propyne and 2-butyne suggest the observed absorption features following 193-nm photolysis of these molecules are likely to be composite with contributions from a number of transient species other than propargyl radicals. Propyne and 2-butyne are structurally similar. The methyl (CH3) and propynyl (CH3C[triple bond]C) radicals are likely to be among the photodissociation products of 2-butyne, and similarly, propynyl is likely to be a photodissociation product of propyne. GC/MS product analysis of photolyzed 2-butyne/He mixtures indicates the formation of C2H6 (formed from the combination of CH3 radicals), and a number of C6H6 and C4H6 isomers formed from self- and cross reactions of C3H3 and CH3 radicals, including 1,5-hexadiyne and 2,4-hexadyine, that are potential products of combination reactions of propargyl as well as propynyl radicals.     

On-Line Monitoring of the Photolysis of Benzyl Acetate and 3,5-Dimethoxybenzyl Acetate by Membrane Introduction Mass Spectrometry

Membrane introduction mass spectrometry (MIMS) allows on-line monitoring of the products of photolysis (254 nm) of benzyl acetate in aqueous methanol and 3,5-dimethoxybenzyl acetate in water. The reaction mixture is continuously exposed to a silicone membrane through which analyte molecules permeate into a triple quadrupole mass spectrometer for qualitative and quantitative analysis. Ionization is achieved by either isobutane or ammonia chemical ionization, and ions characteristic of the reactant ester and its products are monitored simultaneously and continuously. Three products, benzyl methyl ether, ethylbenzene, and bibenzyl are observed in the benzyl acetate photolysis. Two products, 3,5-dimethoxybenzyl alcohol and 3,5-dimethoxyethylbenzene, are formed in the photolysis of 3,5-dimethoxybenzyl acetate. Quantitation is achieved through calibration using external standard solutions and, in the case of benzyl methyl ether, tandem mass spectrometry is used to verify product identification. During the photolysis of benzyl acetate, benzyl methyl ether and ethylbenzene are produced at onset with similar efficiencies. For the 3,5-dimethoxy ester photolysis, performed in aqueous solution, the efficiency of formation of the polar product 3,5-dimethoxybenzyl alcohol is about 300 times greater than that of the nonpolar product 3,5-dimethoxyethylbenzene. The results show that the relative reaction rates are dependent on the solvent and on the photon intensity and are consistent with earlier off-line experiments by Pincock et al. which showed that the photolysis proceeds through both ion and radical pair intermediates. To the best of our knowledge, the work reported here describes the first analysis of the photochemistry of an aralkyl ester in water and the first use of on-line mass spectrometry in a mechanistic study.     

Spectroscopic observation of the resonance-stabilized 1-phenylpropargyl radical

The gas-phase laser-induced fluorescence (LIF) spectrum of a 1-phenylpropargyl radical has been identified in the region 20,800-22,000 cm(-1) in a free jet. The radical was produced from discharges of hydrocarbons including benzene. Disregarding C2, C3, and CH, this radical appears as the most strongly fluorescing product in a visible wavelength two-dimensional fluorescence excitation-emission spectrum of a jet-cooled benzene discharge. The structure of the carrier was elucidated by measurement of a matching resonant two-color two-photon ionization spectrum at m/z = 115 and density functional theory. The assignment was proven conclusively by observation of the same excitation spectrum from a low-current discharge of 3-phenyl-1-propyne. The apparent great abundance of the 1-phenylpropargyl radical in discharges of benzene and, more importantly, 1-hexyne may further underpin the proposed importance of the propargyl radical in the formation of complex hydrocarbons in combustion and circumstellar environments.     

Two-photon chemistry in the laser jet: generation of radical intermidiates and their photochemical transformations

The novel laser jet technique provides sufficiently high photon densities to permit the observation of the photochemistry of photochemically generated radicals (two-photon chemistry) in the liquid phase. Four recent applications of this novel photochemically useful method are presented: these include the photochemistry of hydroxydiphenylmethyl, 9-hydroxyxanthenyl, diphenylmethyl, and benzoyl radicals under laser jet and normal photolysis conditions.

The regioselectivity of cross-coupling reactions of hydroxydiphenylmethyl or 9-hydroxyxanthenyl radicals with solvent-derived radicals changes when these species are electronically excited,i.e. under the high intensity conditions of the laser jet, cross-coupling at the para position (head-to-tail combination) is significantly enhanced relative to the normal coupling mode at the hydroxy-bearing radical site (head-to-head combination). Semiempirical calculations of the spin density distributions for the ground and first excited states of the radicals confirm the change in spin density from the hydroxy-bearing carbon atom to the conjugating benzene rings in these radical species on photoexcitation.

For the diphenylmethyl radical, two reaction pathways have been observed under the high photon densities of the laser jet: the electronically excited diphenylmethyl radical can either abstract a chlorine atom from carbon tetrachloride through an electron transfer process or can be photoionized on further photoexcitation (multiphoton chemistry). The resulting benzhydryl cation was trapped by methanol as the corresponding ether product, which unequivocally demonstrates that carbene formation by photoejection of a hydrogen atom does not take place under laser jet photolysis conditions.

An advantage of the high photon densities produced in laser jet photolysis is the high steady state concentration of short-lived transients that are generated, which enable unprecedented intermolecular reactions to be observed. Thus, about a millimolar concentration of tert-butoxy radicals can be obtained in the laser jet photocleavage of tert-butyl peroxide. When the tert-butoxy radicals are produced in the presence of benzaldehyde, the main product is tert-butyl benzoate. If carbon tetrachloride is also present, chlorobenzene can be detected. This is rationalized as the product derived from chlorine abstraction by phenyl radicals, which are presumably produced by the photodecarbonylation of benzoyl radicals.

An alternative method of obtaining benzoyl radicals is the two-photon cleavage of benzil. The laser jet photolysis of benzil in tert-butyl peroxide yields mainly tert-butyl benzoate, whereas in carbon tetrachloride, benzoyl chloride, chlorobenzene and ,,-trichloroacetophenone are observed. The first two products result from chlorine atom abstraction by the photochemically generated benzoyl and phenyl radicals, and the last product from in-cage cross-coupling between benzoyl and trichloromethyl radicals.

Such product studies provide detailed mechanistic information on the photochemical behaviour of electronically excited, short-lived transients which complements nicely the kinetic and spectral data of time-resolved laser flash studies. Consequently, the laser jet technique constitutes a valuable tool for determining the mechanism of two- photon reactions.     

A study of the unimolecular dissociation of the 2-buten-2-yl radical via the 193 nm photodissociation of 2-chloro-2-butene

This work investigates the unimolecular dissociation of the 2-buten-2-yl radical. This radical has three potentially competing reaction pathways: C-C fission to form CH3 + propyne, C-H fission to form H + 1,2-butadiene, and C-H fission to produce H + 2-butyne. The experiments were designed to probe the branching to the three unimolecular dissociation pathways of the radical and to test theoretical predictions of the relevant dissociation barriers. Our crossed laser-molecular beam studies show that 193 nm photolysis of 2-chloro-2-butene produces 2-buten-2-yl in the initial photolytic step. A minor C-Cl bond fission channel forms electronically excited 2-buten-2-yl radicals and the dominant C-Cl bond fission channel produces ground-state 2-buten-2-yl radicals with a range of internal energies that spans the barriers to dissociation of the radical. Detection of the stable 2-buten-2-yl radicals allows a determination of the translational, and therefore internal, energy that marks the onset of dissociation of the radical. The experimental determination of the lowest-energy dissociation barrier gave 31 +/- 2 kcal/mol, in agreement with the 32.8 +/- 2 kcal/mol barrier to C-C fission at the G3//B3LYP level of theory. Our experiments detected products of all three dissociation channels of unstable 2-buten-2-yl as well as a competing HCl elimination channel in the photolysis of 2-chloro-2-butene. The results allow us to benchmark electronic structure calculations on the unimolecular dissociation reactions of the 2-buten-2-yl radical as well as the CH3 + propyne and H + 1,2-butadiene bimolecular reactions. They also allow us to critique prior experimental work on the H + 1,2-butadiene reaction.     

Product branching ratios in photodissociation of phenyl radical: a theoretical ab initio/Rice-Ramsperger-Kassel-Marcus study

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(6)H(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(6)H(4) + H, l-C(6)H(4) ((Z)-hexa-3-ene-1,5-diyne) + H, and n-C(4)H(3) + C(2)H(2) products, respectively, in a contradiction with recent experimental measurements, which showed C(4)H(3) + C(2)H(2) as the major product. Although two lower energy pathways to the i-C(4)H(3) + C(2)H(2) products are identified, they appeared to be kinetically unfavorable and the computed statistical branching ratio of i-C(4)H(3) + C(2)H(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(6)H(5) and, based on their structures and energies, suggested the following photodissociation mechanism at 193 nm: c-C(6)H(5) 1 → absorption of a photon → electronically excited 1 → internal conversion to the lowest excited state → conversion to the ground electronic state via conical intersections at CI-2 or CI-3 → non-statistical decay of the vibrationally excited radical favoring the formation of the n-C(4)H(3) + C(2)H(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(4)H(3) + C(2)H(2) driven by the dynamical preference.     

Photofragmentation translational spectroscopy of methyl azide (CH3N3) photolysis at 193 nm: molecular and radical channel product branching ratio

We describe molecular-beam photofragment translational spectroscopy (PTS) experiments using electron impact (EI) ionization product detection to investigate the 193 nm photodissociation of methyl azide (CH(3)N(3)) under collision-free conditions. These experiments are used to derive the branching ratio between channels 1 and 2 [(1) radical channel: CH(3)N(3) + hν (λ = 193 nm) → CH(3) + N(3); (2) molecular channel: CH(3)N(3) + hν (λ = 193 nm) → CH(3)N + N(2)], which have been reported in a previous VUV-photoionization based PTS study. (1) Using electron impact ionization cross sections and ion fragmentation ratios for the various detected products, we derive the branching ratio (X(CH(3)-N(3)))/(X(CH(3)N-N(2))) = (0.017 ± 0.004)/(0.983 ± 0.004). Based on analysis of the kinetic energy release in the radical channel, we find that the cyclic form of N(3) is the dominant product in the radical channel. Only a small fraction of the radical channel produces ground state linear N(3).     

Photoionization and photodissociation of HCl(B 1Sigma+, J=0) near 236 and 239 nm using three-dimensional ion imaging

The electronically excited states HCl(*)(E,upsilon(')=0,J(')=0) and HCl(*)(V,upsilon(')=12,J(')=0) have been prepared by two-photon resonant absorption of ground state HCl via Q(0) transitions at 238.719 and at 236.000 nm, respectively. The consequent one-or two-photon excitation at the same wavelength results in the production of H(+), Cl(+), and HCl(+) ions. The speed distributions and anisotropy parameters beta for these ions have been determined by three-dimensional photo-fragment ion imaging based on a position-sensitive delay-line anode assembly. Several results are presented: first, we measured velocity (speed and angle) distributions for HCl(+) due to the electron recoil in the photoionization of HCl(*). Such distributions give information on the photoionization process and on the vibrational distribution of HCl(+) after the laser pulse. Second, the measured beta parameters for Cl(+) and H(+) distributions give information on the symmetries of the upper states in the one-photon photoexcitation of HCl(*). Third, the measured speed distributions for H(+) help to understand the mechanism of the photodissociation of HCl(+) ions.     

Photon-induced unimolecular decay of the benzyl radical: first direct identification of the reaction pathway to C7H6

The thermal unimolecular decay of the benzyl radical has been investigated extensively by several groups. However, the reaction products could not be determined unambiguously. In this work the unimolecular bond fission of the benzyl radical is studied in a molecular beam experiment. The precursor molecules toluene and cycloheptatriene are expanded in a molecular beam and photodissociated with two photons at 248 or 193 nm, yielding in each case hot benzyl radicals. Since the internal energies lie above the dissociation limit, the benzyl radicals decay in a subsequent step. The reaction products are detected in a time-resolved manner with a quadrupole mass spectrometer on the molecular beam axis at low electron energies. The measured time-of-flight spectra provide information on the translational energy distribution of the products. In each case it is found that the hot benzyl radicals C₇H₇ fragment under hydrogen loss to C₇H₆.     

Reactions of C2H with 1- and 2-butynes: an ab initio/RRKM study of the reaction mechanism and product branching ratios

Ab initio CCSD(T)/cc-pVTZ(CBS)//B3LYP/6-311G** calculations of the C(6)H(7) potential energy surface are combined with RRKM calculations of reaction rate constants and product branching ratios to investigate the mechanism and product distribution in the C(2)H + 1-butyne/2-butyne reactions. 2-Ethynyl-1,3-butadiene (C(6)H(6)) + H and ethynylallene (C(5)H(4)) + CH(3) are predicted to be the major products of the C(2)H + 1-butyne reaction. The reaction is initiated by barrierless ethynyl additions to the acetylenic C atoms in 1-butyne and the product branching ratios depend on collision energy and the direction of the initial C(2)H attack. The 2-ethynyl-1,3-butadiene + H products are favored by the central C(2)H addition to 1-butyne, whereas ethynylallene + CH(3) are preferred for the terminal C(2)H addition. A relatively minor product favored at higher collision energies is diacetylene + C(2)H(5). Three other acyclic C(6)H(6) isomers, including 1,3-hexadiene-5-yne, 3,4-hexadiene-1-yne, and 1,3-hexadiyne, can be formed as less important products, but the production of the cyclic C(6)H(6) species, fulvene, and dimethylenecyclobut-1-ene (DMCB), is predicted to be negligible. The qualitative disagreement with the recently measured experimental product distribution of C(6)H(6) isomers is attributed to a possible role of the secondary 2-ethynyl-1,3-butadiene + H reaction, which may generate fulvene as a significant product. Also, the photoionization energy curve assigned to DMCB in experiment may originate from vibrationally excited 2-ethynyl-1,3-butadiene molecules. For the C(2)H + 2-butyne reaction, the calculations predict the C(5)H(4) isomer methyldiacetylene + CH(3) to be the dominant product, whereas very minor products include the C(6)H(6) isomers 1,1-ethynylmethylallene and 2-ethynyl-1,3-butadiene.     

Photodissociation of azulene at 193 nm: ab initio and RRKM study

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of azulene at 6.4 eV (the laser wavelength of 193 nm) upon absorption of one UV photon followed by internal conversion into the ground electronic state. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that photoexcited azulene can readily isomerize to naphthalene and the major dissociation channel is elimination of an H-atom from naphthalene. The branching ratio of this channel decreases with an increase of the photon energy. Acetylene elimination is the second probable reaction channel and its branching ratio rises as the photon energy increases. The main C8H6 fragments at 193 nm are phenylacetylene and pentalene and the yield of the latter grows fast with the increasing excitation energy.     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS—XL PHOTOLYSIS OF CHLORPROMAZINE METABOLITES: A SPIN-TRAPPING STUDY

Abstract. The photochemistry of chlorpromazine (CPZ) and its metabolites, 7-hydroxychlorpromazine (7OHCPZ), desmethylchlorpromazine (DCPZ), didesmethylchlorpromazine (DDCPZ) and chlorpromazine sulfoxide (CPZSO) was studied by the spin trapping technique with 2-methyl-2-nitrosopropane and 5,5-dimethyl-l-pyrroline- N -oxide. 7-Hydroxychlorpromazine generated hydroxyl radicals when excited at 330 nm under either anaerobic or aerobic conditions. 7-Hydroxychlorpromazine, DCPZ and DDCPZ all underwent dechlorination upon photoexcitation which was enhanced in the absence of air. Chlorpromazine sulfoxide did not undergo photodechlorination but instead generated a high yield of the hydroxyl radical. A comparison among CPZ and its derivatives shows that the yield of the photodechlorinated product is directly related to the degree of phototoxicity. This suggests photodechlorination is an important factor in the phototoxicity of CPZ and its metabolites.     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS—XL PHOTOLYSIS OF CHLORPROMAZINE METABOLITES: A SPIN-TRAPPING STUDY

Abstract. The photochemistry of chlorpromazine (CPZ) and its metabolites, 7-hydroxychlorpromazine (7OHCPZ), desmethylchlorpromazine (DCPZ), didesmethylchlorpromazine (DDCPZ) and chlorpromazine sulfoxide (CPZSO) was studied by the spin trapping technique with 2-methyl-2-nitrosopropane and 5,5-dimethyl-l-pyrroline-N-oxide. 7-Hydroxychlorpromazine generated hydroxyl radicals when excited at 330 nm under either anaerobic or aerobic conditions. 7-Hydroxychlorpromazine, DCPZ and DDCPZ all underwent dechlorination upon photoexcitation which was enhanced in the absence of air. Chlorpromazine sulfoxide did not undergo photodechlorination but instead generated a high yield of the hydroxyl radical. A comparison among CPZ and its derivatives shows that the yield of the photodechlorinated product is directly related to the degree of phototoxicity. This suggests photodechlorination is an important factor in the phototoxicity of CPZ and its metabolites.     

Geometries and excited-state dynamics of van der Waals dimers and higher clusters of 1-cyanonaphthalene

Mass-selected resonant two-photon ionization and infrared-ultraviolet double-resonance spectroscopies are combined with correlated (second Moller-Plesset perturbation) quantum chemistry calculation to probe electronic spectra and ground-state geometries of the jet-cooled dimer and higher clusters of 1-cyanonaphthalene. The results indicate that the dimer and trimer have stacked geometries, consistent with the highly efficient, rapid excimer formation that follows photoexcitation of the ground-state clusters.