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The unexpected abundance of HNO in the photodecomposition of the radical 2-nitrosooxy ethyl (CH2CH2ONO) is investigated through calculations of the potential energy surface by the anti-Hermitian contracted Schrödinger equation (ACSE) method, which directly generates the 2-electron reduced density matrix. The ACSE, which is able to balance single-reference (dynamic) and multi-reference (static) correlation effects, reveals some subtle correlation effects along the intrinsic reaction coordinate (IRC) en route to NO + oxirane, an IRC which offers a potential bifurcation to the HNO + vinoxy product channel. These effects were not fully captured by either single-reference techniques, such as coupled cluster, or multi-reference techniques, such as second-order multi-reference perturbation theory. These correlation effects reveal small to moderate energy changes in key transition states, which have implications for the reaction mechanism as related to the production of HNO. [ABSTRACT FROM AUTHOR]

The 234 nm photodissociation dynamics of cyclobutyl bromide is investigated using a two-dimensional photofragment velocity imaging technique. The measured recoil kinetic energies of the c-C4H7 radicals may be stable due to fraction of the internal energy partitioned to rotational energy of the radicals, which is partitioned to rotational energy of the radicals.

The unimolecular dissociation of the methoxycarbonyl, CH3OCO, radical is investigated. Photolysis of methyl chloroformate at 193 nm produced nascent CH3OCO radicals with a distribution of internal energies, determined by the velocities of the momentum-matched Cl atoms, that spans the theoretically predicted barriers to the Ch3O + CO and CH3 + CO2 product channels.

The unimolecular dissociation of the 2-buten-2-yl radical is investigated. Crosses laser-molecular beam studies revealed that 193 nm photolysis of 2-chloro-2-butene produces 2-buten-2-yl in the intial photolytic step.

A molecular beam scattering technique is used to investigate the unimolecular reaction dynamics of C4H7 radical isomers. The unimolecular dissociation of 1-buten-2-yl and other C4H7 radicals is relevant to latest experimental work involving the kinetics of reactions under bulk conditions.

Photofragment translational spectroscopy investigations of the 193 nm dissociation of N2O4 indicate two primary N-N bond fission channels. There is no significant branching to other bond fission or elimination products.

The means by which breakdown of the Born-Oppenheimer approximation at a barrier at an adiabatic reaction coordinate alters dynamics of and expected branching of molecular dissociation pathways has been investigated. To predict branching of energetically allowed product channels, chemists o rely often on statistical transition state theories or precise quantum scattering calculations on one adiabatic potential energy surface. The potential energy surface gives energetic barriers to each chemical reaction and allows rates of reaction to be calculated. The reactions evolve, however, on a single potential energy surface only if the electronic wavefunction can evolve from the reactant electronic configuration to the product electronic configuration fast enough compared to nuclear dynamics through the transition state.

We present a model which accurately predicts the net speed distributions of products resulting from the unimolecular decomposition of rotationally excited radicals. The radicals are produced photolytically from a halogenated precursor under collision-free conditions so they are not in a thermal distribution of rotational states. The accuracy relies on the radical dissociating with negligible energetic barrier beyond the endoergicity. We test the model predictions using previous velocity map imaging and crossed laser-molecular beam scattering experiments that photolytically generated rotationally excited CD2CD2OH and C3H6OH radicals from brominated precursors; some of those radicals then undergo further dissociation to CD2CD2 + OH and C3H6 + OH, respectively. We model the rotational trajectories of these radicals, with high vibrational and rotational energy, first near their equilibrium geometry, and then by projecting each point during the rotation to the transition state (continuing the rotational dynamics at that geometry). This allows us to accurately predict the recoil velocity imparted in the subsequent dissociation of the radical by calculating the tangential velocities of the CD2CD2/C3H6 and OH fragments at the transition state. The model also gives a prediction for the distribution of angles between the dissociation fragments' velocity vectors and the initial radical's velocity vector. These results are used to generate fits to the previously measured time-offlight distributions of the dissociation fragments; the fits are excellent. The results demonstrate the importance of considering the precession of the angular velocity vector for a rotating radical. We also show that if the initial angular momentum of the rotating radical lies nearly parallel to a principal axis, the very narrow range of tangential velocities predicted by this model must be convoluted with a J = 0 recoil velocity distribution to achieve a good result. The model relies on measuring the kinetic energy release when the halogenated precursor is photodissociated via a repulsive excited state but does not include any adjustable parameters. Even when different conformers of the photolytic precursor are populated, weighting the prediction by a thermal conformer population gives an accurate prediction for the relative velocity vectors of the fragments from the highly rotationally excited radical intermediates. [ABSTRACT FROM AUTHOR]

In this study, we computationally investigate the initial and subsequent steps in the chemical mechanism for the gas-phase thermal decomposition of 1,1-diamino-2,2-dinitroethene (FOX-7). We determine the key exothermic step in the gas-phase thermal decomposition of FOX-7 and explore the similarities and differences between FOX-7 and other geminal dinitro energetic materials. The calculations reveal a mechanism for NO loss involving a 3-member cyclic intermediate, rather than a nitro-nitrite isomerization, that occurs in the radical intermediates formed throughout the decomposition mechanism. [ABSTRACT FROM AUTHOR]

We use a combination of crossed laser-molecular beam scattering experiments and velocity map imaging experiments to investigate the three primary photodissociation channels of chloroacetone at 193 nm: C-Cl bond photofission yielding CH3C(O)CH2 radicals, C-C bond photofission yielding CH3CO and CH2Cl products, and C-CH3 bond photofission resulting in CH3 and C(O)CH2Cl products. Improved analysis of data previously reported by our group quantitatively identifies the contribution of this latter photodissociation channel. We introduce a forward convolution procedure to identify the portion of the signal, derived from the methyl image, which results from a two-step process in which C-Cl bond photofission is followed by the dissociation of the vibrationally excited CH3C(O)CH2 radicals to CH3 + COCH2. Subtracting this from the total methyl signal identifies the methyl photofragments that result from the CH3 + C(O)CH2Cl photofission channel. We find that about 89% of the chloroacetone molecules undergo C-Cl bond photofission to yield CH3C(O)CH2 and Cl products; approximately 8% result in C-C bond photofission to yield CH3CO and CH2Cl products, and the remaining 2.6% undergo C-CH3 bond photofission to yield CH3 and C(O)CH2Cl products. [ABSTRACT FROM AUTHOR]

The dissociation dynamics of methoxysulfinyl radicals generated from the photodissociation of CH3OS(O)Cl at 248 nm is investigated using both a crossed laser - molecular beam scattering apparatus and a velocity map imaging apparatus. There is evidence of only a single photodissociation channel of the precursor: S-Cl fission to produce Cl atoms and CH3OSO radicals. Some of the vibrationally excited CH3OSO radicals undergo subsequent dissociation to CH3 + SO2. The velocities of the detected CH3 and SO2 products show that the dissociation occurs via a transition state having a substantial barrier beyond the endoergicity; appropriately, the distribution of velocities imparted to these momentum-matched products is fit by a broad recoil kinetic energy distribution extending out to 24 kcal/mol in translational energy. Using 200 eV electron bombardment detection, we also detect the CH3OSO radicals that have too little internal energy to dissociate. These radicals are observed both at the parent CH3OSO+ ion as well as at the CH3+ and SO2+ daughter ions; they are distinguished by virtue of the velocity imparted in the original photolytic step. The detected velocities of the stable radicals are roughly consistent with the calculated barriers (both at the CCSD(T) and G3B3 levels of theory) for the dissociation of CH3OSO to CH3 + SO2 when we account for the partitioning of internal energy between rotation and vibration as the CH3OSOCl precursor dissociates. [ABSTRACT FROM AUTHOR]

We use a combination of crossed laser-molecular beam experiments and velocity map imaging experiments to investigate the primary photofission channels of chloroacetone at 193 nm; we also probe the dissociation dynamics of the nascent CH3C(O)CH2 radicals formed from C-Cl bond fission. In addition to the C-Cl bond fission primary photodissociation channel, the data evidence another photodissociation channel of the precursor, C-C bond fission to produce CH3CO and CH2Cl. The CH3C(O)CH2 radical formed from C-Cl bond fission is one of the intermediates in the OH + allene reaction en route to CH3 + ketene. The 193 nm photodissociation laser allows us to produce these CH3C(O)CH2 radicals with enough internal energy to span the dissociation barrier leading to the CH3 + ketene asymptote. Therefore, some of the vibrationally excited CH3C(O)CH2 radicals undergo subsequent dissociation to CH3 + ketene products; we are able to measure the velocities of these products using both the imaging and scattering apparatuses. The results rule out the presence of a significant contribution from a C-C bond photofission channel that produces CH3 and COCH2Cl fragments. The CH3C(O)CH2 radicals are formed with a considerable amount of energy partitioned into rotation; we use an impulsive model to explicitly characterize the internal energy distribution. The data are better fit by using the C-Cl bond fission transition state on the S1 surface of chloroacetone as the geometry at which the impulsive force acts, not the Franck-Condon geometry. Our data suggest that, even under atmospheric conditions, the reaction of OH with allene could produce a small branching to CH3 + ketene products, rather than solely producing inelastically stabilized adducts. This additional channel offers a different pathway for the OH-initiated oxidation of such unsaturated volatile organic compounds, those containing a C=C=C moiety, than is currently included in atmospheric models. [ABSTRACT FROM AUTHOR]

This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H2COCH)CH2Cl. The three dominant photoproduct channels analyzed are c-(H2COCH)CH2+Cl, c-(H2COCH)+CH2Cl, and C3H4O+HCl. In the second channel, the c-(H2COCH) photofission product is a higher energy intermediate on C2H3O global potential energy surface and has a small isomerization barrier to vinoxy. The resulting highly vibrationally excited vinoxy radicals likely dissociate to give the observed signal at the mass corresponding to ketene, H2CCO. The final primary photodissociation pathway HCl+C3H4O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H2COC)=CH2; the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C3H5O radical produced from C–Cl bond photofission. When the epoxide radical photoproduct undergoes facile ring opening, it is the radical intermediate formed in the O(3P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C2H4 and H2CO+C2H3 product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C2H4 products at m/e=29 and 28 shows that the dissociation of the radical intermediate imparts a high relative kinetic energy, peaking near 20 kcal/mol, between the products. Similarly, the energy imparted to relative kinetic energy in the H2CO+C2H3 product channel of the O(3P)+allyl radical intermediate also peaks at high-recoil kinetic energies, near 18 kcal/mol. The strongly forward-backward peaked angular distributions and the high kinetic energy release result from tangential recoil during the dissociation of highly rotationally excited nascent radicals formed photolytically in this experiment. The data also reveal substantial branching to an HCCH+H3CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C3H5O potential energy surface calculated with coupled cluster theory, accounting for the rotational and vibrational energy imparted to the radical intermediate and the resulting competition between the H+acrolein, HCO+C2H4, and H2CO+C2H3 product channels. We compare the results of the theoretical prediction with our measured branching ratios. We also report photoionization efficiency (PIE) curves extending from 9.25 to 12.75 eV for the signal from the HCO+C2H4 and H2CO+C2H3 product channels. Using the C2H4 bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C2H4 and HCO yields a value of 11.6+1/-3 Mb for the photoionization cross section of HCO at 11.27 eV. This determination puts the PIE curve of HCO measured here on an absolute scale, allowing us to report the absolute photoionization efficiency of HCO over the entire range of photoionization energies. [ABSTRACT FROM AUTHOR]

These imaging experiments study the formation of the methylsulfonyl radical, CH3SO2, from the photodissociation of CH3SO2Cl at 193 nm and determine the energetic barrier for the radical’s subsequent dissociation to CH3+SO2. We first state-selectively detect the angular and recoil velocity distributions of the Cl(2P3/2) and Cl(2P1/2) atoms to further refine the distribution of internal energy partitioned to the momentum-matched CH3SO2 radicals. The internal energy distribution of the radicals is bimodal, indicating that CH3SO2 is formed in both the ground state and low-lying excited electronic states. All electronically excited CH3SO2 radicals dissociate, while those formed in the ground electronic state have an internal energy distribution which spans the dissociation barrier to CH3+SO2. We detect the recoil velocities of the energetically stable methylsulfonyl radicals with 118 nm photoionization. Comparison of the total recoil translational energy distribution for all radicals to the distribution obtained from the detection of stable radicals yields an onset for dissociation at a translational energy of 70±2 kcal/mol. This onset allows us to derive a CH3SO2→CH3+SO2 barrier height of 14±2 kcal/mol; this determination relies on the S–Cl bond dissociation energy, taken here as the CCSD(T) predicted energy of 65.6 kcal/mol. With 118 nm photoionization, we also detect the velocity distribution of the CH3 radicals produced in this experiment. Using the velocity distributions of the SO2 products from the dissociation of CH3SO2 to CH3+SO2 presented in the following paper, we show that our fastest detected methyl radicals are not from these radical dissociation channels, but rather from a primary S–CH3 bond photofission channel in CH3SO2Cl. We also present critical points on the ground state potential energy surface of CH3SO2 at the //CCSD(T)/aug-cc-pV(Q+d)ZCCSD(T)/6-311++G(2df,p) level. We include harmonic zero-point vibrational corrections as well as core-valence and scalar-relativistic corrections. The CCSD(T) predicted barrier of 14.6 kcal/mol for CH3SO2→CH3+SO2 agrees well with our experimental measurement. These results allow us to predict the unimolecular dissociation kinetics of CH3SO2 radicals and critique the analysis of prior time-resolved photoionization studies on this system. [ABSTRACT FROM AUTHOR]

The dissociation dynamics of methylsulfonyl radicals generated from the photodissociation of CH3SO2Cl at 193 nm is investigated by measuring product velocities in a crossed laser-molecular beam scattering apparatus. The data evidence three primary photodissociation channels of the precursor: S–Cl fission to produce Cl atoms and ground electronic state CH3SO2 radicals, S–Cl fission to produce Cl atoms and electronically excited CH3SO2 radicals, and S–CH3 fission. Some of the vibrationally excited CH3SO2 radicals undergo subsequent dissociation to CH3+SO2, as do all of the electronically excited radicals. The velocities of the SO2 products show that the vibrationally excited ground state CH3SO2 radicals dissociate via a loose transition state having a small exit barrier beyond the endoergicity. Hence, a statistical recoil kinetic energy distribution should and does fit the distribution of velocities imparted to these SO2 products. The electronically excited CH3SO2 radicals also dissociate to CH3+SO2, but with a larger average release to relative kinetic energy. Interestingly, when using 200 eV electron bombardment detection, the ground electronic state CH3SO2 radicals having too little internal energy to dissociate are not observed at the parent CH3SO2+ ion, but only at the CH3+ daughter ion. They are distinguished by virtue of the velocity imparted in the original photolytic step; the detected velocities of the stable radicals are consistent with the calculated barrier of 14.6 kcal/mol for the dissociation of CH3SO2 to CH3+SO2. We present CCSD(T) calculations of the adiabatic excitation energy to the lowest excited state of CH3SO2 radicals, the 1 2A″ state, as well as the vertical energy from the equilibrium geometry of that excited state to the 2 2A″ state, to aid in the experimental assignment. [ABSTRACT FROM AUTHOR]

These experiments study the preparation of and product channels resulting from OCH2CHCH2, a key radical intermediate in the O+allyl bimolecular reaction. The data include velocity map imaging and molecular beam scattering results to probe the photolytic generation of the radical intermediate and the subsequent pathways by which the radicals access the energetically allowed product channels of the bimolecular reaction. The photodissociation of epichlorohydrin at 193.3 nm produces chlorine atoms and c-OCH2CHCH2 radicals; these undergo a facile ring opening to the OCH2CHCH2 radical intermediate. State-selective resonance-enhanced multiphoton ionization (REMPI) detection resolves the velocity distributions of ground and spin-orbit excited state chlorine independently, allowing for a more accurate determination of the internal energy distribution of the nascent radicals. We obtain good agreement detecting the velocity distributions of the Cl atoms with REMPI, vacuum ultraviolet (VUV) photoionization at 13.8 eV, and electron bombardment ionization; all show a bimodal distribution of recoil kinetic energies. The dominant high recoil kinetic energy feature peaks near 33 kcal/mol. To elucidate the product channels resulting from the OCH2CHCH2 radical intermediate, the crossed laser-molecular beam experiment uses VUV photoionization and detects the velocity distribution of the possible products. The data identify the three dominant product channels as C3H4O (acrolein)+H, C2H4+HCO (formyl radical), and H2CO (formaldehyde)+C2H3. A small signal from C2H2O (ketene) product is also detected. The measured velocity distributions and relative signal intensities at m/e=27, 28, and 29 at two photoionization energies show that the most exothermic product channel, C2H5+CO, does not contribute significantly to the product branching. The higher internal energy onset of the acrolein+H product channel is consistent with the relative barriers en route to each of these product channels calculated at the CCSD(T)/aug-cc-pVQZ level of theory, although a clean determination of the barrier energy to H+acrolein is precluded by the substantial partitioning into rotational energy during the photolytic production of the nascent radicals. We compare the measured branching fraction to the H+acrolein product channel with a statistical prediction based on the calculated transition states. [ABSTRACT FROM AUTHOR]

These velocity map imaging experiments characterize the photolytic generation of one of the two radical intermediates formed when OH reacts via an addition mechanism with allene. The CH2CCH2OH radical intermediate is generated photolytically from the photodissociation of 2-chloro-2-propen-1-ol at 193 nm. Detecting the Cl atoms using [2+1] resonance-enhanced multiphoton ionization evidences an isotropic angular distribution for the Cl+CH2CCH2OH photofragments, a spin-orbit branching ratio for Cl(2P1/2):Cl(2P3/2) of 0.28, and a bimodal recoil kinetic energy distribution. Conservation of momentum and energy allows us to determine from this data the internal energy distribution of the nascent CH2CCH2OH radical cofragment. To assess the possible subsequent decomposition pathways of this highly vibrationally excited radical intermediate, we include electronic structure calculations at the G3//B3LYP level of theory. They predict the isomerization and dissociation transition states en route from the initial CH2CCH2OH radical intermediate to the three most important product channels for the OH+allene reaction expected from this radical intermediate: formaldehyde+C2H3, H+acrolein, and ethene+CHO. We also calculate the intermediates and transition states en route from the other radical adduct, formed by addition of the OH to the center carbon of allene, to the ketene+CH3 product channel. We compare our results to a previous theoretical study of the O+allyl reaction conducted at the CBS-QB3 level of theory, as the two reactions include several common intermediates. [ABSTRACT FROM AUTHOR]

This paper reports a series of electronic structure calculations performed on the dissociation pathways of the vinoxy radical (CH2CHO). We use coupled-cluster with single, double, and perturbative triple excitations (CCSD(T)), complete active space self-consistent field (CASSCF), multireference configuration interaction (MRCI), and MRCI with the Davidson correction (MRCI+Q) to calculate the barrier heights of the two unimolecular dissociation pathways of this radical. The effect of state averaging on the barrier heights is investigated at the CASSCF, MRCI, and MRCI+Q levels. The change in mixing angle along the reaction path is calculated as a measure of derivative coupling and found to be insufficient to suggest nonadiabatic recrossing. We also present a new analysis of previous experimental data on the unimolecular dissociation of ground state vinoxy. In particular, an error in the internal energy distribution of vinoxy radicals reported in a previous paper is corrected and a new analysis of the experimental sensitivity to the onset energy (barrier height) for the isomerization reaction is given. Combining these studies, a final “worst case” analysis of the product branching ratio is given and a statistical model using each of the calculated transition states is found to be unable to correctly reproduce the experimental data. [ABSTRACT FROM AUTHOR]

We present photofragment imaging experiments to characterize potential photolytic precursors of three C4H7 radical isomers: 1-methylallyl, cyclopropylmethyl, and 3-buten-1-yl radicals. The experiments use 2+1 resonance enhanced multiphoton ionization (REMPI) with velocity map imaging to state-selectively detect the Br(2P3/2) and Br(2P1/2) atoms as a function of their recoil velocity imparted upon photodissociation of 1-bromo-2-butene, cyclopropylmethyl bromide, and 4-bromo-1-butene at 234 nm as well as the angular distributions of the photofragments. Energy and momentum conservation allows the internal energy distribution of the nascent momentum-matched radicals to be derived. The radicals are detected with single photon photoionization at 157 nm. In the case of the 1-methylallyl radical the photoionization cross section is expected to be independent of internal energy in the range of 7–30 kcal/mol. Thus, comparison of the product recoil kinetic energy distribution derived from the measurement of the 1-methylallyl velocity distribution, detecting the radicals with 157 nm photoionization, with a linear combination of the Br atom recoil kinetic energy distributions allows us to derive reliable REMPI line strength ratios for the detection of Br atoms and to test the assumption that the photoionization cross section does not strongly depend on the internal energy of the radical. This line strength ratio is then used to determine the branching to the Br(2P3/2) and Br(2P1/2) product channels for the other two photolytic systems and to determine the internal energy distribution of their momentum-matched radicals. (We also revisit earlier work on the photodissociation of cyclobutyl bromide which detected the Br atoms and momentum-matched cyclobutyl radicals.) This allows us to test whether the 157 nm photoionization of these radicals is insensitive to internal energy for the distribution of total internal (vibrational+rotational) energy produced. We find that 157 nm photoionization of cyclopropylmethyl radicals is relatively insensitive to internal energy, while 3-buten-1-yl radicals show a photoionization cross section that is markedly dependent on internal energy with the lowest internal energy radicals not efficiently detected by photoionization at 157 nm. We present electronic structure calculations of the radicals and their cations to understand the experimental results. [ABSTRACT FROM AUTHOR]

This paper examines the unimolecular dissociation of propargyl (HCCCH2) radicals over a range of internal energies to probe the CH+HCCH and C+C2H3 bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by 157 nm photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and H2 elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 (+5/-10) kcal/mol as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical’s dissociation and also lower than calculated overall reaction enthalpies. Products from both H2+HCCC and H+C3H2 channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C–H fission channel were also observed in the photolysis of propargyl chloride. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of allyl chloride at 235 nm producing atomic Cl(2PJ;J=1/2,3/2) fragments is investigated using a two-dimensional photofragment velocity ion imaging technique. Detection of the Cl(2P1/2) and Cl(2P3/2) products by [2+1] resonance enhanced multiphoton ionization shows that primary C–Cl bond fission of allyl chloride generates 66.8% Cl(2P3/2) and 33.2% Cl(2P1/2). The Cl(2P3/2) fragments evidenced a bimodal translational energy distribution with a relative weight of low kinetic energy Cl(2P3/2)/high kinetic energy Cl(2P3/2) of 0.097/0.903. The minor dissociation channel for C–Cl bond fission, producing low kinetic energy chlorine atoms, formed only chlorine atoms in the Cl(2P3/2) spin–orbit state. The dominant C–Cl bond fission channel, attributed to an electronic predissociation that results in high kinetic energy Cl atoms, produced both Cl(2P1/2) and Cl(2P3/2) atomic fragments. The relative branching for this dissociation channel is Cl(2P1/2)/[Cl(2P1/2)+Cl(2P3/2)]=35.5%. The average fraction of available energy imparted into product recoil for the high kinetic energy products was found to be 59%, in qualitative agreement with that predicted by a rigid radical impulsive model. Both the spin–orbit ground and excited chlorine atom angular distributions were close to isotropic. We compare the observed Cl(2P1/2)/[Cl(2P1/2)+Cl(2P3/2)] ratio produced in the electronic predissociation channel of allyl chloride with a prior study of the chlorine atom spin–orbit states produced from HCl photodissociation, concluding that angular momentum recoupling in the exit channel at long interatomic distance determines the chlorine atom spin–orbit branching. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This work is a study of the competition between the two unimolecular reaction channels available to the vinoxy radical (CH2CHO), C[Single_Bond]H fission to form H+ketene, and isomerization to the acetyl radical (CH3CO) followed by C[Single_Bond]C fission to form CH3+CO. Chloroacetaldehyde (CH2ClCHO) was used as a photolytic precursor to the vinoxy radical in its ground state; photodissociation of chloroacetaldehyde at 193 nm produces vinoxy radicals with internal energies spanning the G3//B3LYP calculated barriers to the two available unimolecular reaction channels. The onset of the CH3+CO channel, via isomerization to the acetyl radical, was found to occur at an internal energy of 41±2 kcal/mol, agreeing well with our calculated isomerization barrier of 40.8 kcal/mol. Branching to the H+ketene channel was too small to be detected; we conclude that the branching to the H+ketene channel must be at least a factor of 200 lower than what is predicted by a RRKM analysis based on our electronic structure calculations. This dramatic result may be explained in part by the presence of a conical intersection at planar geometries along the reaction coordinate leading to H+ketene, which results in electronically nonadiabatic recrossing of the transition state. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The primary photodissociation dynamics of allyl chloride upon excitation at 193 nm is investigated in a crossed laser-molecular beam scattering apparatus. Tunable vacuum ultraviolet (VUV) photoionization of the products provides a unique ability to learn about the secondary reaction products of the nascent photoproducts formed. The data show evidence for four significant primary reaction channels: a previously unidentified low kinetic energy C–Cl bond fission channel producing unstable allyl radicals, an excited state C–Cl bond fission channel producing Cl atoms with high translational energy, an HCl elimination pathway releasing significant energy to product translation to HCl and its momentum-matched mass 40 partner, and an HCl elimination channel producing low kinetic energy HCl products and predominantly unstable mass 40 products. The measured branching of these primary reaction channels of [all C–Cl] : [fast C–Cl] : [slow C–Cl] : [fast HCl] : [slow HCl] : [all HCl] is 1.00: 0.971: 0.029: 0.291: 0.167: 0.458 (where fast refers to the high recoil kinetic energy channels). The high internal energy allyl radicals formed in the slow C–Cl fission pathway of allyl chloride further dissociate/isomerize, as do the unstable mass 40 products formed in the HCl elimination pathways, and these products are investigated. Photoionization efficiency (PIE) curves of the HCl product suggest that a three-centered elimination mechanism contributes significantly to an observed HCl elimination reaction. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Translational spectroscopy and ion-imaging experiments are studied in combination to analyze the photodissociation of propargyl chloride at 193 nm and to resolve both the HCl elimination and C-Cl fission product channels. It is seen that the time-of-flight data obtained can be primarily associated with HCl elimination and can give gives limits on how much of this signal may be attributed to C(sub 3)H(sub 3) dissociatively ionizing to C(sub 3)(super plus).

Photofragment translational was used to study the photodissociation of propionyl chloride at 248 nm. The detection of both stable propionyl radicals and the secondary dissociation products CH3CH2 + CO of the unstable radicals is reported.

Measures the photofragment velocity and angular distributions from the photodissociation of nitrogen dimethylformamide. Absorption evidences with three competing dissociation channels; Formation of absorption evidences in a ratio determined by use of trimethylamine as calibrant molecule; Occurrence of nitrogen-carbonyl bond fission on a rapid time scale.

Reports on crossed laser-molecular beam scattering experiments measuring the photofragment velocities and product branching in the photodissociation of trimethylamine at 193 nanometer. Observation of two primary N-CH[sub 3] bond fission channels; Contribution of a third minor channel to the dissociation dynamics; Calibration of mass spectrometry sensitivity.

Examines the jet-cooled emission spectroscopy of gaseous carbon disulfide. Barrier between bent and quasilinear excited state geometries along the predissociative state; Observation of preferential emission to bending and symmetric stretch modes; Information on the energy dependent lifetime of the predissociative state.

The emission spectroscopy of acrolein (C3H4O), acrylic acid (C2H3COOH), and acryloyl chloride (C2H3COCl) excited at 199 nm elucidates the dominant electronic character of the excited state reached by the optical transition at this wavelength. Progressions in the C=C and C=O stretching overtones and various combination bands suggest the antibonding orbital has mixed π*(C=C)/π*(C=O) character. We interpret the results in conjunction with ab initio calculations at the configuration interaction singles level to identify the influence of resonance in the excited state of these conjugated molecules. The results on acrylic acid are of particular interest as excitation in this absorption band produces the HOCO intermediate of the OH+CO→H+CO2 reaction that is important in combustion. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation of CH3SH in the first absorption band is studied via ab initio computation of the relevant potential energy surfaces and exact quantum scattering calculations. The effective valence shell Hamiltonian (Hν) ab initio many-body perturbation technique is used to calculate the global ground X 1A’ and 1 1A‘ surfaces as functions of the C–S and S–H internuclear distances. The finite range scattering wave function (FRSW) time-independent quantum scattering method is used to compute the adiabatic dynamics of S–H and C–S bond fission on the 1 1A‘ surface following excitation. Two calculations are performed, one in which the ground state is represented by a cubic spline function fitted to the ab initio data and another in which it is represented as the sum of two uncoupled Morse oscillators. Absorption spectra as well as the branching ratios and photofragment translational energy distributions corresponding to various excitation energies are presented and compared to recent experimental results. A final calculation examines how the branching ratio and product vibrational state distribution changes for the photodissociation of a CH3SH molecule with one quantum of vibrational excitation in the C–S stretch. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation of 1-bromo-3-iodopropane (1,3-C3H6BrI) at 222 nm is studied with crossed laser-molecular beam experiments. Irradiation at this wavelength excites an n(Br)→σ*(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 n(I)→σ*(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 hence 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 β(C–Br)=1.6±0.4 and β(C–I)=0±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. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper extends the McClain formulation of two-photon absorption to show what observables can be independently determined in an emission experiment for which incident and scattered photon polarizations are resolved. Interelectronic state coupling contributions and quantum interference can be extracted from the polarized emission spectrum by determining the three linearly independent contributions to the scattering intensity. This enhances the value of polarized emission spectroscopy as a tool for understanding transitions between coupled electronic states that occur during dissociation dynamics. We demonstrate that the three linearly independent contributions to the polarized emission spectrum can be determined by performing three polarization experiments, at least one of which must involve circularly polarized light. Furthermore, the three polarization experiments must be performed at a scattering angle other than π/2. We also present sample calculations on a model of the methyl iodide system which serve as an illustration of the general theory and demonstrate the sensitivity of this technique to the details of the potential-energy surfaces. [ABSTRACT FROM AUTHOR]

We measured the emission spectra of CH3I and CD3I excited to a predissociative Rydberg state (E,1){2} near 193.3 nm and tabulated emission line positions with their assignments. The emission spectrum of CH3I both provides previously unobserved energies of ground state combination bands in ν2 (CH3 umbrella) and ν3 (C–I stretch), and also suggests the excited level in the (E,1){2} Rydberg state is the previously unobserved 2132 state. These results are discussed in relation to recent comparisons of UV absorption spectra of monomeric and dimeric CH3I. The CD3I emission spectrum measured results from excitation of the combination mode of CD3 rocking (ν6), umbrella motion (ν2), and C–I stretching (ν3) in the (E,1){2} rydberg state of CD3I, followed by emission to a progression of 2n3161 combination modes in the ground electronic state. A simulation of the line shape of the peaks in the CH3I spectrum shows the width of the peaks results from the contribution of both inhomogeneous (K and J rotational structure) and homogeneous (lifetime) broadening. Emission from molecules after crossing to the repulsive surface was not observed. [ABSTRACT FROM AUTHOR]

The assignment of the Br atom is analyzed and comments are provided. It is felt that the assignment of the Br atom product from the photodissociation of cis-1-bromopropene at 193 nm to spin-orbit excited Br [(super 2) P (sub 1/2)] given in the original paper is incorrect and this comment describes the basis for a revised assignment.

The primary photodissociation dynamics of cis-1-bromopropene upon excitation at 193 nm and the unimolecular dissociation dynamics of the nascent 1-propenyl radical are investigated in a crossed laser-molecular beam apparatus. The lowest-energy dissociation barrier of the 1-propenyl radical is experimentally determined for the first time and is found to be 31.5 plus minus 2.0 kcal/mol, in substantial agreement with the theoretical calculations.

The new technique of crossed laser-molecular beam scattering experiments produce 2-propenyl radicals by photolysis of 2-chloropropene and dispersing the radical produced by internal energy. The dominance of the branching of H+ propylene formation over H+ allene formation for this isomer persists at the higher internal energies in agreement with RRKM predictions.