Elimination Channel Research Articles

Branching ratio of gas-phase reaction of CH3C(O)CHCl2 + OH: A theoretical dynamic study

A theoretical study on the atmospheric reaction of hydroxyl radical with CH3C(O)CHCl2 is investigated. Geometries and frequencies calculation have been performed at the MP2/cc-pVDZ level of theory for all stationary points, and energy values have been improved by single-point calculations using the BMC-CCSD level. Two different reaction channels have been found, abstraction and addition–elimination channels. The rate constants for this reaction have been calculated within the temperature range of 200–360K at atmospheric pressure by means of canonical variational transition state theory incorporating with the small-curvature tunneling correction. The overall rate constant of the CH3C(O)CHCl2+OH reaction turns out finally to be the sum of the H-abstraction and OH-addition rate constant. It was found that the contributions of the addition–elimination channel to the overall reaction can be ignore, suggesting that in the upper troposphere and lower stratosphere, this channel will be less important. The reaction proceeds through the H abstraction exclusively from chloromethyl group within the whole range of 200–360K, which may be relevant to atmospheric chemistry. Our results show that the variational effect is small and the small-curvature tunneling effect is only important in the lower temperature range. Agreement between the calculated and experimental data available at 298K is good.

Effects of intramolecular hydrogen bonding on the excited state dynamics of phenol chromophores

The theoretical prediction and experimental confirmation of the 1πσ* excited state of phenol which is repulsive along the O-H bond has a large impact on the interpretation of phenol and tyrosine photochemistry. In this work, we demonstrate that this excited state changes significantly if the OH functional group is involved in the formation of an intramolecular hydrogen bond in the ground state. We investigate the excited state dynamics of 2-, 3-, and 4-hydroxyacetophenone (HAP) separately in a molecular beam at 193 nm using multimass ion imaging techniques. H atom elimination from the repulsive excited state and Norrish type I reactions are the major dissociation channels of 3-HAP and 4-HAP which do not have intramolecular hydrogen bonding. However, the H atom elimination channel is completely quenched for 2-HAP which shows intramolecular hydrogen bonding. In addition, the ground state and the excited state potential energy surfaces (PESs) of HAP, 2-hydroxybenzoyl fluoride, 2-hydroxybenzoyl chloride, and 2-hydroxybenzamide are investigated using ab initio calculations. The results also show that the excited state potential along the O-H bond distance of the hydroxyl group changes significantly for molecules with intramolecular hydrogen bonding. The changes include: (a) the repulsive potential energy surface becomes an attractive potential near the ground state equilibrium geometry, (b) the conical intersection between the first and the second excited states along the O-H bond moves to a much higher energy level, and (c) the conical intersection between the repulsive excited state and the ground state along the O-H bond distance disappears. The results suggest that the interpretation of the photochemistry for molecules with a phenol chromophore must take these effects into consideration.

Molecular elimination of Br2 in photodissociation of CH2BrC(O)Br at 248 nm using cavity ring-down absorption spectroscopy

The primary elimination channel of bromine molecule in one-photon dissociation of CH(2)BrC(O)Br at 248 nm is investigated using cavity ring-down absorption spectroscopy. By means of spectral simulation, the ratio of nascent vibrational population in v = 0, 1, and 2 levels is evaluated to be 1:(0.5 ± 0.1):(0.2 ± 0.1), corresponding to a Boltzmann vibrational temperature of 581 ± 45 K. The quantum yield of the ground state Br(2) elimination reaction is determined to be 0.24 ± 0.08. With the aid of ab initio potential energy calculations, the obtained Br(2) fragments are anticipated to dissociate on the electronic ground state, yielding vibrationally hot Br(2) products. The temperature-dependence measurements support the proposed pathway via internal conversion. For comparison, the Br(2) yields are obtained analogously from CH(3)CHBrC(O)Br and (CH(3))(2)CBrC(O)Br to be 0.03 and 0.06, respectively. The trend of Br(2) yields among the three compounds is consistent with the branching ratio evaluation by Rice-Ramsperger-Kassel-Marcus method. However, the latter result for each molecule is smaller by an order of magnitude than the yield findings. A non-statistical pathway so-called roaming process might be an alternative to the Br(2) production, and its contribution might account for the underestimate of the branching ratio calculations.

Photodissociation Dynamics of Halogenated Thiophenes at 235 nm: A Resonance Enhanced Multiphoton Ionization-Time-of-Flight (REMPI-TOF) Study

The photodissociation dynamics of halogen-substituted thiophenes, namely, 2-chlorothiophene and 2-bromo-5-chlorothiophene, has been studied in a supersonic molecular beam around 235 nm, using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary halogen atoms. A single laser has been used for excitation of halothiophenes, as well as for the REMPI detection of photoproducts, namely, chlorine and bromine atoms, in their spin-orbit states X((2)P(3/2)) and X*((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio, for chlorine and bromine atom elimination channels. State-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies, β(ι). The TOF profiles for Cl, Cl*, Br, and Br* are found to be independent of laser polarization; i.e., the β is well characterized by a value of ~0.0, within the experimental uncertainties. For 2-chlorothiophene, we have observed two components for the Cl and only one component for the Cl* atom elimination channel in the translational energy distributions. The average translational energies for the fast and the slow components of the Cl channel are 3.0 ± 1.0 and 1.0 ± 0.5 kcal/mol, respectively. For Cl*, the average translational energy is 3.5 ± 1.0 kcal/mol. For 2-bromo-5-chlorothiophene, we have observed only one component for Cl, Cl*, Br, and Br* in the translational energy distributions. The average translational energies for the Cl and Cl* channels are 3.5 ± 1.0 and 5.0 ± 1.0 kcal/mol, respectively, whereas the average translational energies for the Br and Br* channels are 2.0 ± 1.0 and 3.5 ± 1.0 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. The ΔH(f)(298) value for 2-chlorothiophene has been estimated theoretically to be 23.5 kcal/mol.

Isomeric C5H11Si+ ions from the trimethylsilylation of acetylene: An experimental and theoretical study

The gas phase offers a unique medium to conduct the electrophilic addition reaction of (CH3)3Si+ (trimethylsilylium ion) to acetylene. However, this deceptively simple reaction displays a remarkable dependence on the gas phase pressure, revealing the interplay of competitive pathways. In FT-ICR mass spectrometry at ca 10−8mbar, the nascent (CH3)3Si+–acetylene complex undergoes a rearrangement process yielding the CH2C(CH3)Si(CH3)2+ ion. This structure has been assigned on the basis of the ion-molecule reactivity displayed by the sampled C5H11Si+ adducts, matching the one of the model ion obtained from 2-(trimethylsilyl)propene. Whereas the absolute values of kinetic rate constants could not discriminate between isomeric species, the branching ratios for competitive addition–elimination channels in the reaction with i-C3H7OH and t-C4H9OCH3 were found to be diagnostic of different structures. The pathways leading from the (CH3)3Si+–acetylene complex primarily formed to the candidate C5H11Si+ isomers have been investigated by ab initio quantum chemical calculations at CCSD(T)/6-311++G(2d,2p)//B3LYP/6-311G(2d,p) level. The energy profiles show that the path to the CH2C(CH3)Si(CH3)2+ isomer is associated to the lowest activation energy barrier, below the reactants energy level. The energy released in the (CH3)3Si+–acetylene association process, remaining stored in the complex formed at low pressure, thus allows the isomerization to a species holding the positive charge on electropositive silicon. Interestingly, the most stable of the conceivable isomers, (E)-(CH3)CHCHSi(CH3)2+, is not accessed because of an activation energy barrier protruding above the reactants energy level. The combined information of ion-molecule reactivity and ab initio calculations of potential isomers and rearrangement pathways has thus afforded a comprehensive view of the (CH3)3Si+ addition reaction to acetylene under various pressure regimes.

Resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) study of phosphorous oxychloride (POCl3) dissociation at 235 nm: Dynamics of Cl(2Pj) formation

In one-color REMPI-TOF experiment, the photodissociation dynamics of POCl3 has been studied by photolyzing POCl3 and probing the chlorine atom photofragments, namely, Cl(2P3/2) and Cl∗(2P1/2) using 2+1 REMPI scheme, in the 234–236nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter, and the spin–orbit branching ratio for chlorine atom elimination channels. The anisotropy parameters for Cl and Cl∗ are the same, and characterized by a value of 0.0±0.05. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl∗. The average translational energies for the Cl and Cl∗ channels for the fast components are 12.5±1.5 and 16.8±1.5kcal/mol, while, for the slow components, the average translational energies are 1.5±1.0 and 2.5±1.0kcal/mol, respectively. Apart from the chlorine atom elimination channel, Cl2 elimination is also observed in the photodissociation of POCl3.

The OH-initiated atmospheric oxidation of divinyl sulfoxide: A theoretical investigation on the reaction mechanism

The potential energy surfaces for the OH+divinyl sulfoxide reaction in the presence of O2/NO are theoretically characterized at the CCSD(T)/6-311+G(d,p)//BH&HLYP/6-311++G(d,p)+ZPE level of theory. Various possible pathways including the direct hydrogen abstraction channels and the addition–elimination channels are considered. The calculations show that the exclusive feasible entrance channel is the formation of adduct CH2(OH)CHS(O)CHCH2 (IM1) in the initial reaction pathways. In the atmosphere, the newly formed adduct IM1 can further react with O2/NO to form the dominant products HCHO+C(O)HS(O)CHCH2 (P9). The calculated results confirm the experimental studies.

Infrared fluorescence from multiphoton dissociation of vinyl bromide: Emission from the products and the parent molecule

Abstract A time-resolved study of the infrared fluorescence emission produced in the infrared multiphoton dissociation of vinyl bromide at different experimental conditions has been carried out. Two different regimes, namely unimolecular and collisional, have been identified and characterized through the parent vinyl bromide and the HBr and C 2 H 2 photoproducts emissions. In the pressure interval studied, the formation of HBr has been determined to be mainly unimolecular, while the C 2 H 2 is produced through a collisionally assisted photodecomposition process. The collisional regime is explained in terms of the 3-centre elimination channel while the unimolecular regime is ascribed to the 4-centre mechanism. Relaxation rate constants for both the vibrationally excited HBr by vinyl bromide and by Ar and for vibrational excited VBr by unexcited VBr, are reported.

Ultraviolet photodissociation dynamics of trichloroethylene at 235 nm

The photodissociation dynamics of trichloroethylene was investigated near 235 nm, (π,π*) transition, by detecting the nascent products, Cl (2P3/2) and Cl* (2P1/2), via 2 + 1 resonance enhanced multiphoton ionization. The photofragment speed distribution, the recoil anisotropy parameter β and the spin–orbit branching ratio for chlorine atom elimination channels were determined from time of flight profiles in polarization experiments. Based on the dynamical information, the features of potential energy surface along the reaction coordinate were discussed.

Product Detection of the CH Radical Reaction with Acetaldehyde

The reaction of the methylidyne radical (CH) with acetaldehyde (CH(3)CHO) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of CHBr(3) and react with acetaldehyde in an excess of helium and nitrogen gas flow. Five reaction exit channels are observed corresponding to elimination of methylene (CH(2)), elimination of a formyl radical (HCO), elimination of carbon monoxide (CO), elimination of a methyl radical (CH(3)), and elimination of a hydrogen atom. Analysis of the photoionization yields versus photon energy for the reaction of CH and CD radicals with acetaldehyde and CH radical with partially deuterated acetaldehyde (CD(3)CHO) provides fine details about the reaction mechanism. The CH(2) elimination channel is found to preferentially form the acetyl radical by removal of the aldehydic hydrogen. The insertion of the CH radical into a C-H bond of the methyl group of acetaldehyde is likely to lead to a C(3)H(5)O reaction intermediate that can isomerize by β-hydrogen transfer of the aldehydic hydrogen atom and dissociate to form acrolein + H or ketene + CH(3), which are observed directly. Cycloaddition of the radical onto the carbonyl group is likely to lead to the formation of the observed products, methylketene, methyleneoxirane, and acrolein.

High-Pressure Rate Rules for Alkyl + O2 Reactions. 1. The Dissociation, Concerted Elimination, and Isomerization Channels of the Alkyl Peroxy Radical

The reactions of alkyl peroxy radicals (RO(2)) play a central role in the low-temperature oxidation of hydrocarbons. In this work, we present high-pressure rate estimation rules for the dissociation, concerted elimination, and isomerization reactions of RO(2). These rate rules are derived from a systematic investigation of sets of reactions within a given reaction class using electronic structure calculations performed at the CBS-QB3 level of theory. The rate constants for the dissociation reactions are obtained from calculated equilibrium constants and a literature review of experimental rate constants for the reverse association reactions. For the concerted elimination and isomerization channels, rate constants are calculated using canonical transition state theory. To determine if the high-pressure rate expressions from this work can directly be used in ignition models, we use the QRRK/MSC method to calculate apparent pressure and temperature dependent rate constants for representative reactions of small, medium, and large alkyl radicals with O(2). A comparison of concentration versus time profiles obtained using either the pressure dependent rate constants or the corresponding high-pressure values reveals that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2).

Branching Fractions of the CN + C3H6 Reaction Using Synchrotron Photoionization Mass Spectrometry: Evidence for the 3-Cyanopropene Product

The gas-phase CN + propene reaction is investigated using synchrotron photoionization mass spectrometry (SPIMS) over the 9.8-11.5 eV photon energy range. Experiments are conducted at room temperature in 4 Torr of He buffer gas. The CN + propene addition reaction produces two distinct product mass channels, C(3)H(3)N and C(4)H(5)N, corresponding to CH(3) and H elimination, respectively. The CH(3) and H elimination channels are measured to have branching fractions of 0.59 ± 0.15 and 0.41 ± 0.10, respectively. The absolute photoionization cross sections between 9.8 and 11.5 eV are measured for the three considered H-elimination coproducts: 1-, 2-, and 3-cyanopropene. Based on fits using the experimentally measured photoionization spectra for the C(4)H(5)N mass channel and contrary to the previous study (Int. J. Mass. Spectrom.2009, 280, 113-118), where it was concluded that 3-cyanopropene was not a significant product, the new data suggests 3-cyanopropene is produced in significant quantity along with 1-cyanopropene, with isomer branching fractions from this mass channel of 0.50 ± 0.12 and 0.50 ± 0.24, respectively. However, similarities between the 1-, 2-, and 3-cyanopropene photoionization spectra make an unequivocal assignment difficult based solely on photoionization spectra. The CN + CH(2)CHCD(3) reaction is studied and shows, in addition to the H-elimination product signal, a D-elimination product channel (m/z 69, consistent with CH(2)CHCD(2)CN), providing further evidence for the formation of the 3-cyanopropene reaction product.

Photodissociation of acryloyl chloride in the gas phase

The 193 nm photodissociation dynamics of CH2CHCOCl in the gas phase has been examined with the technique of time-resolved Fourier transform infrared emission (TR-FTIR) spectroscopy. Vibrationally excited photofragments of CO (ν ⩽ 5), HCl (ν ⩽ 6), and C2H2 were observed and two photodissociation channels, the C-Cl fission channel and the HCl elimination channel have been identified. The vibrational and rotational state distributions of the photofragments CO and HCl have been acquired by analyzing their fully rotationally resolved ν→ν−1 rovibrational progressions in the emission spectra, from which it has been firmly established that the mechanism involves production of HCl via the four-center molecular elimination of CH2CHCOCl after its internal conversion from the S1 state to the S0 state. In addition to the dominant C-Cl bond fission along the excited S1 state, the S1→S0 internal conversion has also been found to play an important role in the gas phase photolysis of CH2CHCOCl as manifested by the considerable yield of HCl.

Kinetics and mechanism of the reaction of fluorine atoms with trifluoroacetic acid

The kinetics of the reaction between fluorine atoms and trifluoroacetic acid has been studied experimentally at T=258–343K. The overall reaction rate constant decreases with temperature: k1(T)=1.09×10−12 exp((+1034K)/T) cm3molecule−1s−1. The potential energy surface of the reaction has been studied using quantum chemistry. The results were used in transition state theory calculations of the temperature dependences of the rate constants of the two channels of the reaction. The abstraction channel ultimately producing HF is dominant at the experimental temperatures; the addition–elimination channel producing CF3 and CF(O)OH becomes important above 1000K.

Computational study on the reaction of CH3SCH2CH3 with OH radical: mechanism and enthalpy of formation

The reaction mechanism of CH3SCH2CH3 with OH radical is studied at the CCSD(T)/6-311+G(3df,p)//MP2/6-31+G(2d,p) level of theory. Three hydrogen abstraction channels, one substitution process and five addition–elimination channels are identified in the title reaction. The result shows hydrogen abstraction is dominant. Substitution process and addition–elimination reactions may be negligible because of the high barrier heights. Enthalpies of formation [ $$ \Updelta_{f} H_{(298.15{\text{K}})}^{o} $$ ] of the reactants and products are evaluated at the CBS-QB3, G3 and G3MP2 levels of theory, respectively. It is found that the calculated enthalpies of formation by the aforementioned three methods are in consistent with the available experimental data. Rate constants and branching ratios are estimated by means of the conventional transition state theory with the Wigner tunneling correction over the temperature range of 200–900 K. The calculation shows that the formations of P1 (CH2SCH2CH3 + H2O) and P2 (CH3SCHCH3 + H2O) are major products during 200–900 K. The three-parameter expressions for the total rate constant is fitted to be $$ k_{\text{total}} = 1.45 \times 10^{ - 21} T^{3.24} \exp ( - 1384.54/T) $$ cm3 molecule−1 s−1 from 200 to 900 K.

Dynamics of Cl (2Pj) Atom Formation in the Photodissociation of Fumaryl Chloride (ClCO − CH = CH − COCl) at 235 nm: A Resonance Enhanced Multiphoton Ionization (REMPI) Time-of-Flight (TOF) Study

The photodissociation dynamics of fumaryl chloride (ClCO-CH═CH-COCl) has been studied in a supersonic molecular beam around 235 nm using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique by detecting the nascent state of the primary chlorine atom. A single laser has been used for excitation of fumaryl chloride and the REMPI detection of chlorine atoms in their spin-orbit states, Cl ((2)P(3/2)) and Cl* ((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, β(i). The TOF profiles for both Cl and Cl* are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distribution, P(E(T)), of Cl and Cl* atoms, and assigned to be formed from different potential energy surfaces. The average translational energies for the fast components of the Cl and Cl* channels are 14.9 ± 1.6 and 16.8 ± 1.6 kcal/mol, respectively. Similarly, for the slow components, the average translational energies of the Cl and Cl* channels are 3.4 ± 0.8 and 3.1 ± 0.8 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. Apart from the chlorine atom elimination channel, molecular hydrogen chloride (HCl) elimination is also observed in the photodissociation process. The HCl product has been detected, using a REMPI scheme in the region of 236-237 nm. The observation of the molecular HCl in the dissociation process highlights the importance of the relaxation process, in which the initially excited parent molecule relaxes to the ground state from where the molecular (HCl) elimination takes place.

Ab Initio and RRKM Study of the HCN/HNC Elimination Channels from Vinyl Cyanide

Ab initio CCSD and CCSD(T) calculations with the 6-311+G(2d,2p) and the 6-311++G(3df,3pd) basis sets were carried out to characterize the vinyl cyanide (C(3)H(3)N) dissociation channels leading to hydrogen cyanide (HCN) and its isomer hydrogen isocyanide (HNC). Our computations predict three elimination channels giving rise to HCN and another four channels leading to HNC formation. The relative HCN/HNC branching ratios as a function of internal energy of vinyl cyanide were computed using RRKM theory and the kinetic Monte Carlo method. At low internal energies (120 kcal/mol), the total HCN/HNC ratio is about 14, but at 148 kcal/mol (193 nm) this ratio becomes 1.9, in contrast with the value 124 obtained in a previous ab initio/RRKM study at 193 nm (Derecskei-Kovacs, A.; North, S. W. J. Chem. Phys.1999, 110, 2862). Moreover, our theoretical results predict a ratio of rovibrationally excited acetylene over total acetylene of 3.3, in perfect agreement with very recent experimental measurements (Wilhelm, M. J.; Nikow, M.; Letendre, L.; Dai, H.-L. J. Chem. Phys.2009, 130, 044307).

The Dynamics of Allyl Radical Dissociation

Dissociation of the allyl radical, CH(2)CHCH(2), and its deuterated isotopolog, CH(2)CDCH(2), have been investigated using trajectory calculations on an ab initio ground-state potential energy surface calculated for 97,418 geometries at the coupled cluster single and double and perturbative treatment of triple excitations, with the augmented correlation consistent triple-ζ basis set level (CCSD(T)/AVTZ). At an excitation energy of 115 kcal/mol, corresponding to optical excitation at 248 nm, the primary channel is hydrogen loss with a quantum yield of 0.94 to give either allene or propyne in a ratio of 6.4:1. The total dissociation rate for CH(2)CHCH(2) is 6.3 × 10(10) s(-1), corresponding to a 1/e time of 16 ps. Methyl and C(2)H(2) are produced with a quantum yield of 0.06 by three different mechanisms: a 1,3 hydrogen shift followed by C-C cleavage to give methyl and acetylene, a double 1,2 shift followed by C-C cleavage to give methyl and acetylene, or a single 1,2 hydrogen shift followed by C-C cleavage to give methyl and vinylidene. In this last channel, the vinylidene eventually isomerizes to give internally excited acetylene, and the kinetic energy distribution is peaked at much lower energy (6.4 kcal/mol) than that for the other two channels (18 kcal/mol). The trajectory results also predict the v-J correlation, the anisotropy of dissociation, and distributions for the angular momentum of the fragments. The v-J correlation for the CH(3) + HCCH channel is strongest for high rotational levels of acetylene, where v is perpendicular to J. Methyl elimination is anisotropic, with β = 0.66, whereas hydrogen elimination is nearly isotropic. In the hydrogen elimination channel, allene is rotationally excited with a total angular momentum distribution peaked near J = 17. In the methyl elimination channel, the peak of the methyl rotational distribution is at J ≈ 12, whereas the peak of the acetylene rotational distribution is at J ≈ 28.

Gas‐Phase Photodissociation of CH3CHBrCOCl at 248 nm: Detection of Molecular Fragments by Time‐Resolved FT‐IR Spectroscopy

By employing time-resolved Fourier transform infrared emission spectroscopy, the fragments HCl (v=1-3), HBr (v=1), and CO (v=1-3) are detected in one-photon dissociation of 2-bromopropionyl chloride (CH(3)CHBrCOCl) at 248 nm. Ar gas is added to induce internal conversion and to enhance the fragment yields. The time-resolved high-resolution spectra of HCl and CO were analyzed to determine the rovibrational energy deposition of 10.0±0.2 and 7.4±0.6 kcal mol(-1), respectively, while the rotational energy in HBr is evaluated to be 0.9±0.1 kcal mol(-1). The branching ratio of HCl(v>0)/HBr(v>0) is estimated to be 1:0.53. The bond selectivity of halide formation in the photolysis follows the same trend as the halogen atom elimination. The probability of HCl contribution from a hot Cl reaction with the precursor is negligible according to the measurements of HCl amount by adding an active reagent, Br(2), in the system. The HCl elimination channel under Ar addition is verified to be slower by two orders of magnitude than the Cl elimination channel. With the aid of ab initio calculations, the observed fragments are dissociated from the hot ground state CH(3)CHBrCOCl. A two-body dissociation channel is favored leading to either HCl+CH(3)CBrCO or HBr+CH(2)CHCOCl, in which the CH(3)CBrCO moiety may further undergo secondary dissociation to release CO.

Photodissociation and photoionization of 2,5-dihydroxybenzoic acid at 193 and 355 nm

Photodissociation and photoionization of 2,5-dihydroxybenzoic acid (25DHBA), at 193 and 355 nm were investigated separately in a molecular beam using multimass ion imaging techniques. Two channels competed after excitation by one 193 nm photon. One channel is dissociation from the repulsive excited state along O-H bond distance, resulting in H atom elimination from meta-OH functional group. The other channel is internal conversion to the ground state, followed by H(2)O elimination. Some of the fragments further proceeded to secondary dissociation. On the other hand, absorption of one 355 nm photon gave rise to H(2)O elimination channel on the ground state. Absorption of more than one 355 nm photon resulted in the three-body dissociation which also occurs on the ground state. Dissociation on the excited state does not play a role at 355 nm. The large concentration ratio (2×10(5)), between neutral fragments and cations produced from 355 nm multiphoton excitation indicates that internal conversion followed by dissociation, is the major channel for 355 nm multiphoton excitation. Multiphoton ionization is a minor channel. Multiphoton ionization of 25DHBA clusters only produces 25DHBA cations. Neither anion nor protonated 25DHBA cation were observed. It is very different from the ions produced from solid matrix-assisted laser desorption/ionization (MALDI), experiments. This suggests that protonated 25DHBA and negatively charged 25DHBA generated in MALDI experiments does not simply result from the ionization following proton transfer reactions or charge transfer reactions of the clusters in the gas phase.