Molecular Elimination Channels Research Articles

Ultraviolet photodissociation of bromoform at 234 and 267 nm by means of ion velocity imaging

The photochemistry of bromoform is of considerable importance to understanding the impact of short-lived halogen species on bromine chemistry in the atmosphere. In the present work, the products of the ultraviolet photodissociation of bromoform at 234 and 267 nm are determined by time-of-flight mass spectrometry and velocity ion imaging. Both ground Br (2P3/2) and spin–orbit excited Br (2P1/2) atoms are found to be formed via resonance-enhanced multiphoton ionization detection. Radical products are detected via vacuum ultraviolet photoionization at 118 nm. The results indicate that there is a primary molecule bromine elimination channel consisting of CHBr+Br2. The quantum yields for atomic Br and molecular Br2 elimination channels are determined from the time-of-flight spectra to be 0.74 and 0.26 at 234 nm, respectively. At 267 nm, they are 0.84 and 0.16, respectively. Energy and angular distributions are deduced from the 2D images of Br, CHBr, and CHBr2. The direct studies described in this paper on the photodissociation of bromoform suggest that the current atmospheric photochemical models that do not anticipate the formation of Br2 need to be reinvestigated to determine their implications for atmospheric bromine chemistry.

Avoided Curve Crossing between the A1 and B1 States in CF2Br2 Photolysis at 234 and 265 nm

The photodissociation dynamics of CF2Br2 has been studied using a two-dimensional photofragment ion image technique for the two excitation wavelengths 234 and 265 nm. At 234 nm, three dissociation channels, i.e., the radical CF2Br + Br (2PJ) (J = 1/2, 3/2), the three-body CF2 + 2Br (2PJ) (J = 1/2, 3/2), and the molecular elimination channel CF2 + Br2, were observed with quantum yields of 0.84, 0.15, and trace, respectively. The difference between β (CF2Br + Br (2P3/2)) = 0.65 and β (CF2Br + Br (2P1/2)) = 0.80 suggests that the excited A1 and B1 states are strongly correlated by avoided curve crossing with a probability of 0.78. At 265 nm, the radical channel, the observed major primary dissociation channel, shows several components due to the transition close to the curve crossing point. On the basis of observations at both wavelengths, we have proposed a photodissociation dynamics model of CF2Br2 after A-band excitation.

Site-specific dissociation dynamics of ethylene at 157 nm: Atomic and molecular hydrogen elimination

The atomic and molecular hydrogen elimination processes from ethylene have been studied using a molecular beam apparatus. Site and isotope effects on the molecular hydrogen elimination from ethylene have been clearly observed from the photodissociation of ethylene at 157 nm. Experimental results show that there are three different types of molecular elimination processes: 1,1 elimination, 1,2-cis elimination, and 1,2-trans elimination. Significant differences have been detected between 1,1 elimination and 1,2 eliminations in their kinetic energy distributions. Noticeable difference is also found between 1,2-cis elimination and 1,2-trans elimination for molecular deuterium elimination. Branching ratios for atomic and molecular hydrogen elimination processes have also been determined for ethylene and its isotopomers. Isotope and site effects on the branching ratios of different molecular elimination channels have been observed. The experimental results are also compared with recent theoretical studies.

Photodissociation Dynamics of Methanol at 157 nm

Photofragment time-of-flight (TOF) spectra at m/e = 1(H), 2(H2, D), 3(HD), and 4(D2) have been measured for CH3OH, CH3OD, and CD3OH at 157 nm excitation. TOF spectra for photofragments at m/e = 12, 13, 14, 15, 29, 30 from the photodissociation of CH3OH at 157 nm were also obtained. Analysis of these experimental results reveals three different atomic H loss processes: one type of direct hydroxyl H elimination and two types of methyl H elimination. Experimental results also indicate that two molecular hydrogen elimination channels are present: 3-center elimination from the methyl group, which displays two different micro-pathways, and 4-center elimination involving hydrogen atoms on both the C and O sites. The relative branching of the atomic versus molecular hydrogen elimination channels was found to be 1:0.21, indicating that the atomic hydrogen process is still much more significant than the molecular hydrogen process. The C−O bond cleavage channel was also experimentally observed. These results prese...

Competing atomic and molecular hydrogen pathways in the photodissociation of methanol at 157 nm

Photofragment translational spectra at m/e=1(H), 2(H2,D), 3(HD), and 4(D2) have been obtained for CH3OH, CH3OD, and CD3OH at 157 nm excitation. Analysis of the time-of-flight spectra reveals two different atomic H loss channels: hydroxyl H elimination, and methyl H elimination. While the hydroxyl H elimination seems to be a single fast process, the methyl H loss exhibits clearly two significantly different mechanisms: one fast and one slow. Experimental results also show two molecular hydrogen elimination channels: three-center elimination from the methyl group, which displays two different micropathways, and four-center elimination involving hydrogen atoms on both the C and O sites. The relative branching of the atomic versus molecular hydrogen elimination channels was found to be 1:0.15. These results present a uniquely clear picture of methanol photodissociation at 157 nm, and thus provide an excellent case for quantitative theoretical investigations.

Potential energy surface for unimolecular dissociations and rearrangements of the ground state of [C2H3FO] systems

The potential energy surface (PES) of [C2H3FO] systems in its electronic ground state has been investigated using density functional theory method, at the B3LYP/6-311++G(d,p) level. Ten stable intermediates, including acetyl fluoride (1), fluoroacetaldehyde (9), 1-fluorovinyl alcohol (4), 2-fluorovinyl alcohol, carbenes and fluorooxiranes, have been located. Most stationary points on the PES corresponding to the molecular elimination and rearrangement channels from these intermediates have been identified. Ketene (8) is found to be the predominant product in the unimolecular dissociations of 1, 4, 9 and fluorooxirane (6). The most probable channels for ketene formation from acetyl fluoride are 1→8 and 1→4→8. In the reactions of both CH3CO and F radicals, both these processes are energetically feasible for the thermal reactants and hence should lead to a spontaneous emission of vibrationally hot HF. The present PES characterises the CH3CO+F reaction to be a capture-limited association–elimination reaction with a very high and pressure-independent rate coefficient. In addition to its direct decomposition to ketene, 9 can give rise to stable rearrangement products, viz., 2-fluorovinyl alcohol (12) and 6. Fluorooxirane (6) decomposes to ketene through its isomerisation to 9 as intermediate and the present study provides an explanation for the non-observation of this intermediate.

Ab initio quantum chemical study of the formation, decomposition and isomerization of the formaldiminoxy radical (CH 2 NO): comparison of the Gaussian-2 and CASPT2 techniques in the calculation of potential energy surfaces

The reaction between triplet methylene and nitric oxide, producing the formaldiminoxy (CH2NO) radical, and the subsequent decomposition and isomerization reactions of CH2NO have been studied using ab␣initio quantum chemical techniques that include the Gaussian-2 (G2), CASSCF and CASPT2 methods. Stationary points on the potential energy surfaces were located at MP2/6-31G(d) and CASSCF/cc-pVDZ levels of theory, while the electronic energies were determined using G2, G2(MP2), QCISD(T)/cc-pVTZ, RCCSD(T)/cc-pVTZ and CASPT2/cc-pVTZ approaches. G2 is believed to be reliable at equilibrium geometries, but the determination of certain transition state geometries and energies requires a MCSCF-based approach. The calculations suggest that CH2NO (2A′) forms in a barrierless reaction and could readily decompose to H+HCNO. A subsequent abstraction reaction then results in H2+CNO. No molecular elimination channel was found. An alternative pathway is the formation of CH2ON, which readily isomerizes to CH2NO.

Ab initio/RRKM approach toward the understanding of ethylene photodissociation

The optimized structures and harmonic frequencies for the transition states and intermediates on the ground state potential energy surfaces of ethylenes, including C2H4, C2D4, D2CCH2, and cis- and trans-HDCCDH, related to the molecular and atomic hydrogen elimination channels of photodissociation in VUV were characterized at the B3LYP/6-311G(d,p) level. The coupled cluster method, CCSD(T)/6-311+G(3df,2p), was employed to calculate the corresponding energies with the zero-point energy corrections by the B3LYP/6-311G(d,p) approach. Ethylidene was found to be an intermediate in the 1,2-H2 elimination channel. The barrier for the 1,1-H2 elimination was computed to be the lowest (4.10–4.16 eV), while the 1,2-H2 elimination and H loss channels have barriers of a similar height (4.70–4.80 eV). The rate constant for each elementary step of ethylene photodissociation at 193 and 157 nm was calculated according to the RRKM theory based on the ab initio surfaces. The rate equations were subsequently solved, and thus the concentration of each species was obtained as a function of time. The concentrations at t→∞ were taken for calculating branching ratios or yields. In accord with previous experimental findings, the calculated branching ratio for the 1,1-H2 elimination process is higher than that for the 1,2-H2 elimination, and the atomic elimination channel is predicted to be favored at increasing excitation energy when competing with the molecular elimination. The significant discrepancies between theoretical and experimental results in the magnitude of the yields and their dependence on the wavelength for the molecular elimination channels suggest the dynamics of either 1,2-H2, or 1,1-H2 elimination, or both channels may be nonstatistical in nature.

Ab initio calculations of potential energy surface and rate constants for ethylene photodissociation at 193 and 157 nm

The transition states and intermediates of the atomic and two molecular hydrogen elimination channels on the ground state potential energy surface of C 2H 4 have been characterized at the B3LYP/6-311G(d, p) level of calculations. The corresponding energies have been computed by the CCSD(T)/6-311+G(3df, 2p) method. Ethylidene was found to be a likely intermediate in 1, 2 H 2 elimination channel. Rate constants of C 2H 4 photodissociation at 193 and 157 nm have been obtained according to the RRKM theory, based on the results from the ab initio calculations. The study indicates that the hydrogen atom loss channel is favored at higher excitation energy, which is consistent with the experimental observations.

Photodissociation of acrylonitrile at 193 nm: A photofragment translational spectroscopy study using synchrotron radiation for product photoionization

We have investigated the photodissociation of acrylonitrile (H2CCHCN) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used tunable vacuum ultraviolet synchrotron radiation for product photoionization. We have identified four primary dissociation channels including atomic and molecular hydrogen elimination, HCN elimination, and CN elimination. There is significant evidence that all of the dissociation channels occur on the ground electronic surface following internal conversion from the initially optically prepared state. The product translational energy distributions reflect near statistical simple bond rupture for the radical dissociation channels, while substantial recombination barriers mediate the translational energy release for the two molecular elimination channels. Photoionization onsets have provided additional insight into the chemical identities of the products and their internal energy content.

Competing Bond Fission and Molecular Elimination Channels in the Photodissociation of CH3NH2 at 222 nm

This paper presents the first experimental investigation under collisionless conditions of the competing photodissociation channels of methylamine excited in the first ultraviolet absorption band. Measurement of the nascent photofragments' velocity distributions and preliminary measurements of some photofragments' angular distributions evidence four significant dissociation channels at 222 nm: N-H, C-N, and C-H bond fission and H[sub 2] elimination. The data, taken on photofragments from both methylamine and methylamine-d[sub 2], elucidate the mechanism for each competing reaction. Measurement of the emission spectrum of methylamine excited at 222 nm gives complementary information, evidencing a progression in the amino wag (or inversion) and combination bands with one quantum in the methyl (umbrella) deformation or with two quanta in the amino torsion vibration. The emission spectrum reflects the forces in the Franck-Condon region which move the molecule toward a ciscoid geometry. The photofragment kinetic energy distributions measured for CH[sub 3]ND[sub 2] show that hydrogen elimination occurs via a four-center transition state to produce HD and partitions considerable energy to relative product translation. In general, the photofragment angular distributions were anisotropic, but the measured $Beta $APEQ -0.4$POM 0.4 for C-N bond fission indicates dissociation is not instantaneous on the time scale of molecular rotation. 38 refs.,more » 13 figs.« less

An ab initio molecular orbital study of the unimolecular dissociation reactions of di- and trichloroethylene

The potential energy surfaces for the unimolecular ground state elimination reactions of dichloroethylene (DCE) and trichloroethylene (TCE) are studied with ab initio molecular orbital calculations. By the gradient optimization with the second-order Mo/ller–Plesset perturbation (MP2) method and single point calculations with the fourth-order Mo/ller–Plesset perturbation (MP4) and the quadratic single and double configuration interaction including a triple contribution [QCISD(T)], many molecular elimination channels including three- and four-center HCl, H2, and Cl2 elimination and H and Cl migration reactions are systematically examined. For cis- and trans-DCE, the three-center HCl elimination with subsequent chlorovinylidene rearrangement has the lowest overall barrier, whereas for 1,1-DCE for which the three-center path is not available, the four-center elimination has a rather low barrier. Another path starting with the rearrangement of DCE isomers to 1,2-dichloroethylidene followed by HCl elimination is not far in energy from these paths, complicating the overall mechanism of HCl elimination. The H2 elimination from DCE isomers also can take either the three-center path or the 1,2-dichloroethylidene path. For TCE, though the overall barrier to produce HCl+dichloroacetylene is the lowest for the four-center HCl elimination pathway, the production of HCl itself is most easily accomplished by the three-center HCl elimination, and the easiest path for disappearance of TCE is its rearrangement via H or Cl migration to give the 1,2,2-trichloroethylidene intermediate. Thus the overall mechanism of HCl elimination reactions from TCE can also be complicated.

Time Evolution of :CF2 in the IR Multiphoton Dissociation of CF2Cl2: RRKM Considerations

AbstractThe CO2 laser induced multiphoton dissociation of CF2Cl2 takes place in atomic and molecular chlorine elimination channels, with about 8% of the reaction by the latter channel. Time resolved experiments show a fast formation of :CF2 from the primary reaction, followed by a secondary slow growth from the decomposition of CF2Cl. The experimental rates are consistent with the RRKM rate constants with an excitation energy of nearly 343 kJ/mol in CF2Cl2. At high laser fluences, addition of argon has improved the dissociation yield, due to rotational hole filling mechanism. The internal energy of CF2Cl2 decreases from 356 to 343 kj/mol on addition of argon as buffer gas. The nascent vibrational temperature of the CF2 is 950 K, whereas an overall temperature of the thermalized ensemble in milliseconds time is computed to be 480 K.

Competition between Hydrogen Fluoride and Hydrogen Chloride Molecular Elimination Channels in the Infrared Multiple-photon Decomposition of 1,2-Dichloro-1,1-difluoroethane

Abstract Time-resolved infrared emission spectra have been measured in the infrared multiple-photon decomposition of CClF2CH2Cl. The spectra were composed of emissions attributable to vibrationally excited HF* and HCl*. The emission yield of HF* increased rapidly with an increase in the fluence, while the yield of HCl* showed a maximum at 20 J cm−2. These results suggest that the HF and HCl molecular elimination channels compete with each other in the unimolecular decomposition of highly vibrationally excited parent molecules. The stochastic trajectory calculations provided a satisfactory description of the fluence dependences of the HF and HCl yields observed here for CClF2CH2Cl and previously for CHClFCHClF.

Competition between atomic and molecular chlorine elimination in the infrared multiphoton dissociation of CF2Cl2

Infrared multiphoton dissociation of CF2Cl2 has been reinvestigated by the crossed laser-molecular beam technique using a high repetition rate CO2 TEA laser. Both the atomic and molecular chlorine elimination channels were observed: (1) CF2Cl2→CF2Cl+Cl and (2) CF2Cl2→CF2+Cl2. No evidence was found for secondary dissociation of CF2Cl at laser energy fluences up to 8 J/cm2. Center-of-mass product translational energy distributions were obtained for both dissociation channels. In agreement with previous work, the products of reaction (1) are found to have a statistical translational energy distribution. The products of reaction (2) are formed with a mean translational energy of 8 kcal/mol, and the distribution peaks rather sharply about this value, indicating a sizeable exit barrier to molecular elimination. The product branching ratio was directly determined. Reaction (2) accounts for roughly 10% of the total dissociation yield in the fluence range 0.3–8 J/cm2. These results provide an additional test of the statistical theory of unimolecular reactions.

Dynamics and mechanisms of CO production from the reactions of CH2 radicals with O(3P) and O2

A cw CO laser has been used to probe the complete CO vibrational energy distributions in the reactions of CH2 radicals with O(3P) and O2 (3∑−g. The CO formed from the reaction with the atom was concluded to be produced via two equally important channels, O+CH2→CO+H2→CO+2H,, with an average vibrational energy of 19 kcal/mole, and an approximate vibrational temperature of ≈104K (for v≤12), which is sufficient to account for previously observed CO stimulated emission from O(3P)+CH2. The population inversion derives mainly from the molecular hydrogen elimination channel. In the reaction of CH2 with molecular oxygen, the CO formed is believed to be produced from both a molecular elimination path giving H2O+CO and from two possible atomic or radical production processes, yielding H+CO+OH. We find from analysis of the CO product vibrational energy distribution that 30% of the CO formed is produced via the former path, and 70% is formed from the latter path. This information is important to the detailed kinetic modeling of acetylene and other hydrocarbon combustion systems.

The photochemistry of acrylonitrile vapour at 213.9 nm

The photolysis of acrylonitrile vapour in the π* ← n transition was carried out at 213.9 nm. Evidence is presented for two molecular elimination channels, one to yield acetylene and hydrogen cyanide (Ф = 0.50), the other to yield cyanoacetylene and hydrogen (Ф = 0.31). Experiments on acrylonitrile-1-d gave evidence that molecular elimination proceeds by α–β elimination following rapid intramolecular scrambling of deuterium by H/D migration in the excited state.

Pulsed CO 2 laser photolysis of CF 2Cl 2

The pulsed infrared laser photolysis of CF 2Cl 2 yields primarily (more than 85%) the CF 2Cl free radical and atomic chlorine. Somewhat smaller amounts (less than 15%) of CF 2 and molecular chlorine are produced in a competing primary process. Depending on experimental conditions, e.g. pressure and intensity, secondary processes can occur that can obscure the primary chemistry. For example, at low intensity the CF 2Cl radical can thermally dissociate to yield CF 2 plus chlorine atoms while at high intensity the CF 2Cl radical can undergo reaction with chlorine atoms to yield CF 2 and molecular chlorine. Quantitative measurements can be made of the relative importance of the primary atomic or molecular chlorine elimination channels under conditions where secondary removal of CF 2Cl is entirely eliminated. Under these conditions, the relative importance of these two channels does not depend on the intensity of the laser or on the laser wavelength. The overall chemistry occurring in this complex system is evaluated in the absence as well as in the presence of atomic and free-radical scavengers using conventional end-product analysis techniques.

Pulsed CO 2 laser photolysis of CF 2Cl 2

The pulsed infrared laser photolysis of CF 2Cl 2 yields primarily (more than 85%) the CF 2Cl free radical and atomic chlorine. Somewhat smaller amounts (less than 15%) of CF 2 and molecular chlorine are produced in a competing primary process. Depending on experimental conditions, e.g. pressure and intensity, secondary processes can occur that can obscure the primary chemistry. For example, at low intensity the CF 2Cl radical can thermally dissociate to yield CF 2 plus chlorine atoms while at high intensity the CF 2Cl radical can undergo reaction with chlorine atoms to yield CF 2 and molecular chlorine. Quantitative measurements can be made of the relative importance of the primary atomic or molecular chlorine elimination channels under conditions where secondary removal of CF 2Cl is entirely eliminated. Under these conditions, the relative importance of these two channels does not depend on the intensity of the laser or on the laser wavelength. The overall chemistry occurring in this complex system is evaluated in the absence as well as in the presence of atomic and free-radical scavengers using conventional end-product analysis techniques.