Elimination Of Hydrogen Atom Research Articles

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.

Dynamics of Atomic and Molecular Hydrogen Elimination from Hydrocarbons at VUV Excitation

AbstractElimination of atomic hydrogen (H) and molecular hydrogen (H2) are important elementary chemical processes in photochemistry and combustion chemistry. Recently, unique and sensitive detection techniques for atomic and molecular hydrogen detection were developed in our laboratory. Using the advanced molecular beam methods, we have studied the photodissociation of a few typical hydrocarbons at 157 nm excitation, especially their atomic and molecular hydrogen elimination processes. In this report, we will briefly describe the results from photodissociation of propane, ethylene, propyne and methanol at 157 nm excitation. These molecules represent different classes of hydrocarbons such as alkane, alkene, alkyne and alcohol. Through careful studies on differently deuterated compounds, clear pictures of selective atomic and molecular hydrogen elimination processes can be constructed for all of the above compounds. These results will help us to understand the dissociation dynamics of the small hydrocarbon molecules.

Hydrogen atom elimination from polycyclic aromatic hydrocarbons with sustained off-resonance irradiation: a new approach to produce carbon/hydrogen cluster cations C nH x+

The stepwise elimination of hydrogen atoms from nine polycyclic aromatic hydrocarbons (PAHs) was investigated by the collision-induced dissociation of the sustained off-resonance irradiation (SORI-CID) technique of Fourier transform ion cyclotron resonance mass spectrometry. In every case the application of sequential SORI-CID resulted in the predominant loss of a single hydrogen atom from the parent ions, and for coronene, perylene, and benz[a]anthracene, this eventually produced pure carbon cluster ions C n ·+. No carbon cluster ions were observed for small PAHs because of competing fragmentations or insufficient intensities of the parent ions. As a common feature of the larger polycyclic aromatic hydrocarbons studied, hydrogenated carbon cluster ions C n H 5 + were produced smoothly after elimination of five hydrogen atoms (from pyrene, C 16H 10, and fluorene, C 13H 10) or of seven hydrogen atoms (from coronene, C 24H 12, perylene, C 20H 12, benz[a]anthracene, C 18H 12, chrysene, C 18H 12, and triphenylene, C 18H 12). However, uncomplicated further loss of hydrogen atoms to produce cluster ions C n H 4 ·+ (or C n H 3 +) was observed only for benz[a]anthracene, chrysene, and triphenylene. The different dissociations of C n H x + with odd and even numbers of hydrogen atoms and the difficulty to form C n H x + (0 ≤ x ≤ 4) by multisteps of single hydrogen elimination are discussed. The latter observation may be attributed to characteristically different structures of hydrogenated carbon cluster ions C n H x + with x ≥ 5 and x ≤ 4.

Photodissociation of propyne and allene at 193 nm with vacuum ultraviolet detection of the products

Vacuum ultraviolet (VUV) laser photoionization is combined with time-of-flight (TOF) mass spectrometry to determine the photofragments produced from the laser photodissociation of allene and propyne in a molecular beam. Detection of C3H3+ confirms that atomic hydrogen elimination is the primary process for both of these molecules. A hydrogen molecule elimination channel and a low mass carbon fragmentation channel of allene to produce C3H2+H2 and CH2+C2H2, respectively, have also been identified. Different ratios of various dissociation channels from these two molecules suggest that the dissociation mechanisms of these two isomers are different. Dissociation must occur before complete isomerization. These results are discussed in terms of recent theoretical calculations on the ground and excited states of these molecules. Secondary photodissociation of the products has been observed, even though the laser energies that have been used are less than 8 mJ/cm2 and the photolysis laser is not focused. Therefore, the present results show how important it is to determine product distributions as a function of the laser energy.

Water and hydrogen atom elimination from ionised n-propanol: extraordinarily large kinetic isotope effects

Hydrogen atom loss from ionised n-propanol occurs specifically from the α-carbon atom and is subject to a kinetic isotope effect of approximately 500∶1; somewhat smaller, though still very large, isotope effects are manifested in water loss, which is initiated by an essentially site-specific 1,4-hydrogen transfer.

Crossed molecular beam studies of the O(1D)+CH4 reaction: Evidences for the CH2OH+H channel

The O(1D)+CH4 reaction has been investigated using a new universal crossed molecular beam apparatus. Hydrogen atom elimination channel has been experimentally observed in this reaction. The pattern of dissociative ionization of the reaction products (from hydrogen loss channel) was compared with that of the methoxy (CH3O) radical produced from the photodissociation of CH3OH at 193 nm which has almost the same total energy deposition as the bimolecular reaction O(1D)+CH4. The experimental results suggest that the main hydrogen loss channel in the O(1D)+CH4 reaction should be CH2OH (hydroxymethyl)+H, while the CH3O (methoxy)+H channel is at most a minor reaction channel. This study provides an excellent experimental example of different dynamical behaviors exhibited in the unimolecular and bimolecular reactions of an essentially same chemical system (excited CH3OH) since the O(1D)+CH4 reaction likely occurs through the insertion mechanism.

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.

Gas-Phase Reactivity of (C5H5Mg)+Complexes: An Experimental and Theoretical Study

The gas-phase reactivity of C5H5Mg+ complexes generated from ionized magnesocene (Cp)2Mg•+ have been investigated by means of tandem mass spectrometry. Decomposition of metastable C5H5Mg+ ions involves the metal−ligand dissociation and a hydrogen atom elimination leading to the formation of Mg+ and C5H4Mg•+ ions. Collisional activation induces further fragmentations, yielding C3H3Mg+ and C3H2Mg•+ ions. To have some insight into the structure and energetics of these four series of complexes, high-level ab initio calculations (G2(MP2)) have been performed. In addition, the nature of the metal ion−ligand interaction, the charge distribution, as well as the characterization of orbitals involved in relevant systems have been analyzed by using the atoms in molecules theory (AIM) of Bader and the natural bond orbital (NBO) analysis. The Cp•···Mg+ binding energy has been estimated to be 74.0 kcal/mol, over the cyclopentadienyl geometry optimized at the CASSCF level both for 2B1 and 2A2 electronic states.

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.

Primary and secondary processes in the 193 nm photodissociation of vinyl chloride

We have investigated the photodissociation of vinyl chloride (H2CCHCl) 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 vacuum ultraviolet synchrotron radiation for product photoionization. We have observed five primary dissociation channels following an initial π*←π excitation. The majority of Cl atoms originate from an excited-state dissociation. The remaining dissociation channels are consistent with competition on the ground electronic state following internal conversion from the optically prepared state. These channels include atomic and molecular hydrogen elimination, HCl elimination, and a translationally slow Cl elimination channel. We have also identified and characterized two secondary decomposition channels: (1) the elimination of Cl from chlorovinyl radicals following the primary atomic hydrogen elimination channel, and (2) hydrogen atom elimination from vinyl radicals following the primary atomic Cl elimination. By measuring the truncation in the translational energy distribution for C2H2Cl products from primary atomic hydrogen elimination we deduce a barrier for the reverse reaction of Cl+acetylene of 11±2 kcal/mol. Since Cl is known to add rapidly to acetylene with no activation barrier, we conclude that H loss primarily forms the ClCCH2 isomer, and that the observed 11 kcal/mol barrier pertains to a concerted addition/rearrangement path to form the α-chlorovinyl radical. Finally, we report low-resolution photoionization spectra for the nascent vinyl radical and HCl photoproducts, in which redshifts in the ionization onsets can be related to the internal energy content.

An unconventional cracking method for hydrocarbon compounds and their derivatives: 1. Liquefaction of polyolefins

Liquefaction of polypropylene, polyethylene and polystyrene by oxidative degradation with gas oil as a carrier and phenol as a catalyst was performed in an autoclave. Air pressure, temperature and reaction time were varied. The advantage of this process is the production of non-oxygenated lighter compounds, mainly saturated hydrocarbons, from the polyolefins. The optimum cracking conditions were 0.7 MPa air pressure, 400°C and 30 min reaction, yielding 32 wt% gasoline, 15 wt% kerosene and 27 wt% gas oil from the degraded oil of polypropylene. These products had petroleum-like properties according to ASTM and IP test methods. Furthermore the oils obtained can be used as chemical feedstocks. From the results, a reaction mechanism is proposed which postulates (1) thermal elimination of hydrogen atoms from hydrocarbon molecules via interaction between phenol and oxygen molecules (initial step), (2) degradation of the resulting hydrocarbon macroradicals through β-scission, producing relatively low-molecular-weight end-chain radicals and (3) free radical transfer reactions via phenoxy radicals, with steps (2) and (3) producing competitive acceleration of the hydrocarbon degradation in a random scission mechanism.

Unraveling the dissociation of dimethyl sulfoxide following absorption at 193 nm

We have studied the photodissociation of dimethyl sulfoxide, DMSO-h6 and DMSO-d6, at 193 nm using the technique of photofragment translational spectroscopy with a tunable vacuum ultraviolet product probe provided by undulator radiation on the Chemical Dynamics Beamline at the Advanced Light Source. In contrast to previous investigations we have found the dissociation to proceed via a stepwise mechanism involving multiple reaction channels. The primary dissociation, S–C bond cleavage to eliminate a methyl radical, was found to have two competing channels with distinct translational energy distributions. The translational energy distribution for the major primary dissociation channel suggests that it proceeds in a statistical manner on the ground electronic surface following internal conversion. In competition with this channel is a primary dissociation that exhibits a translational energy distribution suggestive of dissociation on an excited electronic surface with most of the available energy partitioned into translational and electronic degrees of freedom. Secondary decomposition of the CD3SO intermediate was found to proceed exclusively via C–S bond cleavage, CD3SO→CD3+SO. However, secondary decomposition of the CH3SO intermediate was found to exhibit competition between CH3SO→CH3+SO and CH3SO→CH2SO+H. The dissociation to CH3 and SO was the major secondary decomposition channel with the translational energy distribution indicating a barrier to recombination of >8 kcal/mol. While a minor hydrogen atom elimination channel was found to play a role in secondary decomposition of CH3SO intermediates, no analogous secondary C–D bond cleavage was detected from the CD3SO intermediates indicating the importance of tunneling in the secondary decomposition of CH3SO.

Byproduct identification and mechanism determination in plasma chemical decomposition of trichloroethylene

Plasma chemical behavior of trichloroethylene (TCE) was investigated with a packed-bed ferroelectric pellet reactor and a pulsed corona reactor. Volatile byproducts were identified by gas chromatography and mass spectrometry (GC-MS), and it was shown that reactor type, TCE concentration, flow rate, background gas, and moisture affected TCE decomposition efficiency and product distribution. Byproduct distributions in nitrogen and the negative effect of oxygen and moisture on TCE decomposition efficiency show that TCE decomposition proceeds via initial elimination of chlorine and hydrogen atoms, the addition of which to TCE accelerates its decomposition. Active oxygen species like OH radical is less likely involved in the initial step of TCE decomposition in plasma. Triplet oxygen molecules (/sup 3/O/sub 2/) scavenge intermediate carbon radicals derived from TCE decomposition to give much lower yields of organic byproducts.

Ion/molecule reactions of naphthalene and 1-chloronaphthalene radical cations

Reactions of the naphthalene radical cation, C 10H 8 ·+, and the 1-chloronaphthalene radical cation, C 10H 7Cl ·+, with a series of small organic molecules including amines, alcohols, ethers and simple carbonyl compounds have been studied by the selected ion flow tube (SIFT) technique. Observed reactions for the naphthalene cation include the associated adduct with hydrogen atom elimination from the reactant ion or neutral, association without elimination and charge transfer. Reaction products of the naphthalene cation were identified by running isotopomeric reactions with deuterated naphthalene. Reactions of the chloronaphthalene cation are association with hydrogen atom, chlorine atom or hydrogen chloride molecule elimination from the adducts, association without elimination and charge transfer. Reaction channels depend on the ionization energy of the reactant and vary in the above order with decreasing ionization energy of the neutral. C 10H 7Cl ·+ is generally more reactive than C 10H 8 ·+. Most of the reactions are very slow except for charge transfer reactions with trimethylamine and triethylamine. No proton transfer channel takes place for any of the primary reactions. A naphthyne containing ion adduct is formed in the reaction of C 10H 7Cl ·+ with some molecules due to HCl elimination from the primary adduct.

The ultraviolet photodissociation dynamics of pyrrole

Photofragment translational spectroscopy was used to study the photodissociation of pyrrole at 193 and 248 nm under collision-free conditions. Five primary dissociation channels were observed at 193 nm. Two channels resulted from cleavage of the NH bond to yield H + pyrrolyl radical with one channel following internal conversion (IC) to the ground state (≈21%) and the other originating from electronically excited pyrrole (≈30%). Two dissociation channels involved elimination of HCN following IC. One channel producing HCN + vinylmethylene (≈25%) following ring opening and hydrogen migration and the other proceeding via a bridged 3H-pyrrole intermediate to form HCN+cyclopropene (≈24%). The last channel at 193 nm involved IC to the ground state followed by ring opening and NC bond cleavage to form NH+CHCCHCH 2 (<1%). At 248 nm three dissociation channels were observed, all of which involved the elimination of atomic hydrogen. Analogous to the results at 193 nm, two of these channels resulted from cleavage of the NH bond with one channel following IC (≈42%) and the other dissociating from an excited electronic state (≈47%). The third dissociation channel at 248 nm involved the cleavage of one of the two CH bonds in electronically excited pyrrole (≈11%). Translational energy distributions were determined for all observed dissociation channels. From consideration of the maximum translational energy of the photofragments D 0(NH) =88±2 kcal/mol, D 0(CH) = 112.5±1 kcal/mol and Δ H r(pyrrolyl radical) = 62±2 kcal/mol were determined.

ENDOR Spectroscopic Study of Macroradicals in Irradiated Oriented Nylon-6

Abstract Macroradicals formed by hydrogen atom elimination in uniaxially and biaxially oriented nylon-6 have been investigated at low temperature using a Q-band ENDOR technique. Hyperfine coupling constants were obtained from the observed 1H ENDOR spectra for the protons located at α-, β-, and γ-positions relative to the unpaired electron in the radical -CO-NH-CH-CH2-CH2-. Principal values for α-proton anisotropic coupling tensor have been determined. A degree of angular disorientation of the radical centers -CH- along the axes of biaxially oriented polymer was evaluated. The protons of the β-methylene group were found to be magnetically nonequivalent and therefore the plane of the free carbon was twisted. The deviation from the ideally planar transconfiguration corresponds to the rotation around the Cα-Cβ bond on average angle 11°. A change in β-proton ENDOR spectrum has been revealed under mechanical loading of oriented specimens. The results obtained evidenced for be appearance of reversible torsional ...

Mass spectrometry of transition metal π‐complexes. 40—(η4‐Norbornadiene)‐(η5‐cyclopentadienyl)rhodium derivatives

AbstractElectron impact mass spectra of substituted norbornadienecyclopentadienylrhodium complexes have been studied. The decompositions of molecular ions are (i) metal–ligand bond rupture, (ii) norbornadiene skeleton destruction, (iii) substituent or hydrogen atom elimination from the norbornadiene moiety and (iv) radical or neutral molecule elimination from the substituent(s) on the diene ligand. Norbornadiene ligand loss occurs more easily than loss of the cyclopentadienyl ring. Elimination of C2H2R and C2H3 from the diene ligand leads to the formation of rhodocenium or its corresponding substituted derivatives. It is proposed that bridging CC bond rupture followed by migration of the endo‐H atom around the six‐membered ring precedes the loss of the substituent. Some decomposition pathways of the norbornadiene ligand are explained in terms of specific interaction between the metal atom and substituent.

Stereospecific elimination of hydrogen atoms with opposite absolute orientations during the biosynthesis of orsellinic acid from chiral malonates in Penicillium cyclopium

(R)-[1-13C; 2-2H]Malonate and (S)-[1-13C; 2-2H]malonate are transformed, using succinyl-CoA transferase, into their paired chiral malonyl-CoA derivatives; incorporation of the malonyl-CoA derivatives into orsellinic acid is acomplished using homogeneous orsellinic acid synthase from Penicillium cyclopium; mass spectrometric analysis of the orsellinic acid reveals that the hydrogen atoms eliminated from the methylene groups at the 2- and 4-positions of the putative polyketide intermediate are from opposite absolute orientations in malonyl-CoA.

Dissociation of cyclohexene and 1,4-cyclohexadiene in a molecular beam

Molecular-beam photofragmentation translational spectroscopy of cyclohexene and 1,4-cyclohexadiene was carried out using 193 nm and IR multiphoton excitation. At 193 nm, both the retro-Diels–Alder reaction of cyclohexene and H2 elimination from both molecules were observed in the ground electronic state, indicating the occurrence of internal conversion from the initially excited electronic states. The retro-Diels–Alder reaction is shown to be concerted up to an internal energy higher than 148 kcal/mol. Hydrogen-atom elimination was also observed for both molecules following 193 nm excitation. The H atom is eliminated from an excited state of cyclohexene and is assigned to be from a carbon adjacent to the double bond, with a corresponding C–H bond energy of 87±3 kcal/mol. It is shown that the peak of the translational energy distribution for concerted dissociation in the ground state is determined mainly by the dynamics of the potential-energy release along the reaction coordinate, and is not sensitive to either the amount of internal energy or the form of excitation.

High pressure photochemistry of alkenes. 6. the caee of 1-methylcyclopentene at 184.9 and 147.0 nm

The degradaiion of the 184.9 nm photoexcited 1-methyl-1-cyclopentene molecule shows the involvement of various excited isomers. The most important fragmentation products are 1- and 2-methyl-1,3-cyclopentadiene. These products are probably the result of a one-step elimination process of a hydrogen molecule. This process has also been observed in the case of the 184.9 nm photochemistry of cyclopenten. On the other hand, the involvement of isomers, although possible, is not so obvious at 147.0 nm. Moreover, in this case, the 1- and 2-methyl-1,3-cyclopentadiene formation is the result of hydrogen atom elimination in a two-step process. Cyclopentadiene, ethylene, and various C3 and C4 compounds are formed as well as methyl radicals and hydrogen atoms. These products are probably formed in successive elimination reactions as this is also observed in acyclic alkenes.