Article

Thermal energy reactions of H+3 and H3O+ with a series of small organic molecules

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Abstract

A flowing afterglow has been used to determine the rate coefficients and ion product distributions for the reactions of H+3 and H3O+ with the small organic molecules CH3OH,CH3CHO, C2H5OH, (CH3)2O, (CH3)2 CO and (C2H5)2O at 300 K. The rate coefficients for all the reactions are close to the average dipole orientation collisional values, indicating that the reactions have unit efficiency. However, the products for the H+3 and H3O+ reactions are very different. The H3O+ reactions proceed almost totally by non-dissociative proton transfer, as dictated by reaction energetics. The H+3 reactions are much more exothermic and non-dissociative proton transfer is not the only process, the reactions being dominated by many dissociative processes leading to the production of ions such as CH+3, HCO+, H3CO+, C2H+5 and C2H5O+. The data are compared with previous measurements where possible, and the mechanisms of the reactions are discussed.

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... 4. Previous works have provided information on the rate coefficients and product BRs of the reactions of DME and MF with H + 3 and HCO + (e.g. Tanner et al. 1979;Lee et al. 1992;Lawson et al. 2012), as well as on the dissociative recombination with electrons of methoxymethyl and protonated DME cations (e.g. Hamberg, M. et al. 2010), so that a comparison is possible. ...
... Dimethyl ether: Rate coefficients and BRs for the reaction of H 3 + with DME have been measured using a flowing afterglow technique at 300 K (Lee et al. 1992) and the results are as follows: the overall rate coefficient is 4.7 × 10 −9 cm 3 s −1 and the proton transfer is highly dissociative giving the following fragments, with % BRs in brackets: CH 3 + (29%), CH 5 + (8%), HCO + /C 2 H 5 + (10%), CH 3 O + (26%), C 2 H 5 O + (15%), CH 3 OHCH 3 + (12%). The KIDA data is at odds with these findings, as it exclusively quotes protonated DME (CH 3 OHCH 3 + ) as a product with k = 3.01 × 10 −9 cm 3 s −1 at 298 K (from the modified Arrhenius equation (1) with parameters α=3.00 × 10 −9 , β=−0.50, γ=0.00). ...
... Hereafter, we use the total α=4.70 × 10 −9 , BRs as given by Lee et al. (1992), β=−0.50 and γ=0.00 over the 10-300 K temperature range. In order to avoid introducing new trace ions into the network, we will consider only the two most abundant channels, namely those giving CH + 3 and CH 3 O + as products, and the channel relative to the undissociative proton-trasfer leading to CH 3 OHCH + 3 plus H 2 (already included in the models with a different rate coefficient). ...
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Chapter
This chapter describes the experimental and analytical techniques that have been developed for flowing afterglow applications to the quantitative study of ion–neutral reaction processes. Most other techniques for the measurement of ion–molecule reaction rate constants are inherently unsuited for the examination of an ion reacting with a neutral where the neutral has the lower ionization potential, and sufficient data for this generalization did not exist prior to the flowing afterglow results. Charge transfer reactions of negative ions have sometimes been useful in establishing relative electron affinities of molecules, which are often difficult to measure. Positive ion charge-transfer reactions have, on occasion, been useful in establishing relative ionization potentials of molecules, generally known or better measured in more direct ways. The dc discharge had other advantages over the microwave discharge as well. Its geometrical configuration was more compatible with the detailed flow analysis, and it was more easily incorporated in metal flow tubes, which were soon found to be advantageous.
Chapter
This chapter focuses on the role of Selected Ion Flow Tube (SIFT) in examining ion-neutral reactions. The SIFT technique is a natural extension of the Flowing Afterglow (FA) technique. It builds on and extends the versatility of the FA for the study of ion-neutral reactions under truly thermal conditions. Under this technique, the ions enter a quadrupole mass filter that can be set to pass ions of a given mass-to-charge ratio. These mass selected ions are injected at low energy via a small aperture into a flow tube along which they are convected by a fast-flowing carrier gas at a pressure of typically 0.5 Torr. In the SIFT, the ions are created in a remote ion source and not in the carrier gas-reactant-gas mixture. Thus, crucially, the ion source gas is excluded from the carrier gas and therefore from the reaction zone. Reactions, therefore, between the primary ions and their source gas cannot occur. Various types of ion source have been used in SIFT experiments, the choice depending on the ion types required. The simplest to use are low-pressure sources because they do not offer excessive gas loading to the diffusion pump. They can be built directly into the SIFT chamber.
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Article
The reactions of H3O(+) associated with n H2O ions with CH2O and CH3OH have been studied in flowing afterglow and flow drift reaction systems. Hydrated protonated formaldehyde, CH2O4(+) associated with n H2O, reacts with H2O to form H3O(+) hydrates and CH2O for small n. Therefore protonated formaldehyde ions are not expected to have significant atmospheric concentrations. Methanol reacts rapidly with H3O(+) associated with n H2O ions for n = 0, 1, 2, and 3. The absence of protonated methanol ions in the atmosphere above 40 km, where H3O(+) associated with n H2O (n = 0, 1, 2, and 3) are observed to be the dominant ions, yields a useful upper limit on the atmospheric concentration of methanol.
The reactions of H3+ ions with CH4, NH3, H2O, H2S, C2H2, C2H4, and C2H6 are investigated using ion-cyclotron resonance methods. Except for the reaction H3+H2O, the product distributions for these reactions are highly dependent on the amount of excess vibrational energy in H3+ ions formed by the reaction H2+ + H2. Both the product distributions for the initial distribution of highly vibrationally excited H3+ ions, and the product distributions for the ground state ions, are found by performing experiments as a function of H2 pressure. The rate constant for the deactivation of the initial distribution of excited H3+ ions to ground state ions in collisions with H2 is found to be (2.7±0.6) × 10−10 cm3/sec. The rate constants for the reaction of H3+ ions with the various molecules studied are shown to be independent of excess ion vibrational energy and to agree closely with values calculated using the average-dipole-orientation theory [11].
The proton transfer reaction of H3+ ions with CH3NH2, CH3OH and CH3SH produces the excited intermediate complexes [CH3NH3+]★, [CH3,OH2+]★, and [CH3SH2+]★ which subsequently decompose in two ways; by vicinal hydrogen elimination, and by C-X bond scission to give the methyl cation. The condensation reaction of methyl cations with NH3, H2O, and H2S is the reverse of the latter decomposition pathway and proceeds to give the same excited intermediates followed also either by vicinal hydrogen elimination or by back reaction. The H2+ ions formed by reaction of H2+ with H2 are highly excited, and the presence of excess internal energy in the H3+ ion is shown to be a critical factor in the ratio of decomposition products obtained. A detailed analysis of the deactivation of excited H3+ is made. The rate for deactivation of excited H3+ ions by H2 is measured and the internal energy in H3+ obtained as a function of H2 pressure. Quasiequilibrium calculations are applied to the unimolecular decomposition of the excited intermediate complexes formed in the proton transfer and methyl cation condensation reactions. The experimental product distribution as a function of H3+ internal energy is readily reproduced by quasiequilibrium theory. It is shown that barriers of 2.6, 1.1 and < 1.4 eV are required for vicinal hydrogen elimination from [CH3NH3+]★, and [CH3SH2+]★, respectively. These barriers significantly reduce the apparent exothermicities of the CH3+ condensation reactions and, in particular, the apparent exothermicity for the CH3+-H2O reaction is reduced to near zero. In addition, it appears possible that the excess energy may not be completely randomized before dissociation occurs in the complex resulting from the condensation reactions.
The reactions of H3+ with C2H6, CH3NH2, CH3OH, CH3F, CH3SH, and CH3Cl have been studied using ion cyclotron resonance pulse ejection techniques. The product distribution obtained is strongly dependent upon hydrogen pressure due to a large difference in reactivity between excited and ground state H3+ ions. The H3+ ions originally formed by the reaction of H2+ with H2 are highly excited. At low hydrogen pressures, these excited H3+ ions react mainly by direct processes: by charge transfer and by a process equivalent to hydride ion abstraction. The product distribution changes as the hydrogen pressure is raised due to rapid deactivation of the H3+ ions by collisions with H2 molecules. At intermediate hydrogen pressures, the hydride ion abstraction process disappears and both ground state and partially deactivated H3+ ions react principally by proton transfer to give a longer-lived protonated intermediate. With the exception of ethane, decomposition of the protonated intermediate occurs via vicinal hydrogen elimination and except for C2H6, CH3SH, and CH3Cl, CX bond scission to give the methyl cation is observed as well. From the dependence of the relative rate for charge transfer on hydrogen pressure, evidence is obtained indicating that the excited H3+ ions may not be directly deactivated to the ground state but to some intermediate state(s) still containing a significant amount of internal energy, and that these intermediate H3+ ions are subsequently only very slowly deactivated to the ground state.
Article
The dynamics of the reactions H3+(Ar,H2)ArH+ and ArH+(H2,Ar)H3+ were studied over the initial relative translational energy from 0.87 to 9.7 eV and from 0.18 to 6.7 eV, respectively. The reactions were found to proceed via direct mechanisms at all energies studied. Energetic data are presented that suggest H3+ is not rapidly relaxed vibrationally upon collision with H2 as previously thought. Rather than being relaxed within a few collisions, the lower vibrational states probably are relaxed with a rate constant on the order of 10−12 cm3 molecule−1⋅sec−1.
Article
The flowing afterglow technique has been used to study the reactions of H3+ with a number of neutral reactants at thermal energies. Proton transfer was the only primary reaction observed with N2, CO, CO2, N2O, NO, CH4, C2H2, H2O, and NH3. Both proton transfer and dissociative charge transfer were observed with C2H4 and C2H6, while dissociative charge transfer is the exclusive primary process with NO2. Secondary reactions were observed with NO, C2H6, C2H4, and C2H2. Cluster ions were formed between NO+ and NO2 and H2O, between H3O+ and H2O, CO2, and CO, and between NH4+ and NH3 and H2O. Proton transfer was also observed between HN2+ and CO2, N2O, CH4, and H2O, and between HO2+ and H2 and N2. Rate constants were obtained for these reactions and are discussed. Limits could be placed on the proton affinity (P.A.) of H2 from the failure to observe rapid proton transfer to O2 and the observation of proton transfer to N2. These indicate 4.2 < P.A. (H2)< 4.7 eV with a recommended value of 4.4 eV. The technique can be used to measure relative proton affinities of gases.
Article
The rate coefficients and product ion distributions for the binary reactions of H3O+⋅(H2O)0,1,2 and D3O+⋅(D2O)0,1,2 ions with D2O and H2O, respectively, and with NH3 have been studied at 300 K using a selected ion flow tube (SIFT) apparatus. The ions were created in a flowing afterglow ion source and after mass filtering were injected at low energy into the SIFT. All the reactions proceeded at or near the gas kinetic limit. In the D2O and H2O thermoneutral isotopic exchange reactions, the distribution of H and D amongst the product ions and neutrals was seen to be purely statistical. This implies that these reactions proceed via the formation of an intermediate long‐lived association ion in which total randomization of the H and D atoms takes place prior to unimolecular decomposition. No appreciable isotopic exchange occurred in the exothermic NH3 reactions which apparently proceeded via the simpler mechanisms of D+ (or H+) or D3O+ (or H3O+) transfer. The differing mechanisms for the H2O and D2O reactions compared to the NH3 reactions are rationalized in terms of the thermicities of the reactions and the lifetimes of the respective intermediate ions.
Article
Symmetry of the lowest‐lying internal states of H3+ and the selection rules governing transitions following electron capture by these states are considered. H3+ is a constituent of interstellar medium and lowest‐lying states at temperatures of 3–30°K play a significant role.(AIP)
Article
Revised time-dependent models of the ion-molecule chemistry of dense interstellar clouds (Graedel, Langer, and Frerking, 1982) are used to calculate the abundances of key observational species used to interpret cloud properties and study interstellar chemistry. Consideration is given to nitrogen-, oxygen-, and carbon-bearing molecules and isotopic ratios over a hydrogen density range of 1,000-50,000/cu cm and a temperature range of 10-40 K. The results for over 15 species, including C-13 variants, are presented in tables and graphs. The results are compared to abundance observations in TMC-1 and other interstellar clouds, showing that most observed abundances can be predicted by the models.
Article
The rate coefficient for the ion-molecule radiative association reaction CH3(+) + CH3CHO - (CH3)2CHO(+) has bee calculated in the range 10-300 K with the phase-space techique and the aid of a laboratory measurement of the analogous three-body association at room temperature. It has been suggested by Combes et al. (1987) that this reaction followed by dissociative recombination is responsible for the observed abundance of acetone (CH3COCH3) in Sgr B2. However, it is shown here that the radiative association reaction is probably too slow even at 10 K to lead to the observed abundance of acetone in this source. The question of how acetone is produced in Sgr B2 is thus still unanswered.
Article
Accurate ab initio calculations of the ground and excited H3 potential energy surfaces have been carried out. These surfaces have been examined at geometries appropriate for an analysis of the reaction products of e+H3+ dissociative recombination. It is found that direct recombination of electrons with H3+ ions in their ground vibrational state is very unlikely for low collisional energies. The authors estimate that the currently accepted values of the e+H3+ recombination coefficient for interstellar conditions are two orders of magnitude too large.
The detailed mechanism of formation of protonated dimethyl ether in ionized gaseous methanol has been investigated with a flowing afterglow-triple quadrupole instrument. The structures and unimolecular decomposition reactions of isomeric cluster ions related to those observed in ionized methanol have been examined as models for the proposed bimolecular reaction intermediates. Results from isotope labeling experiments provide evidence for backside nucleophilic attack in the reaction between CH3OH+2 and CH3OH, producing (CH3)2OH+ and H2O. Collision-induced dissociation of the protonated methanol dimer and trimer results in both desolvation and formation of protonated dimethyl ether, the latter presumably via rearrangement of the collisionally activated proton-bound cluster ion to the backside displacement precursor. Both CH3OH)2H+ and (CH3OH)3H+ undergo inefficient but exothermic bimolecular displacement reactions with CH3OH, producing (CH3)2OH+ (CH3OH) and CH3)2OH+ (CH3OH)2, respectively.
A tandem mass spectrometer has been used to measure the ion-product distribution for reaction of H3+ with CH3OH as an explicit function of the average number of H3+-H2+ collisions occurring before reaction. As the number of H3+-H2+ collisions is changed from zero to several, large changes in the product distribution are observed and demonstrate the effectiveness of such collisions in deactivating the vibrationally excited H3+ ions. The present results suggest that 5–10H3+-H2 collisions are required to deactivate the H3+. Isotope studies suggest that reaction occurs initially by proton transfer to give a CH3OH2+ ion which, if energetically possible, either eliminates H2 or cleaves the CO bond. Because the present experiments are performed under conditions of low pressure and long reaction time, observation of the CH3OH2+ product is used to show that the H2 product of the initial protonation reaction carries away a disproportionately large share of the enthalpy of reaction as either vibrational or translational energy.
Article
The reaction of H3+ with ethylene oxide and acetaldehyde yields the following products: H3+ + C2H4O → C2H4O+ + H2 + H (charge exchange), C2H3O+ + 2H2 (hydride abstraction), or [C2H5O+]* + H2 (proton transfer); [C2H5+]* → C2H4O+ + H2, HCO+ + CH4, C2H3+ + H2O, or H3O+ + C2H2. The charge-exchange reaction and hydride-ion-abstraction reaction occur only for H3+ molecules with a large amount of internal energy. Variations in the product distribution with H2 pressure allow investigation of the importance of vibrational energy in H3+ on the reaction dynamics. Comparison is made with variations of the product distribution as a function of H3+ translational energy. The qualitative effects are similar for both vibrationally and translationally excited H3+. The charge-transfer rate increases with energy, as does the hydride-ion-transfer rate. The proton-transfer rate decreases. These energy-dependent studies coupled with the isotopic distributions from D3+-C2H4O and H3+-C2D4O (and their energy dependence) allow a clear elucidation of the detailed mechanism of decomposition of excited [C2H5O+]* ions to yield C2H3O+, HCO+, C2H3+, and H3O+ ions. Comparison is made with data of other workers.
Article
Previous ab initio studies of deuterated H+3 molecular ions are extended to include rotational modes for the zero-point states of vibration. Rotation energies are obtained using direct numerical diagonalization of vibration—rotation hamiltonian matrices, and nuclear wavefunctions as superpositions of mode-coupled anharmonic rotationless vibrators and related prolate symmetric top eigenfunctions. Relevance to recent searches for interstellar H2D+ is noted.
Microwave-afterglow measurements of the electron-temperature dependence of dissociative electron—ion recombination coefficients are subject to some recently discovered complications arising from non-uniformities of the microwave heating fields, inelastic collisions of electrons with molecular additives, and effects of vibrational excitation of ions and neutrals. This paper presents the results of recent experimental and theoretical work and examines consequences for earlier experimental data on electron—ion recombination. It appears that, in some cases, the electron temperatures attained by microwave heating were actually lower than had been thought. As a consequence, the inferred dependences on electron temperature of the recombination coefficients were weaker than the true dependences.
Article
The chemical mechanisms that lead to the formation and destruction of molecules in the interstellar gas are reviewed. Grain surface catalysis is discussed briefly. Detailed attention is given to gas-phase chemical processes and to the modifications they produce in the molecular compositions of interstellar clouds following the initial molecular formation. The effects of chemical fractionation processes on the abundances of molecules containing different isotopes are explored. The review was completed in March 1976.
Article
Interstellar absorption-line spectroscopy of NGC 2264 is reported which shows that the CO molecule has a column density of 5 x 10 to the 18th/sq cm and a rotational excitation temperature of 28 K. A direct upper limit on the H2 column density implies that at least 6 percent of a solar carbon abundance is in the form of CO. The upper limit on the H3(+) abundance implies that the cosmic-ray ionization rate is of the order of 10 to the -16th/s or less. The H3(+) upper limit, together with a previous radio detection of H2D(+) emission, implies either an enormous overabundance of the deuterated molecule or else that most of the radio emission comes from clouds not located directly between use and the infrared source. Observations of the sources AFGL 2591 and NGC 2024 IRS2 indicate that upper limits on H3(+) imply cosmic ray ionization rates of less than 3 and 60 x 10 to the -17th/s, respectively.
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