Thomas O. Tiernan

Theoretical and experimental studies of the N2O− and N2O ground state potential energy surfaces. Implications for the O−+N2→N2O+e and other processes

The ground state potential energy surface of the nitrous oxide negative ion is characterized and related to that of the neutral molecule by a synergetic theoretical–experimental approach. Ab initio multiconfiguration self‐consistent‐field/configuration interaction (MCSCF/CI) and other calculations for N2O−(X 2A′) yield the minimum energy geometry (ReNN, ReNO, AeNNO) = (1.222±0.05 A, 1.375±0.10 A, 132.7±2°), the vibrational frequencies (ν1,ν2,ν3) = (912±100 cm−1, 555±100 cm−1, 1666±100 cm−1), the dipole moment μ =2.42±0.3 D, and other properties. The N2O− molecular ion in the X 2A′ state is found to have a compact electronic wavefunction—one with very little diffuse character. The MCSCF/CI bending potential energy curve from 70° to 180° for the X 1Σ+(1 1A′) state of N2O as well as the bending curve for the X 2A′ state of N2O− are also reported. The dissociation energy D (N2–O−) =0.43±0.1 eV and, thus, the adiabatic electron affinity E.A.(N2O) =0.22±0.1 eV and the dissociation energy D (N–NO−) =5.1±0.1 eV a...

Determination of the Abundance of Excited O+ Ions in Beams Produced by Electron Impact on O2, CO2, N2O, NO2, and H2O

The fraction of excited ions present in O+ ion beams produced by impact of 60‐eV electrons on various oxygenated molecules has been determined approximately 23 μsec after formation. The technique employed involves the measurement of apparent rate coefficients for charge‐transfer reactions of O+ to N2 and CO at very low kinetic energies (∼0.3 eV) in a tandem mass spectrometer. Under these conditions, charge transfer of O+(4S) is endothermic and the formation of N2+ and CO+ products is due solely to excited O+ ions in the reactant beam. By normalizing the rate coefficients for the reactions of O+ from the several sources to that for an O+ beam for which the excited ion abundance is known, the fractional abundance of O+* in each of the respective ion beams is calculated. These fractions for O+ beams from the indicated sources were found to be: CO2, 0.04; N2O, 0.43; NO2, 0.34; H2O, 0.90. The fractional abundance data are utilized for the study of the reactions of both ground‐state and excited‐state O+with Ar ...

Collision‐Induced Dissociation of NO+ and O2+ at Low Kinetic Energies: Effects of Internal Ionic Excitation

The reactions resulting in dissociation of NO+ and O2+ upon impact with various atoms and molecules have been investigated in a double mass spectrometer of in‐line geometry in the reactant ion kinetic energy range below 50 eV. Cross sections measured for these processes range from about 0.02 to 0.3 A2. The dependence of the cross section upon kinetic energy is markedly affected by the energy of the ionizing electrons which produce the reactants. At the lowest electron energy, Ee = 13 eV, kinetic‐energy thresholds are observed for the production of N+ from the NO+/NO reaction, and for O+ production from the O2+/Ar reaction, which are in excellent agreement with known adiabatic dissociation energies for the ground‐state molecule ions. The production of O+ from the NO+/NO interaction is shown to involve a rearrangement process rather than simple collision‐induced dissociation. At higher electron energies, the cross‐section dependence plots exhibit structure which can be interpreted in terms of the participat...

Electron Affinities from Endothermic Negative‐Ion Charge‐Transfer Reactions. II. O2

Electron‐transfer reactions of negative ions were studied utilizing an in‐line tandem mass spectrometer. Cross sections were measured as a function of translational energy for the reactions of D−, O−, S−, SH−, Cl−, Br−, and I− with O2. All of these reactions exhibit thresholds, their cross sections rising sharply with increasing energy above onset. The low energy tails of the electron‐transfer cross sections which are observed are shown to be caused by Doppler broadening of the effective center‐of‐mass energy, owing to the thermal motion of the target molecules. Center‐of‐mass energy distributions were calculated for several of the reactions (O−, S−, Br−, and I−) using an approximate one‐dimensional treatment of the collisions. These energy distributions were convoluted with various trial threshold functions, and the calculated curves were fitted to the experimental cross sections to obtain the true energy thresholds. Within the limits of error of this method, thresholds derived for the various reactions ...

Dissociative Charge‐Exchange Reactions of Rare‐Gas Ions with Propane

Dissociative charge‐exchange reactions of rare‐gas ions with propane and with partially deuterated propanes have been investigated using a modified commercial mass spectrometer. The experimental results for propane are in good agreement with data previously obtained using much more sophisticated instrumentation. Charge‐exchange spectra obtained with 2,2‐dideuteropropane and with 1,1,1,3,3,3‐hexadeuteropropane permit the determination of details of the fragmentation mechanism. The data suggest that both s‐propyl and n‐propyl ions are formed by charge exchange, but that the latter is a relatively unimportant process. Ethylene is formed chiefly by 1,3 elimination of methane from the molecule ion, but 1,2 elimination becomes relatively more important with increasing recombination energy of the rare‐gas ion. In contrast to observations on the vacuum uvphotolysis of deuterated propanes, the present data indicate that elimination of molecular hydrogen as HD is of substantial importance in the over‐all ionic decomposition of both compounds.

Proton‐Transfer Reactions in the Simple Alkanes: Methane, Ethane, Propane, Butane, Pentane, and Hexane

Proton‐transfer reactions from CHO+ and CDO+ to the simple alkanes, methane through hexane, have been investigated in a tandem mass spectrometer. Acetaldehyde is shown to be the molecular source yielding CHO+ reactant with the lowest internal excitation energy. The translational energy dependence observed for the reactions CHO++CH4→CX5++CO, CX5+→CX3++X2 (X=H or D) suggests that the interaction is better represented over the energy range considered as proton stripping rather than as proceeding through an intermediate [CXO+–CX4]* complex. In addition, there is no evidence from isotope effects for a transition in mechanism. A highly simplified model treating the protonated methane species as a collection of closely coupled harmonic oscillators permits application of the classic rate expression for unimolecular decomposition to the dissociation yielding CH3+, and this relation provides a reasonable semiempirical fit to the experimental data. The reaction of CHO+ with ethane at quasithermal energy yields C2H7+...

Ionic Reactions in Gaseous Cyclobutane

Ionic reactions in gaseous cyclobutane have been examined at pressures up to 1 torr in a specially designed ion source fitted to a time‐of‐flight mass spectrometer. It was observed that C4H8+ and C4H7+ are unreactive with cyclobutane, although both of these ions appear to be produced by reactions of other fragment ions. These conclusions were confirmed by experiments in a tandem mass spectrometer in which the reactions of individual ions of the cyclobutane system were identified at impacting ion kinetic energies as low as 0.3 eV. It was shown that ethylene and acetylene ions charge transfer to cyclobutane with high efficiency while the remaining fragment ions react principally by a hydride‐transfer mechanism. Isotopic techniques were employed to aid in establishing the reaction mechanisms. The interactions of cyclobutane ions with various molecules either employed as additives or formed as decomposition products in radiolysis experiments have also been examined by mass‐spectrometric techniques. It was obs...

Rare‐Gas Sensitized Radiolysis of Propane

The radiolysis of mixtures of C3H8 and nitric oxide containing excess amounts of the rare gases Xe, Kr, Ar, and He, and of equimolar amounts of C3H8–C3D8 in the same mixtures has been investigated. The product yields are compared with those expected assuming that the primary process leading to decomposition of the hydrocarbon is charge exchange with the appropriate noble gas, followed by ion—molecule reactions of the hydride‐transfer type. The experiments with deuterated propane demonstrate the formation of ethane and acetylene in bimolecular processes, and the production of methane, ethylene, and propylene chiefly by unimolecular decomposition. If it is assumed that neutralization of propyl ions results in the formation of ethylene and propylene as molecular products, the observations are consistent with the reaction sequence charge transfer, ion—molecule reactions, and neutralization leading to the production of unscavengeable species.

Positive Ion–Molecule Reactions in Perfluoropropane

Positive ion–molecule reactions have been studied in C3F8 by tandem and high‐pressure mass spectrometry. Rate constants at a nominal 0.3‐eV ion energy and kinetic energy dependences of the reactions are reported. The most common reaction is F− transfer which has a typical rate constant of 5 × 10−11 cm3 molecule−1·sec−1. These rate constants are generally more than an order of magnitude lower than those for analogous H− transfer reactions in alkanes. Other reactions, including F transfer to CF2+ and F2 transfer to CF+ as well as several condensation reactions involving C–C bond formation are also observed. Endothermic collision‐induced dissociation (CID) reactions are observed for CF2+, C2F3+, C2F4+, C2F5+, and C3F7+ even at a nominal 0.3‐eV ion kinetic energy. The effects of ionizing electron energy and of changing the molecular source of the ionic species show that the fragment ions are formed with considerable internal excitation. For ions formed by 76‐eV electrons, the ratio of CID to F− transfer rate ...

Dissociation of Molecular Ions Formed by Charge Exchange in an In‐Line Tandem Mass Spectrometer

Dissociations of the molecular ions of several alkanes, alcohols, and ketones, formed in charge‐exchange collisions were studied using an in‐line tandem mass spectrometer. The effects of both incident ion translational energy and collision chamber temperature upon the observed dissociations were examined. For most of the molecules investigated, the extent of fragmentation of the molecular ion increases as the kinetic energy is raised, indicating that energy is being converted from the translational modes into target excitation. The magnitude of this energy conversion seems to be rather small. Higher collision chamber temperatures also lead to more extensive fragmentation, presumably owing to increased vibrational excitation. In certain cases, notably methanol and 2‐hexanone, an increase in the translational energy of incident ions having recombination energies near the onset of known excited ionic states of these molecules results in an increase in the relative intensity of the molecular ion product. This...