Nicholas J. Kirchner

Unimolecular and bimolecular reactions in the C6H6+· system. Experiment and theory

Abstract The results of an experimental and theoretical study of unimolecular and bimolecular reactions in the C6H6+· system are reported. The kinetic energy release distributions for the fragmentation of metastable C6H6+· ions were measured and the bimolecular reactions between C6H4+· and H2, cyclic C4H4+· and C2H2, linear C4H4+· and C2H2, and between C2H2+· and linear C4H4 have been studied. The results of these experiments and absolute unimolecular rate constants and product branching ratios from PIPECO studies are compared with the predictions of the transition state switching model form of statistical rate theory. In general, good agreement was found between the experimental results and the calculations. The calculated total unimolecular rate constants are in excellent agreement with the experimental data (for total energies of 15.0–15.7 eV). Below total energies of 16.5 eV the measured and calculated branching ratios are in good agreement. Above 16.5 eV the agreement becomes increasingly poor. These results were interpreted as indicating that above 16.5 eV the rate of fragmentation is no longer slow compared to the rate of isomerization and so products derived from simple bond cleavages are favored over reactions requiring isomerization to different ion structures. The measured kinetic energy distribution for C6H5+ is broader than the calculated distribution; for C4H4+· the measured and calculated distributions are in good agreement, and for C3H3+ the measured distribution is narrower than the calculated distribution. The measured distribution for C6H4+· peaks at large values of kinetic energy, indicating a substantial barrier along the reaction coordinate in this channel. No reaction was observed between cyclic C4H4+· and C2H2 and between C6H4+· and H2. Reaction between linear C4H4+· and C2H2 occurs at a rate slower than the collision rate, and the experimental and theoretical results indicate that the reaction occurs through a long-lived C6H6+·* intermediate complex. Charge transfer between C2H2+· and C4H4 occurs at the collision rate. Most of the charge transfer is direct. However, the experimental and theoretical results indicate that some trajectories sample the deep C6H6+· well.

An experimental study of the formation and reactivity of ionic hydrogen clusters: The first observation and characterization of the even clusters H+4, H+6, H+8, and H+10

A specially designed coaxial drift tube type ion source has been utilized to generate ionic hydrogen clusters. The cluster spectrum observed is dominated by odd clusters (H+5, H+7, H+9, and H+11 ) in line with past studies on hydrogen ion clusters. However, for the first time even hydrogen cluster ions are formed in a high pressure ion source, with relative intensities (H+6≫H+8>H+10>H+4) . The observation of a relatively intense H+6 peak (H+6/H+5 =0.05) was unexpected. The peak at nominal mass 6 was confirmed to be H+6 (and not H4D+) by high resolution studies. A number of possible formation mechanisms for H+6 are discussed. The most likely mechanism is reaction of odd hydrogen cluster ions (H+3, H+5, H+7,... ) with metastable electronically excited odd hydrogen cluster neutrals (H*3, H*5, H*7,...) . The H*3 species has been experimentally characterized by other research groups (Herzberg; Kuppermann) and is probably formed by electron recombination with H+3 (v≠0), H+5 or H+7 in our source. The H+6 ion is ...

Influence of vibrational excitation and collision energy on the ion‐molecule reaction NH+3(ν2)+ND3

The influence of vibrational excitation and collision energy on the ion‐molecule reaction NH+3(ν2)+ND3 has been investigated using a recently constructed quadrupole‐octopole‐quadrupole mass spectrometer. The NH+3 reagent ions are prepared state selectively with 0–7 quanta in the ν2 umbrella bending mode by (2+1) resonance enhanced multiphoton ionization through the B or C’ Rydberg states of ammonia. Reactive collisions between the mass‐filtered ion beam and a thermal distribution of neutral reagent molecules occur with controlled collision energies (0.5–10.0 eV center of mass) within the octopole ion guide, enabling product ions to be collected independent of scattering dynamics. The reaction of NH+3 with ND3 has three major product channels: (1) deuterium abstraction, (2) charge transfer, and (3) proton transfer. Each of these channels exhibits a strong dependence on ion vibrational excitation and collision energy. Product branching ratios and relative cross sections are reported and compared with prev...

The first experimental observation of stable H4+ ions

Abstract A high pressure, variable temperature ion source is used to generate H 5 + from ionized H 2 . A mass selected H 5 + beam is collisionally dissociated and the products mass and energy analyzed. The dominant product is H 3 + but a significant H 4 + signal is observed. These ions survived at least 10 −6 s from the collision cell to the detector and are thus stable to dissociation. Kinetic energy distributions of the H 3 + and H 4 + product ions are reported and are significantly different. All experiments were also performed using D 5 + generated in ionized D 2 with essentially identical results obtained. The mechanism of H 3 + and H 4 + formation is discussed.

On the structure of CH3O+2 product ions in Ar/O2/CH4 mixtures: the first example of a kinetic surface

Abstract Collision-induced dissociation (CID) studies are used to investigate the structure of the CH 3 O + 2 ions formed by reaction of O + 2 with CH 4 . The evidence unambiguously identifies the product structure as the methylene hydroperoxy ion, CH 2 OOH + , in agreement with Van Doren., who came to the same conclusion using chemical reactivity studies. Earlier studies that identified the O + 2 /CH 4 product ion as protonated formic acid are shown to be in error. We also investigated the structure of CH 3 O + 2 product ions in various Ar/O 2 /CH 4 mixtures. We are able to show that the product structure varies between CH 2 OOH + and the cluster ion CH + 3 ·O 2 in reproducible ways which depend on mixture conditions. It is necessary to run the source at 80 K to observe this effect. The term “kinetic surface” is coined to describe the family of curves which shows the variation of m/z = 47 intensity and structure as the Ar/O 2 /CH 4 mixture varied. A qualitative discussion of the underlying kinetics that explain the phenomena is given. A case is made that the CH + 3 ·O 2 species is a triplet, which probably explains why it does not isomerize to the more stable CH 2 OOH + singlet on the time-scale of our experiment.

Guided ion beam measurement of the product branching ratios for the ion‐molecule reaction N++O2 as a function of collision energy

A quadrupole‐octopole‐quadrupole mass spectrometer has been constructed for comparing ion‐molecule reaction product intensities as both the internal excitation and the kinetic energy of the reactant ion are varied. Such comparisons require an ion beam with a known kinetic energy distribution and, most importantly, they require product intensity measurements made without significant bias in detection of the different product channels. To assess the characteristics of our instrument, we have studied the ion‐molecule reaction N++O2 that is known to yield three different product channels: N+O+2, NO++O, and NO+O+. Ion beam trajectory simulations combined with experimental measurements show that the spread in the kinetic energy of the reagent ions has a fixed value in the range of 0.6 to 1.1 eV full width at half maximum in the center of mass (c.m.). Relative cross sections for the three different product channels are reported as a function of c.m. collision energy. A comparison of the observed product branchin...

Translational energy spectroscopy of beams of C+(2Pu) and C+(4Pg) ions

Abstract Beams of C+ ions are made by electron impact on CO, CO2 and CH4 at 70 eV. Electronic transitions of these ions are observed using TES spectroscopy of the type developed by Illies and Bowers [Chem. Phys., 65 (1982) 281]. He, NO and O2 collision gases were used to induce the electronic transitions. Only energy-loss transitions originating from C+(2Pu) to other states in the doublet manifold were observed for He collision gas. For NO and O2 an energy-loss transition assignable to C+(2Pu) → C+(4Pg) was also observed. The complementary superelastic transition C+(4Pg) → C+(2Pu) was also seen for NO and O2 collision gases. All of the transitions observed were interpreted in terms of state symmetries of the quasi-molecule formed between C+ and the collision gas, where “forbidden” crossings between curves of the same symmetry dominate. From the relative intensities of the C+(2Pu) ⇌ C+(4Pg) transitions it was possible to determine the fractional population of C+(4Pg) in the ion beam. For 70 eV electron impact on CO a value of 16% was obtained and for CO2 a value of 6.3%. The deactivation of C+(4Pg) by CO was measured to be fast, 1±0.5 × 10−9 cm3 s−1. These results were compared with other results available in the literature.