Collision rate constants for polarizable ions

AbstractLangevin described a model for the interaction between an ion and a neutral nearly a century ago and since then, many modifications have been introduced to adjust for specific circumstances. This work discusses the induced dipole–induced dipole interaction between an ion and a neutral without a permanent dipole and introduces an anisotropic adjustment. A point polarizable ion model (PPI) and an orientation dependent polarizable ion model (ODPI) are discussed and applied to systems where the ion is highly polarizable and the neutral is only weakly polarizable. Significant deviations from classical Langevin rate constants and significant differences between PPI and ODPI are observed

Deuterium Kinetic Isotope Effects in Microsolvated Gas-Phase E2 Reactions

This work describes the first experimental studies of deuterium kinetic isotope effects (KIEs) for the gas-phase E2 reactions of microsolvated systems. The reactions of F−(H2O)n and OH−(H2O)n, where n = 0, 1, with (CH3)3CX (X = Cl, Br), as well as the deuterated analogs of the ionic and neutral reactants, were studied utilizing the flowing afterglow-selected ion flow tube technique. The E2 reactivity is found to decrease with solvation. Small, normal kinetic isotope effects are observed for the deuteration of the alkyl halide, while moderately inverse kinetic isotope effects are observed for the deuteration of the solvent. Minimal clustering of the product ions is observed, but there are intriguing differences in the nature and extent of the clustering process. Electronic structure calculations of the transition states provide qualitative insight into these microsolvated E2 reactions

Negative-Ion Photoelectron Spectroscopy, Gas-Phase Acidity, and Thermochemistry of the Peroxyl Radicals CH_3OO and CH_3CH_2OO

Methyl, methyl-d3, and ethyl hydroperoxide anions (CH_3OO-, CD_3OO-, and CH_3CH_2OO-) have been prepared by deprotonation of their respective hydroperoxides in a stream of helium buffer gas. Photodetachment with 364 nm (3.408 eV) radiation was used to measure the adiabatic electron affinities:  EA[CH_3OO, X̃^2A‘‘] = 1.161 ± 0.005 eV, EA[CD_3OO, X̃^2A‘‘] = 1.154 ± 0.004 eV, and EA[CH_3CH_2OO, X̃^2A‘‘] = 1.186 ± 0.004 eV. The photoelectron spectra yield values for the term energies:  ΔE(X̃^2A‘‘−Ã^2A‘)[CH_3OO] = 0.914 ± 0.005 eV, ΔE(X̃^2A‘‘−Ã^2A‘)[CD_3OO] = 0.913 ± 0.004 eV, and ΔE(X̃^2A‘‘−Ã^2A‘)[CH_3CH_2OO] = 0.938 ± 0.004 eV. A localized RO−O stretching mode was observed near 1100 cm^(-1) for the ground state of all three radicals, and low-frequency R−O−O bending modes are also reported. Proton-transfer kinetics of the hydroperoxides have been measured in a tandem flowing afterglow−selected ion flow tube (FA-SIFT) to determine the gas-phase acidity of the parent hydroperoxides: Δ_(acid)G_(298)(CH_3OOH) = 367.6 ± 0.7 kcal mol^(-1), Δ_(acid)G_(298)(CD_3OOH) = 367.9 ± 0.9 kcal mol^(-1), and Δ_(acid)G_(298)(CH_3CH_2OOH) = 363.9 ± 2.0 kcal mol^(-1). From these acidities we have derived the enthalpies of deprotonation: Δ_(acid)H_(298)(CH_3OOH) = 374.6 ± 1.0 kcal mol^(-1), Δ_(acid)H_(298)(CD_3OOH) = 374.9 ± 1.1 kcal mol^(-1), and Δ_(acid)H_(298)(CH_3CH_2OOH) = 371.0 ± 2.2 kcal mol^(-1). Use of the negative-ion acidity/EA cycle provides the ROO−H bond enthalpies: DH_(298)(CH_3OO−H) = 87.8 ± 1.0 kcal mol^(-1), DH_(298)(CD_3OO−H) = 87.9 ± 1.1 kcal mol^(-1), and DH_(298)(CH_3CH_2OO−H) = 84.8 ± 2.2 kcal mol^(-1). We review the thermochemistry of the peroxyl radicals, CH_3OO and CH_3CH_2OO. Using experimental bond enthalpies, DH_(298)(ROO−H), and CBS/APNO ab initio electronic structure calculations for the energies of the corresponding hydroperoxides, we derive the heats of formation of the peroxyl radicals. The “electron affinity/acidity/CBS” cycle yields Δ_fH_(298)[CH_3OO] = 4.8 ± 1.2 kcal mol^(-1) and Δ_fH_(298)[CH_3CH_2OO] = −6.8 ± 2.3 kcal mol^(-1)

Vibrational state dependence of the N₂ ⁺ (v = 0–3) + HCl reaction at thermal energies

N₂ ⁺ (v = 0–3) + HCl reaction rate constant is studied at thermal energies. The rate constants for v = 0, 1, 2 and 3 are 5.3(±1.6)×10⁻¹⁰, 8.3(±2.5)×10⁻¹⁰, 7.9(±2.4)×10⁻¹⁰ and 8.0(±2.4)×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, respectively, showing little or no dependence on vibrational excitation. Vibrational deactivation of the N₂ ⁺ is at most 20% of the total removal rate. The branching ratio for the charge transfer channel to the hydrogen transfer channel is also not altered significantly by N₂ ⁺ vibrational excitation

THE CHEMISTRY OF CANDIDATE MOLECULAR ION CARRIERS OF THE DIFFUSE INTERSTELLAR BANDS

Author Institution: Center for Astrophysics and Space Astronomy, University of Colorado,; 389 UCB, Boulder, CO 80309; Department of Chemistry and Biochemistry,; University of Colorado, 215 UCB, Boulder, CO 80309; Center for Astrophysics and Space Astronomy, University of Colorado,; 389 UCB, Boulder, CO 80309; Department of Chemistry and Biochemistry,; University of Colorado, 215 UCB, Boulder, CO 80309Our group at Colorado has been exploring the diffuse interstellar band (DIB) problem by conducting laboratory experiments to measure chemical reaction rates of molecular ions that have been proposed as DIBs carriers. Our facility consists of a Flowing Afterglow Selected Ion Flow Tube (FA-SIFT), into which we inject ions selected by a quadrupole mass spectrometer which are then allowed to react in the flow tube with neutral species expected to be abundant in the diffuse interstellar environments where the DIBs form. The reaction products are then measured using a second quadrupole mass spectrometer. To date we have focused our attention on PAH cations and carbon chain anions, both of which have been proposed as DIBs carriers. In general we find that PAH cations become hydrogenated by the addition of one or two hydrogen atoms when reacting with atomic or molecular hydrogen, while reaction rates with neutral oxygen and nitrogen typically yield CO and HCN or adducts, again with high reaction rates. We conclude that in the diffuse ISM PAH cations will generally be hydrogenated, so that the protonated forms are the species one should consider as DIBs candidates if PAH cations are viable at all. We find that carbon chain anions, up to C$_9$$^-$, are very quickly destroyed by reactions with H atoms and are not chemically viable candidates as the DIBs carriers. Current work, which will be emphasized in this talk, is aimed at measurements of larger PAH cations than previously studied, to see whether the trends already observed continue as we progress to larger species