Bibliothek

Notizen: We report experimental studies of the formation of CF3O− by ion-molecule and electron attachment reactions, and theoretical investigations of the structure and energetics of CF3O− and its neutral counterpart CF3O. The anion CF3O− is formed from the rapid attachment of free electrons to its neutral dimer, (CF3O)2. Potential sources of CF3O− through ion-molecule reactions of CF3− and F− were surveyed. CF3O− is formed in the bimolecular ion-molecule reaction of CF3− with SO2 and the third-order association reaction of F− with CF2O. In addition, rate constants for the reactions of CF3− with a variety of neutral compounds were measured. A number of cases were found in which formation of CF3O− was energetically allowed but was not observed. The potential energy surfaces of CF3O and CF3O− have been investigated using a variety of density functional theory (DFT) techniques. The ground-state minimum energy structure of CF3O was found to be a 2A′ Jahn–Teller distorted Cs-symmetry structure, while for the anion the ground state is 1A1 with a C3v-symmetry minimum. A search for other low-energy minima for CF3O− was unsuccessful. The DFT methods support a value for the adiabatic electron affinity of CF3O near 4.1 eV. © 1999 American Institute of Physics.

Notizen: Rate constants have been measured for the reactions of O− with CH4, CH2D2, and CD4 as a function of ion-neutral average center-of-mass kinetic energy, 〈KEcm〉, at several temperatures over the range 93 K–565 K using a selected ion flow drift tube apparatus. For the CH4 reaction we also report measurements made using a high-temperature flowing afterglow (HTFA) instrument over the temperature range 300 K–1313 K. The rate constants are found to have a very large isotope effect, with the CH4 rate constant a factor of 15 higher than the CD4 rate constant at 93 K. The rate constants generally have a minimum with respect to temperature and 〈KEcm〉, except for the higher-temperature data for CD4 where the rate constants show only an increase with increasing kinetic energy. The data indicate that increasing rotational temperature decreases the rate constants and that rotational energy behaves similarly to translational energy. Single excitations of bending and twisting vibrations have a negligible effect on the rate constant. Either the stretching vibrations or overtones of the bending vibrations increase the rate constants. If the stretches are responsible for the increase in the rate constants, the derived rate constant for a single quantum of stretch excitation (v=1) is 5×10−10 cm3 s−1, a factor of 6 larger than the rate constant for v=0. The CH2D2 rate constants are approximately equal to the averages of the rate constants for the pure isotopes. The product branching ratio (OH−/OD−) shows no dependence on CH2D2 rotational temperature or low-frequency CH2D2 vibrations. A theoretical study of the minimum energy reaction path was performed to help elucidate the reaction dynamics. The minimum energy reaction surface was characteristic of the standard double minimum pathway for ion molecule reactions. The height of the central barrier was found to be close to the energy of the reactants and varied with isotopic substitution. Conformationally different transition states are found for these isotopic reactions. Theoretical studies at the QCISD(T) level of theory find distinct transition states corresponding to O−+CH4, O−+H-CHD2, O−+D-CH2D, O−+CD4. The transition state barriers increase in the order O−+CH4, O−+H-CHD2, O−+D-CH2D, and O−+CD4, in agreement with experimental reaction rates. The main features of the reactivity are explained by the characteristics of the reaction surface. © 1997 American Institute of Physics.

Notizen: We report the first measurements of rate constants for formation and reaction of the hydrated-hydride ion H3O−. We studied the Kleingeld–Nibbering reaction [Int. J. Mass Spectrom. Ion Phys. 49, 311 (1983)], namely, dehydrogenation of formaldehyde by hydroxide to form hydrated-hydride ion and carbon monoxide. The OD−+H2CO reaction is about 35% efficient at 298 K, with OD−/OH− exchange occurring in about half the reactions. H3O− was observed to undergo thermal dissociation in a helium carrier gas at room temperature with a rate constant of 1.6×10−12 cm3 s−1. We also studied a new reaction in which H3O− is formed: The association of OH− with H2 in a He carrier gas at low temperatures. The rate coefficient for this ternary reaction is 1×10−30 cm6 s−1 at 88 K. Rate coefficients and product branching fractions were determined for H3O− reactions with 19 neutral species at low temperatures (88–194 K) in an H2 carrier. The results of ion-beam studies, negative-ion photoelectron spectroscopy, and ion-molecule reaction data allow us to specify the hydride–water bond energy D0298(H−−H2O)=14.4±1.0 kcal mol−1 (0.62±0.04 eV). The heat of formation of H3O−, −37.5±1.0 kcal mol−1, and the proton affinity of H3O−, 386.0±1.0 kcal mol−1, are derived from these results. Dissociation of H3O− into OH− and H2 requires 4.5±1.0 kcal mol−1 energy.

Notizen: Rate coefficients and product branching fractions have been determined for 31 ion–molecule reactions involving PF5 or PF−5. About half of the reactions studied show an ion–molecule association channel. NH−2 and OH− reaction with PF5 yields HF product. F− and electron transfer channels are also observed in many of the reactions studied. Consideration of the efficiency of the electron transfer channel in these reactions leads to the conclusion that the adiabatic electron affinity of PF5 is 0.75±0.15 eV.

Notizen: The ion–molecule reaction OH−+H2CO→H3O−+CO has been studied at 300 K with isotopic labeling of reactants. The H3O− product is only observed in small abundance because the ion dissociates into OH−+H2 upon multiple collisions in a helium buffer gas. Without isotopic labeling, the pseudo-first-order kinetics plots for the reactions of OH− with H2CO and OD−+D2CO were found to be curved as a result of the regeneration of OH− or OD− reactant. A scavenger technique was used to remove the H3O− (or D3O−) produced prior to dissociation, to reveal the true first-order attenuation of OH− (or OD−) in reaction with H2CO (or D2CO). The rate constant for the OH−+H2CO reaction is 7.6×10−10 cm3 s−1, and for OD−+D2CO is 5.7×10−10 cm3 s−1. For the isotopically mixed cases OH−+D2CO and OD−+H2CO, the rate constants are equal to 1.3×10−9 cm3 s−1, about twice as large as those for the reactions involving only a single hydrogen isotope, indicating that isotopic exchange is an important process. The rate constants for the thermal dissociation of H3O− and D3O− in helium were found to be 1.6×10−12 and 1.1×10−12 cm3 s−1, respectively, within a factor of 2. The results are discussed in terms of other thermal dissociation reactions of ions.

Notizen: A flowing–afterglow Langmuir probe apparatus with mass spectral analysis has been used to measure the rate constant for electron attachment to ClONO2 at 300 K. Electron attachment is efficient with a rate constant of 1.1 (±50%)×10−7 cm3 s−1 and proceeds principally through dissociative channels to produce the major product ions NO−2 (∼50%), NO−3 (∼30%), and ClO− (∼20%). The parent ion ClONO−2 and Cl− are also observed in the mass spectra but are at most minor products in the attachment process, ≤2% and ≤6%, respectively. A description of the secondary ion–molecule chemistry that takes place following electron attachment is given. © 1996 American Institute of Physics.

Notizen: Rate constants were measured for electron detachment reactions of atomic hydrogen with CF−3, C2F−5, and C3F−3. The experiments were performed using a selected ion flow tube (SIFT) instrument. The density of atomic hydrogen was calibrated by studying the reaction of F− with H, and rate constants were measured relative to the known rate constant for this reaction. The reactions with H are fast, proceeding at 17%–66% of the collisional rates. No ionic products were observed for any of the reactions studied. While associative detachment is exothermic for all three anions, other reactive detachment processes are also possible. These anions were found to be unreactive with H2.

Notizen: It has previously been shown that neutral ligands clustered to an alkali ion can react with other neutrals. In fact, the presence of the alkali ion greatly enhances the rate of the corresponding neutral–neutral reaction. Several more reactions that exhibit this phenomenon have been identified experimentally, with rate enhancements ranging from 4 to 30 orders of magnitude. Three possible explanations for the rate enhancement have been explored. Ab initio calculations have been carried out to supplement the experimental measurements and to provide further insight into the mechanism. The calculations indicate that bonding the neutral to the alkali ion causes structural rearrangements that shift the geometry of the neutral molecule toward its transition state geometry in the corresponding neutral–neutral reaction, thereby lowering the activation energy of the reaction.

Notizen: The photodissociation spectrum of SO+2 corresponding to the process SO+2 +hν→SO++O has been measured on a triple quadrupole system for the wavelength ranges: 3000–3400 and 4400–5120 A(ring). The spectrum in the visible has been assigned to the C˜ 2B1←A˜ 2A2 transition and vibrational structure analyzed to yield λ00=4187 A(ring), the harmonic vibrational frequencies ν˜'1=953 cm−1, ν˜''2=499 cm−1, ν˜'1=767 cm−1, and ν˜2 =410 cm−1, and the anharmonicity constants X‘11 =−6.3 cm−1, X''12 =−6.1 cm−1, X‘22 =−4.2 cm−1, X12 =−6.3 cm−1, and X'22 =−7.4 cm−1. Apparent photodissociation cross sections ranged from ∼1×10−20–1.6×10−19 cm2 in the visible spectrum. In the UV spectrum the highly congested vibrational structure could not be resolved sufficiently for analysis; photodissociation cross sections ranged from ∼2–4×10−19 cm2 with broad bands roughly corresponding to expected progression in ν1 and ν'2 within the C˜ 2B1←X˜ 2A1 electronic transition. The process SO+2 +hν→S++O2 showed an onset near 3108 A(ring) and the corresponding photodissociation spectrum showed sufficient vibrational structure to yield ν˜''2∼454 cm−1, ν˜'1 ∼955 cm−1, and ν˜2 ∼411 cm−1. This new process, which had σ≤3×10−20 cm2, was proposed to result from the D˜ 2B2←X˜ 2A1 transition. Several new photodissociation cross section measurements on N2O+ have also been made in the vicinities of 3381 and 4880 A(ring), taking advantage of the high sensitivity (→10−21 cm2) of the triple quadrupole system.

Notizen: Using the flowing afterglow/Langmuir probe (FALP) technique, we have determined (at variously 300 and 570 K) the dissociative attachment coefficients β for the reactions of electrons with the common acids HNO3 (producing NO−2) and H2SO4 (HSO−4), the superacids FSO3H (FSO−3), CF3SO3H (CF3SO−3), ClSO3H (ClSO−3,Cl−), the acid anhydride (CF3SO2)2O (CF3SO−3), and the halogen halides HBr (Br−) and HI (I−). The anions formed in the reactions are those given in the parentheses. The reactions with HF and HCl were investigated, but did not occur at a measurable rate since they are very endothermic. Dissociative attachment is rapid for the common acids, the superacids, and the anhydride, the measured β being appreciable fractions of the theoretical maximum β for such reactions, βmax. The HI reaction is very fast ( β∼βmax) but the HBr reaction occurs much more slowly because it is significantly endothermic. The data indicate that the extreme acidity of the (Bronsted-type) superacids has its equivalence in the very efficient gas-phase dissociative attachment which these species undergo when reacting with free electrons. The anions of the superacids generated in these reactions, notably FSO−3 and CF3SO−3, are very stable (unreactive) implying exceptionally large electron affinities for the FSO3 and CF3SO3 radicals.