Michael Henchman

The isotope exchange reactions H++D2⇄HD+D+ and D++H2⇄HD+H+ in the temperature range 200–300 K

Forward and reverse rate coefficients have been measured in the temperature range 200–300 K for the two reactions H++D2⇄HD+D+ and D++H2⇄HD+H+. Equilibrium constants derived therefrom agree with theoretical van’t Hoff plots calculated from statistical mechanics and confirm the temperature calibration of the SIFT apparatus used. It is suggested that these reactions can be used as kinetic thermometers to measure independently the temperature of ion–molecule reaction cells. The system provides a particularly clear example of the role of statistical factors in chemical kinetics, 1 for the forward reactions and 1/2 for their reverse reactions; and the system illustrates further the relationship between statistical factors in kinetics, symmetry numbers in statistical mechanics, and the corresponding thermodynamic entropy changes. Constraints upon the temperature dependence of the rate coefficients are derived from consideration of thermodynamics and collision dynamics, and the data are seen to conform to these o...

Dissociative attachment reactions of electrons with strong acid molecules

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 ef...

ESR Study of NO2 and NO3 in Irradiated Lead Nitrate

Lead nitrate crystals have been irradiated with electrons at room temperature and their electron spin resonance spectra examined at 77°K to reveal three paramagnetic species. One species shows no hyperfine structure and has an isotropic g tensor with a g value at the free spin value: little can be said of its nature. The others are located, by their spectral anisotropy, at nitrate sites in the crystal with the axes of their axially symmetric g tensors parallel to the nitrate triad axes. One of these species is interpreted as NO3: it shows no nitrogen hyperfine structure but a slight interaction with 207Pb. The other is identified as NO2 rotating in the plane of the nitrate site. Theoretical predictions are in accord with these assignments.Various conclusions are drawn for the radiation chemistry. Other species than the NO2— and O2, revealed by chemical analysis, are significant products. The particular lattice determines the nature of these products, their stability and, in the case of NO2, its mechanism ...

The mechanism of the reaction CH+4 + CH4 = CH+5 + CH3 as a function of energy: rate constants and product distributions for the reactions of CH+4 + CD4 and CD+4 + CH4 at 80 and 300 K

Abstract The collision mechanism of the reaction CH+4 + CH4 = CH+5 + CH3 has been investigated as a function of temperature by measuring rate constants and product distributions for the reactant pairs CH+4 + CD4 and CD+4 + CH4 at 80 and 300 K. At 80 K, both reactions give the same product distribution, which is close to the statistical distribution and consistent with a long-lived complex mechanism. With increasing temperature, the product distributions change dramatically and, at 300 K, they already show strong contributions of the channels which dominate at higher energies (⩾ 0.15 eV)—namely the “direct” transfer of protons and hydrogen atoms. Together with results from ICR, tandem mass spectrometry, crossed beams, photoionization and quantum chemical calculations, the present low-temperature study is able to provide a unified description of the mechanism of this reaction and its energy dependence.

Rate constants and product distributions as functions of temperature for the reaction of OH−(H2O)0,1,2 with CH3CN

Abstract A selected ion flow tube was used to measure the rate constants and product distributions for the reactions of OH− (H2O)n with CH3CN over the temperature range 240–363 K for the case n = 1 and at 298 K for n = 0 and 2. Proton transfer was the only primary reaction channel observed; this process was found to be fast (efficiency ≅ 70%) for n = 0 and 1 but much slower (efficiency ≅ 4%) for n = 2. Interpreting OH− + CH3CN in the context of the general reaction OH− + CH3X, two features are important. First, CN has an abnormally large electron affinity. This gives CN− a large methyl cation affinity and nucleophilic displacement a large barrier: it is not observed, even though exothermic. Second, CH2CN has a large electron affinity and CH2CN− is delocalized. Thus (1) CH3CN shows a low heat of deprotonation, making proton transfer exothermic for OH− + CH3CN, but (2) less than 100% efficient since the product ion is delocalized, and (3) endothermic for OH− (H2O)3 + CH3CN because CH2CN− has a low hydration energy. Solvent switching with CH3CN and the thermal dissociation of the solvated product ions were observed as secondary reactions in the OH− (H2O) + CH3CN system. This study suggests that thermal dissociation can complicate the interpretation of product distributions in flow-tube and comparable experiments.

Rate Constants and Cross Sections

More is known about the rates of ion-molecule reactions than about those of any other family of chemical reactions. This chapter will not, however, consist of a joyful celebration of that pleasant reality, but rather, the reverse—it will take a somewhat severe look at what we need to know, at what we sometimes think we know; and at what in fact we do know. The ultimate objective, complete and mutual overlap of all three domains, is very far from being realized: in reality, despite the proclamation of the first sentence, there is little overlap between the first and third categories at the present time.

Charge transfer reactions between D+, O+, Ar+, Kr+ and Xe+ with CH4, C2H6 and C3H8 at relative energies 1–10 eV

Abstract Fifteen asymmetric resonant charge transfer reactions, involving the five title atomic ions A+ and the three title molecules B have been studied in the relative (center-of-mass) energy range 1–10 eV. Two different tandem mass spectrometers were used to measure limiting cross-sections at the highest energies. Time-of-flight and retarding-potential-analysis techniques were used to characterize the contribution from non-resonant processes, occurring with momentum transfer at low impact parameters. An attempt is made to correlate the magnitudes of the cross-sections for A+ + B → products with molecular parameters of the original reactants: (i) the Franck—Condon factors for B → B+, and (ii) the cross-section for A+ + A → A + A+. The Franck—Condon factors for B → B+ are shown not to correlate quantitatively, although a propensity rule — low factors legislate low cross-sections — does apply. The dependence of the cross-section on the nature of A correlates qualitatively with the square root of the symmetric charge transfer cross-section for A+ + A → A + A+. The incorporation of all these factors, within the context of Forster theory (which treats the transfer of excitation energy A ∗ + A → A + A ∗ ), is considered for charge transfer. Qualitative but not quantitative correlations are found, and a formal justification for this is noted. The implications of these results for the mechanism of these reactions at thermal energies are briefly considered.

THE REACTIONS OF SOME INTERSTELLAR IONS WITH BENZENE, CYCLOPROPANE AND CYCLOHEXANE

Abstract The reactions of the cyclic molecules C6H6 (benzene), c-C3H6 (cyclopropane) and c-C6H12 (cyclohexane) with ArH+ (ArD+), H3+, N2H+, CH5+, HCO+, OCSH+, C2H3+, CS2H+ and H3O+ have been studied at 300 K using a SIFT apparatus. All the reactions except those of C2H3+ proceed via proton transfer and all are fast except the H3O+ and CS2H+ reactions with c-C6H12 which are endothermic and which establish that the proton affinity of c-C6H12 is 160 ± 1 kcal mol−1, which is considerably lower than the published value. In the c-C3H6 and the c-C6H12 reactions multiple products are observed and hence “breakdown curves” for the protonated molecules are constructed and the appearance energies of the various ion products are consistent with available thermochemical data. The reactions of C2H3+ with these cyclic molecules are atypical within this series of reactions in that they appear to proceed largely via hydride ion transfer. The implications of the results of this study to interstellar chemistry are alluded to.

Isotopic fractionation by isotope-exchange reactions as a function of temperature. 2H and 18O in OH–+H2O and H3O++ H2O and 15N in N2H++ N2

A simple model is used to predict the fractionation of isotopes as a function of temperature via ion–molecule reactions. It is developed for reactions proceeding via a single intermediate, where the isotopic mixing is completed within the lifetime of the intermediate such that the reaction is under thermodynamic control. The fractionation efficiency is K/(K+ 1) where K is the equilibrium constant of the isotope-exchange reaction. The model is applied to deuterium and 18O fractionation via the prototypical reactions OH–+ H2O and H3O++ H2O; satisfactory agreement is found with the data available for 300 K. The model is also applied to 15N fractionation via N2H++ N2 and comparison is made with the data available for 80 K. The model predicts simply the fractionation efficiencies for such isotope-exchange reactions at the temperatures of interstellar clouds (10–100 K).

Chemical Pathways for Deuterium Fractionation in Interstellar Molecules

The formation of deuterated interstellar molecules is considered. This contribution (which, in the summarizing remarks of the conference, was admonished for being itself admonitory) has a simple purpose—to apply gas-phase ion chemistry to distinguish deuterium exchange reactions that can occur from those that cannot. Thus the exoergic reaction HCO+ + D → DCO+ + H is facile whereas the exoergic reaction HCO+ + HD —×→ DCO+ + H2 is not. Forbidden pathways involve energy barriers that can result from both reactants showing filled valence electron shells. A procedure is outlined for identifying such.