E.E. Ferguson

An absolute proton affinity scale in the ∼130-140 kcal mol−1 range

The dependences on temperature of the rate coefficients for the endothermic proton transfer reactions of HBr+ with CO2 and CH4 have been obtained in a variable‐temperature selected ion flow tube. The measurements have been used to determine the 300 K proton affinity of CO2, P.A.(CO2), =128.5±1.0 kcal mol−1, utilizing the literature value of P.A.(Br)=131.8 kcal mol−1, obtained from the dissociation energy of HBr+, as a primary standard. The proton affinity difference between CO2 and CH4 has been substantiated by equilibrium constant measurements as a function of temperature for proton transfer between CO2 and CH4. Similar equilibrium constant measurements have been used to determine the proton affinities of HCl, N2 O, HBr, and CO, giving a proton affinity ladder ordered (in kcal mol−1) as CO(141.4), HBr(138.8), N2 O(137.3), HCl(133.0), Br(131.8), CH4 (130.0), and CO2(128.5). Proton affinities have also been determined for Br2(140.0), NO(127.0), and CF4(126.5), the last two values being obtained from select...

Radiative lifetime for v = 1 and v = 2 ground state NO+ ions

Abstract Radiative lifetimes of vibrationally excited ions in their electronic ground state have been measured for the first time. A nonoptical technique has been developed, involving ion cyclotron resonance trapping in conjunction with chemical monitoring of the energy content of the ions. For NO + (X 1 Σ + ) ions, the measured lifetimes of 95 ± 15 ms for v = 1 and 46 ± 10 ms for v = 2 are in excellent agreement with the values obtained from the ab initio calculations of Werner and Rosmus.

Spin non-conservation in low-energy molecular charge-transfer reactions

Abstract The reaction H 2 O + ( 2 B)+NO 2 ( 2 A) → H 2 O( 1 A) + NO 2 + ( 1 Σ) occurs at near the collision rate constant 1.2 × 10 −9 cm 3 s −1 , in spite of the fact that the reactants produce both a singlet and a triplet state and the products correlate only with the singlet state. This would be expected to yield a statistical weight factor of 1 4 to be multiplied by the collision rate constant to obtain the maximum charge-tranfer rate constant. The triplet products of the charge transfer are clearly endothermic. The singlet—triplet intersection has not been identified but the available information about the singlet and triplet states of the intermediate protonated nitric acid molecule is discussed. Four other examples of apparent “spin violation” charge-transfer reactions have been noted H 2 O + + NO, N 2 O + + NO.CO + + NO and CH 4 + + O 2 .

Reactions of C2 and C3 hydrocarbon ions with H, D, H2 and D2 at near-thermal energies

Abstract Reactions of C2H+n (n = 2−6), C+3, C3H+ and C3H+2 ions with H, H2, D and D2 have been investigated in a SIFDT (selected ion flow drift tube) at 300 K in helium buffer gas at low E/N (E is the electric field strength and N is the buffer gas number density) and from 0.15 to 0.35 torr pressure. The rate constants for various processes involved, such as binary reactions, isotope exchange and three-body associations have been determined.

Vibrational quenching of HCl+(ν=1) and DCl+(ν=1) by Ar and Kr

Abstract The vibrational quenching of HCl + (ν=1) and DCl + (ν=1) ions in collisions with Ar and Kr atoms at 300 K has been measured in order to investigate the effect of changing the vibrational and rotational energy levels while keeping the intermolecular interaction potential unchanged. All quenchings are found to be efficient, P > 20%, and the lighter isotope is found to be quenched slightly faster than the heavier one. This is consistent with a dominant role for rotational excitation in the V→T, R process such that the advantage of the larger rotational constant outweighs the disadvantage of the larger vibrational frequency. The same situation had been reported for self-quenching of neutral hydrogen halides and for vibrational predissociation of hydrogen halide directs and supported theoretically in terms of rotational excitation coupling. The present results weigh strongly against a Landau-Teller-type vibrational quenching mechanism for the molecular ions, modified by the existence of the attractive potential.

Flow tube studies of ion-molecule reactions

Publisher Summary This chapter focuses on the advancements in the ion–molecule interaction processes during the past several years of measurements. The chapter considers a few aspects of charge-transfer processes, some chemical reactions, and the recent vibrational energy transfer investigations leading to useful generalities and insights into ion neutral interaction mechanisms. Flow tubes do not play the dominant role in the area of reaction mechanistic studies that they have in their aeronomical and astrophysical applications by virtue of their chemical versatility. Thermal energy ion–molecule reactions have a strong propensity to proceed efficiently along the lowest energy pathways between reactants and products, without steric or Franck–Condon, or sometimes even spin conservation restraints. The negative temperature dependences of charge-transfer and thermal energy ion–atom interchange reactions are invariably weaker than either the experimental temperature dependences of lifetimes deduced from three-body association rate constants or theoretical lifetimes deduced from statistical theories.

Consideration of rotational tunneling in rare gas cluster ions

Abstract Tunneling transmission coefficients through rotational barriers are investigated as a function of argon cluster ion size using the WKB approximation. The results indicate that tunneling coefficients corresponding to transmissions of a few percent, extend over time windows of ten orders of magnitude. Hence, a very broad distribution of metastable lifetimes is expected for decay of cluster ions by tunneling through rotational barriers.

Constraints on the temperature variation of three-body ion-neutral association reaction rate constants

Abstract It is shown that the general finding of activation energies less than dissociation energies for unimolecular decomposition implies a negative activation energy for ion-neutral molecule association reactions at all finite temperatures. Ion-molecule association reactions therefore cannot show maxima in their rate constants, contrary to suggestions in some current experimental data.

Reactions of HNNO+ and NNOH+ ions with CH4 and NO as a function of relative kinetic energy

Abstract Rate constants for the reaction of the two isomers of protonated N 2 O, HNNO + and NNOH + , with CH 4 and NO have been measured as a function of relative kinetic energy. The CH 4 measurements allow a determination of the proton affinity of N 2 O on N of 131.5 ± 1 kcal mol −1 and an isomeric energy difference of 6.2 ± 0.5 kcal mol −1 . This latter value should be compared with the recently obtained theoretical value of 7.1 kcal mol −1 .

Selected ion flow drift tube studies of the reaction of HBr+ with various neutral molecules

The reactions of HBr+ with CF4, NO, CO2, CH4, N2O, SO2, COS, H2S and SF6 have been studied in a SIFDT apparatus. For some reactions the variation of rate constant with centre of mass collision energy Ecm has been investigated. The rate constant for the CH4 reaction exhibits an unusual variation with Ecm which is shown to be due to the different energy variations of the proton transfer, hydrogen abstraction and electron transfer channels. The reduced mobilities of HBr+ and H2Br+ in helium have been determined up to E/N −120 Td.