Joseph J. Grabowski

Gas-phase hydrogen-deuterium exchange reactions of hydroxide and hydroxide-d ions with weakly acidic neutrals

Rate constants for hydrogen-deuterium exchange reactions between HO/sup -/ and DO/sup -/ and a series of weakly acidic neutrals, both organic and inorganic, have been measured in the gas phase by using the selected ion flow tube (SIFT) technique. The reaction efficiencies are discussed in terms of the initial ion-dipole complex energies, the relative acidities of the neutrals, and the change in solvation energy accompanying proton transfer; the effect of these energies on transition-state properties profoundly influences the outcome of the reactions. Exchange occurs rapidly between hydroxide and most aromatic and vinyl compounds but is relatively inefficient for hydrogen. The efficiency for exchange with ammonia is intermediate. Ethylene, dimethyl ether, and methane do not exhibit exchange.

C70: intersystem crossing and singlet oxygen production

Time-resolved, pulsed-laser photoacoustic calorimetry has been utilized to investigate the photophysics of the fullerene C70 in benzene and toluene solutions at room temperature for excitation at three wavelengths from 337 to 510 nm. Analysis of the data indicates that C70 exhibits wavelength-independent population of T1: Φisc = 0.97 ± 0.03. Data taken in the presence of the quencher oxygen show that C70(T1) is an efficient, but not quantitative, sensitizer for singlet oxygen: SΔ = 0.89 ± 0.04.

Mechanistic insight into the reactions of O− with aromatic compounds and the synthesis of didehydroaromatic anions in the gas phase

Abstract The gas-phase ion/molecule reactions of O − with benzene, pyridine, furan, thiophene, pyrrole, and 1,3-cyclopentadiene have been quantitatively characterized under the thermally equilibrated (298 ± 3 K) conditions attainable with the flowing afterglow technique. All atomic oxygen radical anion reactions examined were found to be fast, with reaction occurring on at least three out of four collisions. While reaction rate coefficients are the same for all aromatic compounds, the product distributions for the O − reactions are extremely sensitive to the structure and properties of the neutral. In general, four reaction channels need to be considered: H + 2 -transfer, oxide anion formation (i.e. addition of an oxygen atom and loss of a hydrogen atom), H-atom transfer (i.e. formation of HO − ), and H + -transfer. Formation of the didehydroaromatic radical anion was detected for each neutral while H-atom transfer was a minor pathway and was only found for pyrrole and 1,3-cyclopentadiene. Oxide anion formation was a major pathway for the six-membered aromatic rings. For the five-membered rings, only thiophene produced the corresponding oxide anion (as a trace product) upon reaction with O − . Proton transfer was always important where thermochemically allowed (thiophene, pyrrole, and 1,3-cyclopentadiene). The observations are interpreted with the aid of a reaction coordinate diagram detailing a series of sequential steps for O − with benzene, starting with formation of an ion/neutral complex followed by partitioning into the four reaction channels. The data are supplemented by measurements made for benzene- d 6 and pyridine- d 5 , as well as for HO − reacting with each of the neutrals. In addition to demonstrating the synthetic utility of didehydroaromatic radical anion generation by this method, this work suggests that O − and HO − can be complementary to each other as negative-ion chemical ionization mass spectrometric reagents for analysis of mixtures of aromatic compounds by mass spectrometry.

SIMPLIFIED INJECTOR FLANGES FOR THE SELECTED ION FLOW TUBE

Abstract Dual hole-pattern injector flanges (Venturi inlets) for a selected ion flow tube have been fabricated and tested in order to identify an optimum balance of efficient operational characteristics and simplicity of fabrication, along with reproducible assembly. The diagnostic measurements for each configuration addressed: (a) ability to maintain pressure differences between ion selection and ion reaction regions; (b) ability to inject sufficient ions to carry out ion/molecule reaction studies; and (c) ability to not introduce turbulence in the reaction region of the flow tube. In addition, the sensitivity of these issues to different total gas flow and to partitioning of total flow between inner and outer injectors is considered. All hole-pattern injector configurations ever tried worked without requiring any adjusting. Importantly, extensive kinetic measurements of one ion/molecule reaction for many configurations, gas loads, and partitions, indicate turbulence in the reaction region is never a problem for these injectors. An injector flange with a small total helium inlet area via 12 holes, in a planar surface, with the ion orifice slightly downstream of the helium inlet plane, has provided the best experimental data.

Selected ion flow tube studies of the atomic oxygen radical cation reactions with ethylene and other alkenes

Abstract The chemical ionization reaction of the atomic oxygen radical cation with ethylene have been investigated extensively at 300 K in 0.5 Torr of helium in a selected ion flow tube (SIFT). To help understand the ethylene data, five additional terminal alkenes (propene, isobutene, isoprene, styrene, and vinylidene chloride) were also examined. Considerable care was taken to account for the extremely high reactivity of O·+ (i.e. correction for reaction with trace levels of impurity in the helium) and the possibility of generation of electronically excited reactant ions. Ethylene was found to react on 93% of encounters, and to give 26% of its parent radical cation, 18% of the vinyl cation, 47% of acetylene radical cation, and 9% of protonated carbon monoxide. A mechanistic proposal for how these ions arise is presented. Extension of the mechanistic proposal for ethylene accounts for the reactivity observed for the other alkenes as well. Correlation of the yield of charge transfer product from each alkene with ionization energy (IE) suggests that primary event in the reaction is charge transfer. One exception is styrene: It has the lowest IE of the alkenes examined but the highest yield of molecular radical cation, leading to the suggestion that styrene may yield electronically excited ion products.

A new, general method for the determination of binding constants: photoacoustic titration of dansylamide with carbonic anhydrase

We have developed a new method utilizing the partitioning of energy between radiative and non-radiative pathways following photostimulation for the determination of binding constants in biochemical systems; application of this photoacoustic calorimetry technique to the study of the inhibition of carbonic anhydrase by 1-(dimethylamino)-5-naphthalenesulphonamide yields K11= 1.7 (±0.4)× 106 mol–1 dm3.

The gas-phase basicity and H/D exchange characteristics of the parent thiocarbonyl enolate anions

The enethiolate anions corresponding to thioacetaldehye and thioacetone, synthesized via elimination reactions, undergo H/D exchange reactions, demonstrating the intermediacy of the thiocarbonyl tautomers, and proton transfer reactions, leading to ΔGacid0[CH3CHS]= 341 ± 3 kcal mol–1 and ΔGacid0[(CH3)2CS]= 344 ± 3 kcal mol–1(1 kcal = 4.184 KJ).