Charles H. DePuy

The tandem flowing afterglow-shift-drift

Abstract The design of a new tandem flowing afterglow-SIFT-drift instrument which provides high sensitivity, resolution, and chemical versatility is described. The performance of the instrument is evaluated in terms of (a) the intensities and variety of ions which can be generated, mass-selected, and injected; (b) the efficiency of the dual annulus SIFT injector as a venturi inlet; and (c) the reliability of the kinetic data.

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.

Thermochemistry of the benzyl and allyl radicals and ions

Abstract We have studied the thermochemistry of the resonantly stabilized radicals, C6H5CH2 and CH2CHCH2 and their corresponding cations and anions. A flowing afterglow/selected ion flow tube instrument has been used to measure the rates of reaction: C6H5CH3 + CH3O− ⇌C6H5CH2− + CH3OH C6H5CH3 + CD3O− ⇌C6H5CH2− + CD3OH C6D5CD3 + CH3O− ⇌C6D5CD2− + CH3OD C6D5CD3 + CD3O− ⇌C6D5CD2− + CD3OD CH2CHCH3 + HO− ⇌CH2CHCH2− + H2O The ratio of the rate constants gives a free energy change for each reaction and use of the established gas phase acidity of CH3OH or H2O provides values for the acidities. We calculate the entropy changes, ΔacidS300(C6H5CH3) and ΔacidS300(CH2CHCH3), to extract values for ΔacidH300(C6H5CH3) and ΔacidH300(CH2CHCH3). Earlier photoelectron experiments have provided ionization potentials and electron affinities for the benzyl and allyl radicals. Use of the IPs and EAs, together with the enthalpies of deprotonation, provides values for the C H bond enthalpies at 300 K and the C H bond energies at 0 K. These bond energies are used to compute the heats of formation of the radicals and ions as well as the cation hydride affinities, HA (all values in kcal mol−1): C6H5CH2−H CH2CHCH2−H ΔacidG300(R−H) 374.9 ± 0.2 383.8 ± 0.1 ΔacidH300(R−H) 382.3 ± 0.5 391.1 ± 0.3 DH300(R−H) 89.8 ± 0.6 88.8 ± 0.4 D0(R−H) 88.1 ± 0.6 87.4 ± 0.4 ΔfH0(R) 54.1 ± 0.6 44.4 ± 0.5 ΔfH300(R) 49.7 ± 0.6 41.4 ± 0.4 ΔfH0(R−) 33.1 ± 0.6 33.1 ± 0.5 ΔfH300(R−) 28.7 ± 0.5 30.1 ± 0.4 ΔfH0(R+) 221.3 ± 0.6 231.6 ± 0.7 ΔfH300(R+) 216.8 ± 0.6 228.9 ± 0.6 HA0(R+) 237.9 ± 0.7 254.7 ± 0.7 HA300(R+) 239.5 ± 0.6 258.9 ± 0.7 In addition, we find ΔacidG300(C6D5CD3) = 377.0 ± 0.3 kcal mol−1.

Studies of the reaction of O+2 with deuterated methanes

In the gas phase O+2 reacts with methane at 300 K to produce a hydrogen atom and the CH3O+2 ion. The structure of this ion has recently been determined to be H2COOH+, methylene hydroperoxide ion. The reaction rate coefficients and product distributions have now been measured at 300 K for the CHnD4−n isotopes. The reaction shows both inter‐ and intramolecular isotope effects, e.g., CH2D2 reacts more slowly than methane and more rapidly than CD4, but loses hydrogen or deuterium with equal probability. The ion readily transfers HO+ to alkenes, CS2, and many other neutral molecules. The reaction with CS2 has been used to investigate the isotopic distribution within mixed isotope product ions. In addition, the reaction rate coefficients for both CH4 and CD4 have been measured as functions of temperature between 20 and 500 K; in both cases a clear minimum is observed in the reaction rate coefficient near room temperature. A mechanism for the reaction is proposed which allows us to model the temperature dependen...

Chemical Reactions of Anions in the Gas Phase

Anions of many types, both organic and inorganic, farmiliar and exotic, can be generated in the gas phase by rational chemical synthesis in a flowing afterglow apparatus. Once formed, the rates, products, and mechanisms of their reactions with neutral species of all kinds can be studied, not only at room temperature but at higher energies in a drift field. These completely unsolvated ions undergo a large number of reactions that are analogous to those they undergo in solution, as well as some that are less familiar. New types of ions, for which there are no counterparts in solution, can be produced and their chemical reactions explored.

Photoelectron spectroscopy, gas phase acidity, and thermochemistry of tert-butyl hydroperoxide: Mechanisms for the rearrangement of peroxyl radicals

The 3.531 eV negative ion photoelectron spectra of the hydroperoxide ion and the tert-butylperoxide ion have been studied. We find HO2−+ℏω351.1 nm→HO2+e− EA[HO2,X 2A″]=1.089±0.006 eV, (CH3)3COO−+ℏω351.1 nm→(CH3)3COO+e− EA[(CH3)3COO,X 2A″]=1.196±0.011 eV. The photoelectron spectra show detachment to the ground state of the peroxyl radicals and to a low lying electronic state. The intercombination gaps are measured to be ΔE(X 2A″–A 2A′)[HO2]=0.871±0.007 eV and ΔE(X 2A″–2A′)[(CH3)3COO]=0.967±0.011 eV. The gas phase acidity of (CH3)3COOH was measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to be ΔacidG298=363.2±2.0 kcal mol−1 and we find ΔacidH298[(CH3)3COO–H]=370.9±2.0 kcal mol−1. Use of ΔacidH298[(CH3)3COO–H] and EA[(CH3)3COO] leads to the bond energies DH298[(CH3)3COO–H]=85±2 kcal mol−1 and D0[(CH3)3COO–H]=83±2 kcal mol−1. The thermochemistry of the alkylperoxyl radicals, RO2, is reviewed. A mechanism for the rearrangement of chemically activated peroxyl radicals is proposed [RO2...

Deuterium kinetic isotope effects in gas phase SN2 reactions

Abstract Rate coefficients and secondary α-deuterium kinetic isotope effects (KIEs) for the nucleophilic substitution (S N 2) reactions of Cl − + CH 3 Br → CH 3 Cl + Br − , Cl − + CH 3 I → CH 3 Cl + I − , Br − + CH 3 I → CH 3 Br + I − , and their perdeuterated analogs are remeasured in the gas phase at 300 K. The measurements confirm our previous results [Gronert et al., J. Am. Chem. Soc. 113 (1991) 4009] and substantial inverse KIEs are measured with an improved accuracy: k H / k D = 0.77 (±0.03), 0.84 (±0.01), and 0.76 (±0.01), respectively, for the above systems. Thus, the experimentally observed effect of deuterium substitution in these reactions is considerably more dramatic than predicted previously by transition state theory where KIEs are calculated to be closer to unity. The experimental KIE values for the series of S N 2 reactions, F − , Cl − +CH 3 Br and F − , Cl − , Br − +CH 3 I are both found to decrease (i.e. become more inverse ) with an increase in the transition-state looseness parameter ( R TS ) for larger halide anions, in contrast to those for lighter methyl halides which show the expected positive correlations with R TS .

The heats of formation of cyclic and linear C3H+2

The hydrogen atom abstraction reaction of C3H+ with H2 is shown by gas-phase ion—molecule bracketing reactions to be exothermic, with a small (about 1 kcal mol−1 barrier. The heats of formation of the cyclic and linear isomers of C3H+2 are determined to be 322 ± 4 kcal mol−1 and 334 ± 4 kcal mol−1 respectively. Ab initio calculations confirm that the cyclic isomer is the more stable form by 12 kcal mol−1. The results are compared with the thermochemistry determined by other workers.

Gas phase hydrogen/deuterium exchange reactions of fluorophenyl anions

Hydrogen/deuterium (H/D) exchange reactions of fluorophenyl and difluorophenyl anions (C6H4F−, o-C6H3F2−, m-C6H3F2−, p-C6H3F2−) have been studied using the flowing afterglow-selected ion flow tube technique. The C6H4F− anion exchanges all hydrogens for deuterium upon reaction with D2O. The difluorophenyl anions o-, m-, and p-C6H3F2− exchange three, two, and one hydrogen, respectively, with D2O, whereas they undergo one, two, and three H/D exchanges, respectively, with CH3OD. The structures of the anions and the isotope exchange dynamics within the intermediate ion-dipole complexes are discussed using ab initio molecular orbital calculations. Calculated values for the proton affinities of the most stable anions are 385.2, 378.0, 371.9, and 378.2 kcal/mol for C6H4F−, o-C6H3F2−, m-C6H3F2−, and p-C6H3F2−, respectively, in excellent agreement (within 2 kcal/mol) with the previous experimental values for the acidities of the corresponding fluorobenzenes. The H/D exchange results are explained by the energy differences of the intermediate DO− and CH3O− species within the ion-dipole complexes; CH3O− is mobile within the “hot” intermediate complex, whereas DO− is nearly “frozen” within the complex and cannot migrate across the barriers caused by the fluorine atoms or by the π electrons.

Experimental and computational studies of deuterated ethanols: gas-phase acidities, electron affinities and bond dissociation energies

Abstract The tandem flowing afterglow-selected ion flow tube has been employed to measure the gas-phase acidities of a family of deuterated ethanols. We find that both α and β deuterated ethanols are weaker acids than undeuterated ethanol with α deuteration having a more pronounced effect. The acidities, relative to undeuterated ethanol [ΔHacid,298 K (CH3CH2OH) = 377.5 ± 2kcal mol−1] are ΔΔHacid(CD3CH2OH) = 0.20 ± 0.15kcal mol−1, ΔΔHacid(CH3CD2OH) = 0.35 ± 0.15 kcal mol−1, and ΔΔHacid0(CD3CD2OH) = 0.50 ± 0.15 kcal mol−1. In a separate set of measurements, we have studied the negative ion photoelectron spectroscopy of a set of ethoxide ions to determine the electron affinities of the corresponding ethoxy radicals. The electron affinities decrease with increasing deuterium substitution. We find: EA(CH3CH2O) = 39.55 ± 0.16kcal mol−1, EA(CH3CD2O) = 39.48 ± 0.16kcal mol−1, EA(CD3CH2O) = 39.41 ± 0.16kcal mol−1, and EA(CD3CD2O) = 39.27 ± 0.16 kcal mol−1; the relative error bars are ±0.05 kcal mol−1. The corresponding O-H bond strengths of the deuterated ethanols can be extracted from these acidities and electron affinities. We find that α deuteration increases the OH bond strength by 0.3 ± 0.2 kcal mol−1 while β deuteration has a negligible effect. The bond dissociation energies, relative to undeuterated ethanol [D0,OK (CH3CH2OH) = 102.2kcal mol−1] are D0,OK(CD3CH2OH) = 102.3 ±0.2kcal mol−1, D0,OK(CH3CD2OH) = 102.5 ± 0.2kcal mol−1, and D0,OK(CD3CD2OH) = 102.5 ± 0.2kcal mol−1. Indicated error bars are relative errors; absolute errors are ±2 kcal mol−1. Hartree-Fock-SCF calculations were performed on the various deuterated and undeuterated ethanols, ethoxy radicals, and ethoxide anions to calculate the relative acidities, electron affinities and bond dissociation energies. The results are in good agreement with our experimental values.