Annett M. Lossack

Bio: Annett M. Lossack is an academic researcher from University of Stuttgart. The author has contributed to research in topics: Kinetic isotope effect & Hydrogen atom abstraction. The author has an hindex of 4, co-authored 5 publications receiving 55 citations.

Kinetic isotope effects in H and D abstraction reactions from alcohols by D atoms in aqueous solution.

Abstract: Electron paramagnetic resonance free induction decay attenuation measurements were performed in the range of liquid D{sub 2}O for the reactions of D atoms with undeuterated and deuterated alcohols. Excellent Arrhenius behavior represented by log(k/M{sup -1} s{sup -1}) = (10.97 {+-} 0.14) - [(24.7 {+-} 0.8) kJ mol{sup -1}/2.303RT] for CH{sub 3}OD, log(k/M{sup -1}s{sup -1}) = (11.09 {+-} 0.12) - [(20.8 {+-} 0.6) kJ mol{sup -1}/2.303RT] for CH{sub 3}CH{sub 2}OD, log(k/M{sup -1} s{sup -1}) = (11.45 {+-} 0.10) - [(28.4 {+-} 0.6) kJ mol{sup -1}/2.303RT] for CD{sub 3}CD{sub 2}OD, log(k/M{sup -1} s{sup -1}) = (11.32 {+-} 0.15) - [(21.3 {+-} 0.8) kJ mol-1/2.303RT] for CH{sub 3}CH{sub 2}CH{sub 2}OD, log(k/M-1 s-1) = (11.60 {+-} 0.06) - [(21.2 {+-} 0.3) kJ mol-1/2.303RT] for(CH{sub 3}){sub 2}C(H)OD, log(k/M-1 s-1) = (11.48 {+-} 0.12) - [(24.7 {+-} 0.7) kJ mol-1/2.303RT] for(CD{sub 3}){sub 2}C(D)OD, was found in all cases. Compared with the gas phase, the reactions exhibit slightly higher activation energies, in agreement with expectation for solvation based on a predicted decrease of the dipole moment from the reactant alcohol to the transition state. A pronounced increase in Arrhenius preexponential factors is attributed mostly to equilibrium solvation of the D atom, but a significant difference between H{sub more » 2}O and D{sub 2}O suggests contributions of a dynamic solvent effect. « less

Solvation and kinetic isotope effects in H and D abstraction reactions from formate ions by D, H and Mu atoms in aqueous solution

Abstract: Electron paramagnetic resonance free induction decay attenuation and muon spin rotation measurements were performed in the temperature range of liquid water for the reactions of the hydrogen isotopes D, H, and Mu with undeuterated and deuterated formate ions. Accurate rate constants were determined, and excellent Arrhenius behavior represented by was found in all cases. Ab initio calculations at the MP2 and the QCISD level with the aug-cc-pvDZ basis set reveal that the reaction has no electronic barrier in the gas phase. This contrasts with quite sizeable activation energies observed in aqueous solution, and it suggests that the barrier is entirely solvent induced. Calculations at the above mentioned ab initio level using a polarized dielectric continuum for the solvated reaction system restore a realistic barrier and confirm this interpretation. It is shown that the solvent effect is a consequence of a pronounced change of polarization of the system along the reaction path. It may be more appropriate to describe the reaction as a consecutive electron–proton transfer rather than an H atom abstraction.

Rate constants and kinetic isotope effects in hydrogen abstractions by H from formic acid

Abstract: Competitive kinetic experiments were performed for the reactions of H atoms with HCOOH and DCOOD in aqueous solution. Excellent Arrhenius behavior is represented by log(k/M-1 s-1) = (11.6 ± 0.2) - (34.0 ± 1.1) kJ mol-1/2.303RT ) for HCOOH, log(k/M-1 s-1) = (11.8 ± 0.2) - (39.8 ± 1.2) kJ mol-1/2.303RT ) for DCOOD. Kinetic isotope effects are discussed in terms of transition state theory, supported by ab initio calculations, and in comparison with previous results for the abstraction from the formate ion.

Rate constants for the reaction of the hydrogen atom with aliphatic ketones in water

Abstract: Arrhenius parameters for the reaction of hydrogen atoms with 3-methyl-2-butanone, 3-pentanone, cyclopentanone, 4-methyl-2-pentanone, and 2-butanone in aqueous solution have been directly calculated from electron paramagnetic resonance free induction decay (FID) attenuation measurements. For these compounds, absolute scavenging rate constants at 25.0 °C of (8.84 ± 0.26) × 107, (4.20 ± 0.15) × 107, (4.91 ± 0.28) × 107, (3.25 ± 0.27) × 107, and (2.20 ± 0.32) × 107 dm3 mol−1 s−1, with corresponding activation energies of 17.43 ± 0.29, 20.69 ± 0.31, 18.73 ± 0.36, 22.24 ± 0.80, and 22.30 ± 1.04 kJ mol−1 were determined, respectively. Competition kinetic measurements based on total H2 yields have established that for all of these ketones the dominant hydrogen atom reaction path is by •H atom abstraction. The new activation energy for 2-butanone is much lower than the previously reported value of 40.1 ± 0.7 kJ mol−1 with this difference attributed to interfering reactions from the added bromide previously used as...

Kinetics of the reaction of H and D with methanediol and 1,2-ethanediol in aqueous solution.

Abstract: Kinetic ESR investigation of the reactions of H and D with methanediol (hydrated formaldehyde) and 1,2-ethanediol in liquid water yield rate constants represented by Arrhenius behavior: log (k/ M −1 s −1 )=(12.56±0.20)−[(31.0±0.9) kJ mol −1 /2.303RT)] for H + CH 2 ( OH ) 2 log (k/ M −1 s −1 )=(13.16±0.19)−[(34.6±0.9) kJ mol −1 /2.303RT)] for D + CH 2 ( OD ) 2 log (k/ M −1 s −1 )=(10.95±0.18)−[(21.5±0.9) kJ mol −1 /2.303RT)] for H +( CH 2 OH ) 2 Ethanediol reacts by abstraction whereas the kinetics of methanediol reflects a temperature dependent contribution of parallel reactions.

Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst

Abstract: Formic acid (HCOOH) has great potential as an in situ source of hydrogen for fuel cells, because it offers high energy density, is non-toxic and can be safely handled in aqueous solution. So far, there has been a lack of solid catalysts that are sufficiently active and/or selective for hydrogen production from formic acid at room temperature. Here, we report that Ag nanoparticles coated with a thin layer of Pd atoms can significantly enhance the production of H₂ from formic acid at ambient temperature. Atom probe tomography confirmed that the nanoparticles have a core-shell configuration, with the shell containing between 1 and 10 layers of Pd atoms. The Pd shell contains terrace sites and is electronically promoted by the Ag core, leading to significantly enhanced catalytic properties. Our nanocatalysts could be used in the development of micro polymer electrolyte membrane fuel cells for portable devices and could also be applied in the promotion of other catalytic reactions under mild conditions.

Strong metal–molecular support interaction (SMMSI): Amine-functionalized gold nanoparticles encapsulated in silica nanospheres highly active for catalytic decomposition of formic acid

Abstract: We report the first example of monometallic gold nanoparticles, functionalized with amine and encapsulated in silica nanospheres, as a high-performance catalyst for hydrogen generation from aqueous formic acid for chemical hydrogen storage. Remarkably, the presence of amine in the silica sphere can make the gold nanoparticles highly active although the unsupported or silica-supported gold NPs being inactive for this reaction. The strong metal–molecular support interaction (SMMSI) could be extended, as a general strategy, to the development of nanocatalysts which need necessary environments around the active sites for a variety of catalytic reactions.

Direct Dynamics for Free Radical Kinetics in Solution: Solvent Effect on the Rate Constant for the Reaction of Methanol with Atomic Hydrogen

Abstract: We calculate the rate constant for the reaction •H + CH3OH → H2 + •CH2OH both in the gas phase and in aqueous solution at 298 K. To accomplish this, we apply two different methods to estimate the electronic energies along the reaction path. First, we use specific reaction parameters (SRP) to mix the exchange and correlation energies in Becke's adiabatic connection theory (AC-SRP) to optimize the model for the specific bond-breaking, bond-making combination under consideration. Second, we obtain the potential energy using a linear combination of the Hartree−Fock method and AM1 with specific reaction parameters (HF||AM1-SRP); in this linear mixing method, eight NDDO parameters and the linear mixing parameter are simultaneously optimized by a genetic algorithm. To calculate the reaction rate constants in solution, the solute atomic charges are represented by class IV charges, the electric polarization of the solvent is determined from the electronic charge distribution of the solute self-consistently, and th...

Entropies in Solution from Entropies in the Gas Phase

Abstract: Ab initio calculations at the B3LYP/6-31G(d) level and scaled particle theory, combined with entropies of activation derived from experimental Arrhenius A factors, were applied to examine the origin of the loss of gaseous-phase entropy of a substance upon solution. Eight reactions in water were analyzed: H atom reacting with methanol, ethanol, 2-propanol, methanediol and ethylene glycol; and methanethiol reacting with the radicals, methyl, hydroxymethyl, and 2-hydroxy-2-propyl. The results suggest that the observed entropy loss is entirely due to changes in the solvent. The dominant factor is a loss of entropy due to cavity formation. This is partially offset by a corresponding increase in the disorder of the H-bonding network in the case of the larger species.

Benchmark calculations of reaction energies, barrier heights, and transition-state geometries for hydrogen abstraction from methanol by a hydrogen atom.

Abstract: We report benchmark calculations of reaction energies, barrier heights, and transition state geometries for the reaction of CH 3 OH with H to produce CH 2 OH and H 2 . Highly accurate composite methods, such as CBS, G2, G3, G3X, G3SX, and multi-coefficient correlation methods (MCCMs) are used to calibrate lower-cost methods. We also performed single-level CCSD(T) calculations extrapolated to the infinite-basis limit based on aug-cc-pVXZ (X = 3, 4) correlation consistent basis sets. The benchmark highlevel calculations give consensus values of the forward reaction barrier height and the reaction energy of 9.7 kcal/mol and – 6.4 kcal/mol, respectively. To evaluate the accuracy of cost-efficient methods that are potentially useful for dynamics studies of the title reaction, we further include the results obtained by hybrid density-functional-theory methods and hybrid meta-density-functional-theory methods that have recently been designed for chemical kinetics. Results obtained by popular semiempirical methods are also given for comparison. Based on the benchmark gas-phase results, we suggest MCQCISD/3, MC3BB, and BB1K as reasonably accurate and affordable electronic structure methods for calculating dynamics for the title reaction.