title: Anchoring the Gas-Phase Acidity Scale from Hydrogen Sulfide to Pyrrole. Experimental Bond Dissociation Energies of Nitromethane, Ethanethiol, and Cyclopentadiene.
abstract: A meta-analysis of experimental information from a variety of sources is combined with statistical thermodynamics calculations to refine the gas-phase acidity scale from hydrogen sulfide to pyrrole. The absolute acidities of hydrogen sulfide, methanethiol, and pyrrole are evaluated from literature R-H bond energies and radical electron affinities to anchor the scale. Relative acidities from proton-transfer equilibrium experiments are used in a local thermochemical network optimized by least-squares analysis to obtain absolute acidities of 14 additional acids in the region. Thermal enthalpy and entropy corrections are applied using molecular parameters from density functional theory, with explicit calculation of hindered rotor energy levels for torsional modes. The analysis reduces the uncertainties of the absolute acidities of the 14 acids to within ±1.2 to ±3.3 kJ/mol, expressed as estimates of the 95% confidence level. The experimental gas-phase acidities are compared with calculations, with generally good agreement. For nitromethane, ethanethiol, and cyclopentadiene, the refined acidities can be combined with electron affinities of the corresponding radicals from photoelectron spectroscopy to obtain improved values of the C-H or S-H bond dissociation energies, yielding D298(H-CH2NO2) = 423.5 ± 2.2 kJ mol(-1), D298(C2H5S-H) = 364.7 ± 2.2 kJ mol(-1), and D298(C5H5-H) = 347.4 ± 2.2 kJ mol(-1). These values represent the best-available experimental bond dissociation energies for these species.
authors.affiliation: Department of Chemistry and Chemical Physics Program, University of Nevada, Reno, 1664 North Virginia Street MS 216, Reno, Nevada 89557-0216, United States.
authors.forename: Kent M
authors.initials: KM
authors.lastname: Ervin
authors.affiliation: Department of Chemistry and Chemical Physics Program, University of Nevada, Reno, 1664 North Virginia Street MS 216, Reno, Nevada 89557-0216, United States.
authors.forename: Alex A
authors.initials: AA
authors.lastname: Nickel
authors.affiliation: Department of Chemistry and Chemical Physics Program, University of Nevada, Reno, 1664 North Virginia Street MS 216, Reno, Nevada 89557-0216, United States.
authors.forename: Jerry G
authors.initials: JG
authors.lastname: Lanorio
authors.affiliation: Department of Chemistry and Chemical Physics Program, University of Nevada, Reno, 1664 North Virginia Street MS 216, Reno, Nevada 89557-0216, United States.
authors.forename: Surja B
authors.initials: SB
authors.lastname: Ghale