Conformational Effects on Gas-Phase Acidities of Isomeric C3 and C5 Alkanols.

The competitive threshold collision-induced dissociation technique is used to examine conformational effects on the relative gas-phase acidities of selected alcohols. By use of HF and H2O as reference acids in a local thermochemical network to obtain absolute acidities, the measured 0 K gas-phase acidities for the propanol and pentanol isomers are Δacid H0(CH3CH2CH2O-H) = 1563.9 ± 2.9 kJ/mol, Δacid H0((CH3)2CHO-H) = 1568.2 ± 2.7 kJ/mol, Δacid H0(CH3(CH2)4O-H) = 1556.4 ± 2.9 kJ/mol, and Δacid H0((CH3)2CHCH2CH2O-H) = 1556.5 ± 3.0 kJ/mol. Conformational stabilization during deprotonation results in the observed acidity differences between isomers, which can be compared with the "intrinsic" acidity strength defined as deprotonation of the extended all- anti staggered conformations without relaxation. The intrinsic acidities for the propanol and pentanol isomers are 1567 and 1562 kJ/mol, respectively. The difference in intrinsic and observed acidity is largely due to the result of a twisted geometry of the alkoxide ion, stabilized by electrostatic interaction between the electronegative terminal O atom and a H atom on the γ-carbon. These interactions are primarily due to internal rotation about the Cα-Cß bonds for n-propoxide and the primary pentoxides.

Anchoring the Gas-Phase Acidity Scale from Hydrogen Sulfide to Pyrrole. Experimental Bond Dissociation Energies of Nitromethane, Ethanethiol, and Cyclopentadiene.

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.