Anchoring the Gas-Phase Acidity Scale

Abstract

Theoretical calculations and experimental values from the recent literature are used to construct and evaluate a high precision gas-phase acidity scale. Gas-phase acidities at 0 K are evaluated for 12 reference species with accurately known acidities. Using recent spectroscopic results, small but significant revisions are presented for the acidities of ammonia, water, and formaldehyde. These revised anchor acidities are applied to previous thermokinetic or equilibrium measurements of the acidities of small alkanols, ethene, and benzene. Combined with electron affinities from literature negative ion photoelectron spectroscopy measurements, the revised acidities yield the following improved bond dissociation enthalpies: D298(CH3O−H) = 437.7 ± 2.8 kJ/mol, D298(C2H5O−H) = 438.1 ± 3.3 kJ/mol, D298((CH3)2CHO−H) = 442.3 ± 2.8 kJ/mol, D298((CH3)3CO−H) = 444.9 ± 2.8 kJ/mol, D298(C2H3−H) = 463.0 ± 2.7 kJ/mol, and D298(C6H5BH) = 472.2 ± 2.2 kJ/mol. Calculation of gas-phase acidities at 0 K are investigated for several levels of theory. Excellent performance at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level is found for 16 acids composed of elements through chlorine, with a mean error of −0.2 kJ/mol and a mean absolute error of 1.5 kJ/mol.