Gas phase acidities and associated equilibrium isotope effects for selected main group mono- and polyhydrides, carbon acids, and oxyacids: A G4 and W1BD study

Gas phase standard state (298.15 K, 1 atm) enthalpies (ΔacidH°(g)) and free energies (ΔacidG°(g)) of acid dissociation were calculated at the Gaussian-4 (G4) and W1BD levels of theory for a range of perproteated, perdeuterated, pertritiated, and partially isotopically labeled main group mono- and polyhydrides, carbon acids, and oxyacids. Excellent agreement was obtained between the available experimental datasets and the theoretical estimates, with effectively equivalent ΔacidH°(g)/ΔacidG°(g) prediction accuracy for the G4 and W1BD methods on carbon acids and oxyacids. The W1BD level of theory provided ΔacidH°(g)/ΔacidG°(g) errors about one-half those of the G4 method on main group hydrides. G4 and W1BD predicted primary and secondary equilibrium isotope effects (EIEs) on gas phase acidity for deuterium and tritium substitution exhibit periodic trends among the main group hydrides, as well as acid strength and structure-property relationships for some groups of carbon acids and oxyacids. Primary EIEs calculated at the W1BD level for various isotopologues of the H2, LiH, CH4, NH3, SiH4, SH2, and ClH main group hydrides using the major isotopes for each non-protic element declined in magnitude with increasing atomic number due to decreasing mass differences between the respective isotopologues.