Origins of relative acidity: First and second period hydrides

The origins of the trends of relatively acidity across and between the first and second period hydrides (BH₃, CH₄, NH₃, H₂O, HF, AIH₃, SiH₄, PH₃, H₂S, and HCl) were investigated using molecular and subsystem quantum mechanics at the Hartree-Fock (HF)/6–31 + + G ^**//HF/6–31 + + G ^** level of theory. The total deprotonation energies, Δ E_acid, are interpreted in terms of three component processes: Δ E₁; deprotonation without electronic and nuclear relaxation; Δ E₂, electronic relaxation within the acid geometry; and Δ E₃, nuclear relaxation. Δ E₁ is given from the electrostatic potential at the acidic proton, Δ E₃ + Δ E₂ (= Δ E ^*) is given from the calculated energy of the conjugate anion at the acid geometry. The increased acidity across a given period is shown to be already mostly an inherent property of the acid. © 1996 by John Wiley & Sons, Inc.