Protonated p-Benzoquinone.

The structure and energetics of protonated p-benzoquinone (pBQ) have been investigated using high-pressure mass spectrometry and ab initio calculations. The experimental proton affinity of pBQ is 801.4 +/- 8.9 kJ/mol (191.5 +/- 2.1 kcal/mol) (1sigma) from bracketing measurements and hydration thermochemistry. This value is supported by theory and by analogies with related compounds. In its protonation chemistry, pBQ behaves as an aliphatic ketone, both structurally and energetically. The dissociation of the hydrate (pBQH(+)).(H(2)O) is characterized by DeltaH degrees (D) = 90.0 +/- 2.3 kJ/mol and DeltaS degrees (D) = 123.4 +/- 4.9 J/mol.K (95% confidence).