Considering first the simplest case of phosphinous acid 1H,H, the contribution of the tunneling transmission coefficient (kT) is predicted to be the largest among all the studied reactions, giving a kT value of 138 at 298.15 K. Such a sizable tunneling effect is probably the largest hitherto predicted for a tautomerization reaction at room temperature. This kappa factor results in an apparent reaction barrier ∼3.0 kcal mol -1 below the classical barrier height at 298.15 K, indicative of tunneling from thermally activated states rather than the ground-state vibrational level (v = 0). It is interesting to note that for 1H,H, the semiclassical kSC rate of 5 × 10 [a] The tautomerization reaction was studied from P(V)2w → P(III)2w.