From the best-fit protonation rate constant k p = 10 7 M -1 s -1 and the pKa value of pKa 2 = 3.4, the deprotonation rate constant of SqH -is estimated through equation 32 to be given by k d ≈ 4.0 × 10 3 s -1 . This implies that the mean lifetime of a SqH -molecule, before losing its proton, is