Ultrafast Protonation of Cyanate Anion in Aqueous Solution

We use femtosecond infrared spectroscopy to study the aqueous protonation dynamics of cyanate, OCN-, using a photoacid, 2-naphthol-6,8-disulfonate (2N-6,8S) excited by a 60-fs pulse tuned at 336 nm. The transient response in the spectral range of the cyano-stretching vibrational marker modes of cyanic acid, HOCN, and isocyanic acid, HNCO, reveals how much of both reaction products are formed at early delay times, and whether the on-contact reactive complex between 2N-6,8S and OCN- has a well-defined Structure. Using the Szabo-Collins-Kimball approach to describe bimolecular reaction dynamics subject to the Debye-von Smoluchowski diffusional motions, an on-contact proton transfer reaction rate is derived that follows the correlation between free energy and reaction rates found for a large class of aqueous proton transfer of photoacid dissociation and photoacid-base neutralization reactions.