Hamburg 2009 – scientific programme

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MO: Fachverband Molekülphysik

MO 8: Femtosekundenspektroskopie 2

MO 8.5: Talk

Tuesday, March 3, 2009, 15:00–15:15, VMP 6 HS-F

Ultrafast protonation of cyanate anion in aqueous solution — •Katrin Adamczyk¹, Jens Dreyer¹, Dina Pines², Ehud Pines², and Erik T. J. Nibbering¹ — ¹Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, D-12489 Berlin, Germany — ²Department of Chemistry, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84125, Israel

Protonation of cyanate anion, OCN*, can in principle occur on both the O- and N-site, leading to cyanic acid, HOCN, and isocyanic acid, HNCO, respectively. Resolving this ambiguity in reaction pathways as well as determining the protonation time scale will be of great use in understanding the hydrolysis mechanism of OCN*, producing CO2 and NH3, and the Wöhler synthesis, where urea is formed using NH4+ and OCN*, as well as understanding chemistry in interstellar space. We use femtosecond infrared spectroscopy to study the aqueous protonation dynamics of OCN* using a photoacid, 2-naphthol-6,8-disulfonate excited by a 50 fs UV pulse. Following the transient response of vibrational marker modes of cyanic acid and 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.