Temperature dependence of excited state proton transfer in ice

Pavel Leiderman, Anna Uritski, Dan Huppert

Research output: Contribution to journalArticlepeer-review

Abstract

We have studied the excited-state proton-transfer rate of four photoacids in ice as a function of temperature. For all four photoacids, we have found a non Arrhenius behavior of the proton-transfer rate constant, kPT. d(ln kPT)/d(1/T) decreases as the temperature decreases. The average slope of ln(kPT)versus 1/T depends on the photoacid strength (pK*). The stronger the photoacid is, the smaller the slope. For the strongest photoacid 2-naphthol-6,8-disulfonate (2N68DS) the largest slope is 35 kJ/mol at about 270 K, and the smallest measured slope is about 8 kJ/mol at about 215 K. We propose that the temperature dependence of kPT in ice at the temperature range 270 > T > 200 K can be explained as arising from contributions of two proton-transfer mechanisms over the barrier and tunneling under the barrier. At very low temperatures T < 200 K, the slope of ln(k PT) versus 1/T increases again. At about 170 K, the proton-transfer rate is much slower than the radiative rate, and the deprotonated form of the photoacid cannot be detected in the steady-state emission spectrum. At lower temperatures, T < 200 K, the rate further decreases because of a limitation on the reaction caused by the restrictions on the H2O hydrogen reorientations.

Original languageEnglish
Pages (from-to)4998-5007
Number of pages10
JournalJournal of Physical Chemistry A
Volume111
Issue number23
DOIs
StatePublished - 14 Jun 2007

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