Temperature dependence of excited state proton transfer in ice

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, k_PT. d(ln k_PT)/d(1/T) decreases as the temperature decreases. The average slope of ln(k_PT)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 k_PT 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.