Picosecond dynamics of a small molecule in aqueous solution probed by PAC spectroscopy

111 Ag PAC spectroscopy was applied to explore the rapid dynamics of a small, inorganic probe in water/sucrose solutions with viscosity, ξ , from 1 to 121 mPa∙s at 9 °C or 20 °C. The measured characteristic timescale does not scale linearly with ξ/T , as predicted by the Stokes–Einstein–Debye model for the rotational correlation time, and power-law scaling is also unsatisfactory. In contrast, a model that incorporates local probe-site dynamics on a ~ 400 ps timescale describes the data well. This is the first indication that PAC spectroscopy can resolve both rotational diffusion and picosecond local dynamics via their different ξ/T scaling. • PAC spectroscopy may elucidate rapid (picosecond) local structural fluctuations at the probe site. • Varying viscosity allows for discrimination of pure rotational diffusion from local dynamics. • The estimated time scale of the rotational diffusion is 30 ps – 3 ns, and the estimated time scale of local dynamics of the small inorganic species is ~400 ps.