Infrared Probing of Equilibrium and Dynamics of Metal-Selenocyanate Ion Pairs in N,N-Dimethylformamide Solutions

Abstract

Spectroscopic properties (i.e., peak positions and widths) of vibrational probes are sensitively dependent on their local environments in liquids. Such spectroscopic sensitivities can be utilized for studying the structures and dynamics of a variety of molecular systems. Here, we have studied the ion pairing equilibrium and dynamics of SeCN- ion pairs with Li+ and Mg2+ cations in N,N-dimethylformamide (DMF). SeCN- ion is an excellent vibrational probe for studying ion dynamics in electrolyte solutions, not only because the vibrational lifetime of the CN stretch is substantially long but also because the CN stretch frequency is very sensitive to its local environment. When SeCN- ion forms contact ion pairs (CIPs) with Li+ (Mg2+) ion in DMF solutions, the CN stretch frequency is found to be significantly blue-shifted such that free SeCN- ion is spectrally well distinguished from Li-SeCN CIP and Mg-SeCN+ CIP. This fact allows us to study the ion pairing equilibrium between SeCN- ion and metal ions as well as the dynamics of metal-SeCN- ion pairs. Ion pairing equilibrium between SeCN- ion and Li+ (or Mg2+) was studied by temperature-dependent Fourier transform infrared (FTIR) spectroscopy. The formation of CIPs in DMF was found to be entropically favored. Time-resolved IR pump-probe spectroscopy was used to study the vibrational population relaxation and orientational relaxation dynamics. Vibrational lifetimes of free SeCN- ion, Li-SeCN CIP, and Mg-SeCN+ CIP were determined to be 83.6, 72.3, and 55.6 ps, respectively. Orientational relaxation dynamics were found to get slower in the order free SeCN- ion, Li-SeCN CIP, and Mg-SeCN+ CIP. The orientational anisotropy decays of the CIPs, which were well fit by a biexponential function, were explained by two orientational relaxation processes, that is, a restricted (tethered) orientational relaxation of SeCN- within the CIPs followed by the overall orientational diffusion of the whole CIPs. The orientational relaxation time constants of Li-SeCN CIP and Mg-SeCN+ CIP in DMF were twice different but the orientational diffusion radii calculated by the Debye-Stokes-Einstein equation were found to be almost identical within experimental error. The biexponential decay of the orientational anisotropy was analyzed by the wobbling-in-a-cone model. As a vibrational probe, SeCN- ion and SeCN group can be potentially used for measuring the molecular dynamics on a relatively long time scale because of their long lifetimes.