Molecular Anion Hydrogen Bonding Dynamics in Aqueous Solution.

Abstract

The dynamic nature of hydrogen bonding between a molecular anion, selenocyanate (SeCN(-)), and water in aqueous solution (D2O) is addressed using FT-IR spectroscopy, two-dimensional infrared (2D IR) vibrational echo spectroscopy, and polarization selective IR pump-probe (PSPP) experiments performed on the CN stretching mode. The CN absorption spectrum is asymmetric with a wing on the low frequency (red) side of the line in contrast to the spectrum in the absence of hydrogen bonding. It is shown that the red wing is the result of an increase in the CN stretch transition dipole moment due to the effect of hydrogen bonding (non-Condon effect). This non-Condon effect is similar in nature to observations on pure water and other nonionic systems where hydrogen bonding enhances the extinction coefficient. The 2D IR measurements of spectral diffusion (solvent structural evolution) yield a time constant of 1.5 ps, which is within error the same as that of the OH stretch of HOD in D2O (1.4 ps). The orientational relaxation of SeCN(-) measured by PSPP experiments is long (4.04 ps) compared to the spectral diffusion time. The population decay at or near the absorption line center is a single-exponential decay of 37.4 ± 0.3 ps, the vibrational lifetime. However, on the red side of the line the decay is biexponential with a low amplitude, fast component; on the blue side of the line there is a low amplitude, fast growth followed by the lifetime decay. Both of the fast components have 1.5 ps time constants, which is the spectral diffusion time. The fast components of the population decays are the results of the non-Condon effect that causes the red side of the line to be over pumped by the pump pulse. Spectral diffusion then produces the fast decay component on the red side of the line and the growth on the blue side of the line as the excess initial population on the red side produces a net population flow from red to blue.