Vibrational Frequency Fluctuations of Ionic and Non-ionic Vibrational Probe Molecules in Aqueous Solutions

Abstract

In this chapter we review our studies on vibrational frequency fluctuations in aqueous solutions. Experimentally, the frequency fluctuations were investigated by measuring frequency-frequency time-correlation function (FFTCF) by two-dimensional spectroscopy, and molecular dynamics simulations were carried to obtain molecular pictures for the frequency fluctuations in water. We here compare the two different types of vibrational probes; one is non-ionic, and the other is ionic. For the non-ionic probe, we chose 2-nitro-5-thiocyanate benzoic acid (NTBA), which possesses an electronically neutral vibrational probe (SCN group) and an aromatic ring. For the ionic probe, three atomic ions, SCN− and N3−, were used. Although the charge distributions of the solutes and, consequently, hydration structures around the solutes are different for the ionic and non-ionic vibrational probes, both the ionic and non-ionic probes show similar decay time constants for FFTCF of about 1 ps. It is found that the frequency fluctuation for three atomic ions is almost determined by the electrostatic interaction from the water molecules in the first hydration shell. The collective dynamics of the water molecules in the first-hydration shell are found to be similar to those of bulk water, though the hydrogen bond between the ion and water molecule is very strong. In contrast to the hydration structure and water dynamics in these small ionic solutions, the hydration structure and water dynamics in the vicinity of the vibrational probe of NTBA are found to be similar to those in bulk. We consider that the electrostatic interactions with “bulk-like” water molecules around NTBA induce the slow decay component, i.e., 1-ps decay component, of the FFTCF of NTBA in H2O.