Hydrogen bond dynamics of 9-fluorenone derivatives in water studied by two-dimensional infrared spectroscopy

Abstract

In solutions, a vibrational frequency of solute molecule is sensitive to changes of the solvent environment. Especially, in aqueous solution, hydrogen-bond making and breaking processes play an important role in the dynamics of water molecules surrounding the solute. For the past few decades, vibrational dynamics in aqueous solutions have been investigated by two-dimensional infrared (2D-IR) spectroscopy. 2D-IR spectroscopy is a powerful tool to obtain microscopic information in solution such as ultrafast solvent dynamics and local structure of solvent by probing the changes of vibrational frequency. In the present study, we performed 2D-IR spectroscopic measurements on 9-fluorenone-4-carboxylic acid (FL-4) and 9-fluorenone-2-carboxylic acid (FL-2, molecular structure is shown in Figure 1(a)) in aqueous solutions. By comparing the results of FL-4 and FL-2, we aim to find the relationship between molecular structure and vibrational dynamics. IR absorption D 2 the CO stretching mode is observed. The IR absorption spectra of FL-4 and FL-2 are reproduced by a single Gaussian function and a sum of two Gaussian functions, respectively. the analogy our we assigned the higher and lower bands in the IR spectrum of FL-2 to a complex of FL-2 and one D 2 O and a complex of FL-2 and two D 2 O, respectively. 2D-IR measurement provides information on the correlation of the vibrational spectrum at the pump frequency ( ω 1 ) and probe frequency ( ω 3 ). Figure 1(b) shows 2D-IR spectrum of FL-2 in D 2 O. The off-diagonal component at ( ω 1 , ω 3 ) = (1694 cm -1 , 1709 cm -1 ) may have information about the chemical exchange process occurring between the two hydrogen-bonded complexes. Therefore, we analyzed the 2D-IR spectra to obtain dynamical information of the hydrogen-bond making and breaking processes between the CO group and surrounding water molecules. Moreover, to obtain microscopic details of the system, we also conducted molecular dynamics (MD) simulation. By analyzing the trajectories obtained by MD simulation, we found that hydration structure around the CO group of FL-2 is largely different from that of FL-4. Consequently, the hydrogen bond dynamics between the CO group and water molecules differ between FL-2 and FL-4.