Dynamics of water molecules in an alkaline environment

We report on a two-color mid-infrared pump–probe spectroscopic study of the dynamics of the OH stretch vibrations of HDO molecules dissolved in a concentrated (10 M) solution of NaOD in D₂O. We observe that spectral holes can be created in the broad OH stretch absorption band that change neither position nor width on a picosecond time scale. This behavior differs strongly from that of pure HDO:D₂O where rapid spectral diffusion (t_c^~600 fs) occurs. The long-living inhomogeneity indicates that a concentrated aqueous NaOX (X = H,D) solution has a very static hydrogen-bond network. The results also show that the absorption band of the OH stretch vibration consists of two separate classes of OH groups with very different vibrational lifetimes. For component I, the lifetime of the OH stretch vibration is ~600 fs and increases with OH frequency, which can be explained from the accompanying decrease in the strength of the hydrogen-bond interaction. This component represents HDO molecules of which the OH group is bonded to a D₂O molecule via a DO–H^. OD₂ hydrogen bond. For component II, the lifetime is ~160 fs, and does not show a significant frequency dependence. This component represents HDO molecules that are hydrogen bonded to a D₂O molecule or an OD^– ion. The short, frequency-independent vibrational lifetime of component II can be explained from the participation of the HDO molecule and its hydrogen-bonded partner in deuteron and/or proton-transfer processes.