High pressure study of associated media: Raman scattering of pyridine complexes in aqueous solution

The ν1 and ν12 ring breathing vibrational modes of pyridine in aqueous solution were studied as functions of concentration (up to 12 M) and pressure (up to 35 kbar) in a diamond anvil cell. The pyridine isotopes -h5 and -d5 exhibited marked differences. The pressure dependence of ν1 is quite linear for pyr-h5 but has pronounced curvature for pyr-d5 and this curvature increases as the concentration decreases. This unusual isotope effect is attributed to pyridine–water complexes involving both O–H ⋅ ⋅ ⋅N and C–H ⋅ ⋅ ⋅O hydrogen bonds. For ν12, the pressure dependence does not change with concentration, but it is at least five times more steep for pyr-h5 than pyr-d5. This may occur because the mass at all six ring vertices becomes equal for the -d5 isotope, making the volume change during vibration very small. At high concentrations, ν1 appears as two distinct peaks with different pressure dependence, indicative of uncomplexed and complexed pyridine. For certain combinations of solvent and concentration, a strong pressure dependent Fermi resonance between the ν1 and ν12 modes appears. Similar experiments were done with a weakly associating solvent, dimethylformamide, and nonassociating solvents, toluene and benzene. For these, the pressure dependence did not change with concentration; this and other features indicate which aspects of the aqueous solution results can be attributed to pyridine–water and pyridine–pyridine complexes.