Resonance Raman study of the solvation of p-nitroaniline in supercritical water

Resonance Raman spectra of p-nitroaniline (pNA) have been measured in various states of water. The relative intensities of two bands assigned to the NO₂ stretching mode show a large temperature dependence along the 30.4 MPa isobar between ambient temperature and 669 K. Along the supercritical isotherm at 668.5 K, the higher frequency band shifts by 8 cm⁻¹ over the accessible density range (0.3–1.7ρ_c). We also performed density-functional calculations of the Raman spectra of water clusters of pNA. The calculated changes in electronic structure and Raman spectra with cluster size are used to rationalize these experimental observations.