AbstractThe band intensities of Raman spectrum of PTZ in the solid state show a clear dependence on the laser excitation wavelength. Such a behaviour, not observed when the spectrum is obtained from PTZ in solution, indicates that in the crystal the molecular. orbital of the compound is perturbed to some extent. The PTZ · SO2complex in solution presents the same pre‐resonahce pattern as does solid PTZ, suggesting that the same PTZ orbital is involved in both cases. The most enhanced PTZ modes are those ascribed to the aromatic ring vibrations, indicating an interaction through the π molecular orbitals; it is proposed that PTZ acts as a π‐donor toward SO2on the basis of the pre‐resonance pattern of the Raman spectrum of P