Resonance Raman study of free-base tetraphenylporphine and its dication

Resonance Raman spectra of free-base tetraphenylporphine and its dication obtained with 441.6, 476.5, 488.0 and 514.5 nm excitation lines in the frequency region 100–1625 cm−1 are reported. Some bands due to in-plane and out-of-plane vibrational modes, which are symmetry forbidden under ideal D2hD2h, are also seen in the Raman spectra of these molecules. These bands arise due to dynamic and/or static coupling of out-of-plane modes with the allowed in-plane modes. Dynamic coupling may be facilitated by the proton tunneling, while static coupling is due to out-of-plane distortion in the geometrical structure of the molecule. Shift in the positions for certain bands in the Raman spectra of dication are interpreted on the basis of electronic changes due to sharing of electrons of the B1uB1u orbital by the two added protons.