IR and Raman assignments for zinc phthalocyanine from DFT calculations

Density functional theory (DFT) calculations have been used to predict the IR and Raman spectra for zinc phthalocyanine (ZnPc). There is very good agreement in the frequency data (RMS error of 10 cm(-1) for IR and 12 cm(-1) for Raman scattering, with half of the bands in both cases predicted to within 6 cm(-1)). The precision of the data makes it possible to provide reliable frequency assignments and hence to interpret vibrational changes in terms of structural modifications. The sensitivity of the three IR bands between 700 and 800 cm(-1) to the phthalocyanine polymorphic form is explained by the assignment of two modes to out-of-plane vibrations and one mode to an in-plane vibration. The out-of-plane vibrations are expected to be more sensitive to the solid-state packing. The most intense band in the Raman scattering, predicted at 1517 cm(-1) for ZnPc, has large displacements on the C-N-C bridge bonds and shifts significantly with metal ion size, making it a good marker for changes in the metal ion environment.