Infrared spectra of permanganate anions in potassium perchlorate matrices: vibrational anharmonicity, effective symmetry and vibrational mode mixing effects

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Fourier transform infrared spectra of MnO 4 − anions isomorphously isolated in potassium perchlorate matrices were recorded at room and low temperature (LT, ∼100 K). On the basis of the detected second-order vibrational transitions involving the dopant species ν 3 mode components, anharmonicity constants and harmonic eigenvalues for these modes were calculated. Despite the fact that, rigorously speaking, the appearance of the spectra of dopant permanganate anions may be explained in terms of a C s site symmetry, the ν 3 stretching region resembles an approximate A 1⊕ E splitting (characteristic for a local C 3 v or even higher symmetry), which is not expected, even within the latent symmetry approach. We explain such spectral patterns on the basis of vibrational mode mixing (a ‘Fermi-like’ resonance) of the MnO 4 − ν 1 mode with the ν 3a site-group component. With the results of degenerate case stationary perturbation theory, we show that in the present case the Fermi-like resonance is predominantly responsible for the observed spectral features. The appearance of the region of second-order vibrational transitions in the spectra of dopant permanganate anions may be better explained in terms of the (rigorous) crystallographic C s site group (corresponding to the crystallographic Pnma space group), instead of the ‘latent’ (effective) symmetry site group C 2 v (corresponding to the latent symmetry space group Imma).