Unusual IR ring mode splittings for pyridinium species in H3PW12O40 heteropolyacid: involvement of the δNH internal mode

Abstract

This study aimed at investigating the well-known infrared band splitting affecting the ν8b and ν19b modes of pyridinium species formed by pyridine absorption in tungsten heteropolyacid H₃PW₁₂O₄₀, first reported 30 years ago but the origin of which has not yet been satisfactorily explained. To this aim, IR spectra of isotopically substituted pyridinium species h₅-PyH⁺, h₅-PyD⁺, d₅-PyH⁺ and d₅-PyD⁺ were analysed and compared in the 1700–1300 cm⁻¹ range. DFT calculations were used to assign the IR bands of the 4 isotopomers in this range. The results clearly showed that the splitting specifically affects pyridinium modes presenting a marked δNH character, namely the asymmetric ν8b and ν19b modes of h₅-PyH⁺ and d₅-PyH⁺ species. The Davydov nature of the splittings was eliminated upon using mixtures of isotopic pyridinium species. They are explained by an interaction between the δNH internal mode with the in-plane pyridinium ring frustrated rotation in two potential wells, associated with a tunneling effect.