Combined XRD and infrared studies of pyridinium species in (PyH)(3)[PW(12)O(40)] single crystals.

The pyridinium salts of 12-tungstophosphoric acid (PyH)(3)[PW(12)O(40)]·2CH(3)CN (1) and (PyH)(3)[PW(12)O(40)] (2) have been prepared and studied by single crystal and powder X-ray diffraction (XRD) in order to characterize the crystallographic sites occupied by the pyridinium species. The three PyH(+) species are located on two unequivalent sites. Two species are linearly H-bonded to the oxygen atoms of the Keggins unit (α species), whereas the third one (β) forms a bent H-bond. In order to determine the infrared bands characterizing each type of pyridinium species in the 1650-1300 cm(-1) range, infrared spectra have been recorded from room temperature to 100 K. They reveal that only α pyridinium species give rise to the unusual splitting of the PyH(+)ν8b and ν19b modes, whereas β pyridinium species lead to a classical pyridinium spectrum.