Organic-inorganic hybrid crystals, (2,4,6-CH3PyH)3Sb2Cl9 and (2,4,6-CH3PyH)3Bi2Cl9. Crystal structure characterization and tunneling of CH3 groups studied by 1H NMR and neutron spectroscopy

The crystal structures of (2,4,6-CH3PyH)3Sb2Cl9 (TMPCA) and (2,4,6-CH3PyH)3Bi2Cl9 (TMPCB) (Py – pyridine) have been determined at 100 K by the single crystal X-ray diffraction method. TMPCA and TMPCB crystallize in the monoclinic C2/c and triclinic P1 polar space group, respectively. In both cases the asymmetric part is comprised of three nonequivalent 2,4,6-trimethylpyridinium cations and a discrete M2Cl93− anion. The Bi2Cl93− moiety forms a face-sharing bi-octahedron, whereas in a case of Sb2Cl93− we deal with two pyramids connected by a corner. The inelastic neutron scattering spectra (INS) were recorded for TMPCA at low temperatures (4–50 K). Two peaks on each side of the central elastic line have been observed at ca. 4.8 and 2.9 μeV, the high energy peak exhibits an excitation energy value equal to ca. 6 meV. For TMPCA and TMPCB the 1H NMR spin–lattice relaxation times, T1, have been measured in the temperature region 15–410 K. The flattening of the T1 (spin–lattice) vs. reciprocal temperature, 1/T, dependence between 30 K and 15 K indicates the incoherent tunneling effect of the methyl group being treated as the quantum rotor. The conclusions drawn from the 1H NMR results as regards to the tunneling of the CH3 groups in the pyridinium cations are consistent with the tunneling peaks observed in the INS spectra.