High-field solid-state 35Cl NMR in selenium(IV) and tellurium(IV) hexachlorides

Abstract

We report solid-state 35Cl NMR spectra in three hexachlorides, (NH4)2SeCl6, (NH4)2TeCl6 and Rb2TeCl6. The CQ(35Cl) quadrupole coupling constants in the three compounds were found to be 41.4±0.1 MHz, 30.3±0.1 MHz and 30.3±0.1 MHz, respectively, some of the largest CQ(35Cl) quadrupole coupling constants ever measured in polycrystalline powdered solids directly via 35Cl NMR spectroscopy. The 35Cl EFG tensors are axial in all three cases reflecting the C4v point group symmetry of the chlorine sites. 35Cl NMR experiments in these compounds were only made possible by employing the WURST-QCPMG pulse sequence in the ultrahigh magnetic field of 21.1 T. 35Cl NMR results agree with the earlier reported 35Cl NQR values and with the complementary plane-wave DFT calculations. The origin of the very large CQ(35Cl) quadrupole coupling constants in these and other main-group chlorides lies in the covalent-type chlorine bonding. The ionic bonding in the ionic chlorides results in significantly reduced CQ(35Cl) values as illustrated with triphenyltellurium chloride Ph3TeCl. The high sensitivity of 35Cl NMR to the chlorine coordination environment is demonstrated using tetrachlorohydroxotellurate hydrate K[TeCl4(OH)]∙0.5H2O as an example. 125Te MAS NMR experiments were performed for tellurium compounds to support 35Cl NMR findings.