title: Solid-state dynamics in the closo-carboranes: a (11)B MAS NMR and molecular dynamics study.
abstract: This work explores the dynamic behavior of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B0 = 9.4 T, the (11)B MAS line widths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (τc) calculated by molecular dynamics are on the picosecond time scale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the (11)B spin-lattice relaxation times (T1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the (11)B MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter PQ(eff) derived from "dynamic shift" measurements using (11)B MQMAS NMR spectroscopy. The behavior of PQ(eff) as a function of temperature for p-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures, and this was supported by the molecular dynamics simulations.
authors.affiliation: †School of Chemistry and WestCHEM, University of Glasgow, Glasgow G12 8QQ, United Kingdom.|‡Departamento de Ciencias Básicas, Universidad del Bio-Bio, Casilla 447, Chillán, Chile.
authors.forename: Hernán
authors.initials: H
authors.lastname: Ahumada
authors.affiliation: †School of Chemistry and WestCHEM, University of Glasgow, Glasgow G12 8QQ, United Kingdom.|§Department of Painting Technologies and Techniques, Nicolaus Copernicus University, Toruń, Poland.
authors.forename: Teresa
authors.initials: T
authors.lastname: Kurkiewicz
authors.affiliation: †School of Chemistry and WestCHEM, University of Glasgow, Glasgow G12 8QQ, United Kingdom.|∥Centre for Clinical Brain Sciences, University of Edinburgh, Edinburgh EH16 4SB, United Kingdom.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Thrippleton
authors.affiliation: †School of Chemistry and WestCHEM, University of Glasgow, Glasgow G12 8QQ, United Kingdom.
authors.forename: Stephen
authors.initials: S
authors.lastname: Wimperis
mesh_terms: D001880:Boranes / Q000737:chemistry*; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D009038:Motion; D013997:Time Factors
chemical_list: D001880:Boranes