Solid-State Dynamics in the closo-Carboranes: A ¹¹B 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.

Structural Phase Transitions in closo-Dicarbadodecaboranes C2B10H12

“…Our results do not exclude isotropic rotations in the p -isomer as claimed from NMR studies, but C 5 rotations seem to be preferred, which is in agreement with ref ( 19 ). In contrast, a disagreement between NMR and our results appears for the o -isomer, where NMR studies show predominantly a rotation around C 2 axis with decreasing temperature, while our results show C 3 axis rotations.…”

“…The basic ideas about the thermal polymorphism in the carborane C 2 B 10 H 12 isomers have been clear already from previous spectroscopic studies. 10 − 14 , 19 , 20 With our high-resolution crystallographic studies combined with DFT calculations, we complete the full image of the bonding situation and the crystal structures of the low-temperature phase III in these exciting crystals. Starting from fully ordered phases III, the thermal motion introduces increasing rotational disorder leading to rotatory phases (plastic crystals), first in the phases II with differences between the isomers, and then in the phases I, where the rotation disorder of the molecules is isotropic in all three isomers ( Figure 3 ).…”

“…This is related to the decrease of the reorientation jump frequency along the C 3 and C 5 symmetry axes of the icosahedron leaving prominently rotation along the C 2 axis, which is parallel to their respective dipole moments. The reorientation in the p -isomer has been observed to be isotropic in phases I and II according to ref ( 13 ), while anisotropic according to ref ( 19 ), preferring the C 5 axis in phase II.…”

Site-specific synthesis of a hybrid boron–graphene salt

“…23 It is important to note that the 11 B{ 1 H} NMR peaks of the nido-carborane in Cs n Á2 are significantly broadened as compared to the sharp peaks of the crystalline 7,8-nido-Cs[C 2 B 9 H 12 ]. The latter are expected to give such sharp features because of rapid reorientation dynamics in the solid state, as found in closo-carboranes, 22 but once bound to the graphene surface, as in Cs n Á2, this behavior is severely inhibited and, coupled with multi-faceted surface-induced disorder, gives rise to significant line broadening.…”

“…1b). 22 When attached to the graphene surface in 1, the reorientation dynamics are halted resulting in spectra that are significantly broadened as seen in Fig. 1a.…”