Influence of Polymorphs and Local Defect Structures on NMR Parameters of Graphite Fluorides

Abstract

The role of local molecular structure on calculated 13 C and 19 F NMR chemical shifts for graphite fluoride materials was explored by using gauge-including projector augmented wave (GIPAW) computational methods for different periodic crystal polymorphs and density functional theory (DFT) gauge-including atomic orbital (GIAO) computational methods for individual graphite fluoride platelets, i.e., fluorinated graphene (FG). The impact of stacking sequences, d-spacing, and ring conformations on fully fluorinated graphite fluoride structures was investigated. A range of different defects including Stone−Wales, F and C vacancies, void formation, and F inversion were also evaluated using FG structures. These calculations show that distinct chemical shift signatures exist for many of these polymorphs and defects, therefore providing a basis for spectral assignment and development of models describing the mean local CF structure in disordered graphite fluoride materials.

Modes of Disorder in Poly(carbon monofluoride)

“…These results suggest that a new structural factor is responsible for these deshielding effects in graphite fluorides. Recently, we calculated 19 F and 13 C isotropic chemical shifts in (CF) n by DFT, 79 which considered crystal structures featuring different framework conformations (e.g., chair versus boat) and found that framework conformation alone can be responsible for nearly 50 ppm variability in 19 F isotropic chemical shifts. The 19 F and 13 C NMR isotropic nuclear shielding parameters of both chair and boat conformations were calculated for the ncrown ring perfluorocyclohexane (n-PFCH, for n = 1−4) series of molecules.…”

“…This is broadly consistent with our previous work calculating (CF) n properties by periodic boundary DFT and other results in the fluorographene literature. 79,99 The energy increase by inverting the core pattern ranges from 1.26 eV (bench) to 5.41 eV (zigzag), indicating that the configuration at the core/crown boundary has a significant effect on the overall energy. The ΔE total for randomized core arrangements ranges between 8.77 and 12.20 eV.…”

“…76−78 We have recently published an investigation into fluorographene using models of crystalline (CF) n polymorphs to investigate the effects of framework conformation on isotropic 13 C and 19 F NMR chemical shifts, as calculated by density functional theory (DFT). 79 There we reported, for the first time, a dependence of the isotropic 19 F NMR chemical shift on the configuration of the carbon framework, with boat polymorphs experiencing isotropic 19 F deshielding up to 50 ppm relative to the chair form that is comparable to the strength of deshielding by negative hyperconjugation relative to "ordinary" C−F covalent bonds. As "semi-ionic" C−F content has been linked to improved discharge performance of (CF x ) n cathodes, 11,80−82 this finding exposes the need to reevaluate our understanding of (CF) n structure.…”

“…Our protocol for the calculation of 19 F and 13 C chemical shifts is nearly identical with that used to study fluorographene in our preceding computational investigation, to which the reader is referred Journal of the American Chemical Society for more details. 79 The molecular models of graphite fluoride platelets were optimized in the gas phase by DFT using the B3LYP functional and the 6-311G(d,p) basis set within the Gaussian09 suite of programs (Gaussian Inc., Wallingford, CT). The 19 F and 13 C NMR nuclear shielding tensors were calculated with the gauge-including atomic orbital method.…”

“…We recently reported DFT calculations of 13 C and 19 F chemical shifts in several alkenyl and aromatic defects, which demonstrated a large 40−50 ppm deshielding effect in line with precedent from the literature subfluorinated (CF x ) n . 79 Here, we investigated a set of chair 3-PFCH and 4-PFCH molecules (Figure S13) bearing a wider variety of alkenyl and aromatic defects. Figure 8 summarizes the results of calculations for isotropic 19 F chemical shifts, C−F bond dissociation energies, and C−F bond distances, which can be compared with Figure 5B.…”