Quantification of Uncoupled Spin Domains in Spin-Abundant Disordered Solids

Abstract

Materials often contain minor heterogeneous phases that are difficult to characterize yet nonetheless significantly influence important properties. Here we describe a solid-state NMR strategy for quantifying minor heterogenous sample regions containing dilute, essentially uncoupled nuclei in materials where the remaining nuclei experience heteronuclear dipolar couplings. NMR signals from the coupled nuclei are dephased while NMR signals from the uncoupled nuclei can be amplified by one or two orders of magnitude using Carr-Meiboom-Purcell-Gill (CPMG) acquisition. The signal amplification by CPMG can be estimated allowing the concentration of the uncoupled spin regions to be determined even when direct observation of the uncoupled spin NMR signal in a single pulse experiment would require an impractically long duration of signal averaging. We use this method to quantify residual graphitic carbon using 13 C CPMG NMR in poly(carbon monofluoride) samples synthesized by direct fluorination of carbon from various sources. Our detection limit for graphitic carbon in these materials is better than 0.05 mol%. The accuracy of the method is discussed and comparisons to other methods are drawn.

Modes of Disorder in Poly(carbon monofluoride)

“…We have previously reported how sensitivity enhanced solid-state 13 C MAS NMR can be used to determine the mole fraction of residual fluorine free graphite domains that are too small to be observable by powder XRD. 87 We found the graphite levels of 0.0%, 0.7%, and 4.1% graphite by mole for the PC, CF, and CB derived samples, respectively, such that the graphite content correlates well with the progression of decreasing color brightness across the sample series. The graphitic domains are likely the result of incomplete fluorination of graphite platelets during synthesis.…”

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

“…Such a correlation peak has been observed in the 19 F– 13 C CP-HETCOR spectrum of a sub-fluorinated composition (CF 0.36 ) n , which was assigned to CF adjacent to sp 2 -hybridized carbon . This is an example of a defect that would only be detected by incorporating both 13 C and 19 F NMR results and emphasizes the need to fully quantify low levels of aromatic carbon environments . Larger aromatic domains are easily envisioned, with examples of structures and calculated 19 F and 13 C NMR spectra in Figures S10 and S11, respectively.…”

“…The calculated 13 C NMR chemical shift for the chair (AAA-H) conformation agrees well with the +92 ppm observed experimentally for CF environments (Figure 2b) and is consistent with previously reported 13 C NMR chemical shifts in highly fluorinated (CF x ) n . 51,55,58 These experimental and calculated results differ significantly from those NMR chemical shifts published by Vaḧaḧangas and co-workers, 57 who reported a 13 C δ iso value of +109.3 ppm (with respect to TMS) along with a 19 F δ iso value of −161.2 ppm (with respect to CFCl 3 ) for the periodic AAA-H chair structure or a 19 F δ iso value of −170.7 ppm for their fully relaxed DFT FG cluster calculations. The underlying reason for these discrepancy in the predicted 19 F chemical shifts has not been completely determined but most likely stems from their method of referencing for both their isolated clusters and periodic systems.…”

“…47 This is an example of a defect that would only be detected by incorporating both 13 C and 19 F NMR results and emphasizes the need to fully quantify low levels of aromatic carbon environments. 58 Larger aromatic domains are easily envisioned, with examples of structures and calculated 19 F and 13 C NMR spectra in Figures S10 and S11, respectively. Defects from more localized F vacancies were also explored including the two adjacent fluorine defects to produce an "ene" (ene[2F]) or a four-fluorine vacancy to produce two adjacent "ene" defects (ene[4F]).…”

“…Of interest are the numerous CF environments observed between −140 and −200 ppm and the multiple CF 2 environments between −95 and −150 ppm, which have not been fully assigned to date. We have also recently described an improved method to quantitatively measure low concentrations of the graphitic C environments present in these materials due to defects in the idealized (CF) n structure, 58 which also revealed variations in the 13 C MAS NMR spectra obtained from different material sources. A question remains as to whether graphitic C environments observed can be attributed to specific defects in the (CF) n structure.…”