Measuring 19F shift anisotropies and 1H–19F dipolar interactions with ultrafast MAS NMR

Abstract

A new 19 F anisotropic-isotropic shift correlation experiment is described that operates with ultrafast MAS, resulting in good resolution of isotropic 19 F shifts in the detection dimension. The new experiment makes use of a recoupling sequence designed using symmetry principles that reintroduces the 19 F chemical shift anisotropy in the indirect dimension. The situations in which the new experiment is appropriate are discussed, and the 19 F shift anisotropy parameters in poly(difluoroethylene) (PVDF) are measured. In addition, similar recoupling sequences are shown to be effective for measuring 1 H -19 F distances via the heteronuclear dipolar interaction. This is demonstrated by application to a recently synthesized zirconium phosphonate material that contains one-dimensional chains linked by H -F hydrogen bonds.

Phosphorene and Black Phosphorus: The 31P NMR View

“…Figure 2a,b shows the 31 P static and MAS NMR spectra of tetrahydrofuran suspensions of large fl-bP nanoflakes with an average lateral dimension of 400−700 nm (Figure S1) and a thickness of 10−50 nm (Figure S2). As already reported, 13 the use of MAS allows the broad static signal of fl-bP to be partially resolved in a signal at the isotropic chemical shift of 18.5 ± 0.5 ppm, identified by recording spectra at different MAS frequencies (in the range of 0.5−3 kHz, not shown), and a series of spinning sidebands (indicated by asterisks in Figure 2b), which arise from the anisotropic nuclear spin interactions of 31 P nuclei, i.e., in principle, shielding and homonuclear dipolar interactions. The degree of spectral resolution obtainable at a MAS frequency of 3 kHz and the isotropic peak of fl-bP observed are very similar for a suspension of smaller fl-bP nanoflakes (Figure 2c), having an average lateral dimension of 100−400 nm and a thickness less than 15 nm (Figures S3 and S4).…”

“…This has been done by exploiting an anisotropic -isotropic correlation experiment based on the symmetry sequence R10 1 3 . 26,27 In particular, the sequence R10 selectively recouples the secondrank spatial component of the chemical shielding tensor, while completely suppressing the homonuclear dipole-dipole interaction. 28 In Figure 4 experiments, is about 2100 Hz, thus smaller than the effective main 31 P-31 P homonuclear dipolar interaction (about 3400 Hz).…”

“…Once the strength of the effective dipolar coupling is established, it is interesting to characterize the shielding interaction of 31 P nuclei in bP. This has been done by exploiting an anisotropic–isotropic correlation experiment based on the symmetry sequence R10 1 3 . , In particular, the sequence R10 1 3 selectively recouples the second-rank spatial component of the shielding tensor while completely suppressing the homonuclear dipole–dipole interaction . In Figure , the 2D 31 P anisotropic–isotropic chemical shift correlation spectrum of bP is shown together with the experimental and simulated 31 P line shapes obtained for the 31 P isotropic peak at 20.5 ppm.…”

“…knowledge of these properties is still very poor, despite the great interest received by this layered phosphorus allotrope and its exfoliated 2D form, phosphorene. By combining density functional theory (DFT) calculations and solid-state NMR experiments on suspensions of fl-bP nanoflakes and on solid bP, it has been possible to characterize the 31 P homonuclear dipolar and chemical shift interactions, identifying the network of 31 P nuclei more strongly dipolarly coupled and highlighting two kinds of magnetically nonequivalent 31 P nuclei. These results add an important missing piece of information to the fundamental chemico-physical knowledge of bP and support future extensive applications of NMR spectroscopy to the characterization of phosphorene-based materials.…”

“…24 Thus, the aim of the present work is to identify and quantify the 31 P nuclear spin interactions determining the NMR properties of fl-bP and bP, not only for adding a missing piece to the physico-chemical knowledge of bP but also because NMR can add great value to the future characterization of innovative phosphorene-based materials. We started from the analysis of 31 P MAS NMR spectra of samples of fl-BP with different nanoflake size, suspended in different solvents (sample preparation is reported in the Supporting Information). Then, once the similarity of these spectra with those of crystalline bP was assessed, a thorough analysis of the 31 P spin interactions was carried out on bP by combining solid-state NMR techniques and DFT calculations.…”