MAS NMR

JOURNAL

OF MAGNETIC

RESONANCE

82,367-368 (i 989)

Dimethylphenylphosphine Oxide, a Compoundfor Setting the Magic Angle in 31PCP/MAS NMR RICHARD C. CROSBY AND JAMES F. HAW* Department of Chemistry, Texas A&W University, College Station, Texas77843

ReceivedAugust 11, 1988

In the course of performing solid-state NMR studies of phosphazene polymers (I), we became aware of the lack of a standard compound for adjusting the magic angle in “P experiments. Individual discussions with various workers confirmed the lack of an established reference. Frye and Maciel have suggested (2) that the 7Li resonance of some suitable compound might serve the same role for setting the angle in 31P studies that the 79Br resonance of KBr does for “C CP/MAS studies. Unfortunately, the magnetogyric ratios of 3’P and 7Li differ by 4. I%, and it would almost certainly be necessary to retune the probe, transmitter, and preamplifier for such a large frequency change (e.g., 4.86 MHz at 7.05 T). This large potential inconvenience motivated us to search for a suitable standard with a strongly angle-sensitive 3’P resonance. Listings of 31P chemical-shift anisotropies (3) are not nearly as comprehensive as those for i3C, so a number of phosphorus compounds which were on hand were examined for trends in chemical-shift anisotropy. It was noted that phosphine oxides, particularly those bearing phenyl rings, exhibited large “P chemical-shift anisotropies. We also found that it was desirable for the standard to have at least one methyl group in order to have a short ‘H r,, which is advantageous in cross-polarization

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1.“P cross-polarizationspectraof DMPPO.(A) Spinningat the magicangle.(B)Static.

* To whom correspondenceshouldhe addressed. 367

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NOTES

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2. “P cross-polarization spectra of DMPPO spinning at various angles relative to &.

experiments. For this reason we selected dimethylphenylphosphine oxide (DMPPO) as our standard for setting the magic angle in 3’P CP/MAS experiments. Figure 1 shows 31P cross-polarization spectra of DMPPO with and without spinning at the magic angle. The 3’P powder pattern exhibits a single axially symmetric shielding tensor with an anisotropy of 226 ppm. This compares very favorably with the 168 ppm anisotropy of the i3C resonance of the aromatic carbons of hexamethylbenzene at room temperature (4), a useful standard for setting the magic angle in i3C experiments. DMPPO meets most or all of the criteria for a reference compound for setting the angle. (1) The compound is very sensitive to the spinning angle (Fig. 2). (2) The ‘H T1 is sufficiently short (900 ms at 2.35 T and 298 K) to permit a short pulse delay in cross-polarization experiments. (3) The 31P spectrum shows a single, intense isotropic peak. (4) DMPPO is commercially available (Organometallics, Inc.) and relatively stable. A method for synthesizing it has also been reported (5). The only disadvantage to the use of DMPPO as a standard of which we are aware is that it is somewhat hygroscopic, but we have experienced no difficulties when the sample is kept packed in a magic-angle spinning rotor. ACKNOWLEDGMENT This work was supported by the Office of Naval Research, Contract NOOO14-88-K-0239. REFERENCES 1. 2. 3. 4. 5.

R. C. CROSBY AND J. F. HAW, Macromolecules 15,2324 (1986). J. S. FRYE AND G. E. MACIEL, J. Magn. Reson. 48,125 (1982). M. MEHRING, “High Resolution NMR Spectroscopy in Solids,” Springer-Verlag, Berlin, 1976. A, PINES, M. G. GIBBY, AND J. S. WAUGH, Chem. Phys. Lett. 15,373 (1972). M. L. DENNISTON AND D. R. MARTIN, Znorg. Synth. 17, 183 (1977).