Deuterium substitution as an assignment aid in solid state 13C NMR spectroscopy

Abstract

For many materials 13C NMR lines obtained in solid and solution states differ by no more than a few ppm; however, when many lines are closely spaced, direct correlation between solid and solution lines is impossible. Although many t~c~~i~~e§ exist for the assignment of solution NMR spectra (I), for solids this is not the case. Carbons bonded to 14N can be distinguished (2), as can quaternary carbons when the dipolar dephasing technique is emp loyed (3). Carbon-l 3 labeling (4) also can be used to good advantage. In this communicat ion we show that replacing hydrogen bonded to carbon by deuterium can be used as a technique to aid in the assignment of solid state i3C NMR spectra. For Dianin’s compound (4-hydroxy-2,2,4-trimethylchroman F ig. Ja), a versatile clathrate host (5), a partial assignment of the solid state spectrum can be made (6) by using compiled chemical-shift tables. However, the protonated carbon resonances of the two aromatic rings overlap completely both in the solid and in solution. A complete assignment of this region of the solution spectrum was confirmed by using the two-dimensional heteronuclear chemical-shift correlation experiment (7, 8) as mod ified by Bax and Morris (9). The proton spectrum (see F ig. 1) may be readily assigned from the resolved scalar couplings and the relative upfield shifts of the ortho and paru resonances in phenols. Protons on carbons 12, 16, 13, and 15 give the characteristic AA’BB’ pattern of p-disubstituted benzenes. From the nearly linear relationship between proton and carbon-13 chemical shifts on the aromatic rings, a complete assignment is then possible, as shown. In solids, quaternary carbons and other carbons weakly coupled to protons may be distinguished readily by using the dipolar dephasing technique. When protons are replaced by deuterons, dipolar coupling to an attached carbon13 is reduced by