Investigation of an N center dot center dot center dot H hydrogen bond in a solid benzoxazine dimer by H-1-N-15 NMR correlation techniques under fast magic-angle spinning

The N . . .H distance within the unusual hydrogen-bonding arrangement adopted by a pair of methyl-substituted benzoxazine dimers (C6H3(OH)(2)CH2)(2)N(CH3) has been determined by solid-state NMR to be 194 +/- 5 pm. This indicates that the proton is shared between the nitrogen and oxygen atoms, with a preference for an O-H rather than an N-H bond character. It is to be noted that a previous X-ray single crystal study was unable to localize the position of this hydrogen-bonded proton. The advanced solid-state NMR methods employed utilize REDOR-type recoupling under fast magic-angle spinning to recouple the heteronuclear H-1-N-15 dipole-dipole interaction, such that rotor-encoded spinning-sideband patterns are obtained, the analysis of which yields the H-1-N-15 dipole-dipole coupling and hence the N . . .H distance. Different designs of recoupling pulse sequences are discussed, which allow the experiment to be adapted to the system under investigation in terms of the required N-15 or H-1 chemical shift resolution, conventional (N-15) or inverse (H-1) detection as well as the importance of the perturbing influences of further spins. The chosen recoupling scheme employs inverse, i.e. H-1, detection, because it provides a dramatic increase in signal sensitivity, resulting in savings in measurement time by a factor of at least 20, as well as H-1 chemical-shift resolution in the directly detected spectral dimension. This is the method of choice for cases such as this, where chemical shift resolution is not required in the N-15 dimension. In addition, the perturbing effect of further protons on the N . . .H coupling of interest is minimized, such that a relatively long N... H distance can be determined despite the presence of several other couplings of comparable strength. Copyright (C) 2001 John Wiley & Sons, Ltd.