Investigation of an N⋯H hydrogen bond in a solid benzoxazine dimer by 1H-15N 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)2CH2)2N(CH 3) 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 ¹H-¹⁵N dipole-dipole interaction, such that rotor-encoded spinning-sideband patterns are obtained, the analysis of which yields the ¹H-¹⁵N 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 ¹⁵N or ¹H chemical shift resolution, conventional (¹⁵N) or inverse (¹H) detection as well as the importance of the perturbing influences of further spins. The chosen recoupling scheme employs inverse, i.e. ¹H, 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 ¹H 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 ¹⁵N 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 © 2001 John Wiley & Sons, Ltd.