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 fastmagic-angle spinning

The N . . .H distance within the unusual hydrogen-bonding arrangement adopt ed 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 indica tes 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 posi tion of this hydrogen-bonded proton. The advanced solid-state NMR methods e mployed utilize REDOR-type recoupling under fast magic-angle spinning to re couple the heteronuclear H-1-N-15 dipole-dipole interaction, such that roto r-encoded spinning-sideband patterns are obtained, the analysis of which yi elds the H-1-N-15 dipole-dipole coupling and hence the N . . .H distance. D ifferent designs of recoupling pulse sequences are discussed, which allow t he experiment to be adapted to the system under investigation in terms of t he required N-15 or H-1 chemical shift resolution, conventional (N-15) or i nverse (H-1) detection as well as the importance of the perturbing influenc es 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 we ll as H-1 chemical-shift resolution in the directly detected spectral dimen sion. This is the method of choice for cases such as this, where chemical s hift resolution is not required in the N-15 dimension. In addition, the per turbing effect of further protons on the N . . .H coupling of interest is m inimized, such that a relatively long N... H distance can be determined des pite the presence of several other couplings of comparable strength. Copyri ght (C) 2001 John Wiley & Sons, Ltd.