Investigation of an N…H hydrogen bond in a solid benzoxazine dimer by ~1H-~14N NMR correlation techniques under fast magic-angle spinning

摘要

The N…H distance within the unusual hydrogen-bonding arrangement adopted by a pair of methyl-substituted benzoxazine dimers (C_6H_3(OH)_2CH_2)_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 coupling and hence the N…~15H 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.