An investigation of hydrogen bonding in benzoxazine dimers by fast magic-angle spinning and double-quantum 1H NMR spectroscopy

A combination of solid-state H-1 NMR techniques, namely MAS, CRAMPS, and DQ MAS spectroscopy, is used to investigate the hydrogen-bonding properties of a range of alkyl-substituted benzoxazine dimers in the solid state. These dimers are of interest because they serve as model compounds for a class of recently synthesized polymers, the polybenzoxazines, whose unusual properties, in particular a near-zero shrinkage or volumetric expansion upon curing, have been rationalized in terms of favorable hydrogen-bonding interactions. The enhanced resolution achievable at the only recently available very-fast MAS rotation frequency of 35 kHz coupled with the sensitivity of the H-1 chemical shift to the hydrogen-bonding arrangement means that much useful information can be obtained by simple MAS alone. In addition,: spectra obtained by using a recently introduced CRAMPS approach, suitable for fast MAS and requiring no special experimental setup, are presented. Comparing the methyl- and ethyl- (and n-propyl-) substituted dimers, the significant shifts to low field of the chemical shifts of the hydrogen-bonded protons, as well as the observation of a second aromatic peak only in the latter case, suggest different packing arrangements for the different,dimers. On the basis of the additional information about proton proximities provided by the presence or absence of peaks in two-dimensional DQ MAS spectra, it is shown that, while for the methyl dimer, as previously proposed, pairs of hydrogen-bonded dimers are present; for the ethyl and n-propyl dimer, an alternative linear arrangement of hydrogen-bonded dimers predominates.