AN INVESTIGATION OF HYDROGEN-BONDING IN BENZOXAZINE DIMERS BY FAST MAGIC-ANGLE-SPINNING AND DOUBLE-QUANTUM H-1-NMR SPECTROSCOPY

A combination of solid-state H-1 NMR techniques, namely MAS, CRAMPS, a nd DQ MAS spectroscopy, is used to investigate the hydrogen-bonding pr operties of a range of alkyl-substituted benzoxazine dimers in the sol id state. These dimers are of interest because they serve as model com pounds for a class of recently synthesized polymers, the polybenzoxazi nes, whose unusual properties, in particular a near-zero shrinkage or volumetric expansion upon curing, have been rationalized in terms of f avorable hydrogen-bonding interactions. The enhanced resolution achiev able at the only recently available very-fast MAS rotation frequency o f 35 kHz coupled with the sensitivity of the H-1 chemical shift to the hydrogen-bonding arrangement means that much useful information can b e obtained by simple MAS alone. In addition,: spectra obtained by usin g a recently introduced CRAMPS approach, suitable for fast MAS and req uiring no special experimental setup, are presented. Comparing the met hyl- and ethyl- (and n-propyl-) substituted dimers, the significant sh ifts to low field of the chemical shifts of the hydrogen-bonded proton s, as well as the observation of a second aromatic peak only in the la tter case, suggest different packing arrangements for the different,di mers. On the basis of the additional information about proton proximit ies provided by the presence or absence of peaks in two-dimensional DQ MAS spectra, it is shown that, while for the methyl dimer, as previou sly proposed, pairs of hydrogen-bonded dimers are present; for the eth yl and n-propyl dimer, an alternative linear arrangement of hydrogen-b onded dimers predominates.