X-Ray crystallography, DSC and NMR spectroscopy methods have been used to study the phase transition of D-amphetamine sulfate. The phase transition temperature occurs at about 325 K, and belongs to the discontinuous type with a large temperature hysteresis. A change in the space group is observed from monoclinic P2 1 in the low temperature phase to monoclinic C2 in the high temperature phase. The transformation from phase I to phase II is a result of the ordering of the SO 4 2À groups, allowing a T-shape interaction between D-amphetamine molecules. Proton relaxation rate data obtained for the studied compound have been analysed by assuming two dynamically-inequivalent methyl groups below the phase transition and only one type of ammonium group above the phase transition. The activation energies for the C 3 reorientation of the methyl groups have been determined to be 7.31 and 11.89 kJ mol À1 , whereas for the ammonium group it is 27.29 kJ mol À1 .
The structure and proton tautomerism
of imidazole-doped cellulose
(Cell-Im), an excellent solid state proton conductor, has been studied
by 15N solid-state NMR techniques. 1H–15N HETCOR NMR experiments allowed us to assign the water and
cellulose–OH resonances and to establish 1H–15N connectivities. 15N CPMAS NMR experiments showed
that imidazole is immobile and its tautomerism quenched below 263
K, whereas at higher temperatures, a broad distribution of slow and
fast exchanging protons is observed, where the fraction of the latter
increases with temperature. The tautomerism is found to be coupled
to proton exchange with water molecules. From an analysis of the temperature-dependent
fractions of both phases, a broad distribution of energies of activation
of the tautomerization of Cell-Im is obtained, exhibiting a maximum
at 42 kJ mol–1 and a width of 8.2 kJ mol–1. The tautomerization is slower than in the case of imidazole dissolved
in wet organic solvents. These results indicate that imidazole is
located in an aqueous fluid phase between cellulose microfibrils,
where proton exchange is assisted by a fast molecular reorientation
in transient hydrogen-bonded imidazole–water complexes. The
implications of these findings for the mechanism of proton conductivity
of Cell-Im are discussed. Finally, the potential of Dynamic Nuclear
Polarization (DNP) enhanced 15N-natural abundance CP-MAS
NMR of these heterocyclic systems is evaluated.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.