Molecular motions of α-l-rhamnopyranose and methyl α-l-rhamnopyranoside in the glassy and crystalline states: A proton NMR study

Abstract

Molecular motions of two saccharides, α-L-rhamnopyranose monohydrate (Rha) and methyl α-L-rhamnopyranoside (Me–Rha) in the crystalline and glassy states have been investigated using proton spin lattice relaxation times, T₁ and T_1ρ, and transverse relaxation measurements over the temperature range 110–360 K. The results showed that, in all cases, threefold rotational motion of the methoxyl and methyl groups dominate their proton spin–lattice (T₁ and T_1ρ) relaxation processes at low temperatures. In the crystalline solids the only other process observed to affect T_1ρ was water rotation. In the glasses both water rotation and anisotropic rotation of whole sugar molecules was observed. The anisotropic rotation, which was responsible for T_1ρ relaxation, gave way to a more isotropic rotation at about 50 K above T_g that was responsible for T₁ relaxation. In general the dynamics of the systems could be described by a model which used discrete correlation times and an Arrhenius temperature dependence for motion. There was no need for recourse to a distribution of correlation times model. The results obtained suggest that both the motion of methyl groups and water are not coupled to the motion of the whole sugar molecules and that a correlation time diagram for the motions of whole molecules above T_g and for water and methyl groups below T_g can be prepared.