Solid State NMR Studies of the Molecular Motions in the Polycrystalline α- l -Fucopyranose and Methyl α- l -Fucopyranoside

Molecular dynamics of polycrystalline α- l -fucopyranose and methyl α- l -fucopyranoside have been investigated using 13 C and 1 H spin relaxation times over a range of temperatures (90−400 K). For methyl α- l -fucopyranoside at 300 K, both methoxyl and methyl groups had much shorter 13 C T 1 than the carbons in the pyran ring. 13 C T 1 relaxation measurements (at 74.56 MHz) as a function of temperature enabled characterization of the 3-fold rotations of the methoxyl ( E a ∼ 9 kJ/mol, τ c ∼ 0.7 × 10 -13 s) and methyl ( E a ∼ 9 kJ/mol, τ c ∼ 2 × 10 -13 s) groups. Proton T 1 (100 MHz) measurements of methyl α- l -fucopyranoside showed that the relaxation processes ( E a ∼ 11 kJ/mol, τ c ∼ 0.7 × 10 -13 s), corresponding to rotations of the methoxyl and methyl groups, are not distinguishable and occurred together at about 131 K. For α- l -fucopyranose, proton T 1 showed a relaxation rate maximum at 224 K, attributed to the 3-fold rotation of methyl group ( E a ∼ 15 kJ/mol, τ 0 ∼ 2.6 × 10 -13 s). The rotation processes of the methyl groups dominated T 1 ρ processes for both saccharides and appeared at the low-temperature end (90−150 K) of the temperature range. Contributions of hydroxyl groups to both T 1 and T 1 ρ processes were obscured by the overwhelming relaxation efficiency of methyl groups. The proton second moment showed a reduction to a lower value at about 150 K due to the rotation of methyl groups.