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

Abstract

Molecular dynamics of polycrystalline α-l-fucopyranose and methyl α-l-fucopyranoside have been investigated using 13C and 1H 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 13C T1 than the carbons in the pyran ring. 13C T1 relaxation measurements (at 74.56 MHz) as a function of temperature enabled characterization of the 3-fold rotations of the methoxyl (Ea ∼ 9 kJ/mol, τc ∼ 0.7 × 10-13 s) and methyl (Ea ∼ 9 kJ/mol, τc ∼ 2 × 10-13 s) groups. Proton T1 (100 MHz) measurements of methyl α-l-fucopyranoside showed that the relaxation processes (Ea ∼ 11 kJ/mol, τc ∼ 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 T1 showed a relaxation rate maximum at 224 K, attributed to the 3-fold rotation of methyl group (Ea ∼ 15 kJ/mol, τ0 ∼ 2.6 × 10-13 s). The rotation processes of the methyl groups dom...