A nuclear magnetic resonance study of isomeric pentitols in aqueous and non-aqueous solutions

Abstract

³ J coupling constants for ²H exchanged pentitols in D₂O, [²H₅]pyridine and [²H₆]acetone have been determined at 25 °C after refinement by computer simulation of the fully resolved ¹H n.m.r. spectra obtained at 620 MHz. There are significant differences between the coupling constants for aqueous and non-aqueous solvents but only minor differences between those for pyridine and acetone. A conformational analysis is proposed which is based on the observation that in solid crystals all the H—C—C—H dihedral angles are close to either 60°(gauche) or 180°(trans). From an application of the Karplus equation to the solution ³J values it appears unlikely that single unique conformations of the alditols exist in solution. Instead the results are interpreted in terms of different populations of rotamers that are rapidly equilibrating between dihedral angles of ±60 and 180°. Thermodynamic properties of the alditols in D₂O are shown to be a reflection of the conformation of the specific pentitol in solution rather than that of the crystal structure.