MOLECULAR-DYNAMICS OF 1-DECANOL IN SOLUTION STUDIED BY NMR COUPLED RELAXATION AND STOCHASTIC DYNAMIC SIMULATIONS

The isotopic C-13 labeled 1-decanols at positions 1, 5, and 9 have been synthesized and their dynamics in (CD3OCD2CD2)2O, CD3CD2OD, and CD2Cl2 solvents have en studied by C-13-coupled relaxation methods. The experiments were performed in the temperature range of 245-298 K. The data were fitted using the Redfield theory of nuclear spin relaxation to yield dipolar spectral densities which were then transformed into Cartesian correlation times. The Cartesian correlation times obtained experimentally have a strong bearing on local anisotropic motion and suggest that the size of groups attached to a given carbon and also hydrogen bonding between 1-decanol and the various solvent molecules have a profound effect on local segmental motion. The hydrogen bond anchoring effect is apparently strongest near the hydrogen bonding site. The effects of solvent viscoelastic response, hydrogen bonding, and torsional forces on the motion of Cartesian modes at different locations and end-to-end vectors in 1-decanol are analyzed using both generalized (GLE) and ordinary Langevin equations (OLF) simulations. The asymmetry of Cartesian correlation times as one moves away from the chain center arises from the difference in the torsional potentials of the C-C-C-OH and C-C-C-C linkages at each end and from a hydrogen bond anchoring effect at the first carbon (C1). The stronger retardation effect at C1 observed in ethanol is found from the GLE simulations to be mainly attributable to a large spatial blockage of the motions of the beads near -OH. For a solute molecule surrounded by solvent molecules with internal rotation, its motion is closely correlated with the solvent relaxation rate giving significantly reduced friction forces. Conversely, the local Cartesian relaxation for 1-decanol in methylene chloride fails to correlate effectively with solvent relaxation and can be described satisfactorily by OLEs with a delta-memory kernel. The contributions from overall tumbling and internal motion to the relaxation of local Cartesian modes and to the end-to-end vectors are analyzed by using calculated apparent activation energies.