DETERMINATION OF ROTATIONALREORIENTATION CORRELATION TIME AND 59 Co NUCLEAR QUADRUPOLAR COUPLINGCONSTANT OF POLYAMMONIUM MACROCYCLIC COBALTICYANIDE SUPERCOMPLEXES IN DILUTE AQUEOUS SOLUTIONS--An Application of Dynamic Laser Light Scattering Method
The correlation time, τc, and the nuclear quadrupolar coupling constant (NQCC) of I>1/2 nuclei are important parameters for characterizing local and microdynamic structures of molecules. However, it is often difficult to determine those constants in solution. In this study,τc and NQCC of the 59Co nuclei were determined for a series of supercomplexes［12］aneN4［Co(CN)6］, 18］aneN 6Co(CN) 6］,［24］aneN8［Co(CN)6］,［16］aneN4［Co(CN)6］,［24］aneN6［Co(CN)6］and［32］aneN8［Co(CN)6］in dilute solution. The Dynamic Laser Light Scattering (DLLS) method, which is now widely used in macromolecule research, was first applied to determine the hydrodynamic radii (0.4～0.6 nm) of these molecules. Based on the StokesEinsteinDebye equation and the measured hydrodynamic radii, the rotational reorientation correlation times τc were calculated.τc obtained in this study had a range of 3.8～9.5×10-11s, which are fairly reasonable for molecules with molecular weights of [WT][GK2]about 300～500. Finally, NQCC of the59Co nuclei in the supercomplexes were determined by combining the τc values calculated and the experimentally measured longitudinal relaxation times, T1, of the 59Co nuclei. The NQCC values obtained ranged from 0.5 to 2.2 MHz, which are considerably lower than those extracted from solid tatic NMR lineshape (5 to 7 MHz). The difference is probably due to the structural effects originating from the anisotropy of local electric gradient around 59Co nucleus in solid.