Temperature and solvent dependence of vanadium-51 chemical shifts and line widths of vanadyl-triisopropylate

Line widths Δν1/2 of 51V n.m.r. signals of vanadyl-triisopropylate VO(OPrsup(i))3 dissolved in tetrahydrofurane range from 10 to 610 Hz, increasing with decreasing temperature according to a theoretically proposed T-1/2 exp(E/RT) law, indicating that the molecular correlation time tausub(c) is the main factor influencing line widths in this system. With lowering of temperature, 51V-shielding decreases (the temperature gradient of the chemical shift deltasub(v) is about -0,5 ppm/K over a temperature range of 300-180 K),suggesting a loss of molecular symmetry due to solvation and association processes. In pentane, methylcyclohexane and diethylether, the deltsub(v)/T and Δν/2/T correlations exhibit two inconsistencies at approximately 190 and 240 K which are attributed to the formation of clusters, and of oligomers (VO(OPrsup(i)3)n, respectively. The latter gives rise to a satellite line with an upfield shift relative to the monomeric species. Exchange interactions are discussed as a second factor affecting line widths. (orig.)