Temperature and Solvent Dependence of Vanadium-51 Chemical Shifts and Line Widths of Vanadyl-triisopropylate

Abstract

Line widths ⊿ν1/2 of 51V n.m.r. signals of vanadyl-triisopropylate VO(OPri)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 τc is the main factor influencing line widths in this system. With lowering of temperature, 51V-shielding decreases (the temperature gradient of the chemical shift δV 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 δV/T and ⊿ν1/2/T correlations exhibit two inconsistencies at approximately 190 and 240 K which are attributed to the formation of clusters, and of oligomers {VO(OPri)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.