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Solution-state characterization of sec-butyllithium in [D8]toluene was conducted by utilizing a variety of NMR spectroscopic experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion-coefficient-formula-weight correlation analyses and other one- and two-dimensional NMR spectroscopic techniques. These results suggest that sec-butyllithium exists primarily as a tetramer/hexamer mixture in hydrocarbon solvents. However, the presence of roughly 2-5 % (by mol ratio) lithium sec-butoxide in both commercially available and laboratory-synthesized sec-butyllithium solutions due to the reaction with adventitious dioxygen will lead to the formation of a 1:5 sec-butoxide/ sec-butyllithium mixed hexamer. A sec-butoxide/ sec-butyllithium mixed octamer will emerge when the proportion of butoxide to butyllithium increases. [ABSTRACT FROM AUTHOR]