Evidence for a concentration dependent 6(7)Li NMR contact shift in tetrahydrofuran solutions of lithium naphthalene radical anion and the effect of added 6(7)LiCl

Abstract

The paramagnetic nuclear magnetic resonance shifts for ⁶⁽⁷⁾Li in tetrahydrofuran solutions of lithium biphenyl, naphthalene, phenanthrene, anthracene and trans-stilbene radical anions have been measured as a function of the radical anion concentration. The shift vs. concentration relationships are linear except in the case of naphthalene. Both the observed molar shifts, δ_obs[⁶⁽⁷⁾Li], and the corresponding contact shifts, δ_c, are equal for the two isotopes. The paramagnetic shifts observed for THF solutions of ⁶⁽⁷⁾Li naphthalene radical anion increase with increasing dilution resulting in a non-linear shift vs. concentration relationship. This relationship becomes linear in the presence of ⁶⁽⁷⁾LiCl. A qualitative interpretation is proposed based on dynamic processes involving lithium cation exchange. The relevance of these results to S_RN1 reactions is noted.