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
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, δobs[6(7)Li], and the corresponding contact shifts, δc, 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 SRN1 reactions is noted.