Paramagnetic Solvent Nuclear Magnetic Resonance Shifts in Radical Anion Solutions. 2. Some Cation-Dependent Phenomena in Concentrated Solutions of Aromatic Hydrocarbon Radical Anions and Dianions

Article abstract of DOI:10.1021/jo00150a003

The molar paramagnetic solvent shifts (MPSS's) of (biphenyl)(1-)<*>Li(1+), (p-terphenyl)(1-)<*>M(1+) (M = Li, Na), (naphthalene)(1-)<*>M(1+) (M = Li, Na, K), (2-tert-butylnaphthalene)(1-)<*>Li(1+), (2,6-di-tert-butylnaphthalene)(1-)<*>Li(1+), (phenanthrene)(1-)<*>M(1+) (M = Li, Na, K), (anthracene)(1-)<*>M(1+) (M = Li, Na, K), (pyrene)(1-)<*>M(1+) (M = Li, Na, K),(chrysene)(1-)<*>M(1+) (M = Li, Na, K), (cis-stilbene)(1-)<*>Li(1+), and (trans-stilbene)(1-)<*>Li(1+) have been measured in tetrahydrofuran (THF) at 60 MHz and at 35 +/- 1 deg C.These parameters, which are more or less cation as well as anion dependent, may reflect structural properties of the ion pairs in solution such as covalency in the ion-pair bond or aggregation to diamagnetic species.The effect of alkyl substitution on the magnitude of the MPSS has been studied in the case of lithium napthalene radical anion.A tert-butyl group in the 2-position has no effect on the molar shift.Two tert-butyl groups in the 2,6-positions, however, cause a dramatic drop in the magnitude of the MPSS.It has been shown that this change is due to extensive spin pairing.A double internal reference technique has been used in order to separate contributions due to the bulk paramagnetism and the contact shift.Contrary to previous belief that THF protons undergo only negative contact shifts, it was shown that they may undergo positive contact shifts as well.Thus the α protons of THF in the presence of lithium naphthalene radical anion exhibit a contact shift of -0.25 ppm/mol, whereas in the presence of lithium (p-biphenyl)phenylketyl they exhibit a contact shift of 0.25 ppm/mol.These contact shifts correspond to coupling constants of the order of 10^-3 G.Attention is called, however, to the possibility that in strongly cation-complexing agents such as glymes, the contact shift could correspond to a coupling constant of the order of 0.1 G.In such a case, line broadening in the ESR spectra could be caused by additional small splitting due to the interaction of the unpaired spin with the protons of the coordinated-to-cation ligands.The effect of adding poorly solvating cosolvents, Et2O and Et3N, on the structure of (naphthalene)(1-)<*>M(1+) (M = Li, Na, K) has been studied.Three patterns of behavior have been observed. (1) The added cosolvent does not perturb the structure of the radical anion-cation pair. (2) A change occurs after the composition of the medium reaches a certain point, and the change varies gradually as the medium becomes enriched in cosolvent. (3) An abrupt change occurs at a certain composition of the medium.The above phenomena are strongly cation dependent.Dianions of polynuclear aromatic hydrocarbons, particularly those with angularly condensed rings, are chemically unstable in THF, causing cleavage of the ethereal solvent.Instability appears to depend markedly on the cation and increases in the order K, Na, Li.