Magnetic resonance of intercalation compounds of graphite: Questions of ionicity and mobility of inserted species

Graphite will react at room temperature with Lewis acids as PF₅ and BF₃ in the presence of the oxidant C1F to form intercalation compounds containing closed-shell anions. In the case of “C₁₆BF₄”, the chemical shifts of both ¹¹B and ¹⁹F nuclear magnetic resonances point to the existence of BF^?₄, rather than the initial BF₃, within the graphite planes. The existence of second order quadrupolar coupling of the ¹¹B resonance suggests, however, possible hybrid $\text{BF}{}_3\text{BF}{}^{\mathrm{?}}{}_4$ character, as in B₂F^?₇, a known dimeric anion of BF₄ and BF₃. NMR results on ¹⁹F and ³¹P in the two compounds “C₁₄PF₆” and “C₂₈PF₆” support this hypothesis, as “C₂₈PF₆” shows only the presence of PF^?₆, but the more concentrated “C₁₄PF₆” shows composite $\text{PF}{}_5\text{PF}{}^{\mathrm{?}}{}_6$ character. Our claim for intercalated anions in these systems is reinforced both by radical cation-type signals in the ESR and by deshielding effects in the ¹³C NMR. The narrow linewidths of the nuclear magnetic resonance absorptions of the intercalated species are suggestive of “liquid-like” behavior.