NMR study of partial charge transfer in N-methylphenazinium-tetracyanoquinodimethane (NMP-TCNQ)

The field dependence of the proton NMR second moment $M_2(H, T)$ of powder N-methylphenazinium-tetracyanoquinodimethane (NMP-TCNQ) and (DMP-TCNQ), the analogous material with a deuterated methyl, shows that at 4.2°K there are unpaired electrons localized on ∼ 6% of the NMP sites. This strongly suggests partial ($\gamma =0.94\mathrm{}$) charge transfer in $\mathrm{NM}{\mathrm{P}}^{+\gamma }-\mathrm{TCN}{\mathrm{Q}}^{-\gamma }$, a room-temperature organic conductor previously thought to be fully ionic ($\gamma =1\mathrm{}$). The localized NMP moments are shown to be weakly coupled with a Weiss constant $\ominus \sim 0.6°$K, consistent with static susceptibility data. This local-moment picture is also consistent with the spin dynamics as indicated by low-temperature EPR measurements and the value of $M_2(0, 4.2)$. At higher temperature $M_2(0, T)$ indicates a slowing down of spin fluctuations as $T^{-1\mathrm{}}$, characteristic of one-dimensional systems. These results show cw NMR measurements with selective deuteration provide a useful method for determining the spatial distribution of spin susceptibility and thus the degree of ionicity $\gamma$. This is because static rather than dynamic magnetic properties are measured and as these interactions are well known, the interpretation of the data is simplified.