High Resolution 13C NMR Investigation of the Organic Conductor DMTM (TCNQ)2
We report the result of high resolution 13C NMR experiments performed on the salt N- Dimethylthiomorphoiinium bis-tetracyanoquinodimethane, DMTM(TCNQ)2. Careful analysis of the observed spectrum enables us to identify the inequivalent carbon sites on the compound molecules, by following the temperature dependence of the corresponding resonance frequencies and spin lattice relaxation times, between 275 and 365K.Two resolved fines at -37 and -76 ppm (from TMS) at room temperature, were attributed to the two inequivalent C=C groups on the dimerized TCNQ molecules. Four peaks showing a large negatif and unusual 13C resonance shift between -600 and -900 ppm, were assigned to the inequivalent CN groups on the dimer. Contrary to the other resonances which show no detectable temperature dependence, the resonance positions of the C=C and CN groups show a continuous variation, which is correlated with the total susceptibility of the compound(1). As already pointed out, corresponding shift values lie far outside the range of expected chemical shifts and therefore could be associated with unpaired electron density on the CN groups, suggesting that such a shift is Knight shift.