High-resolution 13C NMR investigation of the organic conductor DMTM(TCNQ) 2

Abstract

We report the results of high-resolution 13C NMR experiments performed on the salt N-dimethylthiomorpholinium bis-tetracyanoquinodimethane, DMTM(TCNQ) 2. Careful analysis of the observed spectrum enabled us to identify seven resonances, in good agreement with the expected seven non-equivalent carbon sites on the molecule. It was possible to attribute the observed resonances to the different carbon sites by following the temperature dependence of the corresponding resonances, between 340 and 275 K. Two resolved lines at −37 ppm and −76 ppm (from TMS) were attributed to the two non-equivalent CN groups on the dimerized TCNQ chains. Contrary to the other observed resonances which show no detectable temperature dependence, the resonance positions of the CN groups show a continuous linear variation. The measured shift values of these lines lie far outside the range of normal chemical shifts and therefore could be associated with unpaired electron density on the CN groups, suggesting that such a shift is a Knight shift.