Knight Shift Tensors and pi-Spin Densities in the Organic Metals alpha(t)-(BEDT-TTF)(2)I-3 and (BEDT-TTF)(2)Cu(NCS)(2)

C-13-MASS spectra of pure BEDT-TTF and of the organic mctals alpha(t)-(BEDT-TTF)(2)I-3 and (BEDT-TTF)(2)Cu(NCS)(2) were recorded at nu(L) = 68 MHz. Isotropic shifts and the principal components of the shift tensors were determined, respectively, from the center and spinning side bands. For pure BEDT-TTF which is a diamagnetic insolator, the measured shifts are chemical shifts while for the organic metals they are the sum of chemical and Knight shifts. In each of the compounds the shifts are assigned in groups to the inner, middle and outer carbons of the BEDT-TTF molecule. For the organic metals the separation of the experimental shifts into chemical and Knight shifts is discussed. From the anisotropic part of the Knight shift tensors the pi-spin densities at the carbon and sulphur positions of the BEDT-TTF molecule are inferred. The result is that the pi-spin density of the unpaired hole is concentrated on the center part of the BEDT-TTF molecule, i.e., on the inner and middle carbons, and on the inner sulphurs. It is argued that the current density is concentrated on this part of the BEDT-TTF molecule as well.