Evidence forS=0-type spin pairing in the nonferromagnetic modification ofTDAE−C60

The X-band electron spin resonance (ESR) spectra, the methyl-proton ${}^1\mathrm{H}$ nuclear magnetic resonance spectra, and the proton spin-lattice relaxation rate $T_1^{-1}$ of the nonferromagnetic modification of tetrakis-dimethylamino-ethilene ${\mathrm{}\left(\mathrm{TDAE}\right)-\mathrm{C}}_{60}$ have been studied between room temperature and 4 K. The results are compared with those previously obtained for the ferromagnetic modification of ${\mathrm{T}\mathrm{D}\mathrm{A}\mathrm{E}-\mathrm{C}}_{60}.$ The nonferromagnetic modification of ${\mathrm{T}\mathrm{D}\mathrm{A}\mathrm{E}-\mathrm{C}}_{60}$ shows only a single proton line and an X-band ESR intensity that vanishes below 10 K. The temperature dependencies of the proton spin-lattice relaxation rate and the ESR intensity are characteristic for a system with a $S=0\mathrm{}$ ground state and a nonzero energy gap between the ground state and the first excited triplet state. The singlet-triplet energy gap is estimated to be of the order of 15 K at $T=5\mathrm{}\mathrm{K}.$ The nonferromagnetic modification seems to be metastable and spontaneously transforms to a ferromagnetic modification at room temperature or above.