~(13)C-labeling and NMR study of Mn_(12)

摘要

The nanomagnet [Mn_(12)O_(12)(CH_3COO)_(16)(H_2O)_4]·2CH_3COOOH·4H_2O, or Mn_(12), was studied by ~(13)C NMR spectroscopy. The sample was ~(13)C-labeled at the methyl carbon position, and magnetically aligned. The spectrum generally consisted of four distinct peaks. The peaks were assigned to methyl carbon positions in the Mn_(12) lattice. Spin-lattice relaxation. T_1, measurements revealed a minimum in T_1 at about 60 K. A fit to a BPP (Bloembergen, Purcell, and Pound) model with two relaxation modes suggests thermally activated behavior on either side of the minimum in T_1. At lower temperature, the activation energy is 64 K, which correlates with the barrier for electronic spin reorientation. At higher temperature, the activation energy is 183 K, which may correspond to a methyl group rotation.