13C-labeling and NMR study of Mn12

The nanomagnet [Mn12O12(CH3COO)(16)(H2O)(4)]. 2CH(3)COOH . 4H(2)O, or Mn-12, was studied by C-13 NMR spectroscopy. The sample was (13)-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. (C) 2001 Elsevier Science Ltd. All rights reserved.