C-13-labeling and NMR study of Mn-12

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 co nsisted of four distinct peaks. The peaks were assigned to methyl carbon po sitions in the Mn-12 lattice. Spin-lattice relaxation, T-1, measurements re vealed 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 b ehavior on either side of the minimum in T-1. At lower temperature, the act ivation energy is 64 K, which correlates with the barrier for electronic sp in reorientation. At higher temperature the activation energy is 183 K, whi ch may correspond to a methyl group rotation. (C) 2001 Elsevier Science Ltd . All rights reserved.