Spin dynamics and energy gap of a Fe dimer from susceptibility and 1H nuclear magnetic resonance

The iron(III) S=5/2 dimer [Fe(OMe)(dbm)(2)](2) (in short Fe2) has a nonmagnetic S=0 ground state. The separation between the singlet ground state and the first excited (triplet) state is determined from susceptibility measurements to be about 22 K; for a dimer this value is equal to the antiferromagnetic exchange constant J. Proton nuclear magnetic resonance measurements were performed on Fe2. The nuclear spin-lattice relaxation rate (NSLR) was studied as a function of temperature at 31 and 67 MHz and as a function of the resonance frequency (10-67 MHz) at T=295 K. At room temperature the H-1 NSLR is independent of frequency contrary to the strong dependence found in planar ring compounds like Fe6 and ferric wheel (Fe10). The temperature dependence of the proton NSLR shows an exponential decrease on lowering the temperature from which we estimate a gap value about double the value obtained from the uniform susceptibility, a result which is unexpected if the NSLR were simply proportional to the concentration of dimers in the triplet excited state. (C) 1999 American Institute of Physics. [S0021-8979(99)27308-2].