Solid-state NMR of 1-propyltetrazole complexes of iron(II) and zinc(II) .1. H-1 spin-lattice relaxation time

Proton spin-lattice relaxation times were measured between 2.2 K and room temperature in [Zn(ptz)(6)](BF4)(2) (ptz = 1-n-propyl-1H-tetrazole) and in the spin-crossover complex [Fe(ptz)(6)](BF4)(2). Three different types of intramolecular motion of the propyl group are suggested in [Zn(ptz)(6)](BF4)(2), namely tunneling and classical rotation of methyl groups and rotation of methylene groups, Correlation times and activation energies are calculated for tunneling rotation of the CH3 group and for the classical (hindered) rotations of -CH3 and -CH2-CH3 as reorientations over a three-well asymmetrical potential, In [Fe(ptz)(6)](BF4)(2) the mechanism for the paramagnetic relaxation is found to be of the rapid-diffusion type according to the theory of Lowe and Tse, and the intramolecular motions are suggested to be the same as for the zinc complex. (C) 1996 Academic Press, Inc.