Electron Spin Resonance Studies of the Reorientational Motion of Ni(mnt)2−

摘要

Electron spin resonance studies of the planar bis(maleonitriledithiolato)nickel complex ion, Ni(mnt)2⁻, have been carried out from the motional narrowing region to the glassy limit in a series of ethers: 2-methyltet-rahydrofuran (MTHF), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), and tetraethylene glycol dimethyl ether (tetraglyme). Analyses of the spectra show that Ni(mnt)2⁻ is reorienting a factor of 3 faster about its long in-plane axis in all of these solvents; i.e., axially symmetric rotational diffusion produces agreement between the experimental and calculated line widths with D||/D⊥= 3.0 ± 0.2; D|| and D⊥ are the diffusion constants for reorientation about the long in-plane (parallel) and perpendicular axes, respectively. The temperature dependence of the reorientational correlation time τ2(0) = (6D⊥)⁻¹ determined from the widths is in agreement with the modified Stokes–Einstein–Debye model; the results indicate that Ni(mnt)2⁻ has relatively strong (but not associative) interactions with the ethers. The experimental values of τ2(0) and the solvents’ viscosities, self-diffusion constants, and dielectric relaxation times are compared and found to have a common temperature dependence. The ESR data also are compared with values of 〈τ〉solv, the correlation time obtained when a fluorescent probe is excited and its emission is monitored as the nonequilibrium solvent distribution relaxes. 〈τ〉solv and τ2(0) are found to have a common temperature dependence in MTHF, tetraglyme, and two other solvents (ethyl alcohol and 1-butanol) in which Ni(mnt)2⁻ has been studied. The factors determining these transport properties are discussed.