Mössbauer spectroscopic studies on molecular rotation and lattice dynamics of cyclodextrin clathrates of bis(η-cyclopentadienyl)iron(II) and some of its ring-substituted derivatives

Abstract

Bis(η-cyclopentadienyl)iron(II) and some its ring-substituted derivatives (guest molecules) are included in the cavities of α-, β-, and γ-cyclodextrins (host molecules). The ⁵⁷Fe Mössbauer spectra for the clathrates have been examined at various temperatures from 78 to 320 K. The temperature-dependent relaxation of the electric field gradient (e.f.g.) tensor, i.e. the reorientation of the guest molecules, was observed in the Mössbauer spectra of the clathrates of bis(η-cyclopentadienyl)iron(II) derivatives with α-cyclodextrin. Activation energies for the reorientation increase with increase in size of the substituents on the cyclopentadienyl rings of the guest molecules. Limiting reorientation times decrease with the increase in dipole moment of the guest molecules. Lattice temperatures (θ₁) and effective vibrating masses (M_eff) for 12 clathrates and four guest molecules were determined from the temperature dependences of the Mössbauer parameters. The lattice temperatures and the effective vibrating masses decrease in the order of the clathrates of α-, β-, and γ-cyclodextrins. Slight decreases of both isomer shifts and quadrupole splittings were observed for the clathrates of α-cyclodextrin, suggesting that there is a difference in the interaction between the guest and host molecules.