ELECTRON-SPIN-RESONANCE STUDIES OF A MOLECULAR SCREW PROPELLER

A paramagnetic (S = 1/2) manganese complex containing two tridentate ligands has been studied in 2-methyltetrahydrofuran (MTHF) using electron spin resonance (ESR). The complex, MnR2, has screw-propeller geometry (approximate D2d symmetry). The ESR line widths have been used to determine the correlation time, tau, for reorientation about the axes perpendicular to the ''propeller Shaft'' axis of the complex. Tau has appreciable contributions from the translational as well as rotational motions of the individual ligands because they are reorienting about axes at a distance r(b) from their centers. The dependence of tau on temperature and viscosity is consistent with a hydrodynamic model that uses stick boundary conditions and approximates the ligands of MnR2 as oblate ellipsoids. Because of the skew symmetry of MnR2 the model predicts a nonzero (but negligibly small) contribution to tau from the coupling between the radical's overall translational and rotational motions. The ligands of MnR2 are approximately the same shape and size as vandyl acetylacetonate (VOAA). The ligand dimensions obtained from tau for MnR2 in MTHF are in reasonable agreement with the dimensions obtained from the translational diffusion constant of VOAA in tetrahydrofuran.