NMR at cryogenic temperatures: A 13C NMR study of ferrocene

A new cryogenic apparatus is described that can be used to obtain NMR spectra at temperatures down to 8-10 K. The static solid ¹³C NMR spectrum of ferrocene is recorded at that temperature. Spectra recorded at higher temperatures show that ferrocene is still freely rotating about its 5-fold symmetry axis on the ¹³C NMR time scale at 45-50 K. A comparison of the principal values of the ¹³C chemical-shift tensor obtained from the room- and low-temperature spectra of ferrocene indicates that the lowest frequency chemical shift principal component, δ33, is tilted off this symmetry axis by approximately 12°. Quantum chemical calculations of the chemical-shift tensor, completed on structures of ferrocene from the literature as well as on optimized structures with the cyclopentadienyl rings locked in both the staggered and eclipsed arrangements, predict the angle between the δ33 direction and the rotation axis to be between 11°and 15°, depending upon the geometry used in the calculation. The calculations also predict the sign of the angular perturbation, information not obtained from the experiment. An explanation of this angular change in the δ33 direction is provided by the composition of the molecular orbitals.

Duke Scholars

Author Julio Cesar Facelli Biostatistics & Bioinformatics, Division of Translational Bi ...