Line-narrowing multiple pulse techniques are applied to a spherical sample crystal of biphenyl. The 10 different proton shielding tensors in this compound are determined. The accuracy level for the tensor components is 0.3 ppm. The assignment of the measured tensors to the corresponding proton sites is given careful attention. Intermolecular shielding contributions are calculated by the induced magnetic point dipole model with empirical atom and bond susceptibilities (distant neighbours) and by a new quantum chemical method (near neighbours). Subtracting the intermolecular contributions from the (correctly assigned) measured shielding tensors leads to isolated-molecule shielding tensors for which there are symmetry relations. Compliance to these relations is the criterion for the correct assignment. The success of this program indicates that intermolecular proton shielding contributions can be calculated to better than 0.5 ppm. The isolated-molecule shielding tensors obtained from experiment and calculated intermolecular contributions are compared with isolated-molecule quantum chemical results. Expressed in the icosahedral tensor representation, the rms differences of the respective tensor components are below 0.5 ppm for all proton sites in biphenyl. In the isolated molecule, the least shielded direction of all protons is the perpendicular to the molecular plane. For the para proton, the intermediate principal direction is along the C-H bond. It is argued that these relations also hold for the protons in the isolated benzene molecule.
Bibliographical noteFunding Information:
Writing up this work was stimulated by a talk of O.L. Malkina and V.G. Malkin presented in late summer 2003 at the EMBL in Heidelberg, Germany. We thank Prof. G. Seifert from the Chemistry Department of the University of Dresden for inspiring discussions and continuous support, and Manfred Hauswirth from the mechanical shop of the MPI in Heidelberg for making, under a magnifying CCD camera, the delicate parts shown in Fig. 2 including the nearly spherical sample crystal. We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) and the Swiss NFS, grant 200020.100070/1.
All Science Journal Classification (ASJC) codes
- Nuclear and High Energy Physics
- Condensed Matter Physics