The proton nuclear magnetic shielding tensors in biphenyl: Experiment and theory

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• 作者：Sch??nborn ; Frank ; Schmitt ; Heike ; Zimmermann ; Herbert ; Haeberlen ; Ulrich ; Corminboeuf ; Cl??mence ; et. al.
• 关键词：Proton shielding tensors ; Intermolecular contributions ; Quantum chemical calculations ; Symmetry relations
• 刊名：Journal of Magnetic Resonance
• 出版年：2005
• 出版时间：July, 2005
• 年：2005
• 卷：175
• 期：1
• 页码：52-64
• 全文大小：503 K

文摘

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.