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

Line-narrowing multiple pulse techniques are applied to a spherical sample crystal of biphenyl. The 10 different proton shielding tensors in this compd. are detd. 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. Intermol. shielding contributions are calcd. by the induced magnetic point dipole model with empirical atom and bond susceptibilities (distant neighbors) and by a new quantum chem. method (near neighbors). Subtracting the intermol. contributions from the (correctly assigned) measured shielding tensors leads to isolated-mol. 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 intermol. proton shielding contributions can be calcd. to better than 0.5 ppm. The isolated-mol. shielding tensors obtained from expt. and calcd. intermol. contributions are compared with isolated-mol. quantum chem. results. Expressed in the icosahedral tensor representation, the root-mean-square differences of the resp. tensor components are