The 13C Chemical Shift of the @so Carbon Atom in Phenyllithium

Phenyllithium was labeled with 6Li and I3C at the ipso carbon atom, and the tetramer was measured by solid-state NMR. The chemical shift tensor data were obtained by a moment analysis of the spinning side bands and were compared with the results obtained by calculations with the IGLO method. Although no splitting by dipolar spin coupling to 6Li was found the very good agreement between IGLO predictions and experimental results allowed alignment of the tensor axis to the molecular frame and interpretation of the data. The large deshielding of the isotropic chemical shift is mainly due to a decrease of the AE term in GP. Phenyllithium plays a role in organometallic chemistry, both as an important reagent for preparative chemistry['] and as a model compound for quantum mechanical calculations[*]. Numerous papers were published on its preparation["], and UV spectra were reportedlb1, ebullioscopic measurements have been performedL71, and several crystal structures of complexes with different ligands forming tetramers, dimers, and monomers are known[s-lOl. Similarly, many NMR studies were reported both in the liquid["-"] and the solid ~tate[*~-~~I. Astonishingly, however, is the lack of understanding of the 13C-chemical shift data with respect of the @so carbon atom and the C-Li bond. Normally, if in an aliphatic compound H is replaced by Li, the corresponding carbon signal is shifted to lower frequencies. This is usually interpreted in terms of the higher electron density at the anionic carbon atom. On replacement of one hydrogen atom by lithium in benzene, however, in phenyllithium the signal of ipso carbon atom is shifted to higher frequencies (A6 = 58 ppm). Several interpretations have been forwarded for this effect. Grant and Fraenkel[21] were the first to argue that a lower averaged excitation energy would be responsible for a larger op term and therefore yielding the high frequency shift. Seebach and cow~rkers[~~l distinguished on a qualitative basis between a CT and a x electron density at the metalated carbon atom and proposed enhanced rs density vs lowered x density. Similar arguments were put forward by Schleyer and coworker~[~~]. Understanding 13C-chemical shift data on a molecular level is only possible, when the chemical shift tensor and its alignment with respect to the molecular frame are known. This approach has been amply forwarded by the research group of Grant, who succeeded in measuring the chemical shift tensors of many basic molecules, such as acetylene, benzene, ethylene, and other~[~~-~~]. We have therefore set up a project to measure the chemical shift tensor of the @so carbon in phenyllithium in order to see whether a congruence with the theoretical predictions by the IGLO method can be obtained. This should lead to a more profound interpretation of the chemical shift in solution. Initially it was hoped that alignment of the principal axis of the chemical shift tensor with respect to the molecular frame would be achievable by an observation of the dipolar 6Li,13C spin coupling.