The ¹³C Chemical Shift of the ipso Carbon Atom in Phenyllithium

Abstract

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 ~t a t e [ *~-~~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 r s 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 o t h e r~ [~~-~~] .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 coupl...

The unexpected roles of σ and π orbitals in electron donor and acceptor group effects on the 13C NMR chemical shifts in substituted benzenes

“…The σ 11 trends are associated with the σ C1–C NLMOs, while the variations in σ 22 are associated with the σ C1–H NLMO. (Sometimes, a shielding analysis in terms of CMOs can also be very insightful, 35 in particular when shielding effects can be tied to the orbital energy gaps Δ ε i , a = ε i – ε a in the denominator of eqn (2). We also performed the shielding analysis in terms of CMOs, but decided to forgo a discussion because the trends were less clear, mainly due to the fact that multiple occupied and unoccupied CMOs contribute to the shielding trends and no obvious relationship to the Δ ε i , a could be identified.)…”

Carbanion Chemistry

“…The contrary reality is nicely illustrated by the 13 C chemical shift of C 1 of phenyl lithium. 18 The isotropic δ value increases as C 1 becomes more negative (i.e., shielding decreases as the electron pair becomes less well bound). Each of the three principal values of the shielding tensor for C 1 changes significantly when its formal charge diminishes from 0 in benzene to −1 in phenyl lithium ( Figure 4).…”

“…The simulated CP-MAS 13 C spectrum of 1-13 C phenyl lithium based on the data of Berger et al18 The principal values of the shift tensor are shown (values for benzene in parentheses) with the electron orbitals, and their associated electron circulations. (See also Ref.…”

Counterintuitive deshielding on the 13C NMR chemical shift for the trifluoromethyl anion

“…This deshielding effect on the δ 13 C for the CF 3 − is counterintuitive, and explanations found in the literature are not complete. Berger et al suggested that the δ 13 C of anions may be understood using the same reason applied by the authors to explain the ipso carbon atom in phenyllithium. The ipso δ 13 C in phenyllithium is larger than in benzene due to smaller energy gap associated to C–Li bond in comparison with C–H bond.…”

“…Several works show that the δ 13 C depends on how the electronic system responds to the magnetic field; therefore, a more complex explanation is required to clarify the anion behaviour. Thus, the present study aims to demystify the deshielding effect on CF 3 − by determining the involved shielding mechanisms as well as the role of each orbital through density functional theory calculations.…”