Interpretation of carbon nuclear magnetic resonance shifts. Part 4.—Effective excitation energies in the shielding of alkene or carbonyl carbon

Abstract

Successive methylations of ethene give ¹³C shifts of the alkene carbons which (when corrected for the ligand diamagnetic contribution) correlate inversely with π→ 3p excitation energies from vacuum u.v. spectroscopy. This correlation is expected since the π→ 3pσ and the π→ 3pπ_y are the lowest magnetically active excitations (and the π→ 3p splitting is small). There are correlations also with the (inactive)π→ 3s excitation energies, and with the ionisation energies I₁(π) and I₂(σ), probably because of bunching of the energies of the virtual orbitals used in the paramagnetic circulations, so that their relative energies are determined by those of the occupied orbitals. Hyperconjugative effects at the β carbon, and the alkene shielding tensor, also point to the importance of the π electron excitations.

For successive chlorination or fluorination, however, the changes in π electron excitation energies do not correlate with the alkene carbon shifts (allowance being made for the increase in radial factor). This is explained by the opposed inductive and conjugative effect of the halogen, as in the “perfluoro effect” which greatly stabilises the σ orbitals but has rather little effect on the π orbitals.

With methylation of carbonyl carbon in formaldehyde, formic acid, methyl formate or formamide, the carbon shifts are commensurate with decrease in ionisation energy of the π_CO electrons. O- or N-methylation of the acids or amides leads to upfield carbon shifts and decrease in I(π_CO), paralleling the hyperconjugative β-carbon effects in the ethenes. There is therefore an overall correlation of ¹³C and ¹⁷O shifts for carbon and oxygen bonded together for a broad range of C—O (CO, CO) groupings; an opposed trend within a particular compound type can be attributed to hyperconjugation as before.