Qualitative Study of Substituent Effects on NMR ¹⁵N and ¹⁷O Chemical Shifts

Abstract

A qualitative approach to analyze the electronic origin of substituent effects on the paramagnetic part of chemical shifts is described and applied to few model systems, where its potentiality can be appreciated. The formulation of this approach is based on the following grounds. The influence of different inter- or intramolecular interactions on a second-order property can be qualitatively predicted if it can be known how they affect the main virtual excitations entering into that second-order property. A set of consistent approximations are introduced in order to analyze the behavior of occupied and virtual orbitals that define some experimental trends of magnetic shielding constants. This approach is applied first to study the electronic origin of methyl-beta substituent effects on both (15)N and (17)O chemical shifts, and afterward it is applied to a couple of examples of long-range substituent effects originated in charge transfer interactions such as the conjugative effect in aromatic compounds and sigma-hyperconjugative interactions in saturated multicyclic compounds.

Applications of nuclear shielding

“…multicyclic compounds. 377 The conformational preorganisation and anioninduced conformational changes of indole-based receptors functionalised with an amide group at the 2-position and a variety of amide, urea and thiourea moieties at the 7-position were studied by the means of NMR spectroscopy. Anion-receptor interactions were evaluated through 1 H and 15 N chemical shift changes.…”

“…This approach is applied first to study the electronic origin of methyl-b substituent effects on both 15 N and 17 O chemical shifts, and afterward it is applied to a couple of examples of long-range substituent effects originated in charge transfer interactions such as the conjugative effect in aromatic compounds and s-hyperconjugative interactions in saturated multicyclic compounds. 431 The authors successfully recorded high resolution 17 O DOR spectra of vitreous B 2 O 3 (v-B 2 O 3 ), a highly effective glass-forming oxide of considerable technology importance. Two distinct oxygen sites are resolved and a complete set of 17 O NMR parameters were determined from the DOR spectra.…”

Effect of protonation on individual hydrogen bonds in the 8-oxoguanine-cytosine base pair: NMR, NBO and AIM analyses

“…Because of that, the nuclear magnetic resonance (NMR) spin-spin coupling constants across the X-H Á Á Á Y hydrogen bonds have obtained a significant interest in recent years. [36][37][38][39][40][41][42][43][44][45][46]. A systematic investigation has not yet been performed on the relationship between the NMR data concerning the H-bonded portion and other characteristics of multi-hydrogen bonded complexes.…”

A DFT‐GIAO and DFT‐NBO study of polar substituent effects on NMR 17O chemical shifts in some rigid polycyclic alkanes

“…The electronic perturbation by remote substituents of the NMR chemical shifts of many common nuclei such as 19 F, 17 O, and 15 N resides in the dominant paramagnetic term ( σ p ) to the shielding constant 1–4. This term is generally expressed in a simplified form as shown in the Eqn (1) for an atom A [where r = mean orbital radius term (related to effective nuclear charge), Δ E = mean excitation energy, and Q AB = bond‐order electron density term].…”

“…Σ Q AB terms, or both. However, another approach is based on the belief that the excitation energy term is the relevant parameter to describe substituent chemical shifts (SCS) i.e., the energy gap between the energies of the antibonding molecular orbital (B−A)* and the appropriate lone pair set(s) of A 4…”