Torsion angle relationship of the ¹⁷O NMR chemical shift in α,β‐unsaturated carbonyl compounds

Abstract

The torsion angle effect on the isotropic shielding of (17)O nucleus in alpha,beta-unsaturated carbonyl groups is studied by means of density functional theory (DFT) calculations using a polarizable continuum model (PCM) for the solvent, employing the PBE0 functional together with the 6-311G(d,p) basis set for geometry optimization, and the 6-311+G(2d,p) basis set for calculating the NMR shielding with the gauge-including atomic orbitals (GIAO) method.This study adds new information on the sensitivity of the (17)O nucleus to conformational changes, revealing a strong dependence of the (17)O NMR chemical shift on the dihedral angle between the carbonyl and the vinyl moiety in all studied compounds; remarkable differences are observed with the data reported for alpha-diketones.

On the Fluxional Behavior of Dess−Martin Periodinane: A DFT and 17O NMR Study

“…For the acetoxy groups, if fast exchange of the oxygen atom pairs is considered, the agreement between the experimental and computed data is excellent, making us confident in the adopted theory level. As already observed in a previous study of cyclic compounds, the computed chemical shift of the carbonyl group of the iodoxolone ring (O 3 ) is significantly lower than the experimental value.…”

“…Calculated absolute shielding data, σ, were converted into the usual chemical shift scale employing the equation δ = σ ref + δ ref − σ, where σ ref and δ ref are respectively the calculated absolute shielding and the experimental chemical shift of a chosen reference. An oxygen atom of the studied molecule (O 4 ) was selected for chemical shift referencing since it was shown that the use of a chemically similar atom as reference cancels systematic errors and permits good chemical shift predictions …”

Recent Advances in Nuclear Shielding Calculations

“…With a fragment model consisting of a dimethyl phosphate and water molecules in the first solvation shell, a description of how 31 P shieldings in DNA are affected by the phosphate backbone torsion angle has been accomplished. 73 Other examples along this line of research involve studies on polymorphs, 74, 75, 76, 77, 78, 79, 80, 81 confined molecules, 82 substituted calixarene systems, 83 sugars, 84, 85 , -unsaturated carbonyl compounds, 86 and phase transitions. 87…”

17o NMR

“…This has been demonstrated, for example, by a series of calculations on α,βunsaturated ketones and esters. 126 The chemical shifts of different conformations of these compounds may be up to 100 ppm apart. In principle, this allows the study of the conformational preference of these and similar systems by 17 O NMR spectroscopy.…”