Contributions of the PO ester and CO torsion angles of the phosphate group to ³¹P‐nuclear magnetic shielding constant in nucleic acids: Theoretical and experimental study of model compounds

The magnetic shielding constant of the 31 P nucleus of the dimethylphosphate anion is calculated by an ab initio method for different values of the torsion angles about the PO ester bond and different orientations of the methyl groups. The results obtained tend to show that both types of conformational parameters contribute to the value of σ 31P . The largest shielding is obtained when the methyl groups are staggered with respect to the PO bond; the smallest, for the eclipsed arrangement. Measurements carried out on the 16 deoxyribodinucleoside monophosphates show that in the majority of cases σ 31P is shifted toward lower field for the dimers having large values of 3 J PH3 ′ and 3 J PH5′(5″) . The theoretical results are discussed in relation to experimental data for polynucleotides and nucleic acids.