13C−H and 13C−13C Spin Coupling Behavior in Aldofuranosyl Rings from Density Functional Theory

Abstract

Ab initio molecular orbital calculations using density functional theory (DFT) have been conducted on the aldopentofuranose, 2-deoxy-{beta}-D-erythro-pentofuranose (1) to evaluate the performance of DFT methodology in structural optimization and NMR spin-spin coupling constant determinations prior to its application in more complex carbohydrate-containing systems. Computed molecular parameters (bond lengths, bond angles, bond torsions) and NMR spin-spin coupling constants (J) in the 10 geometrically optimized envelope forms of 1 are compared to those reported previously from HF/6-31G*-optimized geometries. In earlier work, {sup n}J{sub CH} values were first computed at the HF level using finite-field perturbation theory and a basis set specially designed to economically recover the Fermi-contact contribution to J. Electron correlation effects on the coupling constants were then introduced via second-order Moeller-Plesset perturbation (MP2) calculations. The derived correlation corrections (i.e., the MP2-HF values) were scaled by factors obtained from more elaborate quadratic configuration interaction (QCISD) calculations on related, though necessarily smaller, systems. In the present study, the Fermi-contact components of the J values were computed directly via DFT, presumably recovering the important effects of electron correlation and thus obviating the need for scaling. J{sub CH} values (one-, two-, and three-bond) derived from the DFT treatment are compared to scaled couplings obtained more » previously using HF/MP2 methods. The effect of structural relaxation on J is assessed by direct comparison of HF values for the {sup 13}C-{sup 1}H couplings in both HF- and DFT-optimized geometries. {sup 1}J{sub CC}, {sup 2}J{sub CC}, {sup 3}J{sub CC}, and {sup 2+3}J{sub CC} values are computed (DFT) in 1 as a function of ring conformation for the first time, correlation corrections are evaluated by direct comparison with HF calculations, and new structural interpretations of these couplings are provided. « less