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

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Abstract

Ab initio molecular orbital calculations using density functional theory (DFT) have been conducted on the aldopentofuranose, 2-deoxy-β-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, nJCH 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 Møller−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. JCH values (one-, two-, and three-bond) derived from the DFT treatment are compared to scaled couplings obtained previously using HF/MP2 methods. The effect of structural relaxation on J is assessed by direct comparison of HF values for the 13C−1H couplings in both HF- and DFT-optimized geometries. 1JCC, 2JCC, 3JCC, and 2+3JCC 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.

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