C-13-H-1 and C-13-C-13 spin coupling behavior in aldofuranosyl rings from density functional theory

Ab initio molecular orbital calculations using density functional theory (D FT) have been conducted on the aldopentofuranose, 2-deoxy-beta-D-erythro-pe ntofuranose (1) to evaluate the performance of DFT methodology in structura l optimization and NMR spin-spin coupling constant determinations prior to its application in more complex carbohydrate-containing systems. Computed m olecular parameters (bond lengths, bond angles, bond torsions) and NMR spin -spin coupling constants (J) in the 10 geometrically optimized envelope for ms of 1 are compared to those reported previously from HF/6-31G*-optimized geometries, In earlier work, (n)J(CH) values were first computed at the HF level using finite-field perturbation theory and a basis set specially desi gned to economically recover the Fermi-contact contribution to J. Electron correlation effects on the coupling constants were then introduced via seco nd-order Moller-Plesset perturbation (MP2) calculations. The derived correl ation corrections (i.e., the MP2 - HF values) were scaled by factors obtain ed from more elaborate quadratic configuration interaction (QCISD) calculat ions on related, though necessarily smaller, systems. In the present study, the Fermi-contact components of the J values were computed directly via DF T, presumably recovering the important effects of electron correlation and thus obviating the need for scaling. J(CH) values (one-, two-, and three-bo nd) derived from the DFT treatment are compared to scaled couplings obtaine d previously using HF/MP2 methods. The effect of structural relaxation on J is assessed by direct comparison of HF values for the C-13-H-1 couplings i n both HF- and DFT-optimized geometries. (1)J(CC), (2)J(CC), (3)J(CC), and (2+3)J(CC) values are computed (DFT) in 1 as a function of ring conformatio n for the first time, correlation corrections are evaluated by direct compa rison with HF calculations, and new structural interpretations of these cou plings are provided.