Prediction of vicinal proton-proton coupling constants (3)J(HH) from density functional theory calculations
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Vicinal coupling constants (3)J(HH) have been calculated at the optimized geometries for a series of selected molecules with the aim of developing a practical procedure for predicting this kind of coupling. Calculations of couplings and optimizations of molecular geometries have been carried out at the DFT/B3LYP level using a moderate sized basis set. When the Fermi contact contributions to (3)J(HH) calculated for 25 mono- and 23 1,1-di-substituted ethanes are Multiplied by a factor of 0.904, the corresponding predicted couplings J(pre) are in good agreement with the experimental J(exp) couplings, with standard deviation σ of 0.10 Hz. When such a comparison is carried out for the remaining sets of molecules the σ deviation increases to 0.26 Hz for a dataset of 21 couplings from 11 monosubstituted cyclohexanes, to 0.19 Hz for a dataset of 40 couplings from 6 norbornane type molecules and to 0.25 Hz for a dataset of 54 couplings from 14 three-membered rings. For the complete dataset of 163 couplings the σ deviation amounts to 0.20 Hz. This figure is further reduced to 0.17 Hz by adding to the J(pre) coupling a small correction given by the term -0.15cosφ, depending on the dihedral angle 0 between the coupled protons. A larger σ deviation of 0.31 Hz was reported for the best empirically parameterized extended Karplus equation. DFT J(pre) values Could be further improved by more accurate calculations for the pertinent substituted ethane constituents of the molecule in question by applying a substituent effect model.