Relativistic Force Field: Parametric Computations of Proton–Proton Coupling Constants in ¹H NMR Spectra

Spin–spin coupling constants in ¹H NMR carry a wealth of structural information and offer a powerful tool for deciphering molecular structures. However, accurate ab initio or DFT calculations of spin–spin coupling constants have been very challenging and expensive. Scaling of (easy) Fermi contacts, fc, especially in the context of recent findings by Bally and Rablen (Bally, T.; Rablen, P. R. <i>J. Org. Chem. </i><b>2011</b>, <i>76</i>, 4818), offers a framework for achieving practical evaluation of spin–spin coupling constants. We report a faster and more precise parametrization approach utilizing a new basis set for hydrogen atoms optimized in conjunction with (i) inexpensive B3LYP/6-31G­(d) molecular geometries, (ii) inexpensive 4-31G basis set for carbon atoms in fc calculations, and (iii) individual parametrization for different atom types/hybridizations, not unlike a force field in molecular mechanics, but designed for the fc’s. With the training set of 608 experimental constants we achieved rmsd