Predicting 13C NMR Spectra by DFT Calculations

13C chemical shifts and nJCH coupling constants have been determined both experimentally (by means of J-resolved NMR spectroscopy) and theoretically (by DFT calculations) for a series of organic molecules. With the exception of halogen-bonded carbon nuclei, a good correlation is observed between experimental and calculated data. The magnitude of the most important contributions to the spin-spin coupling constant (Fermi-contact, diamagnetic, and paramagnetic spin-orbit contributions) has been determined. The spin-orbit terms are negligible or cancel out (1JCH and 3JCH), thus leaving the contact term as the only relevant contribution, but become important for 2JCH in aromatic (but not in aliphatic) compounds. Relativistic effects on the 13C chemical shift of carbon bonded to a fairly heavy atom (bromine) have also been investigated. Finally, conformational effects on the long-range nJCH coupling constants has been investigated in a model alkane derivative (n-butyl chloride). The implications to structure prediction and determination by NMR are discussed.