C-HETSERF: distinction of cis/trans-isomers and measurement of long range couplings between chemically equivalent nuclei in polycyclic aromatic hydrocarbons

Abstract

The scalar couplings between chemically equivalent protons in symmetric molecules are not reflected in the NMR spectrum. The present study reports the utility of our previously reported two dimensional spin selective correlation experiment to measure the short and long range homo- and hetero-nuclear scalar couplings in such systems. In weakly coupled spin systems, the experiment also yields the relative signs of the couplings. The relative configurations of double bonded symmetrically disubstituted isomers could be determined from the measured long range vicinal proton-proton couplings (³J_HH), owing to the fact that these couplings are consistently larger for trans isomers. The study also reveals that long range ²J_CH cannot be utilized as an exclusive parameter for the identification of symmetrically disubstituted cis/trans isomers. The application of the methodology for the determination of couplings between the chemically equivalent protons in polycyclic aromatic hydrocarbons and in bigger molecules such as porphyrin, has also been demonstrated. The study, carried out on large number of molecules, shows the generality and wide applicability of the C-HETSERF experiment.