Unambiguous assignment of the 13C NMR spectra of methylindoles

Abstract

The ¹³C NMR spectra of all possible indoles substituted with one, two or three methyl groups on the pyrrole ring were assigned from heteronuclear two‐dimensional ¹³C/¹H contour plots and homonuclear hydrogen‐hydrogen decoupling experiments at a specific concentration of 0.83 M in CDCl₃ and in DMSO‐d₆. The results confirm controversial assignments for some compounds, and provide the assignments for those compounds not previously studied by this technique. In addition, deuterium/hydrogen NMR isotope effect on ¹³C chemical shifts (DHIECS) for indoles, wherein the labile NH was partially exchanged to ND, are reported. In this way DHIECS values over two, three, four and five bonds were measured. The results in each solvent are discussed in terms of the number of bonds between the exchanged hydrogen and the observed carbon, and also according to their geometric dependence. This provides some evidence of the vibrational origin of the DHIECS.