Stereospecific ⁵J_Hortho,OMe couplings in methoxyindoles, methoxycoumarins, and methoxyflavones

Long-range coupling constants ⁵J_Hortho,OMe were measured in series of methoxyindoles, methoxycoumarins, and methoxyflavones by the modified J doubling in the frequency domain method. The COSY and NOESY spectra revealed the coupling of the -OMe group with a specific proton at the ortho position and its preferred conformation. Homonuclear ¹H- ¹H couplings were confirmed by irradiation of the -OMe signal. Density functional theory calculations of ⁵J_Hortho,OMe using the modified aug-cc-pVTZ basis set evidenced that the Fermi contact term shows good agreement with the experimental J values. Accurate chemical shift and coupling constant values followed after iterative quantum mechanical spectral analysis using the PERCH software.