Comparison of experimental and calculated 1H NMR chemical shifts of geometric photoisomers of azo dyes

Quantum chemical calculations based on Density Functional Theory have been used to predict 1H NMR chemical shifts of the cis and trans isomers of three model azo-dye compounds. Calculated absolute chemical shift values, and differences between isomers, were in good agreement with experimentally assigned 1H NMR data. Simulated NMR spectra based on calculated chemical shifts proved useful in identifying differences between predicted and experimental data. The technique could prove valuable in assisting assignment of NMR spectra of more complex dyes of commercial interest, and preliminary investigations on larger systems indicate this is feasible.