NMR studies of water-soluble Dyes

Abstract

Solution- and solid-state Nuclear Magnetic Resonance Spectroscopy (NMR) have been used to investigate a series of substituted, triazinylamino azo dyes. Proton, (^13)C and (^15)N solution-state spectra have been assigned using a variety of techniques and the chemical shifts are discussed. Site-exchange effects have been observed in (^1)H and (^13)C spectra for many of the dyes studied but most prominently for compound 13.The exchange effects were investigated at different temperatures below ambient, for a CD(_3)OD solution of compound 13. The bandshapes were fitted for three-site exchange and the relevant rate constants extracted. The exchange has been attributed to internal rotation of the substituents about the triazine ring. Activation parameters have been calculated: for exchange between one pair of sites H(^#) = 76 ± 8 kJ mol(^-1) and ɅS(^#) = 52 ± 27 J mol(^-1) K-(^-1) and for a second pair, H(^#) = 76 ± 4 kJ mol(^-1), S(^#) = 55 + 15 J mol(^-1) K(^-1). Direct exchange between the remaining pair of sites is negligible. Exchange involving a fourth site affects the spectra at somewhat lower temperatures, which assists in a partial assignment of the observed peaks to the rotamers. The rotations that are causing the effects observable in the NMR spectra are identified, with the help of molecular modelling investigations. Three out of the four possible energy barriers are found to be a combination of rotation and nitrogen inversion. Exploration of the energy surface shows that nitrogen inversion with rotation actually reduces the rotational energy barrier, compared to the planar rotation. The relative barrier heights can be rationalised, although their exact values are not consistent with those measured using NMR. Solid-state (^13)C spectra have also been recorded but are broad, making detailed assignment difficult. The broadness is found to be mainly due to the amorphous nature of the samples. The resolution is not vastly improved on increasing the spectrometer field and the powder X-ray diffraction exhibits only broad reflections.