NMR studies of exchange between triazine rotamers

Abstract

Proton and 13C solution-state spectra were obtained for a substituted triazine and assignments made for the signals using different solvents. Site-exchange effects were observed in 1H spectra at different temperatures below ambient for a CD3OD solution. The bandshapes were fitted for three-site exchange and the relevant rate constants extracted. The exchange was attributed to internal rotation of the substituents about the triazine ring. Activation parameters were calculated: for exchange between one pair of sites Δ H‡ = 71 ± 4 kJ mol−1 and Δ S‡ = 33 ± 15 J mol−1 K−1, and for a second pair Δ H‡ = 72 ± 5 kJ mol−1 and Δ S‡ = 39 ± 26 J mol−1 K−1, whereas direct exchange between the remaining pair of sites is negligible. Exchange involving a fourth rotamer affects the spectra at lower temperatures, which assists in a partial assignment of the observed peaks to the rotamers. These results are discussed in relation to molecular modelling information and literature values. Solution-state and solid-state 13C spectra were also recorded but the latter are broad, making detailed assignment difficult. Copyright © 2000 John Wiley & Sons, Ltd.