Chelate-type intramolecular hydrogen bridging in 2,4-diaryl-6-(2-hydroxy-4-methoxyphenyl)-1,3,5-triazines. A dynamic1H/13C NMR study

Abstract

The energy for breaking the intramolecular hydrogen bond between the phenolic OH and an aza nitrogen in two 2,4-diaryl-6-(2-hydroxy-4-methoxyphenyl)-1,3,5-triazines was determined, by dynamic 1H and 13C NMR spectroscopy, as 55±5 kJ mol-1. The activation parameters ΔH‡ and ΔS‡ for the corresponding internal rotation were derived from a ΔGC‡–TC plot. The strength of the chelate-type hydrogen bond is the rationale for the excellent photostabilizing capacity of this class of triazines. It is demonstrated explicitly that the line separation Δν of the signals under exchange must be extrapolated to the coalescence temperature when ΔGC‡ values are to be calculated by the Gutowsky–Holm equation. Correct lineshape analysis likewise depends on employing the requisite Δν value for each temperature. © 1997 John Wiley & Sons, Ltd.