C-H Bond Strengths and Acidities in Aromatic Systems: Effects of Nitrogen Incorporation in Mono-, Di-, and Triazines

摘要

The negative ion chemistry of five azine molecules has been investigated using the combined experimental techniques of negative ion photoelectron spec- troscopy to obtain electron affinities (EA) and tandem flowing afterglow-selected ion tube (FA-SIFT) mass spectrometry to obtain deprotonation enthalpies (△_(acid)H_(298)). The measured △_(acid)H_(298) for the most acidic site of each azine species is combined with the EA of the corresponding radical in a thermochemical cycle to determine the corresponding C-H bond dissociation energy (BDE). The site-specific C-H BDE values of pyridine, 1,2-diazine, 1,3-diazine, 1,4-diazine, and 1,3,5-triazine are 110.4 ± 2.0, 111.3 ± 0.7, 113.4 ± 0.7, 107.5 ± 0.4, and 107.8 ± 0.7 kcal mol~(-1), respectively. The application of complementary experimental methods, along with quantum chemical calculations, to a series of nitrogen-substituted azines sheds light on the influence of nitrogen atom substitution on the strength of C-H bonds in six-membered rings.