RELATIVE HOMOLYTIC STRENGTHS OF N-H BONDS IN CYCLIC AND ACYCLIC DIACYLHYDRAZIDES, IMIDES AND HYDRAZOIC ACID

With the aid of a thermochemical cycle consisting of acidity and new redox data in dimethyl sulfoxide (DMSO) solution, homolytic N-H bond dissociation energies (DELTA-BDE values) relative to acetamide (1) (where the N-H BDE for 1 is defined as 0 kcal mol-1) were determined for diacetamide (0), biuret (+1), 3,3-dimethylglutarimide (-3), diacetylhydrazine (-16), 4,4-dimethylpyrazolidine-3,5-dione (-25), 4-dimethylurazole (-29), hydrazoic acid (-15), succinimide (-15) and 1,2-dimethylurazole (-13) (all values in kcal mol-1). These DELTA-BDE data provide (a) additional evidence for the minimal N-H bond weakening effects of adjacent carbonyl groups, (b) evidence for relatively large N-H bond weakening effects of adjacent -NHC(O)R moieties in both cyclic and acyclic hydrazides and (c) evidence suggesting that urazolyl radicals are more stable than pyrazolidinedionyl radicals, relative to their hydrogenated precursors. Inserting the appropriate acidity and redox data for hydrazoic acid into a thermochemical cycle that includes a constant that permits comparison of DMSO solution BDEs with gas-phase BDEs yields estimates of 93 kcal mol-1 for the homolytic strengths of the N-H bonds present in succinimide and H-N3. The DMSO N-H BDE determined in this way for H-N3 is in remarkable agreement with a determination of its gas-phase value, whereas the DMSO N-H BDE for succinimide places it intermediate between three published estimates of its gas-phase value.