N—H bond dissociation energies

For decades, the only BDE(N—H) value known was that of the ANI radical cation in aqueous solution179. In a 1982 review, only seven values were reported including the values of 368 and 366 kJmol-1 for PhNH—H and PhMeN—H, respectively180. In 1991, two more BDE values were determined181, namely 386 kJmol-1 for PhNH—H and 366 klmol-1 for Ph2N—H. Since then, both experimental and theoretical results for BDE(N—H) of several series of aniline derivatives have been available182-198. 

In the 1990s, the main electrochemical approach to estimate the BDEs was a combination of the equilibrium acidities (p7THa) and the oxidation potentials of their conjugate 

For the ANI radical cation, an acidity constant pKa = 6.4 was obtained182 however, no experimental BDE of this ion has been reported. A linear correlation of the oxidation potentials of anilines versus the acidities of the corresponding radical cations was observed. Recent ab initio calculations198 derived a value of BDE(N+—H) = 418 10 kl mol-1 for the aniline radical cation, relative to the Ph—N—H+ cation in its singlet ground state. Removal of an electron reinforces the strength of the N—H bond, due to the electron delocalization. 

For the ANI radical anion, an experimental estimate of about 125-180 kJ mol-1 has been put forward for the BDE(N—H ) quantity182. Thus, addition of an excess electron weakens the N—H bond to a large extent. 

For substituted anilines, there has been an apparent variance in the reported results obtained in various solutions (DSMO, water). Such variance likely came from different use of the correlation equation of the type 

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The energy required to break the bond between two covalently bonded atoms is called the bond dissociation energy . In polyatomic molecules this quantity varies with environment. For example, ammonia has three N—H bond dissociation energies ... 

C-H and N-H bond dissociation energies (BDEs) of various five- and six-membered ring aromatic compounds (including 1,2,5-oxadiazole) were calculated using composite ab initio CBS-Q, G3, and G3B3 methods. It was found that all these composite ab initio methods provided very similar BDEs, despite the fact that different geometries and different procedures in the extrapolation to complete incorporation of electron correlation and complete basis set limit were used. A good quantitive structure-activity relationship (QSAR) model for the C-H BDEs of aromatic compounds... 

Heats of Formation and N—H Bond Dissociation Energies for a Series of Aminyl Radicals (in kcal mol-1)"... 

N2H2 N2H + H, the (calculated) N-H bond dissociation energies (Z) ) are 435, 343 and 255 kJ mol. The dissociation of NHj leaves the odd-electron of NH2 located in a nitrogen atomic orbital. However, for each of N Hj and N H, the odd-electron may be delocalized between two nitrogen atomic orbitals, thereby leading to the development of N-N Pauling 3-electron bonds as follows ... 

The bond dissociation energies Do(HNNH-H) = 183 4.6 [14] and 184 kJ/mol [12] were derived from the ionization potential of N2H3. A well-agreeing value of Do(HNNH-H) = 182 kJ/ mol was obtained by a MP2 calculation [2]. A mean N-H bond dissociation energy of 223 13 kJ/mol was derived from the formation enthalpies of diazene and hydrazine [19]. 

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