N—H bond energies

What is known about the enthalpy of formation of the other fragment radical, Ph2C=N The enthalpy of formation of H is well-established and that of Ph2C=NH has been calori-metrically determined. The N—H bond energy in this imine, i.e. roughly the enthalpy of reaction 54, has been reported, twice . ... 

The N—H bond energy, i.e. BDE(H—NHOH), is instead larger and equal to 81-82 kcal moG, thereby explaining why H-atom removal from H2NOH yields H2NO and not the isomeric NHOH radicaP . These two BDE values are well matched by quantum chemical computations ". ... 

Hydrazines and Amines. These substrates are directly oxidized in a base-free matrix (Me2SO or MeCN) at platinum or glassy-carbon electrodes with the potential determined primarily by the / N—H bond energy, and secondarily by die basicity of the substrate (Figures 11.9 and 11.10) 15,16... 

Use the following standard enthalpies of formation to estimate the N—H bond energy in ammonia. Compare this with the value in Table 13.6. 

The standard enthalpy of formation of NH3(g) is -46 kj/mol. Use this and values for the N=N and H—H bond energies to estimate the N—H bond energy. Compare this result with your result from Exercise 43. 

At low energies the abstraction process dominates and at higher energies the exchange mechanism becomes more important. The cross-sections for the two processes crossing at 10 eV. The END calculations yield absolute cross-sections that show the same trend as the experimentally determined relative cross-sections for the two processes. The theory predicts that a substantial fraction of the abstraction product NHjD, which are excited above the dissociation threshold for an N—H bond actually dissociates to NH2D" + H or NH3 during the almost 50-ps travel from the collision chamber to the detector, and thus affects the measured relative cross-sections of the two processes. 

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 ... 

The ionization energy of the hydrogen atom, 313.6 kcal/mole, is quite close to that of fluorine, so a covalent bond between these two atoms in HF is expected. Actually the properties of HF show that the molecule has a significant electric dipole, indicating ionic character in the bond. The same is true in the O—H bonds of water and, to a lesser extent, in the N—H bonds of ammonia. The ionic character of bonds to hydro-... 

In the presence of adsorbed atomic oxygen, the activation barrier is substantially lowered to 132 kJ mol-1, the reaction is endothermic at 48kJmol 1, but the high activation energy suggests that the N-H bond would not be broken. However, at high temperatures it might be achieved. 

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... 

Dissociation Energies of N—H Bonds of Aromatic Amines—continued... 

The comparison of the rate constants of these reactions with BDE of N—H bonds proves that the higher the BDE, the lower rate the constant. The rate constants of the reaction of amine with hydroperoxide calculated by the IPM method are listed in Table 15.18. The parameters of these reactions are given in Table 15.18. The values of activation energies for all reactions of the type... 

Lewis bases having two or more lone-pair-bearing atoms can exhibit a form of isomerism in H-bonding. This can be illustrated most simply with carbon monoxide, which has sigma-type lone pairs both on C and on O, and thus is expected to form n—>-cr H-bonds of either OC HA or CO HA type with a hydride-bearing Lewis acid. For example, complexation of CO with HF gives rise to distinct OC- -HF or CO- -HF complexes.39 The two possible isomeric forms are shown in Fig. 5.3, both bound by rather typical H-bond energies ... 

---