Dinitrogen molecular orbitals

Dinitrogen has a dissociation energy of 941 kj/mol (225 kcal/mol) and an ionisation potential of 15.6 eV. Both values indicate that it is difficult to either cleave or oxidize N2. For reduction, electrons must be added to the lowest unoccupied molecular orbital of N2 at —7 eV. This occurs only in the presence of highly electropositive metals such as lithium. However, lithium also reacts with water. Thus, such highly energetic interactions ate unlikely to occur in the aqueous environment of the natural enzymic system. Even so, highly reducing systems have achieved some success in N2 reduction even in aqueous solvents. 

Triatomic species can be linear, like CO2, or bent, like O3. The principles of orbital overlap do not depend on the identity of the atoms involved, so all second-row triatomic species with 16 valence electrons have the same bonding scheme as CO2 and are linear. For example, dinitrogen oxide (N2 O) has 16 valence electrons, so it has an orbital configuration identical to that of CO2. Each molecule is linear with an inner atom whose steric number is 2. As in CO2, the bonding framework of N2 O can be represented with sp hybrid orbitals. Both molecules have two perpendicular sets of three tt molecular orbitals. The resonance structures of N2 O, described... 

Many researchers have considered models for possible intermediates in the nitrogenase reaction. Two possible dinitrogen attachments to the FeMoco factor of MoFe-protein have been put forward. Symmetric, edge- or side-on modes discussed by Dance48 would lead to a reaction sequence such as is shown in reaction 6.11. In contrast, the asymmetric end-on terminal mode discussed in the work of Nicolai Lehnert50 may be favored thermodynamically and by molecular orbital calculations. Reaction sequence 6.13 below illustrates one scenario for the asymmetric model. 

Molecular nitrogen, 17 271. See also Dinitrogen entries physical properties of, 17 272t Molecular nitrogen lasers, 14 688-691 Molecular orbital (MO) calculations, for boron hydrides, heteroboranes, and their metalla derivatives, 4 183-184 Molecular orbital laser examiner (MOLE), 16 485... 

Extensive research activities have been directed towards elucidating the actual site of activation and reduction of dinitrogen. Extended Htickel molecular orbital calculations carried out on idealised models (Figure 4.28, 45a) of the cofactors FeMco (M = Mo, V, Fe) of the three nitrogenases suggest, based on the similarity of the electronic structures. 

Dinitrogen has an excess of six electrons in bonding orbitals over those in antibonding orbitals. The structural formula of dinitrogen, N=N, shows the two atoms joined by three pairs of electrons in covalent bonds. In molecular orbital theory the number of bonds between two atoms is determined by the excess number of pairs of electrons in bonding orbitals over those in antibonding orbitals. 

The close relationship between diazomethane, hydrazoic acid, and dinitrogen oxide is evident on the basis of energies (kJ mol ) of their highest occupied molecular orbitals (HOMO) and lowest unoccupied orbitals (LUMO) (Houk et al., 1973 a Houk and Yamaguchi, 1984), as well as of their dipole moments (DP, in D) ... 

The activation of dinitrogen for NH3 synthesis, oxidative fixation to nitric acid and fertilizers, organic amine and heterocyclic synthesis, and amino acid synthesis is a topic that has generated much interest. The sequence of filled molecular orbitals of the nitrogen is a-g, lcr , ... 

A number of triatomic radicals can form dimers whose geometries have been well-characterized. A study of the electronic structures of these dimers can illustrate aspects of qualitative valence-bond and molecular orbital theory for electron-rich polyatomic molecules, and interconnections between these theories can be demonstrated. Dinitrogen tetroxide is a molecule par excellence that may be used for these purposes, and here we shall give primary consideration to its electronic structure and bond properties. 

---