Frontier nitrogen atom

The derivatives of the energy correction (of the terms proportional to dP4rr and c)T4r) with respect to the angles J/ J/ yield additional quasi- and pseudotorques (K 4 and (V4, respectively) acting upon the hybridization tetrahedron of the frontier nitrogen atom ... 

The following quantities are presented in Table IX total 77-elec-tronic energy (W), its deviation from that of the parent hydrocarbon (AW), energies of the four frontier 7r-molecular orbitals (k2, klt and Jfc 2 k is the highest occupied orbital, HOMO is the lowest free orbital, LFMO), Fj transition energy [ (A - Ft)] and the atom localization energy for the parent hydrocarbon in the position in which the heterocycle has a nitrogen atom. 

In his classical review, Stucky (1) has already mentioned that in many n complexes the position of the lithium-base fragment is determined by the interaction of the frontier orbitals in the n fragment with the relevant orbitals at the lithium atom. This is nicely demonstrated by a series of cyclopentadienyl-, indenyl-, and fluorenyllithium complexes containing two further nitrogen atoms coordinated to the lithium as portrayed in Figs. 1 and 2. 

However, if we look at the LUMO, we find that it has the form 4.65, namely that of ift4 of benzene, but polarised by the nitrogen atom. This polarisation has reduced the coefficient at C-3, and the coefficient at C-4 is larger than that at C-2, as can be seen from the simple Hiickel calculation for pyridine itself in Fig. 4.11, which gives LUMO coefficients of 0.454 and —0.383, respectively, and an energy of 0.56/3 (compare benzene with 1/3 for this orbital). Thus, soft nucleophiles should attack at C-4, where the frontier orbital term is largest. Again this is the case cyanide ion, bisulfite, enolate ions, and hydride delivered from the carbon atom of the Hantsch ester 4.67 react faster at this site. 

Perhaps solvation is important here the nitrogen, with so little of the charge, must be less crowded by solvent molecules and is therefore more accessible. But this is not the whole story, as we can tell from the complementary case of nitration. In nitration, the important frontier orbital will be the LUMO of N02+, and this is a similar orbital to the HOMO of N02 . The nitronium ion, N02+, always reacts on nitrogen, both with soft nucleophiles like benzene and with hard ones like water. In the nitration of benzene, the solvent is often nonpolar thus differential solvation is not likely to be responsible for the fact that the nitrogen atom is the electrophilic site. 

Tehan et al. [213] used the AMI [211] semiempirical method and parameters from frontier electron theory [214] to model the pK s of 55 anilines, Using the electrophilic superdelocalizability SE, of the nitrogen atom they obtained the relation... 

The abovementioned behaviour has been rationalized,at least partly, by simplified qualitative tt-MO schemes [23] and also by means of extended Huckel calculations [24] which indicate that, although the HOMO of the isocyanide complex is mainly localized at the chloride ligand and at the metal atom, the nitrogen atom carries a considerable negative net atomic charge (in contrast with the metal and the ligating isocyanide carbon atom). Therefore, the 3-protonation appears to be charge-controlled rather than frontier-orbital controlled [24]. 

If we compare for example, the HOMO and LUMO energies of 1,3-butadiene and 2-aza-1,3-butadiene (Fig. 18.2) we will notice how the incorporation of a nitrogen atom in position 2 of the diene has lowered the energies of the frontier orbitals. In particular, the HOMO of the 2-aza-1,3-butadiene is now 0.43 eV lower in energy than the HOMO of 1,3-butadiene and for this reason, it must be less reactive than butadiene toward an electron-deficient dienophile. 

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