Lone-pair interaction, nitrogen

In triaziridine 23, the o-relaxation is appreciable due to the lone pair effect (Sect. 2.1.4) as is the case with triphosphirane 14 which has almost the same SE as tetra-phosphetane 15. Triaziridine 23 has lower SE (31.8 kcal moF ) than tetraazetidine 24 (SE = 33.4 kcal moF ) (Scheme 12) [12]. Lone pairs on nitrogen atoms have high s-character. The hybrid orbitals for the N-N bonds have lower s-character (sp ) in 23 than that (sp ) in triazane 25 [12]. The geminal a-a interaction is less antibonding in 23 (IBP, = -0.004) than in 25 (IBP. = -0.014). The geminal... 

Heteroatomic Substituents. The effect of attaching a heteroatomic substituent to the carbyne carbon, for example, NMe2, is to remove the degeneracy of the 7r-type LUMO s (28). There is extensive mixing (w-donation) from the lone pair on nitrogen into one of the p orbitals on carbon. The ir -orbital for the N - C -interaction is moved to higher energy, and the other p orbital becomes the sole LUMO. 

Analogously, shieldings of antiperiplanar carbons by nitrogen lone pairs have been reported. It was concluded that a considerable part of the 10 ppm difference of the C-2 chemical shifts in the isomers of 8-methyl-8-azabicyclo[3.2.1]octane must be due to lone-pair interaction, since a difference of only 6 ppm is expected from a y-ganche effect of the A-methyl group73. This is supported by the fact that the difference is reduced if the solvent is methanol in this case the lone pairs are involved in hydrogen bridging. 

Photoelectron spectroscopy145 has been particularly useful in the conformational analysis of heterocyclic molecules possessing vicinal electron lone pairs, especially in the hexahydropyridazine system.12,145a In such a system the two lone-pair orbitals on nitrogen do not ionize at the same potential because of interactions with each other and with other orbitals. Lone-pair-lone-pair interactions are expected to dominate and are dependent upon the... 

The existence of ring strain, nitrogen lone-pair interactions, or both seems to be crucial in the absence of aromaticity of 1,2-A3-diazetine (26). 

The s character of the aziridine nitrogen s lone pair has other effects too. The lone pair interacts very poorly with an adjacent carbonyl group, so N-acyl aziridines such as the one you saw on p. 1125 behave not at all like amides. The nitrogen atom is pyramidal and not planar, and the stretching frequency of the C=0 bond (1706 cm-1) is much closer to that of a ketone (1710 cm-1) than that of an amide (1650 cm-1). 

Calculations on 1,2- and 1,3-diazetes (20-22) are discussed in Section 2.5.2.1. Although associated with six -electrons, there is no evidence that the 1,2-dihydro- 1,2-diazetes 63 have any aromatic character, presumably due in part to ring strain and nitrogen lone pair interactions . 

---