Nitrogen lone-pair orbital orientation

These interactions are most commonly observed for divalent chalcogen atoms and the nitrogen atom (the electron donor D) lies within the X-E-Y (E = S, Se, Te) plane, preferably along the extension of one of the covalent bonds as in 15.3. This anisotropy is a clear indication that these short E N contacts have some bonding character, i.e., they are subject to the geometric restrictions of orbital overlap. Eor example, in the diselenide 15.4 the nitrogen lone pairs are clearly oriented towards the Se-Se linkage. ... 

The peptide case constitutes a special situation in so far as the lone-pair of the nitrogen atom is in fact a Pz-orbital which is part of the molecular orbital of the amide systeof. Our data seem to indicate that the variation in J(C,H) coupling constants which must be attributed to lone-pair effects can be as high as 12 Hz. For very favorable relative orientations of the C-H bond and the nitrogen lone-pair even larger effects may be expected. 

A further comparison of structures 41 and 42 shows that the main difference between the structures occurs near this same reactive bond. Structure 42 represents the most stable conformation expected of Dalanyl-Dalanine, where the amide is trans substituted and the nitrogen is oriented so that its lone pair of electrons may overlap with the tt orbitals of the carbonyl group. In... 

Another difference between the molecules is that whereas in benzene each carbon is bonded to a hydrogen atom, in pyridine the nitrogen possesses a lone (unshared) pair of electrons. This lone pair occupies an sp orbital and is orientated in the same plane as the ring moreover, it is available to capture a proton so that pyridine is a base. 

The role of orbital orientation in organic and enzymatic reactions has been of considerable current interest. Deslongchamps and coworkers(1) in studying tetracovalent carbon species have recently demonstrated selective cleavage of bonds which are trans, antiperiplanar (app) to lone pairs on directly bonded oxygen and nitrogen atoms. 

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