Nitrogen bond polarisation

The values of the 15N CP MAS chemical shift of Lys296 nitrogen bonded to retinal via the —C=N bond ( Schiff base) was equal to 155.4 ppm for rhodopsin and 282.8 ppm for metarhodopsin (relative to 5.6 M aqueous NH4C1).70 The results proved the imine bond polarisation, which facilitates Schiff base hydrolysis. The comparison between chemical shifts for metarhodopsin and model compounds suggested that Schiff base linkage of the all-frans retinal chromophore in Metall is in a polar environment. 

In the thermolysis of 2,2-dichloro-l,3-diphenylaziridine, the increased polarisation of the bonds adjacent to the CCI2 group results in the opposite (carbon-nitrogen) bond in the ring undergoing pyrolytic cleavage viz. 

It is often found that imines are stabilised towards hydrolysis by co-ordination of the nitrogen to a 7t-bonding transition metal. Of course, in the absence of significant 7C-bonding interactions, ligand polarisation is expected to have the opposite effect and activate the imine towards nucleophilic attack. 

The nitrile group (CN) is linear in shape with both the carbon and the nitrogen atoms being sp hybridised. The triple bond linking the two atoms consists of one o bond and two n bonds. Nitriles are strongly polarised. The nitrogen is a nucleophilic centre and the carbon is an electrophilic centre. Nucleophiles react with nitriles... 

In 2005, Bielawski and co-workers reported the reaction of a stable carbene with an aryl azide to give the triazine 39 (Scheme 20).48 Both the E-and the Z- isomers were identified in the solid state and were found to have nitrogen-NHC bond lengths of 1.339(3) A and 1.330(3) A respectively, hence is even more polarised than compound 37. 

Indicate the polarisation that is present for each of the single covalent bonds that exist between carbon and nitrogen, oxygen and chlorine respectively. 

The bond system is slightly polarised towards the nitrogen, with each atom bearing only a small partial unit of electric charge. 

Secondly, a triply bonded dinitrogen molecule, in which each nitrogen has an oxidation of zero, bonds to the atomic nitrogen anion to give rise to one of the canonical structures of the azide anion, namely, one terminal nitrogen has one lone pair and three homoatomic bonds, and so has an oxidation number of zero. The central atom has four homoatomic bonds, but one of which is polarised away from it, i.e. the dative bond, and so it has an oxidation number of+1. The other terminal nitrogen started with one extra electron and now also has a homoatomic bond, which is polarised towards it, and so now has an oxidation number of-2. This nitrogen atom has three lone pairs. 

---