Nitrogen hybridisation

No analogous relationships between Edr values and NMR chemical shifts, Sn, of nitro-group nitrogen atoms were found [97], due to less correlation of the shifts with structural details. It must be stated that the aza (amino) nitrogen chemical shifts, Sa, in the nitramino groups are expected to be influenced by the nitrogen hybridisation, the size and conformation of molecule. 

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

The change is also pronounced with C=0, for not only is the nitrogen atom, with its electron pair, bonded to an electron-withdrawing group through an sp2 hybridised carbon atom (c/ p. 59), but an electron-withdrawing mesomeric effect can also operate ... 

The exact reverse of the above is seen with aniline (13), which is a very weak base (pKa = 4-62) compared with ammonia (pKa = 9-25) or cyclohexylamine (pKa = 10-68). In aniline the nitrogen atom is again bonded to an sp2 hybridised carbon atom but, more significantly, the unshared electron pair on nitrogen can interact with the delocalised 7r orbitals of the nucleus ... 

Crystal data for all four amides are uniformly similar. While amides 31b and 31f (IXnI = 65.6° and 65.3°, respectively) are more pyramidal than the urea and carbamate (IxnI = 57.1° and 56.3°, respectively) all four possess sp3-hybridised nitrogens. Average angles at nitrogen for the two amides are smaller than that required by pure tetrahedral geometry as exemplified by the tetrahedral nitrogen in l-aza-2-adamantanone 1. 

Sp3 hybridisation at nitrogen results in decreased lone pair 2pz character, which manifests itself in three ways ... 

The isopropoxy compound 25f reacts about an order of magnitude faster than 25a, 25c and 25g. A measurably larger AS is consistent with additional relief of steric compression in the transition state the protonated intermediate (Fig. 14a) would be sp3 hybridised at nitrogen while the alkoxynitrenium ion (Fig. 14c) would be sp2 hybridised at nitrogen. The isobutoxy compound 25g, in which the branching is one methylene removed from the oxygen atom has similar parameters to straight chain substrates 25a and 25c. 

The physical and spectroscopic properties of /V-acyloxy-A-alkoxyamides confirm pyramidality at nitrogen and the disconnection of the nitrogen lone pair from the amide carbonyl. The presence of an acyloxyl and an alkoxyl group at nitrogen also results in an anomeric interaction between the oxygens, which is facilitated by the sp3-hybridised nitrogen. Experimental observations, including X-ray analysis are fully supported by results from computational chemistry. 

The concept of sp hybridisation can be used to explain the bonding In other molecules, for example, nitrogen. 

---