Trigonal orbital

Fig. 11 (see p. 86/87) contains all trigonal orbitals which were encountered in the molecules considered. The bonding orbitals, in the left column, exhibit the increasing polarization from N2 to LiF. Moreover, the inclination of the contributing (sp) hybrid of the right atom into the bond region diminishes as the polarization increases, i.e., the axis of this hybrid is much closer to being perpendicular to the internuclear axis in LiF than in N2. Clearly, an increase in (p) character accompanies the diminshed inclination. 

In the final 1,4-elimination, the trivalency of a carbon atom does not lead to a double bond formation but to a ring formation sterically facilitated by the fact that carbon atoms participating in double bonds form planar structures characterized by bond angles of 120° (plane trigonal orbitals). After the 1,4-elimination, ejection of a hydrogen ion completes the process. 

As a final application, we discuss an example of a molecular radical, where more symmetry is present than the eye meets. The triphenylmethyl radical, C19H15, is a planar, conjugated, hydrocarbon-radical, with 19 tt-electrons. The molecular point group for the planar configuration is >3a, but, since all valence -orbitals are antisymmetric with respect to the horizontal symmetry plane, the relevant symmetry of the valence shell is only as seen from Fig. 4.10. The molecular symmetry group distributes the 19 atoms over five trigonal orbits of atoms that, under C3 , can solely be permuted with partners in the same orbit. 

Whereas the sp orbital is the key to diamond and aliphatic compounds, the sp (or trigonal) orbital is the basis of all graphitic structures and aromatic compounds. 

The trigonal orbital of the nitrogen atom in pyridine, projecting out from the ring, holds two spin-coupled electrons and so cannot link a hydrogen atom to the nucleus. It can, however, accommodate a proton. In other words, pyridine is basic (see p. 145). 

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