Geometries of Nitrile Ylides

Our initial interest in this area arose from the observation that the orbitals of the commonly accepted planar geometry of the parent nitrile ylide do not correctly account for the cycloaddition regioselectivities observed for these species. Thus, as shown by the examples in Fig. 14, the digonal carbon is the nucleophilic center of the molecules, whereas the HOMO of the planar species has the largest coefficient at the trigonal carbon, which should, therefore, be the more nucleophilic center of the molecule. However, full optimization of the geometry of nitrile ylide using the 

There is a delicate balance between the relative energies of the planar and non-planar species as the component atoms of, or substituents on, various nitrilium betaines are varied. Table 2 summarizes the calculations for substituted derivatives of the parent nitrile ylide32  

Acceptors at C3 stabilize the planar species relative to the bent. This is reflected both in the differences in energy between planar and bent species, and in the optimized geometries, where the XQN angle has increased, rCl N has shortened, and rNC3 has lengthened. By STO-3G, the opposite trend is found for substitution of a 

Nitrile ylide E(planar) - STO-3G3 (kcal/mol) E(bent) MINDO/3b Optimized geometries STO-3G0 (MINDO/3)b XC1N(°) rClN(A) rNC3(A) 

Geometries are those optimized by STO-3G for the parent molecules. Geometries are fully optimized for the substituted compounds by MINDO/3. Only the geometrical parameters listed are optimized by STO-3G. 

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Fig. IS. Fully optimized and planar geometries of nitrile ylide...

Table 2. Geometries and energies of substituted nitrile ylides...

Starting with y= 120° and 0 = 0°, the MNDOC-CI method yields a conical intersection, the structure of which is shown in Figure 6.8. The values 0=0° for the rotational angle and 7 = 143,7° for the bond angle are as expected the bond distances are between those of azirine and nitrile ylide, especially because the distance between nitrogen and the methylene carbon r N = 1.34 A has decreased and is equal to the double-bond value r N = 1.34 A in the nitrile ylide. To completely establish the mechanism, it remains to be shown that the conical intersection is accessible without a barrier from the Franck-Condon geometry and that the nitrile ylide can be reached from the conical intersection. This is easily... 

The energetics of the photoreaction of 2if-azirine as well as the thermal ground-state reaction as obtained from CASPT2 calculations are summarized in Figure 6.14. From this diagram it can be concluded that the photolysis of 2H-azirine to form nitrile ylide occurs from the nji -excited state by way of an S -Sq conical intersection. Because the reaction paths on the surface from the Franck-Condon geometry to the conical intersection as well as on the Sq surface from the conical intersection to the nitrile ylide are... 

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