Azines bond orders

The topological difference between cycl[3,3,3]azine (2) and cycl-[4,3,2]azine (85) follows from a comparison of the isoelectronic anions (86) and (87) using HMO calculations.108 Since the HOMO s of 86 and 87 are nonbonding, anions, cations, and radicals are of comparable stability. Charge density and bond orders of the cation and anion of 86 depend on the symmetry of the molecule the (positive or negative) charges are located in the starred positions (1,3. ..). There are three different bond orders, as shown in formula 86. 

Mathias and Gil have attempted to explain the observed high-field shifts of the signals of protonated azines as arising from one of two effects. Either a decrease of the paramagnetic term in Ramseys equation, or a change in bond order of the bond,... 

Bond orders for 324 bonds in 60 heterocyclic molecules, including 12 azines, were calculated at the HF/6-31G(d) and MP2/6-31G(d) levels of theory <2002IJQ534>. It was found that the MP2-derived bond orders were smaller by an average of 17%, compared to those derived from HF calculations. The ab initio results were further compared to the Gordy bond orders, derived on the basis of an empirical relationship. The latter were found to be much larger than the ab initio values, and the authors recommendation was that Gordy bond orders should not be used in aromaticity studies. 

Isoxazoles are photolabile. The first step in the reaction is believed to be fission of the N—O bond with formation of a diradical which may recyclize. In the case of the isoxazolopyridine (4) the reaction is rationalized as taking place via a 2ZZ-azirine (5) and a carbenoid structure (6) before formation of an oxazole ring (7). A high pressure mercury lamp is used, and the reaction can be run on a preparative scale (79CB3282). Photolysis of the isoxazole (8), which is fused to an azine ring with a different order of its azole heteroatoms, also leads to oxazole formation (10) (78CPB2497), but in this instance another type of intermediate is postulated (9). 

In order to estimate the excited state basicity of the azine group, the absorption and fluorescence spectra have been measured in HCIOa/CHjOH (Table 5). A calculation using eq. 6 showed that pK(j-pKg = -8.4 this corresponds to a greatly enhanced basicity in the excited state of benzaldazine and 2,2 dimethoxy-benzalda-zlne. In sallcyl aldazlne the bathochromlc shift upon acidification is smaller (2800 cm ) due to the already H-bond-lowered... 

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