Molecular-orbital calculations heterocyclic

Amination of heterocyclic nitro compounds such as nitroquinolines, nitroisoquinolines, or nitropyridines can be carried out by means of a liquid ammonia-KMn04 system, which has been studied by Wozniak and coworkers (Eq. 9.44). Frontier molecular orbital calculation can predict the reactivity and regioselectivity of this amination.75 In a similar way, nitroquinolines are methylaminated with a liquid methylamine solution of KMn04.76... 

The dipole moment of the A-phenyl derivative (271, R = Ph) in benzene solution is 6.14 D, ° which may be compared with the molecular orbital calculations by Sundaram and Purcell which give a value of 5.3 D for the meso-ionic l,2,3,4-oxatriazol-5-one heterocyclic ring moment. 

The formation of anion radicals by chemical or electrochemical reduction may provide information about electron delocalisation in an inorganic ring system through the determination of the EPR spectrum in conjunction with molecular orbital calculations. In practice, however, the LUMO of the homo- or heterocycle is often an antibonding orbital. Consequently, occupation of the LUMO... 

The gas phase basicities at both the a- and (3-positions in five-membered heterocycles have been studied by ion cyclotron resonance equilibrium and bracketing experiments on deuteriated substrates. a-Protonation is preferred by 2.8-4.6 kcal mol-1 for both furan and thiophene as compared to 0-2.9 kcal mol 1 for pyrrole (81NJC505) and heteroatom protonation is much less favored than a-protonation. Semiempirical (MNDO) molecular orbital calculations have provided quantitative confirmation of the above conclusions. 

There is little to report on the application of molecular orbital calculations to this class of heterocycles. Yamaguchi et al. <91AX(C)590> have observed that the semiempirical AMI method underestimates the double bond character of the bicyclic C—C bond in 2-phenyl-1,2,3-triazolo[4,5-e][l,2,3,4]tetrazine (24) (Table 1). The AMI method has been reported to overestimate bond lengths in other nitrogen heterocycles <92JCS(P1)2779>, and this is possibly attributable to an overestimation of core-repulsion energies for multinitrogen systems. 

Molecular orbital calculations predict that in unsubstituted imidazoles any nucleophilic attack should take place most easily at C-2. Electron-withdrawing substituents, however, are usually necessary before any such displacements are possible, and these may also modify the position of attack. Quaternization of the heterocycle makes it even more susceptible to such reactions. 

A few molecular orbital calculations have been reported for these metallacycles. An ab initio study of a postulated azoniumrhodacyclopropane produced a structure in good agreement with known azoniummetallacyclopropanes. This as yet unknown heterocycle was studied because it has been proposed as an intermediate in the stereoselective isomerization of allyl amines to enamines... 

The photoelectron spectrum of quinazoline was measured, and had bands between 8 and 16 eV.22,23 Band assignments were proposed by comparison with the spectra of other related nitrogen heterocycles,22,23 and by estimations of the ionization potentials using molecular orbital calculations.24,25 The high-resolution He 584 A photoelectron spectra of quinazoline, 2- and 4-fluoro-, 2,4-difluoro, and hexafluoroquinazoline were measured. Fluorine substitution gave spectra from which the analysis of the bands in the parent compound was more definite.23,26 Unexpected shifts of N lone-pairs were interpreted in terms of through-space and through-bond interactions, and... 

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