N,N-Linked biazoles. III. MNDO calculations on the conformation of N,N-linked biazoles and their quaternary salts

Abstract

A theoretical study of fourteen N,N-linked biazoles has been carried out using the MNDO method with full optimization of the geometries. The variation of energy as a function of the conformation around the central N—N bond shows in all cases the higher stability of the orthogonal or near-orthogonal form. The contribution of the three main types of "ortho" interactions [Formula: see text] and the effect of quaternization have been evaluated. Of the electronic properties calculated, one must emphasize the excellent agreement of dipole moments with experimental values.

Photoinduced Inter- and Intramolecular Proton Transfer in Aqueous and Ethanolic Solutions of 2-(2‘-Hydroxyphenyl)benzimidazole: Evidence for Tautomeric and Conformational Equilibria in the Ground State

“…The closed cis-enol form of HPyBI is very much more favored than the trans conformer; this is suggested by comparison of their intramolecular interactions (the closed cis conformer features both a strong N‚‚‚HsO hydrogen bond and a weaker N‚‚‚HsN bond, whereas in the trans conformer the only attractive interaction, H‚‚‚OsH, is very weak, 48 and there is a repulsive interaction between the lone pairs of the nitrogen atoms) 49 and is corroborated by the results of semiempirical calculations with AM1, according to which the closed cis-enol form of HPyBI is 20 kJ/mol more stable than the trans conformer. 6 For HBI, on the other hand, AM1 calculations predict that the trans conformer is 2 kJ/mol more stable than the closed cis-enol form.…”

“…6 For HBI, on the other hand, AM1 calculations predict that the trans conformer is 2 kJ/mol more stable than the closed cis-enol form. 29,30 This difference in behavior can be attributed to HBI lacking the lone pair-lone pair interaction that destabilizes the trans form of HPyBI and to the N‚‚‚HsN hydrogen bond in the closed cis-enol form of HPyBI being replaced in HBI by a repulsive steric interaction between N-H and C-H. 49 Now, according to the arguments of the preceding paragraphs concerning the origins of the HBI emission bands, the difference between HBI and HPyBI with regard to the relative stabilities of the trans and closed cis-enol conformers implies that whereas HBI will exhibit two emission bands, fluorescence emission by HPyBI ought to be predominantly due to its keto form. Since this is in fact the case, hardly any higherenergy emission by HPyBI being observed, even in ethanol and water, 6 the arguments of the preceding paragraphs allow successful prediction of the experimental facts for HPyBI, which therefore support those arguments.…”

Solid state conformations of symmetrical aromatic biheterocycles: an X-ray crystallographic investigation

“…1 From such investigations it has been proposed that the conformation about the inter-ring bond in biheterocycles is determined by the nature of the adjacent atom groupings. 5 Specifically, two types of interactions have been identified as destabilising [viz C-H … H-C (1a) and N: … :N (1b)] and one as stabilising [C-H … :N (1c)]. Such arguments have been used to rationalise the s-trans conformations of molecules such as 2,2'-bipyridine (2) and 1,1'-bipyrazole (3).…”

“…Such arguments have been used to rationalise the s-trans conformations of molecules such as 2,2'-bipyridine (2) and 1,1'-bipyrazole (3). 5 Considerable computational and experimental evidence supports the importance of these factors in the gas phase and in solution. However, we believe that these factors are likely to be much less important in the solid state.…”

The structure of N-aminoazoles: an experimental (X-ray and15N NMR) and theoretical study

“…In addition to these data, the study of the labelled Naminopyrazole (3b') ["N( 1), I5N (2)] in CDCl, provided some additional coupling constants [N( 1) signal is broad, probably due to the interaction with the I4NH2, and some coupling constants are lost].…”