MNDO semiempirical and 4-31G ab initio SCF-MO calculations of heteroaromatic compounds

Abstract

The performance of the MNDO method, as far as the energies of azines and azoles are concerned, is critically evaluated after comparing its results with some ab initio calculations at the 4-31G level and with available experimental data. The lone-pair electronic repulsions between the neighboring pyridine-like nitrogens are systematically underestimated by MNDO. Nevertheless, when corrections are introduced, even the relative stabilities of quite complex heterocyclic systems (tetrazolotriazine and tetrazolobenzotriazine tautomers) are predicted with reasonable accuracy.Semiempirical SCF-MO methods are rather popular within the organic chemists' community since, with a moderate cost, they offer quite reliable results (in specific areas) of either explanatory or predictive value for medium-to-large-sized molecules. Outstanding among which there is the MNDO method of Dewar and Thiel,1 with parameters now available for the second-period elements and also for Al, Si, P, S, Cl, Br, I, and Sn.2 Its utility for searching equilibrium geometries of unusual species before

Comparative semiempirical calculations of tetrazole derivatives

“…In a number of previous works, it has been noted that among the semiempirical methods MNDO is the most suitable for evaluating the energetic characteristics of five-and six-member nitrogen-containing heterocycles, in particular, of azoles [16,33,34] and azines [16]. The values of the calculated and the experimental heats of formation of the investigated tetrazole derivatives (Table I) confirm this conclusion.…”

“…It should be noted, however, that different authors used various methods of calculation, and, therefore, the published data are difficult to compare and cannot be employed for establishing correlations between calculated and experimental characteristics of tetrazole derivatives. Moreover, a considerable number of published works deal with either the parent compounds of tetrazoles, namely, 1H-and 2H-tetrazoles [2-61, or wide series of azoles [S, 15,16,19,25,26] or azines [17] in which tetrazole compounds are considered only as particular objects. In this connection, it seemed expedient to carry out a systematic quantum chemical investigation of a wide range of tetrazole derivatives with substituents of different natures using a set of semiempirical methods of calculation.…”

On the basicity and π-electron delocalization of ‘hexaazabenzene’ N6 – Quantum-chemical studies

“…However, hexazine N 6 has not yet been prepared, and till now, there are no experimental data. N 6 has been solely investigated by quantum-chemical methods: semiempirical, ab initio, and DFT [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Surprisingly, the results reported in the literature not always confirmed the strong aromatic character of N 6 .…”

Correlations of hip joint space among asymptomatic subjects: Comment on the article by Lanyon et al

“…Triazines vary significantly in stabilities, symmetries, and aromaticity gradation 15, 16. There are three different triazine isomers (i.e., 1,2,3‐triazine, 1,2,4‐triazine, and 1,3,5‐triazine) with different symmetries (Fig.…”