Theoretical investigation on the structures and thermodynamic properties of mixed boron-, nitrogen- and oxygen-containing three- and four-membered rings BnNmOHp (n = 0-3, m = 0-3, p = 0-3)

As a follow-up to our study on the aromatic five-membered ring B2N2OHp, in this paper, the structures, heats of formation (HOF) and enthalpies of combustion, densities, the proton and hydride affinities of the mixed boron-, nitrogen- and oxygen-containing non-aromatic three- and four-membered rings BnNmOHp (n = 0-3, m = 0-3, p = 0-3) have been investigated employing the (U)B3LYP/6-311++G∗∗, (U)B3LYP/aug-cc-pvtz, (U)MP2/6-311++G∗∗, G2, G3 and CBS-QB3 methods. Only twelve five-membered ring structures (a)-(l) were obtained as minima at the potential energy surfaces. The results have shown that the incorporation of the oxygen atom into the ringed high energetic boron-nitrogen compounds can improve the stabilities due to the oxygen atom donating its lone pair of electrons into the empty 2p orbital of the sp2-hybridized boron atom. According to the enthalpies of formation, the thermal stability is predicted to be the order of (k) > (l) > (c) > (h) > (d) > (j) > (a) > (e) > (f) > (b) > (g) > (i). As expected, the density always increases as the boron atoms being replaced by the nitrogen atoms. For the compounds with both of the boron and nitrogen atoms, the hydride affinities are greatly lower than the proton affinities, indicating that the Lewis acidity of them is weaker than the Lewis basicity. For the compound (k), the electrophilic reagent prefers to bind to the nitrogen atom at the ortho-position of the oxygen atom, while for the compounds (c)-(e), the nucleophilic reagent will bind to the boron atom at the meta-position. In particular, the very good linear correlations of the HOFs between the B3LYP and G2, G3 and CBS-QB3 methods for the compounds BnNmOHp have been found, indicating that the accurate HOFs might be obtained from the cheap B3LYP approach by the linear correlation of the HOFs instead of the expensive models. It has been also found that, for the hydride affinities of all the compounds, the values are always the same order of G2 > G3 > CBS-QB3 > MP2/6-311++G∗∗ > B3LYP/6-311++G∗∗ > B3LYP/aug-cc-pvtz. We believe that this investigation must be a new milestone in the development history of explosives and pyrotechnics in theoretical.