APPLICABILITY OF THE EMPIRICAL AND SEMIEMPIRICAL METHODS TO STRUCTURE AND ENERGY CALCULATIONS OF CYCLIC BORON ETHERS

Abstract

UDC 547.87′244:539.194 Stereochemical studies of cyclic ethers of boric acids have revealed a number of interesting features in the structure and chemical behavior of compounds from this class [1]. At the same time, CNDO and INDO determination of the energies and geometrical parameters and the conformational transformations of 1,3,2-dioxaborolane, -borinane, and -borepane molecules and their nitrogen-containing analogs is problematic because these methods fail to ensure an adequate calculation of electron exchange energies in the heteroatom fragment [1-4]. The major problem is the impossibility of adequately recording Coulomb interactions in conjugate systems containing heteroatoms with n-electron pairs [5]. Seeking correct approaches to conformational analysis of cyclic boron ethers, we examined the applicability of the MM+ molecular mechanics method [7] and the MINDO/3 [8], MNDO [9, 10], and AM1 [11, 12] semiempirical methods to calculations of the optimal geometries, formation heats, and free conformation energies of substituents in five-, six-, and seven-membered cyclic boron ethers with oxygen and nitrogen atoms (I-XXX) using HyperChem software [6]. Quantitative estimation of the discrepancy between the calculated and experimental data for each parameter was based on the determination of the relative error Δ = (|Acalc ‐ Aexp|/Aexp)×100% and the total error ΣΔ%/n, where n is the number of objects under study, permitting an unambiguous comparison of the calculation methods to assess the efficiency of each method. Comparison of the experimental (gas phase [13]) and calculated formation heats ( ΔH) of a number of cyclic and acyclic ethers (I-IX, Table 1) shows that AM1 and MNDO methods are more reliable compared to MINDO/3, giving the