Heats of formation for the boronic acids R–B(OH)2 and boroxines R3B3O3 (R=H, Li, HBe, H2B, H3C, H2N, HO, F, and Cl) calculated at the G2, G3, and G4 levels of theory

Abstract

Boronic acids (R–B(OH)2) and their boroxine (R3B3O3) dehydration products have emerged as important classes of compounds with a multitude of diverse applications. However, the available heats of formation for these compounds are not always as accurate as would be required for further use. In this study the heats of formation at 298.15 K of R–B(OH)2 and R3B3O3 (R = H, Li, HBe, H2B, H3C, H2N, HO, F, and Cl) have been calculated at the G2, G3[G3B3], and G4 levels of theory and used to determine the enthalpy changes for the dehydration reactions: 3 R–B(OH)2 → R3B3O3 + 3 H2O; comparisons are made with other rigorous levels of theory, e.g. CBS-Q[CBS-QB3] and W1U, as well as with experimental values wherever possible. Enthalpy changes for the dehydration reactions have also been calculated using second-order Møller-Plesset perturbation theory (MP2) with the Dunning-Woon correlation-consistent aug-cc-pVDZ and aug-cc-pVTZ basis sets, and B3LYP density functional theory with the 6-311++G(2df,2pd) basis set. With the exception of H2N-B(OH)2, the dehydration reactions are consistently predicted to be exothermic. Our results provide a cautionary note for the use of the B3LYP functional in the calculation of structures and energies of boronic acids and boroxines. Where comparisons could be made, the G4 and W1U predictions for the heats of formation of these boron compounds differ significantly.

Spectroscopic and Computational Investigations of The Thermodynamics of Boronate Ester and Diazaborole Self-Assembly

“…S yn conformers of 1 – 6 are predicted to be slightly less stable than endo–exo global minima by 0.7–1.3 kcal/mol, while the anti conformers are 2.4–3.0 kcal/mol less stable (Table ) at the M06-2X/6-31+G­(d,p) level of theory. Results at the B3LYP, CBS-QB3, and MP2 levels give the same trend (see Table S1 of the Supporting Information), and overall results are in line with earlier computational studies of aliphatic boronic acids. ,, The nature of the para-substituent has a small but discernible influence on the structures of arylboronic acids 1 – 6 . For example, the O–H···O intramolecular hydrogen bond present in endo–exo conformers varies with the electron-donating or -withdrawing strength of each para-substituent.…”

“…The energetics of forming condensation products 11 – 14 were studied at four levels of theory, in particular: B3LYP/6-311+G­(d,p), M06-2X/6-31+G­(d,p), CBS-QB3, and MP2/aug-cc-pVDZ. , Several computational investigations of boronic acid-derived assemblies have been carried out previously using a variety of theoretical methods. ,− However, investigations of the thermodynamics of boronate ester, diazaborole, or dithiaborole self-assembly in nonaqueous solutions have not yet been reported. Kua has previously investigated the thermodynamics of boroxine formation and their amine adducts at the B3LYP level, providing insight into the influence of para-substitution and solvent on boroxine and boroxine–amine complex formation. , Bock and co-workers, however, have cautioned against the use of B3LYP to quantitatively describe boroxine thermochemistry, recommending MP2 as a more suitable method. B3LYP was still included in the current study to investigate its applicability to modeling boronate thermochemistry and because it is capable of handling large chemical systems.…”

“…Kua has previously investigated the thermodynamics of boroxine formation and their amine adducts at the B3LYP level, providing insight into the influence of para-substitution and solvent on boroxine and boroxine−amine complex formation. 24a,c Bock and co-workers, however, have cautioned against the use of B3LYP to quantitatively describe boroxine thermochemistry, 40 recommending MP2 as a more suitable method. B3LYP was still included in the current study to investigate its applicability to modeling boronate thermochemistry and because it is capable of handling large chemical systems.…”

“…Results at the B3LYP, CBS-QB3, and MP2 levels give the same trend (see Table S1 of the Supporting Information), and overall results are in line with earlier computational studies of aliphatic boronic acids. 40,41,48 The nature of the para-substituent has a small but discernible influence on the structures of arylboronic acids 1−6. For example, the O−H•••O intramolecular hydrogen bond present in endo−exo conformers varies with the electrondonating or -withdrawing strength of each para-substituent.…”

Monosubstituted Phenylboronic Acids, R–B(OH)2 (R = C6H5, C6H4CH3, C6H4NH2, C6H4OH, and C6H4F): A Computational Investigation

“…In the case of small aliphatic boronic acids, calculations have typically found the endo-exo conformers are lowest in energy [56-63], although exceptions have been reported [64]; energy differences between the endo-exo and anti conformers are often rather small, while the syn conformer is highest in energy. In the case of PBAs, however, substituents on the ring, particularly at the ortho positions, are likely to play a significant role in determining the preferred orientation of the hydroxyl groups in the gas-phase monomeric forms.…”