The Standard Enthalpies of Formation of B(CH3)3, B(C2H5)3, B2(CH3)6, and B2(C2H5)6 Determined Using the Calibration Dependencies, as Well as Their Thermodynamic Properties
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In this study, the most thermochemically stable structures of monomers and dimers of pyrophoric organoboron compounds (X = B(CH 3 ) 3 (TMB), B 2 (CH 3 ) 6 (DTMB), B(C 2 H 5 ) 3 (TEB), B 2 (C 2 H 5 ) 6 (DTEB), B(C 3 H 7 ) 3 , B(C 4 H 9 ) 3 , and their isomers) are determined using quantum-chemical calculations (M062X (i = 1), CBS-QB3 (i = 2), and G3B3 (i = 3)). Their calculated values of H o (X) i are used to verify the consistency of the literature values of Δ f H o (X) lit . Based on the results of comparing the calculated and literature thermochemistry of homodesmotic reactions involving B(i-C 3 H 7 ) 3 , B(sec-C 4 H 9 ) 3 , and B(tert-C 4 H 9 ) 3 , the significant difference between them is concluded, while for the other examined compounds, the calculated and literature values are in good agreement. The latter are used to determine linear calibration relationships (Δ ra H o (X) lit = A i + B i × Δ ra H o (X) i_calc ) between calculated and literature values of standard atomization enthalpies (Δ ra H o (X) i_calc and Δ ra H o (X) lit ), which are used to determine recommended values Δ ra H o (X) i_rec and Δ f H o (X) i_rec (Δ ra H o (X) i_rec = A i + B i × Δ ra H o (X) i_calc ), as well as their errors (SD i ). The values of Δ f H o (B 2 (CH 3 ) 6 ) rec = -185.7 ± 18 kJ/mol and Δ f H o (B 2 (C 2 H 5 ) 6 ) rec = -229.8 ± 18 kJ/mol, determined for the first time, demonstrate the exothermicity and exergonicity of their dissociation reactions (Δ r1 H o = -65.1 kJ/mol and Δ r2 H o = -91.6 kJ/mol). The temperature dependencies of the corresponding values of Δ r1 G o and Δ r2 G o indicate the almost complete dissociation of the dimers even at T < 300 K and negligible contribution of the dimer to the heat of combustion of TMB and TEB.
