Molecular insights into the dehydration mechanism of 1,2-butanediol: A CBS-QB3 and G4MP2 study.

CONTEXT: The present work provides a detailed theoretical investigation of the full dehydration mechanism of 1,2-butanediol (1,2-BDO) using Complete Basis Set-Quadratic Becke3 (CBS-QB3) and Gaussian-4 Møller-Plesset perturbation theory (G4MP2) methods. Bond dissociation energy (BDE) analysis revealed distinct differences between the two hydroxyl groups in 1,2-BDO. The terminal C1-O1 bond is weaker than the internal C2-O2 bond by 1.7 and 1.9 kcal mol⁻1 at the CBS-QB3 and G4MP2 levels, respectively. In contrast, the terminal O1-H bond is stronger than the internal O2-H bond by 1.0 and 1.7 kcal mol⁻1 at the CBS-QB3 and G4MP2 levels, respectively. These results indicate that the internal hydroxyl group is more acidic and therefore more chemically reactive. Thermodynamic analyses of the 19 dehydration pathways indicate that cyclobutene (P12) formation is the most favorable reaction channel. P12 can be generated from intermediates cyclobutanol (P3), but-1-en-1-ol (P4), and but-2-en-1-ol (P5) through transition states TS12, TS15, and TS18, respectively. Among these pathways, the conversion of P5 to P12 via TS18 exhibits the lowest barrier (74.6 kcal mol⁻1), identifying it as the most accessible dehydration route. For the acyclic products, the most favorable pathways leading to but-1-yne (P7) and 1,2-butadiene (P8) proceed through the decomposition of intermediate P1 via TS7 and TS8, respectively. These reactions require barrier energies of 83.4 and 86.2 kcal mol⁻1, which are significantly higher than that of the cyclobutene-forming pathway. Consequently, P12 formation is expected to be favored over the production of P7 and P8.

METHODS

The dehydration mechanism of 1,2-BDO was investigated using the composite ab initio CBS-QB3 and G4MP2 computational methods. The CBS-QB3 approach involves geometry optimization and vibrational frequency calculations at the B3LYP/6-311G(d,p) level, followed by higher-level single-point energy calculations incorporating CCSD(T), MP4SDQ, and MP2 corrections, together with complete-basis-set (CBS) extrapolation to enhance the accuracy of the calculated energies. In the G4MP2 approach, molecular geometries and vibrational frequencies are optimized at the B3LYP/6-31G(2df,p) level, followed by a series of higher-level single-point calculations using CCSD(T,FC)/6-31G(d), MP2(FC)/G3MP2LargeXP, RHF/mod-aug-cc-pVTZ, and RHF/mod-aug-cc-pVQZ levels. Fundamental thermochemical properties, including atomization enthalpies (AE), bond dissociation energies (BDEs), ionization energies (IEs), and electron affinities (EAs), were calculated for 1,2-BDO and the species involved in its decomposition. The reliability of the computational methods was evaluated by comparing the calculated results with available theoretical and experimental data. BDE analysis was used to assess the relative strength and reactivity of the two hydroxyl groups, while thermodynamic analyses were applied to compare the 19 proposed dehydration pathways.