Unveiling the structure‐reactivity relationship involved in the reaction mechanism of the HCl‐catalyzed alkyl t‐butyl ethers thermal decomposition. A computational study

Abstract

The gas‐phase thermal decomposition of a series of alkyl t‐butyl ethers catalyzed by hydrogen chloride is theoretically studied at the ωB97XD/6‐311++g(d)//CCSD(T)/6‐311++g(d) level. The experimental activation free energy for three known systems namely: t‐butyl methyl ether, t‐butyl ethyl ether, and t‐butyl isopropyl ether is used for validation of the proposed theoretical model as the transition state (TS). The chemical process was characterized using intrinsic reaction coordinate, reaction force, and reaction electronic flux profiles. The Cαδ+Oδ− polarization was identified as the determining factor in the rate‐limiting step. Upon functionalization on the Cα, 24 new compounds with different electron‐withdrawing and donating groups were studied. A good multiple‐linear correlation (R2 = 0.88) was found between the Ln(kX/kH) as the response variable and the Taft‐Topsom substituent parameters as attributes. This result supports the reliability and predictability of the proposed transition state model at this level of theory.

Elucidating the Racemization Mechanism of Aliphatic and Aromatic Amino Acids by In Silico Tools

“…Based on these optimization results, the model transition state is validated, and it was used to obtain valuable information about the mechanism [31], especially after corrections on the activation energy were done. Aliphatics require overall higher activation energies than aromatics, which supports aromatic resonance as the main cause for the different racemization rates and energetic values.…”

“…While the extreme regions correspond to structural rearrangement, and the centered regions to electronic reordering, the first two stages correlate to reaching the transition state, and the last stages to relaxation. Integration of the reaction force in each stage gives the reaction work required for each one, according to (Equation ( 5)) [31].…”

“…Given that the highest changes in WBI values show the bonds that break or form "the most", relevant bond transitions can be determined through the NBO analysis, as well as the symmetry of the reaction according to the reactive bonds. The percentage evolution %Evi (Equation ( 7)) of a bond i across the reaction is defined as [31]: Natural bond orbital (NBO) calculations are necessary to find a localized representation of the electrons in each of the atoms in order to estimate the point charges [12,42]. The Wiberg bond index (WBI) shows the average number of electron pairs between atoms, indicating the bond strength [12,43,44], and can be obtained from the NBO results for all the points in the IRC profile.…”

“…Given that the highest changes in WBI values show the bonds that break or form "the most", relevant bond transitions can be determined through the NBO analysis, as well as the symmetry of the reaction according to the reactive bonds. The percentage evolution %Ev i (Equation ( 7)) of a bond i across the reaction is defined as [31]:…”

“…Proton transfer between amide dimers has also been studied to better understand biological enzymatic systems [30]. The methodology has also helped discriminate between two possible mechanisms for the alkyl t-butyl ether thermal decomposition catalyzed by hydrogen chloride [31]. These studies utilize the ωb97xd functional, which calculates separately short-range or bonding interactions, and long-range or non-bonding interactions through a dispersion correction [32].…”