Journal of Theoretical and Computational Chemistry

We perform first-principles calculations to investigate the electronic transport properties of chalcone and flavanone molecules sandwiched between graphene electrodes. These two molecules can be reversibly converted between open and closed states induced by pH, and the significant switching behaviors are observed. The currents and switching ratios are influenced by rotating molecules around the $X$ axis, which are discussed by the transmission eigenstates, electrostatic potential distributions and transmission spectra. The observed negative differential resistance effect is explained in chalcone configuration. The results suggest that spatial distributions of molecules will influence the performance of devices, indicating a potential application in future molecular circuits.

In this study, mechanism and stereochemistry of four-component Ugi reaction was investigated theoretically. Structures of reagents, products, intermediates, and transition states were optimized at B3LYP/6-31$+$G(d,p) level of theory. Mechanism and stereoselectivity of the reaction depended on several processes, including bond rotation, ring closure ring opening, acid-base, nucleophile-electrophile competitions, and rearrangements. These diverse phenomena were studied to provide a clearer picture of the mechanism of this valuable reaction, especially in terms of stereochemistry considerations. According to the results, (E)-oxazolidinols were considered as proper intermediates in Ugi reaction mechanism. In addition, the key point of diastereoselectivity of the reaction was under kinetic and thermodynamic controls of nucleophilic attack of isocyanide to less hindered re-face (E${}_{\mathrm{\text{a}}}=7.31$ compared to 10.19kcal/mol for si-face) of chiral (E)-iminium ion.

The unimolecular dissociation rate constants of the dehydration of Glycerol to Glycidol were calculated at the MP2/6–311G(d,p) level using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The anharmonic effect of the reactions was examined by comparing the rate constants at temperatures (700–3000K) of the canonical case and total energies (25654–53089cm${}^{-1}$) of the microcanonical system. The calculations showed that high temperatures are required for the reaction to proceed. As the temperatures and total energies increased, the rate of reactions increased. However, the growth rate of the unimolecular dissociation rate constants was high and slower both in the canonical and microcanonical systems. Comparative analysis showed that the anharmonic effect was most significant for the reaction $\mathrm{\text{C}}\to \mathrm{\text{TSC}}\to \mathrm{\text{IV}}+{\mathrm{\text{H}}}_2\mathrm{\text{O}}$ and least significant for the reaction $\mathrm{\text{B}}\to \mathrm{\text{TSB}}1\to \mathrm{\text{I}}+{\mathrm{\text{H}}}_2\mathrm{\text{O}}$. The anharmonic effect became more significant as the temperatures and total energies increased. Compared with the microcanonical situation, the anharmonic effect of the canonical system was more pronounced.

The regioselective production of alkenes from (thio)carbonates was calculated by MP2/6-31G(d) method via pyrolysis processes. Four (thio)carbonates were calculated in this paper. They are S-sec-butyl O-methyl thiocarbonate (I), O-sec-butyl S-methyl thiocarbonate (II), sec-butyl methyl thioncarbonate (III), and sec-butyl methyl dithiocarbonate (IV). Thirteen potential thermolysis routes were revealed for the pyrolysis of each substance, including nine routes to produce regioselective alkenes and four rearrangement/decompose alternatives. Among nine alkene generation routes, six-membered ring transition states via a two-step mechanism required the lowest energy, while the other routes exhibited higher energy barriers. The calculation results demonstrated an alkene distribution hierarchy of 1-butene $>$ E-butene $>$ Z-butene for substances I and II, and an order of E-butene $>$ 1-butene $>$ Z-butene for substances III and IV.