Hongqing He

Theoretical study of the reactions CF3CH2OCHF2+OH/Cl and its product radicals and parent ether (CH3CH2OCH3) with OH

A dual‐level direct dynamic method is employed to study the reaction mechanisms of CF3CH2OCHF2 (HFE‐245fa2; HFE‐245mf) with the OH radicals and Cl atoms. Two hydrogen abstraction channels and two displacement processes are found for each reaction. For further study, the reaction mechanisms of its products (CF3CH2OCF2 and CF3CHOCHF2) and parent ether CH3CH2OCH3 with OH radical are investigated theoretically. The geometries and frequencies of all the stationary points and the minimum energy paths (MEPs) are calculated at the B3LYP/6‐311G(d,p) level. The energetic information along the MEPs is further refined at the G3(MP2) level of theory. For reactions CF3CH2OCHF2 + OH/Cl, the calculation indicates that the hydrogen abstraction from CH2 group is the dominant reaction channel, and the displacement processes may be negligible because of the high barriers. The standard enthalpies of formation for the reactant CF3CH2OCHF2, and two products CF3CH2OCF2 and CF3CHOCHF2 are evaluated via group‐balanced isodesmic reactions. The rate constants of reactions CF3CH2OCHF2 + OH/Cl and CH3CH2OCH3 + OH are estimated by using the variational transition state theory over a wide range of temperature (200–2000 K). The agreement between the theoretical and experimental rate constants is good in the measured temperature range. From the comparison between the rate constants of the reactions CF3CH2OCHF2 and CH3CH2OCH3 with OH, it is shown that the fluorine substitution decreases the reactivity of the CH bond.

Insight into the catalytic activity for a series of synthesized and newly designed phosphonium-based ionic liquids on the fixation of carbon dioxide

Abstract Various catalysts have been explored for the coupling reaction of carbon dioxide with propylene oxide in the past decade. However, the search for high-efficiency single-component catalyst in the absence of metal and solvent is still a challenging issue. In this work, the catalytic activity of a series of functionalized phosphonium-based ionic liquids (FPBILs) is investigated by density functional theory. The influences of functional group, chain length of cation, and anion on the catalytic performance have been explored. The carboxyl-functionalized FPBIL with the moderate alkyl chain length presents the best catalytic activity among all investigated catalysts. On the basis of above information, a new sulfonyl hydroxide-functionalized FPBIL is designed. Moreover, its catalytic activity is theoretically examined as compared with that of carboxyl-functionalized FPBIL.

Hydrogen abstraction reactions of OH radicals with CH3CH2CH2Cl and CH3CHClCH3: A mechanistic and kinetic study

The hydrogen abstraction reactions of OH radicals with CH3CH2CH2Cl (R1) and CH3CHClCH3 (R2) have been investigated theoretically by a dual‐level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the B3LYP/6‐311G(d,p) level. To improve the reaction enthalpy and potential barrier of each reaction channel, the single point energy calculation is performed by the BMC‐CCSD method. Using canonical variational transition‐state theory (CVT) with the small‐curvature tunneling correction, the rate constants are evaluated over a wide temperature range of 200–2000 K at the BMC‐CCSD//B3LYP/6‐311G(d,p) level. For the reaction channels with the negative barrier heights, the rate constants are calculated by using the CVT. The calculated total rate constants are consistent with available experimental data. The results show that at lower temperatures, the tunneling correction has an important contribution in the calculation of rate constants for all the reaction channels with the positive barrier heights, while the variational effect is found negligible for some reaction channels. For reactions OH radicals with CH3CH2CH2Cl (R1) and CH3CHClCH3 (R2), the channels of H‐abstraction from –CH2– and –CHCl groups are the major reaction channels, respectively, at lower temperatures. With temperature increasing, contributions from other channels should be taken into account. Finally, the total rate constants are fitted by two models, i.e., three‐parameter and four‐parameter expressions. The enthalpies of formation of the species CH3CHClCH2, CH3CHCH2Cl, and CH2CH2CH2Cl are evaluated by isodesmic reactions.

Theoretical investigations of the structures and electronic spectra of Zn(II) and Ni(II) complexes with cyclohexylamine-N-dithiocarbamate.

The ground-state structures of two ligands cyclohexylamine-N-dithiocarbamate (L) and PPh3 and four complexes [Zn(L)2] (A), [Ni(L)2] (B), [Zn(L)2PPh3] (C), and [Ni(L)2PPh3] (D) are optimized by M06, B3LYP, and B3PW91 methods with the same mixed basis set. As compared with the experimental data of other complexes containing the Ni-P bond, the result obtained by M06/6-31+G(d)-LANL2DZ method is finally regarded as accurate and reliable for this project. Based on the optimized geometries, the compositions of molecular orbitals are analyzed and the absorption spectra are simulated. When one more ligand PPh3 is coordinated, the lowest-lying transition energy presents red-shift; while it shows blue-shift when the metal coordination center change from Ni to Zn with the same ligands. The detailed transition characters related with the absorption spectrum are assigned. In all the key transitions, it is hard to find the contribution from Zn atom. On the contrary, the d orbital of Ni atom contributes a lot for the HOMO and LUMO of complexes B and D. Consequently, the transition characters of Zn(II) and Ni(II) complexes are different.

Mechanism of fixation of CO2 with an epoxide catalyzed by ZnBr2 and a choline chloride co-catalyst: a DFT study

To explore the reason for the high activity of the cycloaddition reaction of PO (propylene oxide) with CO2 catalyzed by ZnBr2/CH (choline chloride) co-catalyst, the mechanism has been constructed using a DFT (density functional theory) method. The combination of CH and ZnBr2 will generate three new stable complexes, i.e., [Ch]2[ZnBr2Cl2], Ch+ZnBr2Cl−, and Ch+ZnBrCl2−. The latter two are derived from the dissociation of [Ch]2[ZnBr2Cl2]. The detailed mechanism of a coupling reaction catalyzed by the more stable complex Ch+ZnBrCl2− is explored. It has been elucidated that the attack from the Zn complex and the Br− anion is the major factor in promoting the cleavage of the C–O bond of PO. Finally, the performance of [Ch]2[ZnBr2Cl2] is also investigated, providing less activity, indicating that it should dissociate to gain better catalytic effect.

Direct Ab initio dynamics study on the reaction of CH3CHF2 (HFC-152a) with the Cl atom.

A direct ab initio dynamics method is used to investigate the hydrogen-abstraction reaction CH(3)CHF(2)+Cl. One transition state is located for alpha-H abstraction, and two are identified for beta-H abstraction. The potential-energy surface (PES) is obtained at the G3(MP2)//MP2/6-311G(d, p) level. Furthermore, the rate constants of the three channels are evaluated by using canonical variational transition-state theory (CVT) with small-curvature tunneling (SCT) contributions over a wide temperature range of 200-2500 K. The dynamic calculations show that the reaction proceeds mainly by alpha-H abstraction over the whole temperature range. The calculated rate constants and branching ratios are both in good agreement with the available experimental values.

Theoretical study on the reactions of CH3NHNH2 with ground state O(P-3) atom and excited state O(D-1) atom

The reaction mechanisms of methylhydrazine (CH3NHNH2) with O(3P) and O(1D) atoms have been explored theoretically at the MPW1K/6-311+G(d,p), MP2/6-311+G(d,p), MCG3-MPWPW91 (single-point), and CCSD(T)/cc-pVTZ (single-point) levels. The triplet potential energy surface for the reaction of CH3NHNH2 with O(3P) includes seven stable isomers and eight transition states. When the O(3P) atom approaches CH3NHNH2, the heavy atoms, namely N and C atoms, are the favourable combining points. O(3P) atom attacking the middle-N atom in CH3NHNH2 results in the formation of an energy-rich isomer (CH3NHONH2) followed by migration of O(3P) atom from middle-N atom to middle-H atom leading to the product P6 (CH3NNH2+OH), which is one of the most favourable routes. The estimated major product CH3NNH2 is consistent with the experimental measurements. Reaction of O(1D) + CH3NHNH2 presents different features as compared with O(3P) + CH3NHNH2. O(1D) atom will first insert into C–H2, N1–H4, and N2–H5 bonds barrierlessly to form the three adducts, respectively. There are two most favourable paths for O(1D) + CH3NHNH2. One is that the C–N bond cleavage accompanied by a concerted H shift from O atom to N atom (mid-N) leads to the product PI (CH2O + NH2NH2), and the other is that the N–N bond rupture along with a concerted H shift from O to N (end-N) forms PIV (CH3NH2 + HNO). The similarities and discrepancies between two reactions are discussed.

Mechanism for H2 release from potential hydrogen storage materials of phosphine alane and phosphine borane in the presence or absence of alane or borane: a theoretical study

Electronic structure calculations have been employed to probe the reaction mechanisms of hydrogen release from phosphine alane and phosphine borane in the presence or absence of alane and borane. Geometries of stationary points are optimized at the MP2/aug-cc-pVDZ level. Then, the energy profiles are refined at the CCSD(T)/aug-cc-pVTZ level based on the MP2 optimized geometries. In both AlH3PH3 and BH3PH3 monomer, H2 elimination cannot be compared with the Al–P and B–P bonds dissociation. The theoretical results demonstrate that both AlH3 and BH3 can facilitate H2 dissociation from AlH3PH3. However, the H2 production process becomes more difficult once the strong adduct is formed. Similarly, AlH3 and BH3 can also take part in a catalytic process for H2 loss from BH3PH3 and induce a substantial reduction of the energy barrier for H2 release. Comparison with the reaction mechanisms of H2 elimination from BH3PH3 with AlH3 or BH3 as the catalyst shows that BH3 is a better catalyst than AlH3. Moreover, BH3PH3 is more favored as a potential hydrogen storage material than AlH3PH3 in the presence of AlH3 or BH3. So a catalyst is required as the generation of H2 from phosphorous systems proceeds with an energy barrier much higher than the X–P (X = Al and B) bond energy.

Catalytic mechanism of human N-acetylserotonin methyltransferase: a theoretical investigation

The methyl-transfer mechanism of human N-acetylserotonin methyltransferase and the roles of several residues around the active sites are investigated by density function theory method. This enzyme will catalyse the conversion of N-acetylserotonin and S-adenosyl-L-methionine (SAM) into melatonin and S-asenosylhomocysteine, which is the terminal step in the melatonin (N-acetyl-5-methoxytryptamine) biosynthesis. The calculated results confirm that the methyl transfer and proton transfer will take place via a SN2 step with a concerted mechanism, which is different from the experimental estimation via a water bridge. The residues H255, D256, E311, and R252 play an important role in reducing the barrier height and inducing methyl transfer. In addition, a full SAM molecule is considered in this work, which is never explored in previous reports. We find that some residues around the SAM in the centre of active site are essential factors to influence the mechanism and barrier height. So a truncated SAM model may not be suitable for all reactions.

Elucidation of hydrogen-release mechanism from methylamine in the presence of borane, alane, diborane, dialane, and borane–alane

The mechanisms of hydrogen release from methylamine with or without borane, alane, diborane, dialane, and borane–alane are theoretically explored. Geometries of stationary points are optimised at the MP2/aug-cc-pVDZ level and energy profiles are refined at the CCSD(T)/aug-cc-pVTZ level based on the second-order Møller–Plesset (MP2) optimised geometries. H2 elimination is impossible from the unimolecular CH3NH2 because of a high energy barrier. The results show that all catalysts can facilitate H2 loss from CH3NH2. However, borane or alane has no real catalytic effect because the H2 release is not preferred as compared with the B–N or Al–N bond cleavage once a corresponding adduct is formed. The diborane, dialane, and borane–alane will lead to a substantial reduction of energy barrier as a bifunctional catalyst. The similar and distinct points among various catalysts are compared. Hydrogen bond and six-membered ring formation are two crucial factors to decrease the energy barriers.