On the Mechanism of Hydrogen Activation by Frustrated Lewis Pairs

We report herein a comprehensive theoretical study of the thermodynamics and kinetics of molecular hydrogen activation by frustrated Lewis pairs (FLPs). A series of intermolecularly combined boranes (Lewis acids) and phosphines (Lewis bases), with experimentally established different reactivities towards H₂, have been subjected to DFT and (SCS‐)MP2 calculations, and analyzed in terms of their structural properties, the energetics of association of the FLPs, and the kinetics of their interactions with H₂ and hydrogenation to the ion‐pair products. The analysis included the following steps: 1) assessment of the ability/inability of the Lewis species to preorganize into FLPs with an optimum arrangement of the acid and base sites for preconditioning the reaction with H₂, 2) comprehension of the different thermodynamics of hydrogenation of the selected FLPs by comparing the Gibbs energies of the overall reactions, and 3) estimation of the mechanism of the activation of H₂ by identifying the reaction steps and the associated kinetic barriers. The results of our studies correlate well with experimental findings and have clarified the reasons for the observed different reactivities of the investigated systems, ranging from reversible or nonreversible activation to no reaction with H₂. The derived predictions could assist the future design of Lewis acid–base systems with desired properties and applicability as metal‐free hydrogenation catalysts.     

乙炔基自由基C2H与氧气反应的密度泛函理论研究

应用量子化学从头算和密度泛函理论(DFT)对C₂H自由基和O₂的反应进行了研究.在B3LYP/6-311G^**水平上优化了反应通道上各驻点(反应物、中间体、过渡态和产物)的几何构型,并计算出它们的振动频率和零点振动能(ZPVE).各物种的总能量由CCSD(T)/6-311G^**//B3LYP/6-311G^**给出,并对能量进行了零点能校正.计算结果表明,反应物中自由基C₂H中的边端C进攻O₂形成了中间体1 (HCCOO),中间体1是一个加合产物.由中间体1经过不同的反应通道可以生成不同的产物P₁ (HCO+CO), P₂ (HCCO+O), P₃(CO₂+CH), P₄ (C₂O+OH)和P₅ (2CO+H).反应通道之间存在着竞争机制.其中P₁, P₂是主要产物,其次还有一定比例的P5生成,而产物P₃, P₄的生成几率较低.各条反应通道化学反应热的计算与实验吻合较好.     

H_2O与NO,CN,OH自由基及负离子相互作用的CP研究

采用量子化学从头算方法在均衡 (Counterpoise)校正和非校正势能面上研究了H2 O与NO ,NO-,CN ,CN-,OH ,OH-之间的相互作用 .比较了CP 梯度优化和非校正梯度优化以及基组的选择对超分子结构和能量的影响 .研究表明 ,6 311++g 基组对于这些体系的研究有很高的效率 .这些自由基和负离子均能与H2 O形成强弱不同的氢键 .按相互作用的强弱次序依次为OH-,CN-,NO-,OH ,CN ,NO ,由相互作用能ΔECP,成键临界点电荷密度 ρ ,二阶稳定化相互作用能E( 2 )分析均得到同样结果 .CP 梯度优化和非校正梯度优化所得能量及BSSE相差不大     

2-酰基-1,3-环二酮类化合物互变异构的理论计算

采用MP2/6-31+G**//B3LYP/6-31+G**方法,系统地研究了2-酰基-1,3-环二酮类化合物的互变异构性质,探讨了环的饱和性、环的大小、环上杂原子以及溶剂效应对各互变异构体相对稳定性的影响。结果表明:(1)未烯醇化的异构体由于无法形成分子内氢键而具有较高的能量;(2)环上杂原子的推电子效应提高了邻位羰基氧上的负电荷,从而有利于相应环外烯醇式异构体的稳定;(3)对化合物3-酰基-1H-吡啶-2,4-二酮,3-酰基吡喃-2,4-二酮,2-酰基环戊-4-烯-1,3-二酮和3-酰基吡咯-2,4-二酮,由互变异构形成的环的芳香性在各异构体的相对稳定性中扮演着决定性的作用。     

Theoretical studies of some sulphonamides as corrosion inhibitors for mild steel in acidic medium

Density functional theory (DFT) at the B3LYP/6‐31G (d,p) and BP86/CEP‐31G* basis set levels and ab initio calculations using the RHF/6‐31G (d,p) methods were performed on four sulfonamides (namely sulfaacetamide (SAM), sulfapyridine (SPY), sulfamerazine (SMR), and sulfathiazole (STI)) used as corrosion inhibitors for mild steel in acidic medium to determine the relationship between molecular structure and their inhibition efficiencies (%IE). The order of inhibition efficiency obtained was SMR > SPY > STI > SAM which corresponded with the order of most of the calculated quantum chemical parameters namely E_HOMO (highest occupied molecular orbital energy), E_LUMO (lowest unoccupied molecular orbital energy), the energy gap (ΔE), the Mulliken charges on the C, O, N, S atoms, hardness (η), softness (S), polarizability (α), dipole moment (μ), total energy change (ΔE_T), electrophilicity (ω), electron affinity (A), ionization potential (I), the absolute electronegativity (χ), and the fraction of electrons transferred (ΔN). Quantitative structure activity relationship (QSAR) approach has been used and a correlation of the composite index of some of the quantum chemical parameters was performed to characterize the inhibition performance of the sulfonamides studied. The results showed that the %IE of the sulfonamides was closely related to some of the quantum chemical parameters but with varying degrees/order. The calculated %IE of the sulfonamides studied was found to be close to their experimental corrosion inhibition efficiencies. The experimental data obtained fits the Langmuir adsorption isotherm. The negative sign of the E_HOMO values and other thermodynamic parameters obtained indicates that the data obtained supports physical adsorption mechanism. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Novel Bonding Character of the Cyclic AlS2 and GaS2 Systems

The geometries, the harmonic vibrational frequencies and the bonding properties have been predicted for cyclic AlS2 and GaS₂ species at the density functional theory (DFT), MPn (n = 2, 3, 4), QCISD(T) and CCSD(T) levels with 6‐311 + G (2df) basis set. The novel bonding character was discussed.     

A Comparative Study of Hydrogen Bonding Using Density Functional Theory

The hydrogen‐bond energies of water dimer and water‐formaldehyde complexes have been studied using density functional theory (DFT). Basis sets up to aug‐cc‐pVXZ (X=D, T, Q) were used. It was found that counterpoise corrected binding energies using the aug‐cc‐pVDZ basis set are very close to those predicted with the aug‐cc‐pVQZ set. Comparative studies using various DFT functionals on these two systems show that results from B3LYP, mPW1PW91 and PW91PW91 functionals are in better agreements with those predicted using high‐level ab initio methods. These functionals were applied to the study of hydrogen bonding between guanine (G) and cytosine (C), and between adenine (A) and thy mine (T) base pairs. With the aug‐cc‐pVDZ basis set, the predicted binding energies of base pairs are in good agreement with the most elaborate ab initio results.     

CHF_2CF_2CH_2OCHF_2与OH自由基反应机理的理论研究

采用从头算和密度泛函方法研究了多通道反应CHF2CF2CH2OCHF2+OH→产物的反应机理.首先在BMK/6-311+G(d,p)水平下优化了稳定点的几何构型并计算了振动频率;然后在BMC-CCSD水平下,对势能面进行高水平能量校正.结果表明,此反应存在提氢和取代两类反应通道,但是无论从动力学还是从热力学分析,提氢反应通道才是主要的反应通道,且从-CH2-基团上提取氢原子的提氢通道是主要的反应通道.     

亚烷基卡宾及取代亚烷基卡宾与环硫乙烷反应的量子化学研究

用量子化学的密度泛函理论(DFT)在6-311G(d, p)水平上对亚烷基卡宾及取代亚烷基卡宾与环硫乙烷的硫转移反应机理进行了系统的研究. 用IRC对过渡态进行了确认. 并用组态混合模型讨论了反应势垒(ΔE^≠)与XYC＝C的单-三态能量差ΔE_ST之间的关系. 结果表明, 取代基的电负性是控制反应的主要因素, 取代基的电负性越大, 取代基越多, π电子给予体越多, 单-三态能量差ΔE_ST就越小, 该反应的活化能就越小, 反应越容易发生. 同时还讨论了该反应中环硫乙烷的C—S键的解离过程. 并与标题化合物和环氧乙烷的氧转移反应进行了比较.     

Hydrogen Bonding Interactions of Radicals

Hydrogen bonding interactions of organic radicals are systematically studied using diverse ab initio and density functional theory (DFT) methods. It is found that open-shell hydrogen bonds with radical proton donors are more difficult to model than those with radical proton acceptors. The DFT methods perform significantly worse than the unrestricted second order Möller-Plesset perturbation (UMP2) method in both geometry optimization and interaction energy calculations for the open-shell hydrogen bonds. The UB3LYP method seriously underestimates the donor-acceptor distances and overestimates interaction energies for the open-shell hydrogen bonds with radical proton donors. Nevertheless, use of the UBH&HLYP functional to study the open-shell hydrogen bonds is still acceptable. Furthermore, it is necessary to use sufficiently flexible basis sets, such as 6-311++G(2df,2p), to get reliable interaction energies for the open-shell hydrogen bonds. The open-shell proton donors are stronger Lewis acids than the corresponding closed-shell proton donors. The open-shell proton acceptors are weaker Lewis bases than the corresponding closed-shell proton acceptors.     

亚烷基卡宾及取代亚烷基卡宾与环氧乙烷反应的量子化学研究

用量子化学的密度泛函理论(DFT)在6-311G(d,p)水平上对亚烷基卡宾及取代亚烷基卡宾与环氧乙烷的氧转移反应机理进行了系统的研究. 用IRC对过渡态进行了确认. 并用组态混合模型讨论了反应势垒(ΔE^≠)与XYC＝C:的单-三态能量差ΔE_ST之间的关系, 结果表明, 取代基的电负性是控制反应的主要因素, 取代基的电负性越大, 取代基越多, π电子给予体越多, 单-三态能量差ΔE_ST就越小, 该反应的活化能就越小, 反应越容易发生. 同时还研究了该反应中环氧乙烷中C—O键的解离过程. 发现两个C—O键解离是一个不同步的协同过程.     

On the effect of the BSSE on intermolecular potential energy surfaces. Comparison of a priori and a posteriori BSSE correction schemes

A comparative study of geometrical parameters is performed on the complexes HF–HF, H₂O–H₂O, and HF–H₂O using 12 different basis sets at the RHF, MP2, and DFT (BLYP and B3LYP) levels of theory. The equilibrium geometries were obtained from uncorrected, a posteriori (counterpoise, CP) and a priori (Chemical Hamiltonian Approach, CHA) BSSE‐corrected potential energy surfaces. The calculation of equilibrium geometries using the CP and CHA schemes is described in details. The effect of the BSSE on various intermolecular parameters is discussed and the performance of the applied theoretical models is critically evaluated from the BSSE point of view. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 765–786, 2001     

Quantum chemical study on the abstraction reaction of alkylidenegermylene with oxirane and thiirane

The mechanisms of theion reaction of alkylidenegermylene with oxirane and thiirane have been characterized detail in using density functional theory, as well as ab initio method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. Energies were calculated by CCSD(T)/6-311G(d)//B3LYP/6-311G(d,p) method for the involved conformations. The results show that the reaction pathways for both reactions consist of two ways: (1) the reactants can yield bent products (P1; P4) by syn-isomers; (2) the reactants can also yield three-membered products (P2; P5) by anti-isomers, which then further react with oxirane and thiirane to form the ultimate products (P3-1, P3-2; P6). Furthermore, a comparison with alkylidenecarbene, oxirane, and thiirane was done.     

Predicting the redox properties of uranyl complexes using electronic structure calculations

A plethora of chemical reactions is redox driven processes. The conversion of toxic and highly soluble U(VI) complexes to nontoxic and insoluble U(IV) form are carried out through proton coupled electron transfer by iron containing cytochromes and mineral surfaces such as machinawite. This redox process takes place through the formation of U(V) species which is unstable and immediately undergo the disproportionation reaction. Thus, theoretical methods are extremely useful to understand the reduction process of U(VI) to U(V) species. We here have carried out the structures and reduction properties of several U(VI) to U(V) complexes using a variety of electronic structure methods. Due to the lack of experimental ionization energies for uranyl (UO₂(V)‐UO₂(VI)) couple, we have benchmarked the current and popularly used density functionals and cost effective ab initio methods against the experimental electron detachment energies of [UO₂F₄]^1‐/2‐ and [UO₂Cl₄]^1‐/2‐. We find that electron detachment energy of U(VI) predicted by RI‐MP2 level on the BP86 geometries correlate nicely with the experimental and CCSD(T) data. Based on our benchmark studies, we have predicted the structures and electron detachment energies of U(V) to U(VI) species for a series of uranium complexes at the RI‐MP2//BP86 level which are experimentally inaccessible till date. We find that the redox active molecular orbital is ligand centered for the oxidation of U(VI) species, where it is metal centered (primarily f‐orbital) for the oxidation of U(V) species. Finally, we have also calculated the detachment energies of a known uranyl [UO₂]¹⁺ complex whose X‐ray crystal structures of both oxidation states are available. The large bulky nature of the ligand stabilizing the uncommon U(V) species which cannot be routinely studied by present day CCSD(T) methods as the system size are more than 20–30 atoms. The success of our efficient computational strategy can be experimentally verified in the near future for the complex as the structures are stable in gas phase which can undergo oxidation.     

The CH_3OH+Cl Hydrogen Abstraction Reactions: An ab initio Study

The hydrogen abstraction from methanol by atomic chlorine can proceed via two possible channels: CH3OH + Cl → CH2OH + HCl (i) → CH3O + HCl (ii)In 2000, Ahmed et al1. reported their investigation for the H abstraction dynamics of the CH3OH+Cl reaction by crossed molecular beams in conjunction with velocity map imaging. Hydroxymethyl (CH2OH) was detected in their experiments. In 1998, Jodkowski et al2. reported their theoretical study on the CH3OH+Cl reactions. They located inte…     

Molecular structure and vibrational spectra of phenobaraitone by density functional theory and ab initio hartree-Fock calculations

Quantum chemistry calculations have been performed using Gaussian03 program to compute optimized geometry, harmonic vibrational frequency along with intensities in IR and Raman spectra at RHF/6-31++G** and B3LYP/6-31++G** levels for phenobarbitone (C₁₂H₁₂N₂O₃) in the ground state. The scaled harmonic vibrational frequencies have been compared with experimental FT-IR and FT-Raman spectra. Theoretical vibrational spectra of the title compound were interpreted by means of potential energy distributions (PEDs) using MOLVIB program. A detailed interpretation of the infrared spectra of the title compound is reported. On the basis of the agreement between the calculated and observed results, the assignments of fundamental vibrational modes of phenobarbitone were examined and some assignments were proposed. The theoretical spectrograms for FT-IR and FT-Raman spectra of the title compound have been constructed.     

Theoretical Study on Gas-phase Pyrolytic Reactions of N-Ethyl, N-Isopropyl and N-t-Butyl Substituted 2-Aminopyrazine

Introduction　　Aminopyrazinecanbeusedasinhibitorforassemblyofprotease1andasantagonist2 forglutamate .Researchresultsshowthataminopyrazineplaysaneffectiveroleinpharmaceu ticalsynthesisofanti cancerandanti HIVvirusdrugs.2 ,3Re cently ,muchattentionwaspaidtoitspropertiesandapplica tionsduetoitsimportanceinchemistryandbiochemistry .Experimentalresults4 7showedthatthegas phasepyrolyticre actionsof 2 alkoxypyrimidinesand 2 alkoxypyrazineswerecarriedoutthroughaunimolecularfirst orderthermalelim…