二氯亚甲基锗烯与乙烯环加成反应机理的理论研究

The mechanism of a cycloaddition reaction between singlet dichloromethylene germylene and ethylene has been investigated with B3LYP/6-31G＊ method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. Energies for the involved conformations were calculated by CCSD（T）//B3LYP/6-31G＊ method. On the basis of the surface energy profile obtained with CCSD（T）// B3LYP/6-31G＊ method for the cycloaddition reaction between singlet dichloromethylene germylene and ethylene, it can be predicted that the dominant reaction pathway is that an intermediate INT1 is firstly formed between the two reactants through a barrier-free exothermic reaction of 61.7 kJ/mol, and the intermediate INT1 then isomerizes to an active four-membered ring product P2.1 via a transition state TS2, an intermediate INT2 and a transition state TS2.1, in which energy barriers are 57.7 and 42.2 kJ/mol, respectively.     

Conformational analysis of cycloheptane,oxacycloheptane, 1,2‐dioxacycloheptane, 1,3‐dioxacycloheptane,and 1,4‐dioxacycloheptane

The conformational analysis of cycloheptane (1), oxacycloheptane (2), 1,2‐dioxacycloheptane (3), 1,3‐dioxacycloheptane (4), and 1,4‐dioxacycloheptane (5) has been carried out using B3LYP, CCD, CCSD, and QCISD with the 6‐311+G(d,p) and cc‐pVDZ basis sets. The twist chair conformers are predicted to be lower in energy than their corresponding boat and chair conformations. All levels of theory predict (4) to be lower in energy than (3) and (5). CCSD predicts remarkably similar activation barriers for the conformational interconversion of the twist chair conformers to their corresponding boat conformers. Small barriers to pseudorotation are also predicted. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Theoretical study on the substitution reactions of silylenoid H2SiLiF with CH4, NH3, H2O, HF, SiH4, PH3, H2S, and HCl

DFT calculations at the B3LYP/6-31G(d,p) level have been performed to explore the substitution reactions of silylenoid H(2)SiLiF with XH(n) hydrides, where XH(n) = CH(4), NH(3), H(2)O, HF, SiH(4), PH(3), H(2)S, and HCl. We have identified a previously unreported reaction pathway on each reaction surface, H(2)SiLiF + XH(n) --> H(3)SiF + LiXH(n-1), which involved the initial formation of an association complex via a five-membered cyclic transition state to form an intermediate followed by the substituted product H(3)SiF with LiXH(n-1) dissociating. These theoretical calculations suggest that (i) there is a very clear trend toward lower activation barriers and more exothermic interactions on going from left to right along a given row in the periodic table, and (ii) for the second-row hydrides, the substitution reactions are more exothermic than for the first-row hydrides and the reaction barriers are lower. The solvent effects were considered by means of the polarized continuum model (PCM) using THF as a solvent. The presence of THF solvent disfavors slightly the substitution reaction. Compared to the previously reported insertions and H(2)-elimination reactions of H(2)SiLiF and XH(n), the substitution reactions should be most favorable.     

亚烷基锡烯与乙烯环加成反应机理的理论研究

The mechanism of a cycloaddition reaction between singlet alkylidenestannylene and ethylene has been investigated with MP2/3-21 G^* and B3LYP/3-21 G* methods, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. Energies for the involved conformations were calculated by CCSD(T)//MP2/3-2 IG^* and CCSD(T)//B3LYP/3-21G^* methods, respectively. The results show that the dominant reaction pathway of the cycloaddition is that an intermediate (INT) is firstly formed between the two reactants through a barrier-free exothermic reaction of 39.7 kJ/mol, and the intermediate then isomerizes to a four-membered ring product (P2.1) via a transition state TS2.1 with a barrier of 66.8 kJ/mol.     

Theoretical investigation of the reactivity of copper atoms with OCS: comparison with CS2 and CO2

The reaction mechanism of the Cu atom with OCS and CO2 has been studied by means of density functional method (B3LYP). The overall energetics has been refined at the CCSD(T) level. In the case of the Cu + OCS reaction, the CS insertion route is found much more favorable than the CO insertion one. This later reaction is direct and involves an activation energy of 83.3 kcal/mol and is endothermic by 50.0 kcal/mol at the CCSD(T) level. The insertion into the CS bond proceeds through the eta1s and eta2cs coordination species as intermediates and is found exothermic by about 20 kcal/mol. The highest transition structure along this route is only 11.5 kcal/mol higher in energy than the reactant's ground states. In the case of the Cu + CO2 reaction, the insertion route into the CO bond is also found direct but with a lower endothermicity (30.6 kcal/mol) and smaller activation energy (61.1 kcal/mol) than that into the CO bond of OCS. In all cases, the insertion mechanism proceeds simultaneously with electron transfer from the Cu atom to OCS (or CO2) molecule.     

亚甲基硅烯与乙烯环加成反应机理的理论研究

The mechanism of a cycloaddition reaction between singlet methylidenesilene and ethylene has been investigated with MP2/6-31G^＊ and B3LYP/6-31G^＊ methods, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. Energies of the involved conformers were calculated by CCSD（T）//MP2/6-31G＊ and CCSD（T）//B3LYP/6-31 G＊ methods, respectively. The results show that the dominant reaction pathway of the cycloaddition reaction is that a complex intermediate is firstly formed between the two reactants through a barrier-free exothermic reaction of 13.3 kJ/mol, and the complex is then isomefized to a four-membered ring product P2,1 via a transition state TS2.1 with a barrier of 32.0 kJ/mol.     

Computational Study of Proton Transfer in Tautomers of 3‐ and 5‐Hydroxypyrazole Assisted by Water

The tautomerism of 3‐ and 5‐hydroxypyrazole is studied at the B3LYP, CCSD and G3B3 computational levels, including the gas phase, PCM–water effects, and proton transfer assisted by water molecules. To understand the propensity of tautomerization, hydrogen‐bond acidity and basicity of neutral species is approached by means of correlations between donor/acceptor ability and H‐bond interaction energies. Tautomerism processes are highly dependent on the solvent environment, and a significant reduction of the transition barriers upon solvation is seen. In addition, the inclusion of a single water molecule to assist proton transfer decreases the barriers between tautomers. Although the second water molecule further reduces those barriers, its effect is less appreciable than the first one. Neutral species present more stable minima than anionic and cationic species, but relatively similar transition barriers to anionic tautomers.     

Theoretical studies on the reaction mechanism of palladium(0)-catalyzed addition of thiocyanates to alkynes

The reaction mechanism of the Pd(0)-catalyzed alkyne cyanothiolation reaction is investigated by MP2, CCSD(T) and the density functional method B3LYP. The overall reaction mechanism is examined. The B3LYP results are consistent with the results of CCSD(T) and MP2 methods for the isomerization, acetylene insertion and reductive elimination steps, but not for the oxidative addition step. For the oxidative addition, the bisphosphine and monophosphine pathways are competitive in B3LYP, while the bisphosphine one is preferred for CCSD(T) and MP2 methods. The electronic mechanisms for the oxidative addition of thiocyanate HS-CN to Pd(PH(3))(2) and Pd(PH(3)) and for the acetylene insertion into Pd-S and Pd-CN are discussed in terms of the electron-donation and back-donation. The chemo-selectivity that acetylene inserts into the Pd-S bond rather than into the Pd-CN bond is due to the involvement of the S p orbital. It is the doubly occupied S p unhybridized orbital that donates an electron to the alkylene pi* anti-bonding orbital, which makes insertion into Pd-S bond more favorable than into the Pd-CN bond. During the insertion into the Pd-S bond, the S sp(2) hybrid orbital and unhybridized p orbital transform into each other, while the C sp hybrid orbital shifts its direction for insertion into Pd-CN bond. By using the monosubstituted acetylenes (CN, Me and NH(2)), the influence of substituents at acetylene on the chemo- and regio-selectivities is analyzed.     

A coupled-cluster study of the structure and vibrational spectra of pyrazole and imidazole

Harmonic force fields were calculated at the corresponding optimized geometries for pyrazole and imidazole at the HF, B3LYP, MP2, CCSD and CCSD(T) levels using the 6-31G* basis set and at the HF and B3LYP levels using the cc-pVTZ basis set. The agreement between the calculated and experimental geometries by the CCSD and CCSD(T) methods was generally similar to that obtained with the B3LYP and MP2 methods. The force fields were scaled using one-scale-factor (1SF), 3SF and 7SF scaling schemes. The scale factors were varied with respect to the experimental frequencies. Using 7SF scaling, the root-mean-square (RMS) deviation of the calculated frequencies from the experimental frequencies by the HF, B3LYP, MP2, CCSD and CCSD(T) methods and the 6-31G* basis set was 16, 7, 13, 11 and 11 cm(-1), respectively. This shows that the B3LYP method is preferred for force field calculations over the perturbative MP2, CCSD and CCSD(T) methods. Using 1SF scaling, the CCSD(T) scale factor was 0.931, the highest among the five methods used but close to that obtained with the B3LYP method and the cc-pVTZ basis set with lower RMS deviation.     

Realistic Energy Surfaces for Real‐World Systems: An IMOMO CCSD(T):DFT Scheme for Rhodium‐Catalyzed Hydroformylation with the 6‐DPPon Ligand

The hydroformylation of terminal alkenes is one of the most important homogeneously catalyzed processes in industry, and the atomistic understanding of this reaction has attracted enormous interest in the past. Herein, the whole catalytic cycle for rhodium‐catalyzed hydroformylation with the 6‐diphenylphosphinopyridine‐(2H)‐1‐one (6‐DPPon) ligand 1 was studied. This catalytic transformation is challenging to describe computationally, since two requirements must be met: 1) changes in the hydrogen‐bond network must be modeled accurately and 2) bond‐formation/bond‐breaking processes in the coordination sphere of the rhodium center must be calculated accurately. Depending on the functionals used (BP86, B3LYP), the results were found to differ strongly. Therefore, the complete cycle was calculated by using highly accurate CCSD(T) computations for a PH₃ model ligand. By applying an integrated molecular orbital plus molecular orbital (IMOMO) method consisting of CCSD(T) as high level and DFT as low‐level method, excellent agreement between the two functionals was achieved. To further test the reliability of the calculations, the energetic‐span model was used to compare experimentally derived and computed activation barriers. The accuracy of the new IMOMO method apparently makes it possible to predict the catalytic potential of real‐world systems.     

Theoretical study of the mechanism and rate constant of the B + CO2 reaction

The different stationary points on the potential energy surface relative to the title reaction have been reinvestigated at the B3LYP/aug-cc-pVDZ level with relative energies computed at the CCSD(T)/aug-cc-pVTZ level with B3LYP/aug-cc-pVDZ optimized geometries and by using the G3B3 composite method. Two entrance channels have been identified. The first one corresponds to boron addition at one of the oxygen atoms of the CO 2 molecule leading to trans-BOCO, which is found to be about 27 kcal/mol exothermic with a potential energy barrier of 16.4 kcal/mol (G3B3). The second channel, which has not been identified in previous theoretical works, corresponds to a direct insertion of the boron atom into a CO bond and leads to OBCO. The B + CO 2 --> OBCO step is found to be about 84 kcal/mol exothermic and needs to overcome a potential energy barrier of only 3.6 kcal/mol (G3B3). The rate constant at 300 K of the insertion step, calculated by using TST theory with G3B3 calculated activation energy value, is 5.4 10 (-14) cm (3) molecule (-1) s (-1), in very good agreement with the experimental data ((7.0 +/- 2.8) 10 (-14) cm (3) molecule (-1) s (-1), DiGiuseppe, T. G.; Davidovits, P. J. Chem. Phys. 1981, 74, 3287). The one corresponding to the addition process is found to be several orders of magnitude smaller because of a much higher potential energy barrier. The addition channel would not contribute to the title reaction even at high temperature. A modified Arrhenius equation has been fitted in the 300-1000 K temperature range, which might be useful for chemical models.     

Thermochemical properties for n‐propyl,iso‐propyl,and tert‐butyl nitroalkanes,alkyl nitrites,and their carbon‐centered radicals

Density functional theory (DFT) based calculations are performed on a series of alkyl nitrites and nitroalkanes representing large‐scale primary, secondary, and tertiary nitro compounds and their radicals resulting from the loss of their skeletal hydrogen atoms. Geometries, vibration frequencies, and thermochemical properties [S°(T) and C°_p(T) (10 K ? T ? 5000 K)] are calculated at the B3LYP/6‐31G(d,p) DFT level. Δ_fH°₂₉₈ values are from B3LYP/6‐31G(d,p), B3LYP/6‐31+G(2d,2p), and the composite CBS‐QB3 levels. Potential energy barriers for the internal rotations have been computed at the B3LYP/6‐31G(d,p) level of theory, and the lower barrier contributions are incorporated into entropy and heat capacity data. The standard enthalpies of formation at 298 K are evaluated using isodesmic reaction schemes with several work reactions for each species. Recommended values derived from the most stable conformers of respective nitro‐ and nitrite isomers include ?30.57 and ?28.44 kcal mol^?1 for n‐propane‐, ?33.89 and ?32.32 kcal mol^?1 for iso‐propane‐, ?42.78 and ?41.36 kcal mol^?1 for tert‐butane‐nitro compounds and nitrites, respectively. Entropy and heat capacity values are also reported for the lower homologues: nitromethane, nitroethane, and corresponding nitrites. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 181–199, 2010     

How computational methods and relativistic effects influence the study of chemical reactions involving Ru‐NO complexes?

Two treatments of relativistic effects, namely effective core potentials (ECP) and all‐electron scalar relativistic effects (DKH2), are used to obtain geometries and chemical reaction energies for a series of ruthenium complexes in B3LYP/def2‐TZVP calculations. Specifically, the reaction energies of reduction ( A ‐ F ), isomerization ( G‐I ), and Cl⁻ negative trans influence in relation to NH₃ ( J ‐ L ) are considered. The ECP and DKH2 approaches provided geometric parameters close to experimental data and the same ordering for energy changes of reactions A ‐ L . From geometries optimized with ECP, the electronic energies are also determined by means of the same ECP and basis set combined with the computational methods: MP2, M06, BP86, and its derivatives, so as B2PLYP, LC‐wPBE, and CCSD(T) (reference method). For reactions A ‐ I , B2PLYP provides the best agreement with CCSD(T) results. Additionally, B3LYP gave the smallest error for the energies of reactions J ‐ L . © 2017 Wiley Periodicals, Inc.     

Spin‐orbit ZORA and four‐component Dirac–Coulomb estimation of relativistic corrections to isotropic nuclear shieldings and chemical shifts of noble gas dimers

Hartree–Fock and density functional theory with the hybrid B3LYP and general gradient KT2 exchange‐correlation functionals were used for nonrelativistic and relativistic nuclear magnetic shielding calculations of helium, neon, argon, krypton, and xenon dimers and free atoms. Relativistic corrections were calculated with the scalar and spin‐orbit zeroth‐order regular approximation Hamiltonian in combination with the large Slater‐type basis set QZ4P as well as with the four‐component Dirac–Coulomb Hamiltonian using Dyall's acv4z basis sets. The relativistic corrections to the nuclear magnetic shieldings and chemical shifts are combined with nonrelativistic coupled cluster singles and doubles with noniterative triple excitations [CCSD(T)] calculations using the very large polarization‐consistent basis sets aug‐pcSseg‐4 for He, Ne and Ar, aug‐pcSseg‐3 for Kr, and the AQZP basis set for Xe. For the dimers also, zero‐point vibrational (ZPV) corrections are obtained at the CCSD(T) level with the same basis sets were added. Best estimates of the dimer chemical shifts are generated from these nuclear magnetic shieldings and the relative importance of electron correlation, ZPV, and relativistic corrections for the shieldings and chemical shifts is analyzed. © 2015 Wiley Periodicals, Inc.     

The equilibrium structure of ferrocene.

The molecular structures of ferrocene in the eclipsed (equilibrium) and staggered (saddle‐point) conformations have been determined by full geometry optimizations at the levels of second‐order Møller–Plesset (MP2) theory, coupled‐cluster singles‐and‐doubles (CCSD) theory, and CCSD theory with a perturbative triples correction [CCSD(T)] in a TZV2P+f basis set. Existing experimental results are reviewed. The agreement between the CCSD(T) results and experiment is in all cases excellent; the calculated structure parameters and the barrier to internal rotation of the ligand rings differ from the most accurate experimental values by less than two estimated standard deviations. The CCSD(T) calculations for single‐configuration‐dominated transition metal complexes such as ferrocene thus appear to have an accuracy comparable to that observed for molecules containing only first‐ and second‐row atoms, and to be of a quality similar to that obtained experimentally. A comparison with previous DFT results indicates that the B3LYP model gives overall the best DFT results, with a deviation of around 2 pm for the metal–carbon distance and smaller errors for the cyclopentadienyl rings.     

Revealing the mechanism of Rh(I)‐catalyzed hydroformylation of 4‐pyridylethene derivatives: DFT study

A comprehensive theoretical investigation into the mechanism of 1‐phenyl‐1‐(4‐pyridyl)ethene hydroformylation, using a rhodium catalyst employing a nonlocal density functional method (B3LYP), was carried out. The calculated results show that it is strongly exothermic by >90 kJ/mol of the whole catalytic cycle, and the rate‐limited step is H₂ oxidative addition. The regioselectivity originates from olefin insertion into the Rh? H bond. The predominant product is the regiospecifically 3‐phenyl‐3‐(4‐pyridal)propanal determined both thermodynamically and kinetically. These are in agreement with practicality experimental studies. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Assessment of density functional methods for reaction energetics: Iridium‐catalyzed water oxidation as case study

We investigate basis set convergence for a series of density functional theory (DFT) functionals (both hybrid and nonhybrid) and compare to coupled‐cluster with single and double excitations and perturbative triples [CCSD(T)] benchmark calculations. The case studied is the energetics of the water oxidation reaction by an iridium‐oxo complex. Complexation energies for the reactants and products complexes as well as the transition state (TS) energy are considered. Contrary to the expectation of relatively weak basis set dependence for DFT, the basis set effects are large, for example, more than 10 kcal mol^?1 difference from converged basis for the activation energy with “small” basis sets (DZ/6‐31G** for Ir/other atoms, or SVP) and still more than 6 kcal mol^?1 for def2‐TZVPP/6‐31G**. Inclusion of the dispersion correction in DFT‐D3 schemes affects the energies of reactant complex (RC), TS, and product complex (PC) by almost the same amount; it significantly improves the complexation energy (the formation of RC), but has little effect on the activation energy with respect to RC. With converged basis, some pure GGAs (PBE‐D3, BP86‐D3) as well as the hybrid functional B3LYP‐D3 are very accurate compared to benchmark CCSD(T) calculations. © 2012 Wiley Periodicals, Inc.