Theoretical study on the reaction mechanism of the OH-initiated oxidation of CH<Subscript>2</Subscript>=C(CH<Subscript>3</Subscript>)CH<Subscript>2</Subscript>CH<Subscript>2</Subscript>OH

In the present work, the detailed reaction mechanism and possible products of the OH-initiated oxidation of CH₂=C(CH₃)CH₂CH₂OH (MBO331) have been revealed theoretically for the first time. The potential energy surfaces of various reaction channels both in the absence and presence of O₂ and NO are evaluated at the CCSD(T)/6−31++G(d,p)//MP2(full)/6−311G(d,p)+ZPE*0.95 level. The major products of HCHO + CH₃C(O)CH₂CH₂OH predicted for the title reaction in the presence of O₂ and NO are in agreement with those of similar reactions of unsaturated alcohols with OH radical.     

Computational study on the reaction of CH<Subscript>3</Subscript>SCH<Subscript>2</Subscript>CH<Subscript>3</Subscript> with OH radical: mechanism and enthalpy of formation

The reaction mechanism of CH₃SCH₂CH₃ with OH radical is studied at the CCSD(T)/6-311+G(3df,p)//MP2/6-31+G(2d,p) level of theory. Three hydrogen abstraction channels, one substitution process and five addition–elimination channels are identified in the title reaction. The result shows hydrogen abstraction is dominant. Substitution process and addition–elimination reactions may be negligible because of the high barrier heights. Enthalpies of formation [ \Updelta_f H_{(298.15\textK)}^o \Updelta_{f} H_{(298.15{\text{K}})}^{o} ] of the reactants and products are evaluated at the CBS-QB3, G3 and G3MP2 levels of theory, respectively. It is found that the calculated enthalpies of formation by the aforementioned three methods are in consistent with the available experimental data. Rate constants and branching ratios are estimated by means of the conventional transition state theory with the Wigner tunneling correction over the temperature range of 200–900 K. The calculation shows that the formations of P1 (CH₂SCH₂CH₃ + H₂O) and P2 (CH₃SCHCH₃ + H₂O) are major products during 200–900 K. The three-parameter expressions for the total rate constant is fitted to be k_\texttotal = 1.45 ×10 ^{- 21} T^3.24 exp( - 1384.54/T) k_{\text{total}} = 1.45 \times 10^{ - 21} T^{3.24} \exp ( - 1384.54/T) cm³ molecule⁻¹ s⁻¹ from 200 to 900 K.     

Molecular structure and conformational preferences of 1-chloro-1-silacyclohexane,CH<Subscript>2</Subscript>(CH<Subscript>2</Subscript>CH<Subscript>2</Subscript>)<Subscript>2</Subscript>SiH-Cl,as studies by gas-phase electron diffraction and quantum chemistry

The molecular structure of axial and equatorial conformer of the 1-chloro-1-silacyclohexane molecule, CH₂(CH₂CH₂)₂SiH-Cl, as well as thermodynamic equilibrium between these species were investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/AUG-cc-PVTZ level of theory. According to electron diffraction data, the compound exists in the gas-phase as a mixture of conformers possessing the chair conformation of the six-membered ring and Cs symmetry and differing in the axial and equatorial position of the Si-Cl bond at 352 K. NBO analysis revealed that axial conformer of 1-chloro-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view steric effects and effects of conjugations and that stabilization is achieved due to electrostatic interactions.     

Searching blue-shifted O–H···Y hydrogen bond in CH<Subscript>3</Subscript>OH complexes with CF<Subscript>4</Subscript>, C<Subscript>2</Subscript>F<Subscript>2</Subscript>, OC,Ne, and He: a theoretical study

The hydrogen-bonded structures of the CH₃OH complexes with CF₄, C₂F₂, OC, Ne, and He are designated as the starting points for geometry optimizations without and with counterpoise (CP) correction at MP2 level of theory with the basis sets 6-31+G*, 6-31++G**, and 6-311++G**, respectively. Tight convergence criteria are applied throughout all geometry optimizations in order to reduce the computational errors. According to the optimizations without CP correction, a blue-shifted O–H···Y (where Y = F, O, Ne, or He) hydrogen bond exists in all these five complexes. The magnitudes of blue shifts of ν(O–H) of the former four complexes with respect to that of CH₃OH are reduced greatly when the polarization and diffuse functions of the hydrogen atoms are added (results from 6-31+G* versus those from 6-31++G**). However, for the complexes CH₃OH–CF₄ and CH₃OH–C₂F₂, our optimizations using the CP corrections did not find the hydrogen-bonded structure to be a stationary point. The energy minimum of both the complexes corresponds to a non-hydrogen-bonded structure.     

DFT studies of the phosphinidene derivative HPNaF and its insertion reaction with R–H(R=F,OH, NH<Subscript>2</Subscript>, CH<Subscript>3</Subscript>)

The formations of the phosphinidene derivative HPNaF and its insertion reactions with R–H (R=F, OH, NH₂, CH₃) have been systematically investigated employing the density functional theory (DFT), such as the B3LYP and MPW1PW91 methods. A comparison with the results of MP2 calculations shows that MPW1PW91 underestimates the barrier heights for the four reactions considered. Similarly, the same is also true for the B3LYP method depending on the selected reactions, but by much less than MPW1PW91, where the barrier heights of the four reactions are 25.2, 85.7, 119.0, and 142.4 kJ/mol at the B3LYP/6-311+G* level of theory, respectively. All the mechanisms of the four reactions are identical to each other, i.e., an intermediate has been located during the insertion reaction. Then, the intermediate could dissociate to substituted phosphinidane(H₂RP) and NaF with a barrier corresponding to their respective dissociation energies. Correspondingly, the reaction energies for the four reactions are −92.2, −68.1, −57.2, and −44.3 kJ/mol at the B3LYP/6-311+G* level of theory, respectively, where both the B3LYP and MPW1PW91 methods underestimate the reaction energies compared with the MP2 results. The linear correlations between the calculated barrier heights and the reaction energies have also been observed. As a result, the relative reactivity among the four insertion reactions should be as follows: H–F > H–OH > H–NH₂ > H–CH₃.     

Quantum-Chemical Study of the Profiles of Reactions that Form Pyrrole Anion N-Adducts with CS<Subscript>2</Subscript> and CO<Subscript>2</Subscript>

The profiles of reactions leading to pyrrole anion N-adducts with CO₂ and CS₂ have been studied by the ab initio (RHF/6-31+G*, MP2/6-31+G*) and density functional (B3LYP/6-31+G*) methods. Addition of the pyrrole anion to the carbon disulfide molecule is accompanied by the appearance of a minimum corresponding to a pre-reaction complex. The transformation of the complex to the N-pyrrolyldithiocarboxylate anion occurs via a low activation barrier, which is due to repolarization of the C=S bonds. The profile of the reaction leading to the pyrrole anion N-adduct with CO₂ does not contain any intermediate stationary points throughout the whole route from reagents to products.Original Russian Text Copyright © 2004 by V. B. Kobychev, N. M. Vitkovskaya, I. L. Zaitseva, and B. A. Trofimov__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 6, pp. 990–993, November–December, 2004.     

Quantum-chemical study of organoxenon molecules Xe(CF<Subscript>3</Subscript>)<Subscript>2</Subscript> and FXeCF<Subscript>3</Subscript>

Charges on the atoms and structural parameters of the Xe(CF₃)₂, FXeCF₃, and XeF₂ molecules were calculated by the MP2(full)/MIDI(d₆)&6-311G(d ₆) quantum-chemical methods. The calculated energy of Xe(CF₃)₂ is greater by 113 kcal/mol than the overall energy of C₂F₆ and Xe, and the energy of FXeCF₃ is greater by 108 kcal/mol than the overall energy of CF₄ and Xe, the barrier to the decomposition being estimated at 40 kcal/mol. Both Xe(CF₃)₂ and FXeCF₃ molecules are stable with respect to spontaneous decomposition with elimination of difluorocarbene.__________Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 12, 2004, pp. 1808–1810.Original Russian Text Copyright © 2004 by Semenov, Sigolaev.     

Ab initio study of mechanism of forming a silicic <Emphasis Type="Italic">bis</Emphasis>-heterocyclic compound between (CH<Subscript>3</Subscript>)<Subscript>2</Subscript>Si=Si: and ethane

The mechanism of the cycloaddition reaction of forming a silicic bis-heterocyclic compound between singlet state (CH₃)₂Si=Si: and ethene has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the reaction has one dominant reaction pathway. The presented rule of this dominant reaction pathway that the 3p unoccupied orbital of Si: in (CH₃)₂Si=Si: and the π orbital of ethane forming a π → p donor-acceptor bond, resulting in the formation of a three-membered ring intermediate (INT1); Then, INT1 isomerizes to a four-membered ring silylene (P1), which driven by ring-enlargement effect; Due to sp ³ hybridization of Si: atom in the four-membered ring silylene (P1), P1 further combines with ethene to form a silicic bis-heterocyclic compound (P2).     

Theoretical study on the reaction mechanism of (CH<Subscript>3</Subscript>)<Subscript>3</Subscript>CO<Superscript>.</Superscript> radical with NO

The reaction mechanism of (CH₃)₃CO^. radical with NO is theoretically investigated at the B3LYP/6-31G* level. The results show that the reaction is multi-channel in the single state and triplet state. The potential energy surfaces of reaction paths in the single state are lower than that in the triple state. The balance reaction: (CH₃)₃CONO⇔(CH₃)₃CO^.+NO, whose potential energy surface is the lowest in all the reaction paths, makes the probability of measuring (CH₃)₃CO^. radical increase. So NO may be considered as a stabilizing reagent for the (CH₃)₃CO^. radical.     

Dual-level direct dynamics studies on the hydrogen abstraction reactions of fluorine atoms with CF<Subscript>3</Subscript>CH<Subscript>2</Subscript>X(X=F,Cl)

The kinetic properties of the hydrogen abstraction reactions of CF₃CH₂F + F → CF₃CHF + HF (R1) and CF₃CH₂Cl + F → CF₃CHCl + HF (R2) have been studied by dual-level direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) were obtained at the B3LYP/6-311 + G(2d,2p) level. Two complexes with energies less than that of the reactants were located in the reactant side of each reaction. The energy profiles were further refined with the interpolated single-point energies (ISPE) method at the G3(MP2) level of theory. Using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) method, the rate constants were evaluated over a wide temperature range of 200–2,000 K. Our calculations have shown that C–H bond activity decreases when one hydrogen atom of CF₃CH₃ is substituted by a fluorine atom, than when substituted with a chlorine atom. This is in good agreement with the experimental results.     

Ab Initio Theoretical Studies on the Kinetics of Hydrogen Abstraction Type Reactions of Hydroxyl Radicals with CH<Subscript>3</Subscript>CCl<Subscript>2</Subscript>F and CH<Subscript>3</Subscript>CClF<Subscript>2</Subscript>

The hydrogen abstraction reactions from CH₃Cl₂F (R-141b) and CH₃CClF₂ (R-142b) by OH radicals are studied theoretically by semi-classical transition state theory. The stationary points for the reactions are located by using KMLYP density functional method along with 6-311++G(2d,2p) basis set and MP2 method along with 6-311+G(d,p) basis set. Single-point energy calculations are performed by the CBS-Q and G4 combination methods on the geometries optimized at the KMLYP/6-311++G(2d,2p) level of theory. Vibrational anharmonicity coefficients, x _ij , which are needed for semi-classical transition state theory calculations, are computed at the KMLYP/6-311++G(2d,2p) and MP2/6-311+G(d,p) levels of theory. The computed barrier heights are slightly sensitive to the quantum-chemical method. Thermal rate coefficients are computed over the temperature range from 200 to 2000 K and they are shown to be in accordance with available experimental data. On the basis of the computed rate coefficients, the tropospheric lifetime of the CH₃CCl₂F and CH₃CClF₂ are estimated to be about 6.5 and 12.0 years, respectively.     

Theoretical study on the potential energy surface of NC<Subscript>3</Subscript>P isomers

Density functional theory (DFT) calculations have been used to study the isomerization process in the NC₃P system. At the DFT/B3LYP/6-311G(d) level, 28 triplet and 28 singlet minima were obtained on their respective potential energy surfaces. The linear triplet ³NCCCP is the lowest-energy structure among the isomers. On the triplet PES, only linear isomers ³NCCCP, ³CNCCP, ³CCCNP, and ³CCNCP possess great kinetic and thermodynamic stabilities to exist under low-temperature conditions (such as in the dense interstellar clouds). At the same time, one chain-like and four three-membered-ring isomers on the singlet PES have been located with high kinetic and thermodynamic stabilities. Further CCSD(T)/6-311G(2df)//QCISD/6-311G(d), CCSD(T)/cc-pVTZ//DFT/B3LYP/cc-pVTZ, and CASPT2(14,12)/cc-pVQZ//CASSCF(14,12)/cc-p VQZ calculations are performed on the structures, frequencies, and energies of the relevant species. The bonding natures were analyzed and the results were compared with the analogous NC₃N and NC₂P molecules so as to aid their future experimental or astrophysical detection.     

Atomic radical—molecule reactions F + CH<Subscript>3</Subscript>C≡CH: mechanistic study

The reaction of atomic radical F with propyne has been studied theoretically using ab initio quantum chemistry methods and transition state theory. The potential energy surface was calculated at the CCSD(T)/aug-cc-pVDZ (single-point) level using the UMP2/6-311++G(d,p) optimized structures. Two reaction mechanisms including the addition–isomerization–elimination reaction mechanism and the directed hydrogen abstraction reaction mechanism are considered. For the hydrogen abstraction reactions, i.e., the most probable evolution pathway in the title reaction, the HF formation occurs via direct abstraction mechanism dominantly and the H atom picked up by the atomic radical F should come mostly from the methyl group of normal propyne. On the other hand, for the addition–isomerization–elimination mechanism, the most feasible pathway should be the atomic radical F attacking on the C≡C triple bond in propyne (CH₃C≡CH) to form a weakly-bound adduct A1 with no barrier, followed by F addition to the C≡C triple bond to form the low-lying intermediate isomer 5. Subsequently, isomer 5 directly dissociates to P3 H₂CCCHF + H via transition state TS₅/P3. The other reaction pathways on the doublet PES are less competitive due to thermodynamical or kinetic factors. Furthermore, based on the analysis of the kinetics of all channels through which the addition and abstraction reaction proceed, we expect that the competitive power of reaction channels may vary with experimental conditions for the title reaction. The present work will provide useful information for understanding the processes of atomic radical F reaction with other unsaturated hydrocarbons. This material is available from author via E-mail.     

Iminopropadienones R–NCCCO and carbon suboxide, C3O2. Theoretical and experimental C NMR spectra

The ¹³C NMR data of five iminopropadienones R–NCCCO as well as carbon suboxide, C₃O₂, have been examined theoretically and experimentally. The best theoretical results were obtained using the GIAO/B3LYP/6-31+G**//MP2/6-31G* level of theory, which reproduces the chemical shifts of the iminopropadienone substituents extremely well while underestimating those of the cumulenic carbons by 5–10 ppm. The computationally faster GIAO/HF/6-31+G**//B3LYP/6-31G* level is also adequate.     

Anharmonic Effect in CH<Subscript>3</Subscript>CH<Subscript>2</Subscript>C(=O)OCH<Subscript>2</Subscript>CH<Subscript>3</Subscript> Decomposition

In this paper, using the B3LYP functional and CCSD(T) method with 6-311++G** basis set, the harmonic and anharmonic rate constants in the unimolecular dissociation of ethyl propanoate have been calculated using Rice–Ramsperger–Kassel–Marcus theory. The anharmonic rate constants of the title reaction have also been examined, the comparison shows that, the anharmonic effect especially in the case of high total energies and temperature for channels 3 to 6 is significant, so that the anharmonic effect cannot be neglected for unimolecular dissociation reaction of CH₃CH₂C(=O)OCH₂CH₃ both in microcanonical and canonical systems.     

The thermodynamic properties of fullerene hydrides C<Subscript>60</Subscript>H<Subscript>2<Emphasis Type="Italic">n</Emphasis></Subscript>

The equilibrium geometric parameters and normal vibration frequencies of the C₆₀H₂, C₆₀H₁₈, and C₆₀H₆₀ molecules were determined using density functional theory at the B3LYP/6-31G* level. The results were used to calculate the thermodynamic properties of the substances in the ideal gas state. The heat capacity of crystalline hydrofullerenes was modeled and the enthalpies of their sublimation estimated. The thermodynamic parameters of the hydrogenation of fullerene C₆₀ were analyzed.     

Molecular Structure and Conformational Preferences of Trimethyl Phosphorotrithioite,P(SMe)<Subscript>3</Subscript>, Evaluated by Gas-Phase Electron Diffraction and Quantum Chemical Calculations

Free P(SMe)₃ molecule was studied by gas electron diffraction (GED) and by B3PW91/6-311+G* (DFT) and MP2/6-31+G* calculations. Each conformer is characterized by three dihedral angles τ(CSPlp), where lp denotes the direction of the lone electron lone pair on the P atom. DFT calculations indicate that the most stable conformer is an anti,gauche+,gauche- (ag+g-) conformer of C _s symmetry; the next are the ag+g+ (ΔE = 2.5 kJ mol⁻¹), g+g+g+ (ΔE = 5.2 kJ mol⁻¹), and aa+g+ (Δ E = 12.5 kJ mol⁻¹) conformers. The MP2 calculations give the similar order, with the relative energies of 0.3, 4.3, and 10.6 kJ mol⁻¹, respectively. The experimental GED data agree well with the presence of only two conformers: χ(ag+g+) = 80(20)% and χ(ag+g-) = 20(10)%.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005, pp. 742–750.Original Russian Text Copyright © 2005 by Belyakov, Khramov, Baskakova, Naumov.     

乙烯醇钠的从头算研究(英文)

在ＲＨＦ／３－２１Ｇ和ＲＨＦ／６－３１＋Ｇ水平上优化了乙烯醇钠的的平衡构型及过渡态，比较了不同构型的稳定性。在ＲＨＦ／６－３１＋Ｇ水平上探讨了气相中由乙醛和氢氧化钠生成乙烯醇钠的机理。并在ＭＰ２（ｆｕｌ）／６－３１＋Ｇ／／ＭＰ２（ｆｕｌ）／６－３１＋Ｇ水平下计算了反应热     

Quantum-mechanical calculations of the structure and vibrational spectra of CH<Subscript>3−<Emphasis Type="Italic">n</Emphasis></Subscript>Cl<Subscript>n</Subscript>SiF<Subscript>3</Subscript> (<Emphasis Type="Italic">n</Emphasis> = 0–3) substituted methylsilanes

Quantum-mechanical calculations of the optimized structures, barriers to internal rotation about the Si-C bond, harmonic force fields, and vibrational frequencies of CCH₃SiF₃, CH₂ClSiF₃, CHCl₂SiF₃, and CCl₃SiF₃ were performed using the Hartree-Fock approximation and density functional theory at the MP2/6-31G*, B3LYP/6-31G*, B3LYP/6-311++G**, B3LYP/aug-cc-pVDZ, and B3LYP/aug-cc-pVTZ levels. A new interpretation of the infrared absorption and Raman spectra of the compounds was suggested on the basis of the theoretical results     

Complexation processes in KF-SbF<Subscript>3</Subscript>-H<Subscript>2</Subscript>O system studied by calorimetric titration

Complexation in a KF-SbF₃-H₂O system is studied in a range of molar ratios of fluorides KF : SbF₃= (0.1–2) : 1 by calorimetric titration. The equilibrium formation constants of complexes KSb₂F₇, KSbF₄, and K₂SbF₅ (5.8×10⁵±800, 3.3×10⁴±500, and 1.9×10⁶± 950, respectively) and the changes in enthalpy (–31.75± 0.74, –28.15±0.44, and –25.5±0.64 J mol^–1, respectively) and entropy (4±7, –8±5, and –35±9 J mol^–1 K^–1, respectively) are determined. The thermodynamic stability of the antimony(III) fluoride complexes is found to increase on going from KSb₂F₇ to K₂SbF₅.Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 3, 2005, pp. 168–171.Original Russian Text Copyright © 2005 by Kovaleva, Zemnukhova, Lebedeva, Fedorishcheva.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.