Ab initio and density function theory computational studies of the CH4 + H → CH3 + H2 reaction

The activation barrier for the CH₄ + H → CH₃ + H₂ reaction was evaluated with traditional ab initio and Density Functional Theory (DFT) methods. None of the applied ab initio and DFT methods was able to reproduce the experimental activation barrier of 11.0-12.0 kcal/mol. All ab initio methods (HF, MP2, MP3, MP4, QCISD, QCISD(T), G1, G2, and G2MP2) overestimated the activation energy. The best results were obtained with the G2 and G2MP2 ab initio computational approaches. The zero-point corrected energy was 14.4 kcal mol⁻¹. Some of the exchange DFT methods (HFB) computed energies which were similar to the highly accurate ab initio methods, while the B3LYP hybrid DFT methods underestimated the activation barrier by 3 kcal mol⁻¹. Gradient-corrected DFT methods underestimated the barrier even more. The gradient-corrected DFT method that incorporated the PW91 correlational functional even generated a negative reaction barrier. The suitability of some computational methods for accurately predicting the potential energy surface for this hydrogen radical abstraction reaction was discussed.     

2D and 3D quantum scattering calculations on the CH4 + OH → CH3 + H2O reaction

The effect on the thermal rate constant and the differential cross-sections of varying the dimensionality of quantum scattering calculations of a polyatomic reaction is investigated. The rotating bond approximation (RBA; 3D) and a rotating line approximation (RLA; 2D) are used for the CH₄ + OH → CH₃ + H₂O reaction. It is found that the RBA and RLA results are in close agreement when an adiabatic treatment is used for the degree of freedom which is treated explicitly in the RBA but not in the RLA.     

The 3ν1 + ν3 vibrational overtone spectrum of CH4

The 3 ν₁ + ν₃ vibrational overtone spectrum of ¹³CH₄ is recorded under Doppler-limited resolution conditions using a titanium sapphire laser-based photoacoustic spectrometer. Data at two temperatures, 100 and 293 K, are presented. The observed spectral congestion is qualitatively similar to that observed for ¹²CH₄, but the detailed ro-vibrational structure of the two isotope variants is completely different. The data reflect the complicating influences of tetrahedral fine structure and vibrational state mixing.     

Reaction dynamics of the Ca(D2, PJ) + CH3I → CaI + CH3 system: chemiluminescence, energy disposal and product polarization

The Ca(¹D₂, ³P_J) + CH₃ → CaI(A,B) + CH₃ reactions system has been studied by measuring its chemiluminescence under beam-gas conditions. Absolute values of the state-to-state reaction cross-sections were determined at low collision energy . In addition, the electronic branching ratio and product energy disposal have been determined for each metastable reaction. The major changed observed in the chemiluminescence when comparing the Ca(¹D₂) reaction versus that of Ca(³P_J) is the total yield associated with the former reaction. To the best of our spectral resolution neither the electronic branching ratio e.g. CaI(A)/CaI(B) nor the internal CaI energy disposal change significantly as the metastable Ca(¹D₂)/Ca(³P_J) ratio is varied. In spite of the fact that the Ca(³P_J) reaction is less exoergic, the CaI product appears with a higher fraction of internal energy than that of Ca(¹D₂) reaction. Thus, the fraction of the total energy appearing in CaI internal energy amounts to 57.5% in the Ca(³P_J) reaction while it is 19.3% only for the Ca(¹D₂) reaction. This difference is discussed in the light of a distinct mechanism associated with the attack of the excited Ca atom into the C---I bond. No significant chemiluminescence yield was found for the energetically open CaCH^*₃ channels.

The product chemiluminescence polarization was also measured as a function of the metastable concentration. A significant degree of polarization was found depending upon the specific electronic excitation. The analysis of the polarization emission associated to the parallel CaI(X ²Σ⁺ ← B ²Σ⁺) emission led into a strong polarization of the product rotational angular momentum. The comparison of the product rotational alignment for the kinematically identical Ca(¹D₂, ³P_J, ¹P₁) + CH₃ → CaI^* (B₂Σ⁺) + CH₃ reaction system showed that the CaI rotational polarization diminishes in the ³P_J → ¹D₂ → ¹P₁ sequence, e.g. as the reaction exothermicity increases. In addition the degree of polarization associated with other emission bands as for example CaI(X ²Σ⁺ ← A ²Π_1/2) indicates the presence of a parallel transition which was been interpreted as mixing of Hund's case (a) and (c) appropriate for this heavy CaI diatom produced with a high rotational excitation.     

Temperature dependence of CH2 (ã A1) removal rates by Ar, NO and H2

Absolute rate constants for the removal of CH₂ (ã ¹A₁) by Ar, NO and H₂ have been measured over the temperature range 295–431 K using laser flash kinetic absorption spectroscopy. For NO and H₂, no marked temperature dependence is observed. For Ar, the rate constants increase by some 50% over this range and the results are discussed in terms of a theoretical model which relates the removal rate constants to the fractional population of CH₂ (ã ¹A₁) in states perturbed by nearby triplet levels.     

Direct dynamics studies on the hydrogen abstraction reactions CF3O+CH4(CD4)→CF3OH(CF3OD)+CH3(CD3)

The dynamics properties of the hydrogen abstraction reaction CF₃O+CH₄→CF₃OH+CH₃ are studied by dual-level direct dynamics method. Optimization calculations are preformed by B3LYP and MP2 with the 6-311G(d,p) basis set, and the single-point calculations are done at the multi-coefficient correction method based on quadratic configuration interaction with single and double excitations (MC-QCISD) method. The rate constants are evaluated by canonical variational transition-state theory with a small-curvature tunneling correction over a wide range of temperature 200–2000 K. The agreement between theoretical and experimental rate constants is good in the measured temperature range. The calculated results show that the variational effect is small and almost neglected over the whole temperature range, whereas, the tunneling correction plays a role in the lower temperature range. The kinetic isotope effect for the reaction is ‘normal’. The value of k_H/k_D is 2.38 at room temperature and it decreases with the temperature increasing.     

Ab initio molecular orbital study on the gas phase SN2 reaction F + CH3Cl → CH3F + Cl

Ab-initio molecular orbital (MO) and direct ab initio dynamics calculations have been applied to the gas phase S_N2 reaction F⁻ + CH₃Cl → CH₃F + Cl⁻. Several basis sets were examined in order to select the most convenient and best fitted basis set to that of high-quality calculations. The Hartree–Fock (HF) 3−21+G(d) calculation reasonably represents a potential energy surface calculated at the MP2/6−311++G(2df,2pd) level. A direct ab initio dynamics calculation at the HF/3−21+G(d) level was carried out for the S_N2 reaction. A full dimensional ab initio potential energy surface including all degrees of freedom was used in the dynamics calculation. Total energies and gradients were calculated at each time step. Two initial configurations at time zero were examined in the direct dynamics calculations: one is a near collinear collision, and the other is a side-attack collision. It was found that in the near collinear collision almost all total available energy is partitioned into two modes: the relative translational mode between the products (40%) and the C − F stretching mode (60%). The other internal modes of CH₃F were still in the ground state. The lifetimes of the early- and late-complexes F⁻ … CH₃Cl and FCH₃ … Cl⁻ are significantly short enough to dissociate directly to the products. On the other hand, in the side-attack collision, the relative translation energy was about 20% of total available energy.     

Synthesis of gibberellins A8 and A56 from gibberellin A3

A mixture of gibberellin A₃ derivatives with 1(10)-ene-2,3-diol and 1(10)-ene-2,3-diol (2:5) groups, readily obtained from gibberellin A₃, has been used for a new and simple synthesis of gibberellin A₈ and its esters. The hydrolysis of GA₃ and the iodolactonization of a mixture of the 2-epimers was carried out in aqueous solution in a single flask, as also was a synthesis of GA₅₆ from GA₃ by a method that we have modified. The mixture of 1-iodides of GA₈ and GA₅₆ was separated by chromatography on SiO₂ in the form of methyl or p-bromophenacyl esters which were then deiodinated and the methyl or p-bromphenacyl ester of GA₈ was isolated. Free GA₈ was obtained by the dephenylation of the latter ester. By two-dimensional NMR spectroscopy we succeeded in assigning all the signals in the¹³C and¹H NMR spectra of the methyl esters of GA₈ and GA₅₆. In an attempt to obtain GA₅ methyl ester by the action of trimethylchlorosilane/sodium iodide on the 2,3-diol system in GA₅₆ methyl ester, the 8,13-epimer of the latter was formed, the structure of its molecule being established from the results of X-ray structural analysis.Novosibirsk Institute of Organic Chemistry, Siberian Division of the Russian Academy of Sciences. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 663–669, September–October, 1994.     

The 1B1, 1B2, 1A1, 1A2, 3A′(2B2), and 2A1 states of the 1,1-difluoromethane ion studied using multiconfiguration second-order perturbation theory

The 1²B₁(X²B₁), 1²B₂, 1²A₁, 1²A₂, 2²B₂, and 2²A₁ states of the ion were studied using CASPT2 and CASSCF methods. Calculations suggest that one should consider the 3²A′ state instead of 2²B₂. The CASPT2 T₀ calculations predict the energy ordering of 1²B₁(X²B₁), 1²B₂, 1²A₁, 1²A₂, 3²A′, and 2²A₁, which is in line with the experimental results by Pradeep and Shirley. The CASPT2 T₀ values for the 1²B₂, 1²A₂, 3²A′, and 2²A₁ states are close to the experimental values. The F-loss and H-loss dissociation processes were studied at the CASPT2//CASSCF level. The energy levels of low-lying states of are compared.     

反常蓝移单电子锂键Y…Li-CH3[Y=CH3, CH2CH3, CH(CH3)2, C(CH3)3]体系的结构与性质

在密度泛函理论B3LYP/6-311++G(d,p)及MP2/6-311++G(d,p)水平上研究了单电子锂键复合物Y…Li—CH3[Y=CH3, CH2CH3, CH(CH3)2, C(CH3)3]的结构与性质. 结果表明, 三种单电子锂键复合物H3CH2C…Li—CH3(II), (H3C)2HC…Li—CH3(III)和(H3C)3C…Li—CH3(IV)单电子锂键强度依II(-26.7 kJ·mol-1)相似文献   

Stereoselective syntheses of the glycosidase inhibitors hyacinthacine A2, hyacinthacine A3 and 5-epi-hyacinthacine A3

Stereoselective syntheses of the naturally occurring glycosidase inhibitors hyacinthacines A₂ and A₃ are reported. In the case of hyacinthacine A₂, the pyrrolizidine system was created from an acyclic precursor via a double cyclization procedure with a one-pot formation of two C-N bonds. In the case of hyacinthacine A₃, the two C-N bonds were created in separate steps. In addition, the non-natural epimer at C-5 of hyacinthacine A₃ was obtained.     

Interaction of CH3OH with α-MoO3 finite clusters, a theoretical study

Methanol chemisorption on various faces of molybdenum trioxide are studied by EHT calculations with model clusters. Both energy values and overlap populations show that molecular and dissociative chemisorption can be envisaged. At a distance fixed at 1.75 Å between the surface molybdenum and the methanol oxygen, the stability of the molecular chemisorption complex increases as we pass from the (100) face to the (001) and (010) ones, if the lastmentioned presents an oxygen vacancy before methanol chemisorption. On the (100) and (001) faces containing unsaturated oxygen, dissociative chemisorption leads to states of greater stability than molecular chemisorption. la contrast, if the superficial oxygen involved is saturated, the states obtained are less stable.     

A rate constant for CH2(3B1) + CH3

Triplet methylene, CH₂(³B₁), and methyl radicals were produced by flash photolysis of a mixture of ketene and azomethane. A computer fit of the product ratios, using the known rate constants for CH₂ + CH₂, and CH₃ + CH₃, requires a rate constant of 5.0 × 10^?11 cm³ molecule^?1s^?1 for the reaction CH₂ + CH₃ ? C₂H₄ + H.     

Nuclear quadrupole resonances of α-(CH3)2 TeX2 (X=Cl, Br, I) and (CH3)2 TeI4

NQR spectra were observed for α-(CH₃)₂ TeX₂ (X=Cl, Br, I) and (CH₃)₂ TeI₄ at various temperatures. The two ⁸¹Br NQR lines were observed above 110 K in α-(CH₃)₂TeBr₂. The characteristic temperature dependence of the ¹²⁷I NQR line in α-(CH₃)₂ TeI. can be explained by the 3c—4e bond of the linear I---Te---I group. The positive temperatures dependence of the lowest ¹²⁷I NQR line in (CH₃)₂TeI₄ is discussed on the basis of the electron population calculated from Townes—Dailey treatment.     

Theoretical calculational studies on the mechanism of thermal dissociations for RN3 (R=CH3, CH3CH2, (CH3)2CH, (CH3)3C)

Mechanisms of RN₃ (R=CH₃, CH₃CH₂, (CH₃)₂CH, (CH₃)₃C) dissociations are proposed based on CAS, MP2 and B3LYP methods. The energy gaps between the ground-state reactants RN₃ and the intersystem crossing (ISC) points are only a little lower than respective potential energy barriers of the spin-allowed reactions, ¹RN₃ → ¹RN + ¹N₂. The ISC point, therefore, is considered as a transition state of the spin-forbidden reactions, ¹RN₃ → ³RN + ¹N₂. The methods of IRC and topological analysis of electron density are used to explain and predict the thermal dissociation pathways of the reactions studied.     

The photoconversion R-CH3 → R-CHO in indazole derivatives

The UV irradiation of 1-(p-nitrophenyl)-3-methyl-indazole leads to the formation of the corresponding 3-formyl derivative as product of the photooxidation R-CH₃ → R-CHO. This method was applied to a series of substituted indazoles and the same reaction was observed with methyl groups attached to the pyrazole and to the benzene ring of the indazole.     

Synthesis of thromboxane A2 analogs—3 : (+)-Thiathromboxane A2

(+)-11a-methano-9,11-thiathiromboxane A₂(1) was synthesized from prostaglandin A₂ and prostaglandin E₂.     

Synthesis of thromboxane A2 analogs—1 : (±)-Dimethanothromboxane A2

The stable thromboxane A₂ analog (±)-dimethanothromboxane A₂ 1 was synthesized from bicyclo[3.1.1]heptane 2 via the tricyclic compound 4.     

Configuration interaction study of the oscillator strenghts for the A1--- A1 and A1--- A1 transitions of the water molecule

Oscillator strengths for transitions between the ¹A₁ ground state of water and its ¹A₁ and ¹A₁ excited states are computed employing two different theoretical approaches. In one series of calculations a common orthonormal one-electron basis is employed for all of the above states, while in the other type of treatment two different, mutually non-orthogonal, sets are used; the multireference single- and double-excitation (MRD CI) method is employed in each case, with configuration selection, to generate the various electronic wavefunctions. It is found that the use of ground-state SCF MOs leads to poor convergence in the wavefunctions of the (Rydberg-type) ¹A₁ and ¹A₁ excited states and consequently also for the corresponding --- and --- f-values; this behavior is seen to be closely related to the near degeneracy of the two excited states, each of which is a mixture of the 3a₁ → 3sa₁ and 1b₁ → 3pb₁ configurations. Analogous computations with the ¹B₁¹b₁ → 3sa₁ MOs show much better convergence properties, and the resulting f-values compare well with what is obtained when state-specific orbital sets are employed separately for ground and excited states and non-orthonormal techniques are applied to compute the desired transition moments. These results tend to confirm previous findings which indicate conceptual and computational advantages for the calculation of excited-state wavefunctions and properties within the context of a state-specific theory. They also show that although the goal of eliminating the dependence of MRD CI calculations on the choice of MO basis is very nearly approached for energy quantities, it is less satisfactorily achieved for other properties, especially when the existence of nearly degenerate electronic states is a critical factor.