Thermochemical and kinetic analysis of the thermal decomposition of monomethylhydrazine: an elementary reaction mechanism

The reaction kinetics for the thermal decomposition of monomethylhydrazine (MMH) was studied with quantum Rice-Ramsperger-Kassel (QRRK) theory and a master equation analysis for pressure falloff. Thermochemical properties were determined by ab initio and density functional calculations. The entropies, S degrees (298.15 K), and heat capacities, Cp degrees (T) (0 < or = T/K < or = 1500), from vibrational, translational, and external rotational contributions were calculated using statistical mechanics based on the vibrational frequencies and structures obtained from the density functional study. Potential barriers for internal rotations were calculated at the B3LYP/6-311G(d,p) level, and hindered rotational contributions to S degrees (298.15 K) and Cp degrees (T) were calculated by solving the Schr?dinger equation with free rotor wave functions, and the partition coefficients were treated by direct integration over energy levels of the internal rotation potentials. Enthalpies of formation, DeltafH degrees (298.15 K), for the parent MMH (CH3NHNH2) and its corresponding radicals CH3N*NH2, CH3NHN*H, and C*H2NHNH2 were determined to be 21.6, 48.5, 51.1, and 62.8 kcal mol(-1) by use of isodesmic reaction analysis and various ab initio methods. The kinetic analysis of the thermal decomposition, abstraction, and substitution reactions of MMH was performed at the CBS-QB3 level, with those of N-N and C-N bond scissions determined by high level CCSD(T)/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) calculations. Rate constants of thermally activated MMH to dissociation products were calculated as functions of pressure and temperature. An elementary reaction mechanism based on the calculated rate constants, thermochemical properties, and literature data was developed to model the experimental data on the overall MMH thermal decomposition rate. The reactions of N-N and C-N bond scission were found to be the major reaction paths for the modeling of MMH homogeneous decomposition at atmospheric conditions.     

Self-discrimination of enantiomers in hydrogen-bonded dimers

The homochiral and heterochiral hydrogen-bonded (HB) dimers of a set of small model molecules (alpha-amino alcohols) have been studied by means of ab initio methods. The gas-phase calculations have been carried out with the hybrid HF/DFT B3LYP method and the 6-311++G** basis set. The electron density of the complexes has been analyzed using the atoms in molecules (AIM) methodology, which allows characterization of the HB interactions and additional intermolecular contacts. To take into account the water solvation effect, the polarized continuum model (PCM) method has been used to evaluate the Delta G(solv). The gas-phase results show that the heterochiral dimers are the most stable ones for each case studied, while in solution for several cases, the relative stability is reversed and the homochiral dimers become more stable. The AIM analysis shows the typical bond critical points characteristic of the HB and additional bond critical points denoting, in this case, destabilization of intermolecular interaction as CF(3)...F(3)C and CH(3)...H(3)C contacts.     

N, P, and As ylides and aza- and arsa-Wittig reactions from topological analyses of electron density

The nature of bonding between N, P, and As constituent atoms in ylide systems with the R(3)XYR' formula (X = N, P, As; Y = N, P, As; R = F, H; R' = H, CH(3)) has been characterized by ab initio (MP2/6-311++G**) and density functional theory (B3LYP/6-311++G**) calculations. Its electronic structure has been analyzed through electron density with the quantum theory of atoms in molecules and the electron localization function (ELF). The characteristics of the central bond are inspected with the calculated rotational barriers. The results show that N has a behavior different from that of the remaining pnicogen atoms (P, As), where the bond is much stronger. Fluorine substituents strengthen the X-Y bond, reduce the bond distance, and increase the electron density in the central bond so that the substituent pulls charge from the bond in the pnicogen X atom. For the N-pnicogen ylides, the results showed different bonding characters between F and X atoms; depending on the position of the F atom, the difference of the bond character is sensed by the basin synaptic order, as it is deduced from the analysis of the ELF basins. The energy profiles of the rotational barriers have been calculated at the MP2/6-311++G** level, indicating that the electronegativity of the substituents is a relevant factor that has consequences in the characteristics of the X-Y bond.     

自由基-分子反应:F+Propene(CH2CHCH3)的从头算研究

The radical-molecule reaction F+propene (CH2CHCH3) was studied in detail by using the Becke's three parameter Lee-Yang-Parr-B3LYP/6-311G(d,p) and coupled cluster with single, double, and triple excitationsCCSD(T)/6-311+G(2d,2p). It is shown that F+propene reaction mainly occurs through complex-formation mechanism: F attacks the double bond of propene leading to the formation of complex 1 and complex 2. As the two radical complexes are metastable, they can quickly dissociate to H+C3H5F, CH3+C2H3F and HF+C3H5. Based on the ab initio calculations, the CH3+C2H3F is the main channel, and the H elimination and HF forming channels also provide some contribution to products. The calculated values are in good agreement with the recently reported experimental results.     

Direct ab initio dynamics calculation of the reaction rates of CH3OCl with OH

A direct ab initio dynamics method was carried out for the reaction CH3OCl + OH --> products. Three abstraction channels from chlorine atom, in-plane hydrogen, and out-of-plane hydrogen atoms at the CH3 group have been found. The optimized geometries and frequencies of the stationary points and the minimum-energy paths (MEPs) were calculated at the MP2/6-311G(d,p) level. To improve the reaction enthalpy and potential barrier, single-point calculations were made at three higher levels of theory, the approximate QCISD(T)/6-311++G(3df,2pd), G3, and G3(MP2) levels. Furthermore, the rate constants for three abstraction channels were evaluated using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) over a wide temperature range of 220-2000 K at above three higher theory levels, respectively. The calculated rate constants as well as branching rates are in reasonable agreement with the experimental values in the temperature region 250-341 K. The present results indicate H-abstraction especially from out-of-plane hydrogen is the main reaction pathway, while Cl-abstraction is much less competitive.     

Ab initio potential energy surface, variational transition state theory, and quasiclassical trajectory studies of the F+CH4-->HF+CH3 reaction

In this work we present a study of the F+CH(4)-->HF+CH(3) reaction (DeltaHdegrees(298 K)=-32.0 kcal mol(-1)) using different methods of the chemical reaction theory. The ground potential energy surface (PES) is characterized using several ab initio methods. Full-dimensional rate constants have been calculated employing the variational transition state theory and using directly ab initio data. A triatomic analytical representation of the ground PES was derived from ab initio points calculated at the second- and fourth-order M?ller-Plesset levels with the 6-311+G(2df,2pd) basis set, assuming the CH(3) fragment to be a 15 a.m.u. pseudoatom in the fitting process. This is suggested from experiments that indicate that the methyl group is uncoupled to the reaction coordinate. A dynamics study by means of the quasiclassical trajectory (QCT) method and employing this analytical surface was also carried out. The experimental data available on the HF internal states distributions are reproduced by the QCT results. Very recent experimental information about the reaction stereodynamics is also borne out by our QCT calculations. Comparisons with the benchmark F+H(2) and analogous Cl+CH(4) reactions are established throughout.     

Hydrogen bonding in acetylacetaldehyde: Theoretical insights from the theory of atoms in molecules

All the possible conformations of tautomeric structures (keto and enol) of acetylacetaldehyde (AAD) were fully optimized at HF, B3LYP, and MP2 levels with 6‐31G(d,p) and 6‐311++G(d,p) basis sets to determine the conformational equilibrium. Theoretical results show that two chelated enol forms have extra stability with respect to the other conformers, but identification of global minimum is very difficult. The high level ab initio calculations G2(MP2) and CBS‐QB3) also support the HF conclusion. It seems that the chelated enol forms have equal stability, and the energy gap between them is probably lies in the computational error range. Finally, the analysis of hydrogen bond in these molecules by quantum theory of atoms in molecules (AIM) and natural bond orbital (NBO) methods fairly support the ab initio results. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A theoretical study of the interactions of NF(3) with neutral ambidentate electron donor and acceptor molecules

A theoretical study of the complexes (dimers and trimers) formed between nitrogen trifluoride (NF(3)) and the ambidentate electron donor/acceptor systems HF, FCl, HCN, and HNC has been carried out using DFT [M05-2x/6-311++G(d,p)] and ab initio methods [(MP2/6-311++G(d,p) and MP2/aug-cc-pVTZ)]. Due to its structure, the NF(3) molecule can interact with both electron acceptors and electron donors through its N and F atoms. Thus, five minimum energy structures have been located for the dimers and four minima structures have been studied for the trimer complexes. New σ-hole bonding complexes have been located.     

Quantum mechanical investigation of the atmospheric reaction CH3O2 + NO

The important stationary points on the potential energy surface of the reaction CH(3)O(2) + NO have been investigated using ab initio and density functional theory techniques. The optimizations were carried out at the B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory while the energetics have been refined using the G2MP2, G3//B3LYP, and CCSD(T) methodologies. The calculations allow the proper characterization of the transition state barriers that determine the fate of the nascent association conformeric minima of methyl peroxynitrite. The main products, CH(3)O + NO(2), are formed through either rearrangement of the trans-conformer to methyl nitrate and its subsequent dissociation or via the breaking of the peroxy bond of the cis-conformer to CH(3)O + NO(2) radical pair. The important consequences of the proposed mechanism are (a) the allowance on energetic grounds for nitrate formation parallel to radical propagation under favorable external conditions and (b) the confirmation of the conformational preference of the homolytic cleavage of the peroxy bond, discussed in previous literature.     

Computational study of the reaction of fluorine atom with acetone

The reaction of F(2P) with acetone has been studied theoretically using ab initio quantum chemistry methods and transition state theory. The potential energy surface was calculated at the G3MP2 level using the MP2/6-311G(d,p) optimized structures. Additionally, to ensure the accuracy of the calculations, optimizations with either larger basis set (e.g., MP2/G3MP2Large) or higher level electron correlation [e.g., CCSD/ 6-311G(d,p)] were also performed. It has been revealed that the F + CH3C(O)CH3 reaction proceeds via two pathways: (1) the direct hydrogen abstraction of acetone by F gives the major products HF + CH3C(O)CH2; (2) the addition of F atom to the >C=O double bond of acetone and the subsequent C-C bond cleavage gives the minor products CH3 + CH3C(O)F. All other product channels are of no importance due to the occurrence of significant barriers. Both abstraction and addition appear to be barrierless processes. Variational transition state model and multichannel RRKM theory were employed to calculate the temperature- and pressure-dependent rate constants and branching ratios. The predicted rate constants for the abstraction channel and the yields of HF + CH3C(O)CH3 and CH3 + CH3C(O)F are both in good agreement with the experimental data at 295 K and 700 Torr. A negative temperature dependence of the overall rate constants was predicted at temperatures below 500 K.     

Synthesis and characterization of the first examples of perfluoroalkyl-substituted trialkyloxonium salts, [(CH3)2OCF3]+[Sb2F11]- and [(CH3)2OCF(CF3)2]+[Sb2F11]-

In the superacidic HF/SbF(5) system, methyl trifluoromethyl ether forms at -78 degrees C the new tertiary oxonium salt [(CH(3))(2)OCF(3)](+)[Sb(2)F(11)](-), which was characterized by Raman and multinuclear NMR spectroscopy and its crystal structure. The same oxonium salt was also obtained by methylation of CH(3)OCF(3) with CH(3)F and SbF(5) in HF solution at -30 to -10 degrees C. Replacement of one methyl group in the trimethyloxonium cation by the bulkier and more electronegative trifluoromethyl group increases the remaining O-CH(3) bond lengths by 0.037(1) A and the sum of the C-O-C bond angles by about 4.5 degrees. Methylation of CH(3)OCF(CF(3))(2) with CH(3)F in HF/SbF(5) solution at -30 degrees C produces [(CH(3))(2)OCF(CF(3))(2)](+)[Sb(2)F(11)](-). The observed structure and vibrational and NMR spectra were confirmed by theoretical studies at the B3LYP/6-311++G(2d,2p) and the MP2/6-311++G(2d,p) levels.     

An ab initio MO study on the structures and electronic states of hydrogen-bonded O3- HF and SO2-HF complexes

Ab initio molecular orbital (MO) calculations have been carried out for base-hydrogen fluoride (HF) complexes (base = O3 and SO2) in order to elucidate the structures and energetics of the complexes. The ab initio calculations were performed up to the QCISD(T)/6-311++G(d,p) level of theory. In both complexes, hydrogen-bonded structures where the hydrogen of HF orients toward one of the oxygen atoms of bases were obtained as stable forms. The calculations showed that cis and trans isomers exist in both complexes. All calculations for the SO2-HF complex indicated that the cis form is more stable in energy than the trans form. On the other hand, in O3-HF complexes, the stable structures are changed by the ab initio levels of theory used, and the energies of the cis and trans forms are close to each other. From the most sophisticated calculations (QCISD(T)/6-311++G(d,p)//QCISD/6-311+G(d) level), it was predicted that the complex formation energies for cis SO2-HF, trans SO2-HF, cis O3-HF, and trans O3-HF are 6.1, 5.7, 3.4, and 3.6 kcal/mol, respectively, indicating that the binding energy of HF to SO2 is larger than that of O3. The harmonic vibrational frequencies calculated for cis O3-HF and cis SO2-HF complexes were in good agreement with the experimental values measured by Andrews et al. Also, the calculated rotation constants for cis SO2-HF agreed with the experiment.     

A theoretical study of hydrogen- and lithium-bonded complexes of F-H∕Li and Cl-H∕Li with NF3, NH3, and NH2(CH3)

Hydrogen- and lithium-bonded complexes of A-H∕Li (A = F, Cl) with the amine analogues NF(3), NH(3), and NH(2)(CH(3)) were studied at the MP2∕6-311++G(d,p) level of theory. Bond extensions and redshifts were obtained for the H-bonded complexes, while bond extensions and blueshifts were obtained for the Li-bonded species. The variation of these and other properties with the basicity of the amines was investigated and rationalized by comparing the ab initio results with predictions from a model derived from perturbation theory.     

Ab initio and DFT studies of the molecular structures and vibrational spectra of succinonitrile

The Molecular structure, conformational stability and vibrational frequencies of succinonitrile NCCH2CH2CN have been investigated with ab initio and density functional theory (DFT) methods implementing the standard 6-311++G* basis set. The potential energy surfaces (PES) have been explored at DFT-B3LYP, HF and MP2 levels of theory. In agreements with previous experimental results, the molecule was predicted to exist in equilibrium mixture of trans and gauche conforms with the trans form being slightly lower in energy. The vibrational frequencies and the corresponding vibrational assignments of succinonitrile in both C2h and C2 symmetry were examined theoretically and the calculated Infrared and Raman spectra of the molecule were plotted. Observed frequencies for normal modes were compared with those calculated from normal mode coordinate analysis carried out on the basis of ab initio and DFT force fields using the standard 6-311++G* basis set of the theoretical optimized geometry. Theoretical IR intensities and Raman activities are reported.     

BeH2与HX(X=F,Cl, Br,I)形成的二氢键复合物的结构特征与本质

对BeH2与HX(X=F, Cl, Br, I)形成的二氢键复合物的结构特征及本质进行了探讨. 在MP2/6-311++G(3d,3p)水平优化、频率验证, 得到复合物的分子结构, 用分子间距离及电子密度拓扑理论确认BeH2与卤化氢已形成了二氢键型复合物. 在MP2/6-311++G(3d, 3p)水平下进行基函数重叠误差(BSSE)校正后的结合能在-14.468 kJ·mol-1到-5.464 kJ·mol-1之间.用对称匹配微扰理论(SAPT)对复合物的结合能进行分解, 结果表明, BeH2…HX二氢键复合物中静电能对总吸引能的贡献都是最主要的, 但交换排斥能、诱导能、色散能对总结合能的贡献也很重要. 从BeH2…HF到BeH2…HI, 诱导能对总吸引能的贡献从37.8%逐渐减小到24.0%. 而色散能对总吸引能的贡献从BeH2…HF体系中的16.0%逐渐增加到BeH2…HI体系中的33.8%.     

Theoretical study of the structures, stability and vibrational spectra of the nitrous acid complexes with CH4

The structures, stability and vibrational spectra of the binary complexes CH4...HONO-trans and CH4...HONO-cis have been investigated using ab initio calculations at the SCF and MP2 levels with 6-311++G(d,p) basis set and B3LYP calculations with 6-31G(d,p) and 6-31+G(d,p) basis sets. Full geometry optimization was made for the complexes studied. It was established that the complex CH4...HONO-trans is more stable by 0.41 kcal mol(-1) than the complex CH4...HONO-cis. The accuracy of the ab initio calculations have been estimated by comparison between the predicted values of the vibrational characteristics (vibrational frequencies and infrared intensities) and the available experimental data. It was established, that the methods, used in this study are well adapted to the problem under examination. The predicted values with the B3LYP calculations are very near to the results, obtained with 6-311++G(d,p)/MP2. The changes in the vibrational characteristics of methane and trans-, cis-nitrous acid upon formation of the hydrogen bond show that the complexes CH4...HONO-trans and CH4...HONO-cis have geometry in which the OH group interacts with a methane molecule forming a single hydrogen bond. This fact is confirmed by relatively strong perturbation of the OH stretching vibration to lower frequencies and an increase of the infrared intensity of this vibration up to three times upon hydrogen bonding.     

Ab initio Hartree-Fock and density functional theory study on molecular structures, energies, and vibrational frequencies of conformations of 2-hydroxy-3-nitropyridine and 3-hydroxy-2-nitropyridine

The optimized molecular structures, vibrational frequencies and corresponding vibrational assignments of conformations of 2-hydroxy-3-nitropyridine and 3-hydroxy-2-nitropyridine molecules have been investigated using ab initio Hartree-Fock (HF) and density functional theory (B3LYP) methods with 6-311++G(d,p) basis set. The comparison of the experimental and calculated spectra of the molecules have shown that they exist in two conformations with the two OH bond angles (110 degrees and 250 degrees ) respective to the CO bond in the ground state and their energy curves having two minimums have been drawn.     

H-Atom abstraction from CH(3)NHNH(2) by NO(2): CCSD(T)/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) and CCSD(T)/6-311+G(2df,p)//CCSD/6-31+G(d,p) calculations

Stationary points of paths for H atom abstraction from CH(3)NHNH(2) (monomethylhydrazine) by NO(2) were characterized via CCSD(T)/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) and CCSD(T)/6-311+G(2df,p)//CCSD/6-31+G(d,p) calculations. Five transition states connecting CH(3)NHNH(2)-NO(2) complexes to a manifold that includes CH(3)NHNH-HONO, CH(3)NNH(2)-HONO, CH(3)NNH(2)-HNO(2), and CH(3)NHNH-HNO(2) complexes were identified. Transition states that connect CH(3)NHNH-HONO, CH(3)NNH(2)-HONO, CH(3)NNH(2)-HNO(2), and CH(3)NHNH-HNO(2) complexes to each other via H atom exchange and/or hindered internal rotation were also identified. The high point in the minimum energy path from the CH(3)NHNH(2) + NO(2) reactant asymptote to the manifold of HONO-containing product states is a transition state 8.6 kcal/mol above the reactant asymptote. From a kinetics standpoint, this value is considerably higher than the 5.9 kcal/mol value that was estimated for it based on theoretical results for H atom abstraction from NH(3) by NO(2).     

A quest for triplet silylenes XHSi3 at ab initio and DFT levels (X = H, F, Cl and Br)

Four ground state triplet silylenes are found among 30 possible silylenic XHSi₃ structures (X = H, F, Cl and Br), at seven ab initio and DFT levels including: B3LYP/6-311++G^∗∗, HF/6-311++G^∗∗, MP3/6-311G^∗, MP2/6-311+G^∗∗, MP4(SDTQ)/6-311++G^∗∗, QCISD(T)/6-311++G^∗∗ and CCSD(T)/6-311++G^∗∗. The latter six methods indicate that the triplet states of 3-flouro-1,2,3-trisilapropadienylidene, 1-chloro-1,2,3-trisilapropargylene and 3-chloro-1,2,3-trisilapropargylene are energy minima. These triplets appear more stable than their corresponding singlet states which cannot even exist for showing negative force constants. Also, triplet state of 1-flouro-1,2,3-trisilapropargylene is possibly accessible for being an energy minimum, since its corresponding singlet state is not a real isomer. Some discrepancies are observed between energetic and/or structural results of DFT vs. ab initio data.     

Vibrational frequencies and infrared intensities of the hydrogen-bonded complexes of nitrous acid with ethers: ab initio and DFT studies

The vibrational spectra of the binary complexes formed by HONO-trans and HONO-cis with dimethyl and diethyl ethers have been investigated using ab initio calculations at the SCF and MP2 levels with 6-311++G(d,p) basis set and B3LYP calculations with 6-31G(d,p) and 6-31+G(d,p) basis sets. Full geometry optimisation was made for the complexes studied. The accuracy of the ab initio calculations have been estimated by comparison between the predicted values of the vibrational characteristics (vibrational frequencies and infrared intensities) and the available experimental data. It was established, that the methods, used in this study are well adapted to the problem under examination. The predicted values with the B3LYP calculations are very near to the results, obtained with 6-311++G(d,p)/MP2. The ab initio and DFT calculations show that the changes in the vibrational characteristics (vibrational frequencies and infrared intensities) upon hydrogen bonding for the hydrogen-bonded complex (CH3)2O...HONO-trans are larger than for the complex (CH3)2O...HONO-cis.