HNNH3, a new possible isomer of N2H4: An ab initio study

Ab initio studies applying the 3-21G, 6-31G, and 6-31G** basis sets and also including the MP2 correction were carried out on H₂NNH₂, HNNH₃, and the transition state of the reaction H₂NNH₂(DOUBLE BOND)HNNH₃. First, the geometries of molecules were optimized using the theoretical methods mentioned in the restricted Hartree–Fock (RHF) scheme. The energies of the molecules corresponding to RHF/6-31G** geometries were subsequently calculated including electron-correlation effects at the level of the second-order Møller–Plesset (MP2) perturbation theory. The vibrational frequencies, net charges, and dipole moments were obtained from the theoretical calculations. The results of our calculations indicate unambiguously that H₂NNH₂ is thermodynamically more stable than is HNNH₃. On the other hand, an isolated HNNH₃ molecule once created would be stable since barriers for its unimolecular isomerization and decomposition are relatively high. But HNNH₃ is unlikely to be isolated in measurable amounts because of bimolecular tautomerization. Nevertheless, HNNH₃ can be considered as an intermediate in chemical processes involving N₂H₄. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64 : 447–452, 1997     

Single‐ and double‐electron reductions of CO2 by using superalkalis: An ab initio study

CO₂, a major contributor to global warming, can be balanced by converting it into fuels. The reduction of CO₂ has been difficult due to its extremely high stability. Recently, single‐electron reduction of CO₂ by superalkalis has been proposed using quantum chemical methods. Herein, we report a systematic study on the single‐reduction of CO₂ by using typical superalkalis. Superalkalis are hypervalent species possessing lower ionization energies than alkali atoms. We have studied the interaction of CO₂ with FLi₂, OLi₃, and NLi₄ superalkalis using ab initio MP2 calculations. We notice that this interaction leads to stable superalkali‐CO₂ complexes in which the structure of CO₂ is bent due to electron transfer from superalkalis. This clearly reveals that the CO₂ can successfully be reduced to the anion. It has been also noticed that the size of superalkalis plays a crucial in the single‐electron reduction of CO₂. For instance, the binding energy of superalkali‐CO₂ complex and charge transfer to CO₂ decreases monotonically with the increase in the size of superalkali. We have also proposed that CO₂ can be further reduced to in case of the anionic complex such as (FLi₂ CO₂)‾. Thus, FLi₂ superalkali is also capable of double‐electron reduction of CO₂. These findings should provide new insights into CO₂‐activation as well as motivate further research in this direction.     

β‐Aminoacrolein: An ab initio,AIM and NBO study

The molecular structure and the intramolecular hydrogen bonding of β‐aminoacrolein and its simple derivatives were investigated at the MP2 and B3LYP levels of theory using the standard 6‐311++G(d, p) basis set. The “atoms in molecules” or AIM theory of Bader which is based on topological properties of the electron density (ρ), was used. Additionally, an analysis of the critical points was performed to study the nature hydrogen bonding in these systems. Natural bond orbital (NBO) analysis was also carried out for to better comprehend the nature of the intramolecular interactions in β‐aminoacrolein and its derivatives. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

High‐level ab initio computational study of doublet and quartet nitrogen reaction with methane

High levels of ab initio calculations were performed with the target of exploring the potential‐energy surface for the doublet and the quartet nitrogen with methane. There is a considerable difference between these two reaction paths in light of the formation of the reactant complex. Doublet nitrogen is an excited state of nitrogen, but it forms a stronger complex with methane, making the activation barrier with the doublet and quartet nitrogen reaction with methane. Activation barriers, heats of the reaction, and bond dissociation energies for minima located in the potential surface of nitrogen and methane transformations into the hydrogen radical and H₂CNH were evaluated, and the most probable reaction pathways were suggested. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 481–490, 1999     

DFT and ab initio calculations on two reactions between hydrogen atoms and the fire suppressants 2-H heptafluoropropane and CF3Br

Reaction enthalpies and barrier heights of the reactions CF3Br+H-->CF3+HBr {reaction (1)} and CF3CHFCF3+H-->CF3CFCF3+H2 {reaction (2)} have been calculated at the near state-of-the-art ab initio level, and also by employing the B3LYP, BH&HLYP, BB1K, MPW1K, MPWB1K and TPSS1KCIS functionals. In addition, the integrated molecular orbital+molecular orbital (IMOMO) method has been used to study reaction (2). The ab initio benchmark values of the reaction enthalpy (298 K) and barrier height (0 K) of reaction (2) are reported for the first time {-(0.7+/-0.7) and 13.3+/-0.5 kcal/mole respectively}. When density functional theory (DFT) results are compared with ab initio benchmarks for both reactions (1) and (2), the MPWB1K functional is found to have the best performance of the six functionals used. The IMOMO method with the RCCSD/aug-cc-pVTZ and/or RCCSD(T)/aug-cc-pVTZ levels, as the high levels of calculation on the model system, gives reaction enthalpies and barrier heights of reaction (2), which agree with ab initio benchmark values to within 1 kcal/mole. Computed key geometrical parameters and imaginary vibrational frequencies of the transition state structures of reactions (1) and (2) obtained at different levels of calculation are compared. The magnitudes of the computed imaginary vibrational frequencies of the transition states of both reactions considered are found to be very sensitive to the levels of calculation used to obtain them. The heat of formation (298 K) of CF3CFCF3 calculated at the near state-of-the-art level has a value of -(318+/-3) kcal/mole.     

Kinetic mechanism of the hydrogen abstraction reactions of the chlorine atoms with CH3CF2Cl and CH3CFCl2: a dual level direct dynamics study

The mechanisms of the reactions: CH(3)CFCl(2) + Cl (R1) and CH(3)CF(2)Cl + Cl (R2) are studied over a wide temperature range (200-3000 K) using the dual-level direct dynamics method. The minimum energy path calculation is carried out at the MP2/6-311G(d,p) and B3LYP/6-311G(d,p) levels, and energetic information is further refined by the G3(MP2) theory. The H-abstraction from the out-of-plane for (R1) is the major reaction channel, while the in-plane H-abstraction is the predominant route of (R2). The canonical variational transition-state theory (CVT) with the small-curvature tunneling (SCT) correction method is used to calculate the rate constants. Using group-balanced isodesmic reactions and hydrogenation reactions as working chemical reactions, the standard enthalpies of formation for CH(3)CFCl(2), CH(3)CF(2)Cl, CH(2)CFCl(2), and CH(2)CF(2)Cl are evaluated at the CCSD(T)/6-311 + G(3df,2p)//MP2/6-311G(d,p) level of theory. The results indicate that the substitution of fluorine atom for the chlorine atom leads to a decrease in the C-H bond reactivity with a small increase in reaction enthalpies. Also, for all reaction pathways the variational effect is small and the SCT effect is only important in the lower temperature range on the rate constants.     

On the kinetic mechanism of the hydrogen abstraction reactions of the hydroxyl radical with CH3CF2Cl and CH3CFCl2: A dual level direct dynamics study

By means of the dual‐level direct dynamics method, the mechanisms of the reactions, CH₃CF₂Cl + OH → products (R1) and CH₃CFCl₂ + OH → products (R2), are studied over a wide temperature range 200–2000 K. The optimized geometries and frequencies of the stationary points are calculated at the MP2/6‐311G(d,p) level, and then the energy profiles of the reactions are refined with the interpolated single‐point energy method at the G3(MP2) level. The canonical variational transition‐state theory with the small‐curvature tunneling (SCT) correction method is used to calculate the rate constants. For the title reactions, three reaction channels are identified and the H‐abstraction channel is the major pathway. The results indicate that F substitution has a significant (reductive) effect on hydrochlorofluorocarbon reactivity. Also, for all H‐abstraction reaction channels the variational effect is small and the SCT effect is only important in the lower temperature range on the rate constants calculation. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

A computational study of base‐catalyzed reactions between isocyanates and epoxides affording 2‐oxazolidones and isocyanurates

Title reactions were investigated with ab initio calculations. Methyl isocyanate and ethylene oxide were adopted as model reactants. The products, 2‐oxazolidones and isocyanurates, cannot be yielded without a base catalyst. The 2‐oxazolidone may be produced by a dual S_N2 reaction, where the catalyst base (e.g., Cl⁻) is a nucleophile and a leaving group on the ethylene–oxide carbon. Isocyanurate is generated by the stepwise association of three isocyanate molecules, where one of the molecules is initially linked with a base. The six‐membered ring isocyanurate is isomerized stepwise into the components isocyanate and 2‐oxazolidone. A tetrahedral type of complex between the isocyanurate and a base‐catalyzed ethylene oxide is the key intermediate for the isomerization. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 316–326, 2001     

Theoretical study of the thermolysis reaction of β‐hydroxynitriles in the gas phase

The gas‐phase thermal decomposition of 3‐hydroxypropionitrile, 3‐hydroxybutyronitrile, and 3‐hydroxy‐3‐methylbutyronitrile has been studied at the MP2/6‐31G(d) level of theory at 683.15 K and 0.06 atm. Results based both in energy and structure data seem to indicate a favorable route of decomposition via a six‐membered cyclic transition state (similar to those suggested for thermal decomposition of other related compounds, such as β‐hydroxyketones, β‐hydroxyalkenes, and β‐hydroxyalkynes) rather than a four‐membered cyclic transition state or even a quasiheterolytic pathway. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Ab initio studies of the conformers and conformational distribution of gas‐phase N,N‐dimethylaminopropanol

Systematic and extensive conformational search has been performed to characterize the gas‐phase N,N‐dimethylaminopropanol structures. A total of 91 unique trail structures were generated by allowing for all the single‐bond rotamers. All the trial structures were initially optimized at the AM1 level, and the resulting structures were optimized at the B3LYP/6‐311G* level of theory and then subjected to further optimization at the B3LYP/6‐311++G**. A total of 36 conformers are found and their zero‐point vibrational enegies, rotational constants, and dipole moments are determined. Vertical ionization energies of 11 low‐lying conformers predicted with the electron propagator theory are in good agreement with the experimental data. The two most stable conformers display intramolecular H bonds (HBs): OH···N. These HBs influence on the molecular electronic structures is exhibited by natural bond orbital analyses. Combined with statistical mechanics principles, conformational distributions at various temperatures are computed and the temperature dependence of photoelectron spectra is interpreted. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

A theoretical ab initio study on the H(2)NO + O(3) reaction

The deviation of the NH(2) pseudo-first-order decay Arrhenius plots of the NH(2) + O(3) reaction at high ozone pressures measured by experimentalists, has been attributed to the regeneration of NH(2) radicals due to the subsequent reactions of the products of this reaction with ozone. Although these products have not yet been characterized experimentally, the radical H(2)NO has been postulated, because it can regenerate NH(2) radicals through the reactions: H(2)NO + O(3) --> NH(2) + O(2) and H(2)NO + O(3) --> HNO + OH + O(2). With the purpose of providing a reasonable explanation from a theoretical point of view to the kinetic observed behaviour of the NH(2) + O(3) system, we have carried ab initio electronic structure calculations on both H(2)NO + O(3) possible reactions. The results obtained in this article, however, predict that of both reactions proposed, only the H(2)NO + O(3) --> NH(2) + O(2) reaction would regenerate indeed NH(2) radicals, explaining thus the deviation of the NH(2) pseudo-first-order decay observed experimentally.     

Carbon‐Centered Radical Addition and β‐Scission Reactions: Modeling of Activation Energies and Pre‐exponential Factors

A consistent set of group additive values ΔGAV° for 46 groups is derived, allowing the calculation of rate coefficients for hydrocarbon radical additions and β-scission reactions. A database of 51 rate coefficients based on CBS-QB3 calculations with corrections for hindered internal rotation was used as training set. The results of this computational method agree well with experimentally observed rate coefficients with a mean factor of deviation of 3, as benchmarked on a set of nine reactions. The temperature dependence on the resulting ΔGAV°s in the broad range of 300–1300 K is limited to ±4.5 kJ mol⁻¹ on activation energies and to ±0.4 on logA (A: pre-exponential factor) for 90 % of the groups. Validation of the ΔGAV°s was performed for a test set of 13 reactions. In the absence of severe steric hindrance and resonance effects in the transition state, the rate coefficients predicted by group additivity are within a factor of 3 of the CBS-QB3 ab initio rate coefficients for more than 90 % of the reactions in the test set. It can thus be expected that in most cases the GA method performs even better than standard DFT calculations for which a deviation factor of 10 is generally considered to be acceptable.     

Electron-spin magnetic moments of the 2Σ+ ions Li2+, Li2−, and Be2+: An ab initio ROHF study

For the ²Σ⁺ ground states of the ions Li₂⁺, Li₂⁻, and Be₂⁺, the dependence of the magnetic moment (parametrized by g-shifts) on the bond length R was studied at the ROHF level. The Δ g-values were calculated via a perturbative approach (complete to second order in Breit-Pauli interactions) using quadruple-zeta AO basis sets augmented by semidiffuse and polarization functions. All Δ g-values in these systems are negative. The parallel component Δ g_∥ generally changes little with R, remaining close to the g-shift of the corresponding ²S atomic dissociation product. For Li₂⁺ and Be₂⁺, the perpendicular component Δ g _⟂ is more sensitive to geometry than is Δ g_∥, mainly because of the second-order magnetic coupling with excited ²Π states. For Li₂⁻, Δ g _⟂ and Δ g_∥ are similar due to the large size of the 2σ_u, SOMO, resulting in g-values close to that of a free electron. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 511–521, 1997     

Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas‐Phase Radical–Radical Reaction O(3P)+(CH3)2CH→C3H6+OH

This paper reports on the gas‐phase radical–radical dynamics of the reaction of ground‐state atomic oxygen [O(³P), from the photodissociation of NO₂] with secondary isopropyl radicals [(CH₃)₂CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O(³P)+(CH₃)₂CH→C₃H₆ (propene)+OH, is examined by high‐resolution laser‐induced fluorescence spectroscopy in crossed‐beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X²Π) products with low‐ and high‐N′′ components in a ratio of 1.25:1. No significant spin–orbit or Λ‐doublet propensities are exhibited in the ground vibrational state. Ab initio computations at the CBS‐QB3 theory level and comparison with prior theory show that the statistical method is not suitable for describing the main reaction channel at the molecular level. Two competing mechanisms are predicted to exist on the lowest doublet potential‐energy surface: direct abstraction, giving the dominant low‐N′′ components, and formation of short‐lived addition complexes that result in hot rotational distributions, giving the high‐N′′ components. The observed competing mechanisms contrast with previous bulk kinetic experiments conducted in a fast‐flow system with photoionization mass spectrometry, which suggested a single abstraction pathway. In addition, comparison of the reactions of O(³P) with primary and tertiary hydrocarbon radicals allows molecular‐level discussion of the reactivity and mechanism of the title reaction.     

Compton profiles and polarizability as two similar probes of the electronic structure of 14 electron diatomic molecules: An ab initio study

High‐ and low‐energy scattering properties, namely, Compton profiles and polarizability, respectively, were calculated at the configuration interaction (CI) level from molecular orbitals expressed in the linear combination of atomic orbitals (LCAO) model for 14 electron diatomic molecules. Extended atomic basis sets including about 100 Gaussian‐type functions (GTFs) were used. The isotropic and directional Compton profiles and the polarizability show that the behavior of CO and N₂ is rather similar but very different from that of BF. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 63–74, 1999