Theoretical study of the mechanism for the ClOO + NO reaction on the singlet potential energy surface

A detailed quantum chemical study is performed on the mechanism of ClOO + NO reaction at the B3LYP/6-311+G (2d) level of theory combined with CCSD (T) single point energy calculation. The possible product channels for the reaction are obtained and discussed on the basis of the singlet [ClNO₃] potential energy surface. The calculation indicates that the dominant product for the title reaction is ClO + NO₂ by the direct dissociation of the initial adduct, and the formation of the other products is much less likely since they are unfavorable kinetically. A comparison is also made between the title reaction and the analogous reaction of FO₂ + NO to gain a deeper insight into the mechanism of the XO₂ + NO reactions.     

Computational mechanistic investigation of the gas phase C2H4 + CO reaction on the singlet and triplet potential energy surfaces

Reaction pathways of ethylene and carbon monoxide on the singlet and triplet potential energy surfaces (PESs) have been calculated at B3LYP/6-311++G (3df, 3dp), G3B3 and CCSD(T)//B3LYP levels. Reaction mechanisms have been investigated by analysis of various structures. Suggested reaction mechanisms reveal that ³P3(CH₂CHCHO) and ³P4(CH₃CCHO) are thermodynamically stable adducts with the negative value in Gibbs free energies on the triplet PES. In addition, results show that one intersystem crossing exists between triplet and singlet PESs, which are obtained by scanning of the C–C bond length in ¹IN3 and ³IN7 species.     

Theoretical studies on the potential energy surfaces and vibrational energy levels of HXeF and HXeCl

The potential energy surfaces for the electronic ground state of the HXeCl and HXeF molecules areconstructed by using the internally contracted multi-reference configuration interaction with theDavidson correction(icMRCI Q)method and large basis sets.The stabilities and dissociation barriersare identified from the potential energy surfaces.The three-body dissociation channel is found to bethe dominate dissociation channel for HXeCl,while two dissociation channels are possible and com-petitive for HXeF.Based on the obtained potentials,vibrational energy levels of HXeCl and HXeF arecalculated using the Lanczos algorithm.Our theoretical results are in good agreement with the avail-able observed values.Particularly,the calculated fundamental frequency of the H—Xe stretching vi-bration including the Xe matrix effect of HXeCl is found to be 1666.6 cm-1,which is only 17.6 cm-1higher than the recently observed value of 1649 cm-1.     

Theoretical study of some bis-verdazyl diradicals: singlet–triplet energy gap

The bis-verdazyl diradical (BVD) system is closely examined by using the multiconfiguration wavefunctions as well as the density functional theory (DFT). The totally symmetric singlet ground state turns out to have strong multiconfiguration character at all levels of theory. The singlet ground state takes on the planar structure while the most stable triplet state corresponds to the twisted form. The MCSCF+MCQDPT2 calculations are shown to be sufficient to predict the singlet–triplet energy gap which is insensitive to the electronic characters of the ring substituents.     

Radical reaction HCNO + 3NH: a mechanistic study

The complex triplet potential energy surface for the reaction of HCNO with NH is investigated at the G3B3 level using the B3LYP/6-311++G(d,p), and QCISD/6-311++G(d,p) geometries. Various possible isomerization and dissociation pathways are probed. The initial association between HCNO and NH is found to be carbon to nitrogen attack leading to HNCHNO 2a, which can convert to 2b, 2c, and 2d. Subsequently, 1,4-H-shift of 2a to form NCHNOH 3a followed by dissociation to P ₂ (¹HCN + ³HON) is the most feasible pathway. Much less competitively, 2d undergoes successive 1,3-H-shift and C-N cleavage to form HNCNOH 8b, and then to product P ₃ (¹HNC + ³HON), the second feasible pathway. 8b can alternatively isomerize to 8c followed by N–O bond rupture to generate P ₆ (²OH + ²HNCN), the lesser followed feasible pathway. In addition, 2b takes continuously 1,3- and 1,2-H-shift to form NC(H)NHO 6a, then to ONHCNH 7a which can convert to 7b. Eventually, 7b may take C-N bond fission to produce P ₅ (¹HNC + ³HNO), the least feasible pathway. The present paper may be helpful for future experimental identification of the product distributions for the title reaction, and may be helpful to deeply understand the mechanism of the title reaction.     

Quantum-chemical study of the singlet potential energy surface of the nitrene—dioxygen system

The potential energy surfaces of the HN—O₂ and PhN—O₂ systems were calculated by the MP2 and B3LYP methods. The mechanism of photooxidation of azides was refined. Photooxidation produces the nitrene—O₂ adducts with dioxaziridine and non-cyclic structures. The parameters of IR spectra of the adducts were calculated. The rearrangement of dioxaziridine to a nitro compound is likely a reason for chemiluminescence accompanying the photooxidation of azides.     

Theoretical Studies on Reaction Mechanisms of HNCS with NH(X~3Σ)

IntroductionThe isothiocyanic acid(HNCS)is a sulfur analogof the well-known isocyanic acid HNCO[1,2].Becausethe latter species plays an important role in both atmos-pheric and combustion processes,it has attracted con-siderable attention[3—8].For example…     

Theoretical study on the reaction mechanism of(CH_3)_3CO+CO

The global environment pollution includes pho-tochemical smog, acid rain and stratospheric ozonedepletion. The short-lived species/radicals in atmos-phere are closely related to these phenomena. Theshort-lived species/radicals bring the photochemicalsmog,…     

Influence of collision energy on the dynamics of the reaction H (2S) + NH (X3Σ−) → N (4S) + H2 (X1Σ g + ) by the state-to-state quantum mechanical study

The conversion of cholesterol to pregnenolone is a physiologically essential process which initiates with two sequential hydroxylation processes catalyzed by cytochrome P450 side-chain cleavage enzyme (P450_SCC). Extensive efforts have been exerted; however, the mechanistic details remain obscure. In this work, we employed the dispersion-corrected density functional theoretical (DFT-D) calculations to investigate the mechanistic details of such hydroxylation processes. Calculated results reveal that the active intermediate Compound I (CpdI) of P450_SCC hydroxylates cholesterol efficiently, which coincides with previous spectrometric observations. The hydrogen bond effect of water molecule within the active site lowers the energy barrier significantly. Intriguingly, the adjacent hydrogen bond (H-bond) between the hydroxyl group of the substrate and the oxo group of CpdI in the second hydroxylation affects the H-abstraction significantly. Such H-bond was weakened during the C–H bond activation process, increasing the energy barriers by approximately 2 kcal/mol, which is different to the intermolecular H-bond effect of water₉₀₃ found by Shaik et al. that decreases the barrier by about 4 kcal/mol. Such adjacent H-bond also affects the transition state by bending the alignment of the C–H–O moiety, and consequently lowering the kinetic isotope effect values. Besides, a series of DFT-D calculations (Grimme’s D2, D3-zero, and D3-BJ methods) were performed and accessed to find out an appropriate protocol for H-bond containing hydroxylation process. Our results show that DFT-D single-point energies (SPE) based on geometries optimized with non-dispersion-corrected DFT varies drastically and sometime presents unreasonable results. DFT-D SPE calculations on DFT-D optimized geometries present stable and reasonable results.     

Global ab initio potential energy surfaces for the O2(3Σg-)+O2(3Σg-) interaction

Completely ab initio global potential energy surfaces (PESs) for the singlet and triplet spin multiplicities of rigid O(2)((3)Σ(g)(-))+O(2)((3)Σ(g)(-)) are reported for the first time. They have been obtained by combining an accurate restricted coupled cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)] quintet potential [Bartolomei et al., J. Chem. Phys. 128, 214304 (2008)] with complete active space second order perturbation theory (CASPT2) or, alternatively, multireference configuration interaction (MRCI) calculations of the singlet-quintet and triplet-quintet splittings. Spherical harmonic expansions, containing a large number of terms due to the high anisotropy of the interaction, have been built from the ab initio data. The radial coefficients of these expansions are matched at long range distances with analytical functions based on recent ab initio calculations of the electric properties of the monomers [M. Bartolomei, E. Carmona-Novillo, M. I. Hernández, J. Campos-Martínez, and R. Hernández-Lamoneda, J. Comput. Chem. (2010) (in press)]. The singlet and triplet PESs obtained from either RCCSD(T)-CASPT2 or RCCSD(T)-MRCI calculations are quite similar, although quantitative differences appear in specific terms of the expansion. CASPT2 calculations are the ones giving rise to larger splittings and more attractive interactions, particularly in the region of the absolute minima (in the rectangular D(2h) geometry). The new singlet, triplet, and quintet PESs are tested against second virial coefficient B(T) data and, their spherically averaged components, against integral cross sections measured with rotationally hot effusive beams. Both types of multiconfigurational approaches provide quite similar results, which, in turn, are in good agreement with the measurements. It is found that discrepancies with the experiments could be removed if the PESs were slightly more attractive. In this regard, the most attractive RCCSD(T)-CASPT2 PESs perform slightly better than the RCCSD(T)-MRCI counterpart.     

Nodal surfaces of the wave functions of the hydrogen molecule in the triplet state 3Σu +

For molecular hydrogen in the triplet state ³Σ_u ⁺, the nodal surfaces of the wave function corresponding to the minimum basis set of Slater orbitals in the Hartree—Fock approximation and those of the wave function used in calculations by the diffusion quantum Monte Carlo method were plotted and analyzed. Taking account of the condition for antisymmetrical wave function of the triplet state ³ S of He atom, the Hartree-Fock approximation in the minimum basis set of one-electron orbitals is inappropriate for a priori determination of the nodal surfaces of many-electron wave functions (MWF). An MWF quantum chemical method developed by the authors is outlined. The alternative nodal surfaces for H₂ (³Σ_u ⁺) a priori specified in this method are presented.     

Theoretical investigation of the CS + OH → Products reaction on an interpolated potential energy surface: reaction dynamic and chemical kinetic insights

Structural Chemistry - An interpolated potential energy surface to describe the gas phase CS+OH reaction has been constructed by quantum chemical ab-initio data. Quasi-classical trajectory...     

The potential energy curves and probability density distributions of the X1Σ+ and A1Σ+ states of RBH

The potential energy curves PMO—RKR—van der Waals of the electronic A¹Σ⁺ and X₁Σ⁺ states of RbH have been determined. The potentials obtained are self-consistent with the experimental data because they have been tested by direct numerical solution of the radial Schrödinger equation. From exact vibrational eigenfunctions probability density distributions and Franck—Condon factors have been calculated over the range of vibrational levels observed. It is observed that the anomalous behaviour of the A¹Σ⁺ state arises in the υ′ = 1, 2 and 3 levels with probability density functions similar to those of a harmonic oscillator.     

Theoretical study on the mechanism of C2Cl3 + NO2 reaction

The radical-molecule reaction of C₂Cl₃ with NO₂ is explored at the B3LYP/6-311G(d,p) and CCSD(T)/6-311+G(d,p) (single-point) levels. On the singlet potential energy surface (PES), the association between C₂Cl₃ and NO₂ is found to be carbon-to-nitrogen attack forming the adduct C₂Cl₃NO₂ (1) without any encounter barrier, followed by isomerization to C₂Cl₃ONO (2). Starting from 2, the most feasible pathway is the N–O1 bond cleavage which lead to P ₁ (C₂Cl₃O + NO). Much less competitively, 2 transforms to the three-membered ring isomer c-OCCl₂C–ClNO (4 ^a ) which can easily interconvert to c-OCCl₂C–ClNO 4 ^b . Then 4 (4 ^a , 4 ^b ) takes direct C1–C2 and C2–O1 bonds cleavage to give P ₂ (COCl₂ + ClCNO). The lesser competitive channel is the 4 ^a isomerizes to the four-membered ring intermediate O-c-CNClOCCl₂ (5) followed by dissociation to P₃ (CO + ClNOCCl₂). The concerted 1,2-Cl shift along with C1–O1 bond rupture of 4 ^b to form ONC(O)CCl₃ (6) followed by dissociation to P ₄ (ClNO + OCCCl₂) is even much less feasible. Moreover, some of P ₃ and P ₄ can further dissociate to P ₅ (ClNO + CO + CCl₂). Compared with the singlet pathways, the triplet pathways may have less contribution to the title reaction. Our results are in marked difference from previous theoretical studies which showed that two initial adducts C₂Cl₃–NO₂ and C₂Cl₃–ONO are obtained. Moreover, in the present paper we focus our main attentions on the cyclic isomers in view of only the chain-like isomers are considered by previous studies. The present study may be helpful for understanding the halogenated vinyl chemistry.     

New ab initio potential energy surface and quantum dynamics of the reaction H(2S) + NH(X3Σ-) → N(4S) + H2

A new global potential energy surface is reported for the ground state ((4)A(")) of the reaction H((2)S) + NH(X(3)Σ(-)) → N((4)S) + H(2) from a set of accurate ab initio data, which were computed using the multi-reference configuration interaction with a basis set of aug-cc-pV5Z. The many-body expansion and neural network methods have been used to construct the new potential energy surface. The topographical features of the new global potential energy surface are presented. The predicted barrier height is lower than previous theoretical estimates and the heat of reaction with zero-point energy is closer to experimental results. The quantum reactive scattering dynamics calculation was carried out over a range of collision energies (0-1.0 eV) on the new potential energy surface. The reaction probabilities, integral cross-section, and rate constants for the title reaction were calculated. The calculated rate constants are in excellent agreement with the available experimental results.     

Effect of internal and translational energy on the NH3+(ν) + D2 ion-molecule reaction

Using 2+1 multiphoton ionization, NH₃⁺ is prepared in selected levels of the ν₂ bending mode (ν = 0−7) and the NH₃⁺(ν) + D₂ reaction is studied as a function of the center-of-mass collision energy (1–10 eV). The exchange channel (NH₂D⁺ + HD or H + D) is enhanced by ion vibrational excitation whereas the addition channel (NH₃D⁺ + D) is almost unaffected.     

Theoretical study on the reaction mechanism of (CH_3)_3CO~· radical with NO

Alkoxy radicals RO? form an important class of hydrocarbon oxidation intermediates in combustion processes, interstellar and atmospheric chemistry[1—8]. The number of NO to NO2 conversions can take place during the oxidation of the parent hydrocarbon in the atmosphere and hence affecting tropospheric ozone production. The experimental and theoretical investi-gations of the reactions of alkoxy radicals will be ad-vantageous for safeguarding the environment. Despite the importance of alkoxy r…     

Theoretical study on the reaction mechanism of NH2(-) with O2 (a1Δg)

A detailed theoretical study of the potential energy surface of poorly understood ion-molecule reaction of NH(2)(-) and O(2) (a(1)Δ(g)) is explored at the density functional theory B3LYP/6-311++G(d,p), ab initio of QCISD/6-311++G(d,p) and CCSD(T)/6-311++G(3df, 2pd) (single-point) theoretical levels for the first time. It is shown that there are six total possible products from P(1) to P(6) on the singlet potential energy surface. Among these, the charge-transfer product P(1) (NH(2) + O(2)(-)) is the most favorable product with predominant abundances, whereas P(4) (NO(-) + H(2)O) and P(2) (HNO + OH(-)) may be the second and third feasible products followed by the almost neglectable P(3) (NO(2)(-) + H(2)), while P(5) (c-NO(2)(-) + H(2)) and P(6) (ONO(-) + H(2)) will not be observed due to their either high barriers or being secondary products. The present theoretical study points out that besides P(1) (NH(2) + O(2)(-)) and P(2) (HNO + OH(-)), P(4) (NO(-) + H(2)O) should be also observed, which is different from the previous experiment study by Anthony Midey et al. in 2008. In addition, almost all of the reaction pathways to products are exothermic and the reaction rate should be very fast since the reaction barriers are very low except for P(5) (c-NO(2)(-) + H(2)) which is in agreement with the measured total reaction rate constant k = 9.0 × 10(-10) cm(3)s(-1) at 300 K in the experiment study. It is expected that the present theoretical study may be helpful for the understanding of the reaction mechanism related to NHX(-), NX(2)(-), PHX(-), and PX(2)(-) (X = H, F, and Cl).     

Self-consistent potential energy curves for the A1Σ+ and X1Σ+ states of isotopic cesium hydrides

From the spectroscopic experimental data available in the literature we have determined the mass-reduced Dunham coefficients for the A¹Σ⁺ ↔ X¹Σ⁺ system of the isotopic species CsH and CsD. Based upon these results, for both ground and excited states of cesium hydride and deuteride we report new hybrid rotationless potential energy curves (PMO-RKR-van der Waals) up to the dissociation. As a consistency check on the accuracy of the potentials the eigenvalues were calculated by direct numerical integration of the radial Schrödinger equation and found to agree within the rms error 0.39 cm⁻¹ (X¹Σ⁺) and 0.41 cm⁻¹ (A¹Σ⁺) with the experimental vibrational energies. From the wavefunctions, the rotational constants B_υ, centrifugal distortion terms D_υ, H_υ and L_υ, Franck—Condon factors, and probability density distributions were obtained. The probability density distributions for the lowest vibrational levels of the A¹Σ⁺ show an anharmonicity associated with the anomalous behavior of that state.     

Ab initio study on mechanisms of singlet oxygen reaction with ethylene

The reaction of singlet oxygen with ethylene has been studied at the ab initio level with both HF/3-21G and HF/6-31G basis sets, fully optimizing the geometries of the critical points. The transition state leading to the intermediate peroxirane from the initial reactants is found,Iocated at 81.71 kJ/mol above the dissociation limit. The vibrational analysis is done with two basis sets. From the results it can be seen that the frequencies have not been made an improvement obviously with augmented 6-31G compared to 3-21G basis; it follows that main reason for too high HF/3-21G frequency could mainly be the vibrational anharmonicity. The eigenvector corresponding to the single imaginary vibrational frequency is dominated by the larger O-O distance. The finding of the transition state confirms that the peroxirane minimum can be reachable passing through a peroxirane-like saddle point. In addition , the mechanisms of the reaction forming oxirane are discussed as well.