Doublet Potential Energy Surface Research Articles

Formation of nitriles and imines in the atmosphere of Titan: combined crossed-beam and theoretical studies on the reaction dynamics of excited nitrogen atoms N(2D) with ethane

The dynamics of the H-displacement channels in the reaction N(2D) + C2H6 have been investigated by the crossed molecular beam technique with mass spectrometric detection and time-of-flight analysis at two different collision energies (18.0 and 31.4 kJ mol(-1)). From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms and the product energy partitioning have been obtained. The interpretation of the scattering results is assisted by new ab initio electronic structure calculations of stationary points and product energetics for the C2H6N ground state doublet potential energy surface. C-C bond breaking and NH production channels have been theoretically characterized and the statistical branching ratio derived at the temperatures relevant for the atmosphere of Titan. Methanimine plus CH3 and ethanimine plus H are the main reaction channels. Implications for the atmospheric chemistry of Titan are discussed.

Theoretical study of B3O radical isomers and their interconversion pathways

Despite the fact that B3O is the second simplest B n O radical after BO, a controversy recently emerged concerning the molecular structure of its global minimum. Two recent theoretical groups predicted the linear quartet BBBO to be the ground isomer. By contrast, another recent theoretical group reported that B3O has a doublet B3-ring ground structure. Moreover, larger B n O clusters usually have low-lying B3-ring isomers. In order to determine the accurate energetic competition between linear and cyclic structures in both the doublet and quartet, and to understand the detailed isomerism between various isomers, which is vital for understanding the formation mechanism of B3O, we report the first potential energy surface (PES) study of B3O at various computational levels, including CCSD(T)/6-311+G(2df), CCSD(T)/aug-cc-pVTZ, CCSD(T)/aug-cc-pVQZ and G3B3 for the single-point energy, as well as B3LYP/6-311+G(d) and QCISD/6-311+G(d) for geometrical optimisation. It is shown that the isomers in the quartet state are all thermodynamically more stable than the corresponding doublet ones, and on both the quartet and doublet PESs, the linear form has the lowest energy. Therefore, our study on both linear and cyclic isomers shows that the linear quartet BBBO 4 01 is definitively the ground isomer. Although being much less stable than the quartet linear BBBO global minimum by >20 kcal mol−1, five cyclic isomers exist as local minima, with the bi-cyclic structure 4 02 possessing the smallest barrier of around 15 kcal mol−1. The dissociation energies for direct combination processes B3 + O, B2 + BO and B + B2O are discussed. The present work may be helpful in obtaining a deep understanding of the doping and oxidation process of pure B n clusters.

A comparative study of hydrogen-atom release dynamics in radical–radical reactions

This paper presents a comparative study of the atomic hydrogen release dynamics in the oxidation reactions of saturated and unsaturated hydrocarbon radicals. The prototypal radical–radical reactive scattering processes were examined by applying a combination of crossed beams and high-resolution vacuum-ultraviolet laser-induced fluorescence spectroscopy. With the aid of ab initio and statistical calculations, remarkable dynamic features were uncovered in the Doppler profile analysis of the atomic hydrogen products. The difference in the reactivity of saturated and unsaturated hydrocarbon radicals could be rationalized in terms of the characteristic geometry of the transition state and activation barriers along the exit channel on the doublet potential energy surface.

Are isomers of the vinyl cyanide ion missing links for interstellar pyrimidine formation?

In the interstellar medium (ISM) there are many regions where the formation of molecules is kinetically driven rather than thermochemically, which can lead to the formation of many isomers even though some may be fairly higher in energy relative to the molecular global minimum. Recent laboratory experiments where noble gas cations are reacted with pyrimidine favored the formation of C(3)H(3)N(+), but the molecular structure(s) of this fragment was not determined. Microscopic reversibility means that pyrimidine could form under interstellar conditions should the required C(3)H(3)N(+) reactant be detected in the ISM. Hence C(3)H(3)N(+) could be a strong candidate for involvement in the formation of heterocyclic biomolecules such as pyrimidine in the ISM. In this study, we have investigated the low energy isomers of the acrylonitrile ion (C(3)H(3)N(+)) using density functional theory as well as high levels of ab initio theory, namely, the singles and doubles coupled-cluster theory that includes a perturbational correction for connected triple excitations, denoted as CCSD(T). An automated stochastic search procedure, Kick, has been employed to find isomers on the ground state doublet potential energy surface. Several new structures, along with all the previously reported minima, have been found. The global minimum H(2)CCCNH(+) is energetically much lower than either H(2)CC(H)CN(+), the acrylonitrile ion, or HCC(H)NCH(+), the most likely intermediate of the reaction between HCCH(+) and HCN. These isomers are connected to the global minimum via several transition states and intermediates. The results indicate that not only the global minimum but also several higher energy isomers of the C(3)H(3)N(+) ion could be important in interstellar pyrimidine formation. The isomeric molecules have the necessary CCNC backbone needed for the reaction with HCN to form the cyclic pyrimidine framework. The structural and rotational parameters of all the isomers studied in this work have been predicted at the CCSD(T) level of theory with the anticipation that it will expedite their laboratory as well as astronomical identification.

Theoretical study on the gas‐phase reaction mechanism between rhodium monoxide and methane for methanol production

The gas-phase reaction mechanism between methane and rhodium monoxide for the formation of methanol, syngas, formaldehyde, water, and methyl radical have been studied in detail on the doublet and quartet state potential energy surfaces at the CCSD(T)/6-311+G(2d, 2p), SDD//B3LYP/6-311+G(2d, 2p), SDD level. Over the 300-1100 K temperature range, the branching ratio for the Rh((4)F) + CH(3)OH channel is 97.5-100%, whereas the branching ratio for the D-CH(2)ORh + H(2) channel is 0.0-2.5%, and the branching ratio for the D-CH(2)ORh + H(2) channel is so small to be ruled out. The minimum energy reaction pathway for the main product methanol formation involving two spin inversions prefers to both start and terminate on the ground quartet state, where the ground doublet intermediate CH(3)RhOH is energetically preferred, and its formation rate constant over the 300-1100 K temperature range is fitted by k(CH3RhOH) = 7.03 x 10(6) exp(-69.484/RT) dm(3) mol(-1) s(-1). On the other hand, the main products shall be Rh + CH(3)OH in the reactions of RhO + CH(4), CH(2)ORh + H(2), Rh + CO +2H(2), and RhCH(2) + H(2)O, whereas the main products shall be CH(2)ORh + H(2) in the reaction of Rh + CH(3)OH. Meanwhile, the doublet intermediates H(2)RhOCH(2) and CH(3)RhOH are predicted to be energetically favored in the reactions of Rh + CH(3)OH and CH(2)ORh + H(2) and in the reaction of RhCH(2) + H(2)O, respectively.

Combined Crossed Molecular Beam and Theoretical Studies of the N(2D) + CH4 Reaction and Implications for Atmospheric Models of Titan

The dynamics of the H-displacement channel in the reaction N((2)D) + CH(4) has been investigated by the crossed molecular beam (CMB) technique with mass spectrometric detection and time-of-flight (TOF) analysis at five different collision energies (from 22.2 up to 65.1 kJ/mol). The CMB results have identified two distinct isomers as primary reaction products, methanimine and methylnitrene, the yield of which significantly varies with the total available energy. From the derived center-of-mass product angular and translational energy distributions the reaction micromechanisms, the product energy partitioning and the relative branching ratios of the competing reaction channels leading to the two isomers have been obtained. The interpretation of the scattering results is assisted by new ab initio electronic structure calculations of stationary points and product energetics for the CH(4)N ground state doublet potential energy surface. Differently from previous theoretical studies, both insertion and H-abstraction pathways have been found to be barrierless at all levels of theory employed in this work. A comparison between experimental results on the two isomer branching ratio and RRKM estimates, based on the new electronic structure calculations, confirms the highly nonstatistical nature of the N((2)D) + CH(4) reaction, with the production of the CH(3)N isomer dominated by dynamical effects. The implications for the chemical models of the atmosphere of Titan are discussed.

Spin-Flip Reaction of Re + CH4—A Relativistic Density Functional Theory Investigation

To explore the reaction mechanisms of methane dehydrogenation by gas-phase Re atom, the sextet, quartet, and doublet potential energy surfaces have been performed using density functional theory (DFT) and zero-order regular approximation relativistic corrections at the PW91/TZ2P level. The minimum energy reaction path is found to proceed through the following steps: (6)Re + CH(4) --> ReCH(4) ((6)1) --> H(3)CReH ((4)2) --> (4)TS2/3 --> H(2)CReH(2) ((4)3) --> (2)TS3/4 --> HCReH(3) ((2)4). Also, the reaction path involves the spin inversion twice in the different reaction steps. To better understand the spin inversion processes, the low energy crossing point is determined with the help of the density functional fractional occupation number approach. The first spin inversion, from the sextet state to the quartet state, makes the activation of the C-H bond energetically spontaneous. The second transition from the quartet state to the doublet state facilitates the cleavage of the second C-H bond, lowering the barrier from 186.1 to 24.2 kJ/mol. The overall reaction is calculated to be exothermic by 149.8 kJ/mol, and the final products in three spin states are investigated by NBO analysis, to compare the Re-C bonds and the C-H bonds.

Interaction of Ga3 cluster with molecular hydrogen: combined DFT and CCSD(T) theoretical study

We have explored the lowest doublet and quartet potential energy surfaces (PES) for the reaction of gallium trimer with H2. This reaction was studied experimentally by Margrave and co-workers in a noble gas matrix. The detailed reaction paths ending up with the low-energy Ga3H2 hydride isomers have been predicted based on the high level ab initio coupled-cluster calculations (CCSD(T)) with large basis set. We have found that the reaction occuring on the lowest doublet PES is described by the activation barrier for H2 cleavage of about 15 kcal/mol, consistent with experiment. In the most stable Ga3H2 hydride structure, whose formation is exothermic by 15 kcal/mol, both H atoms assume three-fold bridged positions. The diterminal planar structure of Ga3H2, proposed experimentally from the observed IR spectra, is found to be only 1 kcal/mol less stable than the dibridged form.

Theoretical study on the reactions of CF/CCl with O 2

The reaction mechanisms of CF and CCl with O 2 have been investigated theoretically on the doublet potential energy surfaces (PESs). The QCISD/6-311++G(d,p) single-point calculations are performed using the B3LYP/6-311++G(d,p) optimized geometries to accurately determine the energetics. The two product channels, F + CO 2 and FCO + O, have been found in the CF + O 2 reaction. The former is more competitive because the formation pathway of it undergoes fewer reaction steps and lower barriers than that of FCO + O. In the case of the CCl + O 2 reaction, the two similar channels of Cl + CO 2 and ClCO + O have also been identified, and the first is less preferable. The title reactions in question are of practical significance in high-temperature processes, because the initial associations are barrier-present. This paper also discusses the effects of halogenations on the reactivity, the reaction pathways, the product channels and the reaction enthalpies of the three CX + O 2 reactions (X = H, F or Cl). The reactivity of CX + O 2 decreases in the order of CH, CF and CCl.

Theoretical study of the mechanism for C-H bond activation in spin-forbidden reaction between Ti+ and C2H4

The mechanism of the spin-forbidden reaction Ti+(4F, 3d24s1) + C2H4→TiC2H2 + (2A2) + H2 on both doublet and quartet potential energy surfaces has been investigated at the B3LYP level of theory. Crossing points between the potential energy surfaces and the possible spin inversion process are discussed by means of spin-orbit coupling (SOC) calculations. The strength of the SOC between the low-lying quartet state and the doublet state is 59.3 cm−1 in the intermediate complex IM1-4B2. Thus, the changes of its spin multiplicity may occur from the quartet to the doublet surface to form IM1-2A1, leading to a sig-nificant decrease in the barrier height on the quartet PES. After the insertion intermediate IM2, two distinct reaction paths on the doublet PES have been found, i.e., a stepwise path and a concerted path. The latter is found to be the lowest energy path on the doublet PES to exothermic TiC2H2 +(2A2) + H2 products, with the active barrier of 4.52 kcal/mol. In other words, this reaction proceeds in the following way: Ti++C2H4→4IC→IM1-4B2→4,2ISC→IM1-2A1→[2TSins]→IM2→[2TSMCTS]→IM5→TiC2H2 +(2A2)+H2.

Theoretical study of the reaction of Mo + with SCX (X=S, O) in gas phase

Density functional theory (DFT) calculations have been performed to explore the sextet, quartet, and doublet potential energy surfaces of C –X (X=S, O) bond activation in CS 2 and SCO molecules by gas-phase Mo + cation, in order to better understanding the reaction mechanism of second-row metal cations reacting with SCX (X=S, O). The minimum energy reaction path is found to involve the spin inversion in the different reaction steps. The crossing points (CPs) of the different potential energy surfaces (PESs) have been localized with the approach suggested by Yoshizawa et al. [K. Yoshizawa, Y. Shiota, T. Yamabe, J. Chem. Phys. 111 (1999) 538]. The potential energy curve-crossing dramatically affects reaction mechanism. The present results show that the reaction mechanism is insertion–elimination mechanism both along the C–S and C–O bond activation branches, but the C–S bond activation is much more favorable than the C–O bond activation in energy. All theoretical results not only support the existing conclusions inferred from early experiment, but also complement the pathway and mechanism for this reaction.

An ab initio and density functional theory study on the mechanism for the reaction of OH with 2-ethylfuran

The mechanism of the reaction of OH + 2-ethylfuran has been investigated using the G3MP2 and G3MP2B3 levels of theory. The geometric parameters of all species involved in the reaction have been optimized at the MP2 and B3LYP levels of theory with 6-311G(d,p) basis set. The overall profile of doublet potential energy surface (PES) for the OH + 2-ethylfuran reaction has been constructed using the G3MP2 and G3MP2B3 methods. The results show that the addition-elimination mechanism dominates the OH + 2-ethylfuran reaction and the major products are CH3CH2C(OH)CHCHCHOH (P8) and CH3CH2COCHCHCHOH (P6).

Theoretical investigations of the cyanogen anion

Large ab initio calculations are performed on neutral cyanogen (C 2N 2) and its negatively charged ion ( C 2 N 2 - ). Four stable isomers are found on the lowest doublet potential energy surface of the anion: trans NCCN − X ∼ 2B u, trans CNCN − X ∼ 2A′, trans CNNC − X ∼ 2B u and trans NNCC − X ∼ 2A′. A set of spectroscopic data is derived for them at the RCCSD(T)/aug cc-pVTZ level of theory. The vertical and the adiabatic electron affinities are also deduced. High level ab initio calculations show that no metastable electronically excited states may exist either in the doublet or in the quartet states manifolds of C 2 N 2 - species.

Reaction Mechanism of HCN+ + C2H4: A Theoretical Study

The complex doublet potential energy surface for the ion-molecule reaction of HCN(+) with C(2)H(4) is investigated at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels. The initial association between HCN(+) and C(2)H(4) forms three energy-rich addition intermediates, 1 (HCNCH(2)CH(2)(+)), 2 (HC-cNCH(2)CH(2)(+)), and 3 (N-cCHCH(2)CH(2)(+)), which are predicted to undergo subsequent isomerization and decomposition steps. A total of nine kinds of dissociation products, including P(1) (HCN + C(2)H(4)(+)), P(2) (HCNCHCH(2)(+) + H), P(3) (NCCH(2) + CH(3)(+)), P(4) (CN + C(2)H(5)(+)), P(5) (NCCHCH(2)(+) + H(2)), P(6) (HNCCHCH(2)(+) + H), P(7) (c-CHCCH(2)N(+) + H(2)), P(8) (c-NHCCH(2)C(+) + H(2)), and P(9) (HNCCCH(+) + H(2) + H), are obtained. Among the nine products, P(1) is the most abundant product. P(2) is the second feasible product but is much less competitive than P(1). P(3), P(4), P(5), and P(6) may have the lowest yields observed. Other products, P(7), P(8), and P(9), may become feasible at high temperature. Because the intermediates and transition states involved in the most favorable pathway all lie below the reactant, the HCN(+) + C(2)H(4) reaction is expected to be rapid, which is confirmed by experiment. The present calculation results may provide a useful guide for understanding the mechanism of HCN(+) toward other unsaturated hydrocarbons.

Ab initio chemical kinetics for the reactions of HNCN with O(3P) and O2

The kinetics and mechanisms of the reactions of cyanomidyl radical (HNCN) with oxygen atoms and molecules have been investigated by ab initio calculations with rate constant prediction. The doublet and quartet state potential energy surfaces (PESs) of the two reactions have been calculated by single-point calculations at the CCSD(T)/6-311+G(3df, 2p) level based on geometries optimized at the CCSD/6-311++G(d, p) level. The rate constants for various product channels of the two reactions in the temperature range of 300–3000K are predicted by variational transition state and RRKM theories. The predicted total rate constants of the O(3P)+HNCN reaction at 760Torr Ar pressure can be represented by the expressions ktotal (O+HNCN)=3.12×10−10×T−0.05 exp (−37/T) cm3molecule−1s−1 at T=300–3000K. The branching ratios of primary channels of the O(3P)+HNCN are predicted: k1 for producing the NO+CNH accounts for 0.72–0.64, k2+k9 for producing the 3NH+NCO accounts for 0.27–0.32, and k6 for producing the CN+HNO accounts for 0.01–0.07 in the temperature range studied. Meanwhile, the predicted total rate constants of the O2+HNCN reaction at 760Torr Ar pressure can be represented by the expression, ktotal(O2+HNCN)=2.10×10−16×T1.28exp (−12200/T) cm3molecule−1s−1 at T=300–3000K. The predicted branching ratio for k11+k13 producing HO2+3NCN as the primary products accounts for 0.98–1.00 in the temperature range studied.

Mechanisms for the Ni +-mediated oxidation of benzene to phenol by N 2O

The gas-phase Ni +-mediated benzene oxidation by N 2O on both quartet and doublet potential energy surfaces are surveyed using density functional theory (DFT). Geometries and energies for all relevant intermediates are calculated. The initial N 2O reduction for forming NiO + takes place only via reaction with the excited Ni +( 4F) through the N–O insertion mechanism. The second step of the oxidation can involve two mechanisms: ‘nonradical’ through benzene H migration following the formation of the boat complex of benzene–NiO + or ‘oxygen-insertion’ via rearrangement to an arenium intermediate from the chair-type benzene–NiO + encounter complex. Both mechanisms are energetically available.

A theoretical investigation on the [formula omitted] clusters by density functional theory methods

The first systematic study of the Ti (CH 3 ) 4 0 , - 1 clusters has been carried out by density functional theory (DFT) using the hybrid B3LYP functional in their singlet, triplet, quintet, doublet and quartet potential energy surfaces. Some molecular properties have been calculated for the stable structures, including optimal geometries, bond lengths, bond angles, total energies, reaction energies, zero-point-corrected binding energies, natural population analysis charges, natural electron configurations, vibrational frequencies, and MO pictures of HOMO and LUMO. The results show that the most stable neutral Ti(CH 3) 4 and its anion are those with spin state S = 0 and 1/2, respectively. Moreover, their reaction energies are −57.68 and −60.80 kcal/mol, and zero-point-corrected binding energies are −9.87 and −10.43 eV. The zero-point-corrected adiabatic electron affinity (AEA) of the ground-state structures is predicted to be 0.80 eV ( 1A ← 2A).

QUANTUM CHEMISTRY STUDY ON THE CLEAVAGE OF METHANOL CATALYZED BY HYDROXYL ZnO

The reaction mechanism of the decomposition of the methanol catalyzed by hydroxyl ZnO has been investigated by density function theory (DFT). The geometries of reactants, intermediates, transition states, and products on both doublet and quartet potential energy surfaces (PESs) have been fully optimized at the B3LYP/6-31G* level. The calculated results show that the reaction is slightly endothermic by 5.3 kJ/mol, which is in good accordance with the previous experiment. The energies and structures of the crossing points (CPs) between two PESs have been determined. The CP appears after the formation of transition states. The two theoretical models chosen to study the reaction mechanism were compared and discussed.

DFT study of the spin-forbidden reaction of N 2O and CO catalyzed by Pt +

The gas-phase reaction of N 2O and CO catalyzed by Pt + has been investigated by means of the relativistic effective core potential (RECP) of Stuttgart basis sets on platinum and UB3LYP/6-311+G(2d) level on nonmetal atoms. The involving crossings between the doublet and quartet potential energy surfaces have been discussed by means of the intrinsic reaction coordinate approach used by Yoshizawa et al, and corresponding MECPs we obtained by the mathematical algorithm proposed by Harvey et al. has also been employed. In addition, the crossing caused by SOC is efficient. And the O-atom affinities (OA) testified that the argumentation is thermodynamically allowed.

DFT study on the gas phase reaction of Ni+ with CS2

The reaction of Ni+ with CS2 has been investigated at B3LYP/TZVP and B3LYP/6-311+G∗ levels of theory. The reaction mechanisms have been explored in detail on both doublet and quartet potential energy surfaces. The products observed in the experiment have been explained according to the cleavage of different bonds in the insertion intermediate S–Ni+–C–S. The product [Ni,C,S]+ observed in experiment was confirmed as Ni–CS+ (2Σ). The spin-forbidden reaction Ni+(2D)+CS2→NiS+(4Σ)+CS (1Σ) was found to proceed through a doublet–quartet surface crossing and the crossing seam was approximately calculated.