Direct dynamics simulation of dioxetane formation and decomposition via the singlet [middle dot]O-O-CH(2)-CH(2)[middle dot] biradical: Non-RRKM dynamics

Electronic structure calculations and direct chemical dynamics simulations are used to study the formation and decomposition of dioxetane on its ground state singlet potential energy surface. The stationary points for (1)O(2) + C(2)H(4), the singlet [middle dot]O-O-CH(2)-CH(2)[middle dot] biradical, the transition state (TS) connecting this biradical with dioxetane, and the two transition states and gauche [middle dot]O-CH(2)-CH(2)-O[middle dot] biradical connecting dioxetane with the formaldehyde product molecules are investigated at different levels of electronic structure theory including UB3LYP, UMP2, MRMP2, and CASSCF and a range of basis sets. The UB3LYP∕6-31G? method was found to give representative energies for the reactive system and was used as a model for the simulations. UB3LYP∕6-31G? direct dynamics trajectories were initiated at the TS connecting the [middle dot]O-O-CH(2)-CH(2)[middle dot] biradical and dioxetane by sampling the TS's vibrational energy levels, and rotational and reaction coordinate energies, with Boltzmann distributions at 300, 1000, and 1500 K. This corresponds to the transition state theory model for trajectories that pass the TS. The trajectories were directed randomly towards both the biradical and dioxetane. A small fraction of the trajectories directed towards the biradical recrossed the TS and formed dioxetane. The remainder formed (1)O(2) + C(2)H(4) and of these ～ 40% went directly from the TS to (1)O(2) + C(2)H(4) without getting trapped and forming an intermediate in the [middle dot]O-O-CH(2)-CH(2)[middle dot] biradical potential energy minimum, a non-statistical result. The dioxetane molecules which are formed dissociate to two formaldehyde molecules with a rate constant two orders of magnitude smaller than that predicted by Rice-Ramsperger-Kassel-Marcus theory. The reaction dynamics from dioxetane to the formaldehyde molecules do not follow the intrinsic reaction coordinate or involve trapping in the gauche [middle dot]O-CH(2)-CH(2)-O[middle dot] biradical potential energy minimum. Important non-statistical dynamics are exhibited for this reactive system.     

Quasiclassical trajectory calculations of the reaction C+C2H2-->l-C3H, c-C3H+H, C3+H2 using full-dimensional triplet and singlet potential energy surfaces

Full-dimensional, density functional theory (B3LYP/6-311g(d,p))-based potential energy surfaces (PESs) are reported and used in quasi-classical calculations of the reaction of C with C(2)H(2). For the triplet case, the PES spans the region of the reactants, the complex region (with numerous minima and saddle points) and the products, linear(l)-C(3)H+H, cyclic(c)-C(3)H+H and c-(3)C(3)+H(2). For the singlet case, the PES describes the complex region and products l-C(3)H+H, c-C(3)H+H and l-(1)C(3)+H(2). The PESs are invariant under permutation of like nuclei and are fit to tens of thousands of electronic energies. Energies and harmonic frequencies of the PESs agree well the DFT ones for all stationary points and for the reactant and the products. Dynamics calculations on the triplet PES find both l-C(3)H and c-C(3)H products, with l-C(3)H being dominant at the energies considered. Limited unimolecular reaction dynamics on the singlet PES find both products in comparable amounts as well as the C(3)+H(2) product.     

A theoretical study of the mechanism and kinetics of F+N3 reactions.

Both the singlet(1A') and triplet(3A') potential energy surfaces (PESs) of F+N(3) reactions are investigated using the complete-active-space self-consistent field (CASSCF) and the multireference configuration interaction (MRCI) methods with a proper active space. The minimum energy crossing point (MECP) at the intersection seam between the 1A' and 3A' PESs is located and used to clarify the reaction mechanisms. Two triplet transition states are found, with one in the cis form and the other one in the trans form. Further kinetic calculations are performed with the canonical unified statistical (CUS) theory on the singlet PES and the improved canonical variational transition-state (ICVT) method on the triplet PES. The rate constants are also reported. At 298 K, the calculated rate constant is in reasonably good agreement with experimental values, and spin-orbit coupling effects lower it by 28 %. The spectroscopic constants derived from the fitted potential-energy curves for the singlet and triplet states of NF are in very good agreement with experimental values. Our calculations indicate that the adiabatic reaction on the singlet PES leading to NF(a(1)Delta)+N(2) is the major channel, whereas the nonadiabatic reaction through the MECP, which leads to NF(X(3)Sigma(-))+N(2), is a minor channel.     

Ab initio study on the oxidation of NCN by O (3P): prediction of the total rate constant and product branching ratios

The reaction of NCN with O is relevant to the formation of prompt NO according to the new mechanism, CH+N2-->cyclic-C(H)NN- -->HNCN-->H+NCN. The reaction has been investigated by ab initio molecular orbital and transition state theory calculations. The mechanisms for formation of possible product channels involved in the singlet and triplet potential energy surfaces have been predicted at the highest level of the modified GAUSSIAN-2 (G2M) method, G2M (CC1). The barrierless association/dissociation processes on the singlet surface were also examined with the third-order Rayleigh-Schr?dinger perturbation (CASPT3) and the multireference configuration interaction methods including Davidson's correction for higher excitations (MRCI+Q) at the CASPT3(6,6)/6-311+G(3df)//UB3LYP/6-311G(d) and MRCI+Q(6,6)/6-311+G(3df)//UB3LYP/6-311G(d) levels. The rate constants for the low-energy channels producing CO+N2, CN+NO, and N(4S)+NCO have been calculated in the temperature range of 200-3000 K. The results show that the formation of CN+NO is dominant and its branching ratio is over 99% in the whole temperature range; no pressure dependence was noted at pressures below 100 atm. The total rate constant can be expressed by: kt=4.23x10(-11) T0.15 exp(17/T) cm3 molecule(-1) s(-1).     

The electronic structure and energetics of V(+)-benzene half-sandwiches of different multiplicities: Comparative multireference and single-reference theoretical study

Multireference [complete active space self-consistent field (CASSCF) and multiconfigurational quasidegenerate perturbation theory (MCQDPT)] and single-reference ab initio (Moller-Plesset second order perturbation theory (MP2) and coupled clusters with singles, doubles and noniterative triples [CCSD(T)]) and density functional theory (PBE and B3LYP) electronic structure calculations of V(C(6)H(6))(+) half-sandwich in the states of different multiplicities are described and compared. Detailed analyses of the geometries and electronic structures of the all found states are given; adiabatic and diabatic dissociation energies are estimated. The lowest electronic state of V(C(6)H(6))(+) half-sandwich was found to be the quintet (5)B(2) state with a slightly deformed upside-down-boat-shaped benzene ring and d(4) configuration of V atom, followed by a triplet (3)A(2) state lying about 4 kcal/mol above. The lowest singlet state (1)A(1)(d(4)) lies much ( approximately 28 kcal/mol) higher. MCQDPT calculated adiabatic dissociation energy (53.6 kcal/mol) for the lowest (5)B(2)(d(4)) state agrees well with the current 56.4 (54.4) kcal/mol experimental estimate, giving a preference to the lower one. Compared to MCQDPT, B3LYP hybrid exchange-correlation functional provides the best results, while CCSD(T) performs usually worse. Gradient-corrected PBE calculations tend to systematically overestimate metal-benzene binding in the row quintet相似文献   

O + C<Subscript>2</Subscript>H<Subscript>4</Subscript> potential energy surface: excited states and biradicals at the multireference level

The focus of this study is to understand the multiconfigurational nature of the biradical species involved in the early reaction paths of the oxygen plus ethylene PES. In previous work (J Phys Chem A 113, 12663, 2009), the lowest-lying O(³P) + C₂H₄ PES was extensively explored at the MCSCF, MRMP2, and MR-AQCC levels of theory. In the current work, ground and excited, triplet- and singlet-state reaction paths for the initial addition of oxygen to ethylene were found at the MCSCF and MRMP2 levels along with five singlet pathways near the ^·CH₂CH₂O^· biradical at the MCSCF, MRMP2, and CR-CC(2,3) levels. One of these five paths can lead to the CH₂CO + H₂ products from CH₃CHO rather than from the ^·CH₂CH₂O^· biradical, and this pathway was investigated with a variety of CAS sizes. To provide further comparison between the MRMP2 and CR-CC(2,3) levels, MR-AQCC single-point energies and optimizations were performed for select geometries. After the initial exploration of this region of the surface, the lowest singlet–triplet surface crossings were explicitly determined at the MCSCF level. Additional MRMP2 calculations were performed to demonstrate the limitations of single-state perturbation theory in this biradical region of the PES, and SO-MCQDPT2 single-point energies using SA MCSCF were calculated on a grid of geometries around the primary surface crossing. In particular, these calculations were examined to determine a proper active space and a physically reasonable number of electronic states. The results of this examination show that at least four states must be considered to represent this very complex region of the PES.     

New insight into the gas-phase bimolecular self-reaction of the HOO radical

The singlet and triplet potential energy surfaces (PESs) for the gas-phase bimolecular self-reaction of HOO*, a key reaction in atmospheric environments, have been investigated by means of quantum-mechanical electronic structure methods (CASSCF and CASPT2). All the reaction pathways on both PESs consist of a first step involving the barrierless formation of a prereactive doubly hydrogen-bonded complex, which is a diradical species lying about 8 kcal/mol below the energy of the reactants at 0 K. The lowest energy reaction pathway on both PESs is the degenerate double hydrogen exchange between the HOO* moieties of the prereactive complex via a double proton transfer mechanism involving an energy barrier of only 1.1 kcal/mol for the singlet and 3.3 kcal/mol for the triplet at 0 K. The single H-atom transfer between the two HOO* moieties of the prereactive complex (yielding HOOH + O2) through a pathway keeping a planar arrangement of the six atoms involves a conical intersection between either two singlet or two triplet states of A' and A" symmetries. Thus, the lowest energy reaction pathway occurs via a nonplanar cisoid transition structure with an energy barrier of 5.8 kcal/mol for the triplet and 17.5 kcal/mol for the singlet at 0 K. The simple addition between the terminal oxygen atoms of the two HOO* moieties of the prereactive complex, leading to the straight chain H2O4 intermediate on the singlet PES, involves an energy barrier of 7.3 kcal/mol at 0 K. Because the decomposition of such an intermediate into HOOH + O2 entails an energy barrier of 45.2 kcal/mol at 0 K, it is concluded that the single H-atom transfer on the triplet PES is the dominant pathway leading to HOOH + O2. Finally, the strong negative temperature dependence of the rate constant observed for this reaction is attributed to the reversible formation of the prereactive complex in the entrance channel rather than to a short-lived tetraoxide intermediate.     

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.     

Trajectory surface hopping study of the O(3P) + ethylene reaction dynamics

Spin-orbit coupling (SOC) induced intersystem crossing (ISC) has long been believed to play a crucial role in determining the product distributions in the O(3P) + C2H4 reaction. In this paper, we present the first nonadiabatic dynamics study of the title reaction at two center-of-mass collision energies: 0.56 eV, which is barely above the H-atom abstraction barrier on the triplet surface, and 3.0 eV, which is in the hyperthermal regime. The calculations were performed using a quasiclassical trajectory surface hopping (TSH) method with the potential energy surface generated on the fly at the unrestricted B3LYP/6-31G(d,p) level of theory. To simplify our calculations, nonadiabatic transitions were only considered when the singlet surface intersects the triplet surface. At the crossing points, Landau-Zener transition probabilities were computed assuming a fixed spin-orbit coupling parameter, which was taken to be 70 cm-1 in most calculations. Comparison with a recent crossed molecular beam experiment at 0.56 eV collision energy shows qualitative agreement as to the primary product branching ratios, with the CH3 + CHO and H + CH2CHO channels accounting for over 70% of total product formation. However, our direct dynamics TSH calculations overestimate ISC so that the total triplet/singlet ratio is 25:75, compared to the observed 43:57. Smaller values of SOC reduce ISC, resulting in better agreement with the experimental product relative yields; we demonstrate that these smaller SOC values are close to being consistent with estimates based on CASSCF calculations. As the collision energy increases, ISC becomes much less important and at 3.0 eV, the triplet to singlet branching ratio is 71:29. As a result, the triplet products CH2 + CH2O, H + CH2CHO and OH + C2H3 dominate over the singlet products CH3 + CHO, H2 + CH2CO, etc.     

Isomerization barriers in bis(4H-thiopyran) and in bithioxanthenes

The inversion and rotation mechanisms for the isomerization of Feringa’s bithioxanthenes existing in two conformations, up/up and up/down, have been calculated at the B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels. The inversion mechanism that maintains the double bond nature of the central bond is a classical one but the rotation mechanisms that break the double bond to form a biradical needs to explore the singlet and triplet states. To do this we have removed the four fused phenyl rings of bithioxanthene and calculated at the CASSCF and CASPT2 levels bis(4H-thiopyran) proving that B3LYP calculations yield reasonable results for the rotation barriers.     

Exploring the Reaction Paths on the Potential Energy Surfaces of the S1 and T1 States in Methylenecyclopropane

The reaction paths of methylenecyclopropane 1 on the potential energy surfaces (PESs) of the lowest triplet (T₁) state and the lowest excited singlet (S₁) state, as well as that of the ground state (S₀), were explored by using the nudged elastic band method at the MRMP2//MCSCF/6‐31++G(d,p) and DFT(B3LYP)/6‐31++G(d,p) levels of theory. After vertical excitation of 1, three transition states on the PES of the lowest triplet state and one transition state on the S₁ PES were found along the reaction path to produce a carbene, cyclobutylidene 2. All of these transition states are lower in energy than the S₁ state produced by vertical excitation at the S₀ energy minimum in 1. Fast transition is predicted to occur from the T₁ state or from the S₁ state to the S₀ state due to strong spin‐orbit coupling or nonadiabatic coupling in the geometrical vicinity of 2. On the MRMP2 S₀ PES, the energy barriers of 5.0, 10.3 and 13.5 kcal mol^?1 were obtained for C migration reaction (backward reaction), 1,2‐H migration reaction to cyclobutene 3, and 1,3‐H migration reaction to bicyclopropane 4, respectively, started at 2. The introduction of phenyl groups makes the energy barriers smaller due to the π conjugation between the carbene center and phenyl groups.     

A computational study of the factors controlling triplet-state reactivity in 1,4-pentadiene

The triplet-state reactions of 1,4-pentadiene have been investigated using density functional theory (UB3LYP) and ab initio (CASSCF) calculations with a 6-31G basis set. Intramolecular [2 + 2] photocycloadditions and three different reaction pathways leading to vinylcyclopropane have been examined. The computed results are in good agreement with the experimental observations, predicting the dominant product to be vinylcyclopropane produced by a di-pi-methane rearrangement, and the favored [2 + 2] cycloaddition product to be bicyclo[2.1.0]pentane. Reaction pathways involving initial C-C or C-H bond cleavage were found to be too high in energy to be significant. Both the [2 + 2] cycloadditions and the di-pi-methane rearrangement proceed through cyclic biradical intermediates formed on the triplet surface. The relative rates of formation of these triplet biradicals are found to depend on three factors: biradical stability, the geometry of the transition structure, and orbital interactions through bonds.     

Reaction mechanism of the CCN radical with nitric oxide

To investigate the possibility of the carbyne radical CCN in removal of nitric oxide, a detailed computational study is performed at the Gaussian-3//B3LYP/6-31G(d) level on the CCN + NO reaction by constructing the singlet and triplet electronic state [C(2)N(2)O] potential energy surfaces (PESs). The barrierless formation of the chain-like isomers NCCNO (singlet at -106.5, triplet cis at -48.2 and triplet trans at -47.6 kcal/mol) is the most favorable entrance attack on both singlet and triplet PESs. Subsequently, the singlet NCCNO takes an O-transfer to form the branched intermediate singlet NCC(O)N (-85.6), which can lead to the fragments CN + NCO (-51.2) via the intermediate singlet NCOCN (-120.3). The simpler evolution of the triplet NCCNO is the direct N-O rupture to form the weakly bound complex triplet NCCN...O (-56.2) before the final fragmentation to NCCN + (3)O (-53.5). However, the lower lying products (3)NCN + CO (-105.6) and (3)CNN + CO (-74.6) are kinetically much less competitive. All the involved transition states for generation of CN + NCO and NCCN + (3)O lie much lower than the reactants. Thus, the novel reaction CCN + NO can proceed effectively even at low temperatures and is expected to play a role in both combustion and interstellar processes. Significant differences are found on the singlet PES between the CCN + NO and CH + NO reaction mechanisms.     

Mapping the triplet potential energy surface of 1-methyl-8-nitronaphthalene

Spin-unrestricted calculations and time-dependent DFT were used to characterize structure and reactivity of 1-methyl-8-nitronaphthalene (1) in the triplet state. Four hybrid models (B3LYP, PBE0, MPW1K, BHLYP) with significantly different amount of the exact exchange were employed. The triplet potential energy surface of 1 was mapped by using the UB3LYP and UMPW1K techniques. Both hybrid models provided qualitatively consistent pictures for the potential energy landscape. Thirty-one stationary points, of which 15 were minima, were found at the UB3LYP level of theory. Three minima corresponding to the nitro form of 1 were located on the triplet surface; just one was found for the singlet ground state. Two reaction paths leading from 1 either to a nitrite-type intermediate (2) or to the aci-form (3) were characterized. For both paths, reaction products were of diradical nature. The lower activation energy was obtained for the triplet-state tautomerization affording 3. The ground state of triplet multiplicity was predicted for two isomers of the aci-form. The triplet diradical 3 is expected to react through the thermal population of a close-lying singlet excited state. The results are discussed in relation to mechanisms of photoinduced rearrangements of peri-substituted nitronaphthalenes that can be used to develop novel photolabile protecting groups.     

Unimolecular decomposition of chemically activated pentatetraene (H2CCCCCH2) intermediates: A crossed beams study of dicarbon molecule reactions with allene

The reactions dynamics of the dicarbon molecule C2 in the 1Sigma (g)+ singlet ground state and 3Pi(u) first excited triplet state with allene, H2CCCH2(X1A1), was investigated under single collision conditions using the crossed molecular beam approach at four collision energies between 13.6 and 49.4 kJ mol(-1). The experiments were combined with ab initio electronic structure calculations of the relevant stationary points on the singlet and triplet potential energy surfaces. Our investigations imply that the reactions are barrier-less and indirect on both the singlet and the triplet surfaces and proceed through bound C5H4 intermediates via addition of the dicarbon molecule to the carbon-carbon double bond (singlet surface) and to the terminal as well as central carbon atoms of the allene molecule (triplet surface). The initial collision complexes isomerize to form triplet and singlet pentatetraene intermediates (H2CCCCCH2) that decompose via atomic hydrogen loss to yield the 2,4-pentadiynyl-1 radical, HCCCCCH2(X2B1). These channels result in symmetric center-of-mass angular distributions. On the triplet surface, a second channel involves the existence of a nonsymmetric reaction intermediate (HCCCH2CCH) that fragments through atomic hydrogen emission to the 1,4-pentadiynyl-3 radical [C5H3(X2B1)HCCCHCCH]; this pathway was found to account for the backward scattered center-of-mass angular distributions at higher collision energies. The identification of two resonance-stabilized free C5H3 radicals (i.e., 2,4-pentadiynyl-1 and 1,4-pentadiynyl-3) suggests that these molecules can be important transient species in combustion flames and in the chemical evolution of the interstellar medium.     

Generation and reactivity of the 4-aminophenyl cation by photolysis of 4-chloroaniline

4-Chloroaniline and its N,N-dimethyl derivative are photostable in cyclohexane but undergo efficient photoheterolysis in polar media via the triplet state and give the corresponding triplet phenyl cations. CASSCF and UB3LYP calculations show that the 4-aminophenyl triplet cation has a planar geometry and is stabilized by >10 kcal mol(-1) with respect to the slightly bent singlet. The triplet has a mixed carbene-diradical character at the divalent carbon. This species either adds to the starting substrate forming 5-chloro-2,4'-diaminodiphenyls (via an intermediate cyclohexadienyl cation) or is reduced to the aniline (via the aniline radical cation) in a ratio depending on the hydrogen-donating properties of the solvent. Transients attributable to the triplet aminophenyl cation as well as to the ensuing intermediates are detected. Chemical evidence for the generation of the phenyl cation is given by trapping via electrophilic substitution with benzene, mesitylene, and hexamethylbenzene (in the last case the main product is a 6-aryl-3-methylene-1,4-cyclohexadiene). Relative rates of electrophilic attack to benzene and to some alkenes and five-membered heterocycles are measured and span over a factor of 15 or 30 for the two cations. The triplet cation formed under these conditions is trapped by iodide more efficiently than by the best pi nucleophiles. However, in contrast to the singlet cation, it does not form ethers with alcohols, by which it is rather reduced.     

A new family of pi-conjugated delocalized biradicals: Electronic structures of 1,4-bis(2,5-diphenylimidazol-4-ylidene)cyclohexa-2,5-diene

A new family of pi-conjugated delocalized biradical compound is developed. The solution of 1,4-bis(2,5-diphenylimidazol-4-ylidene)cyclohexa-2,5-diene shows the ESR signal consisting of a moderately broad unresolved line, Delta H(pp) approximately 1 mT, at room temperature. The presence of the thermal equilibrium between a triplet biradical state and a singlet state is confirmed by the ESR measurements, and the spin concentration is determined as 7.90 x 10(21) spin/mol at 300 K. The spin concentration can also be controlled by modifying the molecular planarity. Moreover, the unrestricted DFT/B3LYP calculations suggest the biradical character of the singlet ground state, and the modulation of the energy gap between the singlet state and the triplet state is investigated from the theoretical point of view. Controlling the equilibrium between a diamagnetic state and a paramagnetic state will provide significant progress in the field of biradical chemistry, and the materials with the biradical character in a ground state will lead to a novel development of molecular-based organic magnets.     

Bonding and correlation analysis of various Si2CO isomers on the potential energy surface

At various levels of theory, singlet and triplet potential energy surfaces (PESs) of Si₂CO, which has been studied using matrix isolation infrared spectroscopy, are investigated in detail. A total of 30 isomers and 38 interconversion transition states are obtained at the B3LYP/6‐311G(d) level. At the higher CCSD(T)/6‐311+G(2d)//QCISD/6‐311G(2d)+ZPVE level, the global minimum ¹1 (0.0 kcal/mol) corresponds to a three‐membered ring singlet O‐cCSiSi (¹A′). On the singlet PES, the species ¹2 (0.2 kcal/mol) is a bent SiCSiO structure with a ¹A′ electronic state, followed by a three‐membered ring isomer Si‐cCSiO (¹A′) ¹3 (23.1 kcal/mol) and a linear SiCOSi ¹4 (¹Σ⁺) (38.6 kcal/mol). The isomers ¹1, ¹2, ¹3 , and ¹4 possess not only high thermodynamic stabilities, but also high kinetic stabilities. On the triplet PES, two isomers ³1 (³B₂) (18.8 kcal/mol) and ³7 (³A″) (23.3 kcal/mol) also have high thermodynamic and kinetic stabilities. The bonding natures of the relevant species are analyzed. The similarities and differences between C₃O, C₃S, SiC₂O, and SiC₂S are discussed. The present results are also expected to be useful for understanding the initial growing step of the CO‐doped Si vaporization processes. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009