Triplet Surface Research Articles

Potential Nonadiabatic Reactions: Ring-Opening 4,6-Dimethylidenebicyclo[3.1.0]hex-2-ene Derivatives to Aromatic Reactive Intermediates

Potential singlet-triplet surface crossings for the ring opening of 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives were explored using density functional theory (DFT) and complete active space self-consistent field (CASSCF) methods. Since these ring openings involve relatively high energy species that lead to relatively stable aromatic species, a good scenario for potential nonadiabatic events, we posited that the reaction paths of these ring openings might come close to or cross excited state surfaces. At the DFT level of theory, all reaction paths exhibited characteristics suggestive of singlet-triplet intersections along their paths. 6-Methylidenebicyclo[3.1.0]hex-3-en-2-one and a closely related derivative (4-methylidenebicyclo[3.1.0]hex-2-en-6-one) were explored at the CASSCF level of theory; CASSCF results were qualitatively similar to DFT results and yielded spin-orbit couplings of 1.1-1.4 cm(-1) at the singlet-triplet crossing points.

Analytical potential energy surface for O + C2H2 system

Full-dimensional analytical potential energy surfaces (PESs) for the O(1P)+C2H2 and O(3P)+C2H2 reactions are presented. The PESs are constructed by fitting thousands of ab initio data points using permutationally invariant polynomials [5]. Electronic structure data are obtained within the restricted coupled cluster theory with single and double excitations with perturbative triple corrections and correlation consistent basis set of triple-ζ with diffuse functions. Motivated by experiments, formation of DOCC and cyclic DCCO at collision energies of 2.5–4eV is examined using quasi-classical trajectory dynamics on the triplet surface.

Signatures of Isomerization in Photodissociation of trans- Crotonaldehyde Probed by Multiphoton Ionization Mass Spectrometry

We report the observation of new isomerization effects in the UV-photodissociation of trans-crotonaldehyde upon multiphoton excitation by the third harmonic (355 nm) pulses of a Nd:YAG laser. A time-of-flight mass spectrometric analysis reveals formation of acetaldehyde, acetyl, and methoxy radical cations as signatures of isomerization processes. A small segment of the multiphoton ionization spectrum of jet-cooled crotonaldehyde is recorded by tuning the laser frequency around 355 nm. An oxetene type transient intermediate in the ground state has been considered for acetaldehyde formation following a photochemical model suggested earlier (Reguero ; et al. J. Am. Chem. Soc. 1994, 116, 2101-2114) for such compounds. Likewise, for methoxy radical formation, a trans-cis isomerization about the C═C double bond has been considered in a triplet surface. Electron ionization mass spectra of the compound are also recorded by varying the electron kinetic energy in the range 11-70 eV. Ionic fragments in the mass spectra of the two ionization processes are dramatically different. Our suggested mechanisms for isomerization and fragmentation channels are substantiated by density functional theory calculations. Combined experimental and calculated data lead us conclude that isomerization occurs in neutral potential energy surfaces prior to dissociation and photoionization.

Experimental and Computational Exploration of Ground and Excited State Properties of Highly Strained Ruthenium Terpyridine Complexes

Dissociative electron transfer reactions are prevalent in one-electron reduced aryl halides; however, calculations applied to charge-transfer excited states of metal complexes suggest that this reaction would be strongly endergonic unless attention is paid to specific structural details. In this current study, we explore the effect of introducing intramolecular strain into a series of halogenated ruthenium(II) polypyridyls. Parent [Ru(tpy)2](2+) (1) (tpy = 2,2':6',2″-terpyridine) is compared with two complexes, [Ru(6,6″-Br2-tpy)(tpy)](2+) (2) and [Ru(6,6″-Br2-tpy)2](2+) (3) (6,6″-Br2-tpy = 6,6″-dibromo-tpy) that incorporate interligand van der Waals strain derived from the large halogen substituents. DFT calculations and the crystal structure of 3 show how this strain distorts the geometry of 3 as compared to 1. Time-dependent DFT calculations are used to explain the effect of this strain on electronic absorption spectra where, in particular, a transition observed in 3 is attenuated in 2 and absent in 1 and heralds interligand charge transfer mediated by the halogen substituent. Ultrafast transient absorption spectroscopy reveals coherent vibrational dynamics particularly in 3 but also in 2 that is interpreted as reflecting heavy-atom motion. Surprisingly, in spite of the additional strain, the excited-state lifetime of 3 is observed to be approximately a factor of 6 longer than 2. Constrained-DFT calculations show that while the excited behavior of 2 is similar to 1, the strain-induced geometric distortions in 3 cause a nesting of excited state triplet surfaces resulting in a longer excited state lifetime. This may afford the additional time needed to engage in photochemistry, and kinetic evidence is observed for the breaking of a C-Br bond in 3 and formation of a contact ion pair state.

Dynamics of Triplet-State Photochemistry of Pentanal: Mechanisms of Norrish I, Norrish II, and H Abstraction Reactions

The photochemistry of aldehydes in the gas phase has been the topic of extensive studies over the years. However, for all but the smallest aldehydes the dynamics of the processes is largely unknown, and key issues of the mechanisms are open. In this article, the photochemistry of pentanal is studied by dynamics simulation using a semiempirical MRCI code for the singlet and triplet potential energy surfaces involved. The simulations explore the processes on the triplet state following intersystem crossing from the initially excited singlet. Test simulations show that the photochemistry takes place on the adiabatic triplet surface only and that no nonadiabatic transitions occur to the other triplets. The main findings include the following: (1) Norrish type I and type II reactions and H detachment have been observed. (2) The time scales of Norrish type I and Norrish type II reactions are determined: Norrish type I reaction tends to occur in the time scale below 10 ps, whereas the Norrish type II reaction is mostly pronounced after 20 ps. The factors affecting the time scales are analyzed. (3) The relative yield for Norrish type I and type II reactions is 34% and 66%, which is close to the experimental observed ones. Bond orders and Mulliken partial charges are computed along the trajectories and provide mechanistic insights. The results throw light on the time scales and mechanisms and competition between different channels in aldehyde photochemistry. It is suggested that direct dynamics simulations using semiempirical potentials can be a very useful tool for exploring the photochemistry of large aldehydes, ketones, and related species.

A Combined Crossed Beam and Ab Initio Investigation of the Gas Phase Reaction of Dicarbon Molecules (C2; X1Σg+/a3Πu) with Propene (C3H6; X1A′): Identification of the Resonantly Stabilized Free Radicals 1- and 3-Vinylpropargyl

The crossed molecular beam reactions of dicarbon, C2(X(1)Σg(+), a(3)Πu), with propene (C3H6; X(1)A') and with the partially deuterated D3 counterparts (CD3CHCH2, CH3CDCD2) were conducted at collision energies of about 21 kJ mol(-1) under single collision conditions. The experimental data were combined with ab initio and statistical (RRKM) calculations to reveal the underlying reaction mechanisms. Both on the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the addition of the dicarbon reactant to the carbon-carbon double bond of propene. These initial addition complexes rearrange via multiple isomerization steps leading ultimately via atomic hydrogen elimination from the former methyl and vinyl groups to the formation of 1-vinylpropargyl and 3-vinylpropargyl. Both triplet and singlet methylbutatriene species were identified as important reaction intermediates. On the singlet surface, the unimolecular decomposition of the reaction intermediates was found to be barrier-less, whereas on the triplet surface, tight exit transition states were involved. In combustion flames, both radicals can undergo a hydrogen-atom assisted isomerization leading ultimately to the thermodynamically most stable cyclopentadienyl isomer. Alternatively, in a third body process, a subsequent reaction of 1-vinylpropargyl or 3-vinylpropargyl radicals with the propargyl radical might yield to the formation of styrene (C6H5C2H3) in an entrance barrier-less reaction under combustion-like conditions. This presents a strong alternative to the formation of styrene via the reaction of phenyl radicals with ethylene, which is affiliated with an entrance barrier of about 10 kJ mol(-1).

Reactivity and Regioselectivity of Methylacetylene Cyclotrimerization over the Phillips Cr/Silica Catalyst: A DFT Study

The mechanism of the methylacetylene cyclotrimerization catalyzed by the Phillips Cr/silica catalyst has been studied by DFT investigations based on a Cr(II)/SiO2 cluster model and a silica supported cluster model. Twenty-one kinds of Cr(II)/SiO2·(C3H4)n (n = 1–3) complexes were first optimized successfully. Starting from the most stable chromium(methylacetylene) complex, the following cyclotrimerization of methylacetylene on the quintet surface is prohibited by the spontaneous coupling of the two coordinated methylacetylenes. Instead of overcoming a much higher Gibbs free energy barrier by about 40 kcal/mol on the quintet surface, a spin flipping to the triplet surface at the chromium(methylacetylene) complex only requires 16.9 kcal/mol in Gibbs free energy. After the spin transition, the methyl-chromacyclopropene species was formed immediately on the triplet surface. The triplet dimethyl-chromacyclopentadiene species was generated by insertion of a coordinated methylacetylene into the 3-membered metallacycle ring. The following reaction may follow two pathways: (a) a concerted [4 + 2] cycloaddition or (b) a stepwise pathway (insertion and reductive elimination) via a trimethyl-chromacycloheptatriene species. All the eight [4 + 2] cycloaddition reaction pathways are favored competing with the stepwise pathways. The reactivity of each reaction pathway can be examined in terms of the calculated TOF using the energetic span model. We found that only four [4 + 2] cycloaddition reaction pathways (PES-T1Da, PES-T2Da, PES-T3Da, and PES-T4Da) are responsible for the cyclotrimerization of methylacetylene. The PES-T4Da leads to the production of 1,3,5-trimethylbenzene, while the other three pathways generate 1,2,4-trimethylbenzene. Furthermore, the effects of the silica support and the dispersion correction have been considered for the most plausible reaction pathways PES-T1Da, PES-T2Da, PES-T3Da, and PES-T4Da, respectively. Finally, with a consideration of the effects of the silica support and inclusion of the dispersion correction in the final calculated energies, the ratio of the 1,3,5- to 1,2,4-TMB is 0.32 at 363 K predicting that the 1,2,4-TMB is the dominant product in the cyclotrimerization of methylacetylene catalyzed by the Phillips Cr/silica catalyst.

Experimental and Theoretical Study of the Product Channels of the C2H + NO Reaction

The product channels of the C2H + NO reaction were studied by transient infrared absorption spectroscopy. HCN, CO, CN, and HNO (produced from HCO secondary chemistry), were successfully detected. Analysis of the obtained product yields provides an estimation of the branching fractions to be φ(1a)(HCN+CO) = 0.77 ± 0.04 and φ(1b)(HCO+CN) = 0.23 ± 0.04 at 298 K. The potential energy surface (PES) of the reaction was investigated by ab initio methods at CCSD(T)/6-311++G(d,p)//DFT-B3LYP/6-311++G(d,p) levels as well as higher levels of theory for selected critical points. The singlet potential energy surface shows a low-energy pathway only to the HCN + CO product channel, but a plausible route to HCO + CN products exists on the triplet surface. The computational results are in qualitative agreement with the experimental observations.

Computational Study of H2 and O2 Production from Water Splitting by Small (MO2)n Clusters (M = Ti, Zr, Hf)

Coupled cluster [CCSD(T)] theory and density functional theory (DFT) have been used to study the production of H2 and O2 from hydrolysis products generated from H2O addition to (MO2)n (M = Ti, Zr, Hf, n = 1-3) clusters on both the lowest singlet and triplet potential energy surfaces (PESs). H2 production occurs via the formation of an M-H containing intermediate followed by H-H recombination and H2 desorption from M(n)O(2n)(OH)2 and M(n)O(2n+2). The hydrogen transfer reactions to form the M-H bond are the rate determining steps and can be considered to be proton coupled, electron transfer (PCET) reactions with one or two electrons being transferred. Oxygen is produced by breaking two weak M-O bonds in an atomic oxygen saturated metal oxide from an M(n)O(2n)•O2 intermediate. On the triplet PES, the activation energies for the first and second H transfer to the metal are calculated to be ~10 to 50 kcal/mol and ~75 to 90 kcal/mol depending on the size of the clusters and the metal. The barriers on the singlet surface for the first and the second H transfer are predicted to be 110 to 140 kcal/mol, in general larger than the H-O bond dissociation energy. The activation barriers for the step of H-H recombination are 15 to 50 kcal/mol, and the H2 desorption energies are less than 10 kcal/mol on the singlet and triplet PESs. The oxygen desorption energies follow the order Ti < Zr < Hf for the triplets and Ti < Zr ≈ Hf for the singlets. The oxygen desorption energy is approximately independent of the size of the cluster for the same metal. The water splitting reactions prefer to take place on the triplet surface. A low excess potential energy is needed to generate 2H2 and O2 from 2H2O after the endothermicity of the reaction is overcome on the triplet PES.

A General Mechanism for the Copper- and Silver-Catalyzed Olefin Aziridination Reactions: Concomitant Involvement of the Singlet and Triplet Pathways

The olefin aziridination reactions catalyzed by copper and silver complexes bearing hydrotris(pyrazolyl)borate (Tp(x)) ligands have been investigated from a mechanistic point of view. Several mechanistic probe reactions were carried out, specifically competition experiments with p-substituted styrenes, stereospecificity of olefins, effects of the radical inhibitors, and use of a radical clock. Data from these experiments seem to be contradictory, as they do not fully support the previously reported concerted or stepwise mechanisms. But theoretical calculations have provided the reaction profiles for both the silver and copper systems with different olefins to satisfy all experimental data. A mechanistic proposal has been made on the basis of the information that we collected from experimental and theoretical studies. In all cases, the reaction starts with the formation of a metal-nitrene species that holds some radical character, and therefore the aziridination reaction proceeds through the radical mechanism. The silver-based systems however hold a minimum energy crossing point (MECP) between the triplet and closed-shell singlet surfaces, which induce the direct formation of the aziridines, and stereochemistry of the olefin is retained. In the case of copper, a radical intermediate is formed, and this intermediate constitutes the starting point for competition steps involving ring-closure (through a MECP between the open-shell singlet and triplet surfaces) or carbon-carbon bond rotation, and explains the loss of stereochemistry with a given substrate. Overall, all the initially contradictory experimental data fit in a mechanistic proposal that involves both the singlet and the triplet pathways.

Computational study of the double C–Cl bond activation of dichloromethane and phosphinealkylation at [CoCl(PR3)3

Density functional theory calculations have been employed to model the double C-Cl bond activation of CH(2)Cl(2) at [CoCl(PR(3))(3)] to give [CoCl(3)(CH(2)PR(3))(PR(3))(2)]. Calculations incorporating dichloromethane solution (PCM approach) on a [CoCl(PMe(3))(3)] model system showed the two C-Cl cleavage steps to involve different mechanisms. The first C-Cl cleavage step occurs on the triplet surface and proceeds via Cl abstraction with a barrier of 19.1 kcal mol(-1). Radical recombination would then give singlet mer,trans-[CoCl(2)(CH(2)Cl)(PMe(3))(3)] with an overall free energy change of +1.8 kcal mol(-1). Alternative C-Cl activation processes based on nucleophilic attack by the Co centre at dichloromethane with loss of Cl(-) have significantly higher barriers. The second C-Cl cleavage occurs via nucleophilic attack of PMe(3) at the CH(2)Cl ligand with formation of a new P-C bond and displacement of Cl(-). This may either occur in an intermolecular fashion (after prior PMe(3) dissociation) or intramolecularly. Both processes have similar barriers of ca. 12 kcal mol(-1). The comproportionation of [CoCl(3)(CH(2)PMe(3))(PMe(3))(2)] with [CoCl(PMe(3))(3)] to give [CoCl(2)(CH(2)PMe(3))(PMe(3))], [CoCl(2)(PMe(3))(2)] and 2 PMe(3) is computed to be strongly exergonic, consistent with the observation of this process in analogous experimental systems.

A theoretical study of the mechanisms of oxidation of ethylene by manganese oxo complexes

The mechanisms of oxidation of ethylene by manganese-oxo complexes of the type MnO3L (L = O(-), Cl, CH3, OCH3, Cp, NPH3) have been explored on the singlet, doublet, triplet and quartet potential energy surfaces at the B3LYP/LACVP* level of theory and the results discussed and compared with those of the technetium and rhenium oxo complexes we reported earlier, thereby drawing group trends in the reactions of this important class of oxidation catalysts. In the reactions of MnO3L (L = O(-), Cl(-), CH3, OCH3, Cp, NPH3) with ethylene, it was found that the formation of the dioxylate intermediate along the concerted [3 + 2] addition pathway on the singlet potential energy is favored kinetically and thermodynamically over its formation by a two-step process via the metallaoxetane by [2 + 2] addition. The activation barriers for the formation of the dioxylate and the product stabilities on the singlet PES for the ligands studied are found to follow the order: NPH3 < Cl(-) < CH3O(-) < Cp < O(-) < CH3. On the doublet PES, the activation barriers for the formation of the dioxylate intermediate for the ligands are found to follow the order: CH3O(-) < Cl(-) < Cp < CH3 while the order of product stabilities is: Cl(-) < CH3O(-) < Cp < CH3. The order of dioxylate product stabilities on the triplet surface for the ligands studied is: Cl(-) < CH3O(-) < Cp < CH3 < NPH3 < O(-) and the order on the quartet surface is O(-) < Cp < CH3 < NPH3 < Cl(-) < CH3O(-). The re-arrangement of the metallaoxetane intermediate to the dioxylate is not a feasible reaction for all the ligands studied. Of the group VII B metal oxo complexes studied, MnO4(-) and MnO3(OCH3) appear to be the best catalysts for the exclusive formation of the dioxylate intermediate, MnO3(OCH3) being better so on both kinetic and thermodynamic grounds. The best epoxidation catalyst for the Mn complexes is MnO3Cl; the formation of the epoxide precursor will not result from the reaction of LMnO3 (L = O(-), Cp) with ethylene on any of the surfaces studied. The trends observed for the oxidation reactions of the Mn complexes with ethylene compare closely with those reported by us for the ReO3L and TcO3L (L = O(-), Cl, CH3, OCH3, Cp, NPH3) complexes, but there is far greater similarity between the Re and Tc complexes than between Mn and either of the other two. There does not appear to be any singlet-triplet or doublet-quartet spin-crossover in any of the pathways studied.

A density functional theory study of the mechanisms of oxidation of ethylene by rhenium oxide complexes

The oxo complexes of group VII B are of great interest for their potential toward epoxidation and dihydroxylation. In this work, the mechanisms of oxidation of ethylene by rhenium-oxo complexes of the type LReO3 (L = O(-), Cl, CH3, OCH3, Cp, NPH3) have been explored at the B3LYP/LACVP* level of theory. The activation barriers and reaction energies for the stepwise and concerted addition pathways involving multiple spin states have been computed. In the reaction of LReO3 (L = O(-), Cl, CH3, OCH3, Cp, NPH3) with ethylene, it was found that the concerted [3 + 2] addition pathway on the singlet potential energy surfaces leading to the formation of a dioxylate intermediate is favored over the [2 + 2] addition pathway leading to the formation of a metallaoxetane intermediate and its re-arrangement to form the dioxylate. The activation barrier for the formation of the dioxylate on the singlet PES for the ligands studied is found to follow the order O(-) > CH3 > NPH3 > CH3O(-) > Cl(-) > Cp and the reaction energies follow the order CH3 > O(-) > NPH3 > CH3O(-) > Cl(-) > Cp. On the doublet PES, the [2 + 2] addition leading to the formation the metallaoxetane intermediate is favored over dioxylate formation for the ligands L = CH3, CH3O(-), Cl(-). The activation barriers for the formation of the metallaoxetane intermediate are found to increase for the ligands in the order CH3 < Cl(-) < CH3O(-) while the reaction energies follow the order Cl(-) < CH3O(-) < CH3. The subsequent re-arrangement of the metallaoxetane intermediate to the dioxylate is only feasible in the case of ReO3(OCH3). Of all the complexes studied, the best dioxylating catalyst is ReO3Cp (singlet surface); the best epoxidation catalyst is ReO3Cl (singlet surface); and the best metallaoxetane formation catalyst is ReO3(NPH3) (triplet surface).

Inside Back Cover: The Triplet Surface of the Zimmerman Di-π-Methane Rearrangement of Dibenzobarrelene (Angew. Chem. Int. Ed. 52/2012)

The di-π-methane rearrangement of dibenzobarrelene, described with a two-step mechanism on the triplet state, was pioneered by Howard Zimmerman (1926–2012). In their Communication on page 13097 ff., R. A. Matute and K. N. Houk give a novel mechanistic insight of competing one-step and two-step pathways on the triplet surface. The picture shows a dromedary and a bactrian camel with the transition states on their humps.

Spectroscopy and dynamics of the predissociated, quasi-linear S2 state of chlorocarbene

In this work, we report on the spectroscopy and dynamics of the quasi-linear S(2) state of chlorocarbene, CHCl, and its deuterated isotopologue using optical-optical double resonance (OODR) spectroscopy through selected rovibronic levels of the S(1) state. This study, which represents the first observation of the S(2) state in CHCl, builds upon our recent examination of the corresponding state in CHF, where pronounced mode specificity was observed in the dynamics, with predissociation rates larger for levels containing bending excitation. In the present work, a total of 14 S(2) state vibrational levels with angular momentum l = 1 were observed for CHCl, and 34 levels for CDCl. The range of l in this case was restricted by the pronounced Renner-Teller effect in the low-lying S(1) levels, which severely reduces the fluorescence lifetime for levels with K(a) > 0. Nonetheless, by exploiting different intermediate S(1) levels, we observed progressions involving all three fundamental vibrations. For levels with long predissociation lifetimes, rotational constants were determined by measuring spectra through different intermediate J levels of the S(1) state. Plots of the predissociation linewidth (lifetime) vs. energy for various S(2) levels show an abrupt onset, which lies near the calculated threshold for elimination to form C((3)P) + HCl on the triplet surface. Our experimental results are compared with a series of high level ab initio calculations, which included the use of a dynamically weighted full-valence CASSCF procedure, focusing maximum weight on the state of interest (the singlet and triplet states were computed separately). This was used as the reference for subsequent Davidson-corrected MRCI(+Q) calculations. These calculations reveal the presence of multiple conical intersections in the singlet manifold.

Reactivity of Compound II: Electronic Structure Analysis of Methane Hydroxylation by Oxoiron(IV) Porphyrin Complexes

The methane hydroxylation reaction by a Compound II (Cpd II) mimic PorFe(IV)=O and its hydrosulfide-ligated derivative [Por(SH)Fe(IV)=O](-) is investigated by density functional theory (DFT) calculations on the ground triplet and excited quintet spin-state surfaces. On each spin surface both the σ- and π-channels are explored. H-abstraction is invariably the rate-determining step. In the case of PorFe(IV)=O the H-abstraction reaction can proceed either through the classic π-channel or through the nonclassical σ-channel on the triplet surface, but only through the classic σ-mechanism on the quintet surface. The barrier on the quintet σ-pathway is much lower than on the triplet channels so the quintet surface cuts through the triplet surfaces and a two state reactivity (TSR) mechanism with crossover from the triplet to the quintet surface becomes a plausible scenario for C-H bond activation by PorFe(IV)=O. In the case of the hydrosulfide-ligated complex the H-abstraction follows a π-mechanism on the triplet surface: the σ* is too high in energy to make a σ-attack of the substrate favorable. The σ- and π-channels are both feasible on the quintet surface. As the quintet surface lies above the triplet surface in the entrance channel of the oxidative process and is highly destabilized on both the σ- and π-pathways, the reaction can only proceed on the triplet surface. Insights into the electron transfer process accompanying the H-abstraction reaction are achieved through a detailed electronic structure analysis of the transition state species and the reactant complexes en route to the transition state. It is found that the electron transfer from the substrate σ(CH) into the acceptor orbital of the catalyst, the Fe-O σ* or π*, occurs through a rather complex mechanism that is initiated by a two-orbital four-electron interaction between the σ(CH) and the low-lying, oxygen-rich Fe-O σ-bonding and/or Fe-O π-bonding orbitals of the catalyst.

Experimental and theoretical studies of the O(3P) + C2H4 reaction dynamics: Collision energy dependence of branching ratios and extent of intersystem crossing

The reaction of O((3)P) with C(2)H(4), of importance in combustion and atmospheric chemistry, stands out as paradigm reaction involving not only the indicated triplet state potential energy surface (PES) but also an interleaved singlet PES that is coupled to the triplet surface. This reaction poses great challenges for theory and experiment, owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Crossed molecular beam (CMB) scattering experiments with soft electron ionization detection are used to disentangle the dynamics of this polyatomic multichannel reaction at a collision energy E(c) of 8.4 kcal∕mol. Five different primary products have been identified and characterized, which correspond to the five exothermic competing channels leading to H + CH(2)CHO, H + CH(3)CO, CH(3) + HCO, CH(2) + H(2)CO, and H(2) + CH(2)CO. These experiments extend our previous CMB work at higher collision energy (E(c) ∼ 13 kcal∕mol) and when the results are combined with the literature branching ratios from kinetics experiments at room temperature (E(c) ∼ 1 kcal∕mol), permit to explore the variation of the branching ratios over a wide range of collision energies. In a synergistic fashion, full-dimensional, QCT surface hopping calculations of the O((3)P) + C(2)H(4) reaction using ab initio PESs for the singlet and triplet states and their coupling, are reported at collision energies corresponding to the CMB and the kinetics ones. Both theory and experiment find almost an equal contribution from the triplet and singlet surfaces to the reaction, as seen from the collision energy dependence of branching ratios of product channels and extent of intersystem crossing (ISC). Further detailed comparisons at the level of angular distributions and translational energy distributions are made between theory and experiment for the three primary radical channel products, H + CH(2)CHO, CH(3) + HCO, and CH(2) + H(2)CO. The very good agreement between theory and experiment indicates that QCT surface-hopping calculations, using reliable coupled multidimensional PESs, can yield accurate dynamical information for polyatomic multichannel reactions in which ISC plays an important role.

Competitive activation of C–H and C–F bonds in gas phase reaction of Ir+ with CH3F: A DFT study

The spin-forbidden reaction mechanism of Ir+ with CH3F to yield H2 or HF has been investigated using density functional theory (DFT) calculations. The overall H2 elimination reaction is calculated to be exothermic, by 36.3 kcal/mol. One pathway is found on three potential energy surfaces (PESs), whereas the HF-elimination reaction is exothermic by 32.8 kcal/mol, two reaction pathways are identified on the quintet and triplet surfaces, only one reaction pathway is identified on the singlet surface. The reaction mechanism can be explained by the simple donor–acceptor model. According to the revealed mechanism and the calculated potential energy surfaces, H2-elimination is energetically much more favorable than HF-elimination. These conclusions are consistent with the experimental observations.

DFT Functional Benchmarking on the Energy Splitting of Chromium Spin States and Mechanistic Study of Acetylene Cyclotrimerization over the Phillips Cr(II)/Silica Catalyst

In this work, a two-state reaction mechanism for the acetylene cyclotrimerization over a cluster model for the Phillips Cr(II)/silica catalyst were systematically investigated using density functional theory (DFT). Since spin crossover phenomenon was confirmed in the catalytic cycle, an accurate prediction of the energy gap between low- and high-spin states is crucial for the description of a reaction involving a two-state reactivity. Therefore, a massive DFT functional benchmarking test has been conducted on the cluster model by taking a CASPT2 energy gap as a reference. Consequently, B3PW91* with 28% Hartree-Fock exchange energy was selected for the following mechanistic investigation. Each of the possible potential energy surface including singlet, triplet, and quintet surfaces was explored. On the quintet surface, the reaction begins with a coordination of an acetylene on the chromium center to generate a π-coordinated complex. The following oxidative coupling through further coordination with a second acetylene was predicted to be a two-step reaction to generate a chromacyclopentadiene species. This transformation was found to be energetically prohibitive by the presence of the transition state (5)TS[C-E] (ΔG(‡) = 31.1 kcal/mol). On the triplet surface, however, the coordination of an acetylene generates a chromacyclopropene species without showing any activation barrier. The second acetylene incorporation proceeding via a coordination on the chromium center followed by an insertion into a Cr-C σ-bond of the chromacyclopropene was predicted to be a facile reaction pathway (ΔG(‡) = 10.2 kcal/mol). The third acetylene was captured by the cluster model through the formation of a hydrogen bond. The later transformation on the triplet surface was found to be an intermolecular [4 + 2] cycloaddition to finish the cyclization. The lack of the aromaticity of the benzene ring in (3)L results in an uncompleted reaction pathway on a single triplet surface. Consequently, a two-state reaction pathway that is connected by two low-lying minimum-energy crossing points (MECPs) of the two surfaces is thus described. It is worthy of note that the third acetylene in the tri(acetylene)chromium complex captured by the cluster model only through the formation of a hydrogen bond rules out the [2 + 2 + 2] concerted one-step reaction pathway proposed by Zecchina et al. [Phys. Chem. Chem. Phys.2003, 5, 4414]. The singlet reaction profile is far higher in energy compared with that proceeded on the triplet and quintet surfaces.

O2(a1Δg) + Mg, Fe, and Ca: Experimental kinetics and formulation of a weak collision, multiwell master equation with spin-hopping

The first excited electronic state of molecular oxygen, O(2)(a(1)Δ(g)), is formed in the upper atmosphere by the photolysis of O(3). Its lifetime is over 70 min above 75 km, so that during the day its concentration is about 30 times greater than that of O(3). In order to explore its potential reactivity with atmospheric constituents produced by meteoric ablation, the reactions of Mg, Fe, and Ca with O(2)(a) were studied in a fast flow tube, where the metal atoms were produced either by thermal evaporation (Ca and Mg) or by pulsed laser ablation of a metal target (Fe), and detected by laser induced fluorescence spectroscopy. O(2)(a) was produced by bubbling a flow of Cl(2) through chilled alkaline H(2)O(2), and its absolute concentration determined from its optical emission at 1270 nm (O(2)(a(1)Δ(g) - X(3)Σ(g) (-)). The following results were obtained at 296 K: k(Mg + O(2)(a) + N(2) → MgO(2) + N(2)) = (1.8 ± 0.2) × 10(-30) cm(6) molecule(-2) s(-1); k(Fe + O(2)(a) → FeO + O) = (1.1 ± 0.1) × 10(-13) cm(3) molecule(-1) s(-1); k(Ca + O(2)(a) + N(2) → CaO(2) + N(2)) = (2.9 ± 0.2) × 10(-28) cm(6) molecule(-2) s(-1); and k(Ca + O(2)(a) → CaO + O) = (2.7 ± 1.0) × 10(-12) cm(3) molecule(-1) s(-1). The total uncertainty in these rate coefficients, which mostly arises from the systematic uncertainty in the O(2)(a) concentration, is estimated to be ±40%. Mg + O(2)(a) occurs exclusively by association on the singlet surface, producing MgO(2)((1)A(1)), with a pressure dependent rate coefficient. Fe + O(2)(a), on the other hand, shows pressure independent kinetics. FeO + O is produced with a probability of only ∼0.1%. There is no evidence for an association complex, suggesting that this reaction proceeds mostly by near-resonant electronic energy transfer to Fe(a(5)F) + O(2)(X). The reaction of Ca + O(2)(a) occurs in an intermediate regime with two competing pressure dependent channels: (1) a recombination to produce CaO(2)((1)A(1)), and (2) a singlet∕triplet non-adiabatic hopping channel leading to CaO + O((3)P). In order to interpret the Ca + O(2)(a) results, we utilized density functional theory along with multireference and explicitly correlated CCSD(T)-F12 electronic structure calculations to examine the lowest lying singlet and triplet surfaces. In addition to mapping stationary points, we used a genetic algorithm to locate minimum energy crossing points between the two surfaces. Simulations of the Ca + O(2)(a) kinetics were then carried out using a combination of both standard and non-adiabatic Rice-Ramsperger-Kassel-Marcus (RRKM) theory implemented within a weak collision, multiwell master equation model. In terms of atmospheric significance, only in the case of Ca does reaction with O(2)(a) compete with O(3) during the daytime between 85 and 110 km.