Computational studies on density function theory for the bimolecular metathesis reaction CH3+HCl⇔︁CH4+Cl

Density function theory has been applied to alkyl radical reaction to get helpful data for its geometric parameters, energy, and vibrational frequency compared with results obtained by ab initio methods and experimental values. The geometry optimization of the transition state, the precursor complex and the successor complex were performed at the 6‐311G* basis set level. The transition state of the CH₄Cl system of the reaction computed was in agreement with the prediction of Benson. From analysis of the vibration frequency and the net charge on the atom of the precursor complex, transition state, successor complex and the isolated state, the reaction mechanism was derived which we complicated with the bond‐rupture electron‐transfer theory. The atom H in molecule HCl attacks the atom C, forming a transition state via the precursor complex and the electron transfer happens in the precursor complex. The reaction rate of the electron transfer determines the rate of the whole reaction to a certain extent, and active energy, electronic coupling matrix element, and reorganization energy were obtained. © 2001 Wiley Periodicals, Inc. Int J Quantum Chem, 2001     

Vibrational spectroscopic study of the electron transfer for the Cl- + CH3Cl S(N)2 reaction in the gas phase

The vibrational spectra study of electron transfer of Cl- + CH3Cl --> ClCH3 + Cl- reaction has been examined by density function theory (DFT) calculations at 6-311 + +G** level in this paper. This reaction includes old bond rupture, new bond formation and electronic transfer in the intermolecular. The vibrational frequencies and vibrational modes of reactant, precursor complex, transition state, successor complex and product are analyzed. The relationship and the change among them can confirm the rupture of bond, the formation of bond and the process of electron transfer.     

Theoretical study on the partial potential energy surface and formation mechanism of the reactive resonance state of HO + CH4 → H2O + CH3 system

In the course of an extensive investigation aimed at understanding the detailed mechanism of a prototypical polyatomic reaction, several remarkable observations were uncovered. To interpret these findings, we surmise the existence of a reactive resonance in this polyatomic reaction. The concerned system is HO + CH₄ → H₂O + CH₃, of which the partial potential energy surface is constructed by the coupling between vibrational models and reactive coordinates. Then we explain the formation mechanism of the reactive resonance state by the partial potential energy surface. Finally, we estimated the lifetime of the resonance state, and it is about 45fs. The study of the reactive resonance in a polyatomic reaction is more than just an extension from a typical atom + diatom reaction. As shown here, it holds great promise to disentangle the elusive intramolecular vibrational dynamics of the transient collision complex in the critical transition‐state region. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Computational study on the reaction CH2CH2 + F --> CH2CHF + H

Previous ab initio studies on reactions involving radical addition to alkenes showed that such reactions are very sensitive to theoretical levels, and thus are difficult to deal with. This motivates us to theoretically reexamine the title reaction thoroughly, which has been studied only at several low levels of theory. In the present work, the geometry optimizations and energy calculations for all species involved in the title reaction were performed at several high levels of theory. The reaction mechanism of the title reaction is discussed at the CCSD(T)/aug-cc-pVDZ//CCSD/6-31G(d,p) theoretical level. According to our study, the fluorine addition to ethylene occurs via the formation of a prereaction complex with C2v symmetry, which is pointed out for the first time. The prereaction complex evolves into a fluoroethyl radical almost without a barrier, with an exothermicity of 41.49 kcal/mol. The fluoroethyl radical can further decompose into a hydrogen atom and fluoroethylene, with an energy release of 10.33 kcal/mol. Besides the direct departure of the hydrogen atom from the fluoroethyl radical, an indirect decomposition pathway may also be open, which has not been reported before. In addition, the formation of a fluoroethyl radical from a separate fluorine atom and ethylene is described pictorially via the molecular intrinsic characteristic contour (MICC) and the electron density mapped on it. Thereby, strong interpolarization and evident electron transfer between the fluorine atom and ethylene are observed as they approach each other. The transition structure for the fluorine addition to ethylene is clearly shown to be reactant-like. This provides new and intuitional insight into the title reaction.     

Indeno[1,2‐b]fluorene‐Based [2,2]Cyclophanes with 4n/4n and 4n/[4n+2] π Electrons: Syntheses,Structural Analyses,and Excitonic Coupling Properties

Indeno[1,2‐b]fluorene‐based [2,2]cyclophanes with 4n/4n and 4n/[4n+2] π‐electron systems were prepared, and their structures were identified by X‐ray crystallography. With short π–π distances around 3.0 Å, [2.2](5,11)indeno[1,2‐b]fluorenophane and its precursor [2.2](5,11)indeno[1,2‐b]fluorene‐6,12‐dionophane exhibit remarkable transannular interactions, leading to their unusual electrochemical and photophysical properties. With the aid of femtosecond transient absorption spectroscopy, the transition from the monomeric excited state to the redshifted H‐type dimeric state was first observed, correlating to the calculated excitonic energy splitting and the steady‐state absorption spectra induced by charge‐transfer‐mediated superexchange interaction.     

Indeno[1,2‐b]fluorene‐Based [2,2]Cyclophanes with 4n/4n and 4n/[4n+2] π Electrons: Syntheses,Structural Analyses,and Excitonic Coupling Properties

Indeno[1,2‐b]fluorene‐based [2,2]cyclophanes with 4n/4n and 4n/[4n+2] π‐electron systems were prepared, and their structures were identified by X‐ray crystallography. With short π–π distances around 3.0 Å, [2.2](5,11)indeno[1,2‐b]fluorenophane and its precursor [2.2](5,11)indeno[1,2‐b]fluorene‐6,12‐dionophane exhibit remarkable transannular interactions, leading to their unusual electrochemical and photophysical properties. With the aid of femtosecond transient absorption spectroscopy, the transition from the monomeric excited state to the redshifted H‐type dimeric state was first observed, correlating to the calculated excitonic energy splitting and the steady‐state absorption spectra induced by charge‐transfer‐mediated superexchange interaction.     

Direct ab initio dynamics study of the reaction of C2(A3Πu) with CH4

The reaction of C₂(A³Π_u) with CH₄ has been investigated over a wide temperature range 200–3,000 K by direct ab initio dynamics method at the BMC‐CCSD//BB1K/6‐311+G(2d,2p) level of theory. The optimized geometries and frequencies of the stationary points are calculated at the BB1K/6‐311+G(2d,2p) level, and then the energy profiles of the reactions are refined using the BMC‐CCSD method. The activation barrier height for H‐abstraction reaction was calculated to be 4.44 kcal/mol in temperature range (337–605 K), and the electron transfer behavior was also analyzed by quasi‐restricted molecular orbital method in detail. The canonical variational transition‐state theory (CVT) with the small curvature tunneling (SCT) correction method is used to calculate the rate constants over a wide temperature range 200–3,000 K. The theoretical results shows that variational effect is to some extent large in lower temperature range, and small curvature and tunneling effect play important roles to the H‐atom abstraction only at lower temperatures. The CVT/SCT rate constants are in good agreement with the available experimental results. Our theoretical study is expected to provide a direct insight into the reaction mechanism and may be useful for estimating the kinetics of the title reaction over a wide temperature range where no experimental data are available so far. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

A quantum electronic theory of chemical processes—The inverted energy profile case: CH3+ + H2 reaction

A quantum approach to chemical processes is developed. The chemical interconversion is described as an electronic process. The reaction corresponds to histories involving quantum states belonging to different stationary molecular Hamiltonians. The system may be embedded in a weak (thermal) and/or external electromagnetic field. The electromagnetic transverse fields lead to transition moments yielding finite probability amplitudes for the system to change from one quantum state to another. Bottleneck subspaces (transition states) are defined; they mediate the interconversions in generic unimolecular and bimolecular processes. Active precursor and successor complexes are introduced to help bridge reactant and product electronic states. The stationary states are modeled with Born-Oppenheimer Hamiltonians. At a qualitative level, the theory is general. The rate, measured as a time derivative of product concentration, is expressed in terms of concentrations of active precursor and successor complexes. The kinetic coefficients are given in terms of quantum processes involving electronic bottleneck states. Stationary structures and vibrational zero-point energies characterizing the reactive CH₃++H₂ system are determined at a Hartree-Fock level of theory with 6-31++G** basis set. The vibrational levels are corrected with anharmonicity effects. The saddle point of index one for hydrogen scrambling reactions has been obtained and shown to be related to the CH₅⁺ molecular complex together with the precursor and successor complexes geometries. The unusual properties of the system with respect to standard transition-state theory are fairly well described within this approach, in particular, isotope scrambling as well as photon emission during formation of the carbocation. The theory suggests that these types of reactions, which are found in outer space, may contribute to the scattering of the cosmic microwave background. © 1997 John Wiley & Sons, Inc.     

Ab initio study on the reaction of Sc+ + CH4 → Sc+(SINGLE BOND)CH2 + H2

An ab initio study on the reaction of the ground state (³D) and the excited state (¹D) of Sc⁺ with methane was performed. Reaction channels on the singlet and triplet potential surface (PES) and the reaction mechanism are examined and discussed. Three regions of the potential surface was studied: the molecular complex, the C(SINGLE BOND)H insertion products, and the transition states for the reaction. Comparisons between singlet and triplet PESs show that the excited state (¹D) of Sc⁺ has more reactivity with methane than does the ground state (³D) due to the spin quantum number conservation with the more stable insertion intermediate. © 1997 John Wiley & Sons, Inc.     

Derivatization of 4‐(Dimethylamino)benzamide to Dual Fluorescent Ionophores: Divergent Spectroscopic Effects Dependent on N or O Amide Chelation

Starting from the pentafluorophenyl ester of 4‐(dimethylamino)benzoic acid, two dual fluorescent amide ligands with aza‐15‐crown‐5 and 2‐(aminomethyl)pyridine were obtained for sensing, respectively, alkali (alkaline‐earth) and transition (heavy) metal cations. The crystal structure of the copper(II) complex is reported. The Cu²⁺ is coordinated through the pyridine N‐ and amide O‐atoms of two symmetry‐related ligands. The azacrown‐directed Ca‐chelation to the N‐atom of the amide leads to a slight quenching of the two fluorescence bands. In contrast, the pyridine directed Cu^II‐chelation to the O‐atom of the amide enhances the short‐wavelength emission 17‐fold over the locally excited state (LE), quenching the twisted intramolecular charge‐transfer (TICT) emission, and, as a result, the intensity ratio I(LE)/I(TICT) provides an accurate and sensitive measurement of the Cu^II concentration. These different cation effects are dependent on which atom (N vs. O) of the amide function participates in cation coordination: while the Ca²⁺ interaction with the N‐atom electron pair leads to the deconjugation of the amide N‐atom from the fluorophore, Cu²⁺ interaction with the lone pair of the O‐atom of the carbonyl group increases the energy of the n‐π* but also of the ¹L_a transition and therefore close the channel to the TICT state.     

Unimolecular Reaction Mechanism of an Imidazolin‐2‐ylidene: An iPEPICO Study on the Complex Dissociation of an Arduengo‐Type Carbene

The photoionization and dissociative photoionization of Im(iPr)₂, 1,3‐diisopropylimidazolin‐2‐ylidene, was investigated by imaging photoelectron photoion coincidence (iPEPICO) with vacuum ultraviolet (VUV) synchrotron radiation. A lone‐pair electron of the carbene carbon atom is removed upon ionization and the molecular geometry changes significantly. Only 0.5 eV above the adiabatic ionization energy, IE_ad=7.52±0.1 eV, the carbene cation fragments, yielding propene or a methyl radical in parallel dissociation reactions with appearance energies of 8.22 and 8.17 eV, respectively. Both reaction channels appear at almost the same photon energy, suggesting a shared transition state. This is confirmed by calculations, which reveal the rate‐determining step as hydrogen‐atom migration from the isopropyl group to the carbene carbon center forming a resonance‐stabilized imidazolium ion. Above 10.5 eV, analogous sequential dissociation channels open up. The first propene‐loss fragment ion dissociates further and another methyl or propene is abstracted. Again, a resonance‐stabilized imidazolium ion acts as intermediate. The aromaticity of the system is enhanced even in vertical ionization. Indeed, the coincidence technique confirms that a real imidazolium ion is produced by hydrogen transfer over a small barrier. The simple analysis of the breakdown diagram yields all the clues to disentangle the complex dissociative photoionization mechanism of this intermediate‐sized molecule. Photoelectron photoion coincidence is a promising tool to unveil the fragmentation mechanism of larger molecules in mass spectrometry.     

Theoretical study of the reactions of the 1Σ+ ground state of MS+ (M = Sc,Y, and La) with oxygen‐transfer reagent MS+ + CO → ScO+ + CS in the gas phase

The reaction mechanisms of the ¹Σ⁺ ground state of MS⁺ (M = Sc, Y, and La) with oxygen‐transfer reagent MS⁺ + CO → MO⁺ + CS in the gas phase has been proposed and investigated by ab initio methods with the 6‐31G* basis set for nonmetal atoms and the effective core potentials of Lanl2dz for the metal atoms. A carbon migration from oxygen atom to sulfur atom via a four‐center transition state is involved on the reaction potential surface. The activation energies of the reactions are 34.0, 24.1, and 36.7 kcal/mol relative to their corresponding reactants and the reaction heats are 15.7, 18.6, and 18.0 kcal/mol (respectively, for M = Sc, Y, and La) at the MP4 (SDTQ)/6‐31G*//MP2/6‐31G* level plus zero‐point energy, which indicates that the cationic yttrium sulfide is more favorable for this type of reaction. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Theoretical Study on the Substituent Effect on the Excited‐State Proton Transfer in the 7‐Azaindole‐Water Derivatives

The first excited‐state proton transfer (ESPT ) in 7AI ‐H₂O complex and its derivatives, in which the hydrogen atom at the C₂ position in pyrrole ring was replaced by halogen atom X (X = F, Cl, Br), were studied at the TD ‐M06‐2X/6‐31 + G(d, p) level. The double proton transfer took place in a concerted but asynchronous protolysis pathway. The vibrational‐mode selectivity of excited‐state double proton transfer in the model system was confirmed. The specific vibrational‐mode could shorten the reaction path and accelerate the reaction rate. The substituent effects on the excited‐state proton transfer process were discussed. When the H atom at C₂ position in 7AI ‐H₂O complex was replaced by halogen atom, some geometrical parameters changed obviously, the barrier height of ESDPT reduced, and the asynchronicity of proton transfer enlarged. The above changes correlated with the Pauling electronegativity of halogen atom.     

Mechanism for the Nonadiabatic Photooxidation of Benzene to Phenol: Orientation‐Dependent Proton‐Coupled Electron Transfer

An efficient catalytic one‐step conversion of benzene to phenol was achieved recently by selective photooxidation under mild conditions with 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) as the photocatalyst. Herein, high‐level electronic structure calculations in the gas phase and in acetonitrile solution are reported to explore the underlying mechanism. The initially populated ¹ππ* state of DDQ can relax efficiently through a nearby dark ¹nπ* doorway state to the ³ππ* state of DDQ, which is found to be the precursor state involved in the initial intermolecular electron transfer from benzene to DDQ. The subsequent triplet‐state reaction between DDQ radical anions, benzene radical cations, and water is computed to be facile. The formed DDQH and benzene‐OH radicals can undergo T₁→S₀ intersystem crossing and concomitant proton‐coupled electron transfer (PCET) to generate the products DDQH₂ and phenol. Two of the four considered nonadiabatic pathways involve an orientation‐dependent triplet PCET process, followed by intersystem crossing to the ground state (S₀). The other two first undergo a nonadiabatic T₁→S₀ transition to produce a zwitterionic S₀ complex, followed by a barrierless proton transfer. The present theoretical study identifies novel types of nonadiabatic PCET processes and provides detailed mechanistic insight into DDQ‐catalyzed photooxidation.     

Imaging the Reaction Dynamics of O(3P)+CH4→OH+CH3

The title reaction was studied in a crossed‐beam experiment, in which the ground‐state methyl products were probed using a time‐sliced velocity‐imaging technique. By taking images over the energy range of chemical significance, from the threshold to about 15 kcal mol^?1, the reactive excitation function as well as the dependences of product angular distributions and of the energy disposal on initial collision energies were determined. All experimental data are consistent with the picture that the ground‐state reaction of O(³P)+CH₄ proceeds via a direct abstraction rebound‐type mechanism with a narrow cone of acceptance. Deeper insights into the underlying mechanism and the key feature of the potential‐energy surface are elucidated by comparing the results with the corresponding observables in the analogous Cl+CH₄ reaction.     

Searching for resonances in the reaction Cl+CH4-->HCl+CH3: quantum versus quasiclassical dynamics and comparison with experiments

A quantum-mechanical (QM) and quasiclassical trajectory (QCT) study was performed on the title reaction, using a pseudotriatomic ab initio based surface. Probabilities and integral cross sections present some clear peaks versus the collision energy E(col), which we assign to Feshbach resonances of the transition state, where the light H atom oscillates between the heavy Cl and CH(3) groups. For ground-state reactants, reactivity is essentially of quantum origin (QCT observables and oscillations are smaller, or much smaller, than QM ones), and the calculated integral cross section and product distributions are in reasonable agreement with the experiment. The reaction occurs through an abstraction mechanism, following both a direct and an indirect mechanism. The quasiclassical trajectory calculations show the participation of a short-lived collision complex in the microscopic reaction mechanism. Finally, QCT differential cross sections of Cl+CH(4)-->HCl (nu(')=0 and 1)+CH(3) oscillate versus E(col), whereas experimentally this only occurs for HCl (nu(')=1). This theoretical result and other oscillating properties found here could, however, be related to the existence of a Feshbach resonance for the production of HCl (nu(')=1), as suggested by experimentalists.     

Kinetics for the hydrogen‐abstraction of CH4 with NO2

The detailed mechanism of the NO₂+CH₄ reaction has been computationally investigated at the M06‐2X/MG3S, B3LYP/6‐311G(2d,d,p), and MP2/6‐311+G(2df,p) levels. The direct dynamics calculations were preformed using canonical transition state theory with tunneling correction and scaled generalized normal‐mode frequencies including anharmonic torsion. The calculated results indicate that the NO₂+CH₄ reaction proceeds by three distinct channels simultaneously, leading to the formation of trans‐HONO (1a), cis‐HONO (1b), and HNO₂ (1c), and each channel involves the formation of intermediate having lower energy than the final product. The anti‐Hammond behavior observed in channel 1a is well analyzed. Proper treatment of anharmonic torsions about the C···H···O (or N) axis in the transition structures greatly improves the accuracy of kinetics predictions. The activation energy for each channel increases substantially with temperature, but is not strictly a linear function of temperature. Therefore, the thermal rate constants are fitted to the four‐parameter expression recommended for this case over the wide temperature range 400–4000 K. © 2012 Wiley Periodicals, Inc.     

Evidence for a Rapid Degenerate Hetero‐Cope‐Type Rearrangement in [Cp*W(S)2S‐CH2‐CHCH2]

Treatment of the salt [PPh₄]⁺[Cp*W(S)₃]^? ( 6 ) with allyl bromide gave the neutral complex [Cp*W(S)₂S‐CH₂‐CH?CH₂] ( 7 ). The product 7 was characterized by an X‐ray crystal structure analysis. Complex 7 features dynamic NMR spectra that indicate a rapid allyl automerization process. From the analysis of the temperature‐dependent NMR spectra a Gibbs activation energy of ΔG^≠ (278 K)≈13.7±0.1 kcal mol^?1 was obtained [ΔH^≠≈10.4±0.1 kcal mol^?1; ΔS^≠≈?11.4 cal mol^?1 K^?1]. The DFT calculation identified an energetically unfavorable four‐membered transition state of the “forbidden” reaction and a favorable six‐membered transition state of the “Cope‐type” allyl rearrangement process at this transition‐metal complex core.     

Thermochemistry and kinetics of the 2-butanone-4-yl CH3C(=O)CH2CH2• + O2 reaction system

Thermochemistry and kinetic pathways on the 2-butanone-4-yl (CH₃C(=O)CH₂CH₂•) + O₂ reaction system are determined. Standard enthalpies, entropies, and heat capacities are evaluated using the G3MP2B3, G3, G3MP3, CBS-QB3 ab initio methods, and the B3LYP/6-311g(d,p) density functional calculation method. The CH₃C(=O)CH₂CH₂• radical + O₂ association reaction forms a chemically activated peroxy radical with 35 kcal mol⁻¹ excess of energy. The chemically activated adduct can undergo RO−O bond dissociation, rearrangement via intramolecular hydrogen transfer reactions to form hydroperoxide-alkyl radicals, or eliminate HO₂ and OH. The hydroperoxide-alkyl radical intermediates can undergo further reactions forming ketones, cyclic ethers, OH radicals, ketene, formaldehyde, or oxiranes. A relatively new path showing a low barrier and resulting in reactive product sets involves peroxy radical attack on a carbonyl carbon atom in a cyclic transition state structure. It is shown to be important in ketones when the cyclic transition state has five or more central atoms.     

Theoretical insights into the excited‐state process of 4‐tert‐butyl‐2‐(5‐(5‐tert‐butyl‐2‐methoxyphenyl)thiazolo[5,4‐d]thiazol‐2‐yl)‐phenol

In this paper, we theoretically explore the motivation and behaviors of the excited‐state intramolecular proton transfer (ESIPT) reaction for a novel white organic light‐emitting diode (WOLED) material 4‐tert‐butyl‐2‐(5‐(5‐tert‐butyl‐2‐methoxyphenyl)thiazolo[5,4‐d]thiazol‐2‐yl)‐phenol (t‐MTTH). The “atoms in molecules” (AIM) method is adopted to verify the formation and existence of the hydrogen bond O? H···N. By analyzing the excited‐state hydrogen bonding behaviors via changes in the chemical bonding and infrared (IR) vibrational spectra, we confirm that the intramolecular hydrogen bond O? H···N should be getting strengthened in the first excited state in four kinds of solvents, thus revealing the tendency of ESIPT reaction. Further, the role of charge‐transfer interaction is addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. Also, the electron distribution confirms the ESIPT tendency once again. The scanned and optimized potential energy curves according to variational O? H coordinate in the solvents demonstrate that the proton transfer reaction should occur in the S₁ state, and the potential energy barriers along with ESIPT direction support this reaction. Based on the excited‐state behaviors reported in this work, the experimental spectral phenomenon has been reasonably explained.