Conical and glancing Jahn-Teller intersections in the cyclic trinitrogen cation

The ground and electronically excited states of cyclic N(3) (+) are characterized at the equilibrium D(3h) geometry and along the Jahn-Teller distortions. Lowest excited states are derived from single excitations from the doubly degenerate highest occupied molecular orbitals (HOMOs) to the doubly degenerate lowest unoccupied molecular orbitals (LUMOs), which give rise to two exactly and two nearly degenerate states. The interaction of two degenerate states with two other states eliminates linear terms and results in a glancing rather than conical Jahn-Teller intersection. HOMO-2-->LUMOs excitations give rise to two regular Jahn-Teller states. Optimized structures, vertical and adiabatic excitation energies, frequencies, and ionization potential (IP) are presented. IP is estimated to be 10.595 eV, in agreement with recent experiments.     

One-electron-transfer reactions of polychlorinated ethylenes: concerted and stepwise cleavages

Reaction barriers were calculated by using ab initio electronic structure methods for the reductive dechlorination of the polychlorinated ethylenes: C2Cl4, C2HCl3, trans-1,2-C2H2Cl2, cis-1,2-C2H2Cl2, 1,1-C2H2Cl2 and C2H3Cl. Concerted and stepwise cleavages of R-Cl bonds were considered. Stepwise cleavages yielded lower activation barriers than concerted cleavages for the reduction of C2Cl4, C2HCl3, and trans-1,2-C2H2Cl2 for strong reducing agents. However, for typical ranges of reducing strength concerted cleavages were found to be favored. Both gas-phase and aqueous-phase calculations predicted C2Cl4 to have the lowest reaction barrier. Additionally, the reduction of C2HCl3 was predicted to show selectivity toward formation of cis-1,2-C2HCl2* over the formation of trans-1,2-C2HCl2*, and 1,1-C2HCl2* radicals.     

Mechanism of halogen-catalyzed Mukaiyama aldol reactions: concerted or stepwise?

The halogen-catalyzed (I₂, Br₂, and Cl₂) Mukaiyama aldol (MA) reactions were investigated by ab initio MO calculations. The halogen-catalyzed MA reaction between a trihydrosilyl enol ether and formaldehyde favors a concerted pathway. In sharp contrast, the I₂-catalyzed reaction between 1-phenyl-1-(trimethylsilyloxy)ethylene and benzaldehyde favors a stepwise mechanism. The nature of the substituent strongly influences the type of mechanism involved.     

Interaction of monovalent cations with salicylaldehyde: Evidence for excited-state ion-pair dissociation

Interaction of monovalent cations with salicylaldehyde resulted in the formation of tight ion-pairs. In these ion-pairs salicylaldehyde acts as a bidentate anion through the phenolate and the carbonyl oxygen atoms. Para substituted phenols, on the other hand, form less tight ion-pairs with monovalent cations. Red shifts in both absorption and emission spectra of these ion-pairs were observed as the ionic radius of the metal ion is increased. In presence of 18-crown-6, solventseparated ion-pairs are formed at the expense of contact ion-pairs. Evidence for excited-state dissociation of the solvent-separated ion-pairs is reported.     

Bridge mediated two-electron transfer reactions: analysis of stepwise and concerted pathways

A theory of nonadiabatic donor (D)-acceptor (A) two-electron transfer (TET) mediated by a single regular bridge (B) is developed. The presence of different intermediate two-electron states connecting the reactant state D-(-)BA with the product state DBA-(-) results in complex multiexponential kinetics. The conditions are discussed at which a reduction to two-exponential as well as single-exponential kinetics becomes possible. For the latter case the rate KTET is calculated, which describes the bridge-mediated reaction as an effective two-electron D-A transfer. In the limit of small populations of the intermediate TET states D-B-A, DB-(-)A, D-BA-, and DB-A-, KTET is obtained as a sum of the rates KTET(step) and KTET(sup). The first rate describes stepwise TET originated by transitions of a single electron. It starts at D-(-)BA and reaches DBA-(-) via the intermediate state D-BA-. These transitions cover contributions from sequential as well as superexchange reactions all including reduced bridge states. In contrast, a specific two-electron superexchange mechanism from D-(-)BA to DBA-(-) defines KTET(sup). An analytic dependence of KTET(step) and KTET(sup) on the number of bridging units is presented and different regimes of D-A TET are studied.     

Thermodynamics of stepwise dissociation of D,L-tryptophan in aqueous solution

The heat effects of the reaction of a solution of D,L-tryptophan with solutions of KOH and HNO₃ were measured by calorimetry at temperatures of 288.15, 298.15, and 308.15 K and ionic strength values of 0.25, 0.5, and 0.75 (KNO₃). The effect of the temperature and concentration of the background electrolyte on the heats of dissociation of amino acid was considered. The standard heat effects of ionization of D,L-tryptophan were determined by extrapolation to zero ionic strength. The standard thermodynamical characteristics of the stepwise dissociation of D,L-tryptophan were calculated.     

Is the decomposition of the primary ozonide a concerted or stepwise process?

An ab initio SCF-MO calculation shows that both concerted and stepwise processes are equally probable for the cleavage of the primary ozonide, leading to the Criegee intermediate. The value of the corresponding activation energy is discussed, and the results are found in agreement with the stereochemistry of the reaction. A very facile ring opening is also predicted.     

A change from stepwise to concerted mechanism in the acid-catalysed benzidine rearrangement: a theoretical study

The monoprotonated mechanism of the benzidine acid-catalysed rearrangement of hydrazobenzene (corresponding to a second order kinetics) is studied and compared with the diprotonated mechanism (corresponding to a third order reaction and previously studied). The nature of the two mechanisms is found to be completely different: a concerted closed-shell sigmatropic shift in the monoprotonated, a stepwise radical cation recoupling in the diprotonated. The activation energies, combined with the energetics of the protonation equilibria, are also very different: 35 kcal mol^?1 for the former and 26 kcal mol^?1 for the latter (in good agreement with the experimental data). These values make the third order diprotonated mechanism favoured with respect to the second order monoprotonated mechanism for the rearrangement of hydrazobenzene, as found at typical experimental acid concentrations.     

Competition between diradical stepwise and concerted mechanisms in chalcogeno-Diels-Alder reactions: a density functional study

The chalcogeno-Diels-Alder reactions of H(2)C=X (X = S, Se, Te) with butadiene, with trans,trans- and cis,trans-2,4-hexadiene, as well as of ethylene with thio-, seleno-, and telluroacrolein and reactions of thioformaldehyde with thioacrolein are examined theoretically. The B3LYP exchange-correlation functional with the 6-31G(d) and LanL2DZ(d) basis sets is employed. Stepwise diradical and concerted pathways are considered for all reactants. A modified concerted mechanism via a pre-reaction complex followed by a concerted transition state is studied for thioformaldehyde reacting with thioacrolein. The stepwise diradical pathways are predicted to be energetically less favorable than the concerted pathways for all cases considered. Even the sterically hindered reaction between selenoformaldehyde and cis,trans-2,4-hexadiene prefers a concerted path. It is a considerable challenge to reverse this energy preference for the concerted reaction given that both electronic and steric factors act to increase or decrease the activation energies of the concerted and diradical stepwise paths in the same way. A modified concerted mechanism operates for reagents with very small HOMO-LUMO gaps such as thioformaldehyde and thioacrolein. This mechanism is completely synchronous, with a vanishingly small barrier.     

Biscyclization reactions in butadiyne- and ethyne-linked triazenes and diazenes: concerted versus stepwise coarctate cyclizations

A series of alkyne-linked bis-2H-indazoles has been prepared by the double cyclization of ethyne- or butadiyne-linked phenyltriazene or phenyldiazene moieties. Even though there are two five-membered ring cyclizations and several triple bond shifts involved, the reactions proceed rapidly under neutral conditions with mild heating, affording the heterocycles in excellent yields. DFT calculations, in agreement with experimental observations, indicate that the reactions: (1) occur via a very short-lived carbene intermediate, (2) are concerted via an asymmetrical transition state, or (3) are even synchronous, with as many as 16 bonds that are made or broken simultaneously. The biscyclizations presented herein strikingly illustrate the concept of coarctate reactions, the stabilization of transition states by coarctate M?bius aromaticity, the ethynologation principle, and the stereochemical rules.     

Thermodynamic characteristics of stepwise dissociation of glycyl-glycyl-glycine in aqueous solutions

The heats of interaction between glycyl-glycyl-glycine and solutions of nitric acid and potassium hydroxide are determined by calorimetry at 298.15 K and ionic strengths of 0.25, 0.50, and 0.75 in the presence of KNO₃. The standard thermodynamic characteristics (Δ_r H°, Δ_r G°, and Δ_r S°) of acid–base interaction in aqueous solutions of the peptide are calculated. The effect of background electrolyte concentration on the enthalpy of dissociation of glycyl-glycyl-glycine is discussed.     

Theoretical studies on the ene reaction mechanisms of propene and cyclopropene with ethylene and cyclopropene: concerted or stepwise

The potential energy surfaces of the ene reactions of propene and cyclopropene with ethylene and cyclopropene were studied by ab initio molecular orbital (MO) methods. The reaction mechanisms were analyzed by CiLC method on the basis of CASSCF MOs. The concerted and stepwise reaction pathways of the ene reaction of propene with ethylene as the parent reaction were located. The energy barrier of the stepwise process is about 4 kcal/mol lower than that of the concerted one. The other reactions can be found only the stepwise mechanism. Although the endo-type reaction of propene with cyclopropene, where cyclopropene is the enophile, probably occurs through a one-step process, the mechanism is divided into the CC bond formations and the hydrogen migration as a stepwise reaction. The CiLC-IRC analysis of the concerted process of propene with ethylene shows the different patterns of the electronic state variation for the CC bond formation/breaking and the hydrogen migration.     

Is the McLafferty rearrangement of ketones concerted or stepwise? The application of kinetic isotope effects

Intramolecular ¹³C and ²H isotope effects have been measured for unimolecular losses of ethene (the McLafferty rearrangement) from metastable molecular ions of 2-ethyl-1-phenylbutanl-1-one, 3-ethylpentan-2-one and heptan-4-one. Primary and secondary deuterium isotope effects are observed at the γ-(terminal) and β-positions, respectively. Large primary ¹³C isotope effects occur at β-positions and for the y positions of γ-ethylpentan-2-one and heptan-4-one. The carbon isotope effects in the cases of the doubly isotopically labelled CH₃COCH(C₂H₅)(¹³CH₂CH₃) and CD₃COCD(C₂D₅)(¹³CD₂CD₃) are 1.17 (±0.01) and 1.04 (±0.01), respectively. All of these isotope effects are consistent with a stepwise mechanism in which more than one step is rate determining.     

Orientational ordering in hard rectangles: The role of three-body correlations

We investigate the effect of three-body correlations on the phase behavior of hard rectangle two-dimensional fluids. The third virial coefficient B3 is incorporated via an equation of state that recovers scaled particle theory for parallel hard rectangles. This coefficient, a functional of the orientational distribution function, is calculated by Monte Carlo integration, using an accurate parametrized distribution function, for various particle aspect ratios in the range of 1-25. A bifurcation analysis of the free energy calculated from the obtained equation of state is applied to find the isotropic (I)-uniaxial nematic (N(u)) and isotropic-tetratic nematic (N(t)) spinodals and to study the order of these phase transitions. We find that the relative stability of the N(t) phase with respect to the isotropic phase is enhanced by the introduction of B3. Finally, we have calculated the complete phase diagram using a variational procedure and compared the results with those obtained from scaled particle theory and with Monte Carlo simulations carried out for hard rectangles with various aspect ratios. The predictions of our proposed equation of state as regards the transition densities between the isotropic and orientationally ordered phases for small aspect ratios are in fair agreement with simulations. Also, the critical aspect ratio below which the N(t) phase becomes stable is predicted to increase due to three-body correlations, although the corresponding value is underestimated with respect to simulation.     

Two paths for the formation of pyrimidines from sym-triazine

The reaction of sym-triazine with the hydrochlorides of N-unsubstituted enaminones gives mixtures of 4-alkyl-5-acyl(ethoxycarbonyl)pyrimidines and 2,6-dialkyl-3,5-diacyl(ethoxycarbonyl)pyridines. The reaction mechanism was studied by means of the ¹⁵N-labelled compounds.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1243–1249, September, 1992.     

McLafferty rearrangement of the radical cations of butanal and 3-fluorobutanal: a theoretical investigation of the concerted and stepwise mechanisms

The stepwise and concerted pathways for the McLafferty rearrangement of the radical cations of butanal (Bu(+)) and 3-fluorobutanal (3F-Bu(+)) are investigated with density functional theory (DFT) and ab initio methods in conjunction with the 6-311+G(d,p) basis set. A concerted transition structure (TS) for Bu(+), (H), is located with a Gibbs barrier height of 37.7 kcal/mol as computed with CCSD(T)//BHandHLYP. Three pathways for the stepwise rearrangement of Bu(+) have been located, which are all found to involve different complexes. The barrier height for the H(gamma) transfer is found to be 2.2 kcal/mol, while the two most favorable TSs for the C(alpha)-C(beta) cleavage are located 8.9 and 9.2 kcal/mol higher. The energies of the 3F-Bu(+) system have been calculated with the promising hybrid meta-GGA MPWKCIS1K functional of DFT. Interestingly, the fluorine substitution yields a barrier height of only 20.5 kcal/mol for the concerted TS, (3F-H). A smaller computed dipole moment, 12.1 D, for (3F-H) compared with 103.2 D for (H) might explain the stabilization of the substituted TS. The H(gamma) transfer, with a barrier height of 4.9 kcal/mol, is found to be rate-determining for the stepwise McLafferty rearrangement of 3F-Bu(+), in contrast to the unsubstituted case. By inspection of the spin and charge distributions of the stationary points, it is noted that the bond cleavages in the concerted rearrangements are mainly of heterolytic nature, while those in the stepwise channels are found to be homolytic.     

Revisiting acido-basicity of the MgO surface by periodic density functional theory calculations: role of surface topology and ion coordination on water dissociation

Low-coordinated (LC) ions at the MgO surface (noted Mg2+LC and O2-LC with L = 1-5), located on monatomic and diatomic steps, corners, step divacancies, and kinks, have been modeled thanks to periodic density functional theory (DFT) calculations (VASP). Ions of lowest coordination induce the strongest surface geometry relaxation and the highest surface energies. The hydration energies of these sites and thermodynamic stabilities of the resulting surfaces were studied. The factors controlling the interaction strength between water and the surface are the possibility for the hydroxyl group to adopt a bridging geometry between two Mg2+ cations in concave areas of the surface, such as the bottom of the monatomic step, and at second order the surface atomic coordination, and especially the presence of three-coordinated ions. The Lewis basicity and acidity of O2-LC and Mg2+LC, respectively, increase as their coordination number decreases, which implies the same trend for the Br?nsted basicity of the Mg2+-O2- pair toward water. However, this trend can be changed if pairs leading to the formation of bridging OH groups are involved, typically on monatomic steps or in step divacancies where O2C-H and O3C-H are obtained, respectively, instead of the expected O1C-H. Thanks to thermodynamic calculations, the state of the surface as a function of temperature can be determined at a given pressure, unraveling the roles of surface topology and ions coordination.     

Understanding the mechanism of non-polar Diels-Alder reactions. A comparative ELF analysis of concerted and stepwise diradical mechanisms

The electron-reorganization along the concerted and stepwise pathways associated with the non-polar Diels-Alder reaction between cyclopentadiene (Cp, 1) and ethylene (2) has been studied using the topological analysis of the electron localization function (ELF) at the B3LYP/6-31G(d) level of theory. ELF results for the concerted mechanism stresses that the electron-reorganization demanded on the diene and ethylene reagents to reach two pseudo-diradical structures is responsible for the high activation energy. A comparative ELF analysis of some relevant points of the non-polar Diels-Alder reaction between Cp and styrene (10) suggests that these concerted mechanisms do not have a pericyclic electron-reorganization.