Symmetrical Transition State Research Articles

Regular reaction dynamics in analytical form in the vicinity of symmetrical transition states. Central barrier crossings in SN2 reactions.

When an activated complex, as defined in transition state theory (TST), has a polyhedral shape, its kinetic energy is found to be diagonal in a system of spherical polar coordinates. If, in addition, the polyhedron is characterized by a high symmetry, then its dynamics considerably simplifies. An application of this approach to the most symmetrical TS known to date, i.e., that which controls the Cl- + CH3Cl → ClCH3 + Cl- SN2 nucleophilic substitution, is presented and an analytical expression of its potential energy surface is provided. In a substantial range around the saddle point, approximate equations of motion for the two components of the reaction coordinate, i.e., the antisymmetrical stretching motion of the ClCCl core and the wagging motion of the hydrogen triad, can be derived in an analytical form. During an extensive period of time, the main component of the reaction coordinate is governed by an unexpectedly simple equation of motion that depends on a single initial condition, irrespective of the other ones and of the internal energy. Reactive trajectories are observed to form a perfectly collimated bundle characterized by undetectable dispersion, thereby giving a spectacular example of regular dynamics in an anharmonic potential. Regularity and collimation are brought about by local symmetry, which is a widespread feature of potential energy surfaces. Anharmonicity is observed to influence the dynamics only at a late stage. As energy increases, trajectories tend to fan out and to deviate from the analytical equation. For the wagging motion, chaos sets in at much lower energies.

Correlation analysis of reactivity in the oxidation of some vicinal and non-vicinal diols by Tributylammonium chlorochromate: A kinetic and mechanistic approach

The kinetics of oxidation of four vicinal, four non-vicinal diols by tributylammonium chlorochromate (TBACC) has been studied in dimethylsulphoxide (DMSO). The main product of oxidation is the corresponding hydroxycarbonyl compound. The reaction is first order each in TBACC. Michaelies-Menten type of kinetics is observed with respect to the diols. The reaction is catalysed by hydrogen ions. The hydrogen ion dependence is taking the form: kobs = a + b[H+]. The oxidation of [1,1,2,2-2H4] ethanediol exhibits a substantial primary kinetic isotope effect (kH/kD = 5.81 at 298 K). The reaction has been studied in nineteen different organic solvents and the solvent effect has been analysed using Taft's and Swain's multiparametric equations. The temperature dependence of the kinetic isotope effect indicates the presence of a symmetrical transition state in the rate-determining step. A suitable mechanism has been proposed.

Parent, 34S, and deuterated triflic acid: Microwave spectra and tunneling splittings due to hydroxyl torsion

Spectra for the parent, 34S, and deuterated forms of triflic acid (CF3SO3H) have been observed by chirped-pulse and cavity Fourier transform microwave spectroscopy. The observation of a-, b-, and c-type transitions definitively establishes that the molecule does not have Cs symmetry, and M06-2X/6-311++G(3df,3pd) calculations concur, yielding a structure in which the OH bond is nearly perpendicular to the C-S-O(H) plane. The rotational spectrum for each isotopologue exhibits a pair of tunneling states resulting from large amplitude motion of the hydroxyl hydrogen between two equivalent structures with opposite directions of the OH bond. The experimentally determined tunneling energies, ΔE, for the parent and 34S species are 52.96784(65) MHz and 52.8761(16) MHz, respectively. For the -OD isotopologue, the tunneling energy decreases significantly, with a value of only ΔE = 0.2460(20) MHz. Curiously, we observe that b-type transitions cross between tunneling states in the parent and 34S spectra, while c-type transitions cross in the spectra of the deuterated species. This likely arises because the molecule is close to a symmetric top, with only the location of the hydrogen defining the orientation of the b- and c-inertial axes, enabling slight structural changes to switch the axis orientations at the transition state. Calculations at the M06-2X/6-311++G(3df,3pd) level of theory predict a 2.8 kcal/mol barrier for the large amplitude motion of the hydroxyl hydrogen rotating around the S-O bond through a Cs symmetric transition state in which the O-H is oriented anti with respect to the S-C bond. A complete scan of the hydroxyl proton around the S-O bond shows an additional transition state of Cs symmetry in the syn orientation with a 6.2 kcal/mol barrier relative to the equilibrium configuration.

OXIDATION KINETICS AND MECHANISTIC STUDIES OF ORGANIC ACIDS BY 2-PICOLINIUM CHLOROCHROMATE

Kinetics and mechanism of oxidation of formic and oxalic acids by 2-picolinium chlorochromate (PiCC) have been studied in dimethylsulphoxide. The main product of oxidation is carbon dioxide. The reaction is first order with respect to PiCC. Michaelis-Menten type of kinetics were observed with respect to the reductants. The reaction is acid-catalysed and the acid dependence has the form: kobs = a + b[H+]. The oxidation of -deuterio formic acid exhibits a substantial primary kinetic isotope effect (kH/kD = 5. 76 at 298 K). The reaction has been studied in nineteen different organic solvents and the solvent effect has been analysed using Taft's and Swain's multiparametric equations. The temperature dependence of the kinetic isotope effect indicates the presence of a symmetrical cyclic transition state in the rate-determining step. Suitable mechanisms have been proposed.

Hetero-Diels-Alder Reactions of In Situ-Generated Azoalkenes with Thioketones; Experimental and Theoretical Studies.

The hetero-Diels-Alder reactions of in situ-generated azoalkenes with thioketones were shown to offer a straightforward method for an efficient and regioselective synthesis of scarcely known N-substituted 1,3,4-thiadiazine derivatives. The scope of the method was fairly broad, allowing the use of a series of aryl-, ferrocenyl-, and alkyl-substituted thioketones. However, in the case of N-tosyl-substituted cycloadducts derived from 1-thioxo-2,2,4,4-tetramethylcyclobutan-3-one and the structurally analogous 1,3-dithione, a more complicated pathway was observed. By elimination of toluene sulfinic acid, the initially formed cycloadducts afforded 2H-1,3,4-thiadiazines as final products. Advanced DFT calculations revealed that the observed high regioselectivity was due to kinetic reaction control and that the (4 + 2)-cycloadditions of sterically less unhindered thiones occurred via highly unsymmetric transition states with shorter C..S-distances (2.27–2.58 Å) and longer N..C-distances (3.02–3.57 Å). In the extreme case of the sterically very hindered 2,2,4,4-tetramethylcyclobutan-1,3-dione-derived thioketones, a zwitterionic intermediate with a fully formed C‒S bond was detected, which underwent ring closure to the 1,3,4-thiadiazine derivative in a second step. For the hypothetical formation of the regioisomeric 1,2,3-thiadiazine derivatives, the DFT calculations proposed more symmetric transition states with considerably higher energies.

Unravelling the Origin of the Linear Structures Distortions of the Acetylene Isoelectronic Compounds (Disilyne, Digermyne, and Distannyne)

The role and contributions of the effective factors on the structural properties of acetylene (1), disilyne (2), digermyne (3), and distannyne (4) were examined by means of LC-ωPBE long-range corrected density functional method with TZP basis set on all atoms. Although the ground state structure of acetylene (1) is linear (D∞h-symmetry), the trans-bent (C2h, as a local minimum) and di-bridged structures (C2v, as a global minimum) were found for compounds 2–4. Plotted electronic energies of the ground and excited states versus the nuclear displacement (by considering the symmetries of the normal modes) revealed that the negative curvatures of the ground state electronic configurations and the positive curvatures of the excited states of the adiabatic potential energy surfaces (APESs) result from mixing of the ground $$^{1}\Sigma _{{\text{g}}}^{ + }$$ and excited 1Πg states, revealing the presence of the strong pseudo Jahn–Teller effect (i.e., PJT ( $$^{1}\Sigma _{{\text{g}}}^{ + }$$ + 1Πg) ⊗ πg problem). The weak PJTE in acetylene (1) does not distort its linear configurations. The distortions of the linear (D∞h) configurations of compounds 2–4 to their corresponding trans-bent structures (C2h) are due to the PJTE associated with the mixing of the ground $$^{1}\Sigma _{{\text{g}}}^{ + }$$ state and third excited Πg states through the mixing of their HOMOs(πu) with their corresponding LUMOs+2(σu) ([HOMO(πu)] + [LUMO+1(σu)]) in compound 2, the mixing of the ground $$^{1}\Sigma _{{\text{g}}}^{ + }$$ state and second excited Πg states ([HOMO(πu)] + [LUMO(σu)]) in compound 3 and the ground $$^{1}\Sigma _{{\text{g}}}^{ + }$$ state and second excited Πg states ([HOMO(πu)] – [LUMO+1(σu)]) in compound 4. The higher- and lower-lying $$^{1}\Sigma _{{\text{u}}}^{ - }$$ , $$^{1}\Sigma _{{\text{u}}}^{ - }$$ , 1Δu, and 1Πu states are not involved in the PJT interactions. The trans-bent local minimum structures could be converted to their corresponding global minimum structures (di-bridge, C2v) by passing from axial symmetrical (C2) transition state geometries. The energy gaps between reference states (∆) in the linear (D∞h) configurations decrease from disilyne (2) to distannyne (4) while the energy difference between the linear (D∞h) and trans-bent (C2h) configurations increases from compound 2 to 3 but decreases from compound to compound 4, revealing the determining impacts of their corresponding vibronic coupling constants (F) which may control their corresponding D∞h → C2h distortion PJT stabilization energies. Importantly, the variations of the M–M-bond length differences in the linear (D∞h) and trans-bent (C2h) structures of compounds 2–4, Δ(rM–M) D∞h–C2h, correlate well with their corresponding PJT-stabilization energies.

Better late than never! Transition state character involved in the neutral solvolysis of an oxalic ester determined by the ionizing power of ethanol/water and methanol/water mixtures

Solvation effects and solvolysis extent are major aspects of the decomposition of esters, particularly in polar protic media. Among the many different ester derivatives, oxalic esters are substances that have two ester groups in the same molecular framework, but with potentially different reactivities. Aryl oxalic esters are commonly used in efficient organic chemiluminescent reactions, leading to important analytical and biological applications. We studied the decomposition of bis(2,4,6-trichlorophenyl) oxalate (TCPO) in 100, 98, 95, 90, 80, 70, 60, and 50% ethanol/water (EtOH/W) and 100, 90, 80, and 70% methanol/water (MeOH/W) mixtures (% in v/v). In these media, one phenolic residue is generated in a fast addition–elimination (step 1), followed by a second slower addition–elimination (step 2), thus, two rate constants are observed. Using a Grunwald–Winstein (G–W) relationship between these rate constants and the solvent ionizing power (Υ), we determined the sensibility (m) to solvation effects and charge development associated with the transition state (TS) of the rate-determining step (rds). We also determined the Gibbs free energy of activation (∆‡G, at 25 °C) associated with each step, and used it to rationalize the TS character of the rds, considering that in both steps a zwitterionic intermediate is generated and that the addition step is rate limiting. In EtOH/W binary mixtures a non-linear G–W plot was observed, and for 100–95% EtOH/W mixtures the sensibility was m = 1.0 (step 1, ∆‡G = 21.6 to 20.6 kcal mol−1) and 1.1 (step 2, ∆‡G = 23.9 to 23.0 kcal mol−1), indicating full ionization in the TS of the addition step. Smaller Y values in 90–50% EtOH/W reflect a more symmetrical TS, with not as many charges being generated (step 1, m = 0.196, ∆‡G = 20.3 to 19.4 kcal mol−1; step 2, m = 0.30, ∆‡G = 22.6 to 21.7 kcal mol−1). In 100%–70% MeOH/W binary mixtures, with Y values similar to those for 90–50% EtOH/W, m = 0.52 (step 1, ∆‡G = 20.3 to 18.8 kcal mol−1) and 0.27 (step 2, ∆‡G = 21.5 to 20.9 kcal mol−1) were observed. Particularly for step 1 in MeOH/W, the observation of a solvent kinetic isotope effect that varies from inverse (0.63 ± 0.09 in pure MeOH) to normal (1.9 ± 0.3 in 70% MeOH/W) is consistent with the significant participation of the solvent in TS stabilization, resembling general acid catalysis.

Synthesis of Flake-shaped [3]Cyclo-4,6-dibenzofuranylene

The cyclic trimer of dibenzofuran linked at the 4,6-positions, named [3]cyclo-4,6-dibenzofuranylene, was synthesized by Ni(0)-mediated reductive coupling of 4,6-dibromodibenzofuran. DFT calculation indicated that the flake-shaped C2 symmetric structure is the most stable conformer and the propeller-shaped D3 symmetric structure is the minor conformer with a higher energy (+19.1 kcal mol−1). The calculated inversion energy of the C2 conformer through a Cs symmetric transition state was +4.1 kcal mol−1, showing rapid fluctuation, which is consistent with the observation of only one set of 1H NMR signals of dibenzofuran of [3]cyclo-4,6-dibenzofuranylene.

On the mechanism of spontaneous thiol-disulfide exchange in proteins.

The thiol-disulfide exchange reaction in model systems and small peptides was investigated by means of a combined QM/MM metadynamics scheme. The free energy landscapes of these systems were generated, providing the structures of reactants and products with atomic detail, as well as the heights of free energy barriers (or, activation energies) opposing the spontaneous exchange. A QM/MM scheme with purely classical water turned out to be an efficient and accurate compromise solution. The calculations yielded the expected symmetric trisulfide transition state at S-S distances of 2.7 Å, interestingly, with a slight deviation from linearity at an S-S-S angle of 165°. The structure of the transition state as well as the free energy barrier were very similar for the intramolecular thiol-disulfide reactions in model peptides. While CXC disulfide bonds were found sterically unfavorable, CXXC were favored over longer-range disulfide bonds along the peptide backbone, in line with the high abundance of CXXC motifs in redox proteins.

Crossing the dividing surface of transition state theory. IV. Dynamical regularity and dimensionality reduction as key features of reactive trajectories.

The atom-diatom interaction is studied by classical mechanics using Jacobi coordinates (R, r, θ). Reactivity criteria that go beyond the simple requirement of transition state theory (i.e., PR* > 0) are derived in terms of specific initial conditions. Trajectories that exactly fulfill these conditions cross the conventional dividing surface used in transition state theory (i.e., the plane in configuration space passing through a saddle point of the potential energy surface and perpendicular to the reaction coordinate) only once. Furthermore, they are observed to be strikingly similar and to form a tightly packed bundle of perfectly collimated trajectories in the two-dimensional (R, r) configuration space, although their angular motion is highly specific for each one. Particular attention is paid to symmetrical transition states (i.e., either collinear or T-shaped with C2v symmetry) for which decoupling between angular and radial coordinates is observed, as a result of selection rules that reduce to zero Coriolis couplings between modes that belong to different irreducible representations. Liapunov exponents are equal to zero and Hamilton's characteristic function is planar in that part of configuration space that is visited by reactive trajectories. Detailed consideration is given to the concept of average reactive trajectory, which starts right from the saddle point and which is shown to be free of curvature-induced Coriolis coupling. The reaction path Hamiltonian model, together with a symmetry-based separation of the angular degree of freedom, provides an appropriate framework that leads to the formulation of an effective two-dimensional Hamiltonian. The success of the adiabatic approximation in this model is due to the symmetry of the transition state, not to a separation of time scales. Adjacent trajectories, i.e., those that do not exactly fulfill the reactivity conditions have similar characteristics, but the quality of the approximation is lower. At higher energies, these characteristics persist, but to a lesser degree. Recrossings of the dividing surface then become much more frequent and the phase space volumes of initial conditions that generate recrossing-free trajectories decrease. Altogether, one ends up with an additional illustration of the concept of reactive cylinder (or conduit) in phase space that reactive trajectories must follow. Reactivity is associated with dynamical regularity and dimensionality reduction, whatever the shape of the potential energy surface, no matter how strong its anharmonicity, and whatever the curvature of its reaction path. Both simplifying features persist during the entire reactive process, up to complete separation of fragments. The ergodicity assumption commonly assumed in statistical theories is inappropriate for reactive trajectories.

A Masked Phosphinidene Trapped in a Fluxional NCN Pincer

The trapping of a phosphinidene (R-P) in an NCN pincer is presented. Stabilized phosphinidene 1 was characterized by 31 P{1 H}, 1 H, and 13 C{1 H} NMR spectroscopy, exhibiting an averaged C2v symmetry in solution between -60 and 60 °C. In the solid state, the phosphinidene is coordinated by one adjacent N atom featuring a formal P-N bond (1.757(2) Å) to give a five-membered ring with some aromatic character, confirmed by DFT calculations (B3LYP-D3/6-311G**++) to be the ground-state structure. Equilibration of the two N ligands occurs rapidly in solution via a "bell-clapper"-type process through an associative symmetric transition state calculated to lie 4.0 kcal mol-1 above the ground state.

Diels-Alder reactions of chiral isoimidium salts: a computational study.

Recently, important efforts have been devoted to asymmetric Diels-Alder (DA) cycloadditions. Several chiral auxiliaries and catalysts were tested, originating a multitude of selectivities. Here, we study the ability of the isoimidium group as a dienophile activator in DA reactions, as well as its behavior in the induction of selectivity, when incorporated in chiral auxiliary units. We study dienophiles attached to isoimidium auxiliaries derived from (2R,5R)-2,5-diphenylpyrrolidine and from (R)-bis((R)-1-phenylethyl)amine, and show that they lead to low activation energies in DA additions to different dienes. Reported experimental regio- and endo/exo selectitivites are also fully rationalized. While diastereoselectivities originated by (2R,5R)-2,5-diphenylpyrrolidine based dienophiles can easily be rationalized by a C2 symmetric transition state, several transition states have to be simultaneously accounted for the rationalization of the selectivity obtained with dienophiles based on (R)-bis((R)-1-phenylethyl)amine. In this case, the C2 symmetric structure leads to opposite selectivities to those experimentally observed. Thus, while the structures of the dienophiles derived from these two amines seem similar, their behavior in the induction of stereoselectivity in DA reactions is quite different. Our models, which substantially differ from those previously proposed in the literature, can also be adapted to other reactions in which this type of chiral auxiliaries is used.

Quantum Chemical Study of the Initial Step of Ozone Addition to the Double Bond of Ethylene

The mechanisms of the initial step in chemical reaction between ozone and ethylene were studied by multireference perturbation theory methods (MRMP2, CASPT2, NEVPT2, and CIPT2) and density functional theory (OPW91, OPBE, and OTPSS functionals). Two possible reaction channels were considered: concerted addition through the symmetric transition state (Criegee mechanism) and stepwise addition by the biradical mechanism (DeMore mechanism). Predicted structures of intermediates and transition states, the energies of elementary steps, and activation barriers were reported. For the rate-determining steps of both mechanisms, the full geometry optimization of stationary points was performed at the CASPT2/cc-pVDZ theory level, and the potential energy surface profiles were constructed at the MRMP2/cc-pVTZ, NEVPT2/cc-pVDZ, and CIPT2/cc-pVDZ theory levels. The rate constants and their ratio for reaction channels calculated for both mechanisms demonstrate that the Criegee mechanism is predominant for this reaction. These results are also in agreement with the experimental data and previous computational results. The structure of DeMore prereactive complex is reported here for the first time at the CCSD(T)/cc-pVTZ and CASPT2/cc-pVDZ levels. Relative stability of the complexes and activation energies were refined by single-point energy calculations at the CCSD(T)-F12/VTZ-F12 level. The IR shifts of ozone bands due to formation of complexes are presented and discussed.

Detection and Kinetic Characterization of a Highly Reactive Heme–Thiolate Peroxygenase Compound I

The extracellular heme-thiolate peroxygenase from Agrocybe aegerita (AaeAPO) has been shown to hydroxylate alkanes and numerous other substrates using hydrogen peroxide as the terminal oxidant. We describe the kinetics of formation and decomposition of AaeAPO compound I upon its reaction with mCPBA. The UV-vis spectral features of AaeAPO-I (361, 694 nm) are similar to those of chloroperoxidase-I and the recently described cytochrome P450-I. The second-order rate constant for AaeAPO-I formation was 1.0 (±0.4) × 10(7) M(-1) s(-1) at pH 5.0, 4 °C. The relatively slow decomposition rate, 1.4 (±0.03) s(-1), allowed the measurement of its reactivity toward a panel of substrates. The observed rate constants, k2', spanned 5 orders of magnitude and correlated linearly with bond dissociation enthalpies (BDEs) of strong C-H bond substrates with a log k2' vs BDE slope of ∼0.4. However, the hydroxylation rate was insensitive to a C-H BDE below 90 kcal/mol, similar to the behavior of the tert-butoxyl radical. The shape and slope of the Brønsted-Evans-Polanyi plot indicate a symmetrical transition state for the stronger C-H bonds and suggest entropy control of the rate in an early transition state for weaker C-H bonds. The AaeAPO-II Fe(IV)O-H BDE was estimated to be ∼103 kcal/mol. All results support the formation of a highly reactive AaeAPO oxoiron(IV) porphyrin radical cation intermediate that is the active oxygen species in these hydroxylation reactions.

Quantum chemical study of the primary step of ozone addition at the double bond of ethylene

The mechanism of the primary step of the interaction between ozone and the double bond of ethylene has been investigated by various methods of quantum chemistry (MP2, QCISD, CCSD, MRMP2) and density functional theory (PBE0, OPTX, CPW91, B3PW91, OLYO, B3LYP, BLYP). The kinetics of two reaction pathways, namely, concerted ozone addition via a symmetric transition state (Criegee mechanism) and nonconcerted ozone addition via a biradical transition state (DeMore mechanism) has been calculated. Both mechanisms are describable well in the single-determinant approximation by the QCISD, CCSD, B3LYP, and PBE0 methods and in the multideterminant approximation by the MRMP2 method. The other methods are less suitable for solving this problem. The calculated data demonstrate that the reaction proceeds via both competing pathways. Rate constant values consistent with experimental data and plausible Criegee-to-DeMore rate constant ratios have been obtained. The concerted addition of ozone to ethylene is significantly more rapid than the nonconcerted addition.

Competition between the concerted and nonconcerted addition of ozone to a double bond

Quantum chemistry methods are used to investigate the mechanism of the reaction of ozone with the double bond of ethylene. It is shown that there are two possible reaction mechanism; concerted addition through a symmetrical transition state (Criegee mechanism) and nonconcerted addition through a biradical transition state (DeMore mechanism). In the single-determinant approximation, both mechanisms were described by using the QCISD, CCSD, and B3LYP methods. These methods give a reasonable ratio between the rates of the two reaction channels, with the rate constants being closer to the experiment when calculated by the CCSD and B3LYP methods. Multiconfiguration calculations are performed at the MRMP2 level. They also show the presence of both channels of the reaction and yield reasonable values of the rate constants for reaction channels and the ratio thereof. It is shown that the reaction of ethylene with ozone via the concerted addition mechanism is much faster.

Experimental and computational determination of Brønsted coefficients for equilibrium transfer of the O,O‐dimethyl phosphorothioyl group between oxyanion nucleophiles

An experimental determination of the βEq value for equilibrium transfer of the O,O‐dimethyl phosphorothioyl group between oxyanion nucleophiles in water and methanol at 25 °C is presented. The respective βEq values in the two solvents are experimentally the same at −1.45 ± 0.08 and −1.39 ± 0.12. Based on the observation that the Brønsted correlation for the nucleophilic reaction of phenoxides in water with substrate 1d (dimethyl 4‐nitrophenyl phosphorothioate, pKaHOAr of 7.14) is linear over the entire range of phenoxides employed (5.53 ≤ pKaNu ≤ 12.38), the reaction for phenoxide nucleophiles displacing phenoxide leaving groups is probably concerted. The obtained data allow one to calculate, for a symmetrical transition state involving 2,4,5‐trichlorophenoxide as a nucleophile and leaving group, an approximately 60% P–OAr cleavage and about 40% P–Nuc bond formation. A computational method is presented for the rapid prediction of the βEq values for such processes in water and methanol, and the results are compared with known values from the literature. Copyright © 2011 John Wiley & Sons, Ltd.

Experimental and Theoretical Characterization of the Aromatization, Epimerization, and Fragmentation Reactions of Bi‐2H‐azirin‐2‐yls Prepared from 1,4‐Diazidobuta‐1,3‐dienes

1,4-Diazidobuta-1,3-dienes (Z,Z)-10, 17, and 21 were photolyzed and thermolyzed to yield the pyridazines 13, 20, and 23, respectively. To explain these aromatic final products, the generation of highly strained bi-2H-azirin-2-yls 12, 19, and 22 and their valence isomerization were postulated. In the case of meso- and rac-22, nearly quantitative formation from diazide 21, isolation as stable solids, and complete characterization were possible. On the thermolysis of 22, aromatization to 23 was only a side reaction, whereas equilibration of meso- and rac-22 and fragmentation, which led to alkyne 24 and acetonitrile, dominated. Prolonged irradiation of 22 gave mainly the pyrimidine 25. The change of the configuration at C-2 of the 2H-azirine unit was observed not only in the case of bi-2H-azirin-2-yls 22 but also for simple spirocyclic 2H-azirines 29 at a relatively low temperature (75 °C). The fragmentation of rac-22 to give alkyne 24 and two molecules of acetonitrile was also studied by high-level quantum chemical calculations. For a related model system 30 (methyl instead of phenyl groups), two transition states TS-30-31 of comparable energy with multiconfigurational electronic states could be localized on the energy hypersurface for this one-step conversion. The symmetrical transition state complies with the definition of a coarctate mechanism.

Studying Double Group Transfer Reactions by Means of Computational Methods

In this article, recent computational studies focused on double group transfer reactions and related processes are summarized. The reported results clearly indicate that these transformations can be considered as a subclass of pericyclic reactions occurring concertedly, with high activation barriers and synchronicity values, and through highly symmetric transition states. Interestingly, the aromatic nature of the latter saddle points has been also studied and discussed showing that they can be viewed as the in-plane analogues of sixmembered hetero-aromatic rings. Finally, the application of the so-called “Strain Model” on these important processes has demonstrated that the strain (the energy required to deform the reactants to the geometry they present in the corresponding transition state) is the major factor controlling the high barrier heights in spite of the stabilizing contribution of the aromaticity.

Symmetric Transition State Analysis: An Analysis of Dissociative Methane Adsorption on Rh{111} Using Quantum Chemical Calculations

The chemical bonding aspects of the transition state (TST) of methane activation on a Rh{111} surface are analyzed. Three methods are compared: The barrier decomposition analysis of Hu et al. in which the bond between CH is assumed completely broken in the TST (Satterfield, Heterogeneous catalysis in industrial practice, 2nd ed., 1996; Chorkendorff and Niemantsverdriet, Concepts of modern catalysis and kinetics, 2003; Somorjai, Introduction to surface chemistry and catalysis, 1994); the activation strain model of Bickelhaupt in which the CH bond is assumed to be equal to the gasphase CH interaction energy (Christmann, Surface science reports, 1988; Nørskov and Christensen, Science, 2006; Forsberg, Chemical engineering progress, 2005); and a model in which the interaction energies between CH, and of the H atom and CH3 with the catalyst are all given equal attention, the symmetric transition state analysis. This symmetric transition state analysis would not yield a result different from the traditional methods if all bonds were additive and decoupled. But, as our results show, that is not in general the case. The position of the maximum in non-additivity can be considered a descriptor for the position of the TST on the reaction coordinate. At the TST, we find that the three interactions are of comparable strength.