Concerted Path Research Articles

Mechanism for the decomposition of 5-aza-2′-deoxycytidine: a theoretical study using Monte Carlo simulation plus local microhydration model

The decomposition mechanism of 5-aza-2′-deoxycytidine (decitabine) has been investigated using the Monte Carlo simulation with free energy perturbation technique based on the local microhydration model. The irreversible deformylation process (process 2) was firstly evaluated. Four possible pathways, two stepwise (paths 1 and 3) and two concerted (paths 2 and 4) were examined. In two stepwise pathways, a tetrahedral intermediate is given by the addition of H2O to the C(1)=O(2) bond, followed by the proton shift to the N(3) atom leading to the final products. In two concerted pathways, the nucleophilic attack of H2O at the C(1) atom and the simultaneous movement of hydroxyl H to the N(3) atom induce the cleavage of the C(1)–N(3) single bond. The potential energy profiles along the minimum energy path for four pathways in the gas phase and in water were obtained. Our computed results exhibit that two stepwise mechanisms (paths 1 and 3) are more favorable than two concerted mechanisms (paths 2 and 4) when the solvent effects of bulk water are taken into account. Then, in collaboration with the previous study (Gao et al. in Theor Chem Acc 131:1108–1123, 2012), the whole decomposition processes of decitabine were discussed. Two possible pathways were considered: pathway t1 be made up of B-path 2 (in process 1) and path 1 as well as pathway t2 be composed of B-path 3 (in process 1) and path 3. The results clearly manifest that pathway t2 is more favorable than pathway t1 in the decomposition reaction of decitabine, which leads to IM-dR2 and GuaUre-dR2 as ring-opening-formylated intermediate and decomposition product, respectively. The third step is the rate-controlling step with the free energy barrier height of 29.8 kcal/mol, which is in agreement with the experimentally determined activation free energy (25.4 kcal/mol).

Novel [2 + 1] Concerted Reaction Path for Disilacyclobutenes with Acetylene

Thermal reactions of benzodisilacyclobutene (1) and disilacyclobutene (2) with acetylene were investigated theoretically. The reactions are thought to proceed via the conventional Diels–Alder reaction of disilabutadiene, the conrotatory ring-opening product of disilacyclobutene, with acetylene. However, this mechanism is incompatible with the observed similar reactivities of 1 and 2 with acetylene and the retention of stereochemistry during the reaction. In our previous paper, we proposed an alternative [2 + 1] cycloaddition pathway that involved the direct addition of acetylene to the Si–Si σ–bond of 1 without ring opening. In this study, we extensively investigated the reaction pathways for both 1 and 2 on a theoretical basis. We found that charge transfer (CT) played a key role in the [2 + 1] cycloaddition pathway. On the basis of natural bond orbital (NBO) analysis, the interaction of the Si–Si σ-bond orbital (donor) with the π* orbital (acceptor) of the acetylene was attributed mainly to the CT proce...

Elucidating Time Course of Structural Changes Leading to Receptor-Ligand Complex Formation with Transient-EPR

Protein functions rely on protein motions. Protein structures, obtained with X-ray diffraction or NMR, reveal mostly static pictures of structure-function relation. To connect structural changes of proteins with function, we need to look at events happening in real time, i.e. the dynamics of the processes.We have chosen mlCNBD as a model protein to study the dynamics of receptor-ligand complex formation. mlCNBD is a cytosolic cyclic nucleotide-binding domain of a bacterial potassium channel (MloK1). mlCNBD binds to cyclic adenosine monophosphate (cAMP) and undergoes conformational changes as evident from the NMR and X-ray structures of the apo and holo conformational states of the protein. Recent kinetic and NMR studies indicate that these structural transitions follow the induced-fit mechanism, i.e. these are a direct consequence of ligand binding. However, the detailed mechanism of these structural rearrangements leading to receptor activation remains elusive.We use transient Electron Paramagnetic Resonance (tr-EPR) spectroscopy in conjunction with Site- Directed Spin Labelling (SDSL) to resolve the dynamics of mlCNBD-cAMP complex formation. We introduce single cysteine residues at different sites in the protein. The mutants are labelled with Methane Thio Sulphonate Spin Label (MTSSL). Binding of cAMP to the mutants is rapidly initiated either via a caged-cAMP approach or through a micro-mixer. The time-resolved EPR data reveals the progression of structural changes taking place at a particular site on millisecond time scale. Collating data across the whole protein will enable us to reconstruct the steps from the apo to the holo state of the protein. It will also provide the answer to the question whether the induced-fit mechanism follows a concerted (single step) or sequential (multi-step) path from the apo to the holo conformation.

Equilibrium and Rate Constants, and Reaction Mechanism of the HF Dissociation in the HF(H2O)7 Cluster by ab Initio Rare Event Simulations

We perform restrained hybrid Monte Carlo (MC) simulations to compute the equilibrium constant of the dissociation reaction of HF in HF(H2O)7. We find that the HF is a stronger acid in the cluster than in the bulk, and its acidity is higher at lower T. The latter phenomenon has a vibrational entropic origin, resulting from a counterintuitive balance of intra- and intermolecular terms. We find also a temperature dependence of the reactions mechanism. At low T (≤225 K) the dissociation reaction follows a concerted path, with the H atoms belonging to the relevant hydrogen bond chain moving synchronously. At higher T (300 K), the first two hydrogen atoms move together, forming an intermediate metastable state having the structure of an eigen ion (H9O4(+)), and then the third hydrogen migrates completing the reaction. We also compute the dissociation rate constant, kRP. At very low T (≤75 K) kRP depends strongly on the temperature, whereas it gets almost constant at higher T’s. With respect to the bulk, the HF dissociation in the HF(H2O)7 is about 1 order of magnitude faster. This is due to a lower free energy barrier for the dissociation in the cluster.

Computational study on the mechanism of N-phenylimine derivatives’ pyrolysis reaction in the gas phase

In this work, quantum chemistry and kinetics calculations have been performed on the retro-cheletropic ene reactions of N-phenyl-1-methyl-6-methylenecyclohexa-2,4-dienylmethanimine (R1) and N-phenyl-2,2-dimethylbut-3-en (R2). Two major possible mechanisms have been considered and rate constants have been calculated using the transition state theory. The simple Wigner, Eckart zero-curvature tunneling and small-curvature tunneling (SCT) methods were evaluated. The best agreement with experimental rate coefficients was found when SCT correction was applied. A mean deviation of a factor 3 on the rate coefficients is found for the studied reactions at the temperatures of 417 and 773 K. Calculated rate coefficients showed that the tunneling corrections played a critical role in obtaining accurate rate coefficients, especially at lower temperature (417 K). Calculated rate coefficients are in good agreement with the reported experimental data and similar compounds in the case of R1 and R2, respectively. These results support the concerted and stepwise paths for the gas phase reactions of R1 and R2, respectively. Computed kinetic parameters confirmed that R1 had greater reactivity than R2. This trend explains the effects of an extra phenyl-like system on the stability of the transition state and hence increases the R1 rate constant. Calculated rate constants especially at the M06 level are in better agreement with the experimental values than the B3LYP ones. Natural bond orbital (NBO) studies of the reactants and their transition states reveal that their electron delocalization change is an important factor in the determination of the reactivity order for these compounds. Finally, the nature of bond making and breaking during the reactions has been investigated using the concepts of electron charge density and Laplacian in atom in the molecule method.

Protected Sulfur Transfer Reactions by the Escherichia coli Suf System

The first step in sulfur mobilization for the biosynthesis of Fe-S clusters under oxidative stress and iron starvation in Escherichia coli involves a cysteine desulfurase SufS. Its catalytic reactivity is dependent on the presence of a sulfur acceptor protein, SufE, which acts as the preferred substrate for this enzyme. Kinetic analysis of the cysteine:SufE sulfurtransferase reaction of the E. coli SufS that is partially protected from reducing agents, such as dithiothreitol and glutathione, was conducted. Under these conditions, the reaction displays a biphasic profile in which the first phase involves a fast sulfur transfer reaction from SufS to SufE. The accumulation of persulfurated/polysulfurated forms of SufE accounts for a second phase of the slow catalytic turnover rate. The presence of the SufBCD complex enhances the activity associated with the second phase, while modestly inhibiting the activity associated with the initial sulfur transfer from SufS to SufE. Thus, the rate of sulfur transfer from SufS to the final proposed SufBCD Fe-S cluster scaffold appears to be dependent on the availability of the final sulfur acceptor. The use of a stronger reducing agent [tris(2-carboxyethyl)phosphine hydrochloride] elicited the maximal activity of the SufS-SufE reaction and surpassed the stimulatory effect of SufBCD. This concerted sulfur trafficking path involving sequential transfer from SufS to SufE to SufBCD guarantees the protection of intermediates at a controlled flux to meet cellular demands encountered under conditions detrimental to thiol chemistry and Fe-S cluster metabolism.

A new intermediate in the Prins reaction.

Two Prins reactions were investigated by the use of DFT calculations. A model composed of R–CH=CH2 + H3O+(H2O)13 + (H2C=O)2, R = Me and Ph, was adopted to trace reaction paths. For both alkenes, the concerted path forming 1,3-diols was obtained as the rate determining step (TS1). TS stands for a transition state. From the 1,3-diol, a bimolecular elimination (TS2) leads to the allylic alcohol as the first channel. In the second channel, the 1,3-diol was converted via TS3 into an unprecedented hemiacetal intermediate, HO–CH2–O–CH(R)–CH2–CH2–OH. This intermediate undergoes ring closure (TS4), affording the 1,3-dioxane product. The intermediate is of almost the same stability as the product, and two species were suggested to be in a state of equilibrium. While the geometry of TS1 appears to be forwarded to that of a carbocation intermediate, the cation disappeared through the enlargement of the water cluster. Dynamical calculations of a classical trajectory using the atom-centered density matrix propagation molecular dynamics model on the four TSs were carried out, and results of IRC calculations were confirmed by them.

DFT Study of the Proton Transfer in the Urethane Formation between 2,4-Diisocyanatotoluene and Methanol

Abstract The proton transfer effects on the urethane formation between 2,4-diisocyanatotoluene (2,4-TDI) and methanol was investigated by using density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level. The calculations show that the direct addition of methanol to 2,4-TDI follows a concerted path with an energy barrier of 31.71 kcal mol−1 and an activation free energy of 39.32 kcal mol−1. With the methanol, diphenylamine, and methyl N-methylcarbamate molecule serving as a proton transporter or a reactive catalyst, the energy barriers were significantly reduced to 12.28, 17.93, and 20.13 kcal mol−1 respectively, and the free energies of activation were lowered to 25.22, 32.54, and 36.86 kcal mol−1 correspondingly. The results indicate that the labile hydrogen-containing compounds play a key role in accelerating the reaction rate and the proton transfer. In the case of one extra methanol molecule serving as a proton transporter, the energy barrier was estimated to be 12.28 kcal mol−1, which is in good agreement with the experimental activation energy between 8.6 and 12.0 kcal mol−1. The urethane formation between 2,4-TDI and methanol is more likely a third-order reaction with respect to the initial reactants.

Reaction Mechanism of Toluene Methylation with Dimethyl Carbonate or Methanol Catalyzed by H-ZSM-5

Para-xylene is an important petrochemical that can be produced by the methylation of toluene. Here, the mechanism of toluene methylation with dimethyl carbonate (DMC) or methanol catalyzed by H-ZSM-5 was studied using theour own N-layered integrated molecular orbital + molecular mechanics (ONIOM) in combination with density functional theory (DFT) methods. The adsorption of reactants and desorption of products are considered, and the structures of important intermediates and transition states are described. Computational rate constants are used to estimate the kinetic activity of toluene methylation reactions. The reaction mechanism of toluene methylation with DMC and that with methanol catalyzed by H-ZSM-5 differ. Toluene methylation with DMC involves full decomposition of DMC prior to methylation to form xylene isomers. In contrast, methanol is more active than DMC as the methylation reagent in toluene methylation. The stepwise and concerted paths of toluene methylation with methanol have similar intrinsic activation energies. At 773 K, the stepwise path has a higher rate constant than the concerted one. For toluene methylation with both reagents, para-xylene formation is kinetically preferred, whereas meta-xylene is the lowest-energy product. The results of our calculations agree well with experimental observations.

Electronic Excited State Paths of Stone–Wales Rearrangement in Pyrene: Roles of Conical Intersections

We investigated the reaction paths of Stone-Wales rearrangement (SWR), i.e., π/2 rotation of two carbon atoms with respect to the midpoint of the bond, in graphene and carbon nanotube quantum chemically. Our particular attention is focused on the roles of electronic excitations and conical intersections (CIs) in the reaction mechanism. We used pyrene as a model system. The reaction paths were determined by constructing potential energy surfaces at the MS-CASPT2//SA-CASSCF level of theory. We found that there are no CIs involved in SWR when both of C-C bond cleavage and formation occur simultaneously (concerted mechanism). In contrast, for the reaction path with stepwise cleavage and formation of C-C bonds, C-C bond breaking and making processes proceed through two CIs. When SWR starts from the ground (S(0)) state, the concerted and stepwise paths have an equivalent reaction barrier ΔE(‡) (9.5-9.6 eV). For the reaction path starting from excited states, only the stepwise mechanism is energetically preferable. This path contains a nonadabatic transition between the S(1) and S(0) states via a CI associated with the first stage of C-C bond cleavage and has ΔE(‡) as large as in the S(0) paths. We confirmed that the main active molecular orbitals and electron configurations for the low-lying electronic states of larger nanocarbons are the same as those in pyrene. This result suggests the importance of the nonadiabatic transitions through CIs in the photochemical reactions in large nanocarbons.

THEORETICAL INVESTIGATIONS ON THE MECHANISM OF ACTIVATION OF AMMONIA BY A p-DIMETHYLAMINOPYRIDINE COORDINATED SI=O DOUBLE BOND

Recently, Xiong et al. reported an activation of ammonia by p-dimethylaminopyridine (DMAP) coordinated silanone (DMAP→Si(L)=O, DS) affording a unique pair of sila-hemiaminal (SH) and silanoic amide (SA) tautomers (Xiong Y, Yao S, Müller R, Kaupp M, Driess M, J Am Chem Soc132:6912, 2010). In this paper, the mechanisms of the activation of ammonia affording SH and SA, the successive generation of hydrogen bonded complex pair SH–SA and the tautomerization between SH and SA have been intensively investigated computationally for the first time at MP2/6-311+G(2d,p)//B3LYP/6-31+G(d,p) level in toluene. The concerted Paths C and D with ammonia assistance are determined by our calculations to be the dominant pathways corresponding to forming SH and SA from DS, respectively. The free energy barrier of Path C affording SH from DS is 14.45 kcal/mol, and that of Path D affording SA is 21.46 kcal/mol. So it is determined theoretically that Path C is dynamically dominant over Path D. And the pair SH–SA is formed then spontaneously by intermolecular hydrogen bond (O–H ⋯ O′) without any barrier. While the tautomerization between SH and SA is nonsignficant resulting from the corresponding relative high barriers (23.79 kcal/mol for process from SH to SA and 26.52 kcal/mol for process from SA to SH). Our results are in good agreement with and good interpretation of the experimental results by Xiong et al.

Shape-Selective Methylation of 2-Methylnaphthalene with Methanol over H-ZSM-5 Zeolite: A Computational Study

Methylation of 2-methylnaphthalene (2-MN) for synthesis of 2,6-dimethylnaphthalene (2,6-DMN) is an industrially important reaction because 2,6-DMN is a key precursor for the advanced polymer material polyethylenenaphthalate. Shape-selective methylation of 2-MN with methanol in an H-ZSM-5 pore was studied using the ONIOM2 model and density functional theory. Two proposed reaction mechanisms, stepwise and concerted, were considered. Computational results reveal that the stepwise path, with methanol dehydration to produce a methoxide intermediate as the rate-limiting step, is kinetically favored. Both the stepwise and concerted path indicated that methylation at the 6-position is favored over methylation at the 7-position; however, the concerted path shows a greater selectivity. 2-MN isomerization and methylation, which may occur on the catalyst external surface and decrease selectivity to the desired 2,6-DMN product were also examined. Isomerization of 2-MN to 1-MN proceeds faster on external surface sites ...

Proton Transfers along Hydrogen Bonds in the Tautomerization of Purine

Tautomerization of purine in the water cluster was investigated by the use of DFT calculations. The correlation between the reaction paths and the number of water molecules (n) was examined. For n = 3 and n = 4, concerted reaction paths were obtained. However, for n = 5, a stepwise path including an ion pair intermediate was found with small activation energies. The n = 4 + 3 and n = 4 + 3 + 9 models were calculated to give further small activation energies, where n = 4 constitutes the reaction center and +3 and +3 + 9 denote the number of catalytic water molecules. The combination of the in-plane deprotonation at the N9 site and the out-of-plane protonation at the N7 site makes the n = 4 model probable. Three protonated n = 4 + 3 + 9 routes, a, b, and c, composed of purineH(+)(H(2)O)(4+3+9) were investigated. The n = 4 + 3 moiety is also included in the three routes, and the route c (with the N1 protonation) was found to be most favorable. The purine tautomerization was found to involve the Zundel cation in the ion pair intermediate.

Atmospheric chemistry of 2-ethyl hexanal: Photochemistry and oxidation in the presence of NO 2

The photolysis and oxidation mechanism of 2-ethyl hexanal (C 4H 9CH(C 2H 5)C(O)H—2-Ethal) have been investigated. The UV absorption cross sections were recorded over the range 200–350 nm at 298 K. The quantum yield for photolysis at 254 nm was determined to be 0.51 ± 0.09. Photolysis of 2-Ethal occurs by two primary paths: a concerted elimination path and a radical formation path, the latter being the main path. CO, heptane, 3-heptanol, and 3-heptanone were identified as the main products. The rate constant for the reaction with Cl atoms was determined using the relative rate technique with isopentane and cyclohexane as reference gases. The average rate constant obtained was k = (2.8 ± 0.9) × 10 −10 cm 3 molecule −1 s −1. The most important path in the oxidation mechanism was the acyl radical forming channel, which was corroborated by the analysis of the products formed via photolysis in the presence of NO 2. Those products were CO 2, CO, peroxyacetyl nitrate, peroxypropionyl nitrate, and ethyl nitrate.

Supra–Supra, Supra–Antara, and Stepwise-Diradical Pathways for an Observed 16-Electron Double-[4 + 4] Cycloaddition within Metal-Templated Dialkyne Dimers (PtX2)2(μ-R2PCCCCPR2)2

Quantum chemistry calculations are used to provide insight into the cycloaddition of two dialkyne chains in initially monocyclic organoplatinum dimers of the type (PtX(2))(2)(μ-R(2)PC(4)PR(2))(2), where X = Cl or Me and R = Ph or Me. Previous experimental studies showed that the cycloaddition occurs with {X, R} = {Cl, Ph} but not {Me, Ph}. Two concerted pericyclic paths, a D(2h)-symmetry double-[π4s+π4s] "Hückel path" and a D(2)-symmetry double-[π4s+π4a] "Möbius path", were explored via orbital energy correlation diagrams (OECDs) computed using a singly occupied molecular orbital technique developed earlier. In accord with pericyclic reaction theory, the 16e(-) rearrangement is forbidden along the D(2h) Hückel path; four electrons would need to change their orbital symmetries. The D(2) Möbius path, afforded by the natural twist in the reactant structure which allows the desired Möbius orbital connectivity for a 4n rearrangement, is concluded to be a borderline forbidden pathway. This Möbius path creates avoided crossings in the OECD, which allows consistent orbital populations throughout the reaction, but it does not cause a change in intended orbital correlation, and the predicted activation barrier is rather high (∼50 kcal mol(-1)). The avoided crossings show strong coupling, but a clear HOMO-LUMO gap for the reaction is not produced. A stepwise path is also presented, with evidence of its diradical character.

A computational study on the relationship between formation and electrolytic dissociation of carbonic acid

Calculations at the B3LYP/6-311+G**, MPW1K/6-311+G**, and MP2/6-311+G** level theory were carried out for the CO2 + nH2O → H2CO3 + (n − 1)H2O chemical reaction, where n denotes the number of water molecules. For n = 1, 2, 3, and 4 water molecules, a concerted path (without intermediates) for the formation of H2CO3 was obtained. For n = 6(3 + 3) and n = 8(4 + 4) water molecules, the MPW1K and B3LYP/6-311+G** SCRF = PCM methods resulted in an ion-pair intermediate being formed. Here, +3 or +4 stands for the three or four catalytic water molecules. The catalytic water molecules are distinguished from the reaction participating ones (either three or four). For n = 3 + 5 and n = 3 + 10, two ion-pair intermediates were obtained using both the B3LYP and MPW1K DFT methods. In order to check the method and model dependence, furthermore, n = 3 + 17 and n = 3 + 27 reacting systems were examined. Here, a likely mechanism of formation and electrolytic dissociation of carbonic acid was found. First, the O2C–OH2 complex is formed. Second, it is isomerized to the Zundel cation H5O2 +. Third, the cation is converted to the carbonic acid. The isomerization, i.e., proton transfer, was computed to occur along the hydrogen-bonded network of the pentagon shape.

Excited-State Tautomerization in the 7-Azaindole-(H2O)n (n = 1 and 2) Complexes in the Gas Phase and in Solution: A Theoretical Study

A systematic study of the excited-state tautomerization of 7-azaindole-(H2O)n (n = 1 and 2) complexes in both gas and solution phases were investigated theoretically. Electronic structures and energies for the reactant, transition state (TS), and product were computed using the time-dependent density functional theory (TDDFT) and complete active space self-consistent field (CASSCF) levels with 6-31G (d,p), 6-311G(d,p), and 6-311+G(d,p) basis sets. Barrier heights and tautomerization energies were corrected by the second-order multireference perturbation theory (MRPT2) to consider the dynamic electron correlation. The solvent effect decreased the tautomerization barrier height in the 7-azaindole-H2O complex. In the 7-azaindole-(H2O)2 complex, two transition states were found for two asynchronous but concerted paths: in the first, the pyrole ring proton moved first to water; in the second, the water proton moved first to the pyridine ring. The CASSCF level with the MRPT2 correction clearly showed that the former path was much preferable to the latter. The preferable barrier height was only 1.6 kcal/mol with a zero-point energy correction, which would make the excited-state tautomerization possible. At all TDDFT levels, the TS structures and barrier heights depended on both the basis set used and the solvent effect. Most TDDFT methods failed to reproduce the CASSCF structures and MRPT2 energies. Only two methods, WB97XD/6-31G(d,p) and M062X/6-311+G(d,p), predicted two TSs for the two asynchronous paths in the 7AI-(H2O)2 complex but failed to reproduce the energetics. Further systematic study is necessary to test whether current TDDFT methods, including solvent effects, can be used to understand excited-state proton transfer reactions.

Reaction mechanism of tert-butylation of phenol with tert-butyl alcohol over H-β zeolite: An ONIOM study

Tert-butylation of phenol with tert-butyl alcohol (TBA) over H-β zeolite was studied using the ONIOM approach with two proposed reaction paths: stepwise and concerted mechanisms. The results obtained by the ONIOM2 (B3LYP/6-31G(d,p):UFF) method reveals that the tert-butylation of phenol preferentially occurs through a co-adsorbed, concerted mechanism without prior dehydration of tert-butyl alcohol, rather than via a stepwise mechanism through dehydration to form tert-butyl carbenium ion as the first step followed by tert-butyl cation attack on the 2- or 4-position on phenol. The kinetic difference between 2- and 4-tert-butylation is more apparent in the concerted path, where 4-tert-butylation proceeds over a lower activation barrier. Decreasing the acid site strength, via substitution of Ga for Al, reduces the H-bonding interaction between the zeolitic proton and tert-butyl alcohol and increases the apparent activation barriers, which slows the overall reaction, and also lowers the selectivity to 4-tert-butyl phenol.

A computational study on the mechanism and the kinetics of urethane formation

The reaction of n-butanol with several aromatic diisocyanates – 2,4 toluene diisocyanate (2,4 TDI), 2,6 toluene diisocyanate (2,6 TDI), 2,4′ dibenzyl diisocyanate (2,4′ DBDI), 4,4′ stilbene diisocyanate, (4,4′ SBDI), m-phenylene diisocyanate ( m-PDI), and p-phenylene diisocyanate ( p-PDI) – resulting in the production of urethane is studied by using density functional theory (DFT) calculations (B3LYP/6-31 + G(d,p)). The addition of alcohol to isocyanate follows either a concerted or stepwise path. The obtained product differs in two mechanisms. Single point solvent calculations were carried out in benzene, by means of the polarizable continuum model (PCM) at the B3LYP/6-31 + G(d,p) level of theory. The calculated free energy profiles showed that the concerted path is more likely to occur than the stepwise route. Natural bond orbital analysis was performed to provide an insight into charge delocalization and the stabilization energies of the molecules. The calculated rate constant ratios ( k 1/ k 2) of the two consecutive alcoholysis reactions of aromatic diisocyanates agree strongly with the experimental results.

Thermolysis of 2-methyloxetane: a computational study

Thermal fragmentation of 2-methyloxetane (2MO), which yields two different sets of products by virtue of ring asymmetry, was studied theoretically by using DFT, MPn and CASPT2//CASSCF methods. At the MPn and DFT theoretical levels, only concerted transition states were located on the ground state potential energy surface (PES). The CASSCF approach leads to different stepwise pathways for the two fragmentation modes, with biradical as intermediates, in addition to the concerted paths, with a very shallow PES for the asynchronous region in which intermediates becomes unstable under CASPT2//CASSCF calculations. Nevertheless, activation barriers thus calculated were quite consistent with experimental values. The reaction pathway that experimentally renders the main set of products was calculated as the lowest-energy path for the fragmentation of the 2-methyloxetane heterocycle, and this evolves with an initial cleavage of the C–O bond of the oxetane ring.