Isomerization pathways from the norbornadiene to the cycloheptatriene radical cation by opening a bridgehead-methylene bond: a theoretical investigation

Three skeletal rearrangement channels for the norbornadiene (N*+) to the 1,3,5-cycloheptatriene (CHT*+) radical cation conversion, initialized by opening a bridgehead-methylene bond in N*+, are investigated using the quantum chemical B3LYP, MP2 and CCSD(T) methods in conjunction with the 6-311 +G(d,p) basis set. Two of the isomerizations proceed through the norcaradiene radical cation (NCD*+), either through a concerted path (N*+ - NCD*+), or by a stepwise mechanism via a stable intermediate (N*+ - I1 - NCD*+). At the CCSD(T)/6-311 +G(d,p)//B3LYP/6-311 +G(d,p) level, the lowest activation energy, 28.9 kcal mol(-1), is found for the concerted path whereas the stepwise path is found to be 2.3 kcal mol(-1) higher. On both pathways, NCD*+ rearranges further to CHT*+ with significantly less activation energy. The third channel proceeds from N*+ through I1 and then directly to CHT*+, with an activation energy of 37.1 kcal mol(-1). The multi-step channel reported earlier by our group, which proceeds from N*+ to CHT*+ via the quadricyclane and the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cations, is 4.6 kcal mol(-1) lower than the most favorable path of the present study. If the methylene group is substituted with C(CH3)2, however, the concerted path is estimated to be 5.6 kcal mol(-1) lower than the corresponding substituted multi-step path at the B3LYP/6-311+(d,p) level. This shows that substitution of particular positions can have dramatic effects on altering reaction barriers in the studied rearrangements. We also note that identical energies are computed for CHT*+ and NCD*+ whereas, in earlier theoretical investigations, the former was reported to be 6-17 kcal mol(-1) more stable than the latter. Finally, a bent geometry is obtained for CHT*+ with MP2/6-311 +G(d,p) in contradiction with the planar conformation reported for this cation in earlier computational studies.     

Theoretical analysis of concerted and stepwise mechanisms of Diels-Alder reactions of butadiene with silaethylene and disilene

The concerted and the stepwise mechanisms of the Diels-Alder reactions of butadiene with silaethylene and disilene were studied by ab initio MO methods. For the reaction of butadiene and silaethylene, an asymmetric concerted process that is almost stepwise and two stepwise processes were located. For the first step of the stepwise process, the C-Si bond formation is more favorable than the C-C bond formation. The activation energy barrier of the concerted transition state is only 0.89 kcal/mol lower than that of the first-step transition state of the C-Si bond formation for the stepwise process by the CASPT2 calculation level. For the reaction of butadiene and disilene, the activation energy barrier of the concerted-type transition state constrained with Cs symmetry is about 9 kcal/mol higher than that of the stepwise transition state by the CASSCF method. The energy barrier of the first step of the stepwise reaction disappears at the CASPT2/6-311++G(d,p) calculation level including the nondynamical correlation energy, although the reaction of the butadiene with disilene occurs through the stepwise-like process.     

Investigation of ethylene dimerization over faujasite zeolite by the ONIOM method.

Ethylene dimerization was investigated by using an 84T cluster of faujasite zeolite modeled by the ONIOM3(MP2/6-311++G(d,p):HF/6-31G(d):UFF) method. Concerted and stepwise mechanisms were evaluated. In the stepwise mechanism, the reaction proceeds by protonation of ethylene to form the surface ethoxide and then C--C bond formation between the ethoxide and the second ethylene molecule to give the butoxide product. The first step is rate-determining and has an activation barrier of 30.06 kcal mol(-1). The ethoxide intermediate is rather reactive and readily reacts with another ethylene molecule with a smaller activation energy of 28.87 kcal mol(-1). In the concerted mechanism, the reaction occurs in one step of simultaneous protonation and C--C bond formation. The activation barrier is calculated to be 38.08 kcal mol(-1). Therefore, the stepwise mechanism should dominate in ethylene dimerization.     

Concerted vs stepwise mechanisms in dehydro-Diels-Alder reactions

The Diels-Alder reaction is not limited to 1,3-dienes. Many cycloadditions of enynes and a smaller number of examples with 1,3-diynes have been reported. These "dehydro"-Diels-Alder cycloadditions are one class of dehydropericyclic reactions which have long been used to generate strained cyclic allenes and other novel structures. CCSD(T)//M05-2X computational results are reported for the cycloadditions of vinylacetylene and butadiyne with ethylene and acetylene. Both concerted and stepwise diradical routes have been explored for each reaction, with location of relevant stationary points. Relative to 1,3-dienes, replacement of one double bond by a triple bond adds 6-6.5 kcal/mol to the activation barrier; a second triple bond adds 4.3-4.5 kcal/mol to the barrier. Product strain decreases the predicted exothermicity. In every case, a concerted reaction is favored energetically. The difference between concerted and stepwise reactions is 5.2-6.6 kcal/mol for enynes but diminishes to 0.5-2 kcal/mol for diynes. Experimental studies on intramolecular diyne + ene cycloadditions show two distinct reaction pathways, providing evidence for competing concerted and stepwise mechanisms. Diyne + yne cycloadditions connect with arynes and ethynyl-1,3-cyclobutadiene. This potential energy surface appears to be flat, with only a minute advantage for a concerted process; many diyne cycloadditions or aryne cycloreversions will proceed by a stepwise mechanism.     

Quadricyclane radical cation rearrangements: a computational study of the transformations to 1,3,5-cycloheptatriene and norbornadiene

An alternative skeletal rearrangement of the quadricyclane radical cation (Q*+) explains the side products formed in the one-electron oxidation to norbornadiene. First, the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, with an activation energy of 14.9 kcal mol(-1), is formed. Second, this species can further rearrange to 1,3,5-cycloheptatriene through two plausible paths, that is, a multistep mechanism with two shallow intermediates and a stepwise path in which the bicyclo[3.2.0]hepta-2,6-diene radical cation is an intermediate. The multistep rearrangement has a rate-limiting step with an estimated activation energy of 16.5 kcal mol(-1), which is 2.8 kcal mol(-1) lower in energy than the stepwise mechanism. However, the lowest activation energy is found for the Q*+ cycloreversion to norbornadiene that has a transition structure, in close correspondence with earlier studies, and an activation energy of 10.1 kcal mol(-1), which agrees well with the experimental estimate of 9.3 kcal mol(-1). The computational estimates of activation energies were done using the CCSD(T)/6-311+G(d,p) method with geometries optimized on the B3LYP/6-311+G(d,p) level, combined with B3LYP/6-311+G(d,p) frequencies.     

Quantum mechanical study of the ring-closing reaction of the hexatriene radical cation

The ring-closing reaction of hexatriene radical cation 1(*)(+) to 1,3-cyclohexadiene radical cation 2(*)(+) was studied computationally at the B3LYP/6-31G* and QCISD(T)/6-311G*//QCISD/6-31G* levels of theory. Both, concerted and stepwise mechanisms were initially considered for this reaction. Upon evaluation at the B3LYP level of theory, three of the possible pathways-a concerted C(2)-symmetric via transition structure 3(*)(+) and stepwise C(1)-symmetric pathways involving three-membered ring intermediate 5(*)(+) and four-membered ring intermediate 6(*)(+)-were rejected due to high-energy stationary points along the reaction pathway. The two remaining pathways were found to be of competing energy. The first proceeds through the asymmetric, concerted transition structure 4(*)(+) with an activation barrier E(a) = 16.2 kcal/mol and an overall exothermicity of -23.8 kcal/mol. The second pathway, beginning from the cis,cis,trans rotamer of 1(*)(+), proceeds by a stepwise pathway to the cyclohexadiene product with an overall exothermicity of -18.6 kcal/mol. The activation energy for the rate-determining step in this process, the formation of the intermediate bicyclo[3.1.0]hex-2-ene via transition structure 9(*)(+), was found to be 20.4 kcal/mol. More rigorous calculations of a smaller subsection of the potential energy hypersurface at the QCISD(T)//QCISD level confirmed these findings and emphasized the importance of conformational control of the reactant.     

The cyclization mechanism of <Emphasis Type="Italic">cis</Emphasis> DAA-DAA dipeptide: an ab initio study

The gas-phase cyclization mechanism of cis DAA-DAA dipeptide (where DAA stands for the earlier described double amino acid molecule of (NH₂)₂C(COOH)₂ formula while DAA-DAA indicates the system formed by two DAAs linked via the peptide bond) is investigated in the absence of any catalysts. Two different paths, concerted and stepwise, each leading to the same cyclo(DAA-DAA) dipeptide product are examined on the basis of theoretical calculations carried out at the CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ level. The final product of the cyclization was found to adopt boat conformation of the six-membered 2,5-diketopiperazine ring and its formation was predicted to be thermodynamically favored by ca. 3.7 kcal/mol. The activation barrier estimated for the concerted mechanism (39 kcal/mol) was found to be higher than each of two barriers (30–33 kcal/mol) on the stepwise route which indicates that the cyclization process leading to the cyclo(DAA-DAA) dipeptide formation is more plausible when operating along the stepwise pathway.     

Substituent effects in pericyclic reactions of radical cations: the ring opening of 3-substituted cyclobutene radical cations

The substituent effects on the ring-opening reaction of cyclobutene radical cations have been studied at the Becke3LYP/6-31G* level of theory. The effect on the reaction energies and activation energies of the concerted and stepwise pathways of electron-donating substituents such as methyl and methoxy as well as electron-withdrawing substituents such as nitrile and carboxaldehyde in the 3-position of the cyclobutene is discussed. The exothermicity of the reaction correlates well with the ability of the substituent to stabilize the 1,3-butadiene radical cation by electron donation or conjugation. The relative stability of the (E) and (Z) isomers of the resulting 1,3-butadiene radical cations depends largely on steric effects. Similarly, steric effects are responsible for the relative energies of the different diastereomeric transition structures. The cyclopropyl carbinyl intermediate of the stepwise pathway resembles the nonclassical carbocation and is stabilized by electron-donating substituents. In the case of electron-donating substituents, this species becomes a minimum on the potential energy hypersurface, whereas unstabilized or destabilized cyclopropyl carbinyl radical cations are not minima on the hypersurface. The stabilization of the cyclopropyl carbinyl radical cation by substituents correlates qualitatively with the Brown-Okamoto substituent parameter sigma+. However, in all cases studied here, the concerted mechanism is the lowest energy pathway.     

Theoretical study on the reaction pathways of HFCO + H2O

For the reaction of methanoyl fluoride with water, both optimized structures and vibrational wavenumbers of reaction intermediates, transition structures and product complexes were calculated and characterized with theory at the MP2/6-311++G(d,p) level. Including a catalytic path and concerted and stepwise hydrolysis paths, possible reaction mechanisms were also investigated. The catalytic reaction of HFCO yielding HF and CO has the smallest activation barrier, 29.6 kcal/mol, whereas for the concerted hydrolysis 33.0 kcal/mol is required to overcome the barrier to form transoid HCOOH + HF, which is less than for the stepwise counterpart, 42.0 kcal/mol.     

Theoretical analysis of concerted and stepwise mechanisms of the hetero-Diels–Alder reaction of butadiene with formaldehyde and thioformaldehyde

The concerted and stepwise mechanisms of the hetero-Diels–Alder reaction of butadiene with formaldehyde and thioformaldehyde were studied by a CASSCF molecular orbital method. The energy barrier of the concerted reaction of butadiene with formaldehyde is about 21 kcal/mol higher than that of butadiene with thioformaldehyde at the CAS-MP2 calculation level. For the stepwise reaction paths, the energy barrier for the first step process of the reaction of butadiene with formaldehyde is about 17 kcal/mol above that of butadiene with thioformaldehyde. The concerted pathways for both systems are more favorable by 9–12 kcal/mol than the stepwise pathways. The electronic mechanisms for the concerted reactions of both reaction systems are also discussed by a CiLC analysis.     

Computational study of the aminolysis of 2-benzoxazolinone

Three possible mechanisms (zwitterionic, neutral stepwise, and neutral concerted) of the ring-opening reaction of 2-benzoxazolinone (BO) upon aminolysis with methylamine were studied at the B3LYP/6-31G* level. In the gas phase, the neutral concerted mechanism is shown to be most favorable, which proceeds via a rate-determining barrier of 28-29 kcal/mol. The transition state, CTS, associated with this barrier is a four-centered one, where 1,2-addition of the N[bond]H of methylamine to the C[bond]O of BO ring occurs. The rate-determining barrier of the neutral stepwise pathway is found to be ca. 42 kcal/mol. The inclusion of solvent effects by a polarizable continuum model (PCM) does not change the conclusions based on the gas-phase study; the barrier at CTS is reduced to 20, 20, and 22 kcal/mol in water, ethanol, and acetonitrile, respectively.     

Reactions of 1,3-cyclohexadiene with singlet oxygen. A theoretical study.

A thorough study of the reaction of singlet oxygen with 1,3-cyclohexadiene has been made at the B3LYP/6-31G(d) and CASPT2(12e,10o) levels. The initial addition reaction follows a stepwise diradical pathway to form cyclohexadiene endoperoxide with an activation barrier of 6.5 kcal/mol (standard level = CASPT2(12e,10o)/6-31G(d); geometries and zero-point corrections at B3LYP/6-31G(d)), which is consistent with an experimental value of 5.5 kcal/mol. However, as the enthalpy of the transition structure for the second step is lower than the diradical intermediate, the reaction might also be viewed as a nonsynchronous concerted reaction. In fact, the concertedness of the reaction is temperature dependent since entropy differences create a free energy barrier for the second step of 1.8 kcal/mol at 298 K. There are two ene reactions; one is a concerted mechanism (DeltaH(double dagger) = 8.8 kcal/mol) to 1-hydroperoxy-2,5-cyclohexadiene (5), while the other, which forms 1-hydroperoxy-2,4-cyclohexadiene (18), passes through the same diradical intermediate (9) as found on the pathway to endoperoxide. The major pathway from the endoperoxide is O-O bond cleavage (22.0 kcal/mol barrier) to form a 1,4-diradical (25), which is 13.9 kcal/mol less stable than the endoperoxide. From the diradical, two low-energy pathways exist, one to epoxyketone (29) and the other to the diepoxide (27), where both products are known to be formed experimentally with a product ratio sensitive to the nature of substitutents. A significantly higher activation barrier leads to C-C bond cleavage and direct formation of maleic aldehyde plus ethylene.     

Density functional study of the retrocyclization of norbornadiene and norbornene catalyzed by Fe(+)

In the presence of Fe(+) catalyst, the retro Diels-Alder reaction of norbornadiene (NBD) is predicted to be stepwise with an activation barrier of 18.8 kcal/mol, which is 3.1 kcal/mol lower than the concerted retro reaction. For norbornene (NBN), the Fe(+)-catalyzed retro reaction is also calculated to be stepwise with an activation barrier of 24.9 kcal/mol, which is 8.5 kcal/mol lower than the uncatalyzed stepwise reaction but 3.8 kcal/mol higher than the concerted reaction. The intermediates from the NBD and NBN retro Diels-Alder reactions, C(5)H(6)FeC(2)H(2)(+) and C(5)H(6)FeC(2)H(4)(+), are predicted to have low activation barriers for ligand-to-ligand hydrogen transfers (through an iron-hydrido intermediate) to form CpFeC(2)H(3)(+) and CpFeC(2)H(5)(+) and, ultimately, vinyl- and ethyl-substituted cyclopentadiene-iron complexes, respectively. In contrast to FeC(2)H(2)(+) and FeC(2)H(4)(+), the lowest-energy pathways on the C(5)H(6)FeC(2)H(2)(+) and C(5)H(6)FeC(2)H(4)(+) potential energy surfaces involve only one multiplicity (quartet). The C(2)H(2) and C(2)H(4) complexes of CpFe(+) and C(5)H(6)Fe(+) are compared.     

NH tautomerism in the natural chlorin derivatives

The NH tautomerism of five Mg-free chlorophyll a and b derivatives 2-6 was studied utilizing NMR spectroscopy and molecular modeling. The results from the dynamic NMR measurements of the chlorins revealed that substituent effects contribute crucially to the free energy of activation (DeltaG(double dagger)) in the NH tautomeric processes. An intermediate tautomer for the total tautomeric NH exchange in a chlorin was observed for the first time, when the (1)H NMR spectra of chlorin e(6) TME (3) and rhodin g(7) TME (4) (TME = trimethyl ester) were measured at lower temperatures. The lower energy barriers (DeltaG(1)(double dagger)) obtained for the formation of the intermediate tautomers of 3 and 4, assigned to the N(22)-H, N(24)-H trans-tautomer, were 10.8 and 10.6 kcal/mol, respectively. The energy barrier (DeltaG(2)(double dagger) value) for the total tautomeric NH exchange in the five chlorins was found to vary from 13.6 kcal/mol to values higher than 18 kcal/mol. The lowest DeltaG(2)(double dagger) value (13.6 kcal/mol) was obtained for rhodochlorin XV dimethyl ester (2), which was the only chlorophyll derivative lacking the C(15) substituent. In the case of chlorins 4 and 5, the steric crowding around the methoxycarbonylmethyl group at C(15) raised the DeltaG(2)(double dagger) activation free-energy to 17.1 kcal/mol. However, the highest energy barrier with DeltaG(2)(double dagger) > 18 kcal/mol was observed for the NH exchange of pyropheophorbide a methyl ester (6), possessing the macrocycle rigidifying isocyclic ring E. Our results demonstrate that the steric strain, arising either from the steric crowding around the bulky substituent at C(15) or the macrocycle rigidifying isocyclic ring E, slows down the NH tautomeric process. We suggest that deformations in the chlorin skeleton are closely connected to the NH tautomeric exchange and that the exchange occurs by a stepwise proton-transfer mechanism via a hydrogen bridge.     

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.     

Mechanisms of the reactions of W AND W+ with H2O: computational studies

The mechanism of the reactions of W and W(+) with the water molecule have been studied for several lower-lying electronic states of tungsten centers at the CCSD(T)/6-311G(d,p)+SDD and B3LYP/6-31G(d,p)+SDD levels of theory. It is shown that these reactions are essentially multistate processes, during which lower-lying electronic states of the systems cross several times. They start with the formation of initial prereaction M(H(2)O) complexes with M-H(2)O bonding energies of 9.6 and 48.2 kcal/mol for M = W and W(+), followed by insertion of the metal center into an O-H bond with 20.0 and 53.3 kcal/mol barriers for neutral and cationic systems, respectively. The overall process of M + H(2)O --> t-HM(OH) is calculated to be highly exothermic, 48.4 and 48.8 kcal/mol for M = W and W(+). From the HM(OH) intermediate the reaction may proceed via several different channels, among which the stepwise HM(OH) --> HMO + H --> (H)(2)MO and concerted HM(OH) --> (H)(2)MO pathways are more favorable and can compete (energetically) with each other. For the neutral system (M = W), the concerted process is the most favorable, whereas for the charged system (M = W(+)), the stepwise pathway is slightly more favorable. From the energetically most favorable intermediate (H)(2)MO the reactions proceed via H(2)-molecule formation with a 53.1 kcal/mol activation barrier for the neutral system. For the cationic system, H-H formation and dissociation is an almost barrierless process. The overall reaction of W and W(+) with the water molecule leading to H(2) + MO formation is found to be exothermic by 48.2 and 39.8 kcal/mol, respectively. In the gas phase with the collision-less conditions the reactions W((7)S) + H(2)O --> H(2) + WO((3)Sigma(+)), and W(+)((6)D) + H(2)O --> H(2) + WO(+)((4)Sigma(+)) are expected to proceed via a 10.4 and 5.1 kcal/mol overall energy barrier corresponding to the first O-H dissociation at the TS1. On the basis of these PESs, we predict kinetic rate constants for the reactions of W and W(+) with H(2)O.     

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.     

A novel hydrogen migration of dialkylphosphonic acid esters using electrospray ionization tandem mass spectrometry

In this paper, attention is focused on analysis of the fragmentation of α-hydroxy-β-amino phosphonate esters designed as inhibitors of protein kinase A. An interesting proton migration mechanism in the cleavage of the P-C bond is investigated by electrospray ionization tandem mass spectrometry. A possible rearrangement mechanism is proposed and verified by high-resolution mass spectra using isotope deuterium/hydrogen-exchange technology and additionally checked by detailed DFT calculation based on Gaussian software. The result clearly indicates that this mechanism proceeds by a five-membered ring concerted transition state with activation energy 11.3 kcal mol(-1) for the compound 3f. The overall reaction is endothermic with an energy 13.2 kcal mol(-1). The effect of different substituents and different metal ions for rearrangement of these esters is studied by experiment and theory. It is concluded that this rearrangement process is energetically unfavorable and hence only occurs in the mass spectrometer.     

Transition states for the dimerization of 1,3-cyclohexadiene: a DFT, CASPT2, and CBS-QB3 quantum mechanical investigation

Quantum mechanical calculations using restricted and unrestricted B3LYP density functional theory, CASPT2, and CBS-QB3 methods for the dimerization of 1,3-cyclohexadiene (1) reveal several highly competitive concerted and stepwise reaction pathways leading to [4 + 2] and [2 + 2] cycloadducts, as well as a novel [6 + 4] ene product. The transition state for endo-[4 + 2] cycloaddition (endo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 28.7 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 19.0 kcal/mol) is not bis-pericyclic, leading to nondegenerate primary and secondary orbital interactions. However, the C(s) symmetric second-order saddle point on the B3LYP energy surface is only 0.3 kcal/mol above endo-2TS. The activation enthalpy for the concerted exo-[4 + 2] cycloaddition (exo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 30.1 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 21.1 kcal/mol) is 1.4 kcal/mol higher than that of the endo transition state. Stepwise pathways involving diallyl radicals are formed via two different C-C forming transition states (rac-5TS and meso-5TS) and are predicted to be competitive with the concerted cycloaddition. Transition states were located for cyclization from intermediate rac-5 leading to the endo-[4 + 2] (endo-2) and exo-[2 + 2] (anti-3) cycloadducts. Only the endo-[2 + 2] (syn-3) transition state was located for cyclization of intermediate meso-5. The novel [6 + 4] "concerted" ene transition state (threo-4TS, DeltaH(double dagger)(UB3LYP(0K)) = 28.3 kcal/mol) is found to be unstable with respect to an unrestricted calculation. This diradicaloid transition state closely resembles the cyclohexadiallyl radical rather than the linked cyclohexadienyl radical. Several [3,3] sigmatropic rearrangement transition states were also located and have activation enthalpies between 27 and 31 kcal/mol.     

DFT方法研究酸性沸石上苯与乙烯烷基化反应的机理

采用B3LYP/6-31G*水平计算来研究酸性沸石上苯与乙烯的烷基化反应历程,从生成能和反应活化能角度分析并讨论了苯与乙烯的反应机理.选取4T簇模型模拟分子筛的酸性位,使用密度泛函理论对烷基化反应三种不同的反应机理(两个联合反应机理和一个分步反应机理)进行计算分析.结果表明,在联合反应机理中,乙烯的质子化和苯与乙烯间C-C键的形成同时发生;分步反应机理中,首先形成一个稳定的乙醇盐中间物种,然后与苯分子反应形成乙苯.联合机理速控步骤的活化能约为160kJ/mol,分步机理速控步骤的活化能为190.24kJ/mol,因此,酸性沸石上苯与乙烯烷基化反应机理主要以联合机理为主,但分步机理与其有一定程度的竞争。