Article

Theoretical Study on Chlorine and Hydrogen Shift in Cycloheptatriene and Cyclopentadiene Derivatives

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Abstract

The transition structures (TSs) for chlorine 1,7-shift and 1,5-shift in 1,7,7-trichlorocycloheptatriene (1) and those of chlorine 1,5-shifts in 1,5,5-trichlorocyclopentadiene (3) and 1,2,5-trichloro-1,3-pentadiene (5) derivatives have been located with density functional theory (DFT) at the Becke3LYP/6-311G [and Becke3LYP/6-311+G] level. The calculational results were compared with those for corresponding hydrogen shifts in nonsubstituted molecules (cycloheptatriene (2), cyclopentadiene (4), and 1,3-pentadiene (6)). The following points were clarified: (1) The activation energy (Delta E(++)) for chlorine 1,7-shift in 1 was evaluated to be only +50.1 [+49.2] kJ/mol, which is smaller than that (+69.9 [+68.3]) for a 1,5-shift, supporting the theory that the conversion between two equivalent A and A' proceeds through a TS for direct chlorine 1,7-shift (Figure 1), rather than through a TS for a 1,5-shift (Figure 2). (2) The considerable amount of charge separation between a migrating chlorine atom (Cl(m)) and a seven-membered ring (-0.53 and +0.47 for Merz-Singh-Kollman scheme) occurs in a chlorine 1,7-shift, which is in good contrast to the result that the migrating hydrogen atom (H(m)) for a 1,7-shift in cycloheptatriene (2) carries almost no charge (Figure 3). This large charge separation can stabilize the TS for the chlorine 1,7-shift pathway. (3) The Delta E(++) values for suprafacial hydrogen 1,7-shift in 2 are quite large (+288.0 [+284.8] kJ/mol), much larger than that (+166.8 [+167.0]) for a 1,5-shift in 4 which is orbital symmetrically allowed (Figure 3). The calculation suggests that the chlorine 1,7-shift in 1 occurs easily at room temperature (actually observed experimentally) by proceeding via concerted suprafacial 1,7-shift through the zwitterionic TS with the significant assistance of Coulomb interaction between charged fragments (negatively charged chlorine atom and positively charged tropylium ring), rather than via a suprafacial 1,5-sigmatropic pathway. Other cases studied in this paper showed usual results predicted by orbital symmetrical consideration.

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... Moreover, Kim inferred that Cl shift in cyclopentadiene ring of dihydrofulvalene [21,22] may be another important reaction route, leading to the formation of additional PCN isomers. As reported, the 1,5-sigmatropic shift of Cl is similar to that of H in chlorinated cyclopentadiene [33]. Besides, H/Cl atom may be directly abstracted. ...
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Not forbidden: Thermal 4n electron electrocyclic reactions of Hückel topology structures proceed via "allowed" conrotatory pathways. However, for a Möbius topology, the Woodward-Hoffmann rules may be reversed and a "forbidden" disrotatory pathway can be preferred as shown theoretically for dodecahexaene 1 that transforms via a Heilbronner-Möbius aromatic transition structure 2 into a cyclic polyene 3.
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The accelerated rates of small-membered heterocycles relative to acyclic analogues are typically rationalized solely in terms of relief of ring strain. The relative rates of attack of ammonia on oxirane, oxetane, thiirane, and thietane were determined computationally in the gas phase at the MP2(Full)/6-31+G(d) level with respect to the model acyclic compounds methoxyethane and thiomethylethane. Because the cyclic ether and thioether pairs have very similar strain energies, they should react at similar rates by the S(N)2 mechanism if the degree of strain energy release in the transition state is approximately equal. The reactivity of the four-membered rings could be explained almost entirely by relief of strain. The three-membered rings reacted at rates at least 10(6) times faster than calculated from ring strain considerations alone. The electronic distribution of the transition states was determined using AIM methodology and found to indicate that bond cleavage was virtually complete, while bond formation was incomplete. Calculation of atomic charges by the Mulliken, AIM, CHELPG, and NBO methods indicated that positive charge at the reaction center was significantly lower for the three-membered rings than other members of the series. A simple electrostatic model identified differences in energy sufficient to account for the observed rate acceleration. The unique topological features of a three-membered ring make it possible for the partially negatively charged oxygen or sulfur to reduce the positive charge on the reaction center.
Article
Intramolecular and radical-promoted mechanisms for the rearrangement of azulene to naphthalene are assessed with the aid of density functional calculations. All intramolecular mechanisms have very high activation energies (>/=350 kJ mol(-1) from azulene) and so can only be competitive at temperatures above 1000 degrees C. Two radical-promoted mechanisms, the methylene walk and spiran pathways, dominate the reaction below this temperature. The activation energy for an orbital symmetry-allowed mechanism via a bicyclobutane intermediate is 382 kJ mol(-1). The norcaradiene-vinylidene mechanism that has been proposed in order to explain the formation of small amounts of 1-phenyl-1-buten-3-ynes from flash thermolysis of azulene has an activation energy of 360 kJ mol(-1); subtle features of the B3LYP/6-31G(d) energy surface for this mechanism are discussed. All intermediates and transition states on the spiran and methylene walk radical-promoted pathways have been located at the B3LYP/6-31G(d) level. Interconversion of all n-H-azulyl radicals via hydrogen shifts was also examined, and hydrogen shifts around the five-membered ring are competitive with the mechanisms leading to rearrangement to naphthalene, but those around the seven-membered ring are not. Conversion of a tricyclic radical to the 9-H-naphthyl radical is the rate-limiting transition state on the spiran pathway, and lies 164.0 kJ mol(-1) above that of the 1-H-azulyl radical. The transition state for the degenerate hydrogen shift between the 9-H-azulyl and 10-H-azulyl radicals is 7.4 kJ mol(-1) lower. Partial equilibration of the intermediates in the spiran pathway via this shift may therefore occur, and this can account for the surprising formation of 1-methylnaphthalene from 2-methylazulene. The rate-limiting transition state for the methylene walk pathway involves the concerted transfer of a methylene group from one ring to the other and lies 182.3 kJ mol(-1) above that of the 1-H-azulyl radical. It is shown that rearrangement via a combination of 31% methylene walk and 69% spiran pathways can account semiquantitatively for all the products from 1-(13)C-azulene, 9-(13)C-azulene, and 4,7-(13)C(2)-azulene, in addition to accounting for the products from methylazulenes, and the formation of naphthalene-d(0) and -d(2) from azulene-4-d. It is also pointed out that a small extension to the spiran pathway could provide an alternative explanation for the formation of 1-phenyl-1-buten-3-ynes.
Article
The effects of aromatic stabilization on the rates of [1,5]-hydrogen shifts in a series of carbo- and heterocyclic dihydroaromatic compounds were estimated by B3LYP/6-31G computations. The aromatic stabilization energy of the product is directly translated into increased exothermicity of these reactions. Relative trends for a significant range of endothermic and exothermic [1,5]-shifts with different intrinsic activation energies are reliably described by Marcus theory. The effects of aromaticity or antiaromaticity are very large and can lead to dramatic acceleration or deceleration of [1,5]-hydrogen shifts and even to complete disappearance of the reaction barrier. Not only the activation energy but the shape and position of the reaction barrier can be efficiently controlled by changes in the aromaticity of the products, making these systems interesting models for studying hydrogen tunneling. Marcus theory can also be applied successfully to other pericyclic shifts such as [1,5]-shifts which involve chlorine and methyl transfer.
Article
Die Übergangszustände elektrocyclischer und sigmatroper Umlagerungen, an denen gleich viele Elektronen beteiligt sind, unterscheiden sich in nichts weiter als einem einzigen Proton (siehe Bild).
Article
To investigate the gas-phase formation of polychlorinated naphthalenes (PCNs) and dibenzofurans (PCDFs) from chlorinated phenols in combustion exhaust gas, experiments were performed with each of the three chlorophenols in a laminar flow reactor over the range of 550-750 degrees C under oxidative conditions. Maximum PCN and PCDF yields were observed between 625 and 725 degrees C. The degree of chlorination of naphthalene and dibenzofuran products decreased as temperature increased, and on average, the naphthalene congeners were less chlorinated than the dibenzofuran congeners. Congener distributions are consistent with proposed PCN and PCDF formation pathways, both involving phenoxy radical coupling at unchlorinated ortho-carbon sites to form a dihydroxybiphenyl keto tautomer intermediate. Tautomerization of this intermediate and subsequent fusion via H2O loss results in PCDF formation, whereas CO elimination and subsequent fusion with hydrogen and/or chlorine loss leads to PCN formation. PCDF isomer distributions were found to be weakly dependent on temperature. PCN isomer distributions were found to be more temperature sensitive, however, with selectivity to particular isomers decreasing with increasing temperature. These results contribute to the understanding of PCN and PCDF formation in combustion and provide information on how to predict and minimize these emissions.
Article
Unlike the synthetically exploited oxiranes and thiiranes, aziridines that lack electron-withdrawing substituents, such as acyl or sulfonyl functionalities at nitrogen, are rather unreactive. As expected, three-membered aziridine 6 was calculated to be significantly more reactive than azetidine 7 in nucleophilic cleavage by ammonia, a typical nucleophile. The reactivity of 7 was about the same as that of an acyclic model compound, 8, when release of ring strain in the transition state was taken into account. Fluorine due to its similar size but vastly different electronegativity has been substituted for hydrogen as a means of modifying chemical properties for varied applications. In the present investigation, the effect of fluorine substitution at aziridine positions other than nitrogen was studied. Computations at the MP2(Full)/6-311++G(d,p)//MP2(Full)/6-31+G(d) level found a vast preference for attack by ammonia at the 3-position of 2-fluoroaziridine in the gas phase at 298 K. When release of ring strain was taken into account, this compound reacted more than 10(11) times faster than 6. The reaction rate with trans-2,3-difluoroaziridine was about twice that of 2-fluoroaziridine, while its diastereomer reacted with ammonia considerably slower. Acyclic fluorinated amine model compounds were employed to assess the generality of the effects produced by fluorine substitution. The results were rationalized by the energy contributions of strain energy releases, stabilization of the leaving group, and the relative electrostatic energies of the heterocycles in the transition states. The more reactive fluoroaziridines underwent nucleophilic attack at rates comparable to those of N-acetylaziridine.
Article
The kinetics of gas-phase thermal [1,5] hydrogen shifts interconverting the five isomeric mono-deuterium-labeled cis,cis-1,3-cyclononadienes have been followed at four temperatures from 240 to 287 degrees C. The activation parameters found were Ea = 37.1 +/- 0.8 kcal/mol, log A = 11.6 +/- 0.3, DeltaH++ = 36.0 +/- 0.8 kcal/mol, and DeltaS++ = -9.0 +/- 0.3 eu. Density functional theory based calculations have provided geometries and energies for the ground-state cyclononadiene conformational isomers, for the transition states linking one to another, and for the transition states for [1,5] hydrogen shifts responsible for isomerizations among the five labeled dienes. A generalized formulation of the Winstein-Holness equation is presented and applied to the complex system, one that involves 11 ground-state conformers, 10 transition states separating them, and five transition states for [1,5] hydrogen shifts. The value for the empirical Ea derived from calculated mole fractions of ground-state conformers and calculated energies for specific ground-state conformers and [1,5] hydrogen shift transition structures was 37.5 kcal/mol, in excellent agreement with the experimentally obtained activation energy. The significance of conformational options in various ground states and transition structures for the [1,5] hydrogen shifts is considerable, an inference that may well have general applicability.
Article
Although heating 2-methoxy-2H-azepine results in a [1,5] sigmatropic hydrogen shift, heating 2-propylthio-2H-azepine results in not only a [1,5] sigmatropic hydrogen shift but also a [1,5] sigmatropic propylthio shift. Kinetic measurements reveal that migratory aptitudes increase in the order of MeO < H, PrS. These [1,5] sigmatropic shifts are discussed on the basis of ab initio DFT calculations. [reaction: see text].
Article
A simple and efficient methodology for the synthesis of a small library of substituted indolizines with different degrees of saturation starting from the racemic 2-formyl-1,4-DHP reagent was described. The large synthetic possibilities of this reagent as well as of its Knoevenagel corresponding 2-dicyanovinyl-1,4-DHP reagent were investigated using four kinds of activated methylenes as nucleophiles. The key step of the sequential reaction was based on the highly diastereoselective tandem Michael addition/intramolecular amino-nitrile cyclization catalyzed by an organic base, which resulted in the formation of 1,7-dihydroindolizines in a diastereoselective manner. The process seems to be a straightforward one and can be extended to numerous active methylenes such as malononitrile, 1,3-diketones, and alkyl acetoacetates. The 1,3-hydrogen shift of partially hydrogenated indolizines was accomplished easily with a base at room temperature, giving rise to the corresponding 7,8-dihydroindolizines in very good yields. Interestingly, when the active methylene bears a leaving group, the latter process could not be accomplished because a rare cis-elimination of phenylsulfinic acid and nitrous acid preceded the hydrogen shift. The resulting 1,7-dihydroindolizines bearing an exo-methylene group at C1 were not isolated in all cases, as they turned rapidly to indolizines as the thermodynamically more stable products. During these investigations, oxidization of 1,7-dihydroindolizines with CuCl(2) resulted in the formation of polysubstituted pyridines. Also, the epimerization of certain 1,7-dihydroindolizines was evidenced in the solution studied by NMR spectroscopy, whereas in the solid state, they existed only in a unique form as shown by X-ray diffraction analysis of a representative structure. Finally, all products reported herein bear a primary amine and a nitrile function crucial for further transformations. These include the introduction of various pharmacophore groups at either NH(2) or CN groups as well as at both groups at the same time to access the more elaborated indolizines fused to N- or N,N-heterocycles.
Article
The geometric and kinetic isotope effects (GIE and KIE) for thermal [1,5]-sigmatropic H and D shifts of (Z)-1,3-pentadiene were studied by including the direct quantum effect of the migrating H or D nucleus in the multi-component molecular orbital-Hartree-Fock (MC_MO-HF) method. Based on the results, the C(1)-D bond lengths are 0.007 Angstrom shorter than the C1-H bond lengths in both the reactant (A) and the transition states (TS), whereas other bond lengths resemble those between H and D. The ratio of the rate constant (k(H)/k(D)) of the reaction for the thermal [1,5]-H and D shifts determined using the MC_MO-HF method (8.28) is closer to the experimental value (12.2) than that determined using either the conventional restricted Hartree-Fock (4.10) or restricted Møller-Plesset second-order perturbation (3.79) methods.
Article
In this paper we report the rearrangement of spirocyclohexadienones into dihydrotropones in basic conditions as a new method for the preparation of seven-membered ring ketones, which are key building blocks for the synthesis of tropoloalkaloids. DFT calculations and deuterium labeling studies support the mechanism we propose for this rearrangement, involving the ring opening of a spirocyclopropane intermediate followed by successive base-catalyzed 1,3-hydrogen shifts. The X-ray structure of the resulting dihydrotropone shows near-perfect planarity and the conjugation gain is likely to be the driving force of the reaction.
Article
The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment.
Article
Orbital symmetry controls in an easily discernible manner the feasibility and stereochemical consequences of every concerted reaction.
Article
A contracted Gaussian basis set (6‐311G∗∗) is developed by optimizing exponents and coefficients at the Møller–Plesset (MP) second‐order level for the ground states of first‐row atoms. This has a triple split in the valence s and p shells together with a single set of uncontracted polarization functions on each atom. The basis is tested by computing structures and energies for some simple molecules at various levels of MP theory and comparing with experiment.
Article
The fully optimized structures and relative energies of all possible methoxy-1,3,5-cycloheptatriene (MCHT) isomers have been determined by semiempirical, ab initio, and density functional theory (DFT) molecular orbital calculations. All methods identify the boat conformation of 1-methoxy-1,3,5-cycloheptatriene as the most stable species in this group of compounds. In order to evaluate boat interconversion barriers, optimizations of the planar isomers were also performed. For comparison purposes, we applied the same computational methodologies to boat and planar conformations of 1,3,5-cycloheptatriene (CHT). Among the semiempirical methods, the SAM1 approximation was found to best reproduce the ab initio and DFT results. Examination of rotational energy profiles allowed for identification of the factors controlling the preferred orientations of the methoxy group in these compounds. The calculations predict that methoxy substitution has little influence on the preferred conformation of the seven-membered ring and only a minor impact on the energy required for interconversion of boat conformations through a planar transition structure. Inclusion of electron correlation makes little difference on the calculated relative energies of the MCHT isomers and optimized geometries but significantly improves the computed reaction barriers involved in the CHT hydrogen transfer transition structures.
Article
A kinetic study of the rearrangement of 5-methylcyclopentadiene to 1-methylcyclopentadiene has been made, using NMR to monitor the reaction. In the absence of a strong base, the rearrangement took place through an intramolecular 1,2 hydrogen shift and showed the characteristics of a homogeneous reaction following the first order rate law. The activation energy and entropy were determined from the temperature dependence of the rate. Base did not catalyze this process, but, in the presence of a sufficiently strong base, an alternative process involving the intermediacy of the methylcyclopentadienide anion became an important route for isomerization. A study of the rearrangement of 1,5-dimethylcyclopentadiene was also made. Results from these experiments and other more approximate data serve as a basis for a discussion of the mechanism of the rearrangement.
Article
The thermal and photochemical [1,3]-H shift in propene and [1,5]-H shift in 1,3-pentadiene have been investigated by means of ab initio calculations. The suprafacial and antarafacial transition states are compared with a transition state (TS) of C2v symmetry, which results from a shift of the migrating hydrogen atom in the plane of the carbon skeleton (planar shift). Full geometry optimizations were performed at the ab initio level of computation using the restricted and unrestricted Hartree—Fock formalism with a 3-21G basis set. All structures were characterized by a complete vibrational analysis. Final state energies were calculated from a configuration interaction among the leading configurations. The effect of all other configurations was treated through a second-order perturbation. The thermal shifts are found to proceed according to the rules of Woodward and Hoffmann, i.e. an antarafacial [1,3]-H shift in propene and a suprafacial [1,5]-H shift in pentadiene. In both cases a second TS of C2v symmetry is found close in energy to the first. These TSs must be described with an open-shell configuration. For the photochemical process, both shifts are found to proceed through a TS of C2v symmetry. This TS correlates with a twisted conformation of the reactant via a planar shift.
Article
Intramolecular rearrangement due to successive 1,5-sigmatropic shifts of chlorine over a five-membered ring were found to occur in 5-chloro-1-alky1-2,3,4,5-tetramethoxycarbonylcyclopentadienes with energy barriers to chlorine migration falling in the range 26.0–27.3 kcal mol–1.
Article
Reexamination of the method of preparation of the title compound has uncovered a thermal rearrangement which converted it into the 5,10-dichloro isomer. The structure of the isomer was established and a mechanism for the transformation is proposed. A convenient synthesis of the title compound has been found.
Article
The thermal rearrangement reactions of 7,7-dideuteriobenzonorcaradiene (1), 5,6-dideuterio-2,3-benzonorbornadiene (7), and 3,5,7,7-tetradeuterio-1,2-benzotropilidene (11) have been studied. Pyrolysis of 1 produced 1,2-benzotropilidene with 0.3 3, 0.5, 0.5, and 0.66 deuteron in positions 3, 4, 6, and 7, respectively, presumably via either a 1,2- or a 1,5-hydrogen shift mechanism. Pyrolysis of 7 gave 1,2-benzotropilidene with, initially, 0.5, 1, and 0.5 deuteron in positions 4, 5, and 6, respectively, ruling out 6,7-benzobicyclo[3.2.0]hept-2,6-diene as a possible intermediate. Further thermal rearrangement of 7 or 11 showed that positions 3, 4, 6, and 7 completely equilibrated their hydrogens. Three mechanisms are postulated for this rearrangement; 1,2- and/or 1,5-hydrogen shift or methylene group rearrangement. In all of these thermal reorganizations 1,5-hydrogen shift, between positions 3 and 7, occurs very rapidly, even below 250°.
Article
In contrast to the terminal phosphinidene complex PhPW(CO)5 (2), which adds to [5]metacyclophane (1) in a 1,4-fashion, dichlorocarbene preferentially adds in a 1,2-fashion to the formal “anti-Bredt” type double bond of the aromatic ring of 1 to afford the norcaradiene 11b, which immediately rearranges to the bridged cycloheptatriene 12b and further by a [1,5] sigmatropic chlorine migration to the isomeric 13b as the first observable product. More slowly, the latter isomerizes via a dissociative mechanism to give 15b. A computational study supports the notion that the [1,5] chlorine migration in the rearrangement 12b → 13b, for which an activation barrier of 70.2 kJ mol-1 was calculated, is essentially concerted with minor charge separation. In contrast, the analogous [1,5] chlorine migration in the flat model compound 7,7-dichlorocycloheptatriene (12a) displays features of a dissociative pathway.
Article
It is demonstrated that 1,3-dienes, in which a vinyl and alkyl group are cis, undergo a reversible thermal isomerization involving the over-all 1,5-transfer of a hydrogen with concomitant migration of both carbon-carbon double.bonds. At 350-450°, depending on the nature of the dienes on hand, an equilibrium mixture is approached in which the more thermodynamically stable isomer predominates. Dienes in which a vinyl group and alkyl group are trans appear to be thermally stable. A means for distinguishing between isomeric cis- and trans-1,3-dienes is suggested on the basis of this difference in thermal behavior. In this manner, geometric configurations have been assigned to α- and β-ocimene.
Article
The present investigation was undertaken with the objective of identifying a (1,5) sigmatropic rearrangement occurring in a cyclic system which is not capable of such distention of the ..pi.. framework. The case for study was the rearrangement of the 9aH-quinolizine to the 4H-quinolizine. The temperature dependence of the isotope effect was again applied as the criterion of TS/sup + +/ geometry in H transfer. It has frequently been demonstrated, though not directly derivable (at present) from conventional transition-state theory of the isotope effect, that a bent TS/sup + +/, i.e., one involving H transfer at an acute angle, can be correlated with a temperature-independent k/sub H//k/sub D/. That is to say, the finding of isotope effect parameters of (..delta..E/sub a/)/sup H//sub D/ approx. = 0 and A/sub H//A/sub D/>>1.2 has been empirically shown to be most congruent with a bent TS/sup + +/.
Article
Ab initio 3-21 G calculations have been performed for the (1,5)-H shift in cis-1,3-pentadiene. A transition state of C/sub s/ symmetry has been compared with one of C/sub 2nu/ symmetry. The lowest energy configuration of this latter structure has B⁠symmetry and must therefore be described by an open-shell calculation. The energy of this structure is favored by 5.2 kcal/mol over the one of C/sub s/ symmetry. Both structures are found to be real transition states. Both the calculated reaction rates and the kinetic isotope effects are found to be considerably smaller than the observed ones. A mechanism is suggested in which tunneling takes place between high-vibrational states of the reactant and the product. It is shown that this mechanism is most likely for the transition state of C/sub 2nu/ symmetry. The calculated tunneling rates indicate that the (1,5)-H shift in cis-1,3-pentadiene mainly takes place via this mechanism.
Article
Previous attempts to combine Hartree–Fock theory with local density‐functional theory have been unsuccessful in applications to molecular bonding. We derive a new coupling of these two theories that maintains their simplicity and computational efficiency, and yet greatly improves their predictive power. Very encouraging results of tests on atomization energies, ionization potentials, and proton affinities are reported, and the potential for future development is discussed.
Article
CNDO/2 and INDO calculations have been carried out in order to construct a suitable model for the activated complex during the reaction. In this reaction model the migrating hydrogen atom moves along an edge of the cyclopentadiene ring. An analysis of this situation suggests a partial electron transfer from the migrating hydrogen to the nascent cyclopentadienyl system. This charge transfer is discussed in terms of aromaticity. The calculated activation enthalpies are 10 kcal/mole (CNDO/2) and 17 kcal/mole (INDO), whereas the experimental value is ca. 24 kcal/mole [1].
Article
INDO calculations have been performed for the activated complex of the [1, 5] H.-shift in 1,3-cyclohexadiene and 1,3,5-cycloheptatriene. During the migration in the cyclohexadiene system a homoconjugation was calculated between the carbon atoms C1 and C5.For cycloheptatriene it could be demonstrated that one double bond does not participate in the reaction.Activation enthalpies are related to (homo)conjugation in the transition state of the reaction for cyclic conjugated dienes and trienes. The electron density on the migrating hydrogen can be related to the electron affinity of the ring system in the transition state.[/p]
Article
Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
Article
Current gradient-corrected density-functional approximations for the exchange energies of atomic and molecular systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy density. Here we report a gradient-corrected exchange-energy functional with the proper asymptotic limit. Our functional, containing only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of atomic systems with remarkable accuracy, surpassing the performance of previous functionals containing two parameters or more.
Article
A correlation-energy formula due to Colle and Salvetti [Theor. Chim. Acta 37, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the local kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
Article
High-level ab initio molecular orbital calculations, using the G2(MP2,SVP) theory (and semiempirical methods) have been used to examine several 1,3- and 1,5-chlorine migrations. It is found that the interaction of chlorine lone pair electrons with a low-lying LUMO accelerates the Cl shift dramatically (lone pair-LUMO-mediated pericyclic reaction). The activation barriers for the 1,3-migration in chloro oxo ketene 1 (Cl(C=O)CH=C=O) and the 1,5-migration in (2-(chlorocarbonyl)vinyl)ketene 2 (Cl(C=O)CH=CHCH=C=O) are only 53 and 61 kJ mol(-)(1), respectively, compared to the 216 and 173 kJ mol(-)(1) barriers for the corresponding unassisted 1,3- and 1,5-sigmatropic shifts of Cl in 3-chloro-1-propene and 5-chloro-1,3-pentadiene. The transition structures for 1 and 2 reveal that migration of the chlorine atoms takes place in the molecular planes. The 1,5-chlorine shift in 6-chlorocyclohexa-2,4-dienone (3) has a significantly higher barrier due to a lack of appropriate orbital interaction. The related 1,3-shift in the (chlorocarbonyl)imine-alpha-chloro isocyanate system is also dramatically accelerated compared with conventional pericyclic 1,3-Cl migration.