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.     

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.     

Corannulene as a Lewis base: computational modeling of protonation and lithium cation binding.

A computational modeling of the protonation of corannulene at B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p) and of the binding of lithium cations to corannulene at B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p) has been performed. A proton attaches preferentially to one carbon atom, forming a sigma-complex. The isomer protonated at the innermost (hub) carbon has the best total energy. Protonation at the outermost (rim) carbon and at the intermediate (bridgehead rim) carbon is less favorable by ca. 2 and 14 kcal mol(-)(1), respectively. Hydrogen-bridged isomers are transition states between the sigma-complexes; the corresponding activation energies vary from 10 to 26 kcal mol(-)(1). With an empirical correction obtained from calculations on benzene, naphthalene, and azulene, the best estimate for the proton affinity of corannulene is 203 kcal mol(-)(1). The lithium cation positions itself preferentially over a ring. There is a small energetic preference for the 6-ring over the 5-ring binding (up to 2 kcal mol(-)(1)) and of the convex face over the concave face (3-5 kcal mol(-)(1)). The Li-bridged complexes are transition states between the pi-face complexes. Movement of the Li(+) cation over either face is facile, and the activation energy does not exceed 6 kcal mol(-)(1) on the convex face and 2.2 kcal mol(-)(1) on the concave face. In contrast, the transition of Li(+) around the corannulene edge involves a high activation barrier (24 kcal mol(-)(1) with respect to the lowest energy pi-face complex). An easier concave/convex transformation and vice versa is the bowl-to-bowl inversion with an activation energy of 7-12 kcal mol(-)(1). The computed binding energy of Li(+) to corannulene is 44 kcal mol(-)(1). Calculations of the (7)Li NMR chemical shifts and nuclear independent chemical shifts (NICS) have been performed to analyze the aromaticity of the corannulene rings and its changes upon protonation.     

Carbene Rearrangements Unsurpassed: Details of the C(7)H(6) Potential Energy Surface Revealed

The rearrangement of phenylcarbene (1) to 1,2,4,6-cycloheptatetraene (3) has been studied theoretically, using SCF, CASSCF, CASPT2N, DFT (B3LYP), CISD, CCSD, and CCSD(T) methods in conjunction with the 6-31G, 6-311+G, 6-311G(2d,p), cc-pVDZ, and DZd basis sets. Stationary points were characterized by vibrational frequency analyses at CASSCF(8,8)/6-31G and B3LYP/6-31G. Phenylcarbene (1) has a triplet ground state ((3)A") with a singlet-triplet separation (DeltaE(ST)) of 3-5 kcal mol(-)(1). In agreement with experiment, chiral 3 is the lowest lying structure on this part of the C(7)H(6) potential energy surface. Bicyclo[4.1.0]hepta-2,4,6-triene (2) is an intermediate in the rearrangement of 1 into 3, but it is unlikely to be observable experimentally due to a barrier height of only 1-2 kcal mol(-)(1). The enantiomers of 3 interconvert via the (1)A(2) state of cycloheptatrienylidene (4) with an activation energy of 20 kcal mol(-)(1). The "aromatic" (1)A(1) state, previously believed to be the lowest singlet state of 4, is roughly 10 kcal mol(-)(1) higher in energy than the (1)A(2) state, and, in violation of Hund's rule, (3)A(2) is also calculated to lie above (1)A(2) in energy. Thus, even if (3)A(2) were populated, it is likely to undergo rapid intersystem crossing to (1)A(2). We suggest (3)B(1)-4 is the metastable triplet observed by EPR.     

The Curtius rearrangement of cyclopropyl and cyclopropenoyl azides. A combined theoretical and experimental mechanistic study

A combined experimental and theoretical study addresses the concertedness of the thermal Curtius rearrangement. The kinetics of the Curtius rearrangements of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate and methyl 1-azidocarbonyl cyclopropane-1-carboxylate were studied by (1)H NMR spectroscopy, and there is close agreement between calculated and experimental enthalpies and entropies of activation. Density functional theory (DFT) calculations (B3LYP/6-311+G(d,p)) on these same acyl azides suggest gas phase barriers of 27.8 and 25.1 kcal/mol. By comparison, gas phase activation barriers for the rearrangement of acetyl, pivaloyl, and phenyl azides are 27.6, 27.4, and 30.0 kcal/mol, respectively. The barrier for the concerted Curtius reaction of acetyl azide at the CCSD(T)/6-311+G(d,p) level exhibited a comparable activation energy of 26.3 kcal/mol. Intrinsic reaction coordinate (IRC) analyses suggest that all of the rearrangements occur by a concerted pathway with the concomitant loss of N2. The lower activation energy for the rearrangement of methyl 1-azidocarbonyl cycloprop-2-ene-1-carboxylate relative to methyl 1-azidocarbonyl cyclopropane-1-carboxylate was attributed to a weaker bond between the carbonyl carbon and the three-membered ring in the former compound. Calculations on the rearrangement of cycloprop-2-ene-1-oyl azides do not support pi-stabilization of the transition state by the cyclopropene double bond. A comparison of reaction pathways at the CBS-QB3 level for the Curtius rearrangement versus the loss of N2 to form a nitrene intermediate provides strong evidence that the concerted Curtius rearrangement is the dominant process.     

Bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation: a theoretical validation of a bishomoaromatic radical cation intermediate

[structure: see text] A natural bond orbital analysis of the distonic bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation interprets its structure and radical character by a three-center two-electron bond between C2, C3, and C7 (a bishomoaromatic stabilization) and a singly occupied orbital on C5, n(5). Moreover, B3LYP/6-311+G(d,p) ESR parameters, which agree excellently with experiment, are interpreted in terms of spin polarization in the natural hybrids of sigma(C5-H5), and a dual hyperconjugative effect involving n(5), sigma(C1-H1a), sigma(C1-H1b), and antibonding counterparts.     

Homoaromaticity in carbene intermediates

Analyses of isodesmic reactions for singlet 7-carbenanorbornene (4S), 8-carbena-endo-tricyclo[3.2.1.0(2,4)]octane (5S), 3-carbenabicyclo[3.1.0]hexane (3S), 2-carbenanorbornene 6S, and 2-carbenabicyclooctadiene 7S at the B3LYP/6-311+G(3df, 2p)//B3LYP/6-31G level provide stabilization energies of 13.83, 13. 50, 3.00, -2.22, and -3.01 kcal/mol, respectively. The C7 carbene in 4S and the C8 carbene center in 5S are strongly bent toward the double bond and cyclopropane ring, respectively, in contrast to their related triplets, 4T and 5T and parent hydrocarbons. The geometric change for 3S compared to 3T or to parent bicyclo[3.1. 0]hexane is minimal. Comparison of the stability of 6S with 2-carbenanorbornane and the geometry of 6S with that of 6T and also with the singlet and triplet 2-carbenanorbornane suggests very modest bridging. The stabilization energy and geometry of 2-carbenabicyclooctadiene 7 resemble antihomoaromatic bicyclooctadienyl cation 9 rather than the related homoaromatic bicyclooctadienyl anion 8. The diamagnetic susceptibility exhaltations (Deltachi) of 3, 4, 5, and 7, calculated at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G level, are -0.7, 22.7, 26.0, and -10.3 cgs-ppm, respectively. The singlet-triplet energy differences, DeltaE(TS), for carbenacyclohexane, carbenacyclopentane, 3, 4, 5, 6, 7, and 2-carbenabicyclo[3.2.1]oct-3-ene at the B3LYP/6-311+G(3df, 2p)//B3LYP/6-31G level are 3.4, 10.3, 8.8, 27.1, 25.9, 12.7, -4.0, and -0.9 kcal/mol, respectively.     

A stable silicon congener of highly strained bicyclo[3.2.0]hepta-1,3,6-triene

A silicon-containing fused bicyclic compound with a highly strained bridgehead double bond, 2,3,6,7-tetra-tert-butyl-4-(tert-butyldimethylsilyl)-5-(tert-butyldimethylsiloxy)-5-silabicyclo[3.2.0]hepta-1,3,6-triene (2), was synthesized quantitatively by the reaction of 1,2-bis-tert-butyl-4,4-bis(tert-butyldimethylsilyl)-4-silatriafulvene (3) with di-tert-butylcyclopropenone (4) at 80 degrees C. An X-ray crystallographic analysis for 2 not only confirmed a bicyclic structure having a silacyclopentadiene (silole) ring fused with a silacyclobutene ring but also the remarkable deformation around the double bonds; the sum of the bond angles around the unsaturated bridgehead carbon was 333 degrees . The strain energy of a model 5-silabicyclo[3.2.0]hepta-1,3,6-triene was calculated at the MP2/6-31+G(d,p)//B3LYP/6-31+G(d) level (30.2 kcal/mol) to be comparable to that for parent bicyclo[3.2.0]hepta-1,3,6-triene (30.7 kcal/mol). Despite the high steric strain, 2 was stable enough to be kept intact for several months in the air. The high stability is ascribed to the effective steric protection of the ring system by the bulky substituents.     

o-Quinone methide as alkylating agent of nitrogen, oxygen, and sulfur nucleophiles. The role of H-bonding and solvent effects on the reactivity through a DFT computational study

The reactivity of the alkylating agent o-quinone methide (o-QM) toward NH(3), H(2)O, and H(2)S, prototypes of nitrogen-, oxygen-, and sulfur-centered nucleophiles, has been studied by quantum chemical methods in the frame of DF theory (B3LYP) in reactions modeling its reactivity in water with biological nucleophiles. The computational analysis explores the reaction of NH(3), H(2)O, and H(2)S with o-QM, both free and H-bonded to a discrete water molecule, with the aim to rationalize the specific and general effect of the solvent on o-QM reactivity. Optimizations of stationary points were done at the B3LYP level using several basis sets [6-31G(d), 6-311+G(d,p), adding d and f functions to the S atom, 6-311+G(d,p),S(2df), and AUG-cc-pVTZ]. The activation energies calculated for the addition reactions were found to be reduced by the assistance of a water molecule, which makes easier the proton-transfer process in these alkylation reactions by at least 12.9, 10.5, and 6.0 kcal mol(-1) [at the B3LYP/AUG-cc-pVTZ//B3LYP/6-311+G(d,p) level], for ammonia, water, and hydrogen sulfide, respectively. A proper comparison of an uncatalyzed with a water-catalyzed reaction mechanism has been made on the basis of activation Gibbs free energies. In gas-phase alkylation of ammonia and water by o-QM, reactions assisted by an additional water molecule H-bonded to o-QM (water-catalyzed mechanism) are favored over their uncatalyzed counterparts by 5.6 and 4.0 kcal mol(-1) [at the B3LYP/6-311+G(d,p) level], respectively. In contrast, the hydrogen sulfide alkylation reaction in the gas phase shows a slight preference for a direct alkylation without water assistance, even though the free energy difference (DeltaDeltaG(#)) between the two reaction mechanisms is very small (by 1.0 kcal mol(-1) at the B3LYP/6-311+G(d,p),S(2df) level of theory). The bulk solvent effect, evaluated by the C-PCM model, significantly modifies the relative importance of the uncatalyzed and water-assisted alkylation mechanism by o-QM in comparison to the case in the gas phase. Unexpectedly, the uncatalyzed mechanism becomes highly favored over the catalyzed one in the alkylation reaction of ammonia (by 7.0 kcal mol(-1)) and hydrogen sulfide (by 4.0 kcal mol(-1)). In contrast, activation induced by water complexation still plays an important role in the o-QM hydration reaction in water as solvent.     

Gaseous reaction mechanism of C2F radical with water

The kinetic properties of the carbon-fluorine radicals are little understood except those of CFn (n =1-3). In this article, a detailed mechanistic study was reported on the gas-phase reaction between the simplest pi-bonded C2F radical and water as the first attempt to understand the chemical reactivity of the C2F radical. Various reaction channels are considered. The most kinetically competitive channel is the quasi-direct hydrogen-abstraction route forming P5 HCCF + OH. At the CCSD(T)/6-311+G(2d,2p)//B3LYP/6-311G(d,p)+ZPVE, CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p)+ZPVE and Gaussian-3//B3LYP/6-31G(d) levels, the overall H-abstraction barriers (4.5, 4.7, and 4.2 kcal/mol) for the C2F + H2O reaction are comparable to the corresponding values (5.5, 3.7, and 5.7 kcal/mol) for the analogous C2H + H2O reaction. This suggests that C2F is a reactive radical like the extensively studied C2H, in contrast to the situation of the CF and CF2 radicals that have much lower reactivity than the corresponding hydrocarbon species. Thus, the C2F radical is expected to play an important role in the combustion processes of the carbon-fluorine chemistry. Furthermore, addition of a second H2O can catalyze the reaction with the H-abstraction barrier significantly reduced to a marginally zero value (0.5 kcal/mol). This is also indicative of the potential relevance of the title reactions in the low-temperature atmospheric chemistry.     

过充电保护添加剂1,4-二甲氧基苯的反应机理

采用密度泛函理论(B3LYP/6-311+G(d,p))和MP2/6-311+G(d,p)方法,研究锂离子电池过充电保护添加剂1,4-二甲氧基苯(p-DMOB)的作用机理.计算结果表明,在过充时,p-DMOB优先于溶剂分子(乙基甲基碳酸酯、二甲基碳酸酯、碳酸乙酯)发生氧化反应.用B3LYP和MP2计算所得的p-DMOB理论氧化电位接近,分别为4.12和4.05V(vsLi/Li+).p-DMOB氧化时首先失去一个电子,生成p-DMOB+·正离子自由基,用B3LYP和MP2方法计算所得的相应能量变化分别为701.24和728.27kJ·mol-1.失去电子后苯环的共轭性受到破坏,随后p-DMOB+·苯环上的C―H键发生断裂,失去H+并形成p-DMOB·自由基.用B3LYP和MP2方法计算所得的相应能量变化分别为1349.78和1810.99kJ·mol-1.p-DMOB·自由基很不稳定,会在电极表面发生聚合反应形成聚合物膜,用B3LYP和MP2方法计算所得的相应能量变化分别为-553.37和-1331.20kJ·mol-1.     

Singlet hydrocarbon carbenes with high barriers toward isomerization: a computational investigation

A prerequisite for a stable singlet hydrocarbon carbene is the existence of high barriers toward isomerization. Four derivatives of cyclopentylidene (1-4) with rigid and varying carbon cages are examined computationally at the B3LYP/6-311+G(d,p) level of theory. Singlet ground states are predicted for carbenes 1-4, with DeltaE(ST)'s = 7-22 kcal/mol. The rearrangement paths considered are 1,3-hydrogen shift, 1,2-carbon shift and beta-CC bond-cleavage. Carbenes 3 and 4 lie in relatively shallow potential-energy wells (around 4 and 6 kcal/mol, respectively) and are expected to rearrange via 1,3-hydrogen shifts to cyclopropane derivatives. For 1 and 2, the lowest energy rearrangement path is beta-CC bond-cleavage requiring about 12 and 20 kcal/mol, respectively, placing 2 in the deepest potential-energy well among the four carbenes.     

Bicyclo[3.2.1]octa-3,6-dien-2-yl cation: a bishomoantiaromate

Antiaromatic compounds with a closed loop of 4n p-electrons are relatively unstable and often difficult to study. We report in this article the synthesis of alcohols 2-(4'-fluorophenyl)bicyclo[3.2.1]octan-2-ol 11, 2-(4'-fluorophenyl)bicyclo[3.2.1]oct-3-en-2-ol 12, and 2-(4'-fluorophenyl)bicyclo[3.2.1]octa-3,6-dien-2-ol 13 and their transformations into corresponding carbocations 14-16, respectively, in a superacidic medium (FSO3H/SO2ClF) at -120 degrees C. Cations 14-16 are characterized by NMR analysis (1H, 13C, 19F), and 15 and 16 are further characterized by quenching in NaOCH3/H3COH at -120 degrees C. The relative stabilities of 14-16 are determined experimentally by 19F NMR spectroscopy. Cation 16 is found to be experimentally less stable than cation 15 by 3.7 kcal/mol. DFT calculations (structure and energy: B3LYP/6-31G(d); NMR: B3LYP/6-311+G(2d,p)) are performed for alcohols 11-13 and bicyclo[3.2.1]octyl cations 6, 7, 9, 14-16, 26, 28, and 30. In the case of 11-16, data from DFT calculations is in good agreement with experimental data. Because 6,7-dimethylenebicyclo[3.2.1]oct-3-en-2-yl cation 26 is more stable than cation 7 by 1.69 kcal/mol, the inductive effect of sp(2)-hybridized carbon atoms C6 and C7 in carbocations 6 and 16 cannot be the reason for the destabilization of 6 relative to 7 and 16 relative to 15. Destabilization of 6 relative to 7 and 16 relative to 15 and the calculated NICS of 6 (+4.17 ppm) and 16(+3.3 ppm) document that 6 and 16 are bishomoantiaromates.     

A theoretical study of the effect of a tetraalkylammonium counterion on the hydrogen bond strength in Z-hydrogen maleate.

High-level ab initio calculations (B3LYP/6-31+G and QCISD(T)/6-311+G**) were carried out to resolve the disagreement between recent experimental and computational estimates of the relative strength of the intramolecular hydrogen bond in Z-hydrogen maleate anion with respect to the normal hydrogen bond in maleic acid. The computational estimates for the strength of the intramolecular hydrogen bond in the gas-phase maleate anion are in a range of 14-28 kcal/mol depending on the choice of the reference structure. Computational data suggest that the electrostatic influence of a counterion such as a tetraalkylammonium cation can considerably weaken the hydrogen bonding interaction (by 1.5-2 times) in the complexed hydrogen maleate anion relative to that in the naked anion. The estimated internal H-bonding energies for a series of Z-maleate/R4N+ salts (R = CH3, C2H5, CH3CH2CH2CH2) range from 8 to 13 kcal/mol. The calculated energy differences between the E- and Z-hydrogen maleates complexed to Me4N+, Et4N+, and Bu4N+ cation are 4.9 (B3LYP/6-31+G(d,p)) and 5.7 and 5.8 kcal/mol (B3LYP/6-31G(d)). It is also demonstrated that the sodium cation exerts a similar electrostatic influence on the hydrogen bond strength in bifluoride anion (FHF-). The present study shows that while low-barrier short hydrogen bonds can exist in the gas phase (the barrier for the hydrogen transfer in maleate anion is only 0.2 kcal/mol at the QCISD(T)/6-311+G//QCISD/6-31+G level), whether they can also be strong in condensed media or not depends on how their interactions with their immediate environment affect their strength.     

Transition state structure and energetics of the N(2)O + X (X = Cl,Br) reactions

The structural and vibrational properties of the transition state of the N(2)O + X (X = Cl,Br) reactions have been characterized by ab initio methods using density functional theory. We have employed Becke's hybrid functional (B3LYP), and transition state optimizations were performed with 6-31G(d), 6-311G(2d,2p), 6-311+G(3d,2p), and 6-311+G(3df,2p) basis sets. For the chlorine atom reaction the coupled-cluster method (CCSD(T)) with 6-31G(d) basis set was also used. All calculations resulted in transition state structures with a planar cis arrangement of atoms for both reactions. The geometrical parameters of transition states at B3LYP are very similar, and the reaction coordinates involve mainly the breaking of the N-O bond. At CCSD(T)/6-31G(d) level a contribution of the O-Cl forming bond is also observed in the reaction coordinate. In addition, several highly accurate ab initio composite methods of Gaussian-n (G1, G2, G3), their variations (G2(MP2), G3//B3LYP), and complete basis set (CBS-Q, CBS-Q//B3LYP) series of models were applied to compute reaction energetics. All model chemistries predict exothermic reactions. The G3 and G2 methods result in the smallest deviations from experiment, 1.8 and 0 kcal mol(-1), for the enthalpies of reaction for N(2)O reaction with chlorine and bromine, respectively. The G3//B3LYP and G1 methods perform best among the composite methods in predicting energies of the transition state, with a deviation of 1.9 and 3.0 kcal mol(-1), respectively, in the activation energies for the above processes. However, the B3LYP/6-311+G(3df,2p) method gives smaller deviations of 0.4 and -1.0 kcal mol(-1), respectively. The performance of the methodologies applied in predicting transition state energies was analyzed.     

Can the ebselen derivatives catalyze the isomerization of peroxynitrite to nitrate?

The reaction of ebselen and its derivatives (1-7) with peroxynitrite anion (ONOO(-); PN) has been studied in gas phase and in aqueous, dichloromethane, benzene, and cyclohexane solutions using B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) and PCM-B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) approaches, respectively. It was shown that the reaction of 2 (R=H) with PN proceeds via 2 + PN --> 2-PN --> 2-TS1 (O-O activation) --> 2-O(NO(2)(-)()) --> 2-SeO + NO(2)(-) pathway with a rate-determining barrier of 25.3 (14.8) kcal/mol at the NO(2)(-) dissociation step (numbers presented without parentheses are enthalpies, and those in parentheses are Gibbs free energies). The NO(3)(-) formation process, starting from the complex 2-O(NO(2)(-)()), requires by (7.9) kcal/mol more energy than the NO(2)(-) dissociation process and is unlikely to compete with the latter. Thus, in the gas phase, the peroxynitrite --> nitrate isomerization catalyzed by complex 2 is unlikely to occur. It is shown that the NO(3)(-) formation process is slightly more favorably than the NO(2)(-) dissociation process for complex 4, with a strongest electron-withdrawing ligand R=CF(3). Therefore, complex 4 (as well as complex 6 with R=OH) is predicted to be a good catalyst for peroxynitrite <--> nitrite isomerization in the gas phase. Solvent effects (a) change the rate-determining step of the reaction 2 + PN from NO(2)(-) dissociation in the gas phase to O-O activation, which occurs with barriers of (13.9), (8.4), (8.4), and (8.2) kcal/mol in water, dichloromethane, benzene, and cyclohexane, respectively, and (b) significantly reduce the NO(2)(-) dissociation energy, while only slightly destabilizing the NO(3)(-) formation barrier, and make the peroxynitrite <--> nitrate isomerization process practically impossible, even for complex 4.     

Mechanism of HCS + O2 reaction: hydrogen- or oxygen-transfer?

In spite of the potential importance of the HCS radical in both combustion and interstellar processes, its chemical reactivity has not been tackled previously. In the present paper, the oxidation reaction of the HCS radical is theoretically investigated for the first time at the CCSD(T)/6-311++G(3df,2p)//BH&HLYP/6-311++G(d,p)+ZPVE and Gaussian-3//B3LYP/6-31G(d) levels. It is shown that the most feasible pathway is the O2 addition to the HCS radical forming the intermediate SC(H)OO which can undergo a subsequent O-extrusion leading to SC(H)O + 3O. This features an indirect O-transfer mechanism with the overall barrier of 4.4 and 3.5 kcal mol(-1), respectively, at the two levels. However, formation of the H-transfer product CS + HO2 is kinetically much less feasible, i.e., the direct mechanism has barriers of 14.3 and 8.7 kcal mol(-1), whereas the indirect mechanism has barriers of 12.6 and 10.7 kcal mol(-1), respectively. This result is in sharp contrast to the analogous HCO + O2 reaction, where the direct (with a barrier of 2.98 kcal mol(-1)) and indirect (2.26 kcal mol(-1)) H-transfer processes are highly competitive over the indirect O-transfer process (the least endothermicity is 19.9 kcal mol(-1)). The possible explanations and implications of the present results are provided.     

A computational study of conformational interconversions in 1,4-dithiacyclohexane (1,4-dithiane)

Ab initio molecular orbital theory with the 6-31G(d), 6-31G(d,p), 6-31+G(d), 6-31+G(d,p), 6-31+G(2d,p), 6-311G(d), 6-311G(d,p), and 6-311+G(2d,p) basis sets and density functional theory (BLYP, B3LYP, B3P86, B3PW91) have been used to locate transition states involved in the conformational interconversions of 1,4-dithiacyclohexane (1,4-dithiane) and to calculate the geometry optimized structures, relative energies, enthalpies, entropies, and free energies of the chair and twist conformers. In the chair and 1,4-twist conformers the C-Hax and C-Heq bond lengths are equal at each carbon, which suggest an absence of stereoelectronic hyperconjugative interactions involving carbon-hydrogen bonds. The 1,4-boat transition state structure was 9.53 to 10.5 kcal/mol higher in energy than the chair conformer and 4.75 to 5.82 kcal/mol higher in energy than the 1,4-twist conformer. Intrinsic reaction coordinate (IRC) calculations showed that the 1,4-boat transition state structure was the energy maximum in the interconversion of the enantiomers of the 1,4-twist conformer. The energy difference between the chair conformer and the 1,4-twist conformer was 4.85 kcal/mol and the chair-1,4-twist free energy difference (deltaG degrees (c-t)) was 4.93 kcal/mol at 298.15 K. Intrinsic reaction coordinate (IRC) calculations connected the transition state between the chair conformer and the 1,4-twist conformer. This transition state is 11.7 kcal/mol higher in energy than the chair conformer. The effects of basis sets on the 1,4-dithiane calculations and the relative energies of saturated and unsaturated six-membered dithianes and dioxanes are also discussed.     

Cheletropic decomposition of cyclic nitrosoamines revisited: the nature of the transition states and a critical role of the ring strain

The cheletropic decompositions of 1-nitrosoaziridine (1), 1-nitroso-Delta(3)-pyrroline (2), 7-nitroso-7-azabicyclo[2.2. 1]hepta-2,5-diene (3), and 6-nitroso-6-azabicyclo[2.1.1]hexa-4-ene (4) have been studied theoretically using high level ab initio computations. Activation parameters of the decomposition of nitrosoaziridine 1 were obtained experimentally in heptane (DeltaH()(298) = 18.6 kcal mol(-)(1), DeltaS()(298) = -7.6 cal mol(-)(1) K(-)(1)) and methanol (20.3 kcal mol(-)(1), 0.3 cal mol(-)(1) K(-)(1)). Among employed theoretical methods (B3LYP, MP2, CCD, CCSD(T)//CCD), the B3LYP method in conjunction with 6-31+G, 6-311+G, and 6-311++G(3df,2pd) basis sets gives the best agreement with experimental data. It was found that typical N-nitrosoheterocycles 2-4 which have high N-N bond rotation barriers (>16 kcal mol(-)(1)) extrude nitrous oxide via a highly asynchronous transition state with a planar ring nitrogen atom. Nitrosoaziridine 1, with a low rotation barrier (<9 kcal mol(-)(1)) represents a special case. This compound can eliminate N(2)O via a low energy linear synperiplanar transition state (DeltaH()(298) = 20.6 kcal mol(-)(1), DeltaS()(298) = 2.5 cal mol(-)(1) K(-)(1)). Two higher energy transition states are also available. The B3LYP activation barriers of the cheletropic fragmentation of nitrosoheterocycles 2-4 decrease in the series: 2 (58 kcal mol(-)(1)) > 3 (18 kcal mol(-)(1)) > 4 (12) kcal mol(-)(1). The relative strain energies increase in the same order: 2 (0 kcal mol(-)(1)) < 3 (39 kcal mol(-)(1)) < 4 (52 kcal mol(-)(1)). Comparison of the relative energies of 2-4 and their transition states on a common scale where the energy of nitrosopyrroline 2 is assumed as reference indicates that the thermal stability of the cyclic nitrosoamines toward cheletropic decomposition is almost entirely determined by the ring strain.     

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.