From sulfoxide precursors to model oligomers of conducting polymers

The gas-phase internal elimination (E(i)) reaction of the sulfoxide (-SO-CH(3)) precursors of ethylene and model oligomers of PPV and PITN has been investigated by means of Hartree-Fock, M?ller-Plesset (second and fourth order), and Density Functional Theory (B3LYP, MPW1K) calculations. Considerable differences between the obtained ground state and transition state geometries and the calculated activation energies are observed from one approach to the other, justifying first a careful calibration against the results of a benchmark CCSD(T) study of the E(i) reaction leading to ethylene. In comparison with the CCSD(T) results, as well as with available experimental data, DFT calculations along with the MPW1K functional are found to be a very appropriate choice for describing the E(i) pathway. The leading conformations of the precursors, the relevant transition state structures, and the energy barriers encountered along the lowest energy path to unsubstituted, alpha and beta chloro-, methoxy-, and cyano-substituted ethylene, styrene, stilbene in its cis and trans forms, and at last trans-biisothianaphthene have therefore been identified and characterized in detail employing DFT (MPW1K). Depending on the substituents attached to the C(alpha) and C(beta) atoms, different reaction mechanisms are observed.     

Theoretical study of the internal conversion of sulfoxide precursors of poly-isothianaphthene and related polymers

In the present contribution, we theoretically investigate the suitability of the sulfoxide route for the synthesis of conjugated polymers of relevance for the fabrication of low-band gap materials with improved characteristics. The study focuses specifically on the internal elimination (E(i)) reactions of sulfoxide precursors of model oligomers of trans- and cis-poly-isothianaphtene (PITN), trans-poly-isothianaphtene vinylene (PITNV), and trans-poly-(ethylene dioxythiophene vinylene) (PEDOTV). These reactions have been characterized in detail by means of Density Functional Theory, along with the MPW1K functional (Modified Perdew-Wang 1-parameter model for kinetics).     

Theoretical study of the internal elimination reactions of xanthate precursors

The gas‐phase internal elimination (E_i) reaction of ethyl xanthate (CH₃‐CH₂‐S‐CS‐O‐CH₃) has been investigated by means of Hartree–Fock, second‐order Møller–Plesset, and density functional theory (DFT) using the Becke three‐parameter Lee–Yang–Parr (B3LYP) functional and the modified Perdew–Wang one‐parameter model for kinetics (MPW1K). Considerable differences between the ground‐ and transition‐state geometries and the calculated activation energies are observed from one approach to the other, which justifies first a careful calibration of the methods against the results of benchmark CCSD(T) calculations. Compared with these, DFT calculations along with the MPW1K functional are found to be an appropriate choice for describing the E_i reaction of xanthate precursors. The precursor conformation and the transition states involved in the internal conversion of xanthate precursors of cyano derivatives of ethylene, and of cis‐ and trans‐stilbene, are then characterized in detail by means of this functional. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 2023–2031, 2003     

MPW1K Performs Much Better than B3LYP in DFT Calculations on Reactions that Proceed by Proton-Coupled Electron Transfer (PCET)

DFT calculations have been performed with the B3LYP and MPW1K functional on the hydrogen atom abstraction reactions of ethenoxyl with ethenol and of phenoxyl with both phenol and alpha-naphthol. Comparison with the results of G3 calculations shows that B3LYP seriously underestimates the barrier heights for the reaction of ethenoxyl with ethenol by both proton-coupled electron transfer (PCET) and hydrogen atom transfer (HAT) mechanisms. The MPW1K functional also underestimates the barrier heights, but by much less than B3LYP. Similarly, comparison with the results of experiments on the reaction of phenoxyl radical with alpha-naphthol indicates that the barrier height for the preferred PCET mechanism is calculated more accurately by MPW1K than by B3LYP. These findings indicate that the MPW1K functional is much better suited than B3LYP for calculations on hydrogen abstraction reactions by both HAT and PCET mechanisms.     

*H atom and *OH radical reactions with 5-methylcytosine

The reactions between either a hydrogen atom or a hydroxyl radical and 5-methylcytosine (5-MeCyt) are studied by using the hybrid kinetic energy meta-GGA functional MPW1B95. *H atom and *OH radical addition to positions C5 and C6 of 5-MeCyt, or *OH radical induced H-abstraction from the C5 methyl group, are explored. All systems are optimized in bulk solvent. The data presented show that the barriers to reaction are very low: ca. 7 kcal/mol for the *H atom additions and 1 kcal/mol for the reactions involving the *OH radical. Thermodynamically, the two C6 radical adducts and the *H-abstraction product are the most stable ones. The proton hyperfine coupling constants (HFCC), computed at the IEFPCM/MPW1B95/6-311++G(2d,2p) level, agree well with B3LYP results and available experimental and theoretical data on related thymine and cytosine radicals.     

Quantum chemical study of hydrogen abstraction reactions of the ethynyl radical with hydrogen compounds (C2H+HX)

We have theoretically investigated the hydrogen abstraction reactions of ethynyl radical with simple hydrogen compounds, C₂H+HX, using quantum chemical computations. Computations have been performed using the density functional theory with the recently proposed MPW1K functional and the 6-311++G(3df,2p) basis set. An analysis of the resulting energy barriers for hydrogen abstraction reactions has been carried out using the bond dissociation energy of the breaking X–H bond and DFT-based reactivity parameters to rationalize the reaction behavior.     

Testing the performance of density functionals for the calculation of energetic properties of complex-forming radical-molecule reactions

The performance of some popular and some more recent density functional methods for the calculation of energies of stationary points on the potential surfaces of radical-molecule reactions was examined. The functionals studied are B3-LYP, BH&H, BH&H-LYP, MPW1K, MPWB1K, TPSS, TPSSh, BB1K, M05 and M05-2X, in conjunction with nine different AO basis sets. The reaction energies, barrier heights and the relative energies of the pre-and post-reaction complexes were compared with those obtained at the CCSD(T)/CBS limit for the reactions of OH radicals with HOOH and CH₃OOH. Very poor barrier heights are provided by the B3-LYP, TPSS and TPSSh functionals. The best overall performance was obtained with the BB1K, MPW1K and MPWB1K functionals. In these reactions all of the studied functionals provide converged results only if they are used with large basis sets like aug-cc-pVTZ and def2-TZVP. The data show that before relying on a functional for a specific reaction, it is desirable to make some test calculations on the performance. The same functional can predict some relative energies very well and some others very poorly even in systems including chemically similar reactants.     

Ab initio thermochemistry and kinetics for carbon-centered radical addition and beta-scission reactions

A quantitative comparison of ab initio calculated rate coefficients using five computational methods and five different approaches of treating hindered internal rotation and tunneling with experimental values of rate coefficients for nine carbon-centered radical additions/beta scissions at 300, 600, and 1000 K is performed. The high-accuracy compound methods, CBS-QB3 and G3B3, and the density functionals, MPW1PW91, BB1K, and BMK, have been evaluated using the following approaches: (i) the harmonic oscillator approximation; (ii) the hindered internal rotor approximation for the internal rotation about the forming/breaking bond in the transition state and product; and the hindered internal rotation approximation combined with (iii) Wigner, (iv) Skodje and Truhlar, and (v) Eckart zero-curvature tunneling corrections. The density functional theory (DFT) based values for beta-scission rate coefficients deviate significantly from the experimental ones at 300 K, and the DFT methods do not accurately predict the equilibrium coefficient. The hindered rotor approximation offers a significant improvement in the agreement with experimental rate coefficients as compared to the harmonic oscillator treatment, especially at higher temperatures. Tunneling correction factors are smaller than 1.40 at 300 K and 1.03 at 1000 K. For both the CBS-QB3 method, including the hindered rotor treatment but excluding tunneling corrections, and the G3B3 method, including hindered rotor and Eckart tunneling corrections, a mean factor of deviation with experimentally observed values of 3 is found.     

Theoretical study of the competitive decomposition and isomerization of 1-hexyl radical

The ground-state potential energy surface of the 1-hexyl system, including the main decomposition and isomerization processes, has been calculated with the MPW1K, BB1K, MPWB1K, MPW1B95, BMK, M05-2X and CBS-QB3 methods. On the basis of these data, thermal rate coefficients of different reaction channels and branching ratios were then calculated using the master equation formulation at 250–2,500 K. The results clearly point out that the 1,5 H atom transfer reaction of 1-hexyl radical with exothermicity proceeds through the lowest reaction barrier, whereas the decomposition processes are thermodynamically unfavorable with large endothermicity. The temperature effect is important on the relative importance of different reactions in the 1-hexyl system. In the low-temperature range of 250–900 K, isomerization reactions, especially 1,5 H atom transfer reaction of 1-hexyl radical, are dominating and responsible for over 82.17% of all the reactions, due to their smaller reaction barriers than those of the decomposition reactions. Furthermore, an equilibrium process involving the isomeric forms of the hexyl radicals appearing at relative low temperature was validated theoretically. However, isomerization and decomposition processes are kinetically competitive and simultaneously important under normal pyrolysis conditions.     

Mechanism and kinetics of the atmospheric degradation of perfluoropolymethylisopropyl ether by OH radical: a theoretical study

In the present work, the mechanism and kinetics of the reaction of perfluoropolymethylisopropyl ether (PFPMIE) with OH radical are studied. The reaction between PFPMIE and OH radical is initiated through breaking of C–C or C–O bond of PFPMIE. These reactions lead to the formation of COF₂ molecules and alkyl radical. The pathways corresponding to the reaction between PFPMIE and OH radical have been modelled using density functional theory methods M06-2X and MPW1K with 6-31G(d,p) basis set. It is found that the C–C bond breaking reaction is most favourable than the C–O bond breaking reaction. The subsequent reactions of the alkyl radicals, formed from the C–C bond breaking reactions, are studied in detail. The rate constant for the initial oxidation reactions is calculated using canonical variational transition state theory with small curvature tunnelling corrections over the temperature range of 278–350 K. From the calculated reaction, potential energy surface and rate constant, the lifetime and global warming potential of PFPMIE are studied.     

Influence of hydrogen bonding on hydrogen-atom abstraction reactions of dehydropyridinium cations in the gas phase

The reactions of several substituted, positively charged dehydropyridinium cations with cyclohexane, methanol, and tetrahydrofuran have been examined in a Fourier-transform ion cyclotron resonance mass spectrometer. All of the charged monoradicals react with the neutral reagents exclusively via hydrogen atom abstraction. For cyclohexane, there is a good correlation between the reaction efficiencies and the calculated electron affinities at the radical sites; that is, the greater the electron affinity of the charged monoradical at the radical site, the faster the reaction. The reaction efficiencies with methanol and tetrahydrofuran, however, do not correlate with the calculated electron affinities. Density functional theory (DFT) calculations indicate that for these reagents a stabilizing hydrogen bonding interaction exists in the hydrogen atom abstraction transition states for some of the charged monoradicals but not for others. At both the MPW1K and G3MP2B3 levels of theory, there is a good correlation between the calculated activation enthalpies and the observed reaction efficiencies, although the G3MP2B3 method provides a slightly better correlation than the MPW1K method. The extent of enhancement in the reaction efficiencies caused by the hydrogen bonding interactions parallels the calculated hydrogen bond lengths in the transition states.     

Theoretical study of the addition and abstraction reactions of hydroxyl radical with uracil

The addition as well as abstraction reactions of hydroxyl radical (OH) with the nucleic acid base, uracil (U), in the gas phase has been explored at the B3LYP/6-31+G(d,p) level of density functional theory (DFT). The energy barrier of the OH addition to both the C5 and C6 positions of the uracil is less than 1 kcal/mol while the hydrogen abstractions (H-abstractions) from either the N1 or the N3 positions are ∼9.5 kcal/mol. Further the energetics of these reactions are assessed by applying the effect of aqueous medium through the polarizable continuum model (PCM). Both the gas and the solution phase data established that the thermodynamic and kinetic factors are more favorable for the OH addition to either C5 or C6 positions of the uracil than the H-abstraction reactions. Moreover, calculations at the MPW1K/6-31+G(d,p), CCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p) and CCSD(T)/6-31+G(d,p)//MPW1K/6-31+G(d,p) levels of theoretical methods qualitatively supported the B3LYP/6-31+G(d,p) results.     

Palladium-catalyzed desulfitative Mizoroki-Heck couplings of sulfonyl chlorides with mono- and disubstituted olefins: rhodium-catalyzed desulfitative heck-type reactions under phosphine- and base-free conditions

New conditions have been found for the desulfitative Mizoroki-Heck arylation and trifluoromethylation of mono- and disubustituted olefins with arenesulfonyl and trifluoromethanesulfonyl chlorides. Thus (E)-1,2-disubstituted alkenes with high stereoselectivity and 1,1,2-disubstituted alkenes with 12:1 to 21:1 E/Z steroselectivity can be obtained. Herrmann's palladacycle at 0.1 mol % is sufficient to catalyze these reactions, for which electron-rich or electron-poor sulfonyl chlorides and alkenes are suitable. If phosphine- and base-free conditions are required, 1 mol % [RhCl(C(2)H(4))(2)] catalyzes the desulfitative cross-coupling reactions. Contrary to results reported for [RuCl(2)(PPh(3))(2)]-catalyzed coupling reactions with sulfonyl chlorides, the palladium and rhodium desulfitative Mizoroki-Heck coupling reactions are not inhibited by radical scavenging agents. Possible sulfones arising from the sulfonylation of alkenes at 60 degrees C are not desulfitated at higher temperatures in the presence of the Pd or Rh catalysts.     

Combined valence bond-molecular mechanics potential-energy surface and direct dynamics study of rate constants and kinetic isotope effects for the H + C2H6 reaction

This article presents a multifaceted study of the reaction H+C(2)H(6)-->H(2)+C(2)H(5) and three of its deuterium-substituted isotopologs. First we present high-level electronic structure calculations by the W1, G3SX, MCG3-MPWB, CBS-APNO, and MC-QCISD/3 methods that lead to a best estimate of the barrier height of 11.8+/-0.5 kcal/mol. Then we obtain a specific reaction parameter for the MPW density functional in order that it reproduces the best estimate of the barrier height; this yields the MPW54 functional. The MPW54 functional, as well as the MPW60 functional that was previously parametrized for the H+CH(4) reaction, is used with canonical variational theory with small-curvature tunneling to calculate the rate constants for all four ethane reactions from 200 to 2000 K. The final MPW54 calculations are based on curvilinear-coordinate generalized-normal-mode analysis along the reaction path, and they include scaled frequencies and an anharmonic C-C bond torsion. They agree with experiment within 31% for 467-826 K except for a 38% deviation at 748 K; the results for the isotopologs are predictions since these rate constants have never been measured. The kinetic isotope effects (KIEs) are analyzed to reveal the contributions from subsets of vibrational partition functions and from tunneling, which conspire to yield a nonmonotonic temperature dependence for one of the KIEs. The stationary points and reaction-path potential of the MPW54 potential-energy surface are then used to parametrize a new kind of analytical potential-energy surface that combines a semiempirical valence bond formalism for the reactive part of the molecule with a standard molecular mechanics force field for the rest; this may be considered to be either an extension of molecular mechanics to treat a reactive potential-energy surface or a new kind of combined quantum-mechanical/molecular mechanical (QM/MM) method in which the QM part is semiempirical valence bond theory; that is, the new potential-energy surface is a combined valence bond molecular mechanics (CVBMM) surface. Rate constants calculated with the CVBMM surface agree with the MPW54 rate constants within 12% for 534-2000 K and within 23% for 200-491 K. The full CVBMM potential-energy surface is now available for use in variety of dynamics calculations, and it provides a prototype for developing CVBMM potential-energy surfaces for other reactions.     

CH3S自由基H迁移异构化及脱H2反应的直接动力学研究

采用密度泛函方法(MPW1PW91)在6-311G(d,p)基组水平上研究了CH₃S自由基H迁移反应CH₃S→CH₂SH (R1), 脱H₂反应CH₃S→HCS＋H₂ (R2)以及脱H₂产物HCS异构化反应HCS→CSH (R3)的微观动力学机理. 在QCISD(t)/6- 311＋＋G(d,p)//MPW1PW91/6-311G(d,p)＋ZPE水平上进行了单点能校正. 利用经典过渡态理论(TST)与变分过渡态理论(CVT)分别计算了各反应在200～2000 K温度区间内的速率常数k^TST和k^CVT, 同时获得了经小曲率隧道效应模型(SCT)校正后的速率常数k^CVT/SCT. 结果表明, 反应 R1, R2 和R3的势垒△E^≠分别为160.69, 266.61和241.63 kJ/mol, R1为反应的主通道. 低温下CH₃S比CH₂SH稳定, 高温时CH₂SH比CH₃S更稳定. 另外, 速率常数计算结果显示, 量子力学隧道效应在低温段对速率常数的计算有显著影响, 而变分效应在计算温度段内对速率常数的影响可以忽略.     

Radical and Heck cyclizations of diastereomeric o-haloanilide atropisomers

The outcomes of radical cyclizations and Heck reactions of N-(cyclohex-2-enyl)-N-(2-iodophenyl)acetamides depend critically on the configurations of the chiral axis and the stereocenter. In substrates without an ortho-methyl group, the diastereomeric precursors interconvert slowly at ambient temperatures. Cyclization of enriched mixtures of diastereomers provided similar yields of acetyl tetrahydrocarbazoles or dihydrocarbazoles, suggesting that interconversion of the radical or organometallic intermediates also occurs. Diastereomers of N-(cyclohex-2-enyl)-N-(2-iodo-4,6-dimethylphenyl)acetamides with an additional ortho-methyl group did not interconvert at ambient temperatures and were readily resolved. In radical cyclizations, syn diastereomers were prone to cyclize, while anti isomers were not. Strikingly, Heck reactions gave the opposite result; anti isomers were prone to cyclization and syn isomers were not. Heck reactions of allylic acetates occur with β-hydride elimination when acetate is trans to palladium and with β-acetoxy elimination when acetate is cis. This is surprising because prior studies have suggested that a trans relationship of palladium and acetoxy is essential for acetate elimination. Analyses of the results provide insights into mechanisms for radical cyclization and for insertion and elimination in the Heck reaction.     

Benchmark database of barrier heights for heavy atom transfer, nucleophilic substitution, association, and unimolecular reactions and its use to test theoretical methods

A benchmark database of forward and reverse barrier heights for 19 non-hydrogen-transfer reactions has been developed by using Weizmann 1 calculations, and 29 DFT methods and 6 ab initio wave-function theory (WFT) methods have been tested against the new database as well as against an older database for hydrogen atom transfer reactions. Among the tested hybrid DFT methods without kinetic energy density, MPW1K is the most accurate model for calculations of barrier heights. Among the tested hybrid meta DFT methods, BB1K and MPWB1K are the two most accurate models for the calculations of barrier heights. Overall, the results show that BB1K and MPWB1K are the two best DFT methods for calculating barrier heights, followed in order by MPW1K, MPWKCIS1K, B1B95, MPW1B95, BH and HLYP, B97-2, mPW1PW91, and B98. The popular B3LYP method has a mean unsigned error four times larger than that of BB1K. Of the methods tested, QCISD(T) is the best ab initio WFT method for barrier height calculations, and QCISD is second best, but QCISD is outperformed by the BB1K, MPWB1K, MPWKCIS1K, and MPW1K methods.     

Pressure dependence and branching ratios in the decomposition of 1-pentyl radicals: shock tube experiments and master equation modeling

The decomposition and intramolecular H-transfer isomerization reactions of the 1-pentyl radical have been studied at temperatures of 880 to 1055 K and pressures of 80 to 680 kPa using the single pulse shock tube technique and additionally investigated with quantum chemical methods. The 1-pentyl radical was generated by shock heating dilute mixtures of 1-iodopentane and the stable products of its decomposition have been observed by postshock gas chromatographic analysis. Ethene and propene are the main olefin products and account for >97% of the carbon balance from 1-pentyl. Also produced are very small amounts of (E)-2-pentene, (Z)-2-pentene, and 1-butene. The ethene/propene product ratio is pressure dependent and varies from about 3 to 5 over the range of temperatures and pressures studied. Formation of ethene and propene can be related to the concentrations of 1-pentyl and 2-pentyl radicals in the system and the relative rates of five-center intramolecular H-transfer reactions and β C-C bond scissions. The 3-pentyl radical, formed via a four-center intramolecular H transfer, leads to 1-butene and plays only a very minor role in the system. The observed (E/Z)-2-pentenes can arise from a small amount of beta C-H bond scission in the 2-pentyl radical. The current experimental and computational results are considered in conjunction with relevant literature data from lower temperatures to develop a consistent kinetics model that reproduces the observed branching ratios and pressure effects. The present experimental results provide the first available data on the pressure dependence of the olefin product branching ratio for alkyl radical decomposition at high temperatures and require a value of <ΔE(down)(1000 K)> = (675 ± 100) cm(-1) for the average energy transferred in deactivating collisions in an argon bath gas when an exponential-down model is employed. High pressure rate expressions for the relevant H-transfer reactions and β bond scissions are derived and a Rice Ramsberger Kassel Marcus/Master Equation (RRKM/ME) analysis has been performed and used to extrapolate the data to temperatures between 700 and 1900 K and pressures of 10 to 1 × 10(5) kPa.     

Activation energies of pericyclic reactions: performance of DFT, MP2, and CBS-QB3 methods for the prediction of activation barriers and reaction energetics of 1,3-dipolar cycloadditions, and revised activation enthalpies for a standard set of hydrocarbon pericyclic reactions

Activation barriers and reaction energetics for the three main classes of 1,3-dipolar cycloadditions, including nine different reactions, were evaluated with the MPW1K and B3LYP density functional methods, MP2, and the multicomponent CBS-QB3 method. The CBS-QB3 values were used as standards for 1,3-dipolar cycloaddition activation barriers and reaction energetics, and the density functional theory (DFT) and MP2 methods were benchmarked against these values. The MPW1K/6-31G* method and basis set performs best for activation barriers, with a mean absolute deviation (MAD) value of 1.1 kcal/mol. The B3LYP/6-31G* method and basis set performs best for reaction enthalpies, with a MAD value of 2.4 kcal/mol, while the MPW1K method shows large errors for reaction energetics. The MP2 method gives the expected systematic underestimation of barriers. Concerted and nearly synchronous transition structures are predicted by all DFT and MP2 methods. Also reported are revised estimated 0 K experimental activation enthalpies for a standard set of hydrocarbon pericyclic reactions and updated comparisons to experiment for DFT, ab initio, and multicomponent methods. B3LYP and MPW1K methods with MAD values of 1.5 and 2.1 kcal/mol, respectively, fortuitously outperform the multicomponent CBS-QB3 method, which has a MAD value of 2.3. The MAD value of the O3LYP functional improves to 2.4 kcal/mol from the previously reported 3.0 kcal/mol.