A DFT study of proton transfers for the reaction of phenol and hydroxyl radical leading to dihydroxybenzene and H2O in the water cluster

Reactions of phenol and hydroxyl radical were studied under the aqueous environment to investigate the antioxidant characters of phenolic compounds. M06‐2X/6‐311 + G(d,p) calculations were carried out, where proton transfers via water molecules were examined carefully. Stepwise paths from phenol + OH^• + (H₂O)_n (n = 3, 7, and 12) to the phenoxyl radical (Ph O^•) via dihydroxycyclohexadienyl radicals (ipso, ortho, meta, and para OH‐adducts) were obtained. In those paths, the water dimer was computed to participate in the bond interchange along hydrogen bonds. The concerted path corresponding to the hydrogen atom transfer (HAT, apparently Ph OH + OH^• → Ph O^• + H₂O) was found. In the path, the hydroxyl radical located on the ipso carbon undergoes the charge transfer to prompt the proton (not hydrogen) transfer. While the present new mechanism is similar to the sequential proton loss electron transfer (SPLET) one, the former is of the concerted character. Tautomerization reactions of ortho or para (OH)C₆H₅=O + (H₂O)_n → C₆H₄(OH)₂(H₂O)_n were traced with n = 2, 3, 4, and 14. The n = 3 (and n = 14) model of ortho and para was calculated to be most likely by the strain‐less hydrogen‐bond circuit.     

Absolute rate calculations: atom and proton transfers in hydrogen-bonded systems.

We calculate energy barriers of atom- and proton-transfer reactions in hydrogen-bonded complexes in the gas phase. Our calculations do not involve adjustable parameters and are based on bond-dissociation energies, ionization potentials, electron affinities, bond lengths, and vibration frequencies of the reactive bonds. The calculated barriers are in agreement with experimental data and high-level ab initio calculations. We relate the height of the barrier with the molecular properties of the reactants and complexes. The structure of complexes with strong hydrogen bonds approaches that of the transition state, and substantially reduces the barrier height. We calculate the hydrogen-abstraction rates in H-bonded systems using the transition-state theory with the semiclassical correction for tunneling, and show that they are in excellent agreement with the experimental data. H-bonding leads to an increase in tunneling corrections at room temperature.     

Hydrolysis of tetrafluorosilane under neutral conditions: A quantum chemical study

A density functional quantum chemical study of the first step of SiF₄ hydrolysis under neutral conditions in the presence of one or two H₂O molecules was carried out. The reaction is endothermic and can follow three different pathways that involve the formation of penta-coordinated (pathway A) and hexacoordinated (pathways B and C) intermediates and transition states as the key steps. Pathway B is the most energetically favorable. All three pathways of the hydrolysis reaction lead to a product with formal retention of the configuration of substituents at the silicon atom. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 749–753, May, 2006.     

3-卤代吡唑质子转移过程的理论研究

采用密度泛函B3LYP/6-311G**方法,对3-卤(-F、-Cl、-Br)代吡唑几何构型进行了全自由度优化,获得了它们的几何结构和电子结构。计算结果显示,N1-H型的稳定性大于N2-H型。计算并考察了3-卤代吡唑进行结构互变的质子转移过程的四种可能途径:(a)分子内质子转移;(b)水助质子转移;(c)同种二聚体双质子转移;(d)异种二聚体双质子转移。计算结果表明(以3-氟代吡唑为例),途径d所需要的活化能最小(54.89 kJ/mol),而途径a所需要的活化能最大(198.83kJ/mol),途径b和c的活化能居中间分别为(104.05 kJ/mol和69.05 kJ/mol)。研究还表明氢键在降低活化能方面起着重要的作用,卤素(-F、-Cl、-Br)对活化能的影响不大。     

Theoretical analysis on the hydrogen bonding and reactivity that associated with the proton transfer reaction of carboxylic acid dimers and their monosulfur derivatives

Carboxylic acid dimers and their monosulfur derivatives are investigated by density functional theory calculations. Basis set superposition error (BSSE) counterpoise correction is included to compare the influence of BSSE on the interaction energies as well as on the geometries. The nature of hydrogen bond is determined on the basis of atoms in molecules (AIM) and natural bond orbital (NBO) analyses. Good correlations have been established between H‐bond length versus AIM topological parameter, orbital interaction, and barrier height for proton transfer. The reactivity behavior along the reaction path of the double proton transfer reaction has also been studied. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Additivity rules using similarity models for chemical reactivity: calculation and interpretation of electrofugality and nucleofugality

A recently proposed, multi-parameter correlation: log k (25 degrees C)=s(f) (Ef + Nf), where Ef is electrofugality and Nf is nucleofugality, for the substituent and solvent effects on the rate constants for solvolyses of benzhydryl and substituted benzhydryl substrates, is re-evaluated. A new formula (Ef=log k (RCl/EtOH/25 degrees C) -1.87), where RCl/EtOH refers to ethanolysis of chlorides, reproduces published values of Ef satisfactorily, avoids multi-parameter optimisations and provides additional values of Ef. From the formula for Ef, it is shown that the term (sfxEf) is compatible with the Hammett-Brown (rho+sigma+) equation for substituent effects. However, the previously published values of N(f) do not accurately account for solvent and leaving group effects (e.g. nucleofuge Cl or X), even for benzhydryl solvolyses; alternatively, if the more exact, two-parameter term, (sfxNf) is used, calculated effects are less accurate. A new formula (Nf=6.14 + log k(BX/any solvent/25 degrees C)), where BX refers to solvolysis of the parent benzhydryl as electrofuge, defines improved Nf values for benzhydryl substrates. The new formulae for Ef and Nf are consistent with an assumption that sf=1.00(,) and so improved correlations for benzhydryl substrates can be obtained from the additive formula: log k(RX/any solvent/25 degrees C)=(Ef + Nf). Possible extensions of this approach are also discussed.     

Computational Study of the Mechanism and Selectivity of Palladium‐Catalyzed Propargylic Substitution with Phosphorus Nucleophiles

The mechanism and sources of selectivity in the palladium‐catalyzed propargylic substitution reaction that involves phosphorus nucleophiles, and which yields predominantly allenylphosphonates and related compounds, have been studied computationally by means of density functional theory. Full free‐energy profiles are computed for both H‐phosphonate and H‐phosphonothioate substrates. The calculations show that the special behavior of H‐phosphonates among other heteroatom nucleophiles is indeed reflected in higher energy barriers for the attack on the central carbon atom of the allenyl/propargyl ligand relative to the ligand‐exchange pathway, which leads to the experimentally observed products. It is argued that, to explain the preference of allenyl‐ versus propargyl‐phosphonate/phosphonothioate formation in reactions that involve H‐phosphonates and H‐phosphonothioates, analysis of the complete free‐energy surfaces is necessary, because the product ratio is determined by different transition states in the respective branches of the catalytic cycle. In addition, these transition states change in going from a H‐phosphonate to a H‐phosphonothioate nucleophile.     

Copper‐based supported catalysts for the atom transfer radical polymerization of methyl methacrylate: How can activity and control be tuned up?

This article is aimed at revisiting the synthesis of copper‐based catalysts immobilized onto crosslinked polystyrene (PS) resins carrying pyridinimine groups (PS–pyridinimine/CuBr). These supported catalytic systems were used for promoting the atom transfer radical polymerization of methyl methacrylate as initiated by ethyl‐2‐bromoisobutyrate. It was evidenced that the control over the polymerization reaction was strongly influenced by the coordination ability and extent of the transition‐metal salt on the supported pyridinimine ligands. For instance, increasing the ligand‐to‐catalyst molar ratio allowed for increasing the polymerization rate and improving the control over the molecular parameters of the synthesized poly(methyl)methacrylate (PMMA) in terms of the molar masses and molecular weight distributions. The PS–pyridinimine/CuBr supported catalyst was recycled and reused for further polymerization reactions. After two recycling steps, the reaction activity appeared to be preserved, and the control was improved in terms of the initiation efficiency. However, a slight increase in the polydispersity indices was observed. Interestingly, the introduction of a flexible polydimethylsiloxane spacer between the PS support and the catalytic sites led to some more improvement of the control over the molecular parameters of PMMA chains, which displayed narrower molecular weight distributions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 744–756, 2006     

Assessment of the extended Koopmans' theorem for the chemical reactivity: Accurate computations of chemical potentials,chemical hardnesses,and electrophilicity indices

The extended Koopmans' theorem (EKT) provides a straightforward way to compute ionization potentials and electron affinities from any level of theory. Although it is widely applied to ionization potentials, the EKT approach has not been applied to evaluation of the chemical reactivity. We present the first benchmarking study to investigate the performance of the EKT methods for predictions of chemical potentials (μ) (hence electronegativities), chemical hardnesses (η), and electrophilicity indices (ω). We assess the performance of the EKT approaches for post‐Hartree–Fock methods, such as Møller–Plesset perturbation theory, the coupled‐electron pair theory, and their orbital‐optimized counterparts for the evaluation of the chemical reactivity. Especially, results of the orbital‐optimized coupled‐electron pair theory method (with the aug‐cc‐pVQZ basis set) for predictions of the chemical reactivity are very promising; the corresponding mean absolute errors are 0.16, 0.28, and 0.09 eV for μ, η, and ω, respectively. © 2015 Wiley Periodicals, Inc.     

Synthesis of methyl methacrylate,styrene, and isobutylene multiblock copolymers using atom transfer and cationic polymerization

ABCBA‐type pentablock copolymers of methyl methacrylate, styrene, and isobutylene (IB) were prepared by the cationic polymerization of IB in the presence of the α,ω‐dichloro‐PS‐b‐PMMA‐b‐PS triblock copolymer [where PS is polystyrene and PMMA is poly(methyl methacrylate)] as a macroinitiator in conjunction with diethylaluminum chloride (Et₂AlCl) as a coinitiator. The macroinitiator was prepared by a two‐step copper‐based atom transfer radical polymerization (ATRP). The reaction temperature, ?78 or ?25 °C, significantly affected the IB content in the resulting copolymers; a higher content was obtained at ?78 °C. The formation of the PIB‐b‐PS‐b‐PMMA‐b‐PS‐b‐PIB copolymers (where PIB is polyisobutylene), prepared at ?25 (20.3 mol % IB) or ?78 °C (61.3 mol % IB; rubbery material), with relatively narrow molecular weight distributions provided direct evidence of the presence of labile chlorine atoms at both ends of the macroinitiator capable of initiation of cationic polymerization of IB. One glass‐transition temperature (T_g), 104.5 °C, was observed for the aforementioned triblock copolymer, and the pentablock copolymer containing 61.3 mol % IB showed two well‐defined T_g's: ?73.0 °C for PIB and 95.6 °C for the PS–PMMA blocks. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3823–3830, 2005     

Azobenzene‐based initiator for atom transfer radical polymerization of methyl methacrylate

2‐Bromopropionic acid 2‐(4‐phenylazophenyl)ethyl ester, 2‐bromopropionic acid 6‐(4‐phenylazophenoxy)hexyl ester (BPA₆), 2‐bromopropionic acid‐(4‐phenylazoanilide), and 2‐bromopropionic acid 4‐[4‐(2‐bromopropionyloxy)phenylazo]phenyl ester (BPPE) were used as initiators with monofunctional or difunctional azobenzene for the heterogeneous atom transfer radical polymerization of methyl methacrylate with a copper(I) chloride/N,N,N′,N″,N″‐pentamethyldiethylenetriamine catalytic system. The rates of polymerizations exhibited first‐order kinetics with respect to the monomer, and a linear increase in the number‐average molecular weight with increasing monomer conversion was observed for these initiation systems. The polydispersity indices of the polymer were relatively low (1.15–1.44) up to high conversions in all cases. The fastest rate of polymerization and the highest initiation efficiency were achieved with BPA₆, and this could be explained by the longer distance between the halogen and azobenzene groups and the better solubility of the BPA₆ initiator. The redshifting of the UV absorptions of the polymers only occurred for the BPPE‐initiated system. The intensity of the UV absorptions of the polymers were weaker than those of the corresponding initiators in chloroform and decreased with the increasing molecular weights of the polymers in all cases. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2358–2367, 2005     

Intramolecular nucleophilicS N 2 substitution at the tetrahedral carbon atom: Anab initio study

Pentacoordination of carbon atom in bicyclic organic compounds of the pentalene type was studied by theab initio RHF/6-31G^** and MP2(full)/6-31G^** methods. It was shown that intramolecularS _N 2 reactions with energy barriers within the energy scale of NMR spectroscopy can occur in systems in which a linear orientation of the attacking and leaving groups is realized. The barrier to the intramolecular nucleophilic substitution reaction in 2,3-dihydro-3-formylmethylenefuran is 36.9 (RHF) and 27.7 kcal mol⁻¹ (MP2) and decreases to 16.4 and 19.4 kcal mol⁻¹, respectively, in the case of diprotonation at the O atoms in this system. For model pentalene type compounds containing electron-deficient B atoms in the ring, theab initio calculations predict a further decrease in the barrier height (down to less than 10 kcal mol⁻¹). Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 7, pp. 1246–1256, July, 1999.     

Ambident Nucleophilic Substitution: Understanding Non-HSAB Behavior through Activation Strain and Conceptual DFT Analyses

The ability to understand and predict ambident reactivity is key to the rational design of organic syntheses. An approach to understand trends in ambident reactivity is the hard and soft acids and bases (HSAB) principle. The recent controversy over the general validity of this principle prompted us to investigate the competing gas-phase S_N2 reaction channels of archetypal ambident nucleophiles CN⁻, OCN⁻, and SCN⁻ with CH₃Cl (S_N2@C) and SiH₃Cl (S_N2@Si), using DFT calculations. Our combined analyses highlight the inability of the HSAB principle to correctly predict the reactivity trends of these simple, model reactions. Instead, we have successfully traced reactivity trends to the canonical orbital-interaction mechanism and the resulting nucleophile–substrate interaction energy. The HOMO–LUMO orbital interactions set the trend in both S_N2@C and S_N2@Si reactions. We provide simple rules for predicting the ambident reactivity of nucleophiles based on our Kohn–Sham molecular orbital analysis.     

The tautomerization and ring closure in the Claisen rearrangement: A DFT study

Elementary processes of the aromatic Claisen rearrangement were investigated by DFT calculations. First, rearrangements of four substrates Ph—O—CH₂—CHCH₂ [A], Ph—O—CH₂—CHCH(OMe) [B], Ph—O—CH₂—CHCH₂····BF₃ [C], and Ph—O—CH—CHCH(OMe)····BF₃ [D] were examined. In these systems, the tautomerization is initiated by the intermolecular proton transfer involving the transient ion‐pair intermediate. An ignition‐propagation chain‐reaction mechanism in the tautomerization was suggested. For [A], the (ortho‐allyl phenol → α‐methyl‐dihydrobenzofuran (α‐methyl‐cumarane)) process was found to be ready and the product of the Claisen rearrangement seems to be the cumarane rather than the phenol. In [D] (activated both by the terminal methoxy group and by the BF₃ catalyst), not the [3,3]‐sigmatropic shift but the tautomerization is the rate determining step. Second, the parent system, Ph—O—CH₂—CHCH₂, was investigated with (H₂O) _n (n = 2, 4, 6, and 10) systematically. The tautomerization takes place by the proton transfer via the water dimer or trimer. Except n = 2, similar changes of Gibbs free energies were obtained from the ether substrate to the cumarane.