Intermolecular Interactions in Li+‐glyme and Li+‐glyme–TFSA− Complexes: Relationship with Physicochemical Properties of [Li(glyme)][TFSA] Ionic Liquids

The stabilization energies (ΔE_form) calculated for the formation of the Li⁺ complexes with mono‐, di‐ tri‐ and tetra‐glyme (G1, G2, G3 and G4) at the MP2/6‐311G** level were ?61.0, ?79.5, ?95.6 and ?107.7 kcal mol^?1, respectively. The electrostatic and induction interactions are the major sources of the attraction in the complexes. Although the ΔE_form increases by the increase of the number of the O???Li contact, the ΔE_form per oxygen atom decreases. The negative charge on the oxygen atom that has contact with the Li⁺ weakens the attractive electrostatic and induction interactions of other oxygen atoms with the Li⁺. The binding energies calculated for the [Li(glyme)]⁺ complexes with TFSA^? anion (glyme=G1, G2, G3, and G4) were ?106.5, ?93.7, ?82.8, and ?70.0 kcal mol^?1, respectively. The binding energies for the complexes are significantly smaller than that for the Li⁺ with the TFSA^? anion. The binding energy decreases by the increase of the glyme chain length. The weak attraction between the [Li(glyme)]⁺ complex (glyme=G3 and G4) and TFSA^? anion is one of the causes of the fast diffusion of the [Li(glyme)]⁺ complex in the mixture of the glyme and the Li salt in spite of the large size of the [Li(glyme)]⁺ complex. The HOMO energy level of glyme in the [Li(glyme)]⁺ complex is significantly lower than that of isolated glyme, which shows that the interaction of the Li⁺ with the oxygen atoms of glyme increases the oxidative stability of the glyme.     

An investigation on the reaction mechanism of the F2+Cl2→2ClF using the B3LYP method

The reaction mechanism of F₂+Cl₂→2ClF has been investigated with the density functional theory at the B3LYP/6‐311G* level. Six transition states have been found for the three possible reaction paths and verified by the normal mode vibrational and IRC analyses. Ab initio MP2/6‐311G* geometry optimizations and CCSD(T)/6‐311G(2df)//MP2/6‐311G* single‐point energy calculations have been performed for comparison. It is found that when the F₂ (or Cl₂) molecule decomposes into atoms first and then the F (or Cl) atom reacts with the molecule Cl₂ (or F₂) nearly along the molecular axis, the energy barrier is very low. The calculated energy barrier of F attacking Cl₂ is zero and that of Cl attacking F₂ is only 15.57 kJ?mol^?1 at the B3LYP level. However, the calculated dissociation energies of F₂ and Cl₂ are as high as 145.40 and 192.48 kJ?mol^?1, respectively. When the reaction proceeds through a bimolecular reaction mechanism, two four‐center transition states are obtained and the lower energy barrier is 218.69 kJ?mol^?1. Therefore, the title reaction F₂+Cl₂→2ClF is most probably initiated from the atomization of the F₂ molecule and terminated by the reaction of F attacking Cl₂ nearly along the Cl? Cl bond. MP2 calculations lead to the same conclusion, but the geometry of TS and the energy barrier are somewhat different. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002     

A quantum mechanical approach to the kinetics of the hydrogen abstraction reaction H2O2 + •OH → HO2 + H2O

The kinetics of the hydrogen abstraction from H₂O₂ by ^?OH has been modeled with MP2/6‐31G*//MP2/6‐31G*, MP2‐SAC//MP2/6‐31G*, MP2/6‐31+G**//MP2/6‐31+G**, MP2‐SAC// MP2/6‐31+G**, MP4(SDTQ)/6‐311G**//MP2/6‐31G*, CCSD(T)/6‐31G*//CCSD(T)/6‐31G*, CCSD(T)/6‐31G**//CCSD(T)/6‐31G**, CCSD(T)/6‐311++G**//MP2/6‐31G* in the gas phase. MD simulations have been used to generate initial geometries for the stationary points along the potential energy surface for hydrogen abstraction from H₂O₂. The effective fragment potential (EFP) has been used to optimize the relevant structures in solution. Furthermore, the IEFPCM model has been used for the supermolecules generated via MD calculations. IEFPCM/MP2/6‐31G* and IEFPCM/CCSD(T)/6‐31G* calculations have also been performed for structures without explicit water molecules. Experimentally, the rate constant for hydrogen abstraction by ^?OH drops from 1.75 × 10^?12 cm³ molecule^?1 s^?1 in the gas phase to 4.48 × 10^?14 cm³ molecule^?1 s^?1 in solution. The same trend has been reproduced best with MP4 (SDTQ)/6‐311G**//MP2/6‐31G* in the gas phase (0.415 × 10^?12 cm³ molecule^?1 s^?1) and with EFP (UHF/6‐31G*) in solution (3.23 × 10^?14 cm³ molecule^?1 s^?1). © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 502–514, 2005     

A quantum-chemical study of the LiH + LiH⋅− ⇄ LiH2− + Li⋅ reaction

Parts of the potential energy surface of the title process and related processes have been investigated at the SCF /6-31G **, SCF /6-31++G **, and MP 2/6-31++G ** levels. The investigated reaction is exothermic (?6.23 kcal/mol, MP 4/6-31++G **//MP 2/6–31++G** level, ZPE included): A linear intermediate radical anion, Li? H? Li? H??, is significantly stabilized with respect to LiH + LiH?? (?38.74 kcal/mol, the same level as above). The BSSE at MP 2/6–31++G **//MP 2/6–31++G ** amounts to 1.8 kcal/mol. The title process seems to be suitable for experimental study in molecular beams.     

Ab initio study on the reaction mechanism of ozone with bromine atom

Ab initio calculations of the potential energy surface (PES) for the Br+O₃ reaction have been performed using the MP2, CCSD(T), and QCISD(T) methods with 6‐31G(d), 6‐311G(d), and 6‐311+G(3df). The reaction begins with a transition state (TS) when the Br atom attacks a terminal oxygen of ozone, producing an intermediate, the bromine trioxide (M), which immediately dissociates to BrO+O₂. The geometry optimizations of the reactants, products, and intermediate and transition states are carried out at the MP2/6‐31G(d) level. The reaction potential barrier is 3.09 kcal/mol at the CCSD(T)/6‐311+G(3df)//MP2 level, which shows that the bromine atom trends intensively to react with the ozone. The comparison of the Br+O₃ reaction with the F+O₃ and Cl+O₃ reactions indicates that the reactions of ozone with the halogen atoms have the similar reaction mechanism. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Isomerism in OBe3F3 + cation: anab initio study

Ab initio MP2/6-31G^*//HF/6-31G^*+ZPE(HF/6-31G^*) calculations of the potential energy surface in the vicinity of stationary points and the pathways of intramolecular rearrangements between low-lying structures of the OBe₃F₃ ⁺ cation detected in the mass spectra of μ₄-Be₄O(CF₃COO)₆ were carried out. Ten stable isomers with di- and tricoordinate oxygen atoms were localized. The relative energies of six structures lie in the range 0–8 kcal mol⁻¹ and those of the remaining four structures lie in the range 20–40 kcal mol⁻¹. Two most favorable isomers, aC _2v isomer with a dicoordinate oxygen atom, planar six-membered cycle, and one terminal fluorine atom and a pyramidalC _3v isomer with a tricoordinate oxygen atom and three bridging fluorine atoms, are almost degenerate in energy. The barriers to rearrangements with the breaking of one fluorine bridge are no higher than 4 kcal mol⁻¹, except for the pyramidalC _3v isomer (∼16 kcal mol⁻¹). On the contrary, rearrangements with the breaking of the O−Be bond occur with overcoming of a high energy barrier (∼24 kcal mol⁻¹). A planarD _3h isomer with a tricoordinate oxygen atom and linear O−Be−H fragments was found to be the most favorable for the OBe₃H₃ ⁺ cation, a hydride analog of the OBe₃F₃ ⁺ ion; the energies of the remaining five isomers are more than 25 kcal mol⁻¹ higher. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 420–430, March, 1999.     

Theoretical study of thermal rearrangements of α‐silylalcohols: Effects of substituents attached to the silicon atom on the reactions

To investigate the effects of substituents attached to the silicon atom on the thermal rearrangement reactions of α‐silyl alcohols, the thermal rearrangement reactions of dimethylsilyl methanol (CH₃)₂SiHCH₂OH and vinylsilyl methanol CH₂?CHSiH₂CH₂OH were studied by ab initio calculations at the G3 level. Geometries of various stationary points were fully optimized at the MP2(full)/6‐31G(d) and MP2(full)/6‐311G(d,p) levels, and harmonic vibrational frequencies were calculated at the same levels. The reaction paths were investigated and confirmed by intrinsic reaction coordinate (IRC) calculations at the MP2(full)/6‐31G(d) level. The results show that two dyotropic reactions could occur when (CH₃)₂SiHCH₂OH or CH₂?CHSiH₂CH₂OH is heated. One is Brook rearrangement reaction (reaction A), and the dimethylsilyl or vinylsilyl groups migrates from carbon atom to oxygen atom coupled with a simultaneous migration of a hydrogen atom from oxygen atom to carbon atom passing through a double three‐membered ring transition state, forming dimethylmethoxylsilane (CH₃)₂SiHOCH₃ or methoxylvinylsilane CH₂?CHSiH₂OCH₃; the other is a hydroxyl group migration (reaction B) from carbon atom to silicon atom, coupled with a simultaneous migration of a hydrogen atom from silicon atom to carbon atom, via a double three‐membered ring transition state, forming trimethylsilanol (CH₃)₃SiOH or methylvinylsilanol CH₃SiH(OH)CH?CH₂. The G3 barriers of the reactions A and B were computed to be 312.8 and 241.4 kJ/mol for (CH₃)₂SiHCH₂OH, and 317.6 and 233.7 kJ/mol for CH₂?CHSiH₂CH₂OH, respectively. On the basis of the MP2(full)/6‐31G(d) optimized parameters, vibrational frequencies, and G3 energies, the reaction rate constants k(T) and equilibrium constants K(T) were calculated using canonical variational transition state theory (CVT) with centrifugal‐dominant small‐curvature tunneling (SCT) approximation over a temperature range of 400–1800 K. The influences of methyl and vinyl groups attached to the silicon atom on reactions are discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Hetero‐π‐systems from 2 + 2 cycloreversion,part 2.1Ab initio thermochemical study of heterocyclobutanes 2 + 2 cycloreversion to form heteroethenes H2C=X (X=NH,O, SiH2, PH,S)

Ab initio and DFT thermochemical study of diradical mechanism of 2 + 2 cycloreversion of parent heterocyclobutanes and 1,3‐diheterocyclobutanes, cyclo‐(CH₂CH₂CH₂X), and cyclo‐(CH₂XCH₂X), where X = NH, O, SiH₂, PH, S, was undertaken by calculating closed‐shell singlet molecules at three levels of theory: MP4/6‐311G(d)//MP2/6‐31G(d)+ZPE, MP4/6‐311G(d,p)//MP2/6‐31G (d,p)+ZPE, and B3LYP/6‐311+G(d,p)+ZPE. The enthalpies of 2 + 2 cycloreversion decrease on going from group 14 to group 16 elements, being substantially higher for the second row elements. Normally endothermic 2 + 2 cycloreversion is predicted to be exothermic for 1,3‐diazetidine and 1,3‐dioxtane. Strain energies of the four‐membered rings were calculated via the appropriate homodesmic reactions. The enthalpies of ring opening via the every possible one‐bond homolysis that results in the formation of the corresponding 1,4‐diradical were found by subtracting the strain energies from the central bond dissociation energies of the heterobutanes CH₃CH₂—CH₂XH, CH₃CH₂—XCH₃, and HXCH₂—XCH₃. The latter energies were determined via the enthalpies of the appropriate dehydrocondensation reactions, using C—H and X—H bond energies in CH₃XH calculated at G2 level of theory. Except 1,3‐disiletane, in which ring‐opening enthalpy attains 69.7 kcal/mol, the enthalpies of the most economical ring openings do not exceed 60.7 kcal/mol. The 1,4‐diradical decomposition enthalpies found as differences between 2 + 2 cycloreversion and ring‐opening enthalpies were negative, the least exothermicity was calculated for ⋅ CH₂SiH₂CH₂CH₂. The only exception was 1,3‐disiletane, which being diradical, CH₂SiH₂CH₂SiH₂, decomposed endothermically. Since decomposition of the diradical containing two silicon atoms required extra energy, raising the enthalpy of the overall reaction to 78.9 kcal/mol, 1,3‐disiletane was predicted to be highly resisting to 2 + 2 cycloreversion. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:704–720, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20377     

The Exceptional Conformational Bias of 1,5-Diene-3,4-diols Is Controlled by Electrostatic Interactions: A Theoretical Elucidation of the Origin in Rotational Isomer Stability

A conformational study of 1,5-hexadiene-3,4-diol (1) and 3-buten-1-ol (2) with the ab initio molecular orbital methods reveals that electrostatic interactions, rather than steric or tosional effects, control the conformational preferences of these compounds. It is found that the 1,2-dioxygen function prefers the anti arrangement due to the lone pair electron repulsion. A pseudo 1,3-diaxial oxygen/pi bond repulsion and a 1,3-diaxial attraction between an oxygen lone pair and a vinyl H are found to be responsible for the profound preference for the CO-eclipsed form in diol 1. Quantitatively, the repulsion between two gauche oxygen atoms is calculated to be approximately 1.5 kcal/mol at the MP2/6-31G//HF/6-31G level of theory. The repulsion between an oxygen atom and a pi bond at the pseudo 1,3-diaxial position is approximately 1 kcal/mol, and the attractive interaction between an oxygen atom and a vinyl H at the pseudo 1,3-diaxial position is approximately 0.5 kcal/mol, respectively, at the MP2/6-31G//MP2/6-31G level.     

Vinyloxyborane and its isomers. An ab initio study of the C2H5BO potential energy surface,the barrier to 1,3-shifts in β-ketoboranes,and the mechanism of the carbonylation reaction of boranes

Vinyloxyboranes, CH₂?CH? ;O? ;BR₂, are shown by ab initio molecular orbital theory to be more stable than the isomeric β-aldoboranes, R₂B? CH₂? CH?O, by ca. 19 kcal/mol. The MP2/6-31G*/6-31G* + ZPE barrier for the [1,3] boron shift is only 10.9 kcal/mol (R ? Me) relative to the aldoborane. Other C₂H₅BO isomers (β-ketoboranes, boraepoxides and organoboron oxides), which are related to the proposed stages in the carbonylation reaction of boranes, are shown to be plausible intermediates. However, some of the computed barriers for methyl group migrations are unrealistically large, up to ca. 63 kcal/mol.     

Thermodynamic and kinetic study of ibuprofen with hydroxyl radical: A density functional theory approach

Ibuprofen, a frequently detected pharmaceutical in natural and engineered waters, was studied in both neutral and anionic forms using density functional theory at the B3LYP/6‐311++G**//B3LYP/6‐31G* level of theory in its reaction with hydroxyl radical ( ? OH). The reaction pathways included ? OH addition to aromatic ring, abstraction of a H‐atom, and nucleophilic attack on the carbonyl group. The results showed that H‐atom abstraction pathways are the most favorable. The free energy change for H‐atom abstraction reaction ranges from ?37.8 to ?15.9 kcal/mol; for ? OH addition ranges from ?3.85 to ?1.23 kcal/mol; and for nucleophilic attack on the carbonyl group is 13.9 kcal/mol. The calculated rate constant between neutral ibuprofen and ? OH, 6.72 × 10⁹ M^?1s^?1, is consistent with the experimental value, 6.5 ± 0.2 × 10⁹ M^?1s^?1. Our results provide direct evidence for byproduct formation and identification on the molecular level. © 2013 Wiley Periodicals, Inc.     

Reactivity of alkyl versus silyl peroxides. The consequences of 1, 2-silicon bridging on the epoxidation of alkenes with silyl hydroperoxides and bis(trialkylsilyl)peroxides

The bond dissociation energies for a series of silyl peroxides have been calculated at the G2 and CBS-Q levels of theory. A comparison is made with the O-O BDE of the corresponding dialkyl peroxides, and the effect of the O-O bond strength on the activation barrier for oxygen atom transfer is discussed. The O-O bond dissociation enthalpies (DeltaH(298)) for bis (trimethylsilyl) peroxide (1) and trimethylsilyl hydroperoxide (2) are 54.8 and 53.1 kcal/mol, respectively at the G2 (MP2) and CBS-Q levels of theory. The O-O bond dissociation energies computed at G2 and G2(MP2) levels for bis(tert-butyl) peroxide and tert-butyl hydroperoxide are 45.2 and 48.3 kcal/mol, respectively. The barrier height for 1,2-methyl migration from silicon to oxygen in trimethylsilyl hydroperoxide is 47.9 kcal/mol (MP4//MP2/6-31G). The activation energy for the oxidation of trimethylamine to its N-oxide by bis(trimethylsilyl) peroxide is 28.2 kcal/mol (B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d)). 1,2-Silicon bridging in the transition state for oxygen atom transfer to a nucleophilic amine results in a significant reduction in the barrier height. The barrier for the epoxidation of E-2-butene with bis(dimethyl(trifluoromethyl))silyl peroxide is 25.8 kcal/mol; a reduction of 7.5 kcal/mol relative to epoxidation with 1. The activation energy calculated for the epoxidation of E-2-butene with F(3)SiOOSiF(3) is reduced to only 2.2 kcal/mol reflecting the inductive effect of the electronegative fluorine atoms.     

ab initio Calculation of the stereoelectronic structure of 1-(1-trichlorogermylethyl)-pyrrolidin-2-one*

RHF/6-31G(d) and MP2/6-31G(d) calculations were carried out to study the stereoelectronic structure of 1-(1-trichlorogermylethyl)pyrrolidin-2-one with a pentacoordinated germanium atom. These results were compared with the X-ray diffraction structural analysis data. Upon formation of the Ge ← O coordination bond in this molecule, the electron density of all the atoms of the coordination polyhedron of the germanium atom, including the oxygen atom, increases, especially the axial chlorine atom, while the electron density of the germanium, nitrogen, and carbonyl group carbon atoms decreases. Different polarization of all three valence p-orbitals of each Cl atom of this molecule was established. ³⁵Cl nuclear quadrupole resonance spectrum parameters were evaluated. The molecule also has stable form, in which the germanium atom is tetracoordinated. The total energy of this form is 2.7 kcal/mol higher than for the structure with a pentacoordinated germanium atom.     

Chemical Reactivity of Oxo‐ and Aza‐γ‐lactam Rings

Different mechanisms for the alkaline hydrolysis of oxo and aza‐γ‐lactam rings have been studied by ab initio calculations at the MP2/6‐31+G*//MP2/6‐31+G* and B3LYP/6‐31+G*//B3LYP/6‐31+G* levels. The tetrahedral intermediate can undergo two different reactions, the cleavage of the C₂−N₂ bond (the classical mechanism) and the cleavage of the C₂−X₆ bond (X=O, N). Both compounds present similar energy barriers for the classical fragmentation, and show considerably lower barriers for the alternative mechanism. Because of this reactivity, the compounds studied are expected to be β‐lactamase inhibitors.     

Interactions of the “piano‐stool” [ruthenium(II) (η6‐arene)(en)CL]+ complexes with water and nucleobases; ab initio and DFT study

Piano stool ruthenium complexes of the composition [Ru(II)(η⁶‐arene)(en)Cl]^+/2+ (en = ethylenediamine) represent an emerging class of cisplatin‐analogue anticancer drug candidates. In this study, we use computational quantum chemistry to characterize the structure, stability and reactivity of these compounds. All these structures were optimized at DFT(B3LYP)/6‐31G(d) level and their single point properties were determined by the MP2/6‐31++G(2df,2pd) method. Thermodynamic parameters and rate constants were determined for the aquation process, as a replacement of the initial chloro ligand by water and subsequent exchange reaction of aqua ligand by nucleobases. The computations were carried out at several levels of DFT and ab initio theories (B3LYP, MP2 and CCSD) utilizing a range of bases sets (from 6‐31G(d) to aug‐cc‐pVQZ). Excellent agreement with experimental results for aquation process was obtained at the CCSD level and reasonable match was achieved also with the B3LYP/6‐31++G(2df,2pd) method. This level was used also for nucleobase‐water exchange reaction where a smaller rate constant for guanine exchange was found in comparison with adenine. Although adenine follows a simple replacement mechanism, guanine complex passes by a two‐step mechanism. At first, Ru‐O6(G) adduct is formed, which is transformed through a chelate TS2 to the Ru‐N7(G) final complex. In case of guanine, the exchange reaction is more favorable thermodynamically (releasing in total by about 8 kcal/mol) but according to our results, the rate constant for guanine substitution is slightly smaller than the analogous constant in adenine case when reaction course from local minimum is considered. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009     

Protonation of phenylboronic acid: Comparison of G3B3 and G2MP2 methods

G3B3 and G2MP2 calculations using Gaussian 03 have been carried out to investigate the protonation preferences for phenylboronic acid. All nine heavy atoms have been protonated in turn. With both methodologies, the two lowest protonation energies are obtained with the proton located either at the ipso carbon atom or at a hydroxyl oxygen atom. Within the G3B3 formalism, the lowest‐energy configuration by 4.3 kcal · mol^?1 is found when the proton is located at the ipso carbon, rather than at the electronegative oxygen atom. In the resulting structure, the phenyl ring has lost a significant amount of aromaticity. By contrast, calculations with G2MP2 show that protonation at the hydroxyl oxygen atom is favored by 7.7 kcal · mol^?1. Calculations using the polarizable continuum model (PCM) solvent method also give preference to protonation at the oxygen atom when water is used as the solvent. The preference for protonation at the ipso carbon found by the more accurate G3B3 method is unexpected and its implications in Suzuki coupling are discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Energetics of proton transfer between carbon atoms (H3CH ? CH3)−

Ab initio calculations were carried out to study the potential energy surface of (H₃C? H? CH₃)^?. The 6–31G* basis set is supplemented by a set of diffuse p functions on both C and H (with a range of exponents for the latter). The binding energy of CH₄ and CH₃^? to form the (H₃CH? CH₃)^? complex is about 2 kcal/mol, much smaller than for comparable ionic H-bonded systems involving O or N atoms. Nearly half of this interaction energy is due to correlation effects, computed at second and third orders of Møller-Plesset perturbation theory. Correlation is also responsible for substantial reductions in the energy barrier to proton transfer within the complex. This barrier is computed to be 13?15 kcal/mol at the MP3 level, depending upon the exponent used for the H p functions.     

Theoretical Study on Gas—phase Pyrolytic Reactions of N—Ethyl,Isopropyl and N—t—Butyl Substituted 2—Aminopyrazine

Density functional theory (DFT) and ab initio methods were used to study gas‐phase pyrolytic reaction mechanisms of iV‐ethyl, N‐isopropyl and N‐t‐butyl substituted 2‐aminopyrazine at B3LYP/6–31G* and MP2/6–31G*, respectively. Single‐point energies of all optimized molecular geometries were calculated at B3LYP/6–311 + G(2d,p) level. Results show that the pyrolytic reactions were carried out through a unimolecular first‐order mechanism which were caused by the migration of atom H(17) via a six‐member ring transition state. The activation energies which were verified by vibrational analysis and correlated with zero‐point energies along the reaction channel at B3LYP/6–311 + G(2d,p) level were 252.02 kJ. mo^?1 (N‐ethyl substituted), 235.92 kJ‐mol^?1 (N‐t‐isopropyl substituted) and 234.27 kJ‐mol^?1 (N‐t‐butyl substituted), respectively. The results were in good agreement with available experimental data.