Theoretical study of the P-Ylide reaction in the carbon nanotube

Recent studies have shown that the inner phase of carbon nanotubes (CNTs) can not only change the properties of molecules inside the tube, but also enhance or restrain the S_N2 reactions. Thus, the CNTs can be considered a form of solid solvent. In this paper, we study the [2+2] cycloaddition reaction between CH₂O and PH₃CH₂ in the gas phase, benzene solution and inner phase of CNT using the density functional theory (DFT). The results indicate that the inner phase of CNT has little effect on the [2+2] cycloaddition reaction. This can be explained as that while taking the linear arrangement for S_N2 reaction, the reactants do not possess the axial symmetry for the studied [2+2] cycloaddition reaction. Therefore, although the CNT has large axial polarizability, it can exert little influence on the [2+2] cycloaddition reaction. Our studies will be helpful for further understanding of the inner phase chemistry of CNTs.     

Theoretical study of DABCO-based ionic liquid: synthesis and reaction mechanism

Quaternary ammonium salt obtained from the Menshutkin reaction between DABCO and benzyl chloride has been used in the synthesis of a novel Bronsted acidic ionic liquid (IL), namely 1-benzyl-4-(sulfobutyl)-diaza-bicyclo-octane hydrogen sulfate. The reaction of DABCO with benzyl chloride is a crucial step in the synthesis of this IL. Density functional theory calculations at B3LYP/6-31G(d,p) level have been employed to investigate the mechanism of Menshutkin reaction by calculating the energy barriers through possible transition states i.e., five-membered ring transition state and S_N2 transition state in gas phase and in diethyl ether as a solvent. It was found that while DABCO reacts with benzyl chloride through the well-known S_N2 transition state mechanism, the corresponding reaction with chlorodiphenylmethane can proceed through both S_N2 and five-membered ring transition state mechanism. However, S_N2 transition state mechanism is still the strongly preferred one out of the two possible mechanisms. The electronic structure analysis shows that solvent effects and enhanced resonance stabilization may play a decisive role in guiding the reaction pathway.     

Solvent effects on SN2 reaction between substituted benzyl chloride and chloride ion

The effects of solvent on S_N2 reaction between some substituted benzyl chlorides and chloride ion have been investigated by DFT and ab initio methods using the polarizabale continuum model. The activation energies are higher in the solution media relative to the gas phase and grow by the increase in the dielectric constant of solvent. The complexation energies in solution media are smaller than those in the gas phase and reduce with increasing the dielectric constant. The energy data are in good agreement with the geometrical parameters and substituent constants. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

The molecular mechanism of low-temperature decomposition of hydrogen sulfide under conjugated chemisorption-catalysis conditions

The molecular mechanism of interaction of two hydrogen sulfide molecules with the (Co^III-H_o)₂S₂(SH₂)₄ model active center containing occluded hydrogen was studied by the density functional theory method with the B3P86 hybrid exchange-correlation functional. The reaction was found to occur in the following elementary steps: molecular adsorption of hydrogen sulfide ? dissociative chemisorption ? S-S bond formation in the surface intermediate {2Co^III ? (μ-S₂) + 2H(ads)} with the release of the first hydrogen molecule into the gas phase H₂(g) ? the release of the second hydrogen molecule into the gas phase H₂(g) ? the formation of cyclooctasulfur in the reaction 4S₂(ads) → S₈(ads). The first three steps occur spontaneously at room temperature, the thermodynamic driving force of the process being the stoichiometric reaction of S-S bond formation at the stage of conjugated chemisorption of two hydrogen sulfide molecules on two adjacent metal ions with the release of the first hydrogen molecule into the gas phase. The catalytic cycle is terminated by the recombination of molecular sulfur S₂ into cyclooctasulfur S₈ in the adsorption layer and the release of the second hydrogen molecule into the gas phase.     

Arenesulfonylation of <Emphasis Type="Italic">N</Emphasis>-alkylanilines: reaction kinetics and mechanism

Kinetic regularities of arenesulfonylation of N-alkylanilines in binary water-organic solvents of variable composition have been studied. The rate constants for these reactions increase with increasing the water content in a system. The steric factor has the decisive influence on reactivity of N-alkylamines. The character of the influence of the composition and nature of a solvent on the rate constants for arenesulfonylation was discussed with the assistance of results of quantum chemical simulation of molecular complexes of the nucleophiles studied with the components of the binary systems. Calculation of potential energy surface for the reaction of N-methylaniline with benzenesulfonyl chloride showed that in the gas phase the process occurs by the S_N2 mechanism.     

Unusual reaction of bromoalkyl trifluoromethyl ketones with symmetrically disubstituted ethylenediamines. Theoretic investigation applying methods of density functional and multiparticle perturbation theory МР-2

Mechanism of a multistage reaction between α-bromotrifluoromethylenones and N,N′-dialkylethylenediamines was examined in detail using quantum chemical methods of density functional (non-empirical functional РВЕ, extended split TZp-basis) and multiparticle perturbation theory МР-2, basis 6-311+G(d,p), in the gas phase approximation, and also including solvents molecules (water and 2,2,2-trifluoroethanol). The specific solvation of transition states owing to the hydrogen bonds formation with solvent molecules is the main factor governing the direction of the reaction. Trifluoroethanol forms a strong H-bond with the carbonyl oxygen atom of the trifluoroacetyl group increasing its electrophilicity. It also solvates the departing bromide anion facilitating the occurrence of the S_N2-substitution reaction. An essential but less important factor is the ability of trifluoroethanol to play the role of a nucleophilic partner by forming hydrogen bonds at the expense of the unshared electron pair of the hydroxyl oxygen facilitating the proton abstraction from the nitrogen atom of the diamine attacking the carbon atom in the course of S_N2-substitution.     

Towards the design of novel boron‐ and nitrogen‐substituted ammonia‐borane and bifunctional arene ruthenium catalysts for hydrogen storage

Electronic‐structure density functional theory calculations have been performed to construct the potential energy surface for H₂ release from ammonia‐borane, with a novel bifunctional cationic ruthenium catalyst based on the sterically bulky β‐diketiminato ligand (Schreiber et al., ACS Catal. 2012, 2, 2505). The focus is on identifying both a suitable substitution pattern for ammonia‐borane optimized for chemical hydrogen storage and allowing for low‐energy dehydrogenation. The interaction of ammonia‐borane, and related substituted ammonia‐boranes, with a bifunctional η⁶‐arene ruthenium catalyst and associated variants is investigated for dehydrogenation. Interestingly, in a number of cases, hydride‐proton transfer from the substituted ammonia‐borane to the catalyst undergoes a barrier‐less process in the gas phase, with rapid formation of hydrogenated catalyst in the gas phase. Amongst the catalysts considered, N,N‐difluoro ammonia‐borane and N‐phenyl ammonia‐borane systems resulted in negative activation energy barriers. However, these types of ammonia‐boranes are inherently thermodynamically unstable and undergo barrierless decay in the gas phase. Apart from N,N‐difluoro ammonia‐borane, the interaction between different types of catalyst and ammonia borane was modeled in the solvent phase, revealing free‐energy barriers slightly higher than those in the gas phase. Amongst the various potential candidate Ru‐complexes screened, few are found to differ in terms of efficiency for the dehydrogenation (rate‐limiting) step. To model dehydrogenation more accurately, a selection of explicit protic solvent molecules was considered, with the goal of lowering energy barriers for H‐H recombination. It was found that primary (1°), 2°, and 3° alcohols are the most suitable to enhance reaction rate. © 2014 Wiley Periodicals, Inc.     

Theoretical investigation of reaction mechanisms of alkaline hydrolysis of 2,3,6-trinitro-β-d-glucopyranose as a monomer of nitrocellulose

The 2,3,6-trinitro-β-d-glucopyranose as a monomer of nitrocellulose in the ⁴C₁ chair conformation was selected for the alkaline hydrolysis of nitrocellulose within S_N2 framework in the gas phase and in the bulk water solution. Both the direct and angular attacks of OH^? in the hydrolysis reactions were considered. Geometries were optimized at the B3LYP/6-311G(d,p) level both in the gas phase and in bulk water solution. Effect of bulk water solution was modeled using the PCM approach. Nature of potential energy surfaces of the local minima and transition states was ascertained through harmonic vibrational frequency analysis. Intrinsic reaction coordinate calculations were also performed to validate the computed transition state structures. Effect of electron correlation on computed energies was considered at the MP2/cc-pVTZ//B3LYP/6-311G(d,p) level. It was found that the angular attack of OH^? in the hydrolysis reaction will require significantly larger activation energy than the direct attack. Computed transition states correspond to the structure where the presence of hydrogen bonds between the OH^? and various sites of nitrocellulose was the necessary stabilizing factor. The ring breaking through the C–O ring bond was not found to be the first step in the alkaline hydrolysis reactions. It was predicted that alkaline hydrolysis would be driven by the addition–elimination (substitution) reaction starting at the C3 site and will progress in the C2 → C6 direction. Entropy of system in water solution will have profound effect on alkaline hydrolysis reaction of nitrocellulose.     

SN2 reactions in the gas phase. Alkyl group structural effects

The gas phase heats and entropies of S_N2-like association of bromide ions with a series of alkyl bromides are reported. Comparison of this data with published data for the same reactions in solution suggests that the alkyl group structural effects on S_N2 reactivity in solution are controlled entirely by the solvent.     

The heterogeneous formation of N2O in the presence of acidic solutions: Experiments and modeling

We investigated the heterogeneous processes that contribute towards the formation of N₂O in an environment that comes as closely as possible to exhaust conditions containing NO and SO₂ among other constituents. The simultaneous presence of NO, SO₂, O₂, and condensed phase water in the liquid state has been confirmed to be necessary for the production of significant levels of N₂O. The maximum rate of N₂O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. The replacement of NO by either NO₂ or HONO significantly increases the rate constant for N₂O formation. The measured reaction orders in the rate law change depending upon the choice of the nitrogen reactant used and were fractional in some cases. The rate constants of N₂O formation for the three different nitrogen reactants reveal the following series of increasing reactivity: NO < NO₂ < HONO, indicating the probable sequential involvement of those species in the elementary reactions. Furthermore, we observed a complex dependence of the rate constant on the acidity of the liquid phase where both the initial rate as well as the yield of N₂O are largest at pH=0 of a H₂SO₄/H₂O solution. The results suggest that HONO is the major reacting N(III) species over a wide range of acidities studied. The N₂O formation in synthetic flue gas may be simulated using a relatively simple mechanism based on the model of Lyon and Cole. The first step of the complex overall reaction corresponds to NO oxidation by O₂ to NO₂ mainly in the gas phase, with the presence of both H₂O and active surfaces significantly accelerating NO₂ production. Subsequently, NO₂ reacts with excess NO to obtain HONO which reacts with S(IV) to result in N₂O and H₂SO₄ through a complex reaction sequence probably involving nitroxyl (HON) and its dimer, hyponitrous acid. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29 : 869–891, 1997.     

Comprehensive mechanistic study of ion pair SN2 reactions of lithium isocyanate and methyl halides

The anionic S_N2 reactions NCO^? + CH₃X and ion pair S_N2 reactions LiNCO + CH₃X (X = F, Cl, Br, and I) at saturated carbon with inversion and retention mechanisms were investigated at the level of MP2/6‐311+G(d,p). There are two possible reaction pathways in the anionic S_N2 reactions, but eight in the ion pair S_N2 reactions. Calculated results suggest that the previously reported T‐shaped isomer of lithium isocyanate does not exist. All the retention pathways are not favorable based on the analysis of transition structures. Two possible competitive reaction pathways proceed via two six‐member ring inversion transition structures. It is found that there are two steps in the most favorable pathway, in which less stable lithium cyanate should be formed through the isomerization of lithium isocyanate and nucleophilic site (N) subsequently attacks methyl halides from the backside. The thermodynamically and kinetically favorable methyl isocyanate is predicted as major product both in the gas phase anionic and the ion pair S_N2 reactions. In addition, good correlations between the overall barriers relative to separated reactants, ΔH , with geometrical looseness parameter %L^≠ and the heterolytic cleavage energies of the C? X and Li? N (or Li? O) bonds are observed for the anionic and ion pair S_N2 reactions. The trend of variation of the overall barriers predicts the leaving ability of X increase in the order: F < Cl < Br < I. The polarized continuum model (PCM) has been used to evaluate the solvent effects on the two inversion pathways with six‐member transition structures for the reactions of LiNCO + CH₃X. The calculations in solution indicate that solvent effects will retard the rate of reactions and the predicted product, methyl isocyanate, is same as the one in the gas phase. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Ab initio and density functional theory studies on the mechanism of nucleophilic vinylic substitution of 4<Emphasis Type="Italic">H</Emphasis>-pyran-4-one and 2-methyl-4<Emphasis Type="Italic">H</Emphasis>-pyran-4-one with ammonia

 Nucleophilic vinylic substitutions of 4H-pyran-4-one and 2-methyl-4H-pyran-4-one with ammonia were calculated by the B3LYP method using the 6-31G(d,p) basis set. Bulk solvent effects of aqueous solution were estimated by the polarized continuum and Poisson–Boltzmann self-consistent reaction field models using the 6-311+G(d,p) basis set. In the gas phase different mechanisms were found for the two reaction systems calculated. The reaction of 4H-pyran-4-one proceeds through enol, whereas a feasible path for the less reactive 2-methyl-4H-pyran-4-one is the mechanism through a keto intermediate. Addition of ammonia in concert with proton transfer is the rate-determining step ofthe reaction. The mechanism proceeding either by a bimolecular nucleophilic substitution (S_N2) or by one involving a tetrahedral zwitterionic intermediate is shown to be unlikely in the gas phase or nonpolar solution. The effects of bulk solvent not only consist in a reduction of the various activation barriers by about 25–40 kJ mol⁻¹ but also in a change in the reaction mechanism. Received 26 May 2002 / Accepted 26 July 2002 / Published online: 14 February 2003     

Theoretical investigation of ion pair SN2 reactions of alkali isothiocyanates with alkyl halides. Part 1. Reaction of lithium isothiocyanate and methyl fluoride with inversion mechanism

The gas‐phase ionic S_N2 reactions NCS^‐ + CH₃F and ion pair S_N2 reaction LiNCS + CH₃F with inversion mechanism were investigated at the level of MP2(full)/6‐311+G**//HF/6‐311+G**. Both of them involve the reactants complex, inversion transition state, and products complex. There are two possible reaction pathways in the ionic S_N2 reaction but four reaction pathways in the ion pair S_N2 reaction. Our results indicate that the introduction of lithium significantly lower the reaction barrier and make the ion pair displacement reaction more facile. For both ionic and ion pair reaction, methyl thiocyanate is predicted to be the major product, but the latter is more selective. More‐stable methyl isothiocyanate can be prepared by thermal rearrangement of methyl thiocyanate. The theoretical predictions are consistent with the known experimental results. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

TDDFT Study of the Electronic Structure,Absorption and Emission Spectra of the Light Emitters of the Amazing Firefly Bioluminescence and Solvation Effects on the Spectra

The ground and excited state properties of luciferin (LH₂) and oxyluciferin (OxyLH₂), the bioluminescent chemicals in the firefly, have been characterized using density functional theory (DFT) and time dependent DFT (TDDFT) methods. The effects of solvation on the electronic absorption and emission spectra of luciferin and oxyluciferin were predicted with a self‐consistent isodensity polarized continuum model of the solvent using TDDFT. The S₀→S₁ vertical excitation energies in the gas phase and in water were obtained. Optimizations of the excited state geometries permitted the first predictions of the fluorescence spectra for these biologically important molecules. Shifts in both of the absorption and emission spectra on proceeding from the gas phase to aqueous solution were also predicted.     

Hydration of the Calcium Dication: Direct Evidence for Second Shell Formation from Infrared Spectroscopy

Infrared laser action spectroscopy in a Fourier‐transform ion cyclotron resonance mass spectrometer is used in conjunction with ab initio calculations to investigate doubly charged, hydrated clusters of calcium formed by electrospray ionization. Six water molecules coordinate directly to the calcium dication, whereas the seventh water molecule is incorporated into a second solvation shell. Spectral features indicate the presence of multiple structures of Ca(H₂O)₇²⁺ in which outer‐shell water molecules accept either one (single acceptor) or two (double acceptor) hydrogen bonds from inner‐shell water molecules. Double‐acceptor water molecules are predominately observed in the second solvent shells of clusters containing eight or nine water molecules. Increased hydration results in spectroscopic signatures consistent with additional second‐shell water molecules, particularly the appearance of inner‐shell water molecules that donate two hydrogen bonds (double donor) to the second solvent shell. This is the first reported use of infrared spectroscopy to investigate shell structure of a hydrated multiply charged cation in the gas phase and illustrates the effectiveness of this method to probe the structures of hydrated ions.     

SN1‐SN2 and SN2‐SN3 mechanistic changes revealed by transition states of the hydrolyses of benzyl chlorides and benzenesulfonyl chlorides

Hydrolysis reactions of benzyl chlorides and benzenesulfonyl chlorides were theoretically investigated with the density functional theory method, where the water molecules are explicitly considered. For the hydrolysis of benzyl chlorides (para‐Z? C₆H₄? CH₂? Cl), the number of water molecules (n) slightly influences the transition‐state (TS) structure. However, the para‐substituent (Z) of the phenyl group significantly changes the reaction process from the stepwise (S_N1) to the concerted (S_N2) pathway when it changes from the typical electron‐donating group (EDG) to the typical electron‐withdrawing one (EWG). The EDG stabilizes the carbocation (MeO? C₆H₄? CH₂⁺), which in turn makes the S_N1 mechanism more favorable and vice versa. For the hydrolysis of benzenesulfonyl chlorides (para‐Z? C₆H₄? SO₂? Cl), both the Z group and n influence the TS structure. For the combination of the large n value (n > 9) and EDG, the S_N2 mechanism was preferred. Conversely, for the combination of the small n value and EWG, the S_N3 one was more favorable. © 2014 Wiley Periodicals, Inc.     

Isotope Effects in the Solvolysis of Sterically Hindered Arenesulfonyl Chlorides

Solvent isotope effects in the ethanolysis of sterically hindered arenesulfonyl chlorides ruled out a proton transfer in the rate‐determining step and agreed with a S_N2 mechanism involving at least a second solvent molecule in the transition state (TS). The lack of a secondary kinetic isotope effect in the o‐alkyl groups allows us to disregard the possible contribution of σ–π hyperconjugation. The measured activation parameters are consistent with a S_N2 mechanism involving the participation of solvent molecules in the TS, possibly forming a cyclic TS through a chain of solvent molecules.     

Transfer of Liquid Cathode Components to the Gas Phase and Their Effect on the Parameters of the Atmospheric Pressure DC Discharge

The transfer of solvent and components of dissolved substances from aqueous solutions to the gas phase under the action of atmospheric pressure air glow discharge was experimentally studied. solutions of NaCl, KCl, CuCl₂, MgCl₂, CaCl₂, SrCl₂, BaCl₂, NaNO₃, KNO₃, Ba(NO₃)₂, Na₂SO₄, K₂SO₄, CuSO₄ with concentrations of 0.1–0.5 mol/L were used as the cathodes of DC discharge at a current of 10–70 mA. The influence of the solution composition on cathode voltage drop and the electric field strength in plasma has been shown. Plasma emission spectra showed the appearance of metal atoms in the plasma requires threshold discharge current or threshold power input to the liquid cathode by ion bombardment. The threshold power values depend on the mass of hydrated cations and their concentration in solution. The efficiency of the transfer processes was characterized by transfer coefficients—the number of particles transferred from the liquid to the gas phase per one ion bombarding the cathode. Dependences of the transfer coefficients on the power dissipated in the cathode region and on the hydration energy of the cations were obtained. Experimental data on the rate of condensate accumulation in the special trap were used to estimate the concentrations of water molecules in the plasma.     

Frontside versus Backside SN2 Substitution at Group 14 Atoms: Origin of Reaction Barriers and Reasons for Their Absence

We have theoretically studied the gas‐phase nucleophilic substitution at group‐14 atoms (S_N2@A) in the model reactions of Cl^?+AH₃Cl (A=C, Si, Ge, Sn, and Pb) using relativistic density functional theory (DFT) at ZORA‐OLYP/TZ2P. Firstly, we wish to explore and understand how the reaction coordinate ζ, and potential energy surfaces (PES) along ζ, vary as the center of nucleophilic attack changes from carbon to the heavier group‐14 atoms. Secondly, a comparison between the more common backside reaction (S_N2‐b) and the frontside pathway (S_N2‐f) is performed. The S_N2‐b reaction is found to have a central barrier for A=C, but none for the other group‐14 atoms, A=Si–Pb. Relativistic effects destabilize reactant complexes and transition species by up to 10 kcal mol^?1 (for S_N2‐f@Pb), but they do not change relative heights of barriers. We also address the nature of the transformation in the frontside S_N2‐f reactions in terms of turnstile rotation versus Berry‐pseudorotation mechanism.