Quantum Theoretical Studies Research Articles

The pattern of bifurcated hydrogen bonds in thiourea cocrystals with diazine derivatives: experimental and quantum theoretical studies.

Cocrystals of thiourea with pyrazine N-oxide as thiourea-pyrazine N-oxide (2/1), C4H4N2O·2CH4N2S, (I), and with phenazine as thiourea-phenazine (6/7), 7C12H8N2·6CH4N2S, (II), both crystallize in the monoclinic space group P21/c. In the crystalline state, molecules of both components are linked by N-H...N hydrogen bonds. In addition, there are R22(8) hydrogen-bond synthons between thiourea molecules in both crystal structures. Furthermore, bifurcated hydrogen bonds between the -NH groups in the thiourea molecule and the N and O atoms in the N-oxide ring [in (I)], as well as the N atom in the central phenazine ring [in (II)], play a significant role in both structures. This emerging motif was thoroughly examined using quantum chemistry methods.

4f/5d Hybridization Induced Single-Electron Delocalization in an Azide-Bridged Dicerium Complex.

Dilanthanide complexes with one-electron delocalization are important targets for understanding the specific 4f/5d-bonding feature in lanthanide chemistry. Here, we report an isolable azide-bridged dicerium complex 3 [{(TrapenTMS)Ce}2(μ-N3)]• [Trapen = tris (2-aminobenzyl)amine; TMS = SiMe3], which is synthesized by the reaction of tripodal ligand-supported (TrapenTMS)CeIVCl complex 2 with NaN3. The structure and bonding nature of 3 are fully characterized by X-ray crystal diffraction analysis, electron paramagnetic resonance (EPR), magnetic measurement, cyclic voltammetry, X-ray absorption spectroscopy, and quantum-theoretical studies. Complex 3 presents a trans-bent central Ce-N3-Ce unit with a single electron of two mixed-valent Ce atoms. The unique low-temperature (2 K) anisotropic EPR signals [g = 1.135, 2.003, and 3.034] of 3 indicate that its spin density is distributed on the central Ce-N3-Ce unit with marked electron delocalization. Quantum chemical analyses show strong 4f/5d orbital mixing in the singly occupied molecular orbital of 3, which allows for the unpaired electron to extend throughout the cerium-azide-cerium unit via a multicentered one-electron (Ce-N3-Ce) interaction. This work extends the family of mixed-valent dilanthanide complexes and provides a paradigm for understanding the bonding motif of ligand-bridged dilanthanide complexes.

Geminal-atom catalysis for cross-coupling.

Single-atom catalysts (SACs) have well-defined active sites, making them of potential interest for organic synthesis1-4. However, the architecture of these mononuclear metal species stabilized on solid supports may not be optimal for catalysing complex molecular transformations owing to restricted spatial environment and electronic quantum states5,6. Here we report a class of heterogeneous geminal-atom catalysts (GACs), which pair single-atom sites in specific coordination and spatial proximity. Regularly separated nitrogen anchoring groups with delocalized π-bonding nature in a polymeric carbon nitride (PCN) host7 permit the coordination of Cu geminal sites with a ground-state separation of about 4 Å at high metal density8. The adaptable coordination of individual Cu sites in GACs enables a cooperative bridge-coupling pathway through dynamic Cu-Cu bonding for diverse C-X (X = C, N, O, S) cross-couplings with a low activation barrier. In situ characterization and quantum-theoretical studies show that such a dynamic process for cross-coupling is triggered by the adsorption of two different reactants at geminal metal sites, rendering homo-coupling unfeasible. These intrinsic advantages of GACs enable the assembly of heterocycles with several coordination sites, sterically congested scaffolds and pharmaceuticals with highly specific and stable activity. Scale-up experiments and translation to continuous flow suggest broad applicability for the manufacturing of fine chemicals.

2,2'-Dithiobispyrazine: about the disulfide bond.

X-ray diffraction studies reveal that pyrazine-2-thiol undergoes condensation to 2,2'-dithiobispyrazine [systematic name: 2-(pyrazin-2-yldisulfanyl)pyrazine], C8H6N4S2 (I), under aerial conditions. In the molecule of I, the pyrazine rings are arranged in an almost perpendicular manner, with an absolute value of the C-S-S-C torsion angle of -91.45 (6)°. A search in the Cambridge Structural Database confirmed that such a conformation is typical for disulfide compounds. Three different rotamers of disulfide I were studied using quantum theoretical studies. The rotamer of lowest energy was observed in the crystalline state in the structure stabilized by hydrogen-bond, chalcogen-bond and stacking interactions. Further quantum chemical computations confirm that 2,2'-dithiobispyrazine can react according to the SN2 mechanism.

Quantum chemical and theoretical kinetics studies on the reactions of hydroperoxy radical with methanethiol and ethanethiol

An exhaustive and theoretical analysis for the CH3SH + HOO• and CH3CH2SH + HOO• reactions has been examined using quantum chemistry theory. The potential energy surface was obtained by the CCSD(T)/CBS//M06-2X/cc-pVTZ level of theory. The overall rate coefficients have been calculated and the energetics and reaction enthalpies were also calculated. Two kinds of reaction channels have been analyzed: hydrogen abstraction channel and CH3-transfer channel. The calculated kinetics demonstrate that the hydrogen abstraction from the CH3 group in CH3SH and the CH2 group in CH3CH2SH are major channels at low temperature region. However, the CH3-transfer channels become competitive reaction channels with the increase in temperature.

The role of sulfur interactions in crystal architecture: experimental and quantum theoretical studies on hydrogen, halogen, and chalcogen bonds in trithiocyanuric acid–pyridine N-oxide co-crystals

Hydrogen, halogen, chalcogen bonds and π interactions of the trithiocyanuric acid ring are responsible for crystal structure architecture and have been classified according to the QTAIM approach as closed-shell interactions.

Quantum chemical and theoretical kinetics studies on the reaction of hydroperoxyl radical with chlorine atom

The singlet and triplet potential energy surfaces for the reaction of HO2 radical with Cl atom are explored by using high-level combination W1BD and W1U methods. Statistical rate theories are employed to compute the rate coefficients of the main product channels including HCl + O2( $$ ^{1} \Sigma _{u}^{ - } $$ ), HCl + O2( $$ ^{3} \Sigma _{g}^{ + } $$ ), OH + ClO and HOOCl as a function of pressure and temperature. It is found that the rate coefficients for the product channels OH + ClO and HOOCl are strongly depended on temperature and pressure. At lower pressures and higher temperatures, the product OH + ClO is important, while at higher pressures and lower temperatures, HOOCl is the dominant product. At some moderate temperatures and pressures, the rate coefficients for the formation of OH + ClO and HOOCl could be equal. The product channel HCl + O2( $$ ^{3} \Sigma _{g}^{ + } $$ ) proceeds via formation of a van der Waals complex Cl..HOO and, next, passes through a saddle point structure with a negative energy of − 5.0 kJ mol−1 relative to the reactants. It is shown that a two transition state model suitably describes the kinetics of this product channel.

Tipping the Balance between Ligand and Metal Protonation due to Relativistic Effects: Unusually High Proton Affinity in Gold(I) Pincer Complexes.

Quantum theoretical studies show that the extremely high proton affinity at the metal center of the unusual T-shaped (LXL)AuI -pincer complex, consisting of a carbazole framework and two mesoionic carbenes, is caused by relativistic effects. This brings the basicity of the AuI center in line with the electron-rich nitrogen atom of the carbazole ring system, resulting in one of the highest proton affinities for a neutral molecule.

Nonquilibrium and Equilibrium Stationary States of Zwitterionic Surfactant Dynamic Adsorption on Limestone Cores at Oil-Reservoir Conditions

Dynamic adsorption of a zwitterionic-type surfactant cocamidopropyl hydroxysultaine on limestone cores at oil reservoir conditions, i.e., under 2000 psi pressure, 130 °C temperature, and in the presence of seawater with 30 800 ppm NaCl, has been studied through core-flood displacement tests and in the absence of oil, at an injection rate of 120 cm3·h–1 up to 60 pore volumes (PV). The amount of adsorbed surfactant has been determined from mass balance by calculating its concentration through high performance liquid chromatography (HPLC). The normalized surfactant concentrations (NSC) have been used in terms of the PV, and then they are obtained the corresponding dynamic adsorption curves. Dynamic adsorption isotherm curves of the zwitterionic surfactant diluted in seawater showed a steep rise between 1 and 3 pore volumes (PV), and the saturation conditions were kept up to 60 PV. An average surfactant adsorption of 0.1138 ± 0.0126 mg·g–1 was determined. Quantum theoretical studies show that the electronic energy gap of (104) the limestone is electrically neutral, preferably acquiring a negative charge tendency, these results imply that the calcite surface will preferentially interact with positive ions, so that the cations present in the solution interacting in the limestone, resulting in a positive charge in the rock that implies that the surfactant cocamidopropyl hydroxysultaine interacts in the rock by the anionic group.

Three centered hydrogen bonds of the type C=O···H(N)···X-C in diphenyloxamide derivatives involving halogens and a rotating CF3 group: NMR, QTAIM, NCI and NBO studies.

The existence of three centered C=O···H(N)···X-C hydrogen bonds (H-bonds) involving organic fluorine and other halogens in diphenyloxamide derivatives has been explored by NMR spectroscopy and quantum theoretical studies. The three centered H-bond with the participation of a rotating CF3 group and the F···H-N intramolecular hydrogen bonds, a rare observation of its kind in organofluorine compounds, has been detected. It is also unambiguously established by a number of one and two dimensional NMR experiments, such as temperature perturbation, solvent titration, (15)N-(1)H HSQC, and (19)F-(1)H HOESY, and is also confirmed by theoretical calculations, such as quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO) and non-covalent interaction (NCI).

Electrochemical synthesis of poly-3-amino-5-mercapto-1,2,4-triazole on copper and its protective effect in 3.5 % NaCl medium

Electropolymerization of 3-amino-5-mercapto-1,2,4-triazole (AMTA) on copper was carried out in methanol/NaOH solution by cyclic voltammetry. Chronoamperometry technique was used to investigate the formation kinetics of the polymer film. The effect of scan rate was also studied. Electropolymerized film of AMTA was duly characterized by Fourier transform infrared spectroscopy. Scanning electron microscopy was used to analyze the surface morphology of the polymer film. The protective ability of poly-3-amino-5-mercapto-1,2,4-triazole (p-AMTA) film was determined by electrochemical impedance spectroscopy and potentiodynamic polarization studies in NaCl medium. The results of the study showed that the p-AMTA film on copper afforded significant corrosion protection in neutral medium. Quantum theoretical studies revealed the adsorptive behavior of AMTA on copper.

Electron Microscopy Study of Gold Nanoparticles Deposited on Transition Metal Oxides

Many researchers have investigated the catalytic performance of gold nanoparticles (GNPs) supported on metal oxides for various catalytic reactions of industrial importance. These studies have consistently shown that the catalytic activity and selectivity depend on the size of GNPs, the kind of metal oxide supports, and the gold/metal oxide interface structure. Although researchers have proposed several structural models for the catalytically active sites and have identified the specific electronic structures of GNPs induced by the quantum effect, recent experimental and theoretical studies indicate that the perimeter around GNPs in contact with the metal oxide supports acts as an active site in many reactions. Thus, it is of immense importance to investigate the detailed structures of the perimeters and the contact interfaces of gold/metal oxide systems by using electron microscopy at an atomic scale. This Account describes our investigation, at the atomic scale using electron microscopy, of GNPs deposited on metal oxides. In particular, high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are valuable tools to observe local atomic structures, as has been successfully demonstrated for various nanoparticles, surfaces, and material interfaces. TEM can be applied to real powder catalysts as received without making special specimens, in contrast to what is typically necessary to observe bulk materials. For precise structure analyses at an atomic scale, model catalysts prepared by using well-defined single-crystalline substrates are also adopted for TEM observations. Moreover, aberration-corrected TEM, which has high spatial resolution under 0.1 nm, is a promising tool to observe the interface structure between GNPs and metal oxide supports including oxygen atoms at the interfaces. The oxygen atoms in particular play an important role in the behavior of gold/metal oxide interfaces, because they may participate in catalytic reaction steps. Detailed information about the interfacial structures between GNPs and metal oxides provides valuable structure models for theoretical calculations which can elucidate the local electronic structure effective for activating a reactant molecule. Based on our observations with HRTEM and HAADF-STEM, we report the detailed structure of gold/metal oxide interfaces.

ChemInform Abstract: Theoretical Studies of Atmospheric Reaction Mechanisms in the Troposphere

AbstractReview: 455 refs.

Quantum chemical and theoretical kinetics studies on the reaction of carbonyl sulfide with H, OH and O(3P)

The potential energy surfaces for the reaction of COS with H, OH and O(3P) are explored by using various quantum chemical methods including W1BD, CBS-QB3, G4, G3B3, CCSD(T), QCISD(T), CCSD, QCISD, M06-2X, B2PLYPD, mPW2PLYPD, ωB97X-D and BB1K. Transition state theory and a modified strong collision/RRKM model are employed to calculate the thermal rate coefficients for the formation of major products as a function of temperature and pressure. The computed rate constants and branching ratios are compared with the available experimental data. It is found that the reaction of hydrogen atom with carbonyl sulfide is the fastest chemical sink for COS degradation. The calculated rate coefficients for COS+OH reaction, at various temperatures and pressures, are in accordance with the experimental observations denying the importance of this reaction in the atmosphere.

Theoretical studies of atmospheric reaction mechanisms in the troposphere

The chemistry of the atmosphere encompasses a vast number of reactions acting on a plethora of intermediates. These reactions, occurring sequentially and in parallel, give rise to intertwined and irreducible mechanisms describing the complex chemical transformations of organic and inorganic compounds in the atmosphere. The complexity of this system is that it requires combined experimental, theoretical, and modeling approaches to elucidate the characteristics of the individual reactions, and their mutual interaction. In this review, we describe recent results from quantum chemical and theoretical kinetic studies of relevance to atmospheric chemistry. The review first summarizes the most commonly used theoretical methodologies. It then examines the VOC oxidation initiation channels by OH, O(3), NO(3) and Cl, followed by the reactions of the alkyl, alkoxy, alkylperoxy and Criegee intermediates active in the subsequent oxidation steps. Specific systems such as the oxidation of aromatics and the current state of knowledge on OH-regeneration in VOC oxidation are also discussed, as well as some inorganic reactions.

Cold collisions of an open-shell S-state atom with a 2Π molecule: N(4S) colliding with OH in a magnetic field

We present quantum-theoretical studies of collisions between an open-shell S-state atom and a (2)Π-state molecule in the presence of a magnetic field. We analyze the collisional Hamiltonian and discuss possible mechanisms for inelastic collisions in such systems. The theory is applied to the collisions of the nitrogen atom ((4)S) with the OH molecule, with both collision partners initially in fully spin-stretched (magnetically trappable) states, assuming that the interaction takes place exclusively on the two high-spin (quintet) potential energy surfaces. The surfaces for the quintet states are obtained from spin-unrestricted coupled-cluster calculations with single, double, and noniterative triple excitations. We find substantial inelasticity, arising from strong couplings due to the anisotropy of the interaction potential and the anisotropic spin-spin dipolar interaction. The mechanism involving the dipolar interaction dominates for small magnetic field strengths and ultralow collision energies, while the mechanism involving the potential anisotropy prevails when the field strength is larger (above 100 G) or the collision energy is higher (above 1 mK). The numerical results suggest that sympathetic cooling of magnetically trapped OH by collisions with ultracold N atoms will not be successful at higher temperatures.

Vacuum ultraviolet laser pulsed field ionization-photoelectron study of allyl radical CH2CHCH2

The pulsed field ionization-photoelectron (PFI-PE) spectrum of allyl radical CH2CHCH2 (C3H5) in the energy range of 65 200-66 600 cm-1 has been measured using vacuum ultraviolet laser. Based on the simulation of the rotational structures resolved in the vibrational PFI-PE bands of C3H5+(X 1A1;0(0+) and nu7+=1), the ionization energies (IEs) of C3H5(X 2A2;0(0)) to form C3H5+(X 1A1;0(0+) and nu7+=1) are determined to be 65 584.6+/-2.0 cm-1 (8.131 46+/-0.000 25 eV) and 66 020.9+/-2.0 cm-1 (8.185 56+/-0.000 25 eV), respectively, where nu7+(a1) is the symmetric C-C-C bending mode of C3H5+(X 1A1). These values are compared to IE(C3H5) values obtained in previous experimental and high-level ab initio quantum theoretical studies.

Catalytic role of enzymes: short strong H-bond-induced partial proton shuttles and charge redistributions.

A two-step reaction mechanism (catalyzed alternatively by acid and base) with partial proton shuttles and charge redistributions promoted by short strong H bonds (SSHBs) (playing a dual role as an amphi-acid/base catalyst) is proposed to explain the enormous rate enhancement observed in enzymatic reactions involving carbanion intermediates. The SSHBs in the two-step reactions are found to be responsible for enhancing enzyme-substrate interactions in favor of the transition state structure over that of reactant. The detailed quantum theoretical studies of ketosteroid isomerase provide evidence of assisting roles of SSHB in enzymatic activity. The understanding of the two-step reaction mechanism would be a useful aid in designing novel functional enzymes and abzymes.

Scattering theory for quasi-one-dimensional tunneling structures

Quantum theoretical studies of semiconductor microstructures are most naturally done in terms of one-dimensional scattering states, which are characterized far from a structure by k-dependent reflection and transmission amplitudes. We have investigated these states using integral forms such as the Lippmann-Schwinger equation. These allow us to obtain global properties of the states and provide the basis for a formal scattering theory of the kind that has been developed for the conventional problem of three-dimensional potential scattering. We find orthonormality relations for the scattering states, the one-dimensional analogues of Wigner's inequality and Levinson's theorem, and associated properties of the complex-momentum transmission amplitude.

Distorted-wave calculations for the three dimensional chemical reaction H + H2(v ⩽ 2, j = 0) → H2(v′ ⩽ 2, j′, mj) + H

Calculations of the vibrational—rotational product state population distributions and differential cross sections for the chemical reaction H + H2(v ⩽ 2, j = 0) → H2(v′ ⩽ 2, j′, mj) + H have been carried out on the Porter—Karplus potential energy surface. The vibrationally-adiabatic-distorted-wave (VADW) method has been used. The relative rotational product distributions, differential cross sections and the helicity mj, dependences of these quantities for the v = 0 reaction agree well with accurate close-coupling results. The absolute integral cross sections are considerably smaller than the accurate quantum values, however. The calculations for the v = 1 reaction agree with the findings of previous sudden quantum, limited close-coupling and quasiclassical theoretical studies and experiments that product H2(v′ = 1) is more likely to be produced than H2(v′ = 0). For the reaction with v = 2, it is found that at high translational energies product H2(v′ = 2) is favoured over H2(v′ = 1) or H2(v′ = 0). The VADW differential cross sections for the v = 1 and v = 2 reactions have a similar shape to those of the v = 0 reaction, with backward peaking when summed over all mj states. The relative rotational distributions for the v = 2, j = 0 → v′ = 2, j and v = 1, j = 0 → v′ = 1, j reactions are also similar to those obtained for the v = 0, j = 0 → v′ = 0, j reaction, with low rotational excitation.