Quantum chemical studies on 1 charged forms of nitroglycerine

Semiempirical quantum chemical calculations at the level of AM1 (UHF) method are performed on the neutral as well as 1 charged systems originated from nitroglycerine. The geometry optimized systems standing for 1 charged forms fragmented. All the neutral and charged systems are found to be stable and exothermic with the exception of cationic system which is endothermic in nature. The fragmentation occurs by the cleavage of ester O–N bond of the terminal ester group in the charged forms while in the cationic form also the rapture of C–C bond occurs.     

The effect of hydrogen adsorption on the electronic structure of gold nanoparticles

It has been demonstrated that hydrogen adsorption has an effect on the electronic structure of gold nanoparticles. The physicochemical properties of separate gold nanoparticles have been studied under an ultrahigh vacuum scanning tunneling microscope. The structure and electronic structure of gold–hydrogen clusters were modeled by the quantum-chemical density functional theory method. Hydrogen adsorption onto gold nanoparticles 4–5 nm is size at room temperature was experimentally revealed, and the lower limit of 1.7 eV for the Au–H bond energy was determined. The interaction of hydrogen with gold leads to a considerable rearrangement of the electronic subsystem of nanoparticles. The experimentally observed effects were supported by quantum-chemical calculations. The rearrangement mechanism is related to strong correlations in the electronic subsystem.     

Competitive adsorption of model charged proteins: the effect of total charge and charge distribution

The adsorption of mixtures of charged proteins on charged surfaces is studied using a molecular theory. The theory explicitly treats each of the molecular species in the system. The mixtures treated in this work are composed by two types of proteins, dissociated monovalent salt and solvent. The intermolecular and surface interactions include electrostatic, van der Waals and excluded volume. The theory is more general than the Poisson-Boltzmann approach since the size and shape of all the molecular components are explicitly treated. The studies presented in this work concentrate on the differences in competitive adsorption when the proteins in the mixtures differ in their total charge or in the spatial distribution of the charges within the proteins. In the cases of mixtures that differ in the number of charges it is found, as expected, that the particles with the larger charge adsorb in excess. The ratio of adsorbed proteins can vary by 3-5 orders of magnitude by varying the bulk salt concentration from 1 to 100 mM. This is the result of an increase on the adsorption of the proteins with larger charge and an even stronger decrease on the adsorption of the less charged particles. The simple model systems studied provide guidelines on how to separate charge ladder proteins and proteins with different charge distributions. In the case of proteins with the same total charge but different charge distribution, it is found that the partition of the proteins depends upon the bulk composition. However, in general the particles with the highest localized charge tend to adsorb more on the surfaces. The proteins are adsorbed in one or more layers. The structure of the second adsorbed layer is determined mostly by the bulk properties of the solution. In all cases it is found that in the range of salt concentrations studied the number of adsorbed ions from the salt is very large. This is due to competitive adsorption with the proteins and their very low bulk concentration compared to the salt. The limitations of the theory and directions for improvement of the approach as well as the model for the proteins are discussed.     

Quantum studies of hydrogen bonding in formic acid and water ice surface

The structure and spectroscopy (electronic and vibrational) of formic acid (HCOOH) dimers and trimers are investigated by means of the hybrid (B3LYP) density-functional theory. Adsorption of single and dimer HCOOH on amorphous water ice surface is modeled using two different water clusters. Particular attention has been given to spectroscopic consequences. Several hypotheses on formic acid film growing on ice and incorporation of a single water molecule in the formic acid film are proposed.     

Quantum chemical investigation of the adsorption of methanol on a cluster model of faujasite

The adsorption of methanol in zeolites of the faujasite type with sodium and calcium counter ions is studied with quantum chemical methods. The zeolite is represented with a cluster model allowing calculations at the Møller-Plesset as well as the DFT level of theory. An adsorption energy of −62.4 kJ/mol is calculated at the MP2/TZVP//BP86/TZVP level of theory for one methanol molecule at one site II sodium cation. Insight into the adsorption process is obtained with a three-body decomposition which reveals a strong destabilisation of the adsorption strength by large, positive three-body terms, which is important for force field development.     

Quantum chemical model for the adsorption of organic molecules on metal surfaces in electrolyte solutions

A quantum chemical model is proposed for the adsorption of organic molecules on metal surfaces in electrolyte solutions. This approach is based on the method of electrostatic images modefied for the case of a double electrical layer within the framework of the PPP method. The calculated data are in good accord with the available experimental results. Donbas State Building and Architecture Academy, 2 Derzhavina ul., Makeevka 339023, Donetsk Oblast', Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 5, pp. 284–288, September–October, 1999.     

C~6~0SiH~2的结构和电子光谱的量子化学研究

用INDO系列方法研究了C~6~0SiH~2的两种结构: 一是SiH~2加在两个六元环之间的键上形成C~2~v构型; 另一是SiH~2加在一个五元环和一个六元环之间的键上形成C~s构型。从总能量和LUMO-HOMO能级差看, C~6~0SiH~2的稳定结构应是C~2~v构型, 其中桥C(15)-C(30)的键长为0.1508nm, 键序为0.9369, 说明不开环, 形成类环丙烷结构。文中计算了两种构型的电子吸收光谱和NMR谱, 此类计算是基于对C~6~0SiH~2的等电子体C~6~0O和C~6~0CH~2的研究之上, 且后两者的研究结果与实验相一致。     

Structure and electronic spectra of 2-phenylbenzimidazoles. Quantum chemical analysis of the intramolecular hydrogen bond

Institute of High-Molecular-Weight Compounds, Russian Academy of Sciences. Leningrad State University Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 5, pp. 20–27, September–October, 1992.     

Quantum chemical modelling of ethene epoxidation with hydrogen peroxide: role of catalytic sites

Ethene epoxidation with hydrogen peroxide was studied on hydroxylated binuclear metal sites, using DFT calculations at the B3LYP/6-311+G(d,p) level of theory. A decrease of the activation enthalpy of approximately 100 kJ mol(-1) was observed compared to the gas phase reaction between hydrogen peroxide and ethene. It was previously shown that micro-solvation with water reduces the activation enthalpy by approximately 77 kJ mol(-1) and only the additional 24 kJ mol(-1) can be attributed to the binuclear site. Three different metal centres were tested, Ti(iv), Si(iv) and Ge(iv), in order to investigate any specific role of the metal centre on the activation enthalpy. The results clearly show that the activation enthalpy is independent on the nature of the metal centre. This emphasises the role of the hydrogen bonded network provided by the hydroxylated metal sites, on the stabilisation of the transitions state. In ref. 1 (A. Lundin, I. Panas and E. Ahlberg, J. Phys. Chem. A, 2007, 111, 9080) it was demonstrated that, at the transition state and upon micro-solvation, the hydrogen peroxide entity becomes polarized within the hydrogen bonding network, forming a negatively-charged fragment distant from the ethene molecule and a positively-charged fragment directly involved in the oxygen insertion step. The same mechanism was found to hold also for the reaction at the binuclear catalytic site, since the required hydrogen bonding is effectively provided by the hydroxylated metal centres. This mechanism is compared to the two-step pathway which employs a metal peroxide intermediate. Both reaction channels were found to be plausible in confined environments.     

Quantum chemical studies on polythiophenes containing heterocyclic substituents: effect of structure on the band gap

Color tuning by the tailoring of substituents at the 3-position of thiophene is very encouraging, and comparative experimental and theoretical studies proved to be powerful in the search for a suitable design for the above. Since the novel polythiophene-based materials substituted with five-membered/six-membered ring containing sulphur and nitrogen at different positions are proven to be potential candidates for electron-transporting hole blocking functions, the structure-property relationship of these systems have been focused in the present study. Molecular-orbital calculations are applied to obtain the optimized geometries and band gaps of the thiophene oligomers. An oligomeric approach has been implemented for calculating the band gaps, and the theoretically obtained band gaps for the different model compounds are compared. Density-functional theory B3LYP6-31G* predicted band-gap values are compared with the experimental band gaps obtained from optical-absorption edge. The predicted values show little deviations from experimental band gaps, but the trend in band gap is found to be the same in experimental and theoretical results in most of the cases. Hence, this study illustrates the usefulness of quantum-mechanical calculations in understanding the effects of various structural parameters on optical band gap.     

Environmental effects on hydrogen bonded systems: A quantum chemical study of proton relay model systems

In this paper an application of a reaction field theory of solvent effects has been made to study proton transfer mechanisms in hydrogen bonded systems coupled to an environment. The latter is simulated with reaction fields having variable strength and direction (defined with respect to the supermolecule's total dipole moment direction), together with superposed uniform external electric fields. Changes in proton potential curves and some other properties of a model water dimer and a water trimer are reported. The results are discussed in relation to relevant phenomena in biology and biochemistry, namely proton relay systems in enzymatic catalysis.     

氨在丝光沸石位置(VI)上的吸附

The adsorption of NH3 on (AO)3SiO*HAl(OA)2OSi(OA)3 cluster from the structure of site (VI) of mordenite Zeolite has been calculated by an adsorption model of hydrogen-bond form, quantum chemical method, CNDO/2 scheme. Variations of adsorption distance between NH3 and O* in the cluster were directly related to initial adsorption heats of NH3 on O*H group, jump frequency of the proton between NH3 and O* and frequency shift of IR band with O*H group. Adsorption distance of NH3 on O*H group has been calculated, the range was about 2.5 to 2.75A, and the most favourable adsorption distance with strong acid strength was 2.5A. It was related to the maximum initial adsorption heat which was 33.80 kcal/mol, the lowest activation energy of proton mobility between NH3 ane O*, and the largest frequency shift of IR bands with O*H group. This result qualitatively corresponded to experimental observations. It showed that the various representations of acidic strength are equivalent to each other which could be explained on basis of adsorptin model of hydrogen bond form.     

Quantum chemical study of the effect of hydrogen bromide-water associates on the bromination of 1-heptene

The MNDO method was used to study the extremal points on the potential energy surface of the reaction of bromine and 1-heptene. The formation of 1,2-dibromoheptane is energetically favored relative to formation of 1-bromo-2-heptene. (H₂O)_nHBr associates reduce the activation energy of the transition state and act as a strong catalytic agent for the molecular reaction. Lvov Research Institute of Judicial Examination, 7 Soborna pl., Lvov 290004, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 34, No. 4, pp. 239–243, July–August, 1998.     

Quantum chemical modeling of ethene epoxidation with hydrogen peroxide: the effect of microsolvation with water

Quantum chemical calculations were performed to study the mechanism of ethene epoxidation with hydrogen peroxide. The calculations were carried out at the B3LYP/6-311+G(d,p) level of theory. The applicability of this functional to the problem at hand, including basis set effects, was validated by CCSD(T) and CASSCF based multireference MP2 calculations. A mechanism was determined where hydrogen peroxide becomes polarized in the transition state upon binding to the ethene molecule. The distant hydroxide fragment of the attached hydrogen peroxide molecule becomes partly negatively charged, while the other part of the molecule involves a proton and becomes partly positively charged. In the absence of water an activation energy of 139.7 kJ mol(-1) was determined for the isolated H(2)O(2) + C(2)H(4) system. By microsolvating with water, the impact of a hydrogen-bonded network on the activation energy was addressed. A 43.7 kJ mol(-1) lowering of the activation energy, DeltaE(a), was observed when including up to 4 water molecules in the model. This effect results from the stabilization of the proton and hydroxide fragments in the transition state. The findings are discussed in the context of previous theoretical studies on similar systems. Effects of adding or removing a proton to mimic acidic and alkaline conditions are addressed and the limitations of the model in solvating the excess charge are discussed.     

Layer-to-layer adsorption of oppositely charged polyelectrolytes: The effect of molecular mass: 2. Bilayer systems

The effect of molecular mass on the formation of a bilayer structure upon the layer-to-layer adsorption of a cationic polyelectrolyte (poly(dimethyldiallylammonium chloride), molecular mass M = 500000 and 100000-200000 Da) and an anionic polyelectrolyte sodium (poly(acrylate), M = 30000 and 2100 Da) on the surface of fused quartz is studied by the capillary electrokinetic method. The time required to reach constant adsorption values and the structure of bilayer systems depend on the ratio between molecular masses of the cationic and anionic polyelectrolytes. The deformability of the bilayer system significantly exceeds that of the first layer in the case when the second layer is formed from an anionic polyelectrolyte with a lower molecular mass, thus suggesting the loosening of the first adsorption layer of the cationic polyelectrolyte. The adsorption of the anionic polyelectrolyte with higher molecular mass insignificantly affects the density of the first layer. Variation in the deformability of the layer with time (its aging) depends on the ratio between molecular masses of the polyelectrolytes.     

Molecular dynamics simulation studies of the transport and adsorption of a charged macromolecule onto a charged adsorbent solid surface immersed in an electrolytic solution

Molecular dynamics simulations were performed in order to study the transport and adsorption of a charged macromolecule (desmopressin) onto a charged solid surface in an electrolytic solution. The strong Coulombic interaction from the charged solid surface represents the major force for accelerating, orienting, entrapping in the electrical double layer, and adsorbing the macromolecule onto the charged solid surface. The macromolecule is flattened as it approaches the charged surface, giving rise to a stronger surface exclusion effect that shields surface sites. When adsorbed, the macromolecule is restrained by a surface interaction more than one hundred times stronger than the thermal energy, of which 99.8% results from the strong dominant Coulombic interaction, and trapped by a hydration layer adjacent to the surface. This leads to zero lateral displacement of the adsorbed macromolecule and indicates that surface diffusion is a physically implausible mechanism in similar systems. Explicit solvent is required for realistic representation of the macromolecular structure and the surface interaction energy. The adsorbed macromolecule also decreased the electrostatic potential gradient perpendicular to the charged solid surface and introduced additional electrostatic potential gradients laterally. The results obtained from the molecular dynamics simulations confirm the importance of electrophoretic migration and support the physical mechanisms used in a macroscopic continuum model that predicts an overshoot in the concentration of a charged macromolecule in the adsorbed phase under certain conditions of pH and ionic strength.     

The effect of electric current on chemical bonding of hydrogen adsorption on an aluminum nanowire

The effect of electric current on hydrogen adsorption on an aluminum nanowire surface is studied by using nonequilibrium Green's function method. We choose the models studied in the previous work of one of the authors as an aluminum nanowire model and a hydrogen-adsorbed one. These nanowire models have conductive ability, because the aluminum part of these models is metallic. It is confirmed that electric current affects the strength of the adsorption of hydrogen atoms, and the change of the bonding of hydrogen to aluminum nanowire surface is larger for larger current. However, the change of the chemical bonding is negligibly small within the bias voltage ≤0.5 V.     

Quantum chemical simulations of water adsorption on a diamond (100) surface with vacancy defects

Results from quantum chemical calculations of the structural, electronic, and energy characteristics of the chemisorption of water on a diamond C(100)-(2 × 1) surface with a vacancy defect are presented. The metastable state of the surface with an adsorbed H₂O molecule and possible configurations of the surface with adsorbed -H and -OH water dissociation fragments are described. It is shown that the presence of a vacancy on the surface decreases the activation energy of the dissociative adsorption of a water molecule.     

Metal bridge effect: V. Quantum chemical studies of some metal chelates

Metal chelates [(C₂H₂X₂)₂M]ⁿ with ligating atoms X = NH, O, S and central atoms M = Be²⁺, Mg²⁺, Ni²⁺, Zn²⁺ (n = 0, ± 2), and M = Li⁺, Cu⁺ (n = ± 1, ?3), have been studied in our Laboratory for some years by ab initio calculations. In this article it is shown that certain features of the complexes are the same for all the different metals in our series. These features include a drastic reduction of the energy gap between unoccupied and occupied orbitals, when electrons are added to the positively charged complexes. This change of the energy gap is shown to be an effect of the ligand dimer [(C₂H₂X₂)₂]ⁿ. But this dimer can only exist when a positive ion, e.g., a metal ion, forms a bridge between the two monomers. The reduced energy gap implies a strong bathochromic shift of the electronic spectrum and low electrical resistivity.