Properties Of Electron Density Distributions Research Articles

Reaction mechanism of dimethyl carbonate synthesis on Cu/β zeolites: DFT and AIM investigations

The mechanism of dimethyl carbonate (DMC) synthesis on Cu-exchanged zeolite β has been investigated employing density functional theory (DFT) calculations and a double numerical plus polarization (DNP) basis set. The adsorption energy (ΔE) and decomposition activation energy (Ea) for O2 are −1.84 and 1.72 eV, respectively, suggesting that the decomposition of O2 occurs readily under reaction conditions on the Cu site. The formed O atom further reacts with methanol to form surface-bound (CH3O)(OH)–Cu(I)/β, in which CH3O and OH were coadsorbed on the Cu+ of the catalyst; this process proceeds without an activation barrier and with an energy release of 1.23 eV. The (CH3O)(OH)–Cu(I)/β species then reacts with another methanol molecule and carbon monoxide to produce DMC through two different reaction pathways. In path I, insertion of carbon monoxide into the (CH3O)(OH)–Cu(I)/β leads to the formation of monomethyl carbonate species (CH3OCOOH), which then reacts with methanol to produce DMC and H2O. The activation energies for both steps are 0.97 and 0.65 eV, respectively. In path II, (CH3O)(OH)–Cu(I)/β reacts with methanol first to produce a dimethoxide species ((CH3O)(CH3O)–Cu(I)/β), and the formation of DMC is via the insertion of carbon monoxide into the (CH3O)(CH3O)–Cu(I)/β. The activation energies for these elementary reactions are 0.65 and 0.70 eV, respectively. The topological properties of electron density distributions for all the related stationary points involved in this reaction have also been examined using the atoms in molecule (AIM) theory for the illustration of the bond paths and weak interactions of all the stationary points in the reaction path.

A comparative study of open‐close and related rotamers methods to evaluate the intramolecular hydrogen bond energies in 3‐imino‐propen‐1‐ol and its derivatives

AbstractMP2 study of OH…N intramolecular hydrogen bond (IMHB) in 3‐imino‐propen‐1‐ol and its derivatives were performed and their IMHB energies were obtained using the related rotamers and open‐close methods. Also the topological properties of electron density distribution and charge transfer energy associated with IMHB were gained by quantum theory of atoms in molecules and natural bond orbital theory, respectively. The computational results reveal that the related rotamers method energies are well correlates with geometrical parameters, topological parameters at hydrogen bond and ring critical points, integrated properties, proton transfer barrier and charge transfer energy of OH…N unit. Surprisingly, it was found that the open‐close hydrogen bond energies cannot represent good linear correlations with these parameters. Consequently, we extrapolate a number of equations that can be used in estimation of OH…N IMHB energy in complex biological systems. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

O [sbnd]H⋯S intramolecular hydrogen bond in thiomalonaldehyde derivatives; a quantum chemical study

O H⋯S intramolecular hydrogen bond (IHB) in thiomalonaldehyde and its various derivatives were studied at MP2/6-311++G (d, p) level of theory and corresponding IHB energies were obtained using the related rotamers and open–close methods. In addition, the topological properties of electron density distribution and charge transfer energy associated with intramolecular hydrogen bond were gained by quantum theory of atoms in molecules and natural bond orbital theory, respectively. The results of this study showed that the related rotamers hydrogen bond energy correlates well with geometrical parameters such as O⋯S and H⋯S distances, topological parameters such as electron density, its Laplacian and electronic potential energy density, V( r cp), at hydrogen bond critical point and ring critical point as well as charge transfer energies in chelated ring. Surprisingly, it was found that the open–close hydrogen bond energies can not represent similar linear or even polynomial correlations with these parameters. Consequently, the open–close method is not suitable for estimation of the IHB energy and erroneous, while the related rotamers method is convenient for estimation of the IHB energy.

Reinvestigation of intramolecular hydrogen bond in malonaldehyde derivatives: An ab initio, AIM and NBO study

AbstractThe RAHB systems in malonaldehyde and its derivatives at MP2/ 6‐311++G(d,p) level of theory were studied and their intramolecular hydrogen bond energies by using the related rotamers method was obtained. The topological properties of electron density distribution in OH···O intramolecular hydrogen bond have been analyzed in term of quantum theory of atoms in molecules (QTAIM). Correlations between the H‐bond strength and topological parameters are probed. The results of QTAIM clearly showed that the linear correlation between the electron density distribution at HB critical point and RAHB ring critical point with the corresponding hydrogen bond energies was obtained. Moreover, it was found a linear correlation between the electronic potential energy density,V(rcp), and hydrogen bond energy which can be used as a simple equation for evaluation of HB energy in complex RAHB systems. Finally, the similar linear treatment between the geometrical parameters, such as O···O or OH distance, and Lp(O)→σ*OHcharge transfer energy with the intramolecular hydrogen bond energy is observed. © 2010 Wiley Periodicals, Inc., Int J Quantum Chem, 2011

Remarkable effect of hydroxylamine anion towards the solvolysis of sarin: A DFT study

The reaction of the nerve agent sarin with hydroperoxide (HOO −) and hydroxylamine anion (NH 2O −) has been studied with a correlated molecular orbital and density functional theory. The profound effect was found for α-nucleophiles on the solvolysis process of sarin compared to alkaline hydrolysis. The enthalpy of activation calculated at MP2/6-31+G∗//MPW1K/MIDI!+ΔG solv (HF/6-31+G∗) level of theory for the reaction of hydroxylamine anion with sarin is 5.2 kcal/mol lower than the barrier for alkaline hydrolysis. The charge descriptions of the α-nucleophiles seem to correlate well with their reactivity towards this organophosphorus compound. Conceptual DFT analysis also showed the similar trend for the solvolysis of sarin with these nucleophiles. The importance of intermolecular hydrogen bonding was also examined with the topological properties of electron density distributions for (– X–H⋯O, X = O, N) using Bader’s theory of atoms in molecules (AIM) and natural bond orbital (NBO) methods.

Solvolysis process of organophosphorus compound P-[2-(dimethylamino)ethyl]-N,N-dimethylphosphonamidic fluoride with simple and α-nucleophiles: a DFT study

Density functional theory (DFT) has been used to study the solvolysis process of the organophosphorus compound P-[2-(dimethylamino)ethyl]-N,N-dimethylphosphonamidic fluoride (GV) with simple nucleophile [hydroxide (HO−)] and α-nucleophiles [hydroperoxide (HOO−) and hydroxylamine anion (NH2O−)]. The lowest energy conformer of GV used for the solvolysis process was identified with Monte Carlo conformational search (MCMM) algorithm employing MMFFs force field followed by DFT calculations. The profound effect was found for α-nucleophiles toward the solvolysis of GV compared to normal alkaline hydrolysis. Incorporation of solvent (water) employing SCRF (PCM) model at B3LYP/6-31+G* showed that solvolysis of GV with hydroperoxide (activation energy = 7.6 kcal/mol) is kinetically more favored compared to hydroxide and hydroxylamine anion (activation energy = 11.0 and 9.2 kcal/mol, respectively). The faster solvolysis of GV with hydroperoxide is achieved due to strong intermolecular hydrogen bonding in the transition state geometry compared to similar α-nucleophile hydroxylamine anion. Assistance of a water molecule in solvolysis of GV affects the activation barriers; however, the hydroperoxidolysis remains the preferential process. The topological properties of electron density distributions for (–X–H···O, X = O, N) intermolecular hydrogen bonding bridges have been analyzed in terms of Bader theory of atoms in molecules (AIM). Further, the analysis was extended by natural bond orbital (NBO) methods for the strength of intermolecular hydrogen bonding in the transition state geometries. This study showed that the reactivity of these α-nucleophiles toward the solvolysis of GV is a delicate balance between the nucleophilicity and hydrogen-bond strength. Solvation governs the overall thermodynamics for the destruction of GV, which otherwise is unfavored in the gas phase studies.

Laplacian and bond critical point properties of the electron density distributions of sulfide bonds; a comparison with oxide bonds

Topological and bond critical point properties of electron density distributions, ρ(r), were calculated for a series of sulfide molecules, containing first- and second-row main group M-cations. Laplacian maps of the distributions, ∇ 2 ρ(r), show that the valence shell charge concentration (VSCC) of the sulfide anion is highly polarized and extended into the internuclear region of the M-S bonds, coalescing with the VSCCs of the more electronegative first-row cations. On the other hand, maps for a corresponding set of oxide molecules show that the oxide anion tends to be less polarized and more locally concentrated in the vicinity of its valence shell, particularly when bonded to second-row M-cations. A search for extrema in the ∇ 2 ρ(r) distributions reveals maxima in the VSCCs that can be ascribed to bonded and nonbonded electron pairs. The different and distinctive properties of sulfides and oxides are examined in terms of the number and the positions of the electron pairs and the topographic features of the Laplacian maps. The evidence provided by the electron density distributions and its topological properties indicates that the bonded interactions in sulfides are more directional, for a given M-cation, than in oxides. The value of the electron density distribution at the bond critical point and the length of a given M-S bond are reliable measures of a bonded interaction, the greater the accumulation of the electron density and the shorter the bond, the greater its shared (covalent) interaction.

SiO bonded interactions in coesite: a comparison of crystalline, molecular and experimental electron density distributions

Bond critical point properties of electron density distributions calculated for representative Si5O16 moieties of the structure of coesite are compared with those observed and calculated for the bulk crystal. The values calculated for the moieties agree with those observed to within ∼5%, on average, whereas those calculated for the crystal agree to within ∼10%. As the SiOSi angles increase and the SiO bonds shorten, there is a progressive build-up in the calculated electron density along the bonds. This is accompanied by an increase in both the curvatures of the electron density, both perpendicular and parallel to each bond, and the Laplacian of the electron density distribution at the bond critical points. The cross sections of the bonds at the critical points become more circular as the angle approaches 180o. Also, the bonded radius of the oxide anion decreases about twice as much as that of the Si cation as the SiO bond length decreases and the fraction of s-character of the bond is indicated to increase. A knowledge of electron density distributions is central to our understanding of the forces that govern the structure, properties, solid state reactions, surface reactions and phase transformations of minerals. The software (CRYSTAL95 and TOPOND) used in this study to calculate the bond critical properties of the electron density and Laplacian distributions is bound to promote a deeper understanding of crystal chemistry and properties.

SiO and GeO bonded interactions as inferred from the bond critical point properties of electron density distributions

The topological properties of the electron den- sity distributions for more than 20 hydroxyacid, geometry optimized molecules with SiO and GeO bonds with 3-, 4-, 6- and 8-coordinate Si and Ge cations were calculated. Electronegativities calculated with the bond critical point (bcp) properties of the distributions indicate, for a given coordination number, that the electronegativity of Ge (1.85) is slightly larger than that of Si (1.80) with the electronegativities of both atoms increasing with de- creasing bond length. With an increase in the electron density, the curvatures and the Laplacian of the electron density at the critical point of each bond increase with de- creasing bond length. The covalent character of the bonds are assessed, using bond critical point properties and elec- tronegativity values calculated from the electron density distributions. A mapping of the (3, u3) critical points of the valence shell concentrations of the oxide anions for bridging SiOSi and GeOGe dimers reveals a location and disposition of localized nonbonding electron pairs that is consistent with the bridging angles observed for sil- icates and germanates. The bcp properties of electron den- sity distributions of the SiO bonds calculated for represen- tative molecular models of the coesite structure agree with average values obtained in X-ray diffraction studies of coesite and danburite to within5%.

Critical point properties of electron density distributions for oxide molecules containing first and second row cations

Minimum energy geometries and electron density distributions, ϱ(r), for ∼40 polyatomic oxide molecules containing first and second row M-cations have been calculated at the Hartree-Fock level with a 6-311++G** basis set. The nature of the bonded interactions in these molecules is examined in terms of the relative electronegativities, χ M , of the M-cations and the properties of the electron density distribution, ϱ(r c ), evaluated at the bond critical points, r c , along each MO bond. As ϱ(r c ) and the Laplacian of ϱ(r c ) increase, χ M increases indicating an increase in the covalent character of the bonded interactions between M and O. The ratios of the curvatures of ϱ(r c ) indicate that the NO bond is predominantly covalent, that the CO and SO bonds are of intermediate type and that the remaining MO bonds are indicated to be predominantly ionic in character. A comparison of the critical point properties of ϱ(r c ) and χ M indicates that the minimum energy MO bond length is an important determinate of the properties of ϱ(r c ) and the character of the MO bonds.

Dynamic electron current induced by molecular vibration

The characteristic feature of the dynamic properties of electron density distribution is studied in the cyclopropenyl cation C 3H 3 + and in the system involving the radical abstraction reaction CH 3 + H 2 → CH 4 + H. First, the dynamic electron current induced by the in-plane vibrational motions of C 3H 3 + accumulates inside the ring and “permeates” through the CC bond region. Secondly, the dynamic spin current induced by the progress of the chemical reaction correlates with the dynamic spin current associated with the specific vibrational motion perpendicular to the reaction coordinate. The isotopic effect is also examined.