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1.
The first and second bond dissociation energies for H2O have been calculated in anab initio manner using a multistructure valence-bond scheme. The basis set consisted of a minimal number of non-orthogonal atomic orbitals expressed in terms of gaussian-lobe functions. The valence-bond structures considered properly described the change in the molecular system as the hydrogen atoms were individually removed to infinity. The calculated equilibrium geometry for the H2O molecule has an O-H bond length of 1.83 Bohrs and an HOH bond angle of 106.5°. With 49 valence-bond structures the energy of H2O at this geometry was ?76.0202 Hartrees. The calculated equilibrium bond length for the OH radical was 1.86 Bohrs and the energy, using the same basis set, was ?75.3875 Hartrees. After correction for zero point energies the calculated bond dissociation energies are: H2O → OH + H, D1=75.38 kcal/mole and OH → O+H, D2=54.79 kcal/mole.  相似文献   

2.
Ab initio calculations using the unscaled 4-31G basis set have been carried out on the cc, tc, and tt conformers of carbonic acid and the bicarbonate ion, with full geometry optimization assuming the structures to be planar. The complete harmonic force field is reported for the (most stable) tt conformer and for the bicarbonate ion, also selected quadratic force constants for the cc and tc conformers. The changes in certain bond lengths and stretching force constants in the cctc, tctt, and cctt conformer conversion reactions are indicative of intramolecular hydrogen bonding, C?O…H? O and H? O…H? O, which is examined in greater detail by partitioning the overall conformer conversion energy into distortion and bonding energy components. The fundamental vibration frequencies for the tt conformer and the bicarbonate ion are calculated from the force constant matrices, and hence, using a scaling factor based on a comparison of calculated and experimental values for the bicarbonate ion and trans-formic acid, a value is predicted for the zero-point energy of the tt conformer. A new estimate of ΔH? for the hydration reaction, H2O + CO2 → H2CO3, at 298 K in the gas phase; is made from thermochemical data, +20.2 ± 3.4 kJ mol?1, which, together with estimates of (H298? – H0?) and the zero-point energy for H2CO3, gives +8.1 ± 7.0 kJ mol?1 for ΔET(expt). ΔET calculated from the 4-31G basis set data is -29.1 kJ mol?1. Comparison of the experimental value, the Hartree–Fock limit value, and values calculated with a variety of basis sets for the bond separation reaction, CO2 + CH4 → 2H2CO, suggests that the differences, ΔET(expt) minus ΔET(SCF ), are due mainly to basis set limitations and not substantial correlation energy contributions.  相似文献   

3.
Single crystals of molybdenum(VI) tricopper(II) tellurium(IV) hepta­oxide dichloride hemihydrate, MoCu3TeO7Cl2·0.5H2O, were synthesized via a transport reaction in sealed evacuated silica tubes. All atoms occupy general positions within the triclinic () unit cell. The building units are irregular CuO4Cl and CuO3Cl2 square pyramids, distorted TeO3+1E trigonal bipyramids (E is the lone pair of TeIV) and irregular MoO5 pyramids. The TeO3+1E, CuO4Cl and CuO3Cl2 polyhedra form (110) layers bridged by Mo atoms. The water mol­ecules are located in [100] channels.  相似文献   

4.
Extended basis set computations on SCF and CEPA level were performed for BH3NH3 and BH3PH3 to determine the complexation energy ΔE and the equilibrium distance r(BX) between the “heavy” atoms. Our CEPA results (SCF in parentheses): ΔE(BH3NH) = ?27(?21.3) kcal/mol, ΔE(BH3PH3) = ?17(?11.8) kcal/mol, r(BN) = 1.65(1.68) Å, r(BP) = 1.95(1.99) Å indicate a marked influence of electron correlation on these properties.  相似文献   

5.
We prepared and isolated a phenalenyl‐based neutral hydrocarbon ( 1 b ) with a biradical index of 14 %, as well as its charge‐transfer (CT) complex 1 b –F4‐TCNQ. The crystal structure and the small HOMO–LUMO gap assessed by electrochemical and optical methods support the singlet‐biradical contribution to the ground state of the neutral 1 b . This biradical character suggests that 1 b has the electronic structure of phenalenyl radicals coupled weakly through an acetylene linker, that is, some independence of the two phenalenyl moieties. The monocationic species 1 b. + was obtained by reaction with the organic electron acceptor F4‐TCNQ. The cationic species has a small disproportionation energy ΔE for the reaction 2× 1 b. +? 1 b + 1 b 2+, which presumably originates from the independence of the phenalenyl moieties. The small ΔE led to a small on‐site Coulombic repulsion Ueff=0.61 eV in the CT complex. Moreover, a very effective orbital overlap of the phenalenyl rings between molecules afforded a relatively large transfer integral t=0.09 eV. The small Ueff/4t ratio (=1.7) resulted in a metallic‐like conductive behavior at around room temperature. Below 280 K, the CT complex showed a transition into a semiconductive state as a result of bond formation between phenalenyl and F4‐TCNQ carbon atoms.  相似文献   

6.
In single crystals of a new monoclinic (C2/m) form of tricopper(II) diselenium(IV) dichloride hexa­oxide, Cu3(SeO3)2Cl2, the Se atom is in the 4i position, while the two Cu atoms are in 2a and 4i positions. The structure is based on layers of CuO4Cl trigonal bipyramids, CuO4 square planes and SeO3E tetra­hedra. The Cu polyhedra are connected by edge‐ and corner‐sharing to form [010] chains and these chains are bridged by the Se atoms to form (001) layers. The compound is isostructural with Cu3(TeO3)2Br2.  相似文献   

7.
The electronic energy band structures of the La-doped superconductors TlBa1.2 La0.8 CuO5 + δ were calculated. The effect of the oxygen content on their electronic structures was studied. The results show that compared with those for TlBa2CuO5, the La doping at the Ba site results in the moderate change in the band structures and the decrease in the densities of states, but the increase in the oxygen content caused by the La doping results in the great change in the band structures and the densities of states near the Fermi surface. The low oxygen content causes the degree of complexity of the band structures and the densities of states near the Fermi level Ef to increase and the high oxygen content causes them to decrease. The oxygen content plays a dominant role in TlBa1.2La0.8CuO5 + δ. In addition, the Cu-O planes are most sensitive to the increase in the oxygen content. © 1996 John Wiley & Sons, Inc.  相似文献   

8.
An Oxygen-rich Neodymium Cuprate: High-pressure Synthesis and Crystal Structure of Nd12Cu6O25 (“Nd2CuO4.17”) Single crystals of Nd12Cu6O25 (“Nd2CuO4.17”) could be obtained by reaction of Nd2O3 und CuO with KO2 in a modified Belt-type apparatus at 40 kbar, 1500°C. The compound crystallizes in the monoclinic space group C2/m with a = 17.128(6), b = 3.7288(6), c = 18.364(6) Å, β = 111.22(1)º and Z = 2. The structure refinement converged at R1 = 0.0302 for 1451 reflections with Fo > 4σ(Fo) and R1 = 0.0903, wR2 = 0.0767 for all 2571 data. The structure-determining feature are Cu(1,2)? O octahedral chains, interrupted by Cu(3) atoms in square-planar coordination. Each of this CuO4 groups is connected to another one, resulting in short copper-copper distances (dCu? Cu = 3.012 Å). A comparison of the structure of “Nd2CuO4.17” with those of La2CuO4 and Nd2CuO4 makes the structural relations of the former with La2CuO4 and, hence, the aristotype K2NiF4 evident, in spite of the close stoichiometric similarity between Nd2CuO4 and “Nd2CuO4.17”.  相似文献   

9.
The local structures of supported CuO/γ-Al2O3 monolayer dispersive catalysts with different CuO loadings have been investigated by EXAFS and multiple scattering XANES simulations. The EXAFS results show that the first nearest neighbors around the Cu atoms in the CuO/γ-Al2O3 catalysts are similar to that of the polycrystalline CuO powder, which is independent of the CuO loadings. Moreover, the Cu K-XANES FEFF8 calculations for CuO reveal that the monolayer-dispersed CuO species are of small distorted (CuO4)mn+ clusters, which is mainly composed of a distorted CuO6 octahedron incorporated in the surface octahedral vacant sites of the γ-Al2O3 support. We consider that the CuO species for the CuO/γ-Al2O3 catalysts with loadings of 0.4 and 0.8 mmol/100 m2 are distorted (CuO4)mn+ clusters composed mainly of a distorted CuO6 octahedron incorporated in the surface octahedral vacant sites of the γ-Al2O3 support after calcinations at high temperature in air for a few hours. On the contrary, for the CuO/γ-Al2O3 with loading of 1.2 mmol/100 m2, the local structure of Cu atoms in CuO/γ-Al2O3 is similar to that of polycrystalline CuO powder.  相似文献   

10.
The stabilization energies (ΔEform) 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 ΔEform increases by the increase of the number of the O???Li contact, the ΔEform 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.  相似文献   

11.
Is it possible to facilitate the formation of a genuine Be?Be or Mg?Mg single bond for the E2 species while it is in its neutral state? So far, (NHCR)Be?Be(NHCR) (R=H, Me, Ph) have been reported where Be2 is in 1Δg excited state imposing a formal Be?Be bond order of two. Herein, we present the formation of a single E?E (E=Be, Mg) covalent bond in E2(NHBMe)2 (E=Be, Mg; NHBMe=(HCNMe)2B) complexes where E2 is in 3u+ excited state having (nσg+)2(nσu+)1((n+1)σg+)1 (n=2 for Be and n=4 for Mg) valence electron configuration and it forms electron‐shared bonding with two NHBMe radicals. The effects of bonding with nσu+ and (n+1)σg+ orbitals will cancel each other, providing the former E?E bond order as one. Be2(NHBMe)2 complex is thermochemically stable with respect to possible dissociation channels at room temperature, whereas the two exergonic channels, Mg2(NHBMe)2 → Mg + Mg(NHBMe)2 and Mg2(NHBMe)2 → Mg2 + (NHBMe)2, are kinetically inhibited by a free energy barrier of 15.7 and 18.7 kcal mol?1, respectively, which would likely to be further enhanced in cases of bulkier substituents attached to the NHB ligands. Therefore, the title complexes are first viable systems which feature a neutral E2 moiety with a single E?E covalent bond.  相似文献   

12.
Is it possible to facilitate the formation of a genuine Be?Be or Mg?Mg single bond for the E2 species while it is in its neutral state? So far, (NHCR)Be?Be(NHCR) (R=H, Me, Ph) have been reported where Be2 is in 1Δg excited state imposing a formal Be?Be bond order of two. Herein, we present the formation of a single E?E (E=Be, Mg) covalent bond in E2(NHBMe)2 (E=Be, Mg; NHBMe=(HCNMe)2B) complexes where E2 is in 3u+ excited state having (nσg+)2(nσu+)1((n+1)σg+)1 (n=2 for Be and n=4 for Mg) valence electron configuration and it forms electron‐shared bonding with two NHBMe radicals. The effects of bonding with nσu+ and (n+1)σg+ orbitals will cancel each other, providing the former E?E bond order as one. Be2(NHBMe)2 complex is thermochemically stable with respect to possible dissociation channels at room temperature, whereas the two exergonic channels, Mg2(NHBMe)2 → Mg + Mg(NHBMe)2 and Mg2(NHBMe)2 → Mg2 + (NHBMe)2, are kinetically inhibited by a free energy barrier of 15.7 and 18.7 kcal mol?1, respectively, which would likely to be further enhanced in cases of bulkier substituents attached to the NHB ligands. Therefore, the title complexes are first viable systems which feature a neutral E2 moiety with a single E?E covalent bond.  相似文献   

13.
A training set of eleven X‐ray structures determined for biomimetic complexes between cucurbit[n]uril (CB[7 or 8]) hosts and adamantane‐/diamantane ammonium/aminium guests were studied with DFT‐D3 quantum mechanical computational methods to afford ΔGcalcd binding energies. A novel feature of this work is that the fidelity of the BLYP‐D3/def2‐TZVPP choice of DFT functional was proven by comparison with more accurate methods. For the first time, the CB[n] ? guest complex binding energy subcomponents [for example, ΔEdispersion, ΔEelectrostatic, ΔGsolvation, binding entropy (?TΔS), and induced fit Edeformation(host), Edeformation(guest)] were calculated. Only a few weeks of computation time per complex were required by using this protocol. The deformation (stiffness) and solvation properties (with emphasis on cavity desolvation) of cucurbit[n]uril (n=5, 6, 7, 8) isolated host molecules were also explored by means of the DFT‐D3 method. A high ρ2=0.84 correlation coefficient between ΔGexptl and ΔGcalcd was achieved without any scaling of the calculated terms (at 298 K). This linear dependence was utilized for ΔGcalcd predictions of new complexes. The nature of binding, including the role of high energy water molecules, was also studied. The utility of introduction of tethered [‐(CH2)nNH3]+ amino loops attached to N,N‐dimethyl‐adamantane‐1‐amine and N,N,N′,N′‐tetramethyl diamantane‐4,9‐diamine skeletons (both from an experimental and a theoretical perspective) is presented here as a promising tool for the achievement of new ultra‐high binding guests to CB[7] hosts. Predictions of not yet measured equilibrium constants are presented herein.  相似文献   

14.
In order to determine the effect of temperature on the chain-transfer reaction in the free-radical polymerization of ethylene, chain-transfer constants were measured for sixteen transfer agents at 130°C and 200°C at 1360 atm. The results were interpreted as ΔE*, the activation energy of the chain-transfer constant. This value is equal to the difference in activation energy between the transfer step (hydrogen abstraction) and the propagation step (addition to the monomer double bond): ΔE* = Es* ? Ep*. Excellent agreement was found between measured ΔE* values determined at 1360 atm pressure and (Es* ? Ep*) data for ethyl radical determined in vacuum gas-phase reactions. Apparently, the ethyl radical is a good model for polyethyl radical. The chain-transfer constant of ethylbenzene was found to be insensitive to temperature changes, indicating that Ep* = Es* for this compound.  相似文献   

15.
Air and moisture sensitive K5[CuO2][CO3] was prepared via the azide/nitrate route from stoichiometric mixtures of the precursors CuO, KN3, KNO3 and K2CO3. According to the single‐crystal X‐ray analysis of the crystal structure [P4/nbm, Z = 2, a = 7.4067(5), c = 8.8764(8) Å, R1 = 0.053, 433 independent reflections] K5[CuO2][CO3] represents an ordered superstructure of Na5[NiO2][CO3]. The structure contains isolated [CuO2]3– dumbbells and CO32– anions, with the latter not connected to the transition element. Raman spectroscopic measurements confirm the presence of CO32– in the structure.  相似文献   

16.
The adsorption of six electron donor–acceptor (D/A) organic molecules on various sizes of graphene nanoflakes (GNFs) containing two common defects, double‐vacancy (5‐8‐5) and Stone–Wales (55‐77), are investigated by means of ab initio DFT [M06‐2X(‐D3)/cc‐pVDZ]. Different D/A molecules adsorb on a defect graphene (DG) surface with binding energies (ΔEb) of about ?12 to ?28 kcal mol?1. The ΔEb values for adsorption of molecules on the Stone–Wales GNF surface are higher than those on the double vacancy GNF surface. Moreover, binding energies increase by about 10 % with an increase in surface size. The nature of cooperative weak interactions is analyzed based on quantum theory of atoms in molecules, noncovalent interactions plot, and natural bond order analyses, and the dominant interaction is compared for different molecules. Electron density population analysis is used to explain the n‐ and p‐type character of defect graphene nanoflakes (DGNFs) and also the change in electronic properties and reactivity parameters of DGNFs upon adsorption of different molecules and with increasing DGNF size. Results indicate that the HOMO–LUMO energy gap (Eg) of DGNFs decreases upon adsorption of molecules. However, by increasing the size of DGNFs, the Eg and chemical hardness of all complexes decrease and the electrophilicity index increases. Furthermore, the values of the chemical potential of acceptor–DGNF complexes decrease with increasing size, whereas those of donor–DGNF complexes increase.  相似文献   

17.
In the framework of the additive SIBFA 2 procedure, the intermolecular interaction energy is computed as a sum of five terms: ΔE = EMTP + Erep + Epol + ECT + Edisp. In order to assess the accuracy of the procedure to compute cation–ligand interactions, the interaction of alkali (Na+, K+) and alkaline-earth (Mg2+, Ca2+) cations with two representative ligands H2O and HCOO? has been studied and the results compared with those of ab initio SCF extended basis set computations. The additive procedure reproduces very satisfactorily the results of ab initio computations as concerns the numerical values of the interaction energies and the equilibrium cation–ligand distances, as well as the evolution of the energy components. A detailed study of these components at different distances helps, in particular, to delineate the relative weights of the charge-transfer and polarization contributions within the second-order energy.  相似文献   

18.
The chemistry of oxygen, hydrogen, water, and other species containing both oxygen and hydrogen atoms on the anatase TiO2 (001) surface is investigated by DFT. The adsorption energy of atoms and radicals depends appreciably on the position and mode of adsorption, and on the coverage. Molecular hydrogen and oxygen interact weakly with the clean surface. However, H2O dissociates spontaneously to give two nonidentical hydroxyl groups, and this provides a model for hydroxylation of TiO2 surfaces by water. The mobility of the hydroxyl groups created by water splitting is initially impeded by a diffusion barrier close to 1 eV. The O2 adsorption energy increases significantly in the presence of H atoms. Hydroperoxy (OOH) formation is feasible if at least two H atoms are present in the direct vicinity of O2. In the adsorbed OOH, the O? O bond is considerably lengthened and thus weakened.  相似文献   

19.
Ab initio SCF LCAO MO calculations for the [H2O…Cl]? complex have been performed. The energy of the linear hydrogen bond has been found to be lower than the energy of the bifurcated one. The difference of the energies is about 3 kcal/mole. The calculated equilibrium distance between the oxygen and chlorine atoms equals 5.75 au. The interaction energy of the chlorine anion and the rigid water molecule amounts to ?19 kcal/mole. The optimization of the OH bond length in the complex (linear hydrogen bond) leads to an interaction energy of ?19.5 kcal/mole (the experimental value equals ?13.1 kcal/mole). As a result of the hydrogen bond formation the OH bond length increases by 0.08 au.  相似文献   

20.
An energy-efficient and environmentally friendly microwave-assisted method was adopted for synthesis of butterfly-like CuO assembled by nanosheets through a Cu2Cl(OH)3 precursor, using no template. Formation mechanism of the butterfly-like CuO was explored and discussed systematically for the first time on the basis of both experimental results and crystal structure transformations in atomic level. The electrochemical sensing properties of the butterfly-like CuO modified electrode to ascorbic acid (AA) were studied for the first time. The results reveal that Cu(OH)2 nanowires were formed once the Cu2+ ions, located in between two CuO4 parallelogram chains of a Cu2Cl(OH)3 precursor, dissolve into the solution as Cu(OH)42− complex ions after ion exchange reactions and simultaneous assemble along a axis. Upon microwave irradiation, the adjacent CuO4 parallelogram chains of the Cu(OH)2 nanowires dehydrate and assemble along c axis, forming CuO nanosheets with (002) as the main exposed facet, which were further assembled to butterfly-like CuO under the action of microwave field, suggesting that microwave field functions like a ‘directing agent’. The butterfly-like CuO modified electrode shows good electrochemical sensing properties to AA with a low detecting limit, short response time and wide linear response range.  相似文献   

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