Intermolecular potential energy extrapolation method for weakly bound systems: Ar2, Ar–H2 and Ar–HF dimers

Two methods are presented and compared for modifying the intermolecular potential energy extrapolation routine SIMPER, to overcome the problems occurring at small intermolecular separation associated with the use of a Coulomb approximation. The first modification uses a charge density overlap pseudopotential added to the effective Hamiltonian of each interacting fragment. The second treats the problem perturbatively, truncating the polarization expansion series at third order. The methods are used to produce potential energy curves for Ar₂ dimer and several one-dimensional cuts through the Ar–H₂ and Ar–HF potential energy surfaces. Both approaches are competitive with supermolecule dimer calculations at high levels of theory, and significantly reduce the computational cost.     

Mass spectrometric and ab initio study of the interaction between 9-methylguanine and amino acid amide group

The temperature dependent field ionization mass spectrometry method combined with ab initio calculations was used to determine the interaction energies and the structures of 9-methylguanine-acrylamide dimers. Acrylamide mimics the side chain amide group of the natural amino acids asparagine and glutamine. The experimental enthalpy of the dimer formation derived from the van't Hoff plot is ?59.5 ± 3.8 kJ mol^?1. The value is higher than interaction energies between acrylamide and other nucleic acid bases which were determined to be ?57.0 for 1-methylcytosine, ?52.0 for 9-methyladenine, and ?40.6 kJ mol^?1 for 1-methyl-uracil. In total, eight hydrogen bonded dimers formed by the three lowest energy 9-methylguanine tautomers and acrylamide were found in the quantum chemical calculations performed at the DFT/B3LYP/6-31++G?? and MP2/6-31++G?? levels of theory. The relative stability and the interaction energies of the dimers were calculated accounting for the basis set superposition error and the zero-point vibrational energy correction. The lowest energy dimer found in the calculations is formed by acrylamide (Ac) with the keto tautomer of 9-methylguanine (Gk). It is stabilized by two intermolecular H bonds, C6=O(Gk) · · · H—N(Ac) and Nl—H(Gk) · · ·O(Ac), and it is more stable than the second lowest energy dimer by ≈ 25 kJ mol^?1. The calculated interaction energies of the lowest energy 9-methylguanine-acrylamide dimer are ?65.0 kJ mol^?1 and ?67.7 kJ mol^?1 at the MP2 and DFT levels of theory, respectively. The experimental enthalpy of the dimer formation is in good agreement with both the calculated interaction energies of the GkAc dimer and much higher than the interaction energies calculated for all other 9-methylguanine-acrylamide dimers. This proved that only one dimer was present in the experimental samples. To verify whether acrylamide is a good model of the amino acid-amide group, we performed direct calculations of the 9-methylguanine-glutamine dimers at the same levels of theory as used for the complexes involving acrylamide. The interaction energies found for the lowest energy 9-methylguanine-glutamine dimer are ?65.1 kJ mon^?1 (MP2/6-31++G??) and ?66.2 kJ mol^?1 (DFT/B3LYP/6-31++G??) and these values are very close (within 0.5 kJ mol^?1) to the interaction energies obtained for the 9-methylguanine-acrylamide dimers.     

The ammonia dimer equilibrium dissociation energy: convergence to the basis set limit at the correlated level

The low-energy region of the intermolecular potential energy hypersurface (PES) of the ammonia dimer was studied at the level of second-order Moller-Plesset perturbation theory (MP2) using a very large basis set. Individual minima were located on the PES employing the counterpoise (CP) correction to account for the basis set superposition error (BSSE). Apart from these canonical MP2 calculations local MP2 (LMP2) calculations were performed. For the latter the BSSE at the correlated level is inherently absent by virtue of the local truncation of the virtual space. Results from canonical and local MP2 calculations are compared and the reliability of the LMP2 method for intermolecular complexes and clusters is discussed. The canonical MP2 calculations predicted five minimum structures, the four most stable ones lying energetically very close. For these four structures single point MP2 energy calculations with a further extended basis set (1024 functions for the ammonia dimer) were performed. The equilibrium dissociation energies so obtained are close to the one-particle basis set limit, as illustrated by a remaining BSSE of less than 0.2 kJ mol^?1. The geometry optimizations at the LMP2 level, using the three most stable canonical MP2 structures as initial geometries, all collapsed to a single minimum corresponding to an asymmetric structural arrangement. A canonical MP2 single point calculation, at that geometry, revealed that the LMP2 minimum structure is virtually as stable as the lowest minima on the canonical MP2 PES. Based on these calculations the global minimum of the ammonia dimer was assigned to a part of the PES represented by an asymmetric structure with an equilibrium dissociation energy of 13.5±0.3 kJ mol^?1     

Total lattice potential energy of sodium bromide dihydrate NaBr · 2H2O

In addition to presenting comparative calculations by two approaches for the total lattice potential energy of sodium bromide dihydrate, NaBr · 2H₂O, found to take the value 803.9 kJ mol^-1, we investigate the influence of the size and nature of the basis set used to generate multipole moments in a Hartee-Fock calculation which are in turn used to calculate the Madelung constant. The requirement is one of critical size of the basis set and once this is reached the electrostatic energy will be reliable.     

Double proton transfer in crystals of 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a] pyrimidine (hppH): 13C and 15N CPMAS NMR study of (hppH)2

In this paper, we report an example of intermolecular solid‐state proton transfer in the bicyclic guanidine, hppH. A combination of X‐ray crystallography, CPMAS NMR (¹³C and ¹⁵N) and theoretical calculations allows us to determine that a double proton transfer takes place in the (hppH)₂ dimer with an activation energy of about 50 kJ mol^?1. According to the B3LYP/6‐311++G(d,p) calculations, the double proton transfer occurs non‐symmetrically through a zwitterion. Copyright © 2009 John Wiley & Sons, Ltd.     

Binding energy analysis of solid hydrogen fluoride

Ab initio energy band structure calculations of infinite single and double HF chains are performed. Interaction energies within and between the linear macromolecules are deduced. As an alternative way for the decomposition of the binding energy tetrameric HF clusters are investigated. Second order perturbation theory is applied to calculate the correlation energy contributions. The interaction of the elementary cell with its neighboring cells in the same layer is repulsive. A binding energy is obtained for the interaction with cells in different layers. The cohesive energy is about - 2 kcal mol^-1 with respect to a single HF dimer. The results show that the binding energy in molecular crystals can be determined with the help of molecular cluster calculations.     

Density functional study of the chemisorption of O2 on the armchair surface of graphite

The reaction between O₂ and the armchair surface of a model graphite molecule has been studied using density functional calculations at the B3LYP/6-31G(d) level of theory. Both equilibrium and transition state geometries were optimized to provide a fundamental understanding of the energetics and kinetics of the chemisorption, desorption, rearrangement, and migration reactions that contribute to carbon gasification. A small barrier of 18 kJ mol⁻¹ was found for the chemisorption reaction, which is 578 kJ mol⁻¹ exothermic overall, producing a stable quinone. A number of reaction pathways with barriers below 578 kJ mol⁻¹ were characterized. Gasification of carbon occurs as CO, with barriers of 296 and 435 kJ mol⁻¹ for the first and second CO loss, respectively. The stable quinone can also undergo a rearrangement reaction to form two ketene groups, with a barrier of 260 kJ mol⁻¹. If the armchair edge is extended to include an adjacent aromatic ring, the oxide can migrate along the surface. This initially forms a furan-like bridge structure, with a barrier of just 89 kJ mol⁻¹. A further barrier of 383 kJ mol⁻¹ leads to CO desorption from the furan. The furan can also rearrange further with a barrier of 212 kJ mol⁻¹ to form a five-membered lactone, the most stable structure identified on the potential energy surface. Rearrangement and migration reactions, which have not generally been incorporated into carbon gasification models, are shown to be potentially important pathways in carbon oxidation reactions.     

Enhancement effect of lithium bonding on the strength of pnicogen bonds: XH2P···NCLi···NCY as a working model (X = F,Cl; Y = H,F, Cl,CN)

MP2 calculations with aug-cc-pVDZ basis set were used to analyse intermolecular interactions in XH₂P···NCLi···NCY triads (X = F, Cl; Y = H, F, Cl, CN) which are connected via pnicogen bond and lithium bond. To understand the properties of the systems better, the corresponding dyads are also studied. Molecular geometries and interaction energies of dyads, and triads are investigated at the MP2/aug-cc-pVDZ computational level. Particular attention is paid to parameters such as cooperative energies and many-body interaction energies. All studied complexes, with the simultaneous presence of a lithium bond and a pnicogen bond, show cooperativity with energy values ranging between ?4.73 and ?8.88 kJ mol^?1. A linear correlation was found between the interaction energies and magnitude of the product of most positive and negative electrostatic potentials. According to energy decomposition analysis, it is revealed that the electrostatic interactions are the major source of the attraction in the title complexes.     

The temperature dependence of the symmetric exchange interaction by optical phonon modulation of the exchange integral

By assuming an anharmonic intermolecular potential for lattice displacement and an exponential form for the exchange energy, the exchange interaction is shown to be temperature dependent in the paramagnetic region. Bond strengths are taken from known tabulated results and overlap integrals are calculated with Slater-type orbitals so there are no adjustable parameters. Agreement with experimental results on K₂CuCl₄ · 2H₂O and other layered compounds is shown.     

反应物振动激发对O(3P)+D2→OD+D立体动力学的影响

运用准经典轨线方法,基于Roger的³A"势能面,在碰撞能为104.5 kJ/mol时对O(³P)+D₂反应的立体动力学性质进行了理论研究. 详细讨论与产物矢量相关的的极化分布函数以及四个极化微分反应截面进行了. 结果表明,产物OD的立体动力学性质对反应物分子H_{2相似文献}   

The adsorption of Xe and CO on a Cu (311) single crystal surface

LEED, electron energy loss spectroscopy and surface potential measurements have been used to study the adsorption of Xe and CO on Cu (311). Xe is adsorbed with a heat of 19 ± 2 kJ mol^/t-1. The complete monolayer has a surface potential of 0.58 V and a hexagonal close-packed structure with an interatomic distance of 4.45 ± 0.05 Å. CO gives a positive surface potential increasing with coverage to a maximum of 0.34 V and then falling to 0.22 V at saturation. The heat of adsorption is initially 61 ± 2 kJ mol^?1, falling as the surface potential maximum is approached to about 45 kJ mol^?1. At this coverage streaks appear in the LEED pattern corresponding to an overlayer which is one-dimensionally ordered in the [011̄] direction. Additional CO adsorption causes the heat of adsorption to decrease further and the overlayer structure to be compressed in the [011̄] direction. At saturation the LEED pattern shows extra spots which are tentatively attributed to domains of a new overlayer structure coexisting with the first. Electron energy loss spectra (EELS) of adsorbed CO show two characteristic peaks at 4.5 and 13.5 eV probably arising from transitions between the electronic levels of chemisorbed CO.     

Electron-electron double resonance in nitroxide radical solutions

All the pair interactions arising from the ground state H₂, N₂, CO₂, HF and LiH molecules are used as models for investigating the validity of the multipolar results (corresponding to the classical interaction of the permanent multipole moments of the interacting molecules) as a representation of the first-order Coulomb interaction energy. Charge overlap effects, the neglect of which are responsible for the misbehaviour of the multipolar results as the intermolecular distance R becomes small, are discussed as a function of the nature of the interacting molecules. Useful analytical formulae are presented which represent the numerical results. Some of the difficulties in using the multipolar results to represent the anisotropic part of a dimer potential are discussed in general and more specifically with respect to the evaluation of the mean square torque of nitrogen molecules.     

Polarisationsmessungen an Elektronenstrahlen nach der Transmission durch magnetisierte Eisenschichten

Magnetized iron foils of a thickness of 500 Å were transmitted by 60 keV electrons. The spin polarization of the electron beam was investigated by Mott scattering after transmission. No polarization effect was found. Since the lowest detectable polarization degree wasP=0·003 the atomic polarization cross section must be assumed to be smaller than 0·8·10^?20cm². This cross section is the sum of the spin exchange cross section and the spin dependent part of the total scattering cross section. If the electron binding energy is neglected, the spin depentend part of the total scattering cross section can be calculated from the theory of Møller scattering. In the case of our conditions-60 keV and an aperture of 10^?3 radian-the calculated cross section is smaller than the experimental upper limit. In further experiments the electron beam was split into the energy spectrum by an electrostatic analyzer placed between the iron foil and the Mott scattering foil. In these measurements only small parts of the energy spectrum were investigated, however, even here no detectable polarization occured.     

Combined Raman scattering and ab initio investigation of the interaction between pyrene and carbon SWNT

Raman spectra of HiPco SWNT and SWNT-pyrene films were measured in the 160–1800 cm^?1 range. Due to the non-covalent interaction between SWNT and pyrene the most intensive component of the SWNT G mode (1590 cm^?1) is downshifted by 2 cm^?1 and becomes narrower. Also the intensity of the low-frequency component of the G mode (1550 cm^?1) decreases by about 30%. Structures and interaction energies in the complexes of pyrene and zigzag (n, 0) SWNTs [6 ≤ n ≤ 20] were determined at the MP2 level of theory. The BSSE-free geometry optimization of the pyrene-zigzag (12,0) SWNT complex converged to a structure with a 1/2staggered conformation and with an intermolecular distance of 3.5 Å. The BSSE-free interaction energy in the complex is ?30.8 kj mol^?1. Increasing of the nanotube diameter leads to a higher interaction energy. This energy becomes equal to ?37.2 kJ mol^?1 in the case of a planar carbon surface.     

Interplay and competition between the lithium bonding and halogen bonding: R3C···XCN···LiCN and R3C···LiCN···XCN as a working model (R = H,CH3; X = Cl,Br)

UMP2 calculations with aug-cc-pVDZ basis set were used to analyse intermolecular interactions in R₃C···XCN···LiCN and R₃C···LiCN···XCN triads (R = H, CH₃; X = Cl, Br) which are connected via lithium bond and halogen bond. To understand the properties of the systems better, the corresponding dyads are also studied. Molecular geometries and binding energies of dyads, and triads are investigated at the UMP2/aug-cc-pVDZ computational level. Particular attention is paid to parameters such as cooperative energies, and many-body interaction energies. All studied complexes, with the simultaneous presence of a lithium bond and a halogen bond, show cooperativity with energy values ranging between ?1.20 and ?7.71 kJ mol^?1. A linear correlation was found between the interaction energies and magnitude of the product of most positive and negative electrostatic potentials (V_S,maxV_S,min). The electronic properties of the complexes are analysed using parameters derived from the atoms in molecules (AIM) methodology. According to energy decomposition analysis, it is revealed that the electrostatic interactions are the major source of the attraction in the title complexes.     

The lattice energy of magnesium hydride

The lattice energy of MgH₂ has been calculated using a Born-Mayer model. The result is U₀ = ?2906.5 kJ mol^?1 which may be compared with the Born-Haber cycle value of ?2721.3 kJ mol^?1. This difference indicates that there is an appriciable covalent contribution to the bonding. The importance of the zero-point energy contribution to the lattice energy in the case of the light metal hydrides is discussed.     

A reliable single parameter interatomic potential for argon

A simple semiempirical approximation previously proposed for the isotropic intermolecular forces between two closed shell systems is tested in detail for the argon-argon interaction. The potential is based on the knowledge of the first-order coulomb interaction energy, a suitably damped three term long range asymptotic expansion of the second order coulomb energy, and a semiempirical representation of the exchange interaction energy which contains one adjustable parameter. The single adjustable parameter can be reliably determined by fitting the second virial coefficient for argon in the 130–773 K temperature range with the long range interaction coefficients being constrained within the theoretical bounds specified by Tang, Norbeck and Certain. The reliability of the potential is compared with that of several literature potentials by comparing the theoretical predictions obtained from the potentials with experimental results for the second virial coefficient, viscosity, thermal conductivity and thermal diffusion ratios for dilute argon gas, and with spectroscopic data for the dimer, and with SCF calculations of the Ar-Ar potential at small interatomic separations. Our best potential predicts these properties with a precision as good as or better than other recent potentials which generally contain more adjustable parameters and/or involve more input data. The results confirm earlier work that suggested that the scheme tested is capable of yielding reliable isotropic potentials for the interaction of closed shell systems for 0·3 ? R/R_m ? ∞ where R_m is the intermolecular distance at the van der Waals minimum. The scheme appears to offer a method for obtaining reliable potentials while avoiding problems associated with optimizing many parameters with respect to fitting experimental constraints.     

The chemisorption of chlorine on zirconium: Neutral and negative ion thermal desorption spectroscopy

Chloride growth on zirconium is investigated by means of the novel thermal desorption spectroscopy of both neutral and negatively ionized products, and interpreted in terms of a single ionic adsorption model. The monohalide desorption energy decreases from a low-coverage value of 320 kJ to 255 kJ mol⁻¹ at the monolayer level, at which point trihalide growth continues indefinitely, indicative of a corrosion layer. In contrast, the desorption energy for the chloride ion remains essentially independent of coverage, at 293 kJ mol⁻¹