An ab initio study of the Ne-CO complex

A new ab initio potential energy surface of the Ne-CO complex is developed using single and double excitation coupled-cluster theory with noniterative treatment of triple excitations [CCSD(T)].The potential has a minimum value of 49.396 cm 1 at R e = 6.40a 0 with approximately T-shaped geometry(θ e = 82.5).Bound state energies are calculated up to J = 12.The theoretically predicted transition frequencies and spectroscopic constants are in good agreement with the available experimental results.     

Experimental and ab initio study of the nuclear quadrupole interaction of 181 Ta-probes in an α-Fe2O3 single crystal

We report perturbed-angular-correlation (PAC) experiments on ¹⁸¹Hf (→¹⁸¹Ta)-implanted corundum α-Fe₂O₃ single crystal in order to determine the magnitude, symmetry and orientation of the electric-field-gradient (EFG) tensor at Ta donor impurity sites of this semiconductor. These results are analyzed in the framework of ab initio full-potential augmented-plane wave plus local orbital (FP-APW+lo) calculations. This combined analysis enables us to quantify the magnitude of the lattice relaxations induced by the presence of the impurity and to determine the charge state of the impurity donor level introduced by Ta in the band gap of the semiconductor.     

Three-dimensional ab initio dipole moment surfaces and stretching vibrational band intensities of the XH3 molecules

Stretching vibrational band intensities of XH₃ (X=N, Sb) molecules are investigated employing three-dimensional dipole moment surfaces combined with the local mode Hamiltonian model.The dipole moment surfaces of NH₃ and SbH₃ are calculated with the density functional theory and at the correlated MP2 level,respectively. The calculated band intensities are in good agreement with the available experimental data. The contribution to the band intensities from the different terms in the polynomial expansion of the dipole moments of four group V hydrides (NH₃, PH₃,AsH₃ and SbH₃) are discussed. It is concluded that the breakdown of the bond dipole approximation must be considered. The intensity “borrowing” effect due to the wave function mixing among the stretching vibrational states is found to be less significant for the molecules that reach the local mode limit.     

The effect of hydrogen-bonding cooperativity on the strength and properties of σ-hole interactions: an ab initio study

Ab initio calculations at the MP2/aug-cc-pVTZ level of theory are performed to investigate the effect of hydrogen-bonding cooperativity on the strength and bonding properties of σ-hole interaction in linear FCl???(NCH)_n=2–5, FHS???(NCH)_n=2–5, FH₂P???(NCH)_n=2–5 and FH₃Si???(NCH)_n=2–5 clusters. It is found that the cooperative effects in the hydrogen-bonding tend to strengthen the σ-hole interaction. However, these effects are almost saturated in the larger clusters (n > 5). For a given cluster, the amount of bond contraction in FCl???(NCH)_n is more important than other systems. A nice linear relationship is found between the σ-hole bond energies and absolute ¹⁵N chemical shieldings or spin–spin coupling constants across the σ-hole bond.     

Structural,electronic and optical properties of ultrathin thallium nanowires – an ab initio study

The energetics and structural, electronic and optical absorption properties of thallium nanowires, Tl _n with n?=?1–18, have been investigated by employing a first-principles density functional theory in the local density approximation. The spin–orbit (SO) interaction has also been considered. We study four types of stable structures: planar, caged, pyramidal and helical. In general, the binding energy increases with the coordination number except in a few cases where the nearest-neighbours lie at comparatively larger separations. The maximum stability is seen for the helical configurations containing pentagons, hexagons, heptagons and octagons. Nanowires containing a core linear chain of atoms on the tube axis are more stable than the corresponding nanotubes having no such chains. All the wires or tubes are found to be metallic with or without consideration of the SO interaction. The electronic structures of the pentagonal-, hexagonal- and octagonal-configuration wires provide a large number of channels, which may give rise to large quantum ballistic conduction. One finds large differences between the optical absorption calculated with and without the SO interaction. Consideration of the SO interaction enhances the number of absorption peaks by approximately a factor of two. A strong and multi-peaked optical absorption, extending up to 4.0?eV including the visible region, appears for wires containing pentagons and octagons. These wires may thus be useful as a source of white radiation.     

An ab initio study on the nature of σ-hole interactions in pnicogen-bonded complexes with carbene as an electron donor

ABSTRACT

A theoretical study of the complexes formed between ZH₂X (Z = P, As, Sb, Bi; X = F, Cl, Br, CN, NC, OH, NH₂) and an N-heterocyclic carbene (imidazol-2-ylidene) is carried out by means of ab initio calculations. According to molecular electrostatic potential analysis, it is inferred that the divalent C atom of the carbene can act as a Lewis base with the pnicogen atom Z of ZH₂X. The pnicogen bond distances (Z–C) are in the range of 2.050–2.911 for these complexes. While the Z?X bonds are longer than the corresponding Z?C bonds in the X = Cl and Br complexes, most of the Z?X bonds are short enough to suggest that they should be considered as covalent bonds which have lost some degree of covalency. For a given Z, the ZH₂Br forms the strongest complex, followed by ZH₂Cl and ZH₂F. On the other hand, the binding energy in the halogenated ZH₂X complexes follows the reverse ranking expected based on the values of the σ-hole of the isolated ZH₂X monomers. The nature of the pnicogen bond interaction in these complexes is analysed by quantum theory of atoms in molecules (QTAIM) and natural bond orbital methods. According to QTAIM analysis, a partially covalent character can be attributed to the pnicogen bonds studied here.     

Mutual influence between triel bond and cation–π interactions: an ab initio study

Using ab initio calculations, the cooperative and solvent effects on cation–π and B···N interactions are studied in some model ternary complexes, where these interactions coexist. The nature of the interactions and the mechanism of cooperativity are investigated by means of quantum theory of atoms in molecules (QTAIM), noncovalent interaction (NCI) index and natural bond orbital analysis. The results indicate that all cation–π and B···N binding distances in the ternary complexes are shorter than those of corresponding binary systems. The QTAIM analysis reveals that ternary complexes have higher electron density at their bond critical points relative to the corresponding binary complexes. In addition, according to the QTAIM analysis, the formation of cation–π interaction increases covalency of B···N bonds. The NCI analysis indicates that the cooperative effects in the ternary complexes make a shift in the location of the spike associated with each interaction, which can be regarded as an evidence for the reinforcement of both cation–π and B···N interactions in these systems. Solvent effects on the cooperativity of cation–π and B···N interactions are also investigated.     

Atomic resonance behavior in laser-induced Rb atom desorption—the effect of ambient gas atoms and molecules

We report an observation that in a typical optical pumping cell containing Rb metal and 150 Torr N₂ gas, a cw laser beam of a few tens of mW and beam size 5.5 mm² can desorb Rb films on cell surfaces when the laser is tuned to the Rb D₁ line. The frequency dependence of the Rb desorption rate displays atomic resonance behavior. The desorption is suggested to be mediated by the ambient gas atoms (Rb) and molecules (N₂). The phenomenon was used to provide evidence for the existence of thin Rb films on seemingly clear cell surfaces.     

Dissociation energy of diatomic molecules — comment on the work of Kaur and Mahajan

When observed spectrum of a diatomic molecule is expressed in terms of the Dunham coefficients Y ₀₀, Y ₁₀, Y ₂₀, Y ₀₁ and Y ₁₁ only, dissociation energy of the molecule is given by Y ₀₀+Y ₁₀ ² /(−4Y ₂₀). Kaur and Mahajan [1] have used the Dunham coefficients Y ₁₀, Y ₂₀, Y ₀₁, and Y ₁₁, for 15 vibrational states of 12 diatomic molecules (Y ₀₀ is zero for the cases accounted for), but their dissociation energy cannot be reproduced by the expression Y ₁₀ ² /(−4Y ₂₀). Probable reason for the discrepancy has been discussed.     

Characterization of Ag adsorption on TiC（001） substrate： an ab initio study

Ag adsorptions at 0.25-3 monolayer （ML） coverage on a perfect TIC（001） surface and at 0.25 ML coverage on C vacancy are separately investigated by using the pseudopotential-based density functional theory. The preferential adsorption sites and the adsorption-induced modifications of electronic structures of both the substrate and adsorbate are analysed. Through the analyses of adsorption energy, ideal work of separation, interface distance, projected local density of states, and the difference electron density, the characteristic evolution of the adatom-surface bonding as a function of the amount of deposited silver is studied. The nature of the Ag/TiC bonding changes as the coverage increases from 0.25 to 3 MLs. Unlike physisorption in an Ag/MgO system, polar covalent component contributes to the Ag/TiC interfacial adhesion in most cases, however, for the case of 1-3 ML coverage, an additional electrostatic interaction between the absorption layer and the substrate should be taken into account. The value of ideal work of separation, 1.55 J/m^2, for a 3-ML-thick adlayer accords well with other calculations. The calculations predict that Ag does not wet TIC（001） surface and prefers a three-dimensional growth mode in the absence of kinetic factor. This work reports on a clear site and coverage dependence of the measurable physical parameters, which would benefit the understanding of Ag/TiC（001） interface and the analysis of experimental data.     

Accurate ab initio potential energy curves and spectroscopic properties of the low-lying electronic states of OH− and SH− molecular anions

ABSTRACT

Multireference configuration interaction method was used in order to generate accurate potential energy curves of the OH, SH, OH^? and SH^? electronic states correlating to the three lowest dissociation limits. These curves were used in addition with core–valence correlation and scalar relativistic corrections for the calculations of accurate spectroscopic constants of bound states, which generally are found in excellent agreement with best available experimental and theoretical values in the literature. The spin–orbit interactions between electronic states have been calculated for the cases in which the couplings were assumed to be responsible for perturbations and used to explain the predissociation of A²Σ⁺ state of OH and SH by dissociative states 1⁴Σ^?, 1²Σ^? and 1 ⁴Π. Dipole moment functions were also computed along internuclear distances and used to explain polarity of these molecules in different calculated electronic states. In addition, stability and metastability of electronic states (X ¹Σ⁺, A¹Π and a³Π) of OH^? and SH^? molecular anions have been studied relatively to curves of neutral parent electronic states. Finally, we have computed adiabatic electron affinity of OH and SH and these values have been found in very good agreement with the best experimental values and resort as among the best achieved values.     

Exploring hydride-π interactions and their tuning by σ-hole bonds: an ab initio study

In the present work, ab initio calculations are performed to investigate the geometry, interaction energy and bonding properties of binary complexes formed between metal-hydrides HMX (M = Be, Mg, Zn and X = H, F, CH₃) and a series of π-acidic heteroaromatic rings. In all the resulting complexes, the heteroaromatic ring acts as a Lewis acid (electron acceptor), while the H atom of the HMX molecule acts as a Lewis base (electron donor). The nature of this interaction, called ‘hydride-π’ interaction, is explored in terms of molecular electrostatic potential, non-covalent interaction, quantum theory of atoms in molecules and natural bond orbital analyses. The results show that the interaction energies of these hydride-π interactions are between ?1.24 and ?2.72 kcal/mol. Furthermore, mutual influence between the hydride-π and halogen- or pnicogen-bonding interactions is studied in complexes in which these interactions coexist. For a given π-acidic ring, the formation of the pnicogen-bonding induces a larger enhancing effect on the strength of hydride-π bond than the halogen-bonding.     

Calculation of the Tautomeric Properties of 3‐Formyl‐Tetrinic Acid by the ab initio and DFT Methods

We have carried out a nonempirical quantumchemical calculation with full optimization of the geometry of all theoretically possible tautomeric forms of 3formyltetrinic acid (FTRA) according to the Møller–Plesset secondorder perturbation theory with the use of a 6–31G(d) double zeta basis set. The correlation corrections to the total energy of molecules for optimized geometric configurations were calculated in the 631G(d,p) basis. All the possible tautomeric forms of FTRA and the harmonic vibrational frequencies were also calculated within the density functional theory (DFT) with the use of the Perdew–Burke–Ernzerhof (PBE) functional in a threeexponent basis. It is shown that FTRA in vapors (in the perfect gas approximation) exists in the form of a mixture of three enol forms, among which two exoforms predominate. The frequencies and forms of normal vibrations for each cisenol tautomer in the region of vibrations of ketogroups and double bonds differ widely, which permits identification of the tautomers present in the mixture. The possible mechanisms of enolenol transformations of 3acyltetrinic acids are discussed.     

A frequency analysis of the ν9 band of CD3CD3: experiment and ab initio calculations

The infrared gaseous spectrum of CD₃CD₃ has been measured in the range of 530–670cm^?1 to investigate vibration—torsion effects in the ν₉ band. Three separate spectra all taken under different experimental conditions were recorded. The lines with (ΔK = ?1) and with high values of K show torsional splittings that are substantially larger than expected from the observed barrier height. These splittings are caused primarily by Coriolis-type interactions between the torsional stack of ν₉ = 1 and the corresponding stack for the ground vibrational state. Because of a near-degeneracy that exists between the states (ν₉ = 0, ν₄ = 3) and (ν₉ = 1, ν₄ = 0), three subbands (K, σ) = (15,1), (16,2), (17,3) are resonantly perturbed. For these cases, perturbation-allowed 3ν₄ torsional transitions have been identified. Here σ= 0, 1, 2 or 3 labels the torsional sublevels. Measurements from the ν₉ and 3ν₄ bands, frequencies from the far-infrared torsional spectra in the ground vibrational state, and lower state combination differences from the ν₉ + ν₄ ? ν₄ band were fitted to within experimental uncertainty using an effective Hamiltonian which considered three torsional stacks; one for the ground vibrational state and two for ν₉ = 1. In all, 22 parameters were determined using a total of 2001 lines. Of these, three parameters were the interstack couplings, eight are from the ground vibrational state and 11 are from the excited vibrational state. Two barrier-dependent torsion—rotation parameters, which were essential for obtaining a satisfactory fit, were calculated by ab initio methods.     

Magneto-electronic properties and spin-resolved I–V curves of a Co/GeSe heterojunction diode: an ab initio study

We present ab initio calculations of magnetoelectronic and transport properties of the interface of hcp Cobalt (001) and the intrinsic narrow-gap semiconductor germanium selenide (GeSe). Using a norm-conserving pseudopotentials scheme within DFT, we first model the interface with a supercell approach and focus on the spin-resolved densities of states and the magnetic moment (spin and orbital components) at the different atomic layers that form the device. We also report a series of cuts (perpendicular to the plane of the heterojunction) of the electronic and spin densities showing a slight magnetization of the first layers of the semiconductor. Finally, we model the device with a different scheme: using semiinfinite electrodes connected to the heterojunction. These latter calculations are based upon a nonequilibrium Green’s function approach that allows us to explore the spin-resolved electronic transport under a bias voltage (spin-resolved I–V curves), revealing features of potential applicability in spintronics.     

Transmission Probability of an Ultracold Atom in the Presence of Atomic Coherence

We investigate the transmission probability of an ultracold V-type three-level atom passing through a micromaser cavity,in the presence of atomic coherence which is established by a coherent driving field.We show that the transmissibility of this micromaser system with the atomic coherence is better than that of the ordinary micromaser system without atomic coherence.When the driving field is strong enough,for any cavity length the ultracold atom can pass through the micromaser cavity freely.     

Feshbach resonances of the C3H− anion: laser autodetachment spectroscopy and ab initio calculations

The electronic spectra of the C₃H^? and C₃D^? anions have been studied above the lowest electron detachment threshold. On the basis of the vibrational, rotational analysis and ab initio calculations, the photodetachment spectrum is assigned to the d³ A″←a³ A″ Feshbach resonance in the bent chain C₃H(D)^? anion. The vibronic system is characterized by a long vibrational progression involving the CCH in plane bending mode ν₄. The potential curves along this coordinate obtained from the spectral analysis and theoretical calculations reveal the importance of vibronic coupling in the electronic excited states. A strong Renner–Teller effect is thought to be the reason for the existence of the Feshbach resonance because the ⁴Σ^? neutral parent and the ³Π anion excited states are close in energy. As for the neutral, ν₄ appears to be the active mode and drives the interaction between the Feshbach and the dipole bound states.     

Nature of M-Ng interaction in the MNg 4 2+ (M = Cu, Ag and Au; Ng = He and Ne) molecules: ab initio calculations

Quantum chemical calculations of the structure and stability of MNg ₄ ²⁺ (M = Cu, Ag and Au; Ng = He and Ne) series at the CCSD(T) theoretical level are performed. The interaction mechanism is investigated using the natural bond orbital (NBO) and electron density analyses. The analyses of the electron density show the weak electrostatic interaction in the system.     

An eclipsed Csp3-CH3 bond? An ab initio investigation of an atypical rotation barrier

In 1987, Seiler et al. (1987, Angew. Chem. Int. Edn Engl., 26, 1175) reported the observation of an eclipsed Csp3-CH3 bond in the low-temperature X-ray analysis of a crystalline trihydrate tricyclic orthoamide. This contrasts with the observed staggered conformation in an anhydrous crystal of the tricyclic orthoamide. Modest quantum chemical calculations estimated that a 5.5 kcal-mol-1 energy difference between the eclipsed form and the expected staggered form could be accounted for by unprecedented hydrogen bonding interaction strengths due to cooperative effects. In 1993, Novoa et al. (1993, Angew. Chem. Int. Edn Engl, 32, 588) showed that the earlier calculations were in error since they did not correct for basis set superposition error (BSSE). The BSSE corrected results of Novoa et al. could not account for the necessary 5.5 kcal mol-1 and this led them to question the original X-ray analysis of Seiler et al. Both sets of calculations suffered from the use of insufficiently accurate levels of theory and an inappropriate choice of model system. A current best estimate for the barrier to rotation is 1.4 kcal mol-1, far below that of the earlier studies. The stabilization of the eclipsed geometry due to hydrogen bonding after correction for BSSE is enough to match this small barrier. At today's best level of theory, the experimentally observed structure is 0.2 kcal mol-1 more stable than the staggered geometry.