Prediction of vibrational constants for gas-phase heteronuclear diatomics and atoms adsorbed on surfaces

An empirical relationship is presented for predicting vibrational constants for gas-phase heteronuclear diatomic molecules and atoms adsorbed on surfaces. Excellent agreement with experimental values is obtained. The method is used for predicting the unknown vibrational constants of AgS and CTe.     

The lowest energy states of the group-IIIA-group-VA heteronuclear diatomics: BN, BP, AlN, and AlP from full configuration interaction calculations

Full configuration interaction (CI) calculations on the group-IIIA-group-VA diatomic molecules BN, BP, AlN, and AlP have been performed with the cc-pVTZ correlation-consistent basis set and compared to CCSD(T) calculations with the same basis set. The CCSD(T) calculations are good to better than 1 kcal/mol in comparison with the full CI results if the T(1) diagnostic is small and to within about 2 kcal/mol if the T(1) diagnostic is large. Inspection of the T(2) amplitudes shows that the simple T(1) diagnostic is providing useful insight into the quality of the starting wave function. The ground state of BN, BP, and AlN is predicted to be the (3)Pi and, for AlP, the ground state is predicted to be (3)Sigma(-). For all molecules except BP, there is an excited state within 1.1 kcal/mol of the ground state. The ordering of the state energies can be explained in terms of simple orbital and bonding models. The results provide little evidence for placing the pi orbital below the sigma orbital for the ground state of these heteronuclear diatomic molecules.     

On the electronic structure of transition-metal complexes II. Bonding characteristics in titanium chloride systems

The electronic structure of [TiCl₄]ⁿ systems (n = 0, ?1, ?2, ?3, and ?4) has been investigated by the INDO method. Calculations show the stability of the tetrahedral structure over the square-planar one, for any value of n, in accord with experimental observation. The occupation of the 4p orbitals of the titanium atom was found to increase regularly as the dihedral angle decreases and reaches a maximum at the square-planar conformation. Energy partitioning analysis enabled much deeper understanding of bonding characteristics of the studied systems. It has been shown that the one-atom energy component may be a very sensitive measure of the stability of the central metal atom in its various oxidation states.     

On the bonding in doubly charged diatomics

Summary The potential energy of interacting atomic ions A⁺+B⁺ often shows a shallow local minimum separated by a broad potential barrier from the dissociation products at much lower energy. Early interpretations of dication potential shapes were based on the similarity of the electronic structure between isoelectronic neutral and ionic species and led to a picture of a chemical bond superimposed on a repulsive Coulomb potential. More recently, barriers in dication potentials have commonly been interpreted as avoided curve crossings involving covalent and ionic structures. In this paper, we demonstrate that the former model is the appropriate one except in cases with very small asymptotic ionic/covalent energy splittings. By deriving dication wavefunctions from their neutral isoelectronic counterparts, we obtain upper bound dication potential curves which show all the characteristic features. By further modeling induction effects, we arrive at an almost quantitative fit of accurateab initio dication potentials. The chemical bond plus electrostatic repulsion interpretation of dication interactions also explains why the accurate calculation of potential curves appears to be much more demanding for dications than for isoelectronic neutrals.Dedicated in the honor of Prof. Klaus Ruedenberg     

Configuration interaction: Molecular orbitals for accurate calculations on diatomics

A procedure is developed to construct an optimum set of molecular orbitals (MO's) to be used in large scale configuration interaction expansion for diatomics. The set is optimum in the sense that a significant energy improvement can be obtained for a relatively short wavefunction expansion. Application of this methodology to the ground state of the LiH molecule gave an energy of –8.06345 a.u. for an expansion with 1852 terms obtained from a set with 16-, 12-, and 6-type MO's.     

Bonding analysis and the mechanisms on the ring-opening of alkoxy-bridged bis(silylene) transition-metal complexes toward MeOH

The mechanistic study on the ring-opening of alkoxy-bridged bis(silylene) transition-metal complexes toward MeOH is performed by using density functional theory. Four steps are predicted to be involved in the reaction, formation of hydrogen bonding between R and a MeOH, ring-opening of the Ru-Si1-O1-Si2 four-membered ring, formation of the six-membered ring, and the hydroxyl hydrogen migration to the metal center. It is found that the reaction is favorable thermodynamically and the hydroxyl hydrogen migration is the rate-determining step. Systematic variations of the structural parameters involved in the reaction mechanism are revealed, which revealed the relationship of the bond strength among Ru-Si, Si-O and O-H bonds.     

Regularities in molecular properties of ground state stable diatomics

A simple relationship is reported between vibrational frequencies, bond lengths, and reduced masses for many families of stable, ground state diatomics: the frequency is proportional to the reciprocal of the product of the bond length and the square root of the reduced mass. This is demonstrated with each of the following related families: the alkali metal diatomics, the group 15 diatomics, the group 16 diatomics, the halogen diatomics, the alkali metal hydrides, the alkaline earth oxides, the group 14 oxides and their sulfides, the diatomics of carbon, of silicon and of germanium with group 16 elements, the hydrogen halides, the halides of lithium, of sodium, of potassium, of rubidium and of cesium, the chlorides of the alkali metals and of silver, and the polyatomic hydrides of groups 14 and 15. Although correlation coefficients of 0.99 or greater in each of the 21 families examined demonstrate the validity of the correlation, the deviations found are significantly larger than can be attributed to experimental uncertainties.     

The simplest heteronuclear metal cluster: LiBe

An electronic transition with an origin near 19203 cm⁻¹ is observed in vapour produced by YAG laser vaporization of a pressed pellet of Li and Be powders and assigned to LiBe diatomic. Vibrational analysis yields a G_1/2″=295 cm⁻¹ and G_1/2′=188 cm⁻¹. A preliminary rotational analysis gives r₀″=2.59 Å and r₀′=3.04 Å. The spectroscopy and the electronic structure are discussed and the experimental molecular constants show a very good agreement with our recent state of the art ab initio calculated values.     

Collisional energy transfer in tin by diatomics and inert gases

Collisional energy transfer between laser-excited atomic tin and both inert-gas and diatomic collision partners has been studied Relative populations of collisionally populated tin levels were determined For certain levels, diatomics transferred nearly two orders of magnitude more population than inert gases; for other levels there was nearly equal population transfer by both groups of partners.     

Modelling of potential energy curves for diatomics: Reference molecular potential method

A method is suggested for modelling the potential energy curves of diatomics. It involves a search for the mapping which transfers the structural details from an analytically prescribed reference potential. For the ground-state potentials of seven covalent molecules, the accuracy in describing the spectroscopic region is comparable with the best empirical potentials, and the correct asymptotic behaviour at R → 0 and R → ∞ are also obtained.     

Bond centred functions in relativistic and non-relativistic calculations for diatomics

In this paper, we discuss the performance of molecular basis sets consisting of atomic centred (AC) functions augmented with bond centred (BC) functions in relativistic and non-relativistic calculations carried out at the Hartree–Fock and several correlated levels of approximation. While usually non-correlated calculations employing BC functions can be performed at a lower computational cost as compared with those making use of energy optimized AC basis sets, the correlated calculations are always more accurate and less expensive with the latter. It is demonstrated that both correlated or non-correlated calculations always benefit from the addition of a few BC functions with a moderate increase of computational effort. The performance of basis sets containing even-tempered BC functions is also studied and their usage is advocated in case of relativistic calculations.     

Bond-length fluctuations in transition-metal oxoperovskites

Bond-length fluctuations in transition-metal oxoperovskites may give rise to two-phase fluctuations in what appears to be a single phase to a diffraction experiment. Orbital disorder at Jahn-Teller ions results in bond-length fluctuations that give 3D-ferromagnetic, vibronic Mn(III)-O-Mn(III) superexchange interactions and allow disproportionation into Mn(IV) and Mn(II) in LaMnO₃; where orbitally ordered and disordered phases coexist, an external magnetic field stabilizes the orbitally disordered, ferromagnetic phase relative to the orbitally ordered, antiferromagnetic phase. Spin-lattice interactions in the paramagnetic phase of charge-transfer compounds give bond-length fluctuations arising from the semicovalent component of the superexchange interactions. At the crossover from localized to itinerant electronic behavior, the coexistence of two-phase fluctuations has been demonstrated in both the single-valent RNiO₃ family (R=rare-earth, A=alkaline-earth) and the mixed-valent R_0.5A_0.5MnO₃ perovskites. “Bad-metal” behavior is found to be associated with bond-length fluctuations.     

Molecular transition-metal phosphonates

Molecular transition-metal phosphonates are of relatively recent origin and can be assembled by several synthetic strategies. The nuclearity and the structure of the metal aggregates can be modulated by several factors including the stoichiometry of the reactants, nature of the metal precursor and the type of phosphonic acid used. This perspective summarizes some of the recent work carried out on copper(II)-, zinc(II)- and cadmium(II) phosphonates with particular emphasis on their synthesis and structure.     

A simple sum rule for total radiative decay rates in diatomics

The quantity (ν³μ_e²), which occurs in expressions for the total radiative decay rate for an excited vibronic state of a diatomic, is evaluated as a simple expectation value for the initial state. In test calculations this expectation value agrees with the exact sum over states within ≈0.2%.     

Astatine Facing Janus: Halogen Bonding vs. Charge-Shift Bonding

The nature of halogen-bond interactions was scrutinized from the perspective of astatine, potentially the strongest halogen-bond donor atom. In addition to its remarkable electronic properties (e.g., its higher aromaticity compared to benzene), C₆At₆ can be involved as a halogen-bond donor and acceptor. Two-component relativistic calculations and quantum chemical topology analyses were performed on C₆At₆ and its complexes as well as on their iodinated analogues for comparative purposes. The relativistic spin–orbit interaction was used as a tool to disclose the bonding patterns and the mechanisms that contribute to halogen-bond interactions. Despite the stronger polarizability of astatine, halogen bonds formed by C₆At₆ can be comparable or weaker than those of C₆I₆. This unexpected finding comes from the charge-shift bonding character of the C–At bonds. Because charge-shift bonding is connected to the Pauli repulsion between the bonding σ electrons and the σ lone-pair of astatine, it weakens the astatine electrophilicity at its σ-hole (reducing the charge transfer contribution to halogen bonding). These two antinomic characters, charge-shift bonding and halogen bonding, can result in weaker At-mediated interactions than their iodinated counterparts.     

Halogen Bonding versus Hydrogen Bonding: A Molecular Orbital Perspective

We have carried out extensive computational analyses of the structure and bonding mechanism in trihalides DX⋅⋅⋅A⁻ and the analogous hydrogen-bonded complexes DH⋅⋅⋅A⁻ (D, X, A=F, Cl, Br, I) using relativistic density functional theory (DFT) at zeroth-order regular approximation ZORA-BP86/TZ2P. One purpose was to obtain a set of consistent data from which reliable trends in structure and stability can be inferred over a large range of systems. The main objective was to achieve a detailed understanding of the nature of halogen bonds, how they resemble, and also how they differ from, the better understood hydrogen bonds. Thus, we present an accurate physical model of the halogen bond based on quantitative Kohn–Sham molecular orbital (MO) theory, energy decomposition analyses (EDA) and Voronoi deformation density (VDD) analyses of the charge distribution. It appears that the halogen bond in DX⋅⋅⋅A⁻ arises not only from classical electrostatic attraction but also receives substantial stabilization from HOMO–LUMO interactions between the lone pair of A⁻ and the σ* orbital of D–X.     

An approximate diatomics in molecules formulation of generalized valence bond theory

The slow computational speed of the generalized valence bond perfect pairing method (GVB-PP) has been an impediment to its routine use. We have addressed this problem by employing a diatomics in molecules Hamiltonian derived from a second quantization perturbation approach. This results in all three- and four-centered two-electron integrals being dropped from the traditional GVB-PP calculation. For moderate sized molecules, as for example C20 computed with a double zeta + polarization basis, there is on average a fifty-fold decrease in computational times. In this article, we present the theory behind our approach and analyze the accuracy and speed of this approximate GVB-PP method for several cases where density functional methods have produced ambivalent results.