The geometry and electronic structure of the ionic defect in a chain of water molecules between a donor and an acceptor

Quantum chemical calculations of the molecular complexes (NH₃)₃Zn²⁺...(H₂O)_n3...NH₃ (C_n, n=11, 16, 21, and 30) that model the proton donor-aqueous chain-acceptor channel in biological molecules were performed. Periodicity of O-H bond lengths in water chains and charges of the H atoms of H-bonds observed earlier were discussed. In C_n complexes, the geometry and electronic structure of the ionic defect in the aqueous chain with an excess proton were studied. The distributions of O-H bond lengths and charges on H-bond H atoms in the region of the ionic defect obtained in ab initio (B3LYP/6-31+G^**) and semiempirical (PM3) calculations are compared. The influence of aqueous chain extension, the position of the protonated water molecule, and the mobility of water molecules in the chain on the structure of the ionic defect was analyzed.     

Character of the electronic ground state and of charge-transfer excited states in ionic solids: An ab initio cluster model approach

The electronic structure of the ground electronic state and of some special charge-transfer excited states in ionic solids is examined from the ab initio cluster model approach. Different ab initio wave functions, including a frozen orbital approach, the Hartree–Fock self-consistent field, and multireference configuration interaction wave functions, are considered and analyzed using different theoretical techniques. We explicitly consider some alkaline–earth oxides such as CaO, a more difficult case such as A1₂O₃, a transition-metal oxide such as NiO, and a system with a more complicated structure such as KNiF₃. Analysis of ab initio wave functions in terms of valence bond components shows that all these compounds are largely ionic, thus supporting the simple picture arising from the ionic model. However, the nature of the excited states is more complex. Alkaline–earth oxides lowest excited states are essentially described as charge-transfer excitations dominated by a single resonant valence bond structure and the calculated energy difference is comparable to the experimental optical gap. In the case of A1₂O₃, the electronic spectra presents excitonic features and the local charge-transfer excitation excited states provide a reasonable representation of these phenomena. Finally, several different valence bond structures are present in the lowest electronic states of KNiF₃. © 1994 John Wiley & Sons, Inc.     

On the role of symmetry in the ab initio hartree-fock linear-combination-of-atomic-orbitals treatment of periodic systems

The symmetry properties of the mono- and bielectronic terms contributing to the Fock matrix in the ab initio Hartree–Fock treatment of periodic systems are discussed. A computational scheme which takes full advantage of the point symmetry is presented; in this respect, it represents a generalization of the scheme proposed in Int. J. Quantum Chem. 17 , 501 (1980). Computational details and numerical examples are reported; it is shown that with respect to two of the bottlenecks of this kind of calculation, namely, computer time and backing storage required for the bielectronic integrals, it is possible to obtain saving factors as large as h and h², respectively, where h is the order of the point group. Preliminary tests are reported which indicate that the study of relatively complicated systems, like quartz or corundum (9 and 10 atoms in the unit cell, respectively) at an ab initio Hartree–Fock level is now within reach.     

On the electronic structure of transition-metal oxide cations:DFTCalculations for VO+ and MoO+

Two transition-metal oxide diatomic cations, VO⁺ and MoO⁺ are considered in this article. Ground- and excited-state properties of the cations are derived from spin-polarized DF calculations, including spectroscopic constants and metal–oxygen bonding features. A set of ionization potentials are calculated and, for vanadium oxide, compared with photoelectron spectroscopy data and a few available ab initio calculations. All calculated properties are close to experiment, the agreement being much better than for other traditional quantum chemical calculations. Present results together with our earlier findings for neutral molecules provide an excellent confirmation of the good performance of DFT in the case of transition-metal systems. © 1995 John Wiley & Sons, Inc.     

Metal Complexes of Pentadentate Macrocyclic Ligands Containing Oxygen and Nitrogen as Donor Atoms

Four macrocyclic ligands have been synthesized: 1-oxa-4,7,10,13-tetraazacyclopentadecane ( 1 ), 1,4-dioxa-7,10,13-triazacyclopentadecane ( 2 ), 1,4-dioxa-7,11,14-triazacyclohexadecane ( 3 ), 1,4-dioxa-7,11,15-triazacycloheptadecane ( 4 ), one of them 3 , for the first time. The protonation constants of the ligands and the stability constants of the complexes formed by the four ligands with some divalent first-series transition-metal ions, Cd²⁺ and Pb²⁺, were determined by potentiometric methods, in aqueous solution, at 25° and I = 0.10M (KNO₃). The sequence of protonation of ligand 1 was studied by ¹H-NMR spectroscopy. The Irving-Williams' order of stability is obeyed for the complexes of all the ligands, and the metal complexes of 1 present the higher values of stability. A drop in the stability of all the metal complexes studied is observed when the metal complexes of 1 are compared with the corresponding complexes of 2 . The effect of the increase of the ring size of the macrocycle can be observed for metal complexes of the series of ligands 2 4 , where, in spite of the slight increase of the overall basicity of the ligands (20.28, 22.25, and 24.96 for 2, 3 , and 4 respectively), small differences in stability are found for the corresponding complexes of 2 and 3 , but a significant drop occurs for all the metal complexes formed with the 17-membered ligand, specially for the larger metal ions like Mn²⁺ and Pb²⁺.     

Equilibrium structure and spectroscopic constants of maleic anhydride

The quadratic, cubic, and semi-diagonal quartic force fields of maleic anhydride have been calculated at the MP2 level of theory employing the cc-pVTZ basis set. The spectroscopic constants derived from the force field are in excellent agreement with the corresponding experimental values. The semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational corrections calculated from the cubic force field. This semi-experimental equilibrium structure is in excellent agreement with the ab initio structures computed at the CCSD(T) level of theory and it is closer to the ab initio structure than the purely experimental (or empirical) structures r ₀, r _m⁽¹⁾, and r _m⁽²⁾ obtained by microwave spectroscopy as well as the equilibrium structure derived from gas-phase electron diffraction data.     

Pyramidal distortions in the ethylene triplet π,π* state

Pyramidal distortions which may stabilize the triplet π,π* state of ethylene are investigated using ab initio molecular orbital calculations. Using a minimal basis set, the calculated stabilization energies are 5.1 kcal mole^?1 for a cis flapped conformation, 3.6 kcal mole^?1 for a trans flapped conformation, and 0.8 kcal mole^?1 for a twisted flapped conformation. With a double zeta basis set the predicted stabilization energies are much smaller: 1.3, 0.7, and 0.0 kcal mole^?1, respectively.     

Studies on mixture of ionic liquid EMIGaCl4 and EMIC

The densities of ionic liquid compound EMIGaCl₄ and its mixtures with EMIC were measured by pycnometry in the temperature range from 308.15 to 343.15 K. The values of apparent molar volume of the mixture, partial molar volume and apparent molar expansibilities of EMIC were determined. Both Raman scattering and ab initio calculations were performed. All results show that GaCl₄⁻ is sole anion in ionic liquid compound EMIGaCl₄ and is predominant one in the mixture composed of EMIC and ionic liquid compound EMIGaCl₄.     

Toward ab initio refinement of protein X-ray crystal structures: interpreting and correlating structural fluctuations

The refinement of protein crystal structures currently involves the use of empirical restraints and force fields that are known to work well in many situations but nevertheless yield structural models with some features that are inconsistent with detailed chemical analysis and therefore warrant further improvement. Ab initio electronic structure computational methods have now advanced to the point at which they can deliver reliable results for macromolecules in realistic times using linear-scaling algorithms. The replacement of empirical force fields with ab initio methods in a final refinement stage could allow new structural features to be identified in complex structures, reduce errors and remove computational bias from structural models. In contrast to empirical approaches, ab initio refinements can only be performed on models that obey basic qualitative chemical rules, imposing constraints on the parameter space of existing refinements, and this in turn inhibits the inclusion of unlikely structural features. Here, we focus on methods for determining an appropriate ensemble of initial structural models for an ab initio X-ray refinement, modeling as an example the high-resolution single-crystal X-ray diffraction data reported for the structure of lysozyme (PDB entry “2VB1”). The AMBER force field is used in a Monte Carlo calculation to determine an ensemble of 8 structures that together embody all of the partial atomic occupancies noted in the original refinement, correlating these variations into a set of feasible chemical structures while simultaneously retaining consistency with the X-ray diffraction data. Subsequent analysis of these results strongly suggests that the occupancies in the empirically refined model are inconsistent with protein energetic considerations, thus depicting the 2VB1 structure as a deep-lying minimum in its optimized parameter space that actually embodies chemically unreasonable features. Indeed, density-functional theory calculations for one specific nitrate ion with an occupancy of 62% indicate that water replaces this ion 38% of the time, a result confirmed by subsequent crystallographic analysis. It is foreseeable that any subsequent ab initio refinement of the whole structure would need to locate a globally improved structure involving significant changes to 2VB1 which correct these identified local structural inconsistencies.     

Correlation effects on energy curves for proton transfer. The cation [H5O2]+

Potential curves for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺ were calculated by both ab initio and semi-empirical LCAO MO SCF CI methods. The energy barrier of the symmetric double minimum potential in [H₅O₂]⁺ is very sensitive to electron correlation. At an OO distance of 2.74 Å it decreases from the HF value of 9.5 kcal/mole to about 7.0 kcal/mole. The results of the semi-empirical calculations agree well with the ab initio data as long as only relative effects are regarded. The partitioning of correlation energy into contributions of individual electron pairs is very similar for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺. In this example the proton transfer appears as a superposition of two “contracted ionic dissociation” processes. An interpretation of the behaviour of correlation during these processes is presented.     

Quantum-chemical study of the geometric and electronic structure of the CoC2 molecule

DFT (B3LYP) calculations have been performed to study the CoC₂ molecule in its different geometric conformations and electronic states. The energies have been refined using ab initio multiconfigurational CASSCF/CASPT2 calculations. Both approaches are in a good semi-quantitative agreement between themselves and predict the symmetric triangular (C_2v) structure to be more stable than the linear (C_∞v) conformation. The ground state has been found to be a quartet, which can formally be regarded as an ionic Co²⁺–C₂²⁻ complex, resulting from a transfer of the two 4s electrons of the cobalt atom to the 3σ_g orbital of the C₂ ligand and distributing the remaining seven valence electrons over the split 3d orbitals.     

Ab initio excited states calculations of Kr 3 + , probing semi-empirical modelling

The accuracy of the diatomics-in-molecules (DIM) model for the krypton ionic trimer is examined in a series of ab initio calculations. In the C_2v symmetry, the ground states of irreducible representations B₂ and A₁ were calculated using partially spin restricted open-shell coupled cluster method with perturbative triple connections (RHF-RCCSD-T), the relativistic effective core potential (RECP) and an extended basis set of atomic orbitals. Internally contracted multireference configuration interaction method (icMRCI) with the extended and restricted basis set was used to generate the potential energy surfaces (PESs) of the nine electronic states of Kr ₃ ⁺ corresponding to Kr(¹S) + Kr(¹S) + Kr⁺(²P) dissociation limit in a wide interval of nuclear geometries. The overall agreement of the accurate ab initio PESs and the diatomics-in-molecules PESs confirms the quality of the DIM Hamiltonian for the Kr ₃ ⁺ clusters and justifies its use in dynamical and spectroscopic studies of the Kr _n ⁺ clusters. Inclusion of the spin–orbit coupling into the ab initio PESs through a semi-empirical scheme is proposed.     

(2+1) REMPI of NO via the D 2Σ+ state: rotational branching ratios

Recent photoelectron spectroscopic studies in a (2 + 1) REMPI of NO via the Rydberg D²Σ⁺ state have revealed anomalous ionic rotational branching ratios. We have performed ab initio calculations of these branching ratios and find that the molecular nature of the ionization continuum plays an essential role in the dynamics. Even though the bound orbital is very atomic-like (⪢ 98% p-like), the photoelectron continuum wavefunction is quite sensitive to the non-spherical nature of the molecular ionic potential and causes a strong persistence of the p-partial wave which, in turn, leads to a large ΔN = 0 peak.     

Application of Static Charge Transfer within an Ionic‐Liquid Force Field and Its Effect on Structure and Dynamics

The effects of linear scaling of the atomic charges of a reference potential on the structure, dynamics, and energetics of the ionic liquid 1,3‐dimethylimidazolium chloride are investigated. Diffusion coefficients that span over four orders of magnitude are observed between the original model and a scaled model in which the ionic charges are ±0.5 e. While the three‐dimensional structure of the liquid is less affected, the partial radial distribution functions change markedly—with the positive result that for ionic charges of ±0.7 e, an excellent agreement is observed with ab initio molecular dynamics data. Cohesive energy densities calculated from these partial‐charge models are also in better agreement with those calculated from the ab initio data. We postulate that ionic‐liquid models in which the ionic charges are assumed to be ±1 e overestimate the intermolecular attractions between ions, which results in overstructuring, slow dynamics, and increased cohesive energy densities. The use of scaled‐charge sets may be of benefit in the simulation of these systems—especially when looking at properties beyond liquid structure—thus providing an alternative to computationally expensive polarisable force fields.     

Experimental and computational study of the pKa of coumaric acid derivatives

The dissociation constant (pK_a) of a drug is a key parameter in drug discovery and pharmaceutical formulation. The hydroxy substituent has a significant effect on the acidity of hydroxycinnamic acid. In this work, the acidic constants of coumaric acids are obtained experimentally by spectrophotometry using the chemometric method and calculated theoretically using ab initio quantum mechanical method at the HF/6‐31G* level of theory in combination with the SMD continuum solvation method. Rank annihilation factor analysis (RAFA) is an efficient chemometric technique based on the elimination of the contribution of one of the chemical components from the data matrix. RAFA cannot be performed because the pure spectrum of HA^? is not available. So, two‐rank annihilation factor analysis (TRAFA) is proposed for the determination of the pK_a OF H₂A. A comparison between the pK_a values obtained previously by TRAFA for the molecules o‐coumaric acid (4.13, 9.58), m‐coumaric acid (4.48, 10.35), and p‐coumaric acid (4.65, 9.92) makes it clear that there is good agreement between the results obtained by TRAFA and ab initio quantum mechanical method.     

Electronic structure and ionic conductivity in calcium- and yttrium-stabilized zirconium dioxide

The electronic structure, chemical bonding, and ionic transport were investigated for ZrO₂ polymorphs and solid solutions ZrO₂-Ca0 and ZrO₂-Y₂O₃ using the LMTO ab initio self-consistent method in a tight binding approximation and Hückel’s semiempirical method. In stabilized zirconium dioxide, ionic conductivity is mainly due to vacancies in the oxygen sublattice. The mechanism of the fluorite stmcture by calcium and yttrium impurities is considered. The nonlinear variation of the ionic conductivity of ZrO₂ is interpreted as a function of the phase composition of solid solutions. The clusterization effect is first supported by ab initio calculations. Translated fromZhumal Stivktumoi Khimii, Vol. 41, No. 2, pp. 229–239, March-April, 2(XX).     

Magnetic Anisotropy of Mononuclear NiII Complexes: On the Importance of Structural Diversity and the Structural Distortions

Mononuclear Ni^II complexes are particularly attractive in the area of single‐molecule magnets as the axial zero‐field splitting (D) for the Ni^II complexes is in the range of ?200 to +200 cm^?1. Despite this advantage, very little is known on the origin of anisotropy across various coordination ligands, coordination numbers, and particularly what factors influence the D parameter in these complexes. To answer some of these questions, herein we have undertaken a detailed study of a series of mononuclear Ni^II complexes with ab initio calculations. Our results demonstrate that three prominent spin‐conserved low‐lying d–d transitions contribute significantly to the D value. Variation in the sign and the magnitude of D values are found to correlate to the specific structural distortions. Apart from the metal–ligand bond lengths, two different parameters, namely, Δα and Δβ, which are correlated to the cis angles present in the coordination environment, are found to significantly influence the axial D values. Developed magneto–structural D correlations suggest that the D values can be enhanced significantly by fine tuning the structural distortion in the coordination environment. Calculations performed on a series of Ni^II models with coordination numbers two to six unfold an interesting observation—the D parameter increases significantly upon a reduction in coordination number compared with a reference octahedral coordination. Besides, if high symmetry is maintained, even larger coordination numbers yield large D values.     

Diabatic investigation for the NaRb molecule

An extensive diabatic investigation of the NaRb species has been carried out for all excited states up to the ionic limit Na^‐Rb⁺. An ab initio calculation founded on the pseudopotential, core polarization potential operators and full configuration interaction has been used with an efficient diabatization method involving a combination of variational effective hamiltonian theory and an effective overlap matrix. Diabatic potential energy curves and electric dipole moments (permanent and transition) for all the symmetries Σ⁺, Π, and Δ have been studied for the first time. Thanks to a unitary rotation matrix, the examination of the diabatic permanent dipole moment (PDM) has shown the ionic feature clearly seen in the diabatic ¹Σ⁺ potential curves and confirming the high imprint of the Na^‐Rb⁺ ionic state in the adiabatic representation. Diabatic transition dipole moments have also been computed. Real crossings have been shown for the diabatic PDM, locating the avoided crossings between the corresponding adiabatic energy curves.     

Magnetic Properties and Electronic Structure of Neptunyl(VI) Complexes: Wavefunctions,Orbitals, and Crystal‐Field Models

The electronic structure and magnetic properties of neptunyl(VI), NpO₂²⁺, and two neptunyl complexes, [NpO₂(NO₃)₃]^? and [NpO₂Cl₄]^2?, were studied with a combination of theoretical methods: ab initio relativistic wavefunction methods and density functional theory (DFT), as well as crystal‐field (CF) models with parameters extracted from the ab initio calculations. Natural orbitals for electron density and spin magnetization from wavefunctions including spin–orbit coupling were employed to analyze the connection between the electronic structure and magnetic properties, and to link the results from CF models to the ab initio data. Free complex ions and systems embedded in a crystal environment were studied. Of prime interest were the electron paramagnetic resonance g‐factors and their relation to the complex geometry, ligand coordination, and nature of the nonbonding 5f orbitals. The g‐factors were calculated for the ground and excited states. For [NpO₂Cl₄]^2?, a strong influence of the environment of the complex on its magnetic behavior was demonstrated. Kohn–Sham DFT with standard functionals can produce reasonable g‐factors as long as the calculation converges to a solution resembling the electronic state of interest. However, this is not always straightforward.     

Molecular orbital study of the bridge bonding in an electron deficient molecule [(CH3)2ALH]2

The electronic distribution in the AlH₂Al bridge of the dimethyl aluminium hybride dimer was computed through ab initio SCF calculation. Comparison with diborane shows an increased role of the ionic Al⁺H ₂ ^2– Al⁺ structures with respect to the usual covalent three-center bonds.