Spectroscopic and quantum chemical study on electronic and geometric properties of free and embedded dithizone molecules

The molecular and electronic structure of the three tautomeric forms of dithizone has been calculated by using semiempirical, density functional theory (DFT), and ab initio methods. Comparison of ground‐state energies shows the symmetric form most stabilized, but there is only a small barrier (<3 kcal/mol) for the hydrogen transfer from N? H toward H? S (enol form). For understanding the origin of the optical transitions intermolecular interactions have to be taken into account. By using the supermolecule method, the absorption band pattern can be rationalized already on the level of the PM3 model. The nuclear magnetic resonance (NMR) solution spectrum is interpreted in terms of an equilibrium between the symmetric and the enol forms of dithizone. The appearance of strong EPR signals only for the solid state reflects a considerable lowering of the triplet state (symmetric form). Experimental features are discussed in view of calculated energies (stabilization), chemical shifts (NMR), and SOMO orbitals (EPR). © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Spectroscopic properties of large molecules in the frenkel exciton approximation

The Frenkel exciton approximation is reviewed and used to calculate the absorption and circular dichroism of large systems composed of weakly-interacting subunits. It is shown that for a system composed of very weakly interacting identical subunits, the lowest-order effect of the interactions is to produce a frequency shift Δω= (l/ħ)tr[H′A]/[A] in the absorption maximum. Here H′ is a matrix representing the interactions between the subunits, while A is an “optical matrix” constructed from the positions and transition dipole moments of the subunits. It is also shown that the total circular dichroism cross section for a band of exciton states is approximately proportional to ϱ′(ω))tr[H′B], where ϱ′(ω) is the derivative of the line-shape function and B is another type of “optical matrix”.     

Spectroscopic and theoretical study of the electronic structure of curcumin and related fragment molecules

The low volatility and thermal instability made the photoelectron (PE), electron transmission (ET), and dissociative electron attachment (DEA) spectroscopy measurements on curcumin (a potent chemopreventive agent) unsuccessful. The filled and empty electronic structure of curcumin was therefore investigated by exploiting the PES, ETS, and DEAS results for representative fragment molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron attachment energies of curcumin was proposed. The pi frontier molecular orbitals (MOs) are characterized by sizable interaction between the two phenol rings transmitted through the dicarbonyl chain and associated with a remarkably low ionization energy and a negative electron attachment energy (i.e., a largely positive electron affinity), diagnostic of a stable anion state not observable in ETS. The lowest energy electronic transitions of half-curcumin and curcumin and their color change by alkalization were interpreted with time-dependent density functional theory (DFT) calculations. For curcumin, it is shown that loss of a phenolic proton occurs in alkaline ethanolic solution.     

Empirical correlations between gas-chromatographic retention data and physical or topological properties of solute molecules

Kováts retention indices (I) and relative retention times (τ) using Silicone oil 550 or Bentone 34 as the stationary phase were measured for 20 hydrocarbons (mainly alkylbenzenes). Statistical analysis of repeated determinations indicated that the empirical distribution function of retention indices is not normal; the relative standard deviations decrease with increasing retention index. Several models were examined to describe the retention data as a function of boiling point (T_B) and molar refraction, molar volume, connectivity indices, etc. A universal fitting equation y_i=A exp (BT_Bi) + Cx_2i + D (where y_i=I or τ, and x_2i is one of the properties mentioned) proved to be best on the basis of statistical characteristics (examination of residuals, multiple correlation coefficient, residual mean square, variance ratio tests) which were also evaluated. The results were compared to equations recommended in the literature.     

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.     

Influence of electronic correlations on the ground-state properties of cerium dioxide

The electron-correlation effects on the ground-state properties of CeO(2) are studied by ab initio quantum-chemical methods. For this purpose the method of increments is applied. It combines Hartree-Fock calculations for periodic systems with correlation calculations requiring only information of the corresponding finite-cluster calculations. Using the coupled-cluster approach for the evaluation of the individual increments, we recover 93% of the experimental cohesive energy. The lattice constant and bulk modulus are found to be in good agreement with experimental values. For comparison also the results obtained with density functional methods are presented.     

Theoretical and experimental investigation of the structural and spectroscopic properties of coumarin 343 fluoroionophores

A joint theoretical-experimental investigation has been carried out to unravel the details of the complexation of cations by fluoroionophores based on coumarin 343 and to interpret the modifications in the ligand and also in the coumarin structural, electronic, magnetic, and vibrational properties. It is confirmed that C343-dea (1) complexes the cations by both the lactone and the amide oxygen atoms whereas for C343-crown (2) and C343-dibenzocrown (3), the cations are complexed by the oxygen atoms of the lactone as well as those of the crown ligand. These complexations induce geometric modifications, which are delocalized over the coumarin backbone and are related to electronic reorganizations that modify the spectroscopic signatures. This paper analyzes these signatures and shows how they are related as well as how they can be used to monitor the complexation process. Upon complexation, the UV-visible absorption spectra display a bathochromic shift of the most intense electronic transition; this shift is generally larger for the most flexible compound 1 as well as when complexing divalent cations. NMR spectra bear many signatures of the complexation, of which the most remarkable ones are the large shielding of C(1) and the large deshieldings of C(9) and C(16). Additional makers of complexation are highlighted in the IR vibrational spectra, in particular the bands associated with the lactone and amide CO vibrations, which are downshifted when the corresponding CO is involved in the complexation mode and, otherwise, upshifted. A high degree of consistency characterizes the different geometrical, electronic, magnetic, and vibrational signatures, which substantiates the assignment of the modes of complexation in 1-3. In addition, the agreement between the experimental data and the theoretical values is rather satisfactory, in that it at least enables us to interpret the spectral signatures.     

Stability and electronic properties of nitrogen nanoneedles and nanotubes

The electronic structures and stability of nitrogen nanostructures, nanotubes, and fiberlike nanoneedles of various diameters, formed by units N2m (m = 2-6), were studied by quantum chemistry computational modeling methods. The geometrical structures with various cross-sections and terminal units, their energetic stability, and their rather peculiar electron density distributions were investigated. The tightest nitrogen nanoneedle (NNN) studied theoretically in this work is the structure (N4n with D2h symmetry, whereas the nitrogen nanotube (NNT) with the largest diameter discussed here is the structure (N12)n with D2 symmetry. These families of NNNs and NNTs can be considered as nanostructures not only for potential applications as devices in nanotechnology or as possible scaffold structures but also as ligands in synthetic chemistry and high-energy density materials (HEDMs). As a consequence of the lone-pair electrons present around the walls of these NNNs and NNTs, these nitrogen nanostructures and the nitrogen nano-bundles (NNB) formed by aligning and combining them using intermediate carbon atoms, can have highly variable electronic properties controlled by the changing charge environment. In particular, for extended systems based on the units studied here, the band gaps of each of these systems can be affected greatly by the local charge of the environment.     

Spectroscopic properties and potential energy curves of low-lying electronic states of RuC

The RuC molecule has been a challenging species due to the open-shell nature of Ru resulting in a large number of low-lying electronic states. We have carried out state-of-the-art calculations using the complete active space multiconfiguration self-consistent field followed by multireference configuration interaction methods that included up to 18 million configurations, in conjunction with relativistic effects. We have computed 29 low-lying electronic states of RuC with different spin multiplicities and spatial symmetries with energy separations less than 38,000 cm(-1). We find two very closely low-lying electronic states for RuC, viz., 1Sigma+ and 3Delta with the 1Sigma+ being stabilized at higher levels of theory. Our computed spectroscopic constants and dipole moments are in good agreement with experiment although we have reported more electronic states than those that have been observed experimentally. Our computations reveal a strongly bound 1Sigma+ state with a large dipole moment which is most likely the experimentally observed ground state and an energetically close 3Delta state with a smaller dipole moment. Overall our computed spectroscopic constants of the excited states with energy separations less than 18,000 cm(-1) agree quite well with those of the corresponding observed states.     

Quantum entanglement between electronic and vibrational degrees of freedom in molecules

We consider the quantum entanglement of the electronic and vibrational degrees of freedom in molecules with tendencies towards double welled potentials. In these bipartite systems, the von Neumann entropy of the reduced density matrix is used to quantify the electron-vibration entanglement for the lowest two vibronic wavefunctions obtained from a model Hamiltonian based on coupled harmonic diabatic potential-energy surfaces. Significant entanglement is found only in the region in which the ground vibronic state contains a density profile that is bimodal (i.e., contains two separate local maxima). However, in this region two distinct types of density and entanglement profiles are found: one type arises purely from the degeneracy of energy levels in the two potential wells and is destroyed by slight asymmetry, while the other arises through strong interactions between the diabatic levels of each well and is relatively insensitive to asymmetry. These two distinct types are termed fragile degeneracy-induced entanglement and persistent entanglement, respectively. Six classic molecular systems describable by two diabatic states are considered: ammonia, benzene, BNB, pyridine excited triplet states, the Creutz-Taube ion, and the radical cation of the "special pair" of chlorophylls involved in photosynthesis. These chemically diverse systems are all treated using the same general formalism and the nature of the entanglement that they embody is elucidated.     

Vibrational relaxation and electronic mutation of metastable nitrogen molecules generated by nitrogen atom recombination on cobalt and nickel

The recombination of nitrogen atoms on polycrystalline samples of cobalt and nickel produces metastable electronically excited nitrogen molecules, probably N₂(W³Δ_u), which are collisionally transferred to the N₂(B³Πg) state. Information about vibrational relaxation of the metastable state by N₂(X¹Σ⁺_g) is inferred from composition dependent changes in the observed first positive emission spectrum [N₂9A³Σ⁺_g)−N₂(B³Π_g] with the aid of multilevel, steady-state, kinetic model.     

Classification of organic nitrogen -containing compounds based on the electronic structure of their molecules

Conclusions It was shown that it is theoretically possible to classify nitrogen-containing compounds on the basis of the obtained quantum-chemical data concerning their electronic structure.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2611–2613, November, 1982.     

Electron correlations in molecules. III. Strength of electron correlations in localized and aromatic bonds of main-group atoms

We investigate electron correlations within a minimal basis of various chemical bonds formed by second- and third-row atoms and hydrogen. The effects of correlations are characterized by two quantities: (i) intrabond correlation energy ϵ_corr and (ii) the correlation strength parameter Σ. The latter describes the relative suppression of the charge fluctuations within a bond. The quantities ϵ_corr and Σ can be associated with chemical bonds due to the local nature of the correlation phenomenon. It is found that one may distinguish between different classes of chemical bonds which are characterized by similar values of Σ parameter within each class. For instance, all considered bonds between hydrogen and main-group atoms fall into one class. The homopolar bonds formed by the atoms of the third row are characterized by a weaker σ but stronger π correlation strength than the respective bonds of the second row. The strongest correlations are found in homopolar single π bonds of third-row atoms. A particular detailed discussion is given of heteropolar bonds. The electron correlations of aromatic π electrons are found to be considerably weaker than those within the corresponding localized bonds. For the SCF input of the correlation calculation a semi-empirical method is used. The bond orbital approximation allows for a transparent interpretation of the computational results.     

Study of the pair correlations between p-nitroaniline molecules in solution by depolarized hyper-Rayleigh scattering

The concentration dependence of the hyper-Rayleigh scattering depolarization ratios of p-nitroaniline in solution was obtained and the results were compared with theory. It was found that the experimental data can be theoretically accounted for by using a pair distribution function that includes only direct correlation, with the molecules interact through a dipolar hard-sphere potential. The results show that short-range dipole-dipole interactions are responsible for the correlation between pairs of p-nitroaniline molecules in solution.     

Vibrational and electronic properties of polyaza chain molecules and their metal complexes

The polyaza chain molecules exhibit a quasi planar backbone with all-trans geometry. The chelation of several metallic ions such as copper (II) and zinc (II) constrains different conformations of the chain molecules. The vibrational and electronic properties are typical of the conformation of the polyaza backbone as well as the spin spin exchange between the metallic ions through the azine bonds.     

Decomposition of electronic properties and calculation of solvation energies for surface-active molecules

For ions and polar molecules located near a surface, we derived and analyzed the general formulas to calculate their solvation energies. St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 6, pp. 3–12, November-December, 1994. Translated by A. Arbuznikov     

The effects of hydrogen-bonding environment on the polarization and electronic properties of water molecules

Adequate representation of the interactions that take place between water molecules has long been a goal of force field design. A full understanding of how the molecular charge distribution of water is altered by adjacent water molecules and by the hydrogen-bonding environment is a vital step toward achieving this task. For this purpose we generated ab initio electron densities of pure water clusters and hydrated serine and tyrosine. Quantum chemical topology enabled the study of a well-defined water molecule inside these clusters, by means of its volume, energy, and multipole moments. Intra- and intermolecular charge transfer was monitored and related to the polarization of water in hydrogen-bonded networks. Our analysis affords a way to define different types of water molecules in clusters.