Al3+, Ca2+, Mg2+, and Li+ in aqueous solution: calculated first-shell anharmonic OH vibrations at 300 K

The anharmonic OH stretching vibrational frequencies, ν(OH), for the first-shell water molecules around the Li(+), Ca(2+), Mg(2+), and Al(3+) ions in dilute aqueous solutions have been calculated based on classical molecular dynamics (MD) simulations and quantum-mechanical (QM) calculations. For Li(+)(aq), Ca(2+)(aq), Mg(2+)(aq), and Al(3+)(aq), our calculated IR frequency shifts, Δν(OH), with respect to the gas-phase water frequency, are about -300, -350, -450, and -750?cm(-1), compared to -290, -290, -420, and -830?cm(-1) from experimental infrared (IR) studies. The agreement is thus quite good, except for the order between Li(+) and Ca(2+). Given that the polarizing field from the Ca(2+) ion ought to be larger than that from Li(+)(aq), our calculated result seems reasonable. Also the absolute OH frequencies agree well with experiment. The method we used is a sequential four-step procedure: QM(electronic) to make a force field+MD simulation+QM(electronic) for point-charge-embedded M(n+) (H(2)O)(y) (second?shell) (H(2)O)(z) (third?shell) clusters+QM(vibrational) to yield the OH spectrum. The many-body Ca(2+)-water force-field presented in this paper is new. IR intensity-weighting of the density-of-states frequency distributions was carried out by means of the squared dipole moment derivatives.     

Effect of partial fluorination on the Lewis sites of microcrystalline γ-alumina studied by adsorption of pyridine as a probe molecule: A quantum chemical cluster model study

The influence of fluorination of microcrystalline γ-alumina on the acidity of Lewis sites was studied by quantum chemical cluster model approach. B3LYP and HF/6-31++G(d,p) levels of theory were employed, considering the standard and counterpoise-corrected potential energy surfaces (PESs). Explicit inclusion of dynamical electron correlation effects along with the elimination of the basis set superposition effects in geometry optimization and harmonic vibrational analysis were found to be crucial to reproduce the experimental trends in the shifts of the pyridine ν_19b and ν_8a modes upon fluorination. The acidity of surface-exposed Al-sites was found to increase upon fluorination, which is manifested as a ~ 10% increase of the interaction energies, and also in the characteristics of the electronic density and density Laplacian at the intermolecular bond critical point. Bader analysis of the electronic density has shown that pyridine adsorption on pure and fluorinated γ-alumina can be classified as non-covalent interaction.     

The crystal and molecular structure of N2H5[M(N2H3COO)3]·H2O (M∈{Co, Zn}) isomorphic compounds—an X-ray crystallographic, vibrational spectroscopic and quantum-chemical study

A systematic study of N₂H₅[M(N₂H₃COO)₃]·H₂O (M∈{Co, Zn}) type of compounds, which are typical model systems for transition metal complexes with α-amino acids (the latter are not obtainable in crystalline form), was carried out. The crystal structures of these compounds were solved by X-ray crystallographic methods. FTIR spectra at room and low temperature (∼100 K) as well as Raman spectra at room temperature were recorded, and analyzed in details. Also, the geometries of the Zn(N₂H₃COO)₃⁻ and N₂H₅⁺ species were fully optimized at ab initio HF and B3LYP/6-31+G(d,p) level of theory, and subsequent vibrational analyses were performed on the basis of which several important reassignments of the IR and Raman bands were proposed. In order to study the binding energetics and the ligand-cation charge-transfer interactions within the Zn(N₂H₃COO)₃⁻ complex, NBO analysis was carried out, employing the second-order perturbation theory analysis of the Fock matrix (i.e., its Kohn-Sham analog) within the NBO basis.     

Fourier transform infrared study of perchlorate (ClO4 and ClO4) anions isomorphously isolated in potassium permanganate matrix. Vibrational anharmonicity and pseudo-symmetry effects

Fourier transform infrared spectra of isotopomeric ³⁵ClO₄⁻ and ³⁷ClO₄⁻ anions isomorphously isolated in potassium permanganate matrix were recorded at room and low temperature (LT, ∼100 K). On the basis of the detected second-order vibrational transitions involving the dopant species ν₃ mode components, anharmonicity constants and harmonic eigenvalues for these modes were calculated. Although the overall appearance of the region of fundamental vibrational transitions in the spectra of dopant perchlorate anions may be better explained in terms of a pseudo-symmetry (the so-called ‘latent’ symmetry) site group C_2v, corresponding to the pseudo-symmetry space group Imma, instead of the (rigorous) crystallographic C_s one (corresponding to the crystallographic Pnma space group), an opposite statement seems to be valid for the region of the second-order vibrational transitions. The vibrational mode mixing (a ‘Fermi-like’ resonance) of the ClO₄⁻ ν₁ mode with the ν_3a site group component is almost negligible.     

Anharmonic vibrational frequency shifts upon interaction of phenol+ with the open shell ligand O2. The performance of DFT methods versus MP2

Anharmonic vibrational frequency shifts of the phenol(+) O-H stretching mode upon complex formation with the open-shell ligand O(2) were computed at several DFT and MP2 levels of theory, with various basis sets, up to 6-311++G(2df,2pd). It was found that all DFT levels of theory significantly outperform the MP2 method with this respect. The best agreement with the experimental frequency shift for the hydrogen-bonded minimum on the potential energy surfaces was obtained with the HCTH/407 functional (-93.7 cm(-1) theoretical vs -86 cm(-1) experimental), which is a significant improvement over other, more standard DFT functionals (such as, e.g., B3LYP, PBE1PBE), which predict too large downshifts (-139.9 and -147.7 cm(-1), respectively). Good agreement with the experiment was also obtained with the mPW1B95 functional proposed by Truhlar et al. (-109.2 cm(-1)). We have attributed this trend due to the corrected long-range behavior of the HCTH/407 and mPW1B95 functionals, despite the fact that they have been designed primarily for other purposes. MP2 method, even with the largest basis set used, manages to reproduce only less than 50% of the experimentally detected frequency downshift for the hydrogen-bonded dimer. This was attributed to the much more significant spin contamination of the reference HF wave function (compared to DFT Kohn-Sham wave functions), which was found to be strongly dependent on the O-H stretching vibrational coordinate. All DFT levels of theory outperform MP2 in the case of computed anharmonic OH stretching frequency shifts upon ionization of the neutral phenol molecule as well. Besides the hydrogen-bonded minimum, DFT levels of theory also predict existence of two other minima, corresponding to stacked arrangement of the phenol(+) and O(2) subunits. mPW1B95 and PBE1PBE functionals predict a very slight blue shift of the phenol(+) O-H stretching mode in the case of stacked dimer with the nearly perpendicular orientation of oxygen molecule with respect to the phenolic ring, which is entirely of electrostatic origin, in agreement with the experimental observations of an additional band in the IR photodissociation spectra of phenol(+)-O(2) dimer [Patzer, A.; Knorke, H.; Langer, J.; Dopfer, O. Chem. Phys. Lett. 2008, 457, 298]. The structural features of the minima on the studied PESs were discussed in details as well, on the basis of NBO and AIM analyses.     

Latent symmetry versus accidental degeneracy effects in the vibrational spectra of dopant chromate anions in M2CrxS1−xO4 solid solutions (M∈{K, Rb, Cs})

Fourier transform IR and Raman spectra of chromate anions isomorphously isolated in potassium, rubidium and cesium sulfate matrices were recorded at room and low temperature (∼100 K). On the basis of the detected second-order vibrational transitions involving the dopant species ν₃ mode components, anharmonicity constants and harmonic eigenvalues for these modes were calculated. Despite the fact that, rigorously speaking, the appearance of the spectra of dopant chromate anions may be explained in terms of a C_s site symmetry, the ν₃ stretching region resembles an approximate (A₁⊕E) splitting (characteristic for a local C_3v symmetry). Although such findings may be rationalized in terms of the latent symmetry concept, we propose an alternative explanation, based on the concept of vibrational mode mixing (a ‘Fermi-like’ resonance) of the CrO₄²⁻ν₁ mode with the ν_3a site-group component. We also derive a quantitative model based on the degenerate case stationary perturbation theory that allows an estimation of the relative importance of the latent symmetry versus vibrational mode mixing effects. In the present case, we show that the Fermi-like resonance is predominantly responsible for the observed spectral features.     

A perturbation theoretical method for determination of the dependence of the intramolecular X–H(D) potential on the hydrogen bond strength

A perturbation theoretical method is proposed that allows determination of the function describing the dependence of the intramolecular XH(D) potential on the hydrogen bond strength. Treating the XH(D) oscillators as mixed cubic–quartic oscillators in which the stretch–stretch couplings of the ν(X–H(D)) mode with the internal modes of the radical X may be neglected, and introducing the hydrogen bonding influence through the changes in the harmonic diagonal force constants (as proposed by Sceats and Rice [41]), it is possible to extract the analytical form of the function ) from the experimental versus correlations. The other parameters obtained by the empirical correlations, within this model, also have an exact physical meaning. The method is applicable to weak hydrogen bonds as well as to hydrogen bonds of intermediate strength, and, strictly speaking, to linear X– systems. However, extension to arbitrary geometry may be easily done. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the origin of the solid-state thermochromism and thermal fatigue of polycyclic overcrowded enes

Aimed at unraveling the relative contribution of the folding, twisting and bending in the mechanism of the solid-state thermochromism of overcrowded polycyclic aromatic enes (PAEs), the structures of two typical heteromeric and homomeric representatives, 2-(thioxanthen-9-ylidene)indane-1,3-dione (1) and 9,9'-bi-9(10H)-anthracenylidene-10,10'-dione (bianthrone, 2), were studied by temperature-resolved single crystal X-ray diffraction (120-530 K) and solid-state UV-visible spectroscopy. Aside from negligibly small unfolding of the tricyclic moiety of 1, this first direct diffraction study of the high-temperature structures of solid PAEs did not unravel any significant and detectable changes in the time- and space-averaged intramolecular structures, thus, showing that the PAE-type thermochromism is not due to phase transitions or to major and permanent molecular distortions of a large portion of the material that would be caused by folding, twisting and/or bending. Instead, the experimental observations and theoretical modeling indicated that the color change is probably due to a dynamic process, where the absorption spectrum changes as a result of enhanced thermal oscillations of the two halves of the molecules around the central bridge. In addition to the reversible coloration, we also observed irreversible processes of thermal fatigue that afford stable chemical products that absorb in the visible region. We showed that the stable products are conductive and they act electrocatalytically toward oxidation of several biomarkers.     

Tamm oscillations in semi-infinite nonlinear waveguide arrays

We demonstrate the existence of nonlinear Tamm oscillations at the interface between a substrate and a one-dimensional waveguide array with either cubic or saturable, self-focusing or self-defocusing nonlinearity. Light is trapped in the vicinity of the array boundary due to the interplay between the repulsive edge potential and Bragg reflection inside the array. In the special case when this potential is linear these oscillations reduce themselves to surface Bloch oscillations.     

Comparison of tunneling times in isotropic and anisotropic media

We present a comparative analysis of the tunneling times of electromagnetic (EM) waves propagating in isotropic and anisotropic media. First, suitable expressions for the tunneling times in a layered periodic material, with anisotropic properties originating from its structure, are derived, followed by numerical calculations performed for a new type of anisotropic semiconductor metamaterial. In the first case, we have considered a layered structure which contains two differently doped In_0.53Ga_0.47As semiconductor layers. The second structure under investigation is made of alternately placed layers of doped In_0.53Ga_0.47As and undoped Al_0.48Ga_0.52As. The investigation of the dwell time as a function of incident wave frequency has revealed the existence of two peaks, one of which may be interpreted as a consequence of anisotropy, while the other one corresponds to the peak related to the absorption and the group delay. Both of these two peaks are affected by variations of layers?? doping densities. Furthermore, at increased incident angles of incoming EM waves, the dwell time peak occurs at the upper boundary of the frequency interval, for which the structure exhibits negative refractive index.