Structure and chemical activity of transition metal and metal oxide catalysts: An insight from theoretical DFT studies

The present theoretical DFT study discusses the structure and chemical activity of transition metal and metal oxide catalysts within the well-known cluster approach. Selective oxidation of carbon monoxide on gold supported on titania (Au/TiO₂ (110)) as well as some key points in understanding the effect of non-metal doping on TiO2 with the aim to increase its photocatalytic functionality have been briefly discussed. It was shown that Au (with formal oxidation state equal to plus one) stabilized on water-assisted and vacancy containing TiO₂ (110) can explain selective oxidation of CO. Here binding of O₂ with the vacancy site is energetically preferable than its adsorption on an Au site. Conversely, CO adsorbs on an Au center of Au/TiO₂ (110) which is energetically much more profitable than its interaction with the oxygen vacancy site. Also, carbon and nitrogen doping on TiO₂ (110) leads to two different structures. Energetically most profitable is that carbon occupies an interstitial position in deep bulk while nitrogen replaces the protruded oxygen atom and forms a surface N-H group.     

A theoretical study on the alkali metal carboxylate-promoted L-Lactide polymerization

Sodium acetate-catalyzed L-lactide polymerization reaction is a simple yet efficient reaction. It provides a robust system to produce poly(L-lactide) in industrial scale. In this study, the mechanism of this reaction has been studied by means of computational chemistry tools based on the experimental results. Basically, the mechanism consists of two steps: lactide coordination and ring opening through O-Acyl bond cleavage. Additionally, the effect of cation size and substituent on carboxylate moiety have been evaluated. The calculations indicate that the larger cation leads to faster reactions. Moreover, the stronger electron-donating group on carboxylate moiety accelerates the reaction rates. To obtain further insights, an orbital analysis has been also carried out. Our calculations are consistent with experimental findings and clarify underlying mechanistic features of the present reaction.     

A DFT study of the complexation behavior of hemispherands toward alkali metal cations

A density functional theory study of the behavior of hemispherands toward alkali metal ions (Li⁺, Na⁺, and K⁺) is performed. The effect of the replacement of the rigid anisyl group(s) by the mobile ether group(s) on the binding energy of hemispherands with alkali metal ions is investigated. The results indicated that the binding energies are inversely proportional to the ionic radius of the cations. Moreover, increasing the flexibility of the ligand results in decreasing the binding toward small ions. The structures of the hosts and the guests are correlated to the binding energies, and the correlations are interpreted in terms of the principle of preorganization. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Selective complexation of alkali metal ions and nanotubular cyclopeptides: a DFT study

A density functional theory based on interaction of alkali metal cations (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) with cyclic peptides constructed from 3 or 4 alanine molecule (CyAla3 and CyAla4), has been investigated using mixed basis set (C, H, O, Li⁺, Na⁺ and K⁺ using 6-31+G(d), and the heavier cations: Rb⁺ and Cs⁺ using LANL2DZ). The minimum energy structures, binding energies, and various thermodynamic parameters of free ligands and their metal cations complexes have been determined with B3LYP and CAM-B3LYP functionals. The order of interaction energies were found to be Li⁺ > K⁺ > Na⁺ > Rb⁺ > Cs⁺ and Li⁺ > Na⁺ > K⁺ ? Rb⁺ > Cs⁺, calculated at CAM-B3LYP level for the M/CyAla3 and M/CyAla4 complexes, respectively. Their selectivity trend shows that the highest cation selectivity for Li⁺ over other alkali metal ions has been achieved on the basis of thermodynamic analysis. The main types of driving force host–guest interactions are investigated, the electron-donating O offers lone pair electrons to the contacting LP* of alkali metal cations.     

DFT theoretical studies of anions of aniline and its several derivatives

Detailed studies of the molecular and electronic structures, vibrational frequencies, and infrared intensities of [M ⁻] and [M-H ⁻] anions of aniline and its derivatives (2-, 3-, 4-fluoroanilines, N-[(2E)-1-methylbut-2-en-1-yl]aniline, (2-cyclopent-2-en-1-ylphenyl)amine, N-[(2E)-1-methylbut-2-en-1-yl]aniline, and N-cyclopent-2-en-1-ylaniline) have been carried out using the density functional method with UB3LYP functional and 6-31G** basis set augmented with sp diffuse functions on nitrogen, fluorine, and three carbon atoms of benzene ring (in 2, 4, and 6 positions). For comparison, similar calculations were carried out for the closed-shell neutral molecules ([M]). The studies have provided detailed insight into the structure changes that take place in negative ions of aniline and its derivatives.     

Experimental and theoretical studies of the structure of alkali halide clusters

We have investigated the structures and properties of alkali halide cluster ions produced by laser vaporization of solid samples. In many alkali halide cluster ions, we observe the appearances of bulk-like characteristics even at sub-nanometer sizes:fcc crystalline structures (including surface terraces), ionic binding, and a susceptibility to common bulk defects such as F and H color-centers. To understand the origins of cluster structures, we have made calculations of ground state energetics, high-temperature molecular dynamics, and the electronic structure of clusters having excess electrons.     

Single-crystal X-ray studies of five alkali metal salts of luminol

Luminol salts of five alkali metals, Li, Na, K, Rb, and Cs, have been prepared and structurally characterized by single-crystal X-ray diffraction. Luminol is deprotonated at the same site whereas each ionic salt has a unique composition and a different number of water molecules. The cation/luminol ion pair to water molecule ratio in the lattices varies as follows: 1 : 0 for K, 1 : 1 for Li, 1 : 2 for Rb, 1 : 3 for Cs, and 1 : 6 for Na. The differences in composition among the five compounds lead to different metal coordination environments in the solid state and distinct 3-D molecular arrangements in the lattice.     

Time-dependent DFT studies of metal core-electron excitations in Mn complexes

Time-dependent density functional theory (TDDFT) has been applied to study core excitations from 1s and 2p Mn orbitals in a series of manganese complexes with oxygen and nitrogen donor ligands. The effect of basis set and functional on the excitation energy was evaluated in detail for one complex, Mn(acac)2 x (H2O)2. The results obtained for a range of compounds, namely, [Mn(Im)6]Cl2, Mn(CH3COO)2 x 4 H2O, Mn(acac)3, Mn(SALADHP)2 and [Mn(SALPN)O]2, show good consistency with the data from X-ray absorption spectroscopy (XAS), confirming the relation between the Mn K-edge energy and the oxidation state of the Mn atom. The energies predicted for 2p core excitations show a dependence on the metal oxidation state very similar to that determined experimentally by 1s2p resonant inelastic X-ray scattering (RIXS) studies for Mn(acac)2 x (H2O)2, Mn(acac)3, and Mn(sal)2(bipy). The reliability of the K-edge energies obtained in the present study indicates that TDDFT can be used in determining the oxidation states of Mn atoms in different computational models of the manganese cluster of photosystem II (PSII).     

Synthesis and alkali metal complexation studies of novel cage-functionalized cryptands

The synthesis of novel cage-functionalized cryptands 1–5 containing adamantane-, 2-oxaadamantane- or noradamantane-moiety [i.e., 1,3-diethyladamantano[2.2.0]cryptand (1), 1,3-diethoxyadamantano[2.2.2]cryptand (2), 1,3-di[(ethyloxy)methyl]adamantano[2.2.2]-cryptand (3), 1,3-di[(ethyloxy)methyl]-2-oxaadamantano[2.2.3]cryptand (4), and 1,2-diethyloxynoradamantano[2.2.2]cryptand (5)] and their alkali metal binding properties are reported. The results obtained by extraction experiments showed that all the cryptands displayed lower extraction capabilities than the parent [2.2.2]cryptand. However, cryptands 1 and 2 showed much higher selectivity toward K⁺ than the reference [2.2.2]cryptand. When the third bridge is enlarged by two additional CH₂-groups as well as by two oxygen atoms, as in cryptands 3 and 4, the complexational abilities for bigger cations (K⁺, Rb⁺ and Cs⁺) are enhanced. Cryptand 5 displayed very good extraction capabilities of all cations, but showed practically no selectivity towards any of the alkali metal cation. The experimental findings are corroborated by calculation studies consisting of force field based conformational search using Monte Carlo method followed by investigation of the stabilities of the complexes of cryptands with Na⁺ and K⁺ metal ions in chloroform by means of quantum chemical calculations at the density functional theory level.     

Kinetic and theoretical studies on alkaline ethanolysis of 4-nitrophenyl salicylate: effect of alkali metal ions on reactivity and mechanism

Pseudo-first-order rate constants (k(obsd)) for reactions of 4-nitrophenyl salicylate (7) with alkali metal ethoxides (EtOM, M = K, Na, and Li) in anhydrous ethanol have been measured spectrophotometrically. Interestingly, the k(obsd) value decreases significantly as the concentration of EtOM increases. Because the phenolic moiety of substrate 7 would be deprotonated and exist as an anionic form (i.e., 7(-)) under kinetic conditions, the ground-state stabilization of 7(-) through formation of a six-membered cyclic complex with M(+) (i.e., 8) is proposed to be responsible for the decreasing k(obsd) trend. The k(obsd) value at a given concentration of EtOK increases steeply upon addition of [18]crown-6 ether (18C6) up to [18C6]/[EtOK] = 1 in the reaction mixture and then remains relatively constant thereafter. In contrast, k(obsd) decreases upon addition of salts (e.g., LiClO(4) or KSCN) to the reaction mixture, which indicates that M(+) ions inhibit the reaction. However, in the presence of 18C6, the k(obsd) value is independent of the concentration of EtOK but remains constant, which indicates that the reaction proceeds through a unimolecular mechanism in the presence of the complexing agent. Although two conceivable unimolecular pathways (formation of ketene 9 and lactone 10) can account for the kinetic results, the reaction has been concluded to proceed via formation of ketene 9 as the reactive intermediate on the basis of theoretical calculations.     

Matrix isolation ESR and theoretical studies of metal phosphides

The ZnP, (67)ZnP, CdP, (111)CdP, and (113)CdP radicals have been formed by laser ablation of the metal with GaP pressed into the metal surface, isolated in an inert neon matrix at 4.3 K and their electronic structure was established using electron spin resonance spectroscopy. The following magnetic parameters were determined experimentally for ZnP/(67)ZnP, g(⊥)=1.9982(2), A(⊥)(P)=111(6)?MHz, A(⊥)((67)Zn)=160(2)?MHz, and D=-29?988(3)?MHz and estimates were made for the following ZnP/(67)ZnP magnetic parameters: g(∥)=1.9941(2), A(∥)(P)=-5(6)?MHz, and A(∥)((67)Zn)=180(50)?MHz. The following magnetic parameters for CdP/(111)CdP/(113)CdP were determined experimentally: g(⊥)=1.9963(2), A(⊥)(P)=97(3)?MHz, A(⊥)((111)Cd)=862(3)?MHz, and A(⊥)((113)Cd)=902(3)?MHz. Evidence for the formation of the MgP radical was also obtained and an approximate hyperfine coupling constant of A(⊥)(P)=157(6)?MHz was determined. The low-lying electronic states of ZnP and MgP were also investigated using the multiconfigurational self-consistent field technique. Potential energy surfaces, binding energies, optimized bond lengths, energy separations, and dissociation energies have been determined. Both radicals are found to have (4)Σ(-) ground states with a leading configuration at r(e) of 10σ(2)11σ(2)5π(1)5π(1)12σ(1) for ZnP and 7σ(2)8σ(2)3π(1)3π(1)9σ(1) for MgP. Significant mixing to this state is calculated for MgP.     

Experimental and theoretical investigation of alkali metal cation interactions with hydroxyl side-chain amino acids

Absolute bond dissociation energies of serine (Ser) and threonine (Thr) to alkali metal cations are determined experimentally by threshold collision-induced dissociation of M+AA complexes, where M+=Li+, Na+, and K+ and AA=Ser and Thr, with xenon in a guided ion beam tandem mass spectrometer. Experimental results show that the binding energies of both amino acids to the alkali metal cations are very similar to one another and follow the order of Li+>Na+>K+. Quantum chemical calculations at three different levels, B3LYP, B3P86, and MP2(full), using the 6-311+G(2d,2p) basis set with geometries and zero-point energies calculated at the B3LYP/6-311+G(d,p) level show good agreement with the experimental bond energies. Theoretical calculations show that all M+AA complexes have charge-solvated structures (nonzwitterionic) with [CO, N, O] tridentate coordination.     

Vibrational spectra of cyclopentadienylphosphine: infrared and theoretical studies from DFT anharmonic potentials

Both experimental and theoretical infrared investigations of cyclopentadienylphosphine (CpP) are reported. The infrared spectra (3500-500 cm(-1)) in the gas phase have been recorded at 0.5 cm(-1) resolution. Infrared absorptions bands of the two lowest stable conformers were observed and assigned. Average integrated intensities of isolated and overlapping vibrational bands were also determined experimentally. The vibrational frequencies of the CpP system and its P-dideuterated isotopologue have been calculated by means of density functional theory. The Becke exchange functional and Lee-Yang-Parr correlation functional method with a combination of the two basis sets, namely 6-31+G(d,p) and the correlation-consistent triple-zeta cc-pVTZ set of Dunning, were used. Hybrid B3LYP/B3LYP//cc-pVTZ/6-31+G(d,p) anharmonic frequencies of the fundamental, overtone, and combination transitions were calculated in the 3500-200 cm(-1) area with the use of a variational approach, implemented in the P_Anhar_v1.1 code, to assign the experimental data for each conformer.     

DFT study on the selective complexation of B12N12 nanocage with alkali metal ions

Density functional theory (DFT) calculations were applied at the M05-2X/6-311++G(d,p) level of the theory to investigate the interaction of the B₁₂N₁₂ nanocage (BN) and alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) in the gas phase and in water. On the basis of the results, BN nanocage is able to form a selective complex with Li⁺. Water, as a solvent, reduces the stability of the metal ion-BN complexes in comparison with the gas phase. Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses, reveal that the electrostatic interaction between the BN and metal ions can be considered as the driving force for complex formation in which the role of water is of significance. Density of states (DOSs) analysis of the BN nanocage structure in the presence of different metal ions showed a noticeable change in the frontier orbitals, especially in the gas phase, and Fermi level shifting toward the lower values.     

An experimental and theoretical study of crystals of calcium fluoride doped with alkali metal cations

Experimental measurements of thermal depolarization in crystals of CaF₂, grown from the melt containing LiF, NaF, KF, or RbF, reveal a common relaxation (designated M2) with an activation energy of 0.51 eV. In addition, the Li⁺- and K⁺-doped crystals exhibit a relaxation (M1) with an activation energy of 0.34eV. A similar relaxation has been found in CaF₂:Rb⁺ by J. Fontanella private communication) and in CaF₂:Na⁺ by R. D. Shelley and G. R. Miller (J. Solid State Chem., 1, 218, 1970). Theoretical calculations on M⁺-doped CaF₂ (where M = Li,Na,K,Rb) are in agreement with the hypothesis that the M2 relaxation is due to Na⁺ in all four systems studied and is associated with the jump of a nearest-neighbor (nn) anion vacancy (F_v^?) around the substitutional Na⁺ ion (Na_s⁺). The assignment of M1 is less certain, but it appears to be associated with similar Li_s⁺&.z.sbnd;F_v^? dipoles resulting from Li⁺ impurity present because of the lower volatility of LiF compared to that of KF and RbF. When LiF dissolves in CaF₂ the Li⁺ ion also forms quadrupoles consisting of a cation vacancy and two Li⁺ interstitials and the reorientation of these quadrupoles has also been studied theoretically.     

A theoretical study of the structure and thermochemical properties of alkali metal fluoroplumbates MPbF3

Molecular constants of MPbF3 (M=Li, Na, K, Rb, and Cs) were calculated theoretically at the MP2(full) and B3LYP levels with the SDD (Pb, K, Rb, and Cs) and cc-aug-pVQZ (F, Li, and Na) basis sets to determine the thermochemical characteristics of the substances. Satisfactory agreement with experiment was obtained, including the unexpected nonmonotonic dependence of substance dissociation energies on the alkali metal atomic number. The bond lengths of the theoretical CsPbF3 model were substantially elongated compared with experimental estimates, likely because of errors in both theoretical calculations and electron diffraction data processing.     

Thermal, UV and FTIR spectral studies in alkali metal cinnamates

A single crystal of sodium and potassium cinnamates was grown by slow evaporation of methanol solution at room temperature. The effect of metals sodium and potassium on the electronic structure of cinnamic acid was studied. In this research many analytical methods such as FTIR, UV, second harmonic generation (SHG) and TG–DTA were used: The spectroscopic studies lead to conclusions containing the distribution of the electronic charge in molecule, the delocalisation of π electrons and the reactivity of metal complexes. The SHG efficiency is more pronounced in the presence of sodium and potassium dopant in the growth medium. Incorporation of sodium and potassium increase the thermal stability ensuring the suitability of material for possible non-linear optical (NLO) application up to 180 °C.     

Influence of halogenation on the properties of uracil and its noncovalent interactions with alkali metal ions. Threshold collision-induced dissociation and theoretical studies

The influence of halogenation on the properties of uracil and its noncovalent interactions with alkali metal ions is investigated both experimentally and theoretically. Bond dissociation energies of alkali metal ion-halouracil complexes, M+(XU), are determined using threshold collision-induced dissociation techniques in a guided ion beam mass spectrometer, where M+ = Li+, Na+, and K+ and XU = 5-fluorouracil, 5-chlorouracil, 6-chlorouracil, 5-bromouracil, and 5-iodouracil. The structures and theoretical bond dissociation energies of these complexes are determined from ab initio calculations. Theoretical calculations are also performed to examine the influence of halogenation on the acidities, proton affinities, and Watson-Crick base pairing energies. Halogenation of uracil is found to produce a decrease in the proton affinity, an increase in the alkali metal ion binding affinities, an increase in the acidity, and stabilization of the A::U base pair. In addition, alkali metal ion binding is expected to lead to an increase in the stability of nucleic acids by reducing the charge on the nucleic acid in a zwitterion effect as well as through additional noncovalent interactions between the alkali metal ion and the nucleobases.