Quantum Chemical Model Calculations Research Articles

On-off-on Control of Molecular Inversion Symmetry via Multi-stage Protonation: Elucidating Vibronic Laporte Rule.

The Laporte rule dictates that one- and two-photon absorption spectra of inversion-symmetric molecules should display alternatively forbidden electronic transitions; however, for organic fluorophores, drawing clear distinction between the symmetric- and non-inversion symmetric two-photon spectra is often obscured due to prevalent vibronic interactions. We take advantage of consecutive single- and double-protonation to break and then reconstitute inversion symmetry in a nominally symmetric diketopyrrolopyrrole, causing large changes in two-photon absorption. By performing detailed one- and two-photon titration experiments, with supporting quantum-chemical model calculations, we explain how certain low-frequency vibrational modes may lead to apparent deviations from the strict Laporte rule. As a result, the system may be indeed considered as an on-off-on inversion symmetry switch, opening new avenues for two-photon sensing applications.

Shock wave and modelling study of the UV spectra of perfluorocarbon iodides and perfluorocarbon radicals

The UV absorption spectrum of C2F5I was studied in shock waves over the temperature range 580–1200 K and, together with room temperature data, was represented in extended Sulzer–Wieland form. As a case study, the work illustrates the properties of high-temperature UV absorption continua for use in kinetics experiments in shock waves. A comparison with experimental data for CF3I and their representation is made. Quantum-chemical model calculations of positions and oscillator strengths are tested and, besides CF3I and C2F5I, applied also to spectra of C2F4I, C2F4, CF2I, C2F2, C2F, CF2, CFI, CF, and IF. Likewise, modelling and experimental results for the high-temperature UV absorption spectra of CF3 and C2F5 radicals are considered.

Non-covalent interactions of imidazolium-based ionic liquids with model pyrrolidones revealed by FTIR spectroscopy and quantum chemical model calculations

Certain ionic liquids (ILs), such as 1-butyl-3-methylimidazolium chloride (bmimCl) and 1-ethyl-3-methylimidazolium acetate (emimAc), are potent cellulose solvents, but the high viscosity of the resulting solutions is problematic in many applications. Organic co-solvents are often employed to alleviate this problem; however, our understanding of the intermolecular interactions determining the IL/co-solvent mixture performance is very limited, hampering the further development of this class of cellulose solvents. Herein, we applied infrared spectroscopy (IR), Raman spectroscopy, and quantum chemical model calculations to investigate the intermolecular interactions in differently concentrated mixtures of bmimCl and emimAc with amidic co-solvents, N-methyl-2-pyrrolidone (NMP) and N-vinyl-2-pyrrolidone (NVP). Based on the detailed analysis of the vibrational spectra of individual mixture components and the improved assignment of the relevant characteristic bands, we determined that bmim+-co-solvent and emim+-co-solvent associates, stabilized by the hydrogen bonds between amidic carbonyl oxygens and the CH groups of the imidazolium rings, are formed in the mixtures. In addition, the data pointed to concomitant disruption of the H-bonds between the IL counterions as the co-solvent concentration was increased, which may indicate partial dissociation of the IL ion pairs. Further, through an extensive deconvolution analysis, we quantified the molar fractions of the co-solvent molecules involved in the associate formation, finding that this fraction is significantly lower for emimAc than for bmimCl mixtures at all the studied concentrations. On the other hand, the influence of the amidic co-solvent structure was negligible, suggesting that also other (aprotic) amides may be applicable as co-solvents. Furthermore, the calculated molar ratio of IL to interacting co-solvent molecules highlighted the possible differences in the associate stoichiometry. The findings indicate that the lower affinity of the IL to co-solvents in emimAc-based mixtures may lead to the retaining of larger IL-rich clusters and, consequently, the comparatively better performance of the system in cellulose dissolution. Finally, increased temperature was found to influence all the studied systems similarly, inflicting about 20% decrease in the fraction of interacting co-solvent molecules at 85 °C as compared to room temperature. In summary, the results of this study provide important implications for the design of new solvent systems for cellulose dissolution.

Non-covalent interactions in bmimCl/co-solvent mixtures: A FTIR spectroscopy and computational study

Ionic liquids such as 1-butyl-3-methylimidazolium chloride (bmimCl) are efficient solvents for cellulose, but high viscosity of the resulting solutions hampers further cellulose shaping and derivatization. To solve this problem, mixtures of ILs with various organic co-solvents are often employed in place of neat ILs. So far, the intermolecular interactions in such mixtures are poorly understood, which hinders further development of this class of cellulose solvents. In this work, we utilized infrared spectroscopy (IR) and quantum chemical model calculations to study the intermolecular interactions in differently concentrated mixtures of bmimCl with two typical co-solvents, dimethyl sulfoxide (DMSO) and N,N-dimethylacetamide (DMAc). First, we refined the assignment of the relevant characteristic bands in the IR spectra of the individual mixture components. This then helped us to reveal that bmim+-DMSO and bmim+-DMAc associates, stabilized by hydrogen bonds between the imidazolium ring CH groups and oxygens of DMSO or DMAc, are formed in the studied mixtures. Importantly, only a relatively small fraction of co-solvent molecules (up to ca. 20 mol% for DMSO and ca. 45 mol% for DMAc) are involved in the associate formation, leaving a large proportion of bmimCl molecules non-solvated. While the maximum co-solvent concentration at which direct cellulose dissolution can be achieved is considerably different for DMSO and DMAc, the corresponding molar ratio of bmimCl to interacting co-solvent molecules was the same for both the studied systems (about 2.5:1). We propose that this ratio might be related to the critical size of bmimCl clusters necessary for achieving cellulose dissolution. Furthermore, increasing the temperature from 25 to 80 °C resulted in a significantly decreased fraction of interacting solvent molecules for bmimCl/DMSO but not for bmimCl/DMAc. This indicates that the comparatively lower tendency of DMSO to form associates with bmimCl is due to the lower strength of the underlying hydrogen bond. The results of this study illustrate how intermolecular interactions influence the macroscopic performance of different IL-based solvent systems.

Intermolecular Interactions in N, N-Dimethylacetamide without and with LiCl Studied by Infrared Spectroscopy and Quantum Chemical Model Calculations.

The mixture of LiCl and N, N-dimethylacetamide (DMAc) is an important laboratory-scale solvent for cellulose. However, the mechanism of cellulose dissolution in DMAc/LiCl could not be fully established due to the limited knowledge about the interactions between DMAc and LiCl. To address this issue, we studied neat DMAc and DMAc/LiCl mixtures by ATR FTIR spectroscopy and quantum chemical model calculations. On the basis of the calculations, we newly assigned the bands at 1660 and 1642 cm-1 in the ν(C═O) region of the spectra to DMAc monomeric and dimeric structures. The latter are presumably stabilized by the C-H···O═C weak hydrogen bonds that prevail in both neat DMAc and DMAc/LiCl mixtures. The analysis of the concentrated (7.9 wt % of LiCl) DMAc/LiCl mixture revealed that only about half of DMAc molecules interact directly with LiCl. The resulting average stoichiometry of about 2.8:1 (DMAc:LiCl), indicating the predominance of [(DMAc)2-LiCl] and [(DMAc)3-LiCl] complexes, was found to be temperature independent. Conversely, the stoichiometry was considerably temperature sensitive for the diluted DMAc/LiCl mixture (2.6 wt % of LiCl), indicating that further DMAc molecules can be incorporated into the primary solvation shell of LiCl at higher temperatures. These results highlight the dynamic character of the DMAc/LiCl system that needs to be considered when studying the cellulose dissolution mechanism.

Vacuum UV Polarization Spectroscopy of p-Terphenyl.

p-Terphenyl is used as a component in a variety of optical devices. In this investigation, the electronic transitions of p-terphenyl are investigated by synchrotron radiation linear dichroism (SRLD) spectroscopy in the range 30000-58000 cm-1 (330-170 nm) on molecular samples aligned in stretched polyethylene, thereby extending the region investigated by polarization spectroscopy into the vacuum UV. The resulting partial absorbance curves reveal that the vacuum UV band system with a maximum at 55000 cm-1 (180 nm) is predominantly short axis-polarized. This result is of interest in the optical applications of p-terphenyl, for example as a wavelength shifter. The observed polarization spectra are compared with the results of quantum chemical model calculations. Convoluted versions of the transitions predicted with the semiempirical ZINDO method and with the long-range-corrected time-dependent density functional theory (TD-DFT) procedures TD-CAM-B3LYP, TD-LC-ωPBE, and TD-ωB97XD are in similar qualitative agreement with the observed partial absorbance curves throughout the investigated spectral regions, while TD-B3LYP fails to predict qualitatively the spectrum of p-terphenyl in the region above 40000 cm-1 (250 nm).

Thermoresponsive behavior of block copolymers of PEO and PNIPAm with different architecture in aqueous solutions: A study by NMR, FTIR, DSC and quantum-chemical calculations

Temperature behavior of D2O solutions of thermoresponsive diblock poly(ethylene oxide) (PEO)-b-poly(N-isopropylacrylamide) (PNIPAm) copolymers and Y-shape triblock PEO-b-(PNIPAm)2 copolymers was investigated and compared with the phase transition of the PNIPAm homopolymer by combination of NMR and ATR FTIR spectroscopy, DSC and model quantum-chemical calculations. It is shown that the phase transition and structures of PNIPAm component (forming micellar cores) are significantly affected not only by the presence of the PEO block but also by copolymer architecture. Both these factors affect energetics of the phase transition, influence of the polymer concentration and behavior of bound water at elevated temperatures. They also affect the degree of dehydration of PNIPAm segments (CO groups) at temperatures above the phase transition which is higher for block copolymers (especially for the diblock) in comparison with PNIPAm homopolymer. NOESY NMR spectra likewise revealed influence of the architecture of the block copolymer on its conformational behavior in the pretransition region.

Influence of ion-exchange on the electrochemical properties of polypyrrole films

Cyclic voltammetry, X-ray microanalysis (EDS), FTIR, and AFM were applied for the systematic examination of the influence of self-exchange and potential stimulated ion-exchange on the electrochemical properties of polypyrrole (PPy) films prepared in the presence of benzene sulfonate (BS), para-toluene sulfonate (pTS), and naphthalene-1, 5-disulfonate (NDS) anions. All electrochemical experiments were carried out in aqueous solutions. It was established that small inorganic anions, like NO3−, Cl−, ClO4− can relatively easily replace an essential part of the sulfonates in PPy/BS and PPy/pTS films simply during a 10min polarization at 0.4V. A single potentiodynamic cycle increases the completeness of the exchange of the anions remarkably. The EDS analysis carried out before and after the anion exchange showed that 5–15% of the sulfonate anions still remain in the PPy films. The ability of the double-charged SO42− anions to replace sulfonate anions is even more pronounced as the exchange process does not need additional electrochemical activation. The quantum chemical model calculations have shown that the interaction of the sulfate anions with charged pyrrole oligomers is essentially stronger compared to other anions under the study.The redox capacitance and the corresponding electrochemical doping level of the redoped PPy films are remarkably higher than those before ion-exchange, showing that the PPy matrix formed in the presence of sulfonate anions does not collapse after ion-exchange and it creates favorable conditions for increased mobility of inorganic anions.

Proton Mobility in Water Clusters

Proton mobility in water occurs quickly according to the so-called Grotthuss mechanism. This process and its elementary reaction steps can be studied in great detail by applying suitable mass spectrometric methods to ionic water clusters. Careful choice of suitable core ions in combination with analysis of cluster size trends in hydrogen/deuterium isotope exchange rates allows for detailed insights into fascinating dynamical systems. Analysis of the experiments has been promoted by extensive and systematic quantum chemical model calculations. Detailed low-energy mechanistic pathways for efficient water rearrangement and proton transfer steps, in particular cases along short preformed "wires" of hydrogen bonds, have been identified in consistency with experimental findings.

Transportation behavior of alkali ions through a cell membrane ion channel. A quantum chemical description of a simplified isolated model

Quantum chemical model calculations were carried out for modeling the ion transport through an isolated ion channel of a cell membrane. An isolated part of a natural ion channel was modeled. The model channel was a calixarene derivative, hydrated sodium and potassium ions were the models of the transported ion. The electrostatic potential of the channel and the energy of the channel-ion system were calculated as a function of the alkali ion position. Both attractive and repulsive ion-channel interactions were found. The calculations - namely the dependence of the system energy and the atomic charges of the water molecules with respect to the position of the alkali ion in the channel - revealed the molecular-structural background of the potassium selectivity of this artificial ion channel. It was concluded that the studied ion channel mimics real biological ion channel quite well.

Raman Optical Activity and Raman Spectra of Amphetamine Species—Quantum Chemical Model Calculations and Experiments

Theoretical calculations and preliminary measurements of vibrational Raman optical activity (ROA) spectra of different species of amphetamine (amphetamine and amphetamine-H+) are reported for the first time. The quantum chemical calculations were carried out as hybrid ab initio DFT-molecular orbital calculations by use of the Gaussian 03W program, based on complete geometry minimizations of the conformational energy of the S-(+)-amphetamine molecule, the S-(+)-amphetamine-H+ ion, and the R-(–)-amphetamine molecule. Following this, harmonic frequency calculations have been made, providing information about the cation vibrational bands, expected in salts of single anions (chlorides) as well as in salts of anions with internal bonds (sulfates, hydrogen phosphates, etc.). It shows that the kind of anion should be given better attention, as so far it has often not been the case, when the spectra are employed for identification purposes. The DFT calculations show that the most stable conformations are those allowing for close contact between the aromatic ring and the amine hydrogen atoms. The internal rotational barrier within the same amphetamine enantiomer has a considerable influence on the Raman and ROA spectra. As predicted the experimental ROA spectra were found to depend on the chirality. Two street samples, provided by the London Police, were also measured and compared to the calculated ROA spectra. The street samples were found to contain different enantiomers of the protonated amphetamine-H+ sulfate. According to the present study the AMPH+ ion in aqueous sulfate solution seems to adopt a conformation in which the phenyl and ammonium groups are in transpositions, similar to what has been found in the solid state.

Chemisorption on cobalt surfaces: The effect of subsurface rhenium atoms from quantum chemical cluster model calculations

We have calculated chemisorption energies for different sorbates on cluster models for a number of sites on pure and subsurface rhenium-doped cobalt surfaces. Bonding energies follow the trend water < CO < propyl < methyl < hydrogen < hydroxyl, and are in good agreement with experimental results where available. The results indicate that for single-bond radicals (hydrogen, alkyl, hydroxyl), rhenium inclusion stabilizes the chemisorbed species. Further, the stabilization leads to a greater number of sites being energetically close to the most stable ones, possibly enhancing surface mobility of chemisorbed species. Hydrogen is less stabilized by rhenium substitution compared to propyl, indicating a possible mechanism for the greater yield of long-chained hydrocarbons afforded by rhenium-doped catalysts. For carbon monoxide, the results are less conclusive as rhenium substitution does not influence chemisorption energies so strongly.

Electronic structure and X-ray spectra of nickel(II) dithiolate chelate complexes

Nickel(II) dithiocarbamate and dithiophosphinate complexes were studied by X-ray and X-ray photoelectron spectroscopy. The X-ray emission and absorption spectra of Ni(II) compounds were interpreted in terms of quantum-chemical model calculations of the electronic spectrum of the complexes. The electronic structure of nickel(II) dithiolates was analyzed on the basis of the experimental and theoretical data.

Oxygen Donor-Mediated Equilibration of Diastereomeric Alkene−Palladium(II) Intermediates in Enantioselective Desymmetrizing Heck Cyclizations

This investigation examines the origin of enantioselection in the desymmetrization of an acyclic prochiral Heck cyclization precursor. High asymmetric induction (97-98% ee) is attributed to a temporary interaction of a Lewis basic oxygen donor with weakly Lewis acidic palladium(II). A series of control experiments combined with quantum-chemical model calculations provided sound evidence for a mechanism involving oxygen donor-mediated, rapid equilibration of diastereomeric alkene-palladium(II) complexes prior to the selectivity-determining ring-closing event, a Curtin-Hammett scenario. Our study also highlights the importance of the cationic pathway (triflate counter anions versus halido ligand) and alkene stereochemistry (E versus Z) in asymmetric Heck reactions.

The absorption spectra of anisole-h8, anisole-d3 and anisole-d8. The assignment of fundamental vibrations in the S0and the S1states

The REMPI spectra of anisole-h8, anisole-d3 and anisole-d8 have been measured. The assignment of the fundamental vibrations of anisole in the S(1) state is supported by quantum chemical model calculations, the isotopic shifts, the comparison with the frequencies of corresponding vibrations in other monosubstituted benzenes, especially phenol, and the overtones, combinations and progressions observed in the spectrum of anisole-h8. The frequencies of the 42 fundamental vibrations of anisole in the S(1) state are evaluated and compared with the frequencies of the corresponding vibrations in the electronic ground state. Some assignments given earlier in the literature have been revised.

Transition State Geometry and Polar Effect in the Addition of Peroxyl Radicals to Olefins

Transition-state interatomic distances in the addition of peroxyl radicals to the double bond of olefins are estimated from experimental data (activation energies and rate constants) using a new algorithm combining quantum chemical and parabolic model calculations. An aromatic ring in the α-position with respect to the double bond elongates the ⋯ distance by 0.02 × 10−10 m. Polar groups adjacent to the double bond affect this distance considerably. The geometric parameters of the transition states in the abstraction and addition of peroxyl radicals are compared.

Tetrahedral Intermediates in Reactions of Carboxylic Acid Derivatives with Nucleophiles

AbstractFor Abstract see ChemInform Abstract in Full Text.

X-ray photoelectron and voltammetry studies of the charged state of the metal atom in nickel(II) chelate complexes with ligands containing CS 2 − and PS 2 − groups and in their mixed-ligand complexes

X-ray photoelectron and voltammetry studies of the charged state of the metal atom in nickel(II) chelate complexes with ligands containing CS 2 − and PS 2 − groups and in heteroligand complexes on their basis were carried out. It is shown that the degree of formal oxidation of the nickel atom in the complexes corresponds to Ni(II). In the case of heteroligand complexes, addition of nitrogen heterocycles causes an increase in the electron density on the atoms of the coordination center. Theoretical data obtained in ab initio quantum chemical model calculations correlate with the experimental results.

Tetrahedral intermediates in reactions of carboxylic acid derivatives with nucleophiles

AbstractTransacylation reactions of carboxylic acids, carboxylic acid esters, carboxylic acid amides and other carboxylic acid derivatives are among the most widespread and most important reactions in chemistry and biochemistry. Already in 1887, Claisen suggested a tetrahedral intermediate in transformations of carboxylic acid derivatives with nucleophiles. A historical overview gives insight into the studies to detect possible tetrahedral intermediates in such reactions. However, only in recent years has detailed information concerning the structures of such species become available. In this review, neutral, cationic and anionic tetrahedral intermediates are described which serve as models for transacylations under neutral, acid‐catalysed or basic conditions. The characteristically different structures correspond nicely with experimental experience with reactions of carboxylic acid derivatives and with quantum chemical model calculations on tetrahedral intermediates. Finally, by means of model calculations, an explanation is given for the fast reactions of Weinreb amides, RC(O)N(CH3)OCH3, with organolithium and even with Grignard reagents: the reactions are determined by comparatively stable chelate transition states. Copyright © 2004 John Wiley &amp; Sons, Ltd.

X-Ray Study of the Electronic Structure of Copper(II) Acetylacetonate

The Cu(acac)2 complex was investigated by X-ray emission and XANES spectroscopy using synchrotron radiation. Structural data for the complex were obtained in chloroform solution and C2F matrix. The experimental spectra and their fitting on a single scale of binding energies point to possible relaxation effects. As shown by model quantum-chemical calculations, the structure and energy position of the LUMOs constructed from the vacant metal 4p, 5p AOs strongly depends on the nature of the nearest environment.