UV Absorption Spectrum of Alternating DNA Duplexes. Analysis of Excitonic and Charge Transfer Interactions

A detailed investigation of the excited states accessed by UV absorption in alternating DNA duplexes was performed by means of an extensive sampling of intra- and intermolecular degrees of freedom. The excited states were computed using the algebraic diagrammatic construction method to second-order (ADC(2)). A realistic DNA environment was included through an electrostatic embedding QM/MM coupling scheme. The results indicate that (i) most excited states are delocalized over at most two bases, (ii) charge transfer states are located at higher energies than the bright states in the Franck–Condon region, but (iii) coupling between locally excited and charge transfer states may provide a route to dynamical charge separation, and (iv) spectral broadening is mainly caused by intramolecular vibrations

Fluorination of an Alumina Surface: Modeling Aluminum–Fluorine Reaction Mechanisms

Density functional theory (DFT) calculations were performed to examine exothermic surface chemistry between alumina and four fluorinated, fragmented molecules representing species from decomposing fluoropolymers: F^–, HF, CH₃F, and CF₄. The analysis has strong implications for the reactivity of aluminum (Al) particles passivated by an alumina shell. It was hypothesized that the alumina surface structure could be transformed due to hydrogen bonding effects from the environment that promote surface reactions with fluorinated species. In this study, the alumina surface was analyzed using model clusters as isolated systems embedded in a polar environment (i.e., acetone). The conductor-like screening model (COSMO) was used to mimic environmental effects on the alumina surface. Four defect models for specific active −OH sites were investigated including two terminal hydroxyl groups and two hydroxyl bridge groups. Reactions involving terminal bonds produce more energy than bridge bonds. Also, surface exothermic reactions between terminal −OH bonds and fluorinated species produce energy in decreasing order with the following reactant species: CF₄ > HF > CH₃F. Additionally, experiments were performed on aluminum powders using thermal equilibrium analysis techniques that complement the calculations. Consistently, the experimental results show a linear relationship between surface exothermic reactions and the main fluorination reaction for Al powders. These results connect molecular level reaction kinetics to macroscopic measurements of surface energy and show that optimizing energy available in surface reactions linearly correlates to maximizing energy in the main reaction

Synthesis and Characterization of Terpyridine-Supported Boron Cations: Evidence for Pentacoordination at Boron

Hypervalent boron centers are proposed to be key intermediates in many stoichiometric and catalytic reactions. However, structurally characterized examples remain rare. We have isolated two new borocations with formal charges of 1+ and 2+. Because the dicationic complex displays evidence of pentacoordination at the boron center, we conclude that the interaction is predominantly electrostatic and is a result of the highly electrophilic dicationic boron atom

Synthesis and Characterization of Terpyridine-Supported Boron Cations: Evidence for Pentacoordination at Boron

Hypervalent boron centers are proposed to be key intermediates in many stoichiometric and catalytic reactions. However, structurally characterized examples remain rare. We have isolated two new borocations with formal charges of 1+ and 2+. Because the dicationic complex displays evidence of pentacoordination at the boron center, we conclude that the interaction is predominantly electrostatic and is a result of the highly electrophilic dicationic boron atom

A Zwitterionic Carbanion Frustrated by Boranes – Dihydrogen Cleavage with Weak Lewis Acids via an “Inverse” Frustrated Lewis Pair Approach

The synthesis, structural characterization, and acid–base chemistry of [C­(SiMe₂OCH₂CH₂OMe)₃]Na (<b>2</b>), a sterically encumbered zwitterionic organosodium compound, is reported. <b>2</b> is a strong Brønsted base that forms frustrated Lewis pairs (FLPs) with a number of boron-containing Lewis acids ranging from weakly Lewis acidic aryl and alkyl boranes to various alkyl borates. These intermolecular FLPs readily cleave H₂, which confirms that even poor Lewis acids can engage in FLP-mediated H₂ cleavage provided that the present bulky base is of sufficiently high Brønsted basicity

Restructuring of a Peat in Interaction with Multivalent Cations: Effect of Cation Type and Aging Time

<div><p>It is assumed to be common knowledge that multivalent cations cross-link soil organic matter (SOM) molecules via cation bridges (CaB). The concept has not been explicitly demonstrated in solid SOM by targeted experiments, yet. Therefore, the requirements for and characteristics of CaB remain unidentified. In this study, a combined experimental and molecular modeling approach was adopted to investigate the interaction of cations on a peat OM from physicochemical perspective. Before treatment with salt solutions of Al³⁺, Ca²⁺ or Na⁺, respectively, the original exchangeable cations were removed using cation exchange resin. Cation treatment was conducted at two different values of pH prior to adjusting pH to 4.1. Cation sorption is slower (>>2 h) than deprotonation of functional groups (<2 h) and was described by a Langmuir model. The maximum uptake increased with pH of cation addition and decreased with increasing cation valency. Sorption coefficients were similar for all cations and at both pH. This contradicts the general expectations for electrostatic interactions, suggesting that not only the interaction chemistry but also spatial distribution of functional groups in OM determines binding of cations in this peat. The reaction of contact angle, matrix rigidity due to water molecule bridges (WaMB) and molecular mobility of water (NMR analysis) suggested that cross-linking via CaB has low relevance in this peat. This unexpected finding is probably due to the low cation exchange capacity, resulting in low abundance of charged functionalities. Molecular modeling demonstrates that large average distances between functionalities (∼3 nm in this peat) cannot be bridged by CaB-WaMB associations. However, aging strongly increased matrix rigidity, suggesting successive increase of WaMB size to connect functionalities and thus increasing degree of cross-linking by CaB-WaMB associations. Results thus demonstrated that the physicochemical structure of OM is decisive for CaB and aging-induced structural reorganisation can enhance cross-link formation.</p></div

Aging effects on WaMB transition temperature (A), Lorentzian line fraction (B) and on the amount of mobilisable water (C), expressed by the difference of the respective parameters after and before aging.

<p>Positive values indicate an increase, and negative values indicate a decrease in the parameter, respectively. Dotted line along zero shown in (C) distinguishes the positive and negative effects.</p

A comparison of cation uptake with respect to the amounts remained in treatment solution in aluminium (SP-Al), calcium (SP-Ca) and sodium (SP-Na) treated samples at different cation addition pH.

<p>A comparison of cation uptake with respect to the amounts remained in treatment solution in aluminium (SP-Al), calcium (SP-Ca) and sodium (SP-Na) treated samples at different cation addition pH.</p

Langmuir fit parameters of sorption curves.

<p>Langmuir fit parameters of sorption curves.</p

Effect of sample treatment by exchange resin followed by addition of specific cations on the major cation content with respect to different charging pH, shown for highest cation loading.

<p>Also the values for untreated (SP) and exchange resin-treated samples (SP-H) are shown.</p