Search results for "Proton"
showing 10 items of 5886 documents
Short hydrogen bonds enhance non-aromatic protein-related fluorescence
2020
AbstractFluorescence in biological systems is usually associated with the presence of aromatic groups. Here, we show that specific hydrogen bonding networks can significantly affect fluorescence employing a combined experimental and computational approach. In particular, we reveal that the single amino acid L-glutamine, by undergoing a chemical transformation leading to the formation of a short hydrogen bond, displays optical properties that are significantly enhanced compared to L-glutamine itself. Ab initio molecular dynamics simulations highlight that these short hydrogen bonds prevent the appearance of a conical intersection between the excited and the ground states and thereby signific…
Influence of structure on the polarizability of hydrated methane sulfonic acid clusters
2015
Abstract: The relationship between polarizability and structure is investigated in methane sulfonic acid (MSA) and in 36 hydrated MSA clusters. The polarizabilities are calculated at B3LYP and MP2 level and further partitioned into molecular contributions using classic and iterative Hirshfeld methods. The differences in the two approaches for partitioning of polarizabilities are thoroughly analyzed. The polarizabilities of the molecules are found to be influenced in a systematic way by the hydrogen bond network in the clusters, proton transfer between MSA and water molecules, and weak interactions between water molecules and the methyl group of MSA.
Proton, Hydroxide Ion, and Oxide Ion Affinities of Closed-Shell Oxides: Importance for the Hydration Reaction and Correlation to Electronic Structure
2019
Phenomenologically, the enthalpy of the dissociative water incorporation (hydration) of oxides is often found to be more favorable for more basic oxides. In the present work, we investigate proton,...
Mechanism of aminocarbene formation by nucleophilic attack on isocyanide ligands in platinum(II)2-pyrazyl and 4-pyridyl complexes
1997
Abstract The reactions of 2-pyrazyl and 4-pyridyl isocyanide complexes [Pt(CNC 6 H 11 )(C 4 H 3 N 2 - C 2 )(dppe)ClO 4 and [Pt(CNC 6 H 11 )(C 5 H 4 N- C 4 ) (dppe)]ClO 4 ( 1 ) with amines involving the formation of aminocarbene derivatives have been studied kinetically in 1,2-dichloroethane by UV—VIS techniques. The kinetics obey the simple second-order rate law rate = k 2 [ 1 ][amine]. Low activation enthalpies and highly negative activation entropies for the k 2 term are observed. A mechanism is proposed involving direct nucleophilic attack of the amine on the isocyanide carbon with concomitant proton transfer from the amine to the isocyanide nitrogen assisted by the heterocyclic nitrogen…
Surface Segregation Entropy of Protons and Oxygen Vacancies in BaZrO3
2016
The perovskite BaZrO3 has attracted considerable attention in the recent decade due to its high temperature proton conducting properties, and possible application as electrolyte in intermediate temperature fuel cells and electrolyzers. In this contribution, we performed, for the first time, first-principles calculations of the phonon contribution to the defect thermodynamics of the ZrO2 terminated (001) surface of BaZrO3. The approach allows us to determine both the segregation enthalpy and entropy of defects, which we apply to two fundamental defects in BaZrO3; fully charged oxygen vacancies (vO••) and protonic defects (OHO•). The calculations show that both defects exhibit favorable segre…
Proton-Induced Multiple Changes of the Absorption and Fluorescence Spectra of Amino-Aza-Oligo(Phenylenevinylene)s
2008
Fluorescent dyes with a high sensitivity of their optical spectra towards changes of the environment were prepared via aldol condensation or Horner olefinations. The main chromophore is a quadrupolar N-substituted 1,4-distyrylbenzene which allows protonation and complexation at various positions resulting in a series of different and significant changes of the optical spectra. The sensitivity of the absorption and emission spectra on solvent polarity, acid, and cations is reported.
A Collective Coordinate to Obtain Free Energy Profiles for Complex Reactions in Condensed Phases.
2015
Exploration of chemical reactions in complex explicit environments has become an affordable task with the use of hybrid quantum mechanics/molecular mechanics potentials which allow calculating free energy profiles of chemical reactions under the influence of the surroundings. Tracing these free energy profiles requires the selection of a reaction coordinate, which can be cumbersome for those processes involving more than a single chemical event in a concerted step. We here propose a collective coordinate to be used in the calculation of free energy profiles for complex reactions in condensed phases. This coordinate is based in the definition of the advance along a path introduced by Brandua…
Radical cations of vitamin E
2002
Radical cations of vitamin E compounds (α-, β-, γ- and δ-tocopherols and α-, β-, γ- and δ-tocotrienols) were for the first time successfully prepared in different solvents. Their unpaired electron distribution was solved by means of EPR and ENDOR spectroscopy and assignment of isotropic hyperfine couplings was performed. The isotropic hyperfine coupling constants calculated theoretically (UB3LYP) were comparable to those studied experimentally. Radical cations were observed to exist as an intermediate product in the formation of the neutral radical. Moreover, loosening the proton from the radical cation of α-tocopherol and from α-tocotrienol produced the neutral radical, which was detected …
Chelation of a proton by oxidized diphosphines
2012
Abstract The chelation of a proton by oxidized diphosphines is studied for the first time both experimentally and theoretically. As a proof of concept the rare case where two different H-bond systems exist in one compound, H[7,8-(OP i Pr 2 ) 2 -7,8- nido -C 2 B 9 H 10 ] is reported. Based on NBO, QTAIM and ELF calculations, the P–O⋯H + ⋯O–P interactions were characterized as strong hydrogen bonds.
Intramolecular Communication in Anionic Oxidized Phosphanes through a Chelated Proton
2015
Oxidation of the 1,2-(PR2 )2 -1,2-closo-C2 B10 H10 (R=Ph, iPr) platform with hydrogen peroxide in acetone is a two-step procedure in which partial deboronation of the closo cluster and oxidation of the phosphorus atoms occur. Based on NMR spectroscopic and kinetic data, we demonstrate that the phosphorus atoms are oxidized in the first step, followed by cluster deboronation. DFT calculations and natural-bond orbital (NBO) analysis were used to obtain insight into the electronic structures of diphosphane ortho-carborane derivatives.