Mass analyzed threshold ionization spectra of phenol⋯Ar2: ionization energy and cation intermolecular vibrational frequencies

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The phenol+⋯Ar2 complex has been characterized in a supersonic jet by mass analyzed threshold ionization (MATI) spectroscopyvia different intermediate intermolecular vibrational states of the first electronically excited state (S1). From the spectra recorded via the S100 origin and the S1βx intermolecular vibrational state, the ionization energy (IE) has been determined as 68 288 ± 5 cm−1, displaying a red shift of 340 cm−1 from the IE of the phenol+ monomer. Well-resolved, nearly harmonic vibrational progressions with a fundamental frequency of 10 cm−1 have been observed in the ion ground state (D0) and assigned to the symmetric van der Waals (vdW) bending mode, βx, along the x axis containing the C–O bond. MATI spectra recorded via the S1 state involving other higher-lying intermolecular vibrational states (σ1s, β3x, σ1sβ1x, σ1sβ2x) are characterized by unresolved broad structures

Ionization-induced pi -> H site-switching in phenol-CH4 complexes studied using IR dip spectroscopy

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.IR spectra of phenol–CH4 complexes generated in a supersonic expansion were measured before and after photoionization. The IR spectrum before ionization shows the free OH stretching vibration (νOH) and the structure of neutral phenol–CH4 in the electronic ground state (S0) is assigned to a π-bound geometry, in which the CH4 ligand is located above the phenol ring. The IR spectrum after ionization to the cationic ground state (D0) exhibits a red shifted νOH band assigned to a hydrogen-bonded cationic structure, in which the CH4 ligand binds to the phenolic OH group. In contrast to phenol–Ar/Kr, the observed ionization-induced π → H migration has unity yield for CH4. This difference is attributed to intracluster vibrational energy redistribution processes

Comparison of Various Means of Evaluating Molecular Electrostatic Potentials for Noncovalent Interactions

The various heterodimers formed by a series of Lewis acids with NH3 as Lewis base are identified. Lewis acids include those that can form chalcogen (HSF and HSBr), pnicogen (H2PF and H2PBr), and tetrel (H3SiF and H3SiBr) bonds, as well as H‐bonds and halogen bonds. The molecular electrostatic potential (MEP) of each Lewis acid is considered in a number of ways. Pictorial versions show broad regions of positive and negative MEP, on surfaces that vary with respect to either the value of the chosen isopotential, or their distance from the nuclei. Specific points are identified where the MEP reaches a maximum on a particular isodensity surface (Vs,max). The locations and values of Vs,max were evaluated on different isodensity surfaces, and compared to the stabilities of the various equilibrium geometries. As the chosen isodensity is decreased, and the MEP maxima drift away from the molecule, some points maintain their angular positions with respect to the molecule, whereas others undergo a reorientation. The lowering isodensity also causes some of the maxima to disappear. In general, there is a fairly good correlation between the energetic ordering of the equilibrium structures and the values of Vs,max. A number of possible Lewis acid sites on the heteroaromatic imidazole ring were also considered and presents some cautions about application of Vs,max as the principal criterion for predicting equilibrium geometries. © 2017 Wiley Periodicals, Inc

A thorough anion-π interaction study in biomolecules:On the importance of cooperativity effects

Noncovalent interactions have a constitutive role in the science of intermolecular relationships, particularly those involving aromatic rings such as π-π and cation-π. In recent years, anion-π contact has also been recognized as a noncovalent bonding interaction with important implications in chemical processes. Yet, its involvement in biological processes has been scarcely reported. Herein we present a large-scale PDB analysis of the occurrence of anion-π interactions in proteins and nucleic acids. In addition we have gone a step further by considering the existence of cooperativity effects through the inclusion of a second noncovalent interaction, i.e. π-stacking, T-shaped, or cation-π interactions to form anion-π-π and anion-π-cation triads. The statistical analysis of the thousands of identified interactions reveals striking selectivities and subtle cooperativity effects among the anions, π-systems, and cations in a biological context. The reported results stress the importance of anion-π interactions and the cooperativity that arises from ternary contacts in key biological processes, such as protein folding and function and nucleic acids-protein and protein-protein recognition. We include examples of anion-π interactions and triads putatively involved in enzymatic catalysis, epigenetic gene regulation, antigen-antibody recognition, and protein dimerization

Consequence of one-electron oxidation and one-electron reduction for aniline

Quantum-chemical calculations were performed for all possible isomers of neutral aniline and its redox forms, and intramolecular proton-transfer (prototropy) accompanied by π-electron delocalization was analyzed. One-electron oxidation (PhNH2 – e → [PhNH2]+•) has no important effect on tautomeric preferences. The enamine tautomer is preferred for oxidized aniline similarly as for the neutral molecule. Dramatical changes take place when proceeding from neutral to reduced aniline. One-electron reduction (PhNH2 + e → [PhNH2]-•) favors the imine tautomer. Independently on the state of oxidation, π- and n-electrons are more delocalized for the enamine than imine tautomers. The change of the tautomeric preferences for reduced aniline may partially explain the origin of the CH tautomers for reduced nucleobases (cytosine, adenine, and guanine)