16 research outputs found
pH and Solvent Influence on p-Aminobenzoic Acid
Through X-ray absorption and emission spectroscopies, the chemical, electronic
and structural properties of organic species in solution can be observed.
Near-edge X-ray absorption fine structure (NEXAFS) and resonant inelastic
X-ray scattering (RIXS) measurements at the nitrogen K-edge of para-
aminobenzoic acid reveal both pH- and solvent-dependent variations in the
ionisation potential (IP), 1s→π* resonances and HOMO–LUMO gap. These changes
unequivocally identify the chemical species (neutral, cationic or anionic)
present in solution. It is shown how this incisive chemical state sensitivity
is further enhanced by the possibility of quantitative bond length
determination, based on the analysis of chemical shifts in IPs and σ* shape
resonances in the NEXAFS spectra. This provides experimental access to
detecting even minor variations in the molecular structure of solutes in
solution, thereby providing an avenue to examining computational predictions
of solute properties and solute–solvent interactions
NEXAFS and XPS of p-Aminobenzoic Acid Polymorphs: The Influence of Local Environment
Nitrogen K-edge XPS and NEXAFS of the two polymorphic forms of para- aminobenzoic acid (PABA) are significantly different reflecting variation in hydrogen bonding. Alteration in hydrogen bonding at the amino group leads to a shift to high energy for both the XPS N 1s core level and the 3π* NEXAFS resonance with β-PABA. Participation of the amine group in the aromatic system causes the 1π* resonance to be sensitive to the nature of the intermolecular bonding at the para-carboxylic acid group, and a shift to low energy for α- PABA is observed due to hydrogen-bonded carboxylic acid dimer formation. FEFF calculations also successfully reproduce both the energy and intensity variations observed for the σ* shape resonance associated with the C-N bond, with the majority of the decrease in energy observed for b-PABA arising from the longer C-N bond
Studies Combining UV-Vis, NEXAFS and RIXS Spectroscopies
NEXAFS-RIXS and home laboratory-based UV-Vis absorption spectroscopy are
combined to examine the speciation and electronic structure of para-
aminobenzoic acid (PABA) in aqueous solution as a function of pH. DFT and TD-
DFT electronic structure calculations reproduce the experimental trends and
provide a correlation between the experimental HOMO↔LUMO gap as well as the
electronic transitions between molecular orbitals in the non-ionic, anionic
and cationic forms of PABA
Incompatibility-Driven Self-Organization in Polycatenar Liquid Crystals Bearing Both Hydrocarbon and Fluorocarbon Chains
The synthesis and liquid crystal properties are reported for tri-and tetra-catenar mesogens in which both hydrocarbon and semiperfluorocarbon chains have been incorporated. In the tricatenar mesogens, the lamellar spacing in the smectic C phase of the all-hydrocarbon mesogen almost doubles when the isolated hydrocarbon chain is replaced by a semiperfluorinated chain on account of the localized segregation in different sublayers between the two chain types. In the tetracatenar materials, the replacement of at least one hydrocarbon chain by semiperfluorocarbon chains is sufficient to promote columnar phase formation, but when the molecule has two hydrocarbon chains at one end and two semiperfluorocarbon chains at the other, the requirement for localized phase segregation leads to the formation of a rectangular phase with very large lattice parameters. The juxtaposition of terminal chains of different nature within the same molecular structure thus leads to a reduction in mesophase symmetry and the emergence of more complex supramolecular organization
The Structure of p-Aminobenzoic Acid in Water: Studies Combining UV-Vis, NEXAFS and RIXS Spectroscopies
NEXAFS-RIXS and home laboratory-based UV-Vis absorption spectroscopy are combined to examine the speciation and electronic structure of para-aminobenzoic acid (PABA) in aqueous solution as a function of pH. DFT and TD-DFT electronic structure calculations reproduce the experimental trends and provide a correlation between the experimental HOMO↔LUMO gap as well as the electronic transitions between molecular orbitals in the non-ionic, anionic and cationic forms of PABA
Intermolecular Bonding of Hemin in Solution and in Solid State Probed by N K-edge X-ray Spectroscopies
X-ray absorption/emission spectroscopy (XAS/XES) at the N K-edge of iron protoporphyrin IX chloride (FePPIX-Cl, or hemin) has been carried out for dissolved monomers in DMSO, dimers in water and for the solid state. This sequence of samples permits identification of characteristic spectral features associated with the hemin intermolecular bonding. These characteristic features are further analyzed and understood at the molecular orbital (MO) level based on the DFT calculations
NEXAFS Sensitivity to Bond Lengths in Complex Molecular Materials: A Study of Crystalline Saccharides
Detailed analysis of the C K near-edge X-ray absorption fine structure (NEXAFS) spectra of a series of saccharides (fructose, xylose, glucose, galactose, maltose monohydrate, α-lactose monohydrate, anhydrous β-lactose, cellulose) indicates that the precise determination of IPs and σ* shape resonance energies is sensitive enough to distinguish different crystalline saccharides through the variations in their average C–OH bond lengths. Experimental data as well as FEFF8 calculations confirm that bond length variations in the organic solid state of 10–2 Å can be experimentally detected, opening up the possibility to use NEXAFS for obtaining incisive structural information for molecular materials, including noncrystalline systems without long-range order such as dissolved species in solutions, colloids, melts, and similar amorphous phases. The observed bond length sensitivity is as good as that originally reported for gas-phase and adsorbed molecular species. NEXAFS-derived molecular structure data for the condensed phase may therefore be used to guide molecular modeling as well as to validate computationally derived structure models for such systems. Some results indicate further analytical value in that the σ* shape resonance analysis may distinguish hemiketals from hemiacetals (i.e., derived from ketoses and aldoses) as well as α from β forms of otherwise identical saccharides
New Strategies in Modeling Electronic Structures and Properties with Applications to Actinides
This chapter discusses contemporary quantum chemical methods and provides
general insights into modern electronic structure theory with a focus on
heavy-element-containing compounds. We first give a short overview of
relativistic Hamiltonians that are frequently applied to account for
relativistic effects. Then, we scrutinize various quantum chemistry methods
that approximate the -electron wave function. In this respect, we will
review the most popular single- and multi-reference approaches that have been
developed to model the multi-reference nature of heavy element compounds and
their ground- and excited-state electronic structures. Specifically, we
introduce various flavors of post-Hartree--Fock methods and optimization
schemes like the complete active space self-consistent field method, the
configuration interaction approach, the Fock-space coupled cluster model, the
pair-coupled cluster doubles ansatz, also known as the antisymmetric product of
1 reference orbital geminal, and the density matrix renormalization group
algorithm. Furthermore, we will illustrate how concepts of quantum information
theory provide us with a qualitative understanding of complex electronic
structures using the picture of interacting orbitals. While modern quantum
chemistry facilitates a quantitative description of atoms and molecules as well
as their properties, concepts of quantum information theory offer new
strategies for a qualitative interpretation that can shed new light onto the
chemistry of complex molecular compounds.Comment: 43 pages, 3 figures, Version of Recor