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The origin of the exceptional stability of molecular glasses grown by physical vapor deposition (PVD) is not well understood. Differences in glass density have been correlated with thermodynamic stability for thin films of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) grown by PVD at specific substrate temperatures below the glass transition temperature. However, the relationship between the internal conformation of glass molecules and the thermodynamic properties of molecular glasses is not well studied. We use carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectroscopy to examine different TPD sample preparations in which differences in the thermodynamic stability of the glass are known. Density functional theory simulations of the NEXAFS spectra of TPD allow us to attribute spectroscopic differences to changes in the internal conformation of the TPD molecule and relate this conformation to the stability of the TPD glass. This provides a direct experimental measurement of the internal conformation of molecules forming an organic glass. [ABSTRACT FROM AUTHOR]

We present high resolution oxygen K near-edge x-ray absorption spectra of the acenaphthenequinone (ANQ) derivative 3,8-dibromo-5,6-dichloro-ANQ (Br2Cl2-ANQ). The spectral features exhibit an almost identical vibronic fine structure compared to that shown by ANQ. The unequal distances of the vibronic levels as derived from the least-squares fit analysis of the vibronic progressions allows us to determine the anharmonicity of the excited state potentials involved. We conclude that a single vibrational progression couples to the resonant excitation of O 1s core electrons preferentially. Comparison of the two ANQ derivatives gives a clear indication that the vibronic mode corresponds to a C=O stretching mode, rather than coupling to a C-H mode as suggested previously. These conclusions are supported by density functional theory calculations. [ABSTRACT FROM AUTHOR]

The speciation and quantification of sulfur species based on sulfur K-edge x-ray absorption spectroscopy is of wide interest, particularly for biological and petroleum science. These tasks require a firm understanding of the sulfur 1s near-edge x-ray absorption fine structure (NEXAFS) spectra of relevant species. To this end, we have examined the gas phase sulfur 1s NEXAFS spectra of a group of simple thiol and thioether compounds. These high-resolution gas phase spectra are free of solid-state broadening, charging, and saturation effects common in the NEXAFS spectra of solids. These experimental data have been further analyzed with the aid of improved virtual orbital Hartree-Fock ab initio calculations. The experimental sulfur 1s NEXAFS spectra show fine features predicted by calculation, and the combination of experiment and calculation has been used to improve assignment of spectroscopic features relevant for the speciation and quantification of the sulfur compounds. [ABSTRACT FROM AUTHOR]

We have recorded the iron 2p and carbon 1s near-edge x-ray absorption fine structure (NEXAFS) spectra of decamethyl ferrocene (Fe 3d6) and ferrocenium hexafluorophosphate (Fe 3d5), two low spin compounds with closed- and open-shell electron configurations, respectively. These species have a similar covalent bonding environment but a different electron configuration, allowing us to examine the role of ligand-metal interactions and electron correlation responsible for their NEXAFS spectra. We have interpreted these spectra with the aid of ab initio configuration interaction (CI) calculations. The CI calculations indicate that double excitation is essential to interpret the Fe 2p NEXAFS spectra of not only the open-shell ferrocenium ion but also the closed-shell ferrocene species, even though the ground states of both species are well described within the Hartree–Fock single-configuration approximation. [ABSTRACT FROM AUTHOR]

The carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectra of simple gaseous alkane molecules differ from the spectra of the same alkane molecules in the condensed phase. The origin of these large, systematic differences is poorly understood. The NEXAFS spectra of gaseous alkanes are interpreted as a progression of core→Rydberg transitions with distinctive vibronic structure. The interpretation of the NEXAFS spectra of condensed phase alkanes is varied. Specifically, the degree of Rydberg character in the preedge core excited states of condensed alkanes is controversial. We determined the character of core excited states in condensed alkanes with a combination of experiment and computational study. From this, we have determined the nature of matrix effects for these species. The high-resolution carbon 1s NEXAFS spectrum of gaseous neopentane is dramatically different from its condensed phase spectrum, a striking illustration of the dramatic spectroscopic changes that occur upon condensation. High quality ab initio calculations of a cluster designed to model the solid phase environment provide definitive evidence for the reduction of Rydberg character and support the assignment of σC–H* valence character in the preedge features in the NEXAFS spectra of condensed alkanes. [ABSTRACT FROM AUTHOR]

NEXAFS spectroscopy has potential for study of packing and order in organic materials but only if intermolecular effects are understood. This work studies how π-π interactions between adjacent unsaturated groups affect their NEXAFS spectra, with a broader goal of building a general understanding of the role of intermolecular effects in NEXAFS spectroscopy. These effects are examined using paracyclophane (PCP) molecules, in which the benzene-benzene separation distance is controlled by varying the length of the alkyl groups separating the benzene rings. NEXAFS spectroscopy and density functional theory (DFT) simulations are used to examine spectroscopic changes related to the strength of these π-π interactions. A characteristic red shift is observed as adjacent benzene rings get closer together. This shift is attributed to Coulombic and orbital interactions between the adjacent benzene rings, mediated through overlapping π/π* orbitals. [ABSTRACT FROM AUTHOR]

Highlights • Linear dichroism in the carbon 1s NEXAFS spectra of n -alkane single crystals show crystal structure differences. • A 'backbone' component in the 'C–H' band is confirmed, contradicting simple 'building block' models. • The 'backbone' spectroscopic component appears in one polarization, reflecting the monoclinic crystal symmetry. Abstract Linear Dichroism in Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is a useful tool for studying molecular orientation and for clarifying spectroscopic assignments. n- Alkane molecules can form single crystals with different polymorphs; orthorhombic and monoclinic in the case of even chain lengths between n- C 28 H 58 and n- C 40 H 82. These polymorphs exhibit different linear dichroism in their NEXAFS spectra; in particular a backbone-oriented transition contributes to the low-energy "C–H" band in the monoclinic polymorph. [ABSTRACT FROM AUTHOR]

Highlights: [•] We examine the sulfur 1s X-ray absorption spectra of sulfone and sulfoxide species. [•] Improved virtual orbital calculations complement high energy resolution experimental measurement. [•] Characteristic chemical fingerprints are modified by oxidation state and ligand electronic interactions. [Copyright &y& Elsevier]

Highlights: [•] We examine the sulfur 1s X-ray absorption spectra of disulfide species. [•] Improved Virtual Orbital calculations complement high energy resolution experimental measurement. [•] Characteristic chemical fingerprints are modified by symmetry, inductive and orbital interactions. [Copyright &y& Elsevier]

• First NEXAFS spectra of liquid phase n -alkanes are obtained. • Distinct spectroscopic differences between gas, liquid and solid phase n -alkanes. • Nuclear motion broadening is required to explain observed trends. The carbon 1s near edge X-ray absorption fine structure (NEXAFS) spectra of liquid phase n -alkanes have been measured for the first time. Clear trends are observed in the liquid phase spectra as a function of chain length. Specifically, the 'C-H band' in the NEXAFS spectra shifts to lower energy as the chain length increases. Characteristic differences between the gas, liquid and solid phase spectra are observed. Differences in the liquid phase n -alkane NEXAFS spectra are examined through molecular dynamics simulations coupled with density functional theory simulations of the NEXAFS spectra. [ABSTRACT FROM AUTHOR]

Sulfurplays an important role in the rubber industry as a vulcanizingagent, providing an enhancement in mechanical properties relativeto unvulcanized rubber. The sulfur used in industry originates fromnatural and petroleum sources. Petroleum-based sulfur (PS), usuallya by-product from a refinery process, was used as the vulcanizingsulfur in the present work. The chemical structure of PS was characterizedand compared to that of a commercial rhombic sulfur (RS) using sulfurK-edge X-ray absorption near-edge structure (XANES) spectroscopy.The results reveal that sulfur K-edge XANES spectroscopy has goodsensitivity to the chemical structure of sulfur, and the chemicalstructures of sulfur in rubbers vulcanized with PS and RS are verysimilar. Furthermore, different types of rubbers vulcanized with PSor RS exhibit comparable mechanical properties. This work shows thatPS can be used to replace RS as a vulcanizing agent for rubber. [ABSTRACT FROM AUTHOR]