Site-Dependent Spectral Shifts in Core-to-π* Excitations of Pyridine Clusters

Abstract

Site- and element-selective core-to-pi* excitation in free pyridine clusters is investigated. The experimental results indicate the occurrence of site- and size-dependent spectral shifts in the C 1s and N 1s --> pi* excitation regime. Specifically, we observe in the C 1s regime a substantial and site-dependent redshift of the low energy slopes of the C 1s --> pi* band by 90 meV in clusters relative to the bare molecule, whereas the high energy slopes of this band remain almost unchanged. In contrast, a size-dependent blueshift of the same order of magnitude is found for the entire N 1s --> pi* band. This is distinctly different from previous results on van der Waals clusters, where exclusively redshifts in 1s --> pi* transitions are observed. The experimental results are compared to ab initio calculations, which serve to simulate the 1s --> pi*( v = 0) transitions. These results clearly indicate that the spectral shifts are primarily a result of electrostatic interactions between the molecular moieties and that an antiparallel orientation of molecular units preferably dominates in variable-size pyridine clusters.

Gas-to-cluster effects in S 2p-excited SF6

“…According to Eqs. (10) and (11), the gas-to-cluster effects are proportional to (i) the Lorentzian widths W L,m of the molecular shape resonances and (ii) the module |B c | of the backscattered electron waves at neighboring molecules in clusters. The analysis of the experimental data supports the occurrence of the cluster W L,m |B c |-effect (WB-effect).…”

“…[7][8][9] Even larger spectral shifts have been observed for aromatic molecules with a substantial dipole moment, whereby for C 1s → π * -excited pyridine a spectral redshift of 90 meV is observed, whereas a spectral blueshift of the same magnitude is observed for the N 1s → π * -excitation. 11 The gas-to-cluster redshifts have been on the one hand attributed to the freezing of molecular rotations and the dynamic stabilization of the core-excited clustered molecules. 7 On the other hand, ab initio calculations show for individual sites in clusters site-specific stabilization or destabi-lization effects, which explain the overall observed spectral shifts.…”

Nucleation of Organic Crystals—A Molecular Perspective

“…From a CNT perspective, this research direction reveals the nature of the building units of the [86,88] 5-fluorouracil (nitromethane/water) molecular modeling yes (exp. XPS studies of core-level binding energies associated with hydrogen bonding and protonation have already been carried out [72][73][74][75][76][77][78]97] and together with previous studies of core-level shifts caused by van der Waals and dipole interactions [98][99][100] have prepared the ground for more detailed structural studies of solute-solvent interactions, self-association, and cluster formation. [94] benzophenone (methanol, toluene) NMR yes [91] diphenylamine (methanol, toluene) NMR yes [91] p-acetanisidide (chloroform) NMR yes [84] isonicotinamide (methanol, nitromethane) FTIR, Raman yes [89] carbamazepine (methanol, chloroform) NMR yes [105,106] nucleus, and the nature of the building units is related to the attachment frequency of the molecules.…”

“…Dielectric spectroscopy is an emerging technique that reveals information about molecular movement during crystallization from the amorphous state, [95,96] whereas spectroscopic methods with soft X-rays, such as XPS and NEXAFS, have been shown to be sensitive to bond lengths, coordination numbers, and the geometric arrangement of coordinating species, as well as the oxidation and charge state of solute species. XPS studies of core-level binding energies associated with hydrogen bonding and protonation have already been carried out [72][73][74][75][76][77][78]97] and together with previous studies of core-level shifts caused by van der Waals and dipole interactions [98][99][100] have prepared the ground for more detailed structural studies of solute-solvent interactions, self-association, and cluster formation.…”

Keimbildung organischer Kristalle aus molekularer Sichtweise

“…Dielektrische Spektroskopie ist ein neues Verfahren, um Informationen über die Molekülbewegung während der Kristallisation aus dem amorphen Zustand zu gewinnen,95, 96 während Spektroskopie mit weichen Röntgenstrahlen, beispielsweise XPS und NEXAFS, empfindlich für Bindungsabstände, Koordinationszahlen und die geometrische Anordnung von koordinierenden Spezies ist, ebenso wie für den Oxidations‐ und Ladungszustand der gelösten Spezies. XPS‐Untersuchungen von Rumpfelektronen‐Bindungsenergien in Verbindung mit Wasserstoffbrücken und Protonierung72–78, 97 und früheren Untersuchungen von Verschiebungen der Rumpfelektronen‐Energieniveaus durch Van‐der‐Waals‐ und Dipol‐Wechselwirkungen98–100 bilden die Grundlage für ausführlichere Strukturuntersuchungen von Wechselwirkungen zwischen gelöstem Stoff und Lösungsmittel, Selbstorganisation und Clusterentstehung.…”