Six-fold-symmetry internal rotation in toluenes: the low barrier challenge of 2,6-and 3,5-difluorotoluene

Pure six-fold symmetry (V6) internal rotation poses significant challenges to experimental and theoretical determination, as the very low torsional barriers result in huge tunneling splittings difficult to identify and to model. Here we resolved the methyl group internal rotation dynamics of 2,6- and 3,5-difluorotoluene using a newly developed computer code especially adapted to V6 problems. The jet-cooled rotational spectra of the title molecules in the 5–25 GHz region revealed internal rotation tunneling doublings of up to 3.6 GHz, which translated in methyl group potential barriers of V6 = 0.14872(24) and 0.0856(10) kJ mol−1, respectively, in the vibrational ground-state. Additional information on Stark effects and carbon isotopic species in natural abundance provided structural data and the electric dipole moments for both molecules. Ab initio calculations at the MP2 level do not reproduce the tiny torsional barriers, calling for experiments on other systems and additional theoretical models.DFGMINECO/CTQ2012-39132-C02-0

H\u3csub\u3e2\u3c/sub\u3e Oxidation Over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H\u3csub\u3e2\u3c/sub\u3e Activation at the Metal-Support Interface

Water adsorbed at the metal-support interface (MSI) plays an important role in multiple reactions. Due to its importance in CO preferential oxidation (PrOx), we examined H2 oxidation kinetics in the presence of water over Au/TiO2 and Au/Al2O3 catalysts, reaching the following mechanistic conclusions: (i) O2 activation follows a similar mechanism to that proposed in CO oxidation catalysis; (ii) weakly adsorbed H2O is a strong reaction inhibitor; (iii) fast H2 activation occurs at the MSI, and (iv) H2 activation kinetics are inconsistent with traditional dissociative H2 chemisorption on metals. Density function theory (DFT) calculations using a supported Au nanorod model suggest H2 activation proceeds through a heterolytic dissociation mechanism, resulting in a formal hydride residing on the Au and a proton bound to a surface TiOH group. This potential mechanism was supported by infrared spectroscopy experiments during H2 adsorption on a deuterated Au/TiO2 surface, which showed rapid H-D scrambling with surface hydroxyl groups. DFT calculations suggest that the reaction proceeds largely through proton-mediated pathways and that typical Brønstednsted-Evans Polanyi behavior is broken by introducing weak acid/base sites at the MSI. THe kinetics data were successfully reinterpreted in the context of the heterolytic H2 activation mechanism, tying together the experimental and computational evidence and rationalizing the observed inhibition by physiorbed water on the support as blocking the MSI sites required for heterolytic H2 activation. In addition to providing evidence for the unusual H2 activation mechanism, these results offer additional insight into why water dramatically improves CO PrOx catalysis over Au

Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran–Formaldehyde Complex

The 1:1 benzofuran–formaldehyde complex has been chosen as model system for analyzing π→π* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. A joint “rotational spectroscopy–quantum chemistry” strategy unveiled the dominant role of π→π* interactions in tuning the intermolecular interactions of such adduct. The exploration of the intermolecular potential energy surface led to the identification of 14 low-energy minima, with 4 stacked isomers being more stable than those linked by hydrogen bond or lone-pair→π interactions. All energy minima are separated by loose transition states, thus suggesting an effective relaxation to the global minimum under the experimental conditions. This expectation has been confirmed by the experimental detection of only one species, which was unambiguously assigned owing to the computation of accurate spectroscopic parameters and the characterization of 11 isotopologues. The large number of isotopic species opened the way to the determination of the first semi-experimental equilibrium structure for a molecular complex of such a dimension

Conformational steering in dicarboxy acids: the native structure of succinic acid

Succinic acid, a dicarboxylic acid molecule, has been investigated spectroscopically with computational support to elucidate the complex aspects of its conformational composition. Due to the torsional freedom of the carbon backbone and hydroxy groups, a large number of potentially plausible conformers can be generated with an indication that the gauche conformer is favored over the trans form. The microwave and millimeter wave spectra have been analyzed and accurate spectroscopic constants have been derived that correlate best with those of the lowest energy gauche conformer. For an unambiguous conformational identification measurements were extended to the monosubstituted isotopologues, precisely determining the structural properties. Besides bond distances and angles, particularly the dihedral angle has been determined to be 67.76(11)°, confirming the anomalous tendency of the methylene units to favor gauche conformers when a short aliphatic segment is placed between two carbonyl groups.Spanish Ministry of Science and Innovation/CTQ2011-22923Spanish Ministry of Science and Innovation/CGL2011-2244

Parameteridentifikation und Virtuelles Prototyping von Nanopositionier- und Nanomessmaschinen basierend auf Methoden der Mehrkörperdynamik

In Nanopositionier- und Nanomessmaschinen sollen hochdynamische Präzisionspositionierungen durch Kombination von Positioniersystemen mit großen und kleinen Bewegungsbereichen erreicht werden. Mit einer modellgestützten Simulation des dynamischen Verhaltens in frühen Phasen des konstruktiven Entwicklungsprozesses können Positionierbereiche von 450 mm x 450 mm mit Nanometerreproduzierbarkeit bei Bahngeschwindigkeiten bis 0,5 m/s erreicht werden. Aymptotische Methoden werden auf Minimalmodelle der NPM-Maschine angewendet, um analytische Lösungen für Schwingungsprobleme zu entwickeln. Dabei wird der Fokus nicht vorrangig auf die Lösung der Direkten Aufgabe der Dynamik gelegt. Vielmehr wird durch den Vergleich der analytischen Ausdrücke für die stationären Amplituden mit experimentell ermittelten Werten die Bestimmung von Parametern, die der unmittelbaren messtechnischen Erfassung nicht zugänglich sind, möglich. Zur Ermittlung dynamischer Steifigkeiten und Dämpfungen wird die Methode der Inertialkrafterregung entwickelt

Electron penetration in the nucleus and its effect on the quadrupole interaction

A series expansion of the interaction between a nucleus and its surrounding electron distribution provides terms that are well-known in the study of hyperfine interactions: the familiar quadrupole interaction and the less familiar hexadecapole interaction. If the penetration of electrons into the nucleus is taken into account, various corrections to these multipole interactions appear. The best known one is a scalar correction related to the isotope shift and the isomer shift. This paper discusses a related tensor correction, which modifies the quadrupole interaction if electrons penetrate the nucleus: the quadrupole shift. We describe the mathematical formalism and provide first-principles calculations of the quadrupole shift for a large set of solids. Fully relativistic calculations that explicitly take a finite nucleus into account turn out to be mandatory. Our analysis shows that the quadrupole shift becomes appreciably large for heavy elements. Implications for experimental high-precision studies of quadrupole interactions and quadrupole moment ratios are discussed. A literature review of other small quadrupole-like effects is presented as well

Search for corannulene (C20H10) in the Red Rectangle

Polycyclic Aromatic Hydrocarbons (PAHs) are widely accepted as the carriers of the Aromatic Infrared Bands (AIBs), but an unambiguous identification of any specific interstellar PAH is still missing. For polar PAHs, pure rotational transitions can be used as spectral fingerprints for identification. Combining dedicated experiments, detailed simulations and observations, we explore d the mm wavelength domain to search for specific rotational transitions of corannulene (C20H10). We performed high-resolution spectroscopic measurements and a simulation of the emission spectrum of ultraviolet-excited C20H10 in the environment of the Red Rectangle (RR), calculating its synthetic rotational spectrum. Based on these results, we conducted a first observational campaign at the IRAM 30-m telescope towards this source to search for several high-J rotational transitions of C20H10. The laboratory detection of the J = 112 ← 111 transition of corannulene showed that no centrifugal splitting is present up to this line. Observations with the IRAM 30-m telescope towards the RR do not show any corannulene emission at any of the observed frequencies, down to a rms noise level of Tmb= 8 mK for the J =135 → 134 transition at 137.615 GHz. Comparing the noise level with the synthetic spectrum, we are able to estimate an upper limit to the fraction of carbon locked in corannulene of about 1.0 × 10−5 relative to the total abundance of carbon in PAHs. The sensitivity achieved in this work shows that radio spectroscopy can be a powerful tool to search for polar PAHs. We compare this upper limit with models for the PAH size distribution, emphasizing that small PAHs are much less abundant than predicted. We show that this cannot be explained by destruction but is more likely related to the chemistry of their formation in the environment of the R