1,720,960 research outputs found
Phenyl- vs Cyclohexyl-Substitution in Methanol: Implications for the OH Conformation and for Dispersion-Affected Aggregation from Vibrational Spectra in Supersonic Jets
No full textThe monomers and hydrogen-bonded dimers of benzyl alcohol, cyclohexyl methanol, and 2-methyl-1-propanol are investigated by jet-FTIR spectroscopy, complemented by Raman spectra and quantum chemical calulations, including CCSD(T) corrections. A large variety of London dispersion effects from the interacting carbon cycles is revealed, sometimes adding to and sometimes competing with the alcoholic hydrogen bonds. Conformational (in-)flexibility provides the key for understanding these effects, and this requires accurate predictions of monomer conformational preferences, which are shown to be subtly at variance with experiment even for some triple-zeta MP2 calculations. In some observed dimers, cooperative OH center dot center dot center dot OH center dot center dot center dot pi patterns are sacrificed to optimize sigma-pi dispersion interactions. In other competitive dimers, dispersion interactions are far from maximized, because that would imply a substantial weakening of the hydrogen bond. In the series from methanol dimer to 1-indanol dimer, which this contribution bridges, B3LYP-D3 appears to switch from an overestimation to a slight underestimation of cohesion, but overall it provides a very useful modeling tool for vibrational spectra of systems affected by both hydrogen bonds and London dispersion
Control over the Hydrogen‐Bond Docking Site in Anisole by Ring Methylation
No full textThe supramolecular docking of methanol to anisole may occur via an OH center dot center dot center dot O hydrogen bond or via an OH center dot center dot center dot pi contact. The subtle balance between these two structures can be varied in supersonic jets by one order of magnitude through single to triple methylation of the aromatic ring and introduction of a single tert-butyl substituent, as evidenced by infrared spectroscopy. This steep variation makes it possible to assess the accuracy of relative quantum-chemical energy predictions on a kJ mol(-1) level, promising insights into inductive, mesomeric, and dispersive effects. The zero-point-corrected B3LYP-D3/aVTZ level is shown to provide an accurate relative description of the two very different hydrogen bonds, similar to a wavefunction-based protocol including CCSD(T) corrections applied to the same structures. M06-2X alone systematically overestimates the stability of pi coordination
Chirality-dependent balance between hydrogen bonding and London dispersion in isolated (+/-)-1-indanol clusters
No full textThe aggregation behavior of racemic and enantiopure 1-indanol has been studied by FTIR spectroscopy, resonant ion dip IR spectroscopy, and spontaneous Raman scattering in supersonic jets. This triple experimental approach, augmented by homology to related molecular fragments and dispersion-corrected DFT predictions, allows disentangling the complex spectroscopic signature in the OH stretch range. Evidence for chirality-sensitive aggregation via isolated OH center dot center dot center dot pi bonds in competition with cooperative center dot center dot center dot OH center dot center dot center dot OH center dot center dot center dot pi patterns is collected. An accurate description of London dispersion forces provides the key to its explanation
To pi or not to pi - how does methanol dock onto anisole?
No full textAnisole offers two similarly attractive hydrogen bond acceptor sites to an incoming hydrogen bond donor: its oxygen atom and its delocalized pi electron system. Electronic structure calculations up to the CCSD(T)/AVTZ level suggest an isoenergetic situation for methanol after harmonic zero point energy correction, within less than 1 kJ mol(-1). Linear infrared absorption spectroscopy in the OH stretching fundamental range applied to a cold supersonic jet expansion of anisole and methanol in helium shows that the oxygen binding site is preferred, with about 20 times less pi-bonded than O-bonded dimers despite the non-equilibrium collisional environment. Accidental band overlap is ruled out by OH overtone and OD stretching spectroscopy. Furthermore, the diagonal anharmonicity constant of the OH stretching mode is derived from experiment and reaches 80% of the monomer distortion found in the methanol dimer, as expected for a weaker hydrogen bond to the aromatically substituted oxygen. To reconcile these experimental findings with ab initio theory, accurate nuclear and electronic structure calculations involving AVQZ basis sets are required. Dispersion-corrected double-hybrid density functional theory provides a less expensive successful structural approach.Deutsche Forschungsgemeinschaft (DFG) [SU 121/4, SPP 1807
Molecular Recognition in Glycolaldehyde, the Simplest Sugar: Two Isolated Hydrogen Bonds Win Over One Cooperative Pair
No full textCarbohydrates are used in nature as molecular recognition tools. Understanding their conformational behavior upon aggregation helps in rationalizing the way in which cells and bacteria use sugars to communicate. Here, the simplest a-hydroxy carbonyl compound, glycolaldehyde, was used as a model system. It was shown to form compact polar C-2-symmetric dimers with intermolecular O-H center dot center dot center dot O=C bonds, while sacrificing the corresponding intramolecular hydrogen bonds. Supersonic jet infrared (IR) and Raman spectra combined with high-level quantum chemical calculations provide a consistent picture for the preference over more typical hydrogen bond insertion and addition patterns. Experimental evidence for at least one metastable dimer is presented. A rotational spectroscopy investigation of these dimers is encouraged, also in view of astrophysical searches. The binding motif competition of aldehydic sugars might play a role in chirality recognition phenomena of more complex derivatives in the gas phase.Open-Access-Publikationsfonds 201
IR-UV spectroscopy of jet-cooled 1-indanol: Restriction of the conformational space by hydration
No full textThe effect of hydration on a flexible amphiphilic molecule has been studied on the example of 1-hydroxyindan (1-indanol). Studies in jet-cooled conditions by means of resonance-enhanced two-photon ionization and IR-UV double resonance experiments show that the mono-hydrate 1-indanol(H2O) is formed in a dominant isomer, as well as the di-hydrate 1-indanol(H2O)(2). 1-Indanol(H2O) favors a cooperative hydrogen bond pattern with -OH center dot center dot center dot O(H)-H center dot center dot center dot pi it topology, while 1-indanol(H2O)(2) forms a cyclic hydrogen bond network with three OH center dot center dot center dot O interactions. The single conformation observed for the hydrates contrasts with the bare molecule which shows two dominant conformations, with the hydroxyl in axial or in equatorial position, respectively. Hydration therefore results in a restriction of the conformational space and conformational locking. (C) 2014 Elsevier B.V. All rights reserved.Gottingen University; ANR [ANR-08-BLAN-0158
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The Guinness Molecules for the Carbohydrate Formula
No full textA systematic review and analysis of the most stable spatial arrangements of n carbon, n oxygen, and 2n hydrogen atoms including vibrational zero-point energy up to n=5 shows that small-molecule aggregates win, typically followed by thermally unstable molecules, before kinetically stable molecules and finally carbohydrates are found. Near n approximate to 60 a crossover to carbon allotropes and ice as the global minimum structure is expected and the asymptotic limit is most likely graphite and ice. Implications for astrochemical and fermentation processes are discussed. Density functionals like B3LYPD3 are found to describe these energy sequences quite poorly, mostly due to an overestimated stability of carbon in high oxidation states
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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