1,720,972 research outputs found
Comment on: "Quantum Confinement in Hydrogen Bond" by Carlos da Silva dos Santos, Elso Drigo Filho, and Regina Maria Ricotta, Int. J. Quantum Chem. 2015, 115, 765-770.
No full textMolecular Docking via Olefinic OH center dot center dot center dot pi Interactions: A Bulky Alkene Model System and Its Cooperativity
No full textComplexes of t-butyl alcohol with norbornene and its monocyclic constituents cyclopentene and cyclohexene are studied via their OH stretching fundamental transitions in supersonic jet expansions. Compared to OH....OH hydrogen bonds, the spectral shifts due to OH.....pi bonding in the mixed dimers are reduced by a factor of 2. Mixed trimers show substantially different spectral signatures due to cooperative effects. Regioselective docking on the two sides of the double bond in norbornene is observed. Harmonic modeling of the spectra using dispersion-corrected hybrid functionals is quite successful, suggesting a high predictive power for this poorly explored class of complexes between alcohols and alkenes.German Research Foundation (DFG) [Su 121/4-1
From hydrogen bond donor to acceptor: the effect of ethanol fluorination on the first solvating water molecule
No full textSupersonic jet FTIR spectra of the OH stretching vibrations in complexes of mono-, di- and trifluoroethanol with water are presented. In contrast to the non-fluorinated ethanol case, the fluorinated alcohols are all shown to act as O-H center dot center dot center dot O hydrogen bond donors towards water. This is found to be mostly a consequence of the intramolecular electron-withdrawing effect of the fluorine atoms and, with decreasing importance for increasing fluorination, due to the attractive intermolecular contact between one of the dangling water OH groups and the fluorine atoms. The findings provide a stepwise rationalization for the hydrophobic properties of the pharmaceutically important trifluoromethyl group.DFG (German Research Foundation) [SFB 357, Su121/4
Soft hydrogen bonds to alkenes: the methanol–ethene prototype under experimental and theoretical scrutiny
No full textAn FTIR spectroscopic study of the elusive hydrogen-bonded methanol–ethene complex, the most elementary example for weak intermolecular alcohol hydrogen bonding to a p cloud, is presented. By isolating the complex in a supersonic jet, the rigorous comparability to high-level quantum chemical calculations is ensured. In stark contrast to classical hydrogen bonds, experimental overtone analysis reveals the harmonic oscillator approximation for the OH red shift to be accurate. Harmonic calculations up to explicitly correlated local coupled-cluster level are thus found to agree very well with experiment. The experimental OH values for the red shift (45 cm 1), the small change in diagonal anharmonicity ( 3 cm 1) and the overtone intensity attenuation (2 102-fold) together with theoretical predictions for the preferred structural arrangement and the zero-point-corrected dissociation energy (8 kJ mol 1) may thus be regarded as definitive reference values for related systems and for more approximate computational methods. In particular, MP2 calculations are shown to fail for this kind of weak intermolecular interaction.German Research Foundation [Su 121/4
Communication: Towards the binding energy and vibrational red shift of the simplest organic hydrogen bond: Harmonic constraints for methanol dimer
No full textThe discrepancy between experimental and harmonically predicted shifts of the OH stretching fundamental of methanol upon hydrogen bonding to a second methanol unit is too large to be blamed mostly on diagonal and off-diagonal anharmonicity corrections. It is shown that a decisive contribution comes from post-MP2 electron correlation effects, which appear not to be captured by any of the popular density functionals. We also identify that the major deficiency is in the description of the donor OH bond. Together with estimates for the electronic and harmonically zero-point corrected dimer binding energies, this work provides essential constraints for a quantitative description of this simple hydrogen bond. The spectroscopic dissociation energy is predicted to be larger than 18 kJ/mol and the harmonic OH-stretching fundamental shifts by about -121 cm(-1) upon dimerization, somewhat more than in the anharmonic experiment (-111 cm(-1)). (C) 2014 AIP Publishing LLC.German Research Foundation [Su 121/4-1
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
Ringmethylierung kontrolliert die Wasserstoffbrücken‐Andockstelle bei Anisol
No full textEin Methanolmolekül kann mit seiner OH-Gruppe entweder über einen OH⋅⋅⋅O- oder einen OH⋅⋅⋅π-Kontakt an ein Anisolmolekül binden. Wie durch FTIR-Absorptionsspektroskopie nachgewiesen wird, lässt sich das Verhältnis der Häufigkeiten dieser beiden Motive in Überschallexpansionen um eine Größenordnung verändern, indem Anisol am aromatischen Ring ein- bis dreifach methyliert oder mit einer tert-Butylgruppe substituiert wird. Diese starke Abhängigkeit vom Methylierungsgrad liefert einen auf kJ mol−1 genauen Maßstab für die Vorhersagequalität quantenchemischer Methoden und kann zu einem besseren Verständnis von induktiven, mesomeren und dispersionsgesteuerten Effekten beitragen. Auf B3LYP-D3/aVTZ-Niveau mit Schwingungsnullpunktkorrektur gelingt eine ausgewogene Beschreibung der beiden Wasserstoffbrückenmotive, wie auch mit einem Wellenfunktions-basierten Protokoll, das auf Einzelpunktrechnungen auf MP2- und CCSD(T)-Niveau an den optimierten Strukturen basiert. Dagegen überschätzt das M06-2X-Funktional systematisch die Stabilität des OH⋅⋅⋅π-Motivs
Bracketing subtle conformational energy differences between self-solvated and stretched trifluoropropanol.
No full textThe intramolecular OH···F hydrogen bond in 3,3,3-trifluoropropanol (TFP) exerts a subtle stabilizing effect that, when compared to the non-fluorinated analog, reorders the five distinguishable conformers and widens the gap between the two most stable structures. Here, we combine findings from Raman spectroscopy in supersonic expansions and high-level quantum-chemical calculations to bracket the energy difference between the two most stable TFP structures at 1.7(5) kJ mol(-1). The torsional potential energy surface suggests consecutive backbone and OH torsional motions for the conformer interconversion, which are discussed in the framework of supersonic jet cooling as a function of nozzle temperature. The picture of a bistable cold molecule with trans or gauche backbone emerges, in which the OH group controls the energy difference and modulates the high barrier separating the heavy atom frames.German Research Foundation [DFG SU 121/2-2
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
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