1,721,153 research outputs found
Diffusion mechanisms in smectic ionic liquid crystals: Insights from coarse-grained MD simulations
No full textWe present the results of MD simulations, using a coarse grained force field, concerning the mechanism of diffusion in ionic liquid crystals. The dynamical properties of the recently characterized (G. Saielli, Soft Matter, 2012, 8, 10279) model system of 1-hexadecyl-3-methylimidazolium nitrate in the smectic A phase are analysed in detail. Comparison is made with the dynamical behaviour of thermotropic non-ionic smectic liquid crystals (LCs) and with that of lamellar phases of surfactant-water mixtures. The self-diffusion anisotropy, that is the ratio between the parallel and perpendicular diffusion coefficients, is consistent with that of non-ionic smectic LCs, though a significant contribution to the parallel diffusion is given by "pore" defects, similarly to what was observed in lamellar phases. © 2013 The Royal Society of Chemistry
Computational Spectroscopy of Ionic Liquids for Bulk Structure Elucidation
No full text“Computational spectroscopy” refers to quantum chemistry protocols capable of predicting the electronic and/or magnetic spectra of molecules. The most common techniques used for structural assignment are infrared, electronic, and NMR spectroscopies. Chemists can normally deduce the chemical structure of an unknown substance by using a vast collection of empirical relationships linking the spectral features with the presence or absence of functional groups and, this part mostly by NMR, the connectivity between them and the relative stereochemistry. Computational spectroscopy is a powerful aid for structural elucidation when empirical relationships do not suffice to unambiguously assign the structure. In these cases, the calculated spectrum of a putative structure is compared with the experimental one and the match, or lack thereof, between the two, measured by several statistical parameters, indicates whether or not that structure is the correct one. Is it possible to extend such protocols to bulk phases of complex fluids, such as ionic liquids, rather than covalent molecules, in order to get insights into the average structure of the fluid? It is the aim of this Progress Report to highlight recent advances in this field through the discussion of specific case studies
Computational NMR spectroscopy of ionic liquids: [C4C1Im]Cl/water mixtures
Get PDFIn this work, I have analyzed the structure of binary mixtures of 1‐butyl‐3-methylimidazolium chloride ionic liquid, [C4C1im]Cl, and water, using computational NMR spectroscopy. The structure of the complex fluid phase, where the ionic and hydrophobic nature of ionic liquids is further complicated by the addition of water, is first generated by classical Molecular Dynamics (MD) and then validated by calculating the NMR properties with DFT at the ONIOM(B3LYP/cc‐pVTZ//B3LYP/3‐21G) on clusters extracted during the MD trajectories. Three ionic liquid/water mixtures have been considered with the [C4C1im]Cl mole fraction of 1.00, 0.50, and 0.01, that is the pure ionic liquid [C4C1im]Cl, the equimolar [C4C1im]Cl/water mixture, and a diluted solution of [C4C1im]Cl in water. A good agreement is obtained with published experimental data that, at the same time, validates the structural features obtained from the MD and the force field used, and provides an example of the power of NMR spectroscopy applied to complex fluid phases
Td-dft prediction of the intermolecular charge-transfer uv-vis spectra of viologen salts in solution
Get PDFThe absorption spectrum of viologen salts in a medium or low polar solvent is an essential feature that influences all its “chromic” applications, whether we are considering thermochromic, electrochromic, photochromic or chemochromic devices. The prediction by quantum chemical methods of such absorption bands, typically observed in the visible range and due to charge transfer (CT) phenomena, is a very challenging problem due to strong solvent effects influencing both the geometry and the electronic transitions. Here we present a computational protocol based on DFT to predict with very high accuracy the absorption maxima of the CT bands of a series of viologen salts in solvents of low and medium polarity. The calculations also allow a clear dissection of the solvent effects, direct and indirect, and orbital contributions to the CT band
One-bond 1J(15N─19F) spin–spin coupling constants of cationic fluorinating reagents: Insights from DFT calculations
No full textWe have investigated, by means of density functional theory protocols, the one-bond 1J(15N─19F) spin–spin coupling constants in a series of fluorinating reagents, containing the N─F bond, recently studied experimentally. The results of the calculations show a very good linear relationship with the experimental values, even though only the M06-2X(PCM)/pcJ-2//B3LYP/6-311G(d,p) level affords a very low mean absolute error. The calculations allow to analyze the various molecular orbitals contributions to the J coupling and to rationalize the observed positive sign, corresponding to a negative sign of the reduced spin–pin coupling constant K(N─F). Moreover, of the four Ramsey contributions, only the diamagnetic spin orbit is negligible, whereas the paramagnetic spin orbit and spin dipole terms decrease the magnitude of the Fermi contact (FC) term by an amount that goes from a minimum of 35% up to more than 60% of the FC term itself. Several effects have been investigated, namely, the contribution of the long-range solvent reaction field, relativistic corrections, and conformational and vibrational effects
Differential Solvation Free Energies of Oxonium and Ammonium Ions: Insights from Quantum Chemical Calculations
No full textWe have employed computational protocols to determine the differential solvation free energy in water of oxonium and ammonium ions. We have focused our analysis on pairs of onium ions having the same hydrocarbon content and substitutional pattern (HCSP pairs). In agreement with previous suggestions (Taft, R. W.; Wolf. J. F.; Beauchamp, J. L.; Scorrano, G.; Arnett, E. M. J. Am. Chem. Soc. 1978, 100, 1240), we found that cavitation and van der Waals terms do not contribute to the differential solvation free energy. Moreover, we observe that oxonium ions are more strongly solvated than the analogous ammonium ions even though the former ones have fewer H-bond donor sites. The performance of two different continuum solvation models, PCM and SMD, is discussed. © 2010 American Chemical Society
Comparison of the ionic liquid crystal phase of [C12C1im][BF4] and [C12C1im]Cl by atomistic MD simulations
Get PDFWe present fully atomistic molecular dynamics (MD) simulations at 450 K of two ionic liquid crystals in the smectic phase: 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C12C1im][BF4]) and 1-dodecyl-3-methylimidazolium chloride ([C12C1im]Cl). The two systems experimentally exhibit different ranges of thermal stability of the ionic smectic phase: The chloride salt has a more stable LC phase, between 270.3 K and 377.6 K, with a range of SmA of more than 107 K. In contrast, the tetrafluoroborate salt has a smectic phase between 299.6 K and 311.6 K, with a range of mesophase of just 12 K. The MD simulations show that the chloride system is stable in the smectic phase for the 5 ns of simulation, while the tetrafluoroborate salt melts into an isotropic ionic liquid, in qualitative agreement with the experiments. Comparison of the electrostatic and van der Waals energetic contributions enables one to rationalize the observed behavior as being due to the large size of the [BF4] anion: a larger size of the anion means a lower charge density, and therefore a weaker electrostatic interaction in the ionic layer
DFT study of the interaction free energy of π-π complexes of fullerenes with buckybowls and viologen dimers
No full textWe present a theoretical investigation, by means of DFT protocols, of the complexation thermodynamics of (i) complexes of C70 and C 60 fullerenes with bowl-shaped hexabenzocoronene derivatives and (ii) complexes of C60 with viologen dimers. The recent functionals of the M06 family, accounting for π-π interactions to a good level of approximation, have been used to calculate the interaction free energies. For the former complexes, the good agreement between the calculated results and the experimental data confirms the reliability of the protocol used. On these grounds, we then checked the stability of a series of complexes of C 60 with some viologen dimers, 1BPnBP1 (n = 6-9), where two N-methylated bipyridinium units are linked by an alkyl chain of variable length, acting as molecular tweezers. Both forms of the viologen cores, that is the cation (each core doubly charged) and the reduced neutral form, were considered. For a suitable chain length a free energy gain upon complexation is predicted for the neutral form while the complexation of C60 by the cationic form is disfavoured mainly for entropic reasons. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique
Ion-pairing of octyl viologen diiodide in low-polar solvents: An experimental and computational study
No full textWe have investigated the ion-pairing and solvent effect on the NMR and UV/vis spectra of 1,1′-di-n-octyl-4,4′-bipyridinium diiodide in various solvents. A strikingly different behavior is observed in the low polar solvent dichloromethane. A large deshielding of the meta bipyridinium core resonance occurs and charge transfer (CT) transitions are observed in the visible region due to the formation of ion-pairs. The CT bands show a marked blue-shift as the polarity of the solvent is increased. Experimental data have been compared with the results of DFT calculations of proton's chemical shifts and TD-DFT calculations of the vertical electronic transitions of model ion-pairs (using the smaller methyl viologen dication) in the gas phase and after the inclusion of the solvent reaction field by means of the PCM scheme. Different geometrical arrangements of the ion-pairs have been investigated, and the direct and indirect solvent effect has been elucidated. A good agreement is obtained which allows one to get insights concerning the CT transitions of this system and the geometry of the ion-pairs in solution of low-polar solvents. © 2008 American Chemical Society
Fully Atomistic Simulations of the Ionic Liquid Crystal [C16mim][NO3]: Orientational Order Parameters and Voids Distribution
No full textWe present a fully atomistic molecular dynamics simulation of the smectic phase of the ionic liquid crystal (ILC) 1-hexadecyl-3-methylimidazolium nitrate, [C16MIm][NO3]. We have characterized the structure of the phase by means of a set of radial distribution functions resolved along the director and in the plane of the smectic layers. The results obtained allow us to discuss the similarities in the microscopic structure of ionic liquids (ILs) and ILCs. In addition to this, we have calculated the orientational order parameters, S, of the methylene groups of the alkyl chain and compared them with the results obtained for phospholipidic membranes from 2H NMR experiments. We also discuss the orientational order parameters of the imidazolium ring. Finally, we analyze the distribution of voids in the ILC phase. We have found that voids of considerable volume to host a nonpolar gas, e.g. xenon, are localized in the hydrophobic layers and almost absent in the ionic layers
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