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Paolo Lazzeretti - One of the best experts on this subject based on the ideXlab platform.
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correlation between the out of plane components of magnetizability and central magnetic shielding in unsaturated cyclic molecules
Journal of Physical Chemistry A, 2011Co-Authors: Stefano Pelloni, Paolo LazzerettiAbstract:A simple classical model of magnetic-field induced π-electron flow is discussed, showing that the contribution to the σ∥ out-of-plane component of the virtual magnetic shielding provided by π-Ring Currents, at points P along the Cn axis of cyclic planar unsaturated hydrocarbons CnHn with Dnh symmetry, in the presence of a magnetic field Bext at right angles to the σh plane, is, with good approximation, connected with the π-electron contribution to the out-of-plane component of the magnetizability, ξ∥. The relationship is σ∥(h) = −(μ0/2π)(s2 + h2)−3/2ξ∥, where s is the distance of a C nucleus from the center of the carbon Ring, and h is the distance of P from σh. The Ring current susceptibility, that is, the strength of the π Currents, expressed in nA/T (nano ampere per tesla) within the SI system of units, is given by ∂I/∂Bext = −ξ∥/(πs2), which can be used as a reliable virtual measure of magnetotropicity and relative π-electron mobility in isoelectronic systems. Criteria for the practicality of the prop...
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Ring-current models from the differential Biot-Savart law.
Organic Letters, 2004Co-Authors: Stefano Pelloni, Andrea Ligabue, Paolo LazzerettiAbstract:The differential Biot-Savart law provides simple models for the π Ring Currents induced in diatropic and paratropic planar conjugated molecules by a perpendicular magnetic field. The model predictions are confirmed by ab initio maps of nuclear magnetic shielding density. The effects on the protons and on the Ring carbon atoms from the closest and furthest segments of the current loop are easily interpreted.
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are Ring Currents still useful to rationalize the benzene proton magnetic shielding
Organic Letters, 2004Co-Authors: Rosario G Viglione, Riccardo Zanasi, Paolo LazzerettiAbstract:The conventional interpretation of proton NMR chemical shifts is supported by large basis set ab initio quantum mechanical calculations. The benzene protons are predicted to lie within the deshielding zone defined in terms of the out-of-plane magnetic shielding domain. However, Ring Currents by themselves are not sufficient to account quantitatively for the observed benzene proton downfield chemical shift. σ-Electron contributions must also be taken into account. The conventional explanation for the ethyne proton chemical shift is valid.
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current density maps magnetizability and nuclear magnetic shielding tensors of bis heteropentalenes ii furo furan isomers
Journal of Chemical Physics, 2004Co-Authors: Garcia I Cuesta, Soriano R Jartin, Sanchez A De Meras, Paolo LazzerettiAbstract:Magnetic susceptibility and nuclear magnetic shielding at the nuclei of bis-heteropentalenes formed by two furan units ([2,3-b], [3,2-b], [3,4-b], and [3,4-c] isomers) have been computed by several approximated techniques and a large Gaussian basis set to achieve near Hartree–Fock estimates. Ab initio models of the Ring Currents induced by a magnetic field normal to the molecular plane were obtained for the three isomeric systems of higher symmetry, showing that the π electrons give rise to intense diamagnetic circulation. The π Currents are responsible for enhanced magnetic anisotropy and strong out-of-plane proton deshielding. The theoretical findings are used to build up a “diatropicity matrix” for two fused five-membered heterocyclic systems.
Alexander C. Forse - One of the best experts on this subject based on the ideXlab platform.
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mesoscopic simulations of the in situ nmr spectra of porous carbon based supercapacitors electronic structure and adsorbent reorganisation effects
Physical Chemistry Chemical Physics, 2021Co-Authors: Anagha Sasikumar, Alexander C. Forse, Patrice Simon, Anouar Belhboub, Camille Bacon, John M Griffin, Clare P Grey, Celine MerletAbstract:In situ NMR spectroscopy is a powerful technique to investigate charge storage mechanisms in carbon-based supercapacitors thanks to its ability to distinguish ionic and molecular species adsorbed in the porous electrodes from those in the bulk electrolyte. The NMR peak corresponding to the adsorbed species shows a clear change of chemical shift as the applied potential difference is varied. This variation in chemical shift is thought to originate from a combination of ion reorganisation in the pores and changes in Ring current shifts due to the changes of electronic density in the carbon. While previous Density Functional Theory calculations suggested that the electronic density has a large effect, the relative contributions of these two effects is challenging to untangle. Here, we use mesoscopic simulations to simulate NMR spectra and investigate the relative importance of ion reorganisation and Ring Currents on the resulting chemical shift. The model is able to predict chemical shifts in good agreement with NMR experiments and indicates that the Ring Currents are the dominant contribution. A thorough analysis of a specific electrode/electrolyte combination for which detailed NMR experiments have been reported allows us to confirm that local ion reorganisation has a very limited effect but the relative quantities of ions in pores of different sizes, which can change upon charging/discharging, can lead to a significant effect. Our findings suggest that in situ NMR spectra of supercapacitors may provide insights into the electronic structure of carbon materials in the future.
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nmr spectroscopy reveals adsorbate binding sites in the metal organic framework uio 66 zr
Journal of Physical Chemistry C, 2018Co-Authors: Aditya Nandy, Alexander C. Forse, Velencia J. Witherspoon, Jeffrey A. ReimerAbstract:We assign 1H and 13C NMR resonances emanating from acetone, methanol, and cyclohexane adsorbed inside the pores of UiO-66(Zr). These results are informed by density functional theory (DFT) calculations, which probe the role of two competing effects inside of the pore environment: (i) nucleus independent chemical shifts (NICSs) generated by Ring Currents in conjugated linkers and (ii) small molecule coordination to the metal-oxyhydroxy cluster. These interactions are found to perturb the chemical shift of in-pore adsorbate relative to ex-pore adsorbate (which resides in spaces between the MOF particles). Changes in self-solvation upon adsorption may also perturb the chemical shift. Our results indicate that cyclohexane preferentially adsorbs in the tetrahedral pores of UiO-66(Zr), while acetone and methanol adsorb at the Zr–OH moieties on the metal-oxyhydroxy clusters in a more complex fashion. This method may be used to probe molecular adsorption sites and material void saturation with selected adsorbates...
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NMR Spectroscopy Reveals Adsorbate Binding Sites in the Metal–Organic Framework UiO-66(Zr)
2018Co-Authors: Aditya Nandy, Alexander C. Forse, Velencia J. Witherspoon, Jeffrey A. ReimerAbstract:We assign 1H and 13C NMR resonances emanating from acetone, methanol, and cyclohexane adsorbed inside the pores of UiO-66(Zr). These results are informed by density functional theory (DFT) calculations, which probe the role of two competing effects inside of the pore environment: (i) nucleus independent chemical shifts (NICSs) generated by Ring Currents in conjugated linkers and (ii) small molecule coordination to the metal-oxyhydroxy cluster. These interactions are found to perturb the chemical shift of in-pore adsorbate relative to ex-pore adsorbate (which resides in spaces between the MOF particles). Changes in self-solvation upon adsorption may also perturb the chemical shift. Our results indicate that cyclohexane preferentially adsorbs in the tetrahedral pores of UiO-66(Zr), while acetone and methanol adsorb at the Zr–OH moieties on the metal-oxyhydroxy clusters in a more complex fashion. This method may be used to probe molecular adsorption sites and material void saturation with selected adsorbates, and with further development may eventually be used to trace in-pore chemistry of MOF materials
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NMR Spectroscopy Reveals Adsorbate Binding Sites in the Metal-Organic Framework UiO-66(Zr)
eScholarship University of California, 2018Co-Authors: Nandy A, Alexander C. Forse, Vj Witherspoon, Ja ReimerAbstract:© 2018 American Chemical Society. We assign 1H and 13C NMR resonances emanating from acetone, methanol, and cyclohexane adsorbed inside the pores of UiO-66(Zr). These results are informed by density functional theory (DFT) calculations, which probe the role of two competing effects inside of the pore environment: (i) nucleus independent chemical shifts (NICSs) generated by Ring Currents in conjugated linkers and (ii) small molecule coordination to the metal-oxyhydroxy cluster. These interactions are found to perturb the chemical shift of in-pore adsorbate relative to ex-pore adsorbate (which resides in spaces between the MOF particles). Changes in self-solvation upon adsorption may also perturb the chemical shift. Our results indicate that cyclohexane preferentially adsorbs in the tetrahedral pores of UiO-66(Zr), while acetone and methanol adsorb at the Zr-OH moieties on the metal-oxyhydroxy clusters in a more complex fashion. This method may be used to probe molecular adsorption sites and material void saturation with selected adsorbates, and with further development may eventually be used to trace in-pore chemistry of MOF materials
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new insights into the structure of nanoporous carbons from nmr raman and pair distribution function analysis
Chemistry of Materials, 2015Co-Authors: Alexander C. Forse, John M Griffin, Celine Merlet, Phoebe K Allan, Elizabeth K Humphreys, Mesut Aslan, Marco Zeiger, Volker Presser, Yury Gogotsi, Clare P GreyAbstract:The structural characterization of nanoporous carbons is a challenging task as they generally lack long-range order and can exhibit diverse local structures. Such characterization represents an important step toward understanding and improving the properties and functionality of porous carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function (PDF) analysis as new tools to probe the local structures of porous carbons, alongside more conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived carbons (CDCs) synthesized at higher temperatures. The Ring Currents induced in the NMR experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent on the size of the aromatic carbon domains. We exploit this property and use co...
Jeffrey A. Reimer - One of the best experts on this subject based on the ideXlab platform.
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nmr spectroscopy reveals adsorbate binding sites in the metal organic framework uio 66 zr
Journal of Physical Chemistry C, 2018Co-Authors: Aditya Nandy, Alexander C. Forse, Velencia J. Witherspoon, Jeffrey A. ReimerAbstract:We assign 1H and 13C NMR resonances emanating from acetone, methanol, and cyclohexane adsorbed inside the pores of UiO-66(Zr). These results are informed by density functional theory (DFT) calculations, which probe the role of two competing effects inside of the pore environment: (i) nucleus independent chemical shifts (NICSs) generated by Ring Currents in conjugated linkers and (ii) small molecule coordination to the metal-oxyhydroxy cluster. These interactions are found to perturb the chemical shift of in-pore adsorbate relative to ex-pore adsorbate (which resides in spaces between the MOF particles). Changes in self-solvation upon adsorption may also perturb the chemical shift. Our results indicate that cyclohexane preferentially adsorbs in the tetrahedral pores of UiO-66(Zr), while acetone and methanol adsorb at the Zr–OH moieties on the metal-oxyhydroxy clusters in a more complex fashion. This method may be used to probe molecular adsorption sites and material void saturation with selected adsorbates...
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NMR Spectroscopy Reveals Adsorbate Binding Sites in the Metal–Organic Framework UiO-66(Zr)
2018Co-Authors: Aditya Nandy, Alexander C. Forse, Velencia J. Witherspoon, Jeffrey A. ReimerAbstract:We assign 1H and 13C NMR resonances emanating from acetone, methanol, and cyclohexane adsorbed inside the pores of UiO-66(Zr). These results are informed by density functional theory (DFT) calculations, which probe the role of two competing effects inside of the pore environment: (i) nucleus independent chemical shifts (NICSs) generated by Ring Currents in conjugated linkers and (ii) small molecule coordination to the metal-oxyhydroxy cluster. These interactions are found to perturb the chemical shift of in-pore adsorbate relative to ex-pore adsorbate (which resides in spaces between the MOF particles). Changes in self-solvation upon adsorption may also perturb the chemical shift. Our results indicate that cyclohexane preferentially adsorbs in the tetrahedral pores of UiO-66(Zr), while acetone and methanol adsorb at the Zr–OH moieties on the metal-oxyhydroxy clusters in a more complex fashion. This method may be used to probe molecular adsorption sites and material void saturation with selected adsorbates, and with further development may eventually be used to trace in-pore chemistry of MOF materials
Celine Merlet - One of the best experts on this subject based on the ideXlab platform.
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mesoscopic simulations of the in situ nmr spectra of porous carbon based supercapacitors electronic structure and adsorbent reorganisation effects
Physical Chemistry Chemical Physics, 2021Co-Authors: Anagha Sasikumar, Alexander C. Forse, Patrice Simon, Anouar Belhboub, Camille Bacon, John M Griffin, Clare P Grey, Celine MerletAbstract:In situ NMR spectroscopy is a powerful technique to investigate charge storage mechanisms in carbon-based supercapacitors thanks to its ability to distinguish ionic and molecular species adsorbed in the porous electrodes from those in the bulk electrolyte. The NMR peak corresponding to the adsorbed species shows a clear change of chemical shift as the applied potential difference is varied. This variation in chemical shift is thought to originate from a combination of ion reorganisation in the pores and changes in Ring current shifts due to the changes of electronic density in the carbon. While previous Density Functional Theory calculations suggested that the electronic density has a large effect, the relative contributions of these two effects is challenging to untangle. Here, we use mesoscopic simulations to simulate NMR spectra and investigate the relative importance of ion reorganisation and Ring Currents on the resulting chemical shift. The model is able to predict chemical shifts in good agreement with NMR experiments and indicates that the Ring Currents are the dominant contribution. A thorough analysis of a specific electrode/electrolyte combination for which detailed NMR experiments have been reported allows us to confirm that local ion reorganisation has a very limited effect but the relative quantities of ions in pores of different sizes, which can change upon charging/discharging, can lead to a significant effect. Our findings suggest that in situ NMR spectra of supercapacitors may provide insights into the electronic structure of carbon materials in the future.
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new insights into the structure of nanoporous carbons from nmr raman and pair distribution function analysis
Chemistry of Materials, 2015Co-Authors: Alexander C. Forse, John M Griffin, Celine Merlet, Phoebe K Allan, Elizabeth K Humphreys, Mesut Aslan, Marco Zeiger, Volker Presser, Yury Gogotsi, Clare P GreyAbstract:The structural characterization of nanoporous carbons is a challenging task as they generally lack long-range order and can exhibit diverse local structures. Such characterization represents an important step toward understanding and improving the properties and functionality of porous carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function (PDF) analysis as new tools to probe the local structures of porous carbons, alongside more conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived carbons (CDCs) synthesized at higher temperatures. The Ring Currents induced in the NMR experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent on the size of the aromatic carbon domains. We exploit this property and use co...
Patrick W. Fowler - One of the best experts on this subject based on the ideXlab platform.
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concurrence between current density nucleus independent chemical shifts and aromatic stabilization energy the case of isomeric 4 and 5 phenylenes
Journal of Organic Chemistry, 2013Co-Authors: Renana Gershoniporanne, Christopher M Gibson, Patrick W. Fowler, Amnon StangerAbstract:The 17 isomers of the [4]- and [5]phenylenes have been studied with three different computational levels of current-density analysis (CDA) and by calculation of the out-of-plane contribution to nucleus-independent chemical shifts (NICSπzz). Current-density maps for these isomeric phenylenes are typically dominated by strong paratropic Ring Currents in four-membered Rings. The relative energies of the isomers, which differ only through the effects of differential strain and aromaticity, were computed at the B3LYP/6-311G* computational level. It was found that the three levels of CDA correlate well among themselves and with NICSπzz. The latter correlation is improved when the Ring sum ∑NICSπzz for each isomer is correlated to the Ring-current sum ∑J extracted from CDA. The strain-corrected relative energies of the isomers correlate linearly with ∑NICSπzz. In particular, the compatibility of different summed quantities with easily computed Huckel–London Ring Currents suggests a simply calculated measure for ...
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the anthracene problem closed form conjugated circuit models of Ring Currents in linear polyacenes
Journal of Physical Chemistry A, 2011Co-Authors: Patrick W. Fowler, Wendy MyrvoldAbstract:Conjugated-circuit models for induced π Ring Currents differ in the types of circuit that they include and the weights attached to them. Choice of circuits for general π systems can be expressed compactly in terms of matchings of the circuit-deleted molecular graph. Variants of the conjugated-circuit model for induced π Currents are shown to have simple closed-form solutions for linear polyacenes. Despite diffeRing assumptions about the effect of cycle area, all the models predict the most intense perimeter current in the central Rings, in general agreement with ab initio current–density maps. All tend to overestimate the rate of increase with N of the central Ring current for the [N]polyacene, in comparison with molecular-orbital treatments using ipsocentric ab initio, pseudo-π, and Huckel–London approaches.
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aromaticity of substituted fulvene derivatives substituent dependent Ring Currents
Physical Chemistry Chemical Physics, 2010Co-Authors: Tadeusz M Krygowski, Patrick W. Fowler, Wojciech P Oziminski, Marcin Palusiak, Andrew MckenzieAbstract:Fulvene is a non-aromatic molecule, but variation of the electron-donating/withdrawing power of substituents exo to the five-membered Ring can drive the system between the extremes of aromatic and antiaromatic, as judged by prediction of fully developed diatropic and paratropic Ring Currents through ab initio calculations made at the ipsocentric 6-31G**/CTOCD-DZ CHF level.
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induced Currents and electron counting in aromatic boron wheels b82 and b9
Inorganic Chemistry, 2007Co-Authors: Patrick W. Fowler, Benjamin R GrayAbstract:The newly discovered atom-centered polygonal wheels B82- and B9- are predicted to show Ring Currents characteristic of aromatic systems. Ipsocentric mapping of induced current density for both mole...
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the shell structure of π Ring Currents in the expanded porphyrin amethyrin
Organic and Biomolecular Chemistry, 2006Co-Authors: Erich Steiner, Patrick W. FowlerAbstract:Direct ab initio mapping of induced current density in the π system of the expanded porphyrin amethyrin shows a picture at variance with the conventional 4n π ascription of antiaromaticity. The ipsocentric orbital model interprets the pattern of Currents as a superposition of shell contributions: an intense paratropic Ring current concentrated on the inner 20-site cycle, counteracted by a weaker diatropic circulation on a 24-site conjugation pathway and a combination of local 5-site diatropic circulations on the six pyrrolic Rings.