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Ion exchange chromatography separates charged molecules from a solution by reversibly exchanging them with mobile, or 'active', ions associated with the oppositely charged stationary phase. This method can be used to separate ions, soften and deionize water, and purify solutions. The polymers comprising the ion-exchange column are high-molecular-weight and chemically stable polymers, crosslinked to be porous and essentially insoluble. They are also functionalized with either acidic or...
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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
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Isomerism in Complexes
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis...
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An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0,...
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In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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Merging Ion Concentration Polarization between Juxtaposed Ion Exchange Membranes to Block the Propagation of the Polarization Zone
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Coherent polarization transfer in chemically exchanging systems.

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Summary

Signal Amplification by Reversible Exchange (SABRE) simulations were compared using Markov chain Monte Carlo and superoperator methods. Both simulation approaches accurately reproduced experimental data for NMR signal enhancement.

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Area of Science:

  • Nuclear Magnetic Resonance (NMR) Spectroscopy
  • Quantum Chemistry
  • Computational Physics

Background:

  • Signal Amplification by Reversible Exchange (SABRE) enhances NMR signals using para-hydrogen.
  • Optimizing SABRE and in silico descriptions remain active research areas.
  • Simulating SABRE is challenging due to complex chemical exchange and polarization transfer.

Purpose of the Study:

  • To compare two distinct simulation methods for SABRE.
  • To assess the accuracy and efficiency of Markov chain Monte Carlo (MC) and superoperator (SO) approaches.
  • To simulate SABRE in various spin systems and experimental conditions.

Main Methods:

  • Markov chain Monte Carlo (MC) simulations.
  • Liouville von Neumann equation with superoperators of chemical exchange (SO).
  • Simulation of three- and four-spin-½ systems under continuous polarization (constant/alternating B0 field).

Main Results:

  • MC and SO simulations yielded comparable results for SABRE.
  • Both methods successfully reproduced previously reported experimental data.
  • SO simulations were significantly faster (orders of magnitude) than MC simulations.

Conclusions:

  • Both MC and SO are suitable for simulating SABRE.
  • The SO method offers a substantial speed advantage for SABRE simulations.
  • Accurate in silico modeling of SABRE is achievable with these computational approaches.