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Related Concept Videos

Molecular Models02:00

Molecular Models

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Physical models representing molecular architectures of chemical compounds play essential roles in understanding chemistry. The use of molecular models makes it easier to visualize the structures and shapes of atoms and molecules.
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Heteronuclear single-quantum correlation spectroscopy (HSQC) is a 2D NMR technique that reveals one-bond correlations between hydrogen and a heteronucleus. The HSQC experiment is similar to the heteronuclear correlation experiment (HETCOR) but is more sensitive. In the HSQC spectrum, the proton chemical shift is plotted on the horizontal F2 axis, while the 13C chemical shift is plotted on the vertical F1 axis. The corresponding proton and 13C spectra are also shown. The HSQC contour plot does...
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¹H NMR: Complex Splitting01:13

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A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied...
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Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

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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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¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons01:03

¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons

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Protons in identical electronic environments within a molecule are chemically equivalent and have the same chemical shift. The replacement test is a useful tool to identify chemical equivalence and predict NMR spectra. A substituent replaces each of the protons being examined and the resulting molecules are compared. If the same molecule is obtained, the protons are equivalent or homotopic. Replacement of any hydrogens in ethane by chlorine yields chloroethane because all six protons are...
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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1.3K
Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
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Multicomponent CASSCF Revisited: Large Active Spaces Are Needed for Qualitatively Accurate Protonic Densities.

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  • 1Department of Chemistry, University of Missouri, Columbia, Missouri 65203, United States.

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New multicomponent quantum chemistry methods accurately describe proton behavior. This study introduces an enhanced method for larger systems, improving calculations of protonic properties.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Multicomponent methods treat nuclei quantum mechanically alongside electrons.
  • Hartree-Fock (HF) fails for electron-proton interactions, yielding incorrect protonic properties.
  • Existing multicomponent CASSCF methods are limited to small active spaces.

Purpose of the Study:

  • To develop an improved multicomponent CASSCF method for larger systems.
  • To enable accurate calculations of protonic properties by addressing limitations of previous methods.
  • To extend the applicability of quantum mechanical treatment for nuclei.

Main Methods:

  • Derivation and implementation of a two-step multicomponent CASSCF method.
  • Utilizing multicomponent heat-bath configuration interaction for the configuration interaction step.
  • Enabling calculations with significantly larger active spaces (up to 16 electrons in 48 orbitals).

Main Results:

  • The new method allows for very large active space calculations.
  • Accurate protonic densities were obtained for HCN and FHF- molecules.
  • Demonstrated the necessity of large electronic active spaces for accurate results.

Conclusions:

  • The enhanced multicomponent CASSCF method significantly expands the scope of quantum mechanical nuclear treatments.
  • This approach is crucial for accurately describing systems with strong electron-proton correlation.
  • Potential applications include double-well protonic potentials and electronically multireference systems.