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Chemical Shift: Internal References and Solvent Effects

Chemical Shift: Internal References and Solvent Effects

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.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...

Polymer Classification: Stereospecificity

Polymer Classification: Stereospecificity

Polymerization generates chiral centers along the entire backbone of a polymer chain. Accordingly, the stereochemistry of the substituent group has a significant effect on polymer properties. Polymers formed from monosubstituted alkene monomers feature chiral carbons at every alternate position in the polymer backbone. Relative to the predominant orientation of substituents at the adjacent chiral carbons, the polymer can exist in three different configurations: isotactic, syndiotactic, and...

Polymer Classification: Architecture

Polymer Classification: Architecture

Polymers are classified as linear or branched on the basis of their chain architecture. The polymer chains in linear polymers have a long chain-like structure with minimal to no branching at all. Even if a polymer features large substituent groups on the monomer, which appear as branches to the skeleton, it is not considered a branched polymer. A branched polymer contains secondary polymer chains that arise from the main polymer chain. The branching occurs when the polymer growth shifts from...

E1 Reaction: Stereochemistry and Regiochemistry

E1 Reaction: Stereochemistry and Regiochemistry

One of the critical aspects of the E1 reaction mechanism, as also observed in E2, is the regiochemistry, with multiple regioisomers obtained as products. In the example discussed, the presence of water as a weak base favors elimination over substitution to generate two alkenes. Given that alkenes’ stability increases with the number of alkyl groups across the double bond, typically, E1 reactions lead to the Zaitsev product, for this is more substituted and stable than the Hofmann product.

Polymers: Molecular Weight Distribution

Polymers: Molecular Weight Distribution

For any given polymer, the weight average molecular weight (Mw) is higher than, if not equal to, the number average molecular weight (Mn). The only situation in which the weight average molecular weight and the number average molecular weight are equal is when a polymer consists only of chains with equal molecular weight. However, this never happens in a synthetic polymer, since it is difficult to control the polymerization process up to a molecular level with accuracy to a hundred percent.

E2 Reaction: Stereochemistry and Regiochemistry

E2 Reaction: Stereochemistry and Regiochemistry

Elimination reactions of alkyl halides can yield one or more alkenes depending on the specific regiochemical and stereochemical considerations. While the regiochemistry of the reaction governs the location of the double bond in the product, the stereochemical requirements often influence the geometry.
When a substrate with two different β hydrogens undergoes an E2 elimination, the presence of a strong base can yield two regioisomeric alkenes. The more-substituted alkene is the major...

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Updated: Apr 4, 2026

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

Published on: June 7, 2018

Multireference Methods for Chemistry and Materials Science: Automated Active Spaces, Efficient Dynamic Correlation,

Jacob J Wardzala¹, Matthew R Hennefarth¹, Valay Agarawal¹

¹Department of Chemistry and Chicago Center for Theoretical Chemistry, University of Chicago, Chicago, Illinois 60637, United States.

Chemical Reviews

|April 3, 2026

Summary

This summary is machine-generated.

Recent advancements in multiconfigurational methods are making complex chemical system calculations more routine. These robust approaches improve accuracy for modeling excitations, reactivity, and extended systems in chemistry and materials science.

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Area of Science:

Quantum Chemistry
Computational Materials Science

Background:

Multiconfigurational approaches were historically complex, requiring expert intervention.
Recent developments aim to democratize these powerful computational tools.

Purpose of the Study:

To review the state-of-the-art in multiconfigurational methods.
To highlight advancements making these methods more robust, efficient, and applicable to challenging chemical systems.

Main Methods:

Automated active-space selection for large-scale multireference calculations.
Extensions of pair-density functional theory for correlation energy recovery.
Product-form wave functions for localized active space methods.

Main Results:

Enabling accurate modeling of vertical excitations and chemical reactivity.
Efficient treatment of excited-state dynamics and training machine-learned potentials.
Computation of multiconfigurational band structures for extended systems.

Conclusions:

Multiconfigurational methods are becoming more routine and accessible.
These advancements facilitate high-impact applications in chemical and materials science.