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Related Concept Videos

Polymer Classification: Crystallinity01:21

Polymer Classification: Crystallinity

Unlike ionic or small covalent molecules, polymers do not form crystalline solids due to the diffusion limitations of their long-chain structures. However, polymers contain microscopic crystalline domains separated by amorphous domains.
Crystalline domains are the regions where polymer chains are aligned in an orderly manner and held together in proximity by intermolecular forces. For example, chains in the crystalline domains of polyethylene and nylon are bound together by van der Waals...
Polymer Classification: Stereospecificity01:26

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...
Factors Affecting Dissolution: Polymorphism, Amorphism and Pseudopolymorphism01:21

Factors Affecting Dissolution: Polymorphism, Amorphism and Pseudopolymorphism

Polymorphism refers to the existence of a drug substance in multiple crystalline forms, known as polymorphs. Recently, this term has been expanded to include solvates (forms containing a solvent), amorphous forms (non-crystalline forms), and desolvated solvates (forms from which the solvent has been removed).
Some polymorphic crystals possess lower aqueous solubility than their amorphous counterparts, leading to incomplete absorption. For instance, the oral suspension of Chloramphenicol, which...
Conformations of Cyclohexane02:11

Conformations of Cyclohexane

Cyclohexane does not exist in a planar form due to the high angle and torsional strain it would experience in the planar structure. Instead, it adopts non-planar chair and boat conformations.
The chair form is the most stable and derives its name from its resemblance to the “easy chair.” In the chair conformation, two carbon atoms are arranged out-of-plane — one above and one below, minimizing the torsional strain. In the chair form, the bond angle is very close to the ideal tetrahedral value,...
Chair Conformation of Cyclohexane02:02

Chair Conformation of Cyclohexane

The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.
The hydrogen atoms linked to carbons are arranged in two different axial and equatorial orientations to achieve this staggered...
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...

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High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal
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Conformational polymorphism in organic crystals.

Ashwini Nangia1

  • 1School of Chemistry, University of Hyderabad, Hyderabad 500 046, India. ashwini_nangia@rediffmail.com

Accounts of Chemical Research
|March 20, 2008
PubMed
Summary
This summary is machine-generated.

Conformational polymorphism arises from molecular shape changes influencing crystal structures. Molecular flexibility and energy trade-offs between conformers and crystal packing drive the formation of diverse polymorphs.

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Area of Science:

  • Solid-state chemistry
  • Crystallography
  • Organic chemistry

Background:

  • Polymorphs are distinct crystalline forms of the same substance.
  • Conformational polymorphism occurs when different molecular shapes (conformers) form different crystal structures.
  • Multiple conformers can sometimes coexist within a single crystal lattice.

Purpose of the Study:

  • To investigate how molecular conformation changes impact the formation and stability of polymorphs.
  • To analyze the interplay between intramolecular (conformer) and intermolecular (lattice) energies in polymorphic systems.
  • To understand the role of hydrogen bonding and crystal packing in conformational polymorphism.

Main Methods:

  • Analysis of X-ray crystal structures of conformational polymorphs.
  • Utilized the Cambridge Structural Database to study hydrogen bond interactions (O-H...O, N-H...O, C-H...O).
  • Calculated conformer and lattice energies using computational tools (Gaussian 03, Cerius^2) for 23 polymorph sets.

Main Results:

  • Polymorphs with higher numbers of symmetry-independent molecules (high Z') often exhibit better intermolecular interactions, suggesting conformational differences.
  • In two-thirds of analyzed cases, strained conformers (higher E(conf)) were stabilized by favorable crystal packing (lower U(latt)).
  • Organic molecules with flexible torsions and low-energy conformers are more prone to polymorphism due to varied packing and bonding possibilities.

Conclusions:

  • Molecular conformation plays a critical role in the emergence and stability of polymorphs.
  • The balance between conformer energy and lattice energy significantly influences polymorphic outcomes.
  • Understanding conformational polymorphism is crucial for designing pharmaceutical polymorphs and predicting crystal structures.