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Related Concept Videos

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

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The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
1.1K
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

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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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¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

1.3K
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...
1.3K
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.0K
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
1.0K
Polymers: Defining Molecular Weight01:01

Polymers: Defining Molecular Weight

2.9K
Unlike small molecules with definite molecular weights, polymers are a mixture of individual polymer chains of varying lengths, each with a unique molecular weight.  So, the molecular weight of a polymer is expressed as an average value based on the average size of the polymer chains. The two most common forms of averages used for polymers are the number average molecular weight and weight average molecular weight.
The number average molecular weight (Mn) is the summation of the number...
2.9K
Polymers: Molecular Weight Distribution01:10

Polymers: Molecular Weight Distribution

3.4K
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.
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Related Experiment Video

Updated: Jul 1, 2025

Characterization of Synthetic Polymers via Matrix Assisted Laser Desorption Ionization Time of Flight MALDI-TOF Mass Spectrometry
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Polymorph Identification for Flexible Molecules: Linear Regression Analysis of Experimental and Calculated Solution-

Mohammed Rahman1,2, Hugh R W Dannatt3, Charles D Blundell3

  • 1Department of Physics, University of Warwick, Coventry CV4 7AL, U.K.

The Journal of Physical Chemistry. A
|March 1, 2024
PubMed
Summary

The advanced Δδ regression approach accurately identifies solid forms of molecules with multiple conformations, like furosemide polymorphs, using NMR and DFT. This method offers greater dynamic range than RMSE for precise solid-state characterization.

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Area of Science:

  • Solid-state chemistry
  • Computational chemistry
  • Nuclear Magnetic Resonance (NMR) spectroscopy

Background:

  • Distinguishing between solid forms (polymorphs) is crucial in pharmaceutical development.
  • Traditional methods for solid form characterization can be time-consuming and complex.
  • NMR spectroscopy combined with computational methods offers a powerful approach for solid form analysis.

Purpose of the Study:

  • To extend the Δδ regression approach for discriminating solid forms of molecules with multiple conformational degrees of freedom.
  • To accurately identify furosemide polymorphs using a combination of experimental NMR data and density functional theory (DFT) calculations.
  • To evaluate the effectiveness of Monte Carlo sampling and an alternative ensemble method for estimating solution-state NMR chemical shifts.

Main Methods:

  • Utilized the Δδ regression approach comparing experimental solution- and solid-state NMR chemical shifts (Δδexperimental) with gauge-including projector augmented wave (GIPAW) calculations (Δδcalculated).
  • Employed Monte Carlo random sampling to efficiently calculate solution-state NMR chemical shifts for molecules with multiple conformations.
  • Investigated an alternative method using an ensemble of crystal structures from the Cambridge Structural Database (CSD) to approximate solution-state dynamics.

Main Results:

  • The Δδ regression approach successfully and precisely identified furosemide polymorphs.
  • Monte Carlo sampling reduced computation time by avoiding exhaustive conformational landscape sampling.
  • The alternative ensemble method also proved successful in estimating solution-state NMR chemical shifts, suggesting a potential workaround for dependency on pre-determined solution structures.

Conclusions:

  • The extended Δδ regression approach is effective for discriminating solid forms of molecules with multiple conformational degrees of freedom.
  • The method demonstrates a greater dynamic range compared to the root mean squared error (RMSE) method.
  • New avenues for estimating solution-state NMR chemical shifts were identified, potentially overcoming current limitations.