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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.
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...
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Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

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The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
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NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
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Thermal Sigmatropic Reactions: Overview01:16

Thermal Sigmatropic Reactions: Overview

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Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in 1,5-hexadiene, referred...
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Most acid-base titrations are performed in an aqueous medium. In aqueous titrations, water competes with weaker acids or bases for proton donation or acceptance, leading to ambiguous endpoints in the titration curve. Water also affects the partial ionization of weak acids or bases. For example, water accepts a proton from acetic acid to form hydronium and acetate ions. The hydronium ion formed is a stronger acid than acetic acid, and the acetate ion is a stronger base than water. As a result,...
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Leveling Effect and Non-Aqueous Acid-Base Solutions02:11

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This lesson defines the leveling effect in acidic and basic solutions and its role in aqueous and non-aqueous solutions. It is essential to understand the competing nature of various species in a chemical system.
The Leveling Effect of a Solvent
A generic acid (HA) reacts with the generic base (B-) to yield the corresponding conjugate base (A-) and conjugate acid (HB):
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Comparison of induction methods for supersaturation: pH shift versus solvent shift.

Jakob Plum1, Christoffer Bavnhøj1, Henrik Palmelund1

  • 1Department of Pharmacy, University of Copenhagen, Universitetsparken 2, DK-2100 Copenhagen, Denmark.

International Journal of Pharmaceutics
|November 24, 2019
PubMed
Summary

This study compared solvent shift and pH shift methods for inducing drug supersaturation. For most basic drugs, both methods yielded similar supersaturation levels and precipitation behaviors, suggesting comparable results when drug solubility is adequate at low pH.

Keywords:
Poorly soluble drugsPrecipitationSolvent shiftSupersaturationpH-shift

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Area of Science:

  • Pharmaceutical Sciences
  • Physical Chemistry
  • Drug Delivery

Background:

  • Supersaturation is crucial for drug absorption.
  • Solvent shift is a common method to induce supersaturation.
  • pH shift is a more biorelevant method for weak bases, but its impact is understudied.

Purpose of the Study:

  • To investigate the impact of supersaturation induction method (solvent shift vs. pH shift) on drug precipitation.
  • To compare four key parameters: highest apparent degree of supersaturation, induction time, precipitation rate, and precipitate solid form.
  • To evaluate this impact for nine different basic drugs using a standardized small-scale method.

Main Methods:

  • Developed and utilized a novel, standardized small-scale method for supersaturation and precipitation.
  • Applied both solvent shift and pH shift methods to induce supersaturation for nine basic drugs.
  • Quantified and compared the highest apparent degree of supersaturation, induction time, precipitation rate, and solid form of the precipitate.

Main Results:

  • Eight out of nine drugs showed identical highest apparent degree of supersaturation regardless of the induction method.
  • No systematic differences in induction time, precipitation rate, or precipitate solid form were observed between the two methods.
  • Individual drug responses varied, indicating method-specific effects for certain compounds.

Conclusions:

  • For basic drugs with sufficient solubility at low pH, solvent shift and pH shift methods for inducing supersaturation produce comparable results.
  • The induction method's impact on supersaturation and precipitation is drug-dependent.
  • This study provides a standardized approach for evaluating supersaturation induction methods in a pharmaceutical context.