Related Experiment Video
Updated: Jun 2, 2026
06:25
Post Column Derivatization Using Reaction Flow High Performance Liquid Chromatography Columns
Published on: April 26, 2016
Deviation from van't Hoff dependence in RP-LC induced by tautomeric interconversion observed for four compounds
1Department of Analytical Chemistry, Faculty of Chemistry, University of Bucharest, Bucharest, Romania.
Journal of Separation Science
|May 4, 2011
Summary
Certain compounds undergoing tautomeric interconversion deviate from the standard van
Area of Science:
- Analytical Chemistry
- Chromatography
- Physical Chemistry
Background:
- Reversed-phase liquid chromatography (RPLC) typically exhibits linear van't Hoff plots for temperature dependence of retention.
- Deviations occur when studying compounds with structural modifications or complex behaviors.
Purpose of the Study:
- To investigate and report on compounds exhibiting non-linear van't Hoff plots due to tautomeric interconversion.
- To propose a thermodynamic model explaining this anomalous retention behavior.
Main Methods:
- Analysis of temperature dependence of retention factors (ln k) versus inverse temperature (1/T) in RPLC.
- Examination of compounds like piroxicam, drotaverine, vincamine, and epivincamine.
Main Results:
- Observed deviations from linear van't Hoff plots for tautomeric compounds.
- Identified instances where increased temperature led to increased retention.
- Developed a thermodynamic model predicting polynomial dependence of ln k on 1/T.
Conclusions:
- Tautomeric equilibria in the mobile phase can cause non-linear van't Hoff behavior in RPLC.
- The proposed thermodynamic model successfully explains these deviations.
Related Concept Videos
01:34Woodward–Hoffmann Selection Rules and Microscopic Reversibility
Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
01:13¹H NMR: Complex Splitting
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 first.
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 first.
01:02¹H NMR: Interpreting Distorted and Overlapping Signals
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 slanted or...
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 slanted or...
01:22Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)
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...
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...
01:15¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR
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.
00:52¹H 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...