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Chemistry
Posted 3 months ago

Demonstrates an exceptional level of knowledge and understanding of the material. Answers are complete, scientifically accurate and based upon accepted understanding.Briefly describe what is the relevance of each of the following with respect to NMR spectroscopy, including a sketch to illustrate each answer. (i) A HMQC spectrum (ii) Karplus rule for three-bond couplings between hydrogen atoms.
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Answer from Sia
Posted 3 months ago
Solution
1
Relevance of HMQC spectrum in NMR spectroscopy: HMQC (Heteronuclear Multiple Quantum Coherence) spectrum is a two-dimensional NMR technique that is used to correlate protons with directly bonded carbon-13 or other heteronuclei
2
Sketch of HMQC spectrum: The HMQC spectrum typically displays two axes, one for the ^1H NMR chemical shifts and the other for the ^13C NMR chemical shifts. Cross-peaks appear at coordinates that correspond to the chemical shifts of directly bonded ^1H-^13C pairs
3
Relevance of Karplus rule in NMR spectroscopy: The Karplus rule is an empirical relationship that describes the correlation between the dihedral angle (θ) of three-bond couplings (vicinal couplings) between hydrogen atoms and the coupling constant (J). It is particularly useful in determining the conformation of organic molecules
4
Sketch of Karplus curve: The Karplus curve is a graph that plots the coupling constant (J) against the dihedral angle (θ). It typically shows that J-coupling constants are larger for dihedral angles around 0° and 180° (cis and trans conformations) and smaller for angles around 90° (gauche conformation)
1 Answer
(i) HMQC spectrum is relevant for identifying direct proton-carbon couplings in molecules. (ii) Karplus rule is relevant for determining the dihedral angles between vicinally coupled hydrogens, aiding in the elucidation of molecular conformation.
Key Concept
HMQC spectrum and Karplus rule are tools in NMR spectroscopy for structural analysis of organic compounds.
Explanation
HMQC provides insights into the connectivity between protons and heteronuclei, while the Karplus rule helps in understanding the spatial arrangement of hydrogen atoms within a molecule.

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