Organic NMR 1 Flashcards
Different atoms in a molecule have different:
Resonance frequencies (chemical shifts) Different nuclei absorb EM radiation at different wavelengths Nuclei of a given type resonate at different energies depending on chemical and electronic environment
Shielding is caused by:
A diamagnetic current opposing the applied field, thereby reducing the effective magnetic field at any nearby nucleus.
High frequency:
Low electron density
Deshielded nucleus
Why is TMS (tetramethylsilane) used as a reference in NMR spectra?
The protons and carbons of its methyl groups are more shielded than most other known compounds.
Chemical shift is calculated by:
δ=ν-ν(ref) Hz /ν(measuring freq) MHz
What is the pulsed Fourier transformation?
Short pulse applied
After pulse excited nuclei begin to relax - lose energy and return to original spin state
As relaxing - emit radiation
Many different nuclei relax - different radiation frequencies
FID - free induction decay
What range are sp3 protons observed at?
0-2ppm if no electronegative π bonded groups are nearby
At what range are sp2 protons observed?
-C=C-H ethylene protons observed at 4.5-7ppm
In an sp2 bond, carbon has 33% s character, more electronegative than sp3 carbon so holds electrons closer, therefore less shielding for proton nucleus than in sp3 bond
At what range are sp hydrogens observed?
Appear anomalously at 2-3ppm
Due to anisotropy
What is magnetic anisotropy?
All groups in a molecule that have π electrons generate secondary anisotropic (non-uniform) fields, so protons resonate at lower frequency than expected.
What are mesomeric effects?
Ortho and para positions are more strongly shielded than those in meta positions, so ortho and para have lower resonance frequencies than meta positions
What is meant by equivalence?
Chemically and magnetically equivalent nuclei resonate at the same energy and give a single signal or pattern.
