NMR
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What does NMR stand for?
Nuclear Magnetic Resonance
How does NMR work?
-certain nuclei absorb specific radio frequencies (energies) when in the presence of a strong magnetic field -these nuclei are said to resonate when exposed to certain quantized energies -the energy absorbed depends on the type of nucleus as well as its electronic environment in a molecule
Intensity of the absorption is related to ______.
the number of nuclei in that environment present
Hydrogens are indistinguishable due to …
-single bond rotations -symmetry
Hydrogens are different due to …
-distance from EN atoms -number of hydrogen’s attached to the same carbon
“H type” is the same as …
-peaks -signals -resonances
What is N? How is it calculated?
-N is the number of H’s of a different type ‘nearby’ -# of peaks = N + 1
Singlet (s)
-no neighboring hydrogens -N+1=1
Doublet (d)
-1 neighboring hydrogen -N+1 = 2
Triplet (t)
-2 neighboring hydrogens -N+1 = 3
Quartet (q)
-3 neighboring hydrogens -N+1=4
______ is standard reference for electron density in H NMR
TMS (tetramethylsilane)
What determines chemical shift?
-sigma bond electron density -C-H bonds are shorter–electron density is concentrated -C-C bonds are longer–electron density is more stretched out
What type of shift does alpha to N result in?
+1
What type of shift does alpha to EN atoms (O, Cl, Br, etc.) result in?
+2
H’s attached to sp2-C exhibit __________
increased chemical shift (aldehyde>aromatic>alkene)
H’s attached to sp-C exhibit _________.
an unexpected chemical shift
N-H and O-H groups experience ______ due to acidity, which blurs or “broadens” coupling constants; coupling effects can be ignored
fast proton exchange
Carbon 13 NMR summary
-number of signals is proportional to number of carbon types -only C-type information is provided -no coupling/splitting -no integration
_____ ~ 1 Carbon 13
1 H ppm x 20