Nuclear Magnetic Resonance Spectroscopy (NMR) • Spectrum represents the different interactions of stereochemically different protons ( 1 H) with the applied magnetic field. • We will focus on 1 H NMR (proton, H + ) • 4 general rules for 1 H NMR spectra 1. Only stereochemically different 1 Hs give different signals. 2. Area covered under the signal is proportional to the number of 1 Hs causing the signal and is usually represented by integrals. 3. The Chemical Shift (where on spectrum each peak appears) depends on the “chemical environment” of each proton. (see above picture) a. 1 Hs close to electronegative atoms (O, N, X (halogen)) or aromatics shift to the left (deshielded, downfield shifted) b. The larger the number of 1 Hs on the same carbon the more to the right (shielded, upfield shifted) the NMR signal is. CH 3 CH 2 -Cl Different H Different NMR signal CH 3 CH 2 CH 2 CH 3 Different H Different NMR signal Same H, symmetric Same H, symmetric Br Br 0 1 2 3 4 5 6 PPM 1 2 3
3
Embed
Nuclear Magnetic Resonance Spectroscopy (NMR) · 2011-12-02 · Nuclear Magnetic Resonance Spectroscopy (NMR) • Spectrum represents the different interactions of stereochemically
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Nuclear Magnetic Resonance Spectroscopy (NMR)
• Spectrum represents the different interactions of stereochemically different protons
(1H) with the applied magnetic field.
• We will focus on 1H NMR (proton, H+)
• 4 general rules for 1H NMR spectra
1. Only stereochemically different 1Hs give different signals.
2. Area covered under the signal is proportional to the number of 1Hs causing the
signal and is usually represented by integrals.
3. The Chemical Shift (where on spectrum each peak appears) depends on the
“chemical environment” of each proton. (see above picture)
a. 1Hs close to electronegative atoms (O, N, X (halogen)) or aromatics shift to
the left (deshielded, downfield shifted)
b. The larger the number of 1Hs on the same carbon the more to the right
(shielded, upfield shifted) the NMR signal is.
CH3CH2-Cl
Different HDifferent NMR signal
CH3CH2CH2CH3
Different HDifferent NMR signal
Same H, symmetric
Same H, symmetric
Br
Br
0123456
PPM
1
2
3
4. The multiplicity of the NMR peak depends on the number of 1Hs on neighboring
carbons, NOT the same carbon. 1Hs attached to adjacent carbons split each other
into:
a. Exchangeable, acidic 1H (-OH, NH2) DO NOT split 1Hs on adjacent carbons
and show on the spectrum as broad singlets.
H3C
H2C
CH2
O
CH3
t (triplet)2+1
sextet5+1
t (triplet)2+1
s (singlet)0+1
0123
PPM
(n+1) peaks
n = number of 1Hs on adjacent carbons
(not the same C)
3
2 2
3
t (triplet)2+1
sextet5+1
broad singlet
2
H3CCH
CH2
CHCH3
OHNH2sextet5+1
d (doublet)1+1
d (doublet)1+1
H3C
H2C
CH2
CH3
a. b.
0123456
PPM
1
identical Hssymmetric
q (quartet) [3+1]
t (triplet)2+1
t (triplet)2+1
1
3
3
21
b. Only non identical 1Hs split each other.
The shape/relative intensity of the peaks follows the algorithm of Pascal’s
Triangle:
• Calculating the degree of unsaturation for a compound (number of RDBs, Rings and
Double Bonds) when the molecular formula is known.
i.e. C9H9OCl : RDBs = (2x9 + 2 – 9 – 1)/2 = 5
1
1 1
11 2
3 31 1
44 61 1
10 105 51 1
1 peak, singlet (s)
2 peaks, doublet (d)
3 peaks, triplet (t)
4 peaks, quartet (q)
5 peaks, quintet/pentet
6 peaks, sextet/sixtet
7 peaks, septet
>8 peaks, multiplet
Pascal's Triangle
#RDBs =2n + 2 - #Hs - #Halides + # N atoms
2
n = # Cs(Oxygens do not participate in this equation and can be ignored when