Spectroscopy Problem Solving
Dr. Chris, UP Feb 2016
Chemical Shift δ
equivalent protons
Protons that can be transferred to another one by mirror or rotation are equivalent => They have the same δ and do not couple
How many signals ?
Identify the protons of ethylacetate
Find the position of Br and I in bromo-iodo-benzene
13C (decoupled)
Solution
Less deshielded, doublet
Less deshielded, triplet
More deshielded, doublet
More deshielded, triplet
Find the structure by IR, 1H and 13C
Find functional groups
Compare to:
More de-shielding
CDCl3
Solution
CH3 singlet
1.9 ppm
CH2 triplet
3.9 ppm CH2 quintet
1.45 ppm
CH2 multiplet 1.25 ppm
CH3 triplet
0.8 ppm
δ goes down
13C NMR Because the 13C isotope is present at only 1.1% natural abundance, the probability of finding two adjacent 13C carbons in the same molecule of a compound is very low. As a result spin-spin splitting between adjacent non-equivalent carbons is not observed. However, splitting of the carbon signal by directly bonded protons is observed, and the coupling constants are large, ranging from 125 to 250 Hz. Methyl groups appear as quartets, methylenes as triplets, methines as doublets, and unprotonated carbons as singlets. Commonly, splitting of the signal by protons is eliminated by a decoupling technique which involves simultaneous irradiation of the proton resonances at 300 MHz while observing the carbon resonances at 75 MHz.
Identify the 13C peaks of Ethylacetate:
CDCl3
How many 19F signals ?
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