This document discusses spectroscopy problem solving techniques including: 1) Chemical shifts and how equivalent protons will have the same shift and not couple. 2) Identifying the number of signals in a spectrum by considering equivalent protons. 3) Examples of assigning proton and carbon signals in spectra of compounds like ethylacetate and determining structures. 4) Factors that influence chemical shifts like functional groups and solvents. 5) Details on 13C NMR spectra and how coupling to protons is observed but can be removed through decoupling.

1. Spectroscopy Problem Solving
Dr. Chris, UP Feb 2016

2. Chemical Shift δ

3. 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

4. How many signals ?

6. Identify the protons of ethylacetate

7. Find the position of Br and I in
bromo-iodo-benzene
13C (decoupled)

8. Solution
Less deshielded,
doublet
Less deshielded,
triplet
More deshielded,
doublet
More deshielded,
triplet

9. Find the structure by IR, 1H and 13C

10. Find functional groups
Compare to:

12. More de-shielding
CDCl3

13. 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

14. 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.

15. Identify the 13C peaks of Ethylacetate:
CDCl3

16. How many 19F signals ?