CHEM106: Assignment 13
NMR Spectra
1. Which of the following nuclei would not be NMR active?
A. 15N
B. 1H
C. 16O
D. 13C
To be active in NMR, the nuclei need to possess a non-zero nuclear spin quantum
number (I). The nuclei 15N, 1H, and 13C all have nuclear spin quantum number I = l/2,
and thus are NMR active. But 16O has a nuclear spin quantum number I = 0, and is
NMR inactive.
2. How many nuclear spin states are allowed for the 1H nucleus?
The 1H nucleus has a nuclear spin quantum number I = 1/2. The number of spin states
is given by 2I +1 = 2.
3. Give two reasons why 13C NMR is less sensitive than 1H NMR.
The 13C NMR is less sensitive than 1H NMR for the following two reasons:
A. Natural isotopic abundance of 13C is about 100 times less than that of 1H.
B. The gyromagnetic ratio  of 13C is 6.73  107 rad  T 1  s 1 , which is much smaller than
that of 1H ( 26.75  107 rad  T 1  s 1 ). At the same magnetic field strength, 13C atoms
come into resonance at approximately one-quarter the frequency of 1H atoms. Since
the energy split is not as large, the population difference is not as large, and
therefore the 13C NMR experiment is not as sensitive.
4. What is the value of the chemical shift δ for an absorption at 1200 Hz from
tetramethylsilane (TMS) using an NMR spectrometer with an operating frequency of 300
MHz?
The chemical shift  is defined as the difference between the frequency of a nuclear
spin flip of the nucleus in question and the nuclear spin flip of a reference molecule,
divided by the operating frequency of the NMR spectrometer.
dH =
=(
Re sonance frequency of nucleus H relative to TMS
´10 6
Spectrometer frequency
n H - n TMS
) ´10 6
n spectrometer
Plug in the numbers, and we get
H 
1200 Hz
 10 6  4 ppm
6
300  10 Hz
5. Arrange the following compounds in order of decreasing chemical shift for the
hydrogens highlighted in bold face (largest δ value first, smallest value last).
A. CH3CH2CH3
B. CH3OCH2CH3
C. Cl2CHCH2CH3
D. ClCH2CH2CH3
The chemical shift of hydrogen reflects the extent of shielding by nearby atoms in the
compound. High
electron
density
of
neighboring
atoms
may
shield the hydrogen
nucleus
from
an
external
magnetic
field. In general, the higher the electron density
of the neighboring atom, the lower the chemical shift. So the order goes as follows:
b > c > d > a.
6. Which proton(s) of the compound below would appear as a triplet in the 1H NMR
spectrum?
CH3CH2CH2-O-CH(CH3)2
Coupling of a magnetic nucleus A to N equivalent nuclei with spin I will split the signal
from A into a multiplet consisting of 2NI + 1 peaks. In the case of 1H nucleus, I = 1/2, so
the multiplet contains N + 1 peaks. Applying this to the compound above, we identify
two protons that will appear as triplet in 1H NMR. The two protons are shown in boldface
type here: CH3CH2CH2-O-CH(CH3)2.