CH310 Industrial ad Organic Chemistry
Mentor: Dr. Kabaso Kalebaila
Email: kkalebai@gmail.com
School of Mathematics and Natural Sciences
Chemistry Department
Nuclear Magnetic Resonance (NMR)
Spectroscopy
NMR: most powerful tool for organic structure elucidation.
Most atomic such as 1H, 13C, 19F, 31P have nuclear spin.
Nuclear spin creates magnetic moments: like small magnets.
In an external magnetic field, nuclear spin either lines in same
direction as applied field or opposite direction.
NMR Spectroscopy
Source of energy in NMR is radiowaves: long wavelengths,
hence low energy and frequency.
The waves can change the nuclear spins of some elements.
Absorption of radiowaves with right energy creates α-state
(same direction) and -state (opposite) direction
Difference in energy levels is: E= h = hc/
NMR Spectroscopy
Criteria for NMR: rules for net nuclear spin
1) When the number of neutrons and the number of protons are
both even, then the nucleus has NO spin i.e. not NMR active.
2) When the number of neutrons and the number of protons are
both odd, then the nucleus has an integer spin (i.e. 1, 2, 3
3) When the sum of neutrons and protons (n+p) is odd: the
nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2)
Atoms like 12C have no nuclear spins (6 protons +6 neutrons):
spins are paired against each other, no overall spin: rule 1
No difference in energy between α-state and -states in a
magnetic field.
Cannot perform NMR on 12C.
The NMR Spectrometer
Major Components
Energy difference is proportional to the magnetic field strength.
E = h = h B0
2
= gyromagnetic constant 26,753 s-1gauss-1 for H).
Often 14,092 gauss field & 60 MHz energy needed to spin
proton
Solvents used in NMR
Nonviscous and able to dissolve sample analyte.
Should not absorb within spectral range of analysis.
Solvents must be aprotic: must not contain protons (i.e. no H`s).
Most common solvent for is deuterated chloroform-d (CDCl3).
General NMR Procedure
Dissolve solid sample in test tube using CDCl3
Transfer the dissolved sample into a NMR Tube filter with glass
wool as needed. Close the NMR tube with the lid
Insert NMR tube into spectrometer
Turn on software and collect the final graph.
Final spectra is referenced to an internal standard
tetramethylsilane (TSM)
Tetramethylsilane
• TMS internal reference.
• Silicon (Si) less electronegative than carbon
• Implies electrons reside more on C than Si hence, TMS
protons are highly shielded i.e. feel a weak magnetic field.
• To get a signal from these protons, a high field is needed
i.e. very upfield
• Thus, TMS signal appears at 0 ppm/high field) as reference.
• Organic protons absorb downfield (to the left) of the TMS
signal.
CH3
H3C Si CH3
CH3
Magnetic Shielding & Shielded Protons
Protons are surrounded by electrons that shield them from
the external field.
Circulating electrons create an induced magnetic field that
opposes the external magnetic field.
Magnetic field strength must be increased for a shielded
proton to flip at the same frequency as unshielded proton.
Protons NMR of Molecules
Protons in a molecule are shielded by different
amounts.
Higher magnetic field implies higher chemical
shift
Chemical Shift
• An NMR spectrum is a plot of the radio frequency
applied against absorption.
• A signal in the spectra is called a resonance.
• Chemical shift, , : the frequency of the
resonance signal with reference to a standard
compound (TMS) defined at 0 ppm.
• Chemical shift has units of parts per million (ppm)
Chemical Shift
Peak at chemical shift, δ, of 9 ppm is downfield or deshielded
Peak at say 3 ppm is upfield or shielded.
downfield
or deshielded upfield or shielded
Increasing frequency
Increasing chemical shift
Increasing magnetic field
NMR Signals in a Molecule
• The number of signals shows how many
different kinds of protons are present.
• The location of the signals shows how
shielded or deshielded the proton is.
• The intensity of the signal shows the number
of protons of that type.
• Signal splitting shows the number of protons
on adjacent atoms.
The NMR Graph
Location of Signals
• More electronegative atoms deshield
more and give larger shift values.
• As more Chlorine atoms (very
electronegative) are added the H
becomes less and less shielded i.e.
more deshielded and appears more
downfield (7.2ppm, HCCl3) vs 0.2ppm
(CH4)
• Effect decreases with distance.
• Additional electronegative atoms
cause increase in chemical shift.
Typical Values
Typical Values
O-H and N-H Signals
• Chemical shift depends on concentration.
• Hydrogen bonding in concentrated solutions
deshield the protons, so signal is around
3.5 for N-H and 4.5 for O-H.
• Proton exchanges between the molecules
broaden the peak.
Carboxylic Acid Proton, 10+
• Acid proton (H) very deshielded by the O-C=O group: H has low
electron cloud and easy to flip so appears downfield (chemical
shift 10 or more ppm) i.e. low magnetic field.
• H on CH3 are more shielded C is less electronegative: H appear
upfield (chemical shift 2.1 ppm)need more magnetic field.
Number of Signals
• Equivalent hydrogens have the same chemical shift.
• Three signals: CH3 (a) are different from CH2(b) and from CH3(c)
• CH3(a) less shielded followed by CH2(b) then CH3 (c)
Intensity of Signals
• The area under each peak is proportional to
the number of protons.
• Shown by integral trace.
How Many Hydrogens?
When the molecular formula is known, each integral
rise can be assigned to a particular number of H`s.
Type of H: a b c d
Integral rise 3 1.5 1.0 0.5
Divide by smallest 3/0.5 1.5/0.5 1.0/0.5 0.5/0.5
Ratio 6 3 2 1
Hence 2CH3 CH3 CH2 OH
Spin-Spin Splitting
• Nonequivalent protons on adjacent carbons have magnetic
fields that may align with or oppose the external field.
• Signals are often split into different number of peaks
depending on the adjacent protons.
• n-equivalent neighboring hydrogens will split a 1H signal into
an ( n + 1 ), the n +1 rule.
• Thus, for 3 equivalent neighbors, a proton will split into 4 lines.
• The intensity of lines is determined by Pascal’s triangle:
Pascal Triangle: Spin-Spin Splitting
no. of neighbors relative intensities pattern example
0 1 singlet (s)
H H
1 1 1 doublet (d)
C C
H H
2 1 2 1 triplet (t) C C H
H H
3 1 3 3 1 quartet (q) C C H
H H H H
4 1 4 6 4 1 pentet C C C H
H H H H
5 1 5 10 10 5 1 sextet H C C C H
H H H H
6 1 6 15 20 15 6 1 septet H C C C H
H H
the alcohol hydrogen –OH usually does not split neighboring
hydrogen signals nor is itself split.
R-OH often singlet peak from 1-1.55 ppm).
1H- 1H Coupling: Neighboring Effect
Why splitting of signals happens
Consider the structure shown below
The two Ha protons are equivalent & different from Hb.
Normally only two signals in NMR graph expected.
But because Ha and Hb are nonequivalent, spin-spin
interaction occurs leading to signal splits.
Continued
Ha have same chemical shift: equivalent/same environment
and are deshield by 2Cl so appear at higher chemical shift.
But the line is split into two by spin coupling from nearby Hb
Hb occurs at lower shift/more shield but signal is split into
three lines by the spin field of two Ha protons.
1H - 1H Coupling: Neighboring Effect
Another Example
Three H neighbors: N+1 = 4 lines Two H neighbors: N+1 = 3 lines
Origin of the triplet for HB?
HB feels the splitting of both HA and HA’.
Initially NMR signal for HB is a single line
Coupling from one Ha splits into two lines
These two lines are further split by coupling from the other Ha
The two lines in the middle overlap, intensity is twice as much
HB If uncoupled, H B would appear as a
singlet where the dashed line indicates
the chemical shift of the singlet.
Now, let's "turn on" HB - HA coupling. This splits
the single line into two lines
HA HB
Now, let's "turn on" HB - HA' coupling. This
HA' C C
splits each of the two new lines into two lines,
but notice how the two lines in the middle
overlap. Overall, we then have three lines.
Important Facts & Coupling Constant
• Equivalent protons do not split each other: Ha or Hb
will not split themselves.
• Protons that are four or more bonds will not couple.
• Protons bonded to the same carbon will split each
other only if they are nonequivalent: Hb, Hc.
• Protons on adjacent carbons normally will couple: Ha &
Hb or Ha & Hc
a c
H H
C C
Hb
2 3 4
1 5
Nonequivalent H
1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
Explanation: Doublet: 1 Adjacent Proton
Triplet: 2 Adjacent Protons
Example 1: Identify the peaks observed and explain the
splitting patterns.
Solution to Example 1:
Methyls protons Ha equivalent and more shielded occur
upfield: same chemical shift.
Hb deshielded by electronegative oxygen: downfield same
chemical shift i.e. equivalent
Methyls (CH3) at (a) forms a doublet: split by the Hb
H at (b) forms quartet (4 peaks) has 3 neighboring H from
the two CH3 (a): 6 : 2 integration ratio.
Hb is downfield (higher ppm) due to neighbor O:electron
withdrawing
H(c) are less shielded than those at (d): Hc are closer to C=O
that is electron withdrawing hence less shielded.
Hd is split into 5 peaks (quintet): has 4 adjacent H
Example 2: Aromatic hydrogen and an aldehyde H, and
another characteristic alkyl splitting pattern.
Solution to Example 2:
Methyls (CH3) at (a) forms a doublet: split by the Hb
H at (b) forms septet (7 peaks) because of 6 neighboring H from
the two CH3 (a): 6 : 1 integration ratio.
The regions are expanded to clearly see splitting patterns.
Hydrogens on the benzene ring, (c) are more shielded than
those at (d): Hd are closer to C=O that is electron withdrawing
hence less shielded.
Aldehyde H in R-CHO is normally at shifted downfield at 9.5 ppm
i.e. highly deshielded by the C=O.
Further shift to 10ppm is observed due to further deshieldding
from the benzene ring.
Complex Splitting: Nonequivalent Protons
a c
H H c CH3
C C
H OHa
Hb
H Cl
dH Hb aH Hb
Cl
styrene
• Signals may be split by adjacent protons, different
from each other, with different coupling constants.
• Example: Ha of styrene which is split by an
adjacent H trans to it (J = 17 Hz) and an adjacent H
cis to it (J = 11 Hz).
Styrene Splitting Pattern
a c
H H
C C
Hb
doublet of doublet
NMR Spectrum for Styrene
