Nuclear magnetic resonance

• IR reveal the type of functional groups present in a molecule.

• Through NMR one can determine the number each of the distinct
type of hydrogen nuclei, information regarding immediate
environment of each type.
• Similar information can be determined for carbon nuclei.
• Combination of IR and NMR is sufficient to determine completely the
structure of unknown compound.
Nuclear spin states
• Atomic nuclei have a property called spin. They behave as if they were
spinning.
• Atomic nuclei possesses odd mass and odd atomic number have spin
angular momentum or magnetic moment.
• C-12 & O-16 don’t have the spin, however C-13 & O-17 have the spin.
• H-1 and H-2 both have the spin.
• For each nucleus the allowed spin state is determined by a nuclear
spin quantum number I.
Nuclear spin state

• In the absence of magnetic field all of the spin states of a given

nucleus are of equivalent energy (degenerate).
Nuclear magnetic moment
• Spin state are not equivalent energy in applied magnetic field because
nucleus is a charged particle, charged particle generate magnetic field
of its own.
• Hydrogen nucleus has clockwise (+1/2) and counterclockwise (-1/2)
spin, nuclear magnetic moment pointed in opposite direction.
• In the presence of magnetic field they are either aligned with the field
or opposed to it.
• +1/2 is the lower energy they are aligned with the field and -1/2 is
higher energy they are opposed to the field.
Nuclear magnetic moment
Mechanism of absorption (resonance)
• In the applied magnetic field, the proton begin to preces.
• The frequency at which the proton preces is directly proportional to
the strength of the applied magnetic field.
• Stronger the applied field, higher the rate of precession.
• Since the nucleus is charged precession generate oscillating electric
field of the same frequency.
• When the frequency of oscillating electric field component of
incoming radiation matches the frequency of electric field generated
by precessing nucleus, two field couple, energy transferred, this cause
spin change such condition called resonance.
Mechanism of absorption (resonance)
• Nucleus is said to be in resonance with the incoming electromagnetic
wave.
Chemical shift and shielding
• Not all protons have resonance at same frequency.
• Protons are surrounded by electrons, valence shell electron densities
vary from one proton to another.
• So, the protons are shielded by electrons.
• In applied magnetic field, valence electrons are caused to circulate.
• This circulation called a local diamagnetic current, generate a counter
magnetic field that opposes the applied magnetic field.
• This effect is called magnetic shielding or anisotropy.
Chemical shift and shielding

• So greater electron density around a nucleus, greater will be induced

field that opposes the applied field.
• Induced field diminish the applied field that nucleus experience so as
a result the nucleus precesses at lower frequency. It mean it absorb
radiofrequency radiation at lower frequency.
• Each proton in slightly different chemical environment and slightly
different electronic shielding which result in slightly different
resonance frequency.
• Resonance frequency of each proton is measured relative to the
resonance frequency of reference substance.
• The standard substance that is used is tetramethyl silane (TMS)
(CH3)4Si.
• This was initially chosen as standard because proton of methyl are
more shielded than most of the known compounds.
• It mark one end of the range, when another compound is measured,
resonance of proton reported how far they are shifted from TMS.
Chemical shift
• Shift from TMS depend on the strength of applied magnetic field.
• In the applied field of 1.41 Tesla the resonance of proton is 60 MHz,
whereas in an applied field of 2.35 Tesla the resonance appear at 100
MHz.
• So the ratio is

For a given proton the shift is 5/3 time larger in 100 MHz than in 60
MHz.
• This is confusing for the worker trying to compare the data if they
have spectrometer that differ in strength of applied magnetic field.
• This can be overcome by defining a new parameter independent of
field strength.
• Field independent measure called chemical shift is obtained by
dividing shift in Hertz with frequency in megahertz.
Chemical equivalence
• All protons in a chemically identical environment are chemically
equivalent. They exhibit same chemical shift.
• Here are the examples
Chemical equivalence
Chemical equivalence

• Number of peaks observed correspond to number of chemically

distinct type of protons in the molecule.
Integrals and integration
• NMR spectrum not only distinguish between different types of proton
but tell the number of protons as well.
• In NMR spectrum the area under the peak is proportional to
hydrogens generating that peaks.
• NMR spectrometer has the capability to electronically integrate the
area under each peak.
• In older NMR instruments integral lines does not give the absolute
number of hydrogens and through ratio we find the number of
hydrogens.
• Modern FT-NMR there are integral lines with digitalized integral
values printed below the peaks.
Chemical environment and chemical shift
• Different type of proton have different chemical shifts and each has
characteristic value of chemical shift.
• Numerical value of chemical shift for a proton give clues regarding the
type of proton originating the signal.
Example-Phenylacetone
• Both have the aromatic protons with chemical shift almost at 7.3 ppm
• Methyl group attached directly to carbonyl have resonance about 2.1
ppm.
• It is important to learn chemical shifts of common types of protons
which have resonance.
• Correlation chart contain most essential and frequently encountered
protons.
Example-Benzyl acetate
Factor effecting the chemical shift
• Electronegativity
• As the electronegativity of attached element increases chemical shift
increases.

• Electronegative element have little effect on proton that more than

three carbon distant.
Factor effecting the chemical shift

• Electrons around the protons shield them from applied field this is called
local diamagnetic shielding.
• Electronegative element because of the electron withdrawing effect reduce
the electron density around proton attached to carbon.
• They reduce the diamagnetic shielding and deshield the protons.
• More the deshielding, greater will be the chemical shift.
Hybridization effects
• Sp3 hydrogens
• Hydrogens attached to carbon in highly strained ring like cyclopropyl
hydrogens occur at 0-1ppm.
• Methyl group occur at near 1 ppm.
• Methylene group hydrogens occur at greater chemical shift 1.2 –
1.4ppm
• Tertiary methine occur at higher chemical shift.
Sp2 hydrogens
• In sp2-1s the C-H bond has more s character so the s orbitals hold
electrons closer to the nucleus so this result in less shielding of
hydrogen nucleus than sp3-1s C-H bond.
• They vinyl has greater chemical shift 5-6 ppm and aliphatic hydrogen
on sp3 carbon has 1-4 ppm.
• Aromatic hydrogens appear in the range further downfield 5-6 ppm.
• The downfield of vinyl and aromatic are greater then one would
expect on the base of hybridization.
• Aldehyde protons appear even downfield than the aromatic protons
• This is one due to the inductive effect of carbonyl and the anomalous
anisotropy behavior like in aromatic and alkene protons responsible
for chemical shift.
sp hybridization
• Acetylenic hydrogens C-H (sp-1s) appear anomalously at 2-3 ppm due
to anisotropy.
• On the bases of hybridization one would expect the acetylenic proton
to have greater chemical shift then that of they vinyl protons.
• Because sp carbon behave they were more electronegative than sp2
carbon.
• Here it is opposite of what is observed previously.
Acidic and exchangeable protons: H-bonding
• Some of the least shielded protons are those attached to carboxylic
acid. They have resonance at 10-12 ppm.

• Both resonance and electronegative oxygen withdraw electrons from

acid protons.
• The more hydrogen bonding take place the more deshielded a proton
become.
• Amount of hydrogen bonding is a function of temperature and
concentration.
• The more concentrated the solution, the more molecules come in
contact with each other and hydrogen bond.
• At high dilution hydroxyl protons have dilution near 0.5-1.0 ppm.
• In concentrated solution their absorption is closer to 4-5 ppm.
Magnetic anisotropy
• Aryl protons have chemical shift as large as that of the chloroform.
• Alkene, alkyne and aldehyde also have protons with resonance value
that are not in line with the expected values due to the electron
withdrawing and hybridization effect.
• This behavior is due to the presence of unsaturated system in the
vicinity of proton in question.
• For example benzene
• It hydrogen are deshielded by the diamagnetic anisotropy of ring.
• Isotropic field is the one that is uniform, have uniform density or
spherical symmetric distribution.
• Anisotropic field in comparison to that is the one that is nonuniform.
• Protons attached to benzene is influenced by three magnetic fields.
One the strong magnetic field applied by the electromagnets of NMR
spectrometer, two weaker field one due to the usual shielding by
valence electron around the proton and other due to the anisotropy
generated by the ring system pi electrons.
• These protons lie in the deshield region of the anisotropic field.
• If the protons placed in the center of the ring rather than the
periphery it would be found to be shielded and there these lines have
opposite direction from those at the periphery.
Example acetylene
• All molecules that have pi electrons generate secondary anisotropic
fields.
• In acetylenes magnetic fields generated by pi electron has geometry
such that the acetylenic hydrogens are shielded.
• So these hydrogens have resonance at higher field than the expected.
Spin-spin splitting (n+1) rule
• As learned previously about the chemical shift and integrals, they give
information about the types and number of hydrogens.
• A third type of information is derived from the spin-spin splitting
phenomena.
• Each type of proton in a molecule rarely give a single resonance peak.
• Example
• 1,1,2-trichloroethane
• On the basis of information one expect the two peaks in the spectrum
with area ratio of 2:1.
• But in reality a high resolution NMR of this compound has five peaks:
a group of three (called triplet) at 5.77 ppm and a group of two (called
doublet) at 3.95 ppm.
• Methine resonance at 5.77 split in to triplet, methylene resonance at
3.95 ppm split in to doublet.
• Area under triplet is 1 and area under doublet is two.
• This phenomena is called spin-spin splitting and can be explained by
the rule n+1 rule.
• Examine the case 1,1,2-trichloroethane using the n+1 rule
• Methine proton has next carbon situated bearing two protons so two
equivalent neighbors (n=2) so split according to rule n+1 =3.
• Methylene protons are situated next to a carbon bearing only one
methine hydrogen so it has one neighbor (n=1) so split into n+1 =2.
• So, note spin-spin splitting give a new type of structural information, it
reveals how many hydrogen are adjacent to each type of hydrogen
that is giving an absorption peak.
Origin of spin-spin splitting
• Spin-spin splitting arises because hydrogen on adjacent carbon atoms
can sense one another.
• Hydrogen on carbon B in some molecule it has spin +1/2 and in other
molecule it has spin -1/2.
• Proton A is influenced by the direction of spin in proton B and it has
slightly different type of chemical shift value in X than Y type of
molecules so have doublet.
Coupling constant
• The distance between the peaks in simple multiplet is called coupling
constant.
• The spacing between multiplet peak is measured on same scale as
chemical shift, so coupling constant is expressed in Hz.
• If the spectrum get at 60 MHz so each ppm of chemical shift represent
60Hz. There are 12 grid lines per ppm, each grid line represent 60Hz/12 =
5Hz. It is calibrated in cycles per second (cps), which are the same as Hz,
there are 20 chart division per 100 cps, so one division equal to 100cps/20
= 5cps = 5Hz. The spacing between multiplet line is almost 1.5 chart
division. So
• J = 1.5 div x 5Hz/1div = 7.5Hz
Coupling constant
• Here is the coupling constant determined at 60MHz and 100MHz
instruments, you can see despite the expansion of spectrum in
100MHz the coupling constant in both remain the same.
Alkane
• R-CH3, the methyl group show the peak at 0.7-1.3 ppm.
• Methylene absorption such as R-CH2-R, it show in the region 1.2-
1.4ppm.
• Methine hydrogen has largest chemical shift than the methyl and
methylene so it is R3-CH at 1.4-1.7 ppm.
 Alkene
• Hydrogen attached to double bond C=C-H vinyl hydrogen are
deshielded by the anisotropy of adjacent double bond it show shift at
4.5-6.5 ppm.
• Hydrogen attached to carbon adjacent to double bond allylic
hydrogen are also deshielded by the anisotropy of the double bond
but it is distant and effect smaller. So give peak at 1.6-2.6 ppm.
Aromatic compound
• The hydrogen attached to aromatic ring have shift near 6.5-8.0 ppm.
They are deshielded by the anisotropy of the ring pi system.

• Benzylic hydrogens are also deshielded by the anisotropic field of the

ring but they are distant and have smaller effect.
Spectra of aromatic compound
Alkynes
• The terminal or acetylenic hydrogen have chemical shift near 1.7-2.7
ppm due to the anisotropic shielding by the adjacent pi bond.

• Protons on carbon next to triple bond also effected by the pi system.

They show shift at 1.6 – 2.6 ppm.
Spectra of alkyne
Alkyl halide
• The chemical of hydrogen attached to same carbon as halide
increases.
• The deshielding effect is due to the electronegativity of attached
halogen atom. Extent of shift increased as electronegativity of
attached atom increases with larger shift observed in compound
containing fluorine.
• CH-I show shift at 2.0-4.0 ppm
• CH-Br show shift at 2.7-4.1 ppm
• CH-Cl show shift at 3.1-4.1 ppm
• CH-F show shift at 4.2-4.8 ppm
Alkyl halide
Alcohols
• Proton such as C-OH, its chemical shift is variable and it depend on
the concentration, solvent and temperature and show shift at 0.5-5.0
ppm.
• Protons on alpha carbon such as CH-OH are deshielded by the
electronegative oxygen and show shift at 3.2-3.8 ppm
Alcohol
Ether
Amines
Amine example
Nitriles
Aldehyde
Aldehyde example
Ketone
Ester
Ester example
Carboxylic acid
Carboxylic acid example
Amide
Amide example
Nitro alkane
C-13 shifts chart