NMR study of molecules

MAJOR PROJECT (SYNOPSIS)

Submitted in fulfillment of the partial requirements
For the award of the degree
Of
BACHELOR OF SCIENCE
IN
CHEMISTRY HONOURS

UNIVERSITY SCHOOL OF SCIENCES

RAYAT BAHRAUNIVERSITY
MOHALI, INDIA
DEC2019
SUBMITTED BY SUPERVISOR
KASHISH MAHAJAN Ms.Rijul.
 DECLARATION

This is to declare that work embodied in the report for Dissertation entitled “NMR study of

molecules” is being submitted by KASHISH MAHAJAN in partial fulfillment of the

requirements for the award for the degree of Bachelor of Science (Chemistry). All the used or

quoted sources have been indicated and acknowledged by means of complete references. Further,

it is mentioned that the content in this synopsis is free from plagrism.

KASHISH MAHAJAN
 ACKNOWLEDGMENT

I would like to express my gratitude towards Dr. HarvinderKaur, Dean, University School of

Sciences, Rayat Bahra University, Mohali and Dr. Rajni Garg, Head, Department of Chemistry

for this work. I also thank our teachers for their kind co-operation and encouragement which

helped me in completion of literature review and writing the synopsis.

My thanks and appreciations also go to my friends who have willingly helped me out with their

abilities.

KASHISH MAHAJAN
 CONTENTS

SR.NO TITLE PAGE

NO.

1. INTRODUCTION 1-4

2. PRINCIPLE 5-6

3. ORIGIN OF SIGNALS 7

4. INSTRUMENTATION OF SPECTROPHOTOMETER 8-9

5. NMR SPECTRA OF MOLECULES 10

6. RESULTS AND DISCUSSION

7. CONCLUSION
 INTRODUCTION

During the last six decades or so, spectroscopic methods such as ultraviolet (UV), infrared (IR),
nuclear magnetic resonance (NMR) and mass spectrometry have emerged as powerful tools for
identification and structure determination of organic molecules. NMR which was discovered by
Felix Block and Edward Purcell in 1946.

The field of NMR spectroscopy has been further divided into number of subfields depending
upon the magnetic nucleus being examined. For example , proton magnetic resonance (PMR or
1
H NMR) , carbon magnetic resonance (13C NMR) , fluorine magnetic resonance (19F NMR) ,
phosphorous magnetic resonance (31P NMR), etc.

The nucleus of a hydrogen atom (proton) behaves as a spinning bar magnet because it possesses
both electric and magnetic spin. Like any other spinning charged body, the nucleus of hydrogen
atom also generates a magnetic field.

Nuclear magnetic resonance involves the interaction between an oscillating magnetic field of
electromagnetic radiation and the magnetic energy of the hydrogen nucleus or some other type of
nuclei when these are placed in an external static magnetic field.

The sample absorbs electromagnetic radiations in radio wave region at different frequencies
since absorption depends upon the type of protons or certain nuclei contained in the sample.
Consider a spinning top. It also performs a slower waltz like motion, in which the spinning axis
of the top moves slowly around the vertical. This is precessional motion and top is said to be
precessing around the vertical axis of earth’s gravitational field. The precession arises from the
interactions of spin with earth’s gravity acting vertically downwards. It is called Gyroscopic
motion. Let us consider a peculiarity of a small magnet spinning in an external field. It has been
found that the proton (tiny magnet) presesses about axis of the external magnetic field in the
same manner in which a spinning gyroscope precesses under the influence of gravity.

It has been found that-

ω = γ H0 ------- (1)

ω = angular precessional velocity

H0 = applied field in gauss :

γ = Gyromagnetic ratio = 2πμ/hI

μ = magnetic moment of the spinning bar magnet.

I = spin quantum number of the spinning magnet.

h = Planck’s constant
According to the fundamental NMR equation which correlates electromagnetic frequencies with
the magnetic field, we say that

γH0 = 2πν -------(2)

Here ν is frequency of electromagnetic radiation

From (1) and (2)

Angular precessional velocity ω = 2πν

The value of this frequency (ν) inserted is called Precessional frequency.

The precessional frequency may be defined as the number of revolutions per second made by the
magnetic moment vector of the nuclear around the external field Ho. Alternatively the
precessional frequency of the spinning bar magnet (nucleus) may be defined as equal to the
frequency of electromagnetic radiations in megacycles per second necessary to induce a
transition from one spin state to another.

All nuclei carry a charge. So, they will possess spin angular momentum. The moment of the spin
angular momentum is quantized, i.e., only those nuclei which have a finite value of spin quantum
number (I>0) will precess along the axis of rotation. It is known that the spin quantum number I
is associated with mass number and atomic number of the nuclei.

Mass number Atomic number Spin quantum number (l)

odd Odd or even ½ , 3/2 , 5/2 ….
Even even 0
even odd 1,2,3…….

The circulation of the nuclear charge generates a magnetic moment along the axis. The intrinsic
magnitude of the generated dipole is expressed in terms of magnetic moment μ.

If a proton is placed in a magnetic field, then it starts precessing at a certain frequency in the
radiowave region and thus, will be capable of taking up one of the two orientations with respect
to the axis of the external field.

1. Alignment with the field and

2. Alignment against the field.

If a proton is precessing in the aligned orientation, it can pass into the opposed orientation by
absorbing energy. From the high energy opposed orientation, it comes back to the low energy
aligned orientation (more stable) by losing energy. The transition from one energy state to
the other is called flipping of the proton. The transition between the two energy states can be
brought about by the absorption of a quantum of electromagnetic radiation in the radiowave
region with energy hν.

The energy required to bring about the transition (∆E = hν) or to flip the proton depends upon
the strength of the external field. Stronger the field, greater will be the tendency of the
nuclear magnet to remain lined up with it and higher will be the frequency of the radiation
needed to flip the proton to the higher energy state.

We know that,

ν = γH0/2π

ν = frequency in cycles per sec or Hz.

H0 = strength of the magnetic field in gauss.

γ = nuclear constant or Gyromagnetic ratio and is equal to 26750 for the proton.

In a field of 14092 gauss, the energy required to cause flipping corresponds to

electromagnetic radiation of frequency 60 million cycles per second or 60 MHz (Radiowave
region). This energy is much lower than that possessed by infra-red radiation. If we irradiate
the processing nuclei with a beam of electromagnetic radiation of desired frequency, then the
low energy nuclei will absorb it and move to higher energy state. The processing proton will
absorb energy from the radio frequency source only if the precessing frequency is the same
as the frequency of the radio frequency beam, i.e., when the quantum energy (hν) of
electromagnetic radiation matches up the energy difference between the two energy states at
the field strength H0. When this occurs, the nucleus and the radio-frequency beam are said to
be in resonance. Hence, the term nuclear magnetic resonance.

This technique consistes of exposing the protons (placing the substance) in an organic
molecule to a powerful field. The protons will precess at different frequencies. Now, we
irradiate these precessing protons with steadily changing frequencies (for promoting or
flipping protons from the low energy state to high energy state) and observe the
frequency/frequencies at which absorptions occur. It is generally more convenient to keep the
radio-frequency constant and the strength of the magnetic field is constantly varied. At some
value of the field strength, the energy required to flip the protons matches the energy of the
radiation. Absorption occurs and a signal is observed. Such a spectrum is called Nuclear
magnetic resonance spectrum.

It may be noted that all protons do not absorb at the same applied field but absorption
depends upon the magnetic field which a particular proton feels. Clearly, the effective field
strength is different for different sets of protons as one set of protons will have slightly
different environment from any other set of protons. Thus, at a given radio-frequency ,
different protons (different sets of equivalent protons) will require slightly different applied
field strengths to produce the same effective field strength which causes absorption. In the
nmr spectrum, we measure applied field strengths for each set of prons and the absorption
peaks are plotted. The number of signals at different applied field strengths is equal to the
different sets of equivalent protons.
 PRINCIPLE OF NUCLEAR MAGNETIC RESONANCE

To understand the principle of nuclear magnetic resonance, let us consider the motion of a
common spinning top. When the spinning top is disturbed a little, its spinning axis will trace out
a circle perpendicular to the earth’s gravitational field. Such a motion is called precession.

In a similar way, when a nuclear magnet is placed in an external field, it will experience a torque
(force perpendicular to the axis of the nuclear magnet) which will tend to align it with the field.
Since the nuclear magnet is spinning, it will not align with the external magnetic field but will
precess around it with a certain angular frequency ω, called the precessional frequency or the
Larmor frequency. The frequency at which a proton will precess will depend upon the strength of
the applied magnetic field. Stronger the applied field, higher is the precessional frequency. For a
proton, if the applied magnetic field is 14100 gauss, the precessional frequency will be 60 MHz
cycles per second).

Since a proton has two allowed spin states ( I = + ½ and I = - ½ ), it can precess in two principal
orientations – one aligned with the applied field (low energy) and the other opposed to the
applied field (high energy).

It is possible to induce transitions between these two orientations by irradiating precessing nuclei
(protons) with radiowaves (electromagnetic radiations) of correct frequency. When the
precessional frequency of the nucleus (proton) is exactly equal to the frequency of the
electromagnetic radiations, the absorption occurs and the nuclei in lower energy spin state ‘flip’
to the higher energy spin state. When this occurs, the nucleus and radiofrequency (RF) beam are
said to be in resonance and since this resonance is due to magnetic properties of the nuclei, it is
called nuclear magnetic resonance abbreviated as NMR.

The exact amount of the radiofrequency energy needed for rrsonance depends upon the strength
of the external magnetic field and on the nucleus being irradiated, i.e.

∆E = hν = γ h/2π . H0

Where, ∆E = energy difference between the two spin states, h = Planck’s constant, γ =
gyromagnetic ratio. It is a ratio of magnetic moment to angular momentum and is constant for
each nucleus. For a proton, the value of γ = 26,750.

It may, however, be noted here that a NMR instrument with a magnetic field strength of 1.41 T
and operating at a constant RF of 60 MHz, can induce transitions only among proton spin states
in amolecule and is not useful for other nuclei. Therefore, separate instruments are required to
observe transitions in the nuclei of other elements such as carbon, phosphorus, fluorine, etc.
however, the modern more expensive Fourier transform (FT) instruments which are commonly
used these days can be used to observe the resonance of nuclei of several different elements in a
single instrument. Instruments with 200 and 300 MHz are now quite common and instruments
with frequencies upto 800 MHz are found in the advanced research institutions and bigger
universities.

We have discussed above that the energy difference between the two spin states of a proton at
60MHz is just 2.39 × 10-5 kJ mol-1. Since this energy difference is too small, thermal energy due
to room temperature is sufficient to popular both the spin states. There is, however, a slight
excess nuclei in the lower energy spin state. The magnitude of this difference can be calculated
by using Boltzmann distribution equations. The following equation gives the Boltzmann ratio of
nuclear spins in the upper and lower states.

Nupper/Nlower = e-∆E/kT = e-hν/kT

Where ∆E is the energy difference between the upper and the lower spin states and k is the
molecular (not molar) gas constant.

Using this equation, we can calculate that, at 298 K, for an instrument operating at 60 MHz,
there are 1,000,009 nuclei in the lower spin state for, 1,000,000 that occupy the upper spin state.
In the other words, in approx 2 milllion nuclei, there are only 9 more nuclei in the lower spin
state. This excess nuclei are the ones which are responsible for the NMR signal. When the 60
MHz radiation is applied, it not only induces transitions upward but also stimulates transitions
downward .

When the populations of the two states become exactly equal, we observe no net signal. This
situation is called saturations. Therefore, while performing an NMR experiment, care must be
taken to avoid saturation. Saturation is achieved quickly if the power of the RF signal is very
high.

If we increase the operating frequency of the NMR instrument, the energy difference between
the two spin states increases which causes an increase in excess population e.g. 16 at 100 MHz,
32 at 200 MHz , 48 at 300 MHz and 96 at 600 MHz. In other words, at higher operating
frequencies (200 MHz, 300 MHz, 600 MHz etc.), more nuclei can undergo transitions, therefore,
the sensitivity of the instrument increases and the resonance signals are stronger.
 ORIGIN OF SIGNALS

In principle, a PMR spectrum can be obtained by placing the substance containing hydrogen
nuclei or protons in a magnetic field of constant strength and passing electromagnetic radiations
of varying frequency through the substance and observe the frequency at which radiation is
absorbed. In practice, however, it has been found more convenient to keep the frequency of the
electromagnetic radiation constant and vary the strength of the magnetic field. At some value of
the field strength, the energy required to spin the proton matches the energy of the
electromagnetic radiations. At this stage, absorption occurs and a signal is observed. Such a
spectrum is called PMR spectrum.

The PMR spectrum is a plot of intensity of absorption along Y-axis against magnetic field
strength along X-axis. By convention, magnetic field strength increases from left to right across
the chart. Since the magnetic field strength and the frequency are related by the equation,

ν = μH0/hI = γH0/2π

the position of any absorption signal along the X-axis can be expressed in either field or the
frequency units.

We have discussed above that a PMR signal is obtained at some value of the field strength at
which the energy required to flip the proton becomes equal to the energy of the electromagnetic
radiation. Therefore, we may expect that all the protons in an organic molecule will absorb at
exactly the same field strength. Consequently, the PMR spectrum would consist of a single
signal and it will be of little use of the organic chemist. In practice, it has been found that all the
protons, do not absorb at the same frequency. This is due to the reason that the frequency at
which a proton absorbs depends upon the magnetic field which that proton actually feels and this
effective field strength is not exactly the same as the applied magnetic field. This effective field
strength which a proton feels depends upon its local environment, i.e., electron density around
that proton and the presence of other protons in its neighbourhood. In other words, each proton
or more precisely, each set of equivalent protons will have slightly different environment than
every other set of equivalent protons and hence will absorb at slightly different applied field
strengths to experience the same effective field strength at which the absorption occurs. Thus, it
follows that at a given radiofrequency, the protons having different environments experience the
same effective field strength but absorb at different applied field strengths. Since in a PMR
spectrum, absorption is plotted against the applied field strength, a PMR spectrum will consist of
a number of absorption signals which would be very useful in determining the structure of
molecules.
 INSTRUMENTATION OF SPECTROPHOTOMETER

We have discussed above that there are two ways to scan an NMR spectrum either (1) by
keeping the magnetic field (H0) constant and changing the frequency of the radiofrequency
oscillator or by (2) keeping the frequency of the radiofrequency oscillator constant and changing
the magnetic field. In practice, however, NMR instruments which work at constant
radiofrequency are used since they are easier and cheaper to construct. Such instruments are
called continuous wave or CW spectrometer.

The schematic diagram of an CW NMR spectrometer. It consists of the following parts-

1. A powerful electromagnet whose strength can be varied.

2. A radiofrequency generator.
3. A detector which monitors the absorption of energy and amplifies the electronic signal.
4. A recorder which records the spectrum on a chart paper which is calibrated in frequency
and ppm units.
5. A 5 mm wide long sample tube.

The sample is dissolved in a suitable solvent which does not contain protons (usually CCl 4) and
is spun around its axis to ensure that all part of the solution experience a relatively uniform
magnetic field. In between the poles of the magnet, there is a coil which is attached to a 60 MHz
RF generator. This coil supplies the electromagnetic energy used to change the spin orientation
of the protons. Perpendicular to the RF oscillator coil is a detector coil.

Nuclear magnetic resonance spectrophotometer makes use of a magnet, a radio-frequency, a

detector and an amplifier. The detection system is used to note that energy is being transformed
from the radio-frequency beam to the nucleus.

The sampler under investigation is taken in a glass tube which is placed between the pole faces
of a magnet. A radio-frequency source (ν = 60 mega cycles sec -1) is made to fall on the sample. It
can be done by feeding energy (Radio-frequency source) into a coil wound round the sample
tube. A signal is detected if the nuclei in the sample resonates with the source, i.e., ∆E, energy
required to flip the proton is the same as that of the source. Energy is transferred from the source
via nuclei to the detector coil. The output from the detector can be fed to a cathode ray
oscillograph or to a strip chart recorder after amplication etc.

Protons being in different electronic environments in a molecule cannot resonate at exactly 60

mega cycles sec-1. For practical purposes, radio-frequency source is held steady at the said
frequency and field strength is varied by placing small electromagnet to the pole faces of the
main magnet. By increasing the current flowing through these electromagnets, the total field
strength is increased.
As the field strength increases, the precessional frequency of each proton increases until
resonance with the radio-frequency source takes place. As a proton (or a set of equivalent
protons) comes to resonance, the signal from the detector produces a peak on the chart paper.
The nmr spectrum consists of series of peaks that correspond to different applied field strengths.
Each peak means a set of protons.
 NMR SPECTRA OF MOLECULES

1.1-Dibromethane (CH3-CHBr2 )

Class – Halides

Structure -

NMR SPECTRA-
2. 1,1,2-Tribromoethane (CH2Br-CHBr2)

Class – Halides

Structure –

NMR spectrum-
3.Ethanol (CH3CH2OH)

Class – Alcohol

Structure –

NMR spectrum-
4.Acetaldehyde (CH3-CHO)

Class- Aldehyde

Structure-

NMR spectrum-
5..Ethyl acetate

Class – ester

Structure –

NMR spectra –
6.Toluene (C6H5-CH3)

Class- aromatic hydrocarbon

Structure-

NMR spectra-
7.Benzaldehyde (C6H5-CHO)

Class – Aldehyde

Structure –

NMR spectra –
8.Acetophenone (C6H5-CO-CH3)

Class – Aromatic Ketone

Structure -

NMR spectra –
9.p-Anisidine (CH3O-C6H4-NH2)

Class- Aminophenyl ethers (phenyl ethers)

Structure-

NMR spectrum –
10.p-Nitrotoluene (O2N-C6H4-CH3)

IUPAC name – 1-Methyl-4-nitrobenzene

Class – Nitro aromatic compounds

Structure –

NMR spectra –