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NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

Dr.P.L.Prasunamba

Retired Professor in Chemistry

Osmania University

Hyderabad

NMR Spectroscopy

STRUCTURE ELUCIDATION OF Organic compounds

• Chemical degradation

• Spectral methods

uv Spectroscopy

IR Spectroscopy

NMR Spectroscopy

Mass Spectrometry

• Synthesis

Electromagnetic Spectrum

Radiofrequency waves

wavelength (λ ) 1012 Ao / 108 µ / 104 cm / 100 m

Wave-number (ν = 1/λ) 10-4 cm-1

Frequency (c/ λ) 3 x 106 Hz = 3 MHz

Energy (E = hv) 10-8 eV / 10-4 cal

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

1. This technique is based on Magnetic properties of Nuclei,

hence the name Nuclear Magnetic Resonance Spectroscopy.

2. Different energy levels for absorption of radiant energy and

excitation are not existing in the molecule. They are to be

generated.

3. The two different energy states involved are

Nuclear Magnetic Resonance

Definition:

Nuclei with I>0 under the influence of magnetic field when exposed to Radiofrequency waves resonance occurs

Which Nuclei

• Nuclei = Protons + Neutrons

• Nuclear spin quantum number/ Angular momentum of nuclei = I

• Spin quantum number of H = +1/2 & -1/2

• Are all nuclei magnetic ?

Magnetic & Non-magnetic Nuclei

Nuclei

Odd mass number1H,13C,17O,19F

I=1/2,3/2,5/2

MAGNETIC

Even mass numberOdd(Z)+Odd(n)

2D,10B,14NI=1,2,3

MAGNETIC

Even mass numberEven(Z)+ Even(n)

12C, 18OI=0

NON - MAGNETIC

1H-NMR / PMR spectroscopy

1H-NMR spectrum of a molecule gives all the

information about the hydrogens present in it.

number of signals

position of the signals

intensities of the signals

splitting of a signal

8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0

Applications of PMR

• Structural elucidation

• Reaction mechanisms

• Electronegativity information

• Aromaticity

• Sterochemistry

• Confirmational analysis

• Optical purity

Explaining NMR

The NMR Spectrometer

A Modern NMR Instrument

Radio Wave

Transceiver

Principles of NMR

• Measures nuclear magnetism or changes in nuclear magnetism in a molecule

• NMR spectroscopy measures the absorption of light (radio waves) due to changes in nuclear spin orientation

• NMR only occurs when a sample is in a strong magnetic field

• Different nuclei absorb at different energies (frequencies)

Protons (and other nucleons) Have Spin

Spin up Spin down

Each Spinning Proton is Like a “Mini-Magnet”

Spin up Spin down

magnetic moment

A Spinning Gyroscope

in a Gravity Field

A Spinning Charge

in a Magnetic Field

How these two orientations of protons of different

energy generated?

This has become possible due to magnetic properties of nucleus.

Nuclei are spinning around their own axis just like the earth. The

nuclei are positively charged. The circulating charge generates

magnetic field, hence nucleus is equivalent to a tiny magnet

Bigger Magnets are Better

low frequency high frequency

Increasing magnetic field strength

Different Isotopes Absorb at Different Frequencies

low frequency high frequency

2H 15N 13C 19F 1H

30 MHz 50 MHz 125 MHz 480 MHz 500 MHz

Because of the behavior of nucleus as a tiny bar magnet, it will be oriented

in two different ways.

One of this is aligned with the magnetic field (parallel) and the other

aligned against the applied magnetic field (anti parallel).

In the absence of applied magnetic field the nuclei are oriented

randomly

When we apply an external magnetic

field the magnets alter direction.

Nuclear magnet

External magnet

Resonance

• The nuclei is precessing in the magnetic field.

• To allow the nuclei to flip from E1 to E2 one must irradiate at the precessional frequency,

• ω is a function of H0

14,000 25,000

Resonance

• Irradiate with radio frequency to equalize the population then allow to relax back to normal distribution.

r.frelax

Two Energy States

The magnetic fields of the spinning nuclei will align either withthe external field, or against the field.

A photon with the right amount of energy can be absorbed and cause the spinning proton to flip.

NMR RequiReMeNt (cONtd…)

E1 -µHO

E2 µHO

Δ E = E2 – E1

= μH0 – ( - μH0 )

= 2 μH0

E = hn , hn = 2 μH0

n= 2 μ /h. H0

NMR equation (contd..)

• At Resonance n= 2 μ /h. H0

• For Hydrogen μ = 1.41x 10-23 erg/gauss

h = 6.624 x 10-27

• If H0 = 14,000 gauss

• n = 2 x 1.41 x 10-23 x 14000/6.624 x 10-27

= 60 x 106 Hz or 60 MHz

Magnetic Shielding

• If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained.

• But 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.

Shielded Protons

Magnetic field strength must be increased for a shielded proton to flip at the same frequency.

Spin Relaxation

• There are two primary causes of spin relaxation:

• Spin – lattice relaxation, T1, longitudinal relaxation

lattice

Spin- spin relaxation, T2, transverse relaxation.

Chemical Shift

• Measured in parts per million.

• Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (Hz).

• Same value for 60, 100, or 300 MHz machine.

• Called the delta scale.

Tetramethylsilane

• We need a reference peak to measure from.

• TMS is added to the sample.

• Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero.

• Organic protons absorb downfield (to the left) of the TMS signal.

Delta Scale

NMR Units of Measurement

• Parts per million (d) 0 - 12 ppm (for 1H)

nobs - nref

ppm is proportional to frequency

x 106ppm =

1H NMR Spectra Exhibit...

• Chemical Shifts (peaks at different frequencies or ppm values)

• Splitting Patterns (from spin coupling)

• Different Peak Intensities (# 1H)

8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 41

The NMR Graph

1H-NMR spectrum of methyl acetate

Downfield: the shift of an NMR signal to the left on the

chart paper

Upfield: the shift of an NMR signal to the right on the

chart paper

Typical Values

Factors affecting chemical shift

Location of Signals

• More electronegative atoms deshield more and give larger shift values.

• Effect decreases with distance.

• Additional electronegative atoms cause increase in chemical shift.

Vinyl Protons, d5-d6

Aromatic Protons, d7-d8

Acetylenic Protons, d2.5

Aldehyde Proton, d9-d10

Electronegative

oxygen atom

O-H and N-H Signals

• Chemical shift depends on concentration.

• Hydrogen bonding in concentrated solutions deshield the protons, so signal is around d3.5 for N-H and d4.5 for O-H.

• Proton exchanges between the molecules broaden the peak.

Carboxylic Acid Proton, d10+

NMR Signals

• 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.

Equivalent hydrogens: have the same chemical environment

– a molecule with 1 set of equivalent hydrogens gives 1 NMR signal

H3 C CH3

CH3 CCH3 ClCH2 CH2 Cl

Propanone(Acetone)

1,2-Dichloro-ethane

Cyclopentane 2,3-Dimethyl-2-butene

a molecule with 2 or more sets of equivalent hydrogens gives

a different NMR signal for each set

CH3CHCl

Cyclopent-anone

(2 s ignals)

1,1-Dichloro-ethane

(2 signals )

(Z)-1-Chloro-propene

(3 signals)

Cyclohexene (3 signals)

Number of SignalsEquivalent hydrogens have the same chemical

shift.

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 hydrogens.

NMR Peak Intensities

C - CH C - CH2 C - CH3

X Z X Z X Z

AUC = 1 AUC = 2 AUC = 360

Spin-Spin Coupling

• Many 1H NMR spectra exhibit peak splitting (doublets, triplets, quartets)

• This splitting arises from adjacent hydrogens (protons) which cause the absorption frequencies of the observed 1H to jump to different levels

• These energy jumps are quantized and the number of levels or splittings = n + 1 where “n” is the number of nearby 1H’s

Spin-Spin Splitting

• Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field.

• This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed.

• All possibilities exist, so signal is split.

Spin-Spin Coupling

C - Y C - CH C - CH2 C - CH3

singlet doublet triplet quartet

X ZX Z X Z X Z

1,1,2-Tribromoethane

Nonequivalent protons on adjacent carbons.

Doublet: 1 Adjacent Proton

Triplet: 2 Adjacent Protons

1H-NMR spectrum of 1,1-dichloroethane

CH3 -CH-Cl

For these hydrogens, n = 1;their signal is split into(1 + 1) = 2 peaks; a doublet

For this hydrogen, n = 3;its s ignal is split into(3 + 1) = 4 peaks; a quartet

The N + 1 Rule

If a signal is split by N equivalent protons,

it is split into N + 1 peaks.

Spin Coupling Intensities

1 3 3 1

1 4 6 4 1

1 5 10 10 5 1

1 1 1 12

Pascal’s Triangle

1 13 3

Range of Magnetic Coupling

• Equivalent protons do not split each other.

• Protons bonded to the same carbon will split each other only if they are not equivalent.

• Protons on adjacent carbons normally will couple.

• Protons separated by four or more bonds will not couple.

Splitting for Ethyl Groups

Splitting for Isopropyl Groups

Time Dependence

• Molecules are tumbling relative to the magnetic field, so NMR is an averaged spectrum of all the orientations.

• Axial and equatorial protons on cyclohexane interconvert so rapidly that they give a single signal.

• Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule.

Hydroxyl Proton

• Ultrapure samples of ethanol show splitting.

• Ethanol with a small amount of acidic or basic impurities will not show splitting.

N-H Proton

• Moderate rate of exchange.

• Peak may be broad.

Understanding the Protons Song

We can derive the following information:

• The number of different types of protons (the number of signals)

• The environment of the protons (the chemical shift)

• The number of protons on the neighbouring carbons (the signal multiplicity)

• The number of protons responsible for each signal (the integral)

We shall use Paracetamol as an example

Paracetamol from an NMR Experiment

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