Presented by:Asif Mohammad Salehin

Jatan TanchangyaSulov Saha

Md. Mohsin Ali

SpectroscopyDefinition: Spectroscopy is the study of of the interaction between matter and electromagnetic radiation.

Types of spectroscopy:• Absorption spectroscopy – uses electromagnetic spectra

in which a substance absorbs: IR, NMR

• Emission spectroscopy – uses electromagnetic spectra in which a substance emits: Luminescence

• Scattering spectroscopy – measures the amount of light that a substance scatter at certain wavelengths, incident angles and polarization angles: Raman

Infrared Spectroscopy

• IR spectroscopy is concerned with the study of absorption of infrared radiation, which causes vibrational transition in the molecule.

• IR spectra mainly used to identify functional groups present in molecules.

Infrared Molecular vibration

Infrared spectrosco

py

Principle of IR spectroscopy

• Molecules are made up of atoms linked by chemical bonds. The movement of atoms and the chemical bonds like spring and balls (vibration).

• This characteristic vibration are called Natural frequency of vibration.

• When energy in the form of infrared radiation is applied then it causes the vibration between the atoms of the molecules and when,

Applied infrared frequency = Natural frequency of vibration

Then, absorption of IR radiation takes place and a peak is

observed.

Different functional groups absorb characteristic frequencies of IR

radiation. Hence gives the characteristic peak value.

Therefore, IR spectrum of a chemical substance is a finger

print of a molecule for its identification.

Criteria for a compound to absorb IR radiation

• Correct wavelength of radiation A molecule to absorb IR radiation, the natural frequency of vibrations of some part of a molecule is the same as the frequency of incident radiation.

• Change in dipole moment A molecule can only absorb IR radiation when its absorption cause a change in its electric dipole.

Types of Molecular Vibrations

1.Stretching

Symmetrical

Asymmetrical

2.Bending

In plane

Out plane

a) Symmetrical stretching:

2 bonds increase or decrease in length simultaneously.

H

H

C

b) Asymmetrical stretching: In this, one bond length is increased and other is decreased.

H

H

C

a) In plane bending: i. Scissoring ii. Rocking

b) Out plane bending:i. Wagging

ii. Twisting

H

H

CC

H

H

CC

Instrumentation

Interpreting an Infrared Spectrum

• Ethanoic acid (it contains C-C, C-H, C=O, C-O and O-H bonds.)

Fig: IR Spectra of common functional groups

Applications• Identification of functional group and structure

elucidation• Identification of substances• Studying the progress of the reaction• Detection of impurities• Quantitative analysis

Pros and Cons of IR Spectroscopy

Largely qualitative Difficult to analyze complex solutions in infraredSimilar infrared spectra can misidentify the fingerprint region

NMR Spectroscopy

The study of absorption of radiofrequency

radiation by nuclei in a magnetic field is called

Nuclear Magnetic Resonance.

• NMR spectroscopy is commonly used technique for organic compound structure determination.

Radiowaves

Nuclear spin

NMR spectrosco

py

Principle of NMR spectroscopy

• A spinning charge creates a magnetic moment, so these nuclei can be thought of as tiny magnets.

• If we place these nuclei in a magnetic field, they can line up with or against the field by spinning clockwise or counter clockwise.

NMR active nuclei

Number of nucleons-Must be odd for residual magnetism For e.g. Hydrogen exhibits, Deuterium does not1H, 13C and 15N nuclei-have a very small magnetic moment: “half integer spin”

Resonance condition

• Transition from low to high energy state occurs

• hע = gßH

• Either a constant magnetic field is applied and appropriate region of radio frequency is swept

• Or a constant frequency is employed & the external magnetic field is swept

Chemical shifts- Arises from applied field inducing secondary fields at the proton by interacting with adjacent bonding electrons- Is measured in ppm relative to an internal standard – TMS- Trimethylsilane (TMS) is the accepted internal standard for calibrating chemical shift for 1H, 13C and 29Si NMR spectroscopy in organic solvents.

H

3.6 ppm

1.2 ppm

Splitting or MultiplicityScalar coupling constants- Protons on adjacent carbons will interact and “split” each others resonances into multiple peaks (multiplets)- Measured in Hz (“Hertz”, s-1) - Caused by different spin states of neighboring spins (parallel or antiparallel)

H

- Equivalent protons do not show spin-spin coupling- Doublet: 1 coupling partner- Triplet: 2 coupling partners- Quartet: 3 coupling partners

When energy in the form of radiofrequency is applied

When applied frequency is equal to processional frequency

Absorption of energy occurs

Nucleus is in resonance

NMR signal is recorded

Steps involved in the process

Instrumentation

• Fourier Transform NMR Instrument

• Continuous Wave NMR Instrument

Two types of NMR spectroscopy instruments

1H NMR Spectroscopy

Study of 1H NMR Spectra

13C NMR Spectroscopy

Interpretation of NMR Spectrum

Number of main signal = number of equivalent protons in unknown compound

Chemical shift indicates the type of H atoms. E.g. methylene, methyl groups, etc.

Spin-spin splitting ---- arrangement of groups in the molecule.

Area of peaks ---- no. of H nuclei present in each group. For e.g. relative areas of methyl peaks in propane would be 6:2. In butane it would be 6:4

E.g. of ethanola) Low resolution spectrum b) High resolution

spectrum

Applications• To study molecular structure and interactions• For determining 3D structure of proteins and

other macromolecules• Solid-state NMR is used to study variety of

materials• Magnetic Resonance Imaging (MRI)• Oil and natural gas exploration • Polymer production, cosmetics and food

manufacturing

ppm

H2O

Protein NMR

MRI

Eliminates risk of X-radiation

Excellent and contrast resolution

Detecting disease at earlier stages

Magnetic resonance imaging (MRI) is a noninvasive medical test that physicians use to diagnose and treat medical conditions.

Pros and Cons of NMR Spectroscopy

Provides high resolution informationDoes not require a protein crystal and is not affected by crystal contactsCan be used to study flexible proteinsReflects conformational averaging

Requires high concentrations of soluble proteinCan not be applied to large proteins (800kD max so far)Can not be used with amyloid fibrils