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Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation as a function of frequency or wavelength due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a function of frequency, and this variation is the called as absorption spectrum. Absorption spectroscopy is performed across the electromagnetic spectrum.

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When continuous radiation passes through a transparent material, a

portion of the radiation may be absorbed. If that is passed through a

prism, yields a spectrum, called as absorption spectrum.

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Types of Energy transitions in each

region of the electromagnetic spectrum

Region of Spectrum Energy Transitions

X Rays Bond Breaking

UltraViolet Electronic

Infrared Vibrational

Microwave Rotational

Radiofrequencies Nuclear Spin (Nuclear Magnetic

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Infrared spectroscopy (Vibrational Spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy.

• The instrument used to measure the Infrared radiations is called as

infrared spectrometer.

• Only those bonds that have a dipole moment that change with respect to time are capable of absorbing in the infrared portion of electromagnetic spectrum.

• The frequency () depends on the energy gap between vibrational levels

• E = h= hc/(cm-1)

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Fingerprinting technique: Functional groups present in a

molecule can be deduced from an IR spectrum, Almost

any compound having covalent bonds, whether organic or

inorganic, absorbs various frequencies of electromagnetic

radiation in the infrared region of the electromagnetic

spectrum.

Provides information about the vibrations of functional

groups in a molecule

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• A basic IR spectrum is essentially a graph of infrared light absorbance (or transmittance) on the vertical axis vs. frequency or wavelength on the horizontal axis. Typical units of frequency used in IR spectra are reciprocal centimetres. (Wave numbers, cm−1)

• Units of IR wavelength are commonly given in

micrometers (formerly called "microns"), symbol μm

The laboratory instrument that uses this technique is a Fourier transform Infrared Spectrometer (FTIR).

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Francis A. Carey, Organic Chemistry,Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.

2000

3500 3000 2500 1500 1000 500

Wave number, cm-1

Infrared Spectrum of 2-Hexanol

H—C

O—H

OH

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A molecule to be "IR active", it must be associated with changes in the dipole.

• A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules with N number of atoms in them, linear molecules have 3N – 5 degrees of vibrational modes, whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called vibrational degrees of freedom).

• For example H2O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.

• Simple diatomic molecules have only one bond and only one vibrational band. If the molecule is symmetrical, e.g. N2, the band is not observed in the IR spectrum

• Asymmetrical diatomic molecules, e.g. CO, absorb in the IR spectrum.

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The absorption intensity depends on how efficiently the energy of an electromagnetic wave of frequency  can be

transferred to the atoms involved in the vibration

The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon

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Dipole Moment Must Change during

for a vibration to be “IR active”!

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

1

2

k

m

r



m

r

m

1

m

2

m

1

m

2

Where,

=frequency,

k = Force constant (bond strength)

m

r

= reduced mass (~ mass of largest

atom)

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Directly on the strength of the bonding

between the two atoms (

~ k)

Inversely on the reduced mass of the two

atoms (v ~ 1/m)

Expect:

will increase with increasing bond

strength (bond order) and decreasing mass

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region of infrared that is most useful lies between

2.5-16

m

m (4000-625 cm

-1

)

depends on transitions between vibrational

energy states

stretching

bending

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Stretching Vibrations of a CH

2

Group

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Bending Vibrations of a CH

2

Group

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Bending Vibrations of a CH

2

Group

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Structural unit

Frequency, cm

-1

Stretching vibrations (carbonyl groups)

Aldehydes and ketones 1710-1750

Carboxylic acids

1700-1725

Acid anhydrides

1800-1850 and 1740-1790

Esters

1730-1750

Amides

1680-1700

Infrared Absorption Frequencies

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Structural unit

Frequency, cm

-1

Bending vibrations of derivatives of benzene

Monosubstituted

730-770 and 690-710

Ortho-disubstituted

735-770

Meta-disubstituted

750-810 and 680-730

Para-disubstituted

790-840

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Structural unit

Frequency, cm

-1

Stretching vibrations (single bonds)

O—H (alcohols)

3200-3600

O—H (carboxylic acids)

3000-3100

N—H

3350-3500

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Ultra Violet Spectroscopy

Ultraviolet–visible spectroscopy (UV-Vis) refers to absorption spectroscopy in the ultra violet spectral region, As a Molecule absorbs energy, an electron is promoted from an occupied orbital to an unoccupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).

The Π orbitals lie at somewhat higher energy levels, and orbitals that hold

unshared pairs, the non bonding (n) orbitals, lie at even higher energies. The

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Electronic Transitions

Molecules containing π-electrons or non-bonding electrons

(n-electrons) can absorb the energy in the form of ultraviolet or visible

light to excite these electrons to higher anti-bonding molecular

orbitals.

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The Beer Lambert Law states that the absorbance of a solution is directly proportional to the concentration of the absorbing species in the solution and the path length.

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Thus, for a fixed path length, UV/Vis spectroscopy can be used to determine the concentration of the absorber in a solution.

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Applications of UV Spectroscopy

UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as transition metal ions, highly conjugated compounds and biological macromolecules.

Solutions of transition metal ions can be coloured (i.e., absorb visible light) because d electrons within the metal atoms can be excited from one electronic state to another. The colour of metal ion solutions is strongly affected by the presence of other species, such as certain anions/ligands (λmax).

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