BY

G. RAM KUMAR

Department of Chemistry

Pydah College P.G. Courses

INTRODUCTION TO SPECTROSCOPY

•It is the most powerful tool available for the study of atomic and molecular structures.

•It is used in the analysis of wide range of samples.

What is spectroscopy?

Spectroscopy may be defined as the study of interactions between the matter and Electro Magnetic Radiations.

TYPES OF SPECTROSCOPY

•Atomic Spectroscopy : It is concerned with interactions of Electro Magnetic Radiations with atoms.

•Molecular Spectroscopy : It is concerned with interactions of

Electro Magnetic Radiations with molecules .

In general all the Spectroscopic methods are classified into two.

•Non-Destructive methods Ex:UV-Visible , IR , NMR.

•Destructive methods Ex:FES , AAS , ICP-MS.

ELECTROMAGNETIC RADIAIONS

Electromagnetic radiation is an oscillating electric and magnetic disturbance that spreads as a wave through empty space, the vacuum

CHARACTERISTICS OF ELECTROMAGNETIC RADIATIONS

•Wave length(λ): It is the distance between two crests or troughs.

Units: Å Angstroms or Nanometers nm

1 Å=10-8cm=10-10m

1nm=10 Å=10-9cm

• Frequency(υ): It is defined as the number of waves which can pass through a point in one second.

Units: Hertz(Hz); Fresnel

1Hz = 1cycle/sec; 1Fresnel = 1012 Hz

•Wave Number(ΰ): It is the reciprocal of wave length and is defined as the total number of waves which can pass through a space of 1cm.

Units: cm-1 or m-1

1cm-1 is sometimes called 1KAYSER(K)

•Velocity(v): It is the distance travelled by a wave in 1sec.

Units: cm/sec or m/sec

DISPERSION OF LIGHT

When visible light (White light) is passed through prism, it split up into seven colors which corresponds to definite wave lengths.This is known as dispersion of light

Visible and Ultraviolet Spectroscopy

SPECTRUM:

The series of color bands obtained by dispersion of light is known as spectrum.

In a spectrum all these bands are arranged in the increasing order of wave length

CLASSIFICATION OF SPECTRA:

I 1.LINE SPECTRA : Spectrum obtained by interaction of electromagnetic radiations with atoms. Line spectra consists of sharp well defined lines that correspond to a definite frequency

2.BAND SPECTRA: Spectrum obtained by interaction of electromagnetic radiations with molecules.Band spectra consists of different bands just as visible spectrum.

II

CONTINUOUS SPECTRUM:

In a spectrum, if one color merges into another without a gap, then the spectrum is known as Continuous spectrum.

DISCONTINUOUS SPECTRUM:

In a spectrum , if one color will not merge into other is known as Discontinuous spectrum.

III

ABSORPTION SPECTRUM:

If the electromagnetic radiations are passed through a substance, the dark pattern of lines that are obtained corresponding to wave lengths absorbed is known as Absorption spectrum .

EMISSION SPECTRUM:

If electromagnetic radiations are passed through a substance, the pattern of lines recorded after the emission of the absorbed wave lengths are known as emission spectrum.

ELECTROMAGNETIC SPECTRUM

The arrangement of electromagnetic waves or radiations in the order of their increasing wave lengths or decreasing frequencies is called electromagnetic spectrum.

Interactions of Different types of Electromagnetic Radiations produce different types of Spectroscopy

Regions of EM spectrum

Radio frequency region

Microwave region

Infrared region

UV & Visible region

Type of sepctroscopy

Nuclear magnetic resonance or electron spin resonance

Rotational spectroscopy

Vibrational spectroscopy

Electronic spectroscopy

X-ray: core electron excitation

UV: valance electronic excitation

IR: molecular vibrations

Radio waves:Nuclear spin states(in a magnetic field)

Electronic Excitation by UV/Vis Spectroscopy :

The wavelength and amount of light that a compound absorbs depends on

its molecular structure and the concentration of the compound used.

The concentration dependence follows Beer’s Law.

A=bcWhere A is absorbance (no units, since A = log10 P0 / P ) is the molar absorbtivity with units of L mol-1 cm-1 b is the path length of the sample - that is, the path length of the cuvette in which the sample is contained (typically in cm). c is the concentration of the compound in solution, expressed in mol L-1

Molecules have quantized energy levels:

ex. electronic energy levels. en

ergy

hv

ener

gy

}= hv

Q: Where do these quantized energy levels come from?A: The electronic configurations of associated with bonding.

Each electronic energy level (configuration) has associated with it the many vibrational energy levels we examined with IR.

C C

hv

C CH

HH H

HH

max = 135 nm (a high energy transition)

Absorptions having max < 200 nm are difficult to observe because everything (including quartz glass and air) absorbs in this spectral region.

C C

hv

Example: ethylene absorbs at longer wavelengths:max = 165 nm = 10,000

= hv =hc/

hv

n

n

C O

n

The n to pi* transition is at even lower wavelengths but is not as strong as pi to pi* transitions. It is said to be “forbidden.”Example:

Acetone: nmax = 188 nm ; = 1860nmax = 279 nm ; = 15

C C

C C

C O

C OH

135 nm

165 nm

n183 nm weak

150 nmn188 nmn279 nm weak

A

180 nm

279 nm

C O

C C

HOMO

LUMO

Conjugated systems:

Preferred transition is between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO).

Note: Additional conjugation (double bonds) lowers the HOMO-LUMO energy gap:Example:

1,3 butadiene: max = 217 nm ; = 21,0001,3,5-hexatriene max = 258 nm ; = 35,000

O

O

O

Similar structures have similar UV spectra:

max = 238, 305 nm max = 240, 311 nm max = 173, 192 nm

Woodward-Fieser Rules for Dienes  Homoannular HeteroannularParent=253 nm=214 nm

=217 (acyclic)Increments for:Double bond extending conjugation +30  Alkyl substituent or ring residue +5  Exocyclic double bond    +5Polar groupings:  -OC(O)CH3    +0  -OR    +6  -Cl, -Br   +5  -NR2 +60  -SR +30

CC C

C

Homoannular heteroannular acyclic exocyclic

For more than 4 conjugated double bonds: max = 114 + 5(# of alkyl groups) + n(48.0-1.7n)

O

Parent: 214 (heteroannular)

3 alkyls +15 (3x5)

+5 (exocyclic)

TOTAL 234 nm

(Actual = 235 nm)

Parent: 253 (homoannular)

3 alkyls +15 (3x5)

+5 (exocyclic)

TOTAL 273 nm

(Actual = 275 nm)

O

HOParent: 202 (5-member ring ketone)

+35 (alpha hydroxyl)

+12 (beta alkyl - note part of ring)

Total: 249

Lycopene:

max = 114 + 5(8) + 11*(48.0-1.7*11) = 476 nm

max(Actual) = 474.