MOLECULAR ABSORPTION SPECTROSCOPY : THEORY, INSTRUMENTATION & APPLICATION CHAPTER 2

COMPONENTS OF INSTRUMENTS FOR OPTICAL SPECTROSCOPY

General Design of Optical Instruments Absorption

Emission

Five Basic Optical Instrument Components 1) Source – A stable source of radiant energy at the desired wavelength (or  range). 2) Sample Container – A transparent container used to hold the sample (cells, cuvettes, etc).

3) Wavelength Selector – A device that isolates a restricted region of the EM spectrum used for measurement (monochromators, prisms & filters). 4) Detector/Photoelectric Transducer – Converts the radiant energy into a useable signal (usually electrical). 5) Signal Processor & Readout – Amplifies or attenuates the transduced signal and sends it to a readout device as a meter, digital readout, chart recorder, computer, etc.

I. Sources of Radiation • Generate a beam of radiation that is stable and has sufficient power. A. Continuum Sources – emit radiation over a broad wavelength range and the intensity of the radiation changes slowly as a function of wavelength.

This type of source is commonly used in UV, visible and IR instruments. - Deuterium lamp is the most common UV source. - Tungsten lamp is the most common visible source. - Glowing inert solids are common sources for IR instruments.

B. Line Sources – Emit a limited number lines or bands of radiation at specific wavelengths. - Used in atomic absorption spectroscopy - Usually provide radiation in the UV and visible region of the EM spectrum. - Types of line source: 1) Hollow cathode lamps 2) Electrodeless discharge lamps 3) Lasers-Light – amplification by stimulated emission of radiation

II. Wavelength Selectors • Wavelength selectors output a limited, narrow, continuous group of wavelengths called a band. • Two types of wavelength selectors: 1) Filters 2) Monochromators

A. Filters - Two types of filters: 1) Interference Filters 2) Absorption Filters B. Monochromators - Wavelength selector that can continuously scan a broad range of wavelengths - Used in most scanning spectrometers including UV, visible, and IR instruments.

III. Radiation Transducer (Detectors) • Early detectors in spectroscopic instruments were the human eye, photographic plates or films. Modern instruments contain devices that convert the radiation to an electrical signal.

• Two general types of radiation transducers: a. Photon detectors b. Thermal detectors

A. Photon Detectors - Commonly useful in ultraviolet, visible, and near infrared instruments. - Several types of photon detectors are available: 1. Vacuum phototubes 2. Photomultiplier tubes 3. Photovoltaic cells 4. Silicon photodiodes 5. Diode array transducers 6. Photoconductivity transducers

B. Thermal Detectors - Used for infrared spectroscopy because photons in the IR region lack energy to cause photoemission of electrons. - Three types of thermal detectors: 1. Thermocouples 2. Bolometers 3. Pyroelectric transducers

IV. Sample Container • Sample containers, usually called cells or cuvettes must have windows that are transparent in the spectral region of interest. • There are few types of cuvettes: - quartz or fused silica - silicate glass - crystalline sodium chloride Quartz or fused silica - required for UV and may be used in visible region Silicate glass - Cheaper compared to quartz. Used in UV.

Crystalline sodium chloride - Used in IR.

 Spectrometer - is an instrument that provides information about the intensity of radiation as a function of wavelength or frequency.  Spectrophotometer - is a spectrometer equipped with one or more exit slits and photoelectric transducers that pemits the determination of the ratio of the radiant power of two beams as a function of wavelength as in absorption spectroscopy.

SUMMARY Types of source, sample holder and detector for various EM region REGION

SOURCE

SAMPLE HOLDER

DETECTOR

Ultraviolet

Deuterium lamp

Quartz/fused silica

Phototube, PM tube, diode array

Visible

Tungsten lamp

Glass/quartz

Phototube, PM tube, diode array

Infrared

Nernst glower (rare earth oxides or silicon carbide glowers)

Salt crystal e.g. crystal sodium chloride

Thermocouples, bolometers

ULTRAVIOLET-VISIBLE SPECTROSCOPY

In this lecture, you will learn:

• Absorption process in UV/VIS region in terms of its electronic transitions • Molecular species that absorb UV/VIS radiation • Important terminologies in UV/VIS spectroscopy

INSTRUMENTATION Important components in a UV-Vis spectrophotometer 1

2

Source lamp

Sample holder

UV region: -Deuterium lamp; H2 discharge tube

Visible region: - Tungsten lamp

Quartz/fused silica

Glass/quartz

3  selector

Prism/monochromator

Prism/monochromator

4 Detector

Phototube, PM tube, diode array

Phototube, PM tube, diode array

5 Signal processor & readout

Instrumentation • UV-Visible instrument 1. Single beam 2. Double beam

Single beam instrument

• Single beam instrument -

One radiation source Filter/monochromator ( selector) Cells Detector Readout device

Single beam instrument • Disadvantages: – Two separate readings has to be made on the light. This result in some error because the fluctuations in the intensity of the light do occur in the line voltage, the power source and in the light bulb btw measurements. – Changing of wavelength is accompanied by a change in light intensity. Thus spectral scanning is not possible.

Double beam instrument

Double-beam instrument with beams separated in space

• Double-beam instrument

Advantages: 1. Compensate for all but most short-term fluctuations in the radiant output of the source as well as for drift in the transducer and amplifier. 2. Compensate for wide variations in source intensity with . 3. Continuous recording of transmittance or absorbance spectra.

INORGANIC SPECIES

ORGANIC COMPOUNDS

MOLECULAR SPECIES THAT ABSORB UV/VISIBLE RADIATION

CHARGE TRANSFER

Definitions: • Organic compounds – Chemical compound whose molecule contain carbon – E.g. C6H6, C3H4

• Inorganic species – Chemical compound that does not contain carbon. – E.g. transition metal, lanthanide and actinide elements. – Cr, Co, Ni, etc

• Charge transfer – A complex where one species is an electron donor and the other is an electron acceptor. – E.g. iron (III) thiocyanate complex

PERIODIC TABLE OF ELEMENTS

ULTRAVIOLET-VISIBLE SPECTROSCOPY • In UV/VIS spectroscopy, the transitions which result in the absorption of EM radiation in this region are transitions between electronic energy levels.

Molecular absorption • In molecules, not only have electronic level but also consists of vibrational and rotational sub-levels. • This result in band spectra.

Types of transitions • 3 types of electronic transitions - ,  and n electrons - d and f electrons - charge transfer electrons

What is σ,

 and n electrons? single covalent bonds (σ)

H

+

O

+

H

H

O

H

or

H

H

O

lone pairs(n) O

C

O

or

O

O

C

double bonds ()

N

N

or

N

N

triple bond ()

Sigma () electron  Electrons involved in single bonds such as those between carbon and hydrogen in alkanes.  These bonds are called sigma () bonds.  The amount of energy required to excite electrons in  bond is more than UV photons of wavelength. For this reason, alkanes and other saturated compounds (compounds with only single bonds) do not absorb UV radiation and therefore frequently very useful as transparent solvents for the study of other molecules. For example, hexane, C6H14.

Pi () electron • Electrons involved in double and triple bonds (unsaturated). • These bonds involve a pi () bond. • For exampel: alkenes, alkynes,conjugated olefins and aromatic compounds. • Electrons in  bonds are excited relatively easily; these compounds commonly absorb in the UV or visible region.

• Examples of organic molecules containing  bonds. H

CH2CH3 H

H

C

C

C

C

H

ethylbenzene

H

C

C

C

CH3

C

H

propyne

H

H

C

H C

benzene H

H C

C

H H

1,3-butadiene

n electron • Electrons that are not involved in bonding between atoms are called n electrons. • Organic compounds containing nitrogen, oxygen, sulfur or halogens frequently contain electrons that re nonbonding. • Compounds that contain n electrons absorb UV/VIS radiation.

• Examples of organic molecules with nonbonding electrons. : NH2

..

O: C

R

H3C

H

C aminobenzene

.. : Br ..

C H

Carbonyl compound If R = H  aldehyde If R = CnHn  ketone

2-bromopropene

ABSORPTION BY ORGANIC COMPOUNDS

• UV/Vis absorption by organic compounds requires that the energy absorbed corresponds to a jump from occupied orbital to an unoccupied orbital of greater energy. • Generally, the most probable transition is from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).

Electronic energy levels diagram *

Antibonding

Unoccupied levels

n  *

n  *

  *

Antibonding

  *

Energy

*

n

Nonbonding



Bonding



Bonding

Occupied levels

Electronic transitions

Increasing energy

  *

In alkanes

  *

In alkenes, carbonyl compounds, alkynes, azo compounds

n  *

In oxygen, nitrogen, sulfur and halogen compounds

n  *

In carbonyl compounds

Electronic transitions 

* transitions

• The energy required to induce a  * transition is large (see the arrow in energy level diagram). • Never observed in the ordinarily accessible ultraviolet region. • This type of absorption corresponds to breaking of C-C, C-O, C-H, C-X, ….bonds

n

* transitions

- Saturated compounds containing atoms with unshared electron pairs (non-bonding electrons). - Compounds containing O, S, N and halogens can absorb via this type of transition. - Absorption are typically in the  range, 150 - 250 nm region and are not very intense. -  range: 100 – 3000 cm-1mol-1 - Absorption maxima tend to shift to shorter  in polar solvents. e.g. H2O, CH3CH2OH

Some examples of absorption due to n * transitions max (nm)

max

H2O

167

1480

CH3OH

184

150

CH3Cl

173

200

CH3I

258

365

(CH3)2O

184

2520

CH3NH2

215

600

Compound

n

* transitions

- Unsaturated compounds containing atoms with unshared electron pairs (nonbonding electrons) - These result in some of the most intense absorption in  range, 200 – 700 nm - Unsaturated functional group - to provide the  orbitals -  range: 10 – 100 Lcm-1mol-1



* transitions

- Compounds with unsaturated functional groups to provide the  orbitals. - These result in some of the most intense absorption in  range, 200 – 700 nm -  range: 1000 – 10,000 Lcm-1mol-1

* and 

Examples n H H

C H

O C H

  * at 180 nm n  * at 290 nm

*

MOLECULAR SPECIES THAT ABSORB UV/VISIBLE RADIATION (A) Absorption by organic compounds 2 types of electrons are responsible: i. Shared electrons that participate directly in bond formation ( and  bonding electrons) ii. Unshared outer electrons (nonbonding or n electrons)

Absorption by organic compounds • The shared electrons in single bonds, C-C or C-H ( electrons) are so firmly held. Therefore, not easily excited to higher E levels. Absorption (  *) occurs only in the vacuum UV region (  180 nm). • Electrons in double & triple bonds (electrons) are more loosely held. Therefore, more easily excited by radiation. Absorptions (   *) for species with unsaturated bonds occur in the UV/VIS region (   180 nm)

Absorption by organic compounds

CHROMOPHORES Unsaturated organic functional groups that absorb in the UV/VIS region.

Typical organic functional groups that serve as chromophores Chromophores

Chemical structure

Type of transition

Acetylenic

-CC-

  *

Amide

-CONH2

  *, n  *

Carbonyl

>C=O

  *, n  *

Carboxylic acid

-COOH

  *, n  *

Ester

-COOR

  *, n  *

Nitro

-NO2

  *, n  *

Olefin

>C=C