S.M.Shazzad RahmanLecturer, Textile

Engineering DepartmentNorthern University

BangladeshCourse Title: ChemistryCourse Code:

CH1201,CH1101

Spectroscopy

What is Spectroscopy

The study of structure and properties of atoms and molecule by means of the spectral information obtained from the interaction of electromagnetic radiant energy with matter

It is the base on which a main class of instrumental analysis and methods is developed & widely used in many areas of modern science

What to be discussed

◦ Theoretical background of spectroscopy◦ Types of spectroscopy and their working principles in brief ◦ Major components of common spectroscopic instruments◦ Applications in Chemistry related areas and some

examples

Electromagnetic radiation (e.m.r.) ◦ Electromagnetic radiation is a form of energy◦ Wave-particle duality of electromagnetic radiation

Wave nature - expressed in term of frequency, wave-length and velocity Particle nature - expressed in terms of individual photon, discrete packet of energy

when expressing energy carried by a photon, we need to know the its frequency

Characteristics of wave ◦ Frequency, v - number of oscillations per unit time, unit: hertz (Hz) - cycle per

second◦ velocity, c - the speed of propagation, for e.m.r c=2.9979 x 108 m×s-1 (in vacuum)◦ wave-length, l - the distance between adjacent crests of the wave

wave number, v’, - the number of waves per unit distance v’ =l-1

The energy carried by an e.m.r. or a photon is directly proportional

to the frequency, i.e. where h is Planck’s constant h=6.626x10-34J×s

Electromagnetic Radiation

Electromagnetic radiation

X-ray, light, infra-red, microwave and radio waves are all e.m.r.’s, difference being their frequency thus the amount of energy they possess

Spectral region of e.m.r.

Electromagnetic Radiation

Interaction of electromagnetic radiant with matter◦ The wave-length, l, and the wave number, v’, of e.m.r. changes

with the medium it travels through, because of the refractive index of the medium; the frequency, v, however, remains unchanged

◦ Types of interactions

Absorption Reflection Transmission Scattering Refraction

◦ Each interaction can disclose certain properties of the matter

◦ When applying e.m.r. of different frequency (thus the energy e.m.r. carried) different type information can be obtained

Interaction of e.m.r. with Matter

Spectrum is the display of the energy level of e.m.r. as a function of wave number of electromagnetic radiation energy

The energy level of e.m.r. is usually expressed in one of these terms

◦ absorbance (e.m.r. being absorbed)

◦ transmission (e.m.r. passed through)

◦ Intensity

The term ‘intensity’ has the meaning of the radiant power that carried by an e.m. r.

Spectrum

1.0

0.5

0.0350 400 450

wave length cm-1

inte

nsi

ty

What an spectrum tells

◦ A peak (it can also be a valley depending on how the spectrum is constructed) represents the absorption or emission of e.m.r. at that specific wavenumber

The wavenumber at the tip of peak is the most important, especially when a peak is broad

A broad peak may sometimes consist of several peaks partially overlapped each other - mathematic software (usually supplied) must be used to separate them case of a broad peak (or a valley) observed

The height of a peak corresponds the amount absorption/emission thus can be used as a quantitative information (e.g. concentration), a careful calibration is usually required

The ratio in intensity of different peaks does not necessarily means the ratio of the quantity (e.g. concentration, population of a state etc.)

Spectrum

1.0

0.5

0.0350 400 450

wave length cm-1

inte

nsi

ty

Spectral properties, applications, and interactions of electromagnetic radiation

absorptionemission

fluorescence

Magneticallyinduced spin

states

Electronparamagnetresonance

Infrared

Wave numberv’

cm-1

Wavelengthl

cm

Frequencyv

Hz

Energy

kcal/molElectronvole eV

Type of radiation

Type of spectroscopy

Type of quantum transition

9.4x107 4.1x106 3.3x1010 3.0x10-11 1021

9.4x105 4.1x104 3.3x108 3.0x10-9 1019

9.4x103 4.1x102 3.3x106 3.0x10-7 1017

9.4x101 4.1x100 3.3x104 3.0x10-5 1015

9.4x10-1 4.1x10-2 3.3x102 3.0x10-3 1013

9.4x10-3 4.1x10-4 3.3x100 3.0x10-1 1011

9.4x10-5 4.1x10-6 3.3x10-2 3.0x101 109

9.4x10-7 4.1x10-8 3.3x10-4 3.0x103 107

Gamma ray

X-ray

Ultra Violet

Visible

Microwave

Radio

X-rayabsorption emission

NuclearGamma ray

emission

Electronic(outer shell)

Molecularrotation

Molecularvibration

Nuclear magneticresonance

Microwaveabsorption

UV absorption

IR absorptionRaman

VacUVVis

Electronic(inner shell)

1. A laser emits light with a frequency of 4.69x1014 s-1. (h = 6.63 x 10-

34Js)A) What is the energy of one photon of the radiation from this laser in kcal? B) If the laser emits 1.3x10-5J during a pulse, how many photons are

emitted during the pulse?

Ans: A) Ephoton = 3.11 x 10-22 kJ

B) No. of photons = 4.2x1013

2. The brilliant red colours seen in fireworks are due to the emission of red light at a wave length of 650nm. What is the energy of one photon of this light? (h = 6.63 x 10-34Js)

Ans: Ephoton = 3.06x10-19J

3: Compare the energies of photons emitted by two radio stations, operating at 92 MHz (FM) and 1500 kHz (MW)?

Ans: Ephoton = 6.1 x 10-26J, 9.9 x 10-28J

Problems

Shell structure & energy level of atoms

◦ In an atom there are a number of shells and of subshells where e-’s can be found

◦ The energy level of each shell & subshell are different and quantised

The e-’s in the shell closest to the nuclei has the lowest energy. The higher shell number is, the higher energy it is

The exact energy level of each shell and subshell varies with substance

Ground state and excited state of e-’s

◦ Under normal situation an e- stays at the lowest possible shell - the e- is said to be at its ground state

◦ Upon absorbing energy (excited), an e- can change its orbital to a higher one - we say the e- is at is excited state.

Atomic Spectra

Electron excitation◦ The excitation can occur at

different degrees low E tends to excite the outmost

e-’s first when excited with a high E (photon

of high v) an e- can jump more than one levels

even higher E can tear inner e-’s away from nuclei

◦ An e- at its excited state is not stable and tends to return its ground state

◦ If an e- jumped more than one energy levels because of absorption of a high E, the process of the e- returning to its ground state may take several steps, - i.e. to the nearest low energy level first then down to next …

Atomic Spectra

Atomic spectra◦ The level and quantities of energy

supplied to excite e-’s can be measured & studied in terms of the frequency and the intensity of an e.m.r. - the absorption spectroscopy

◦ The level and quantities of energy emitted by excited e-’s, as they return to their ground state, can be measured & studied by means of the emission spectroscopy

◦ The level & quantities of energy absorbed or emitted (v & intensity of e.m.r.) are specific for a substance

◦ Atomic spectra are mostly in UV (sometime in visible) regions

Atomic Spectra

Motion & energy of molecules◦ Molecules are vibrating and rotating all

the time, two main vibration modes being

stretching - change in bond length (higher v)

bending - change in bond angle (lower v) (other possible complex types of stretching & bending are: scissoring / rocking / twisting

◦ Molecules are normally at their ground state (S0)

S (Singlet) - two e-’s spin in pair E

T (Triplet) - two e-’s spin parallel J

◦ Upon exciting molecules can change to high E states (S1, S2, T1 etc.), which are associated with specific levels of energy

◦ The change from high E states to low ones can be stimulated by absorbing a photon; the change from low to high E states may result in photon emission

Molecular Spectra

S0

T1

S2

S1

v1

v2

v3

v4

v1

v2

v3

v4

v1

v2

v3

v4

v1

v2

v3

v4

Excitation of a molecule◦ The energy levels of a molecule at each

state / sub-state are quantised ◦ To excite a molecule from its ground

state (S0) to a higher E state (S1, S2, T1 etc.), the exact amount of energy equal to the difference between the two states has to be absorbed. (Process A)

i.e. to excite a molecule from S0,v1 to S2,v2, e.m.r with wavenumber v’ must be used

◦ The values of energy levels vary with the (molecule of) substance.

◦ Molecular absorption spectra are the measure of the amount of e.m.r., at a specific wavenumber, absorbed by a substance.

Molecular Spectra

v1

v2

v3

v4

S0

T1

S2

S1

v1

v2

v3

v4

v1

v2

v3

v4

v1

v2

v3

v4

absorptionA

A

Energy change of excited molecules

An excited molecules can lose its excess energy via several processes

◦ Process B - Releasing E as heat when changing from a sub-state to the parental state occurs within the same state

◦ The remaining energy can be release by one of following Processes (C, D & E)

◦ Process C - Transfer its remaining E to other chemical species by collision

◦ Process D - Emitting photons when falling back to the ground state - Fluorescence

◦ Process E1 - Undergoing internal transition within the same mode of the excited state

◦ Process E2 - Undergoing intersystem crossing to a triplet sublevel of the excited state

◦ Process F - Radiating E from triplet to ground state (triplet quenching) - Phosphorescence

Molecular Spectra

S0

T1

S2

S1

v1

v2

v3

v4

v1

v2

v3

v4

v1

v2

v3

v4

v1

v2

v3

v4

Inter- systemcrossing

Internaltransition

B

B

E1

E2

C

F

A

B

Fluorescence

D

Phosphorescence

Jablonsky diagram

Two types of molecular emission spectra

◦ Fluorescence In the case fluorescence the energy emitted

can be the same or smaller (if heat is released before radiation) than the corresponding molecular absorption spectra.

e.g. adsorption in UV region - emission in UV or visible region (the wavelength of visible region is longer than that of UV thus less energy)

Fluorescence can also occur in atomic adsorption spectra

Fluorescence emission is generally short-lived (e.g. ms)

◦ Phosphorescence

Phosphorescence generally takes much longer to complete (called metastable) than fluorescence because of the transition from triplet state to ground state involves altering the e-’s spin. If the emission is in visible light region, the light of excited material fades away gradually

Molecular Spectra

S0

S2

v1

v2

v3

v4

v1

v2

v3

v4

B

Aphosphor-enscence

D

Fluore-scence

T1

v1

v2

v3

v4

F

Comparison of atomic and molecular spectra

Atomic Spectra & Molecular Spectra

Quantum mechanics is the basis of atomic & molecular spectra

The transitional, rotational and vibrational modes of motion of objects of atomic / molecular level are well-explained.

UV & Visible Spectrophotometry

Observations

When a light of intensity I0 goes through a liquid of concentration C & layer thickness b

◦ The emergent light, I, has less intensity than the incident light I0 scattering, reflection absorption by liquid

◦ There are different levels of reduction in light intensity at different wavelength detect by eye - colour change detect by instrument

The method used to measure UV & visible light absorption is called spectrophotometry

(colourimetry refers to the measurement of absorption of light in visible region only)

ultraviolet visible infra-red

200 - 400 400 - 800 800 - 15nm nm nm nm nm mm

Incident light, I0

(UV or visible)Emergent light, I

C

b

Theory of light absorption

Quantitative observation◦ The thicker the cuvette

- more diminishing of light in intensity

◦ Higher concentration the liquid- the less the emergent light intensity

These observations are summarised by Beer’s Law:

Successive increments in the number of identical absorbing molecules in the path of a beam of monochromatic radiation absorb equal fraction of the radiation power travel through them

Thus

UV & Visible Spectrophotometry

I0

dx

bx

s

sI

light absorbed

I'kdxNcs

dI

2

fraction of light

number of moleculesN-Avogadro number

acdxdxNcs'kI

dI 2

acbI

Idxac

I

dI bbI

I

b 0

0ln

0

AabcI

I 0log

Absorbance

Terms, units and symbols for use with Beer’s Law

Name alternative name symbol definition unit

Path length - b (or l) - cm

Liquid concentration - c - mol / L

Transmittance Transmission T I / I0 -

Percent transmittance - T% 100x I / I0 %

Absorbance Optical density, A log(I / I0) -

extinction

Absorptivity Extinction coeff., a (or e, k) A/(bc) [bc]-1

absorbance index

Molar absorptivity Molar extinction coeff., a A/(bc)molar absorbancy index [or aM AM/(bc’) ] M-molar weight

c’ -gram/L

UV & Visible Spectrophotometry

Use of Beer’s Law Beer’s law can be applied to the absorption of UV, visible, infra-red & microwave

The limitations of the Beer’s Law◦ Effect of solvent - Solvents may absorb light to a various extent,

e.g. the following solvents absorb more than 50% of the UV light going through them

180-195nm sulphuric acid (96%), water, acetonitrile

200-210nm cyclopentane, n-hexane, glycerol, methanol, ethanol

210-220nm n-butyl alcohol, isopropyl alcohol, cyclohexane, ethyl ether

245-260nm chloroform, ethyl acetate, methyl formate

265-275nm carbon tetrachloride, dimethyl sulphoxide/formamide, acetic acid

280-290nm benzene, toluene, m-xylene

300-400nm pyridine, acetone, carbon disulphide

◦ Effect of temperature Varying temperature may cause change of concentration of a solute because of

thermal expansion of solution changing of equilibrium composition if solution is in equilibrium

UV & Visible Spectrophotometry

What occur to a molecule when absorbing UV-visible photon?

◦ A UV-visible photon (ca. 200-700nm) promotes a bonding or non-bonding electron into antibonding orbital - the so called electronic transition

Bonding e-’s appear in s & p molecular orbitals; non-bonding in n

Antibonding orbitals correspond to the bonding ones

e-’s transition can occur between variousstates; in general, the energy of e-’stransition increases in the following order: (n®p*) < (n®s*) < (p ®p*) < (s ®s*)

Molecules which can be analysed by UV-visible absorption◦ Chromophores

functional groups each of which absorbs a characteristic UV or visible radiation.

UV & Visible Spectrophotometry

*

*

n

Antibonding Antibonding

non-bonding

Bonding

Energ

y

*

*

n *

n *

The functional groups & the wavelength of UV-visible absorption

Group Example lmax, nm Group Example lmax, nm

C=C 1-octane180 arene benzene 260

naphthalene 280

C=O methanol 290 phenenthrene 350

propanone 280 anthracene 375

ethanoic acid 210 pentacene 575

ethyl ethanoate 210

ethanamide 220 conjugated 1,3-butadiene 220

1,3,5-hexatriene 250

C-X methanol 180 2-propenal 320

trimethylamine 200 b-carotene (11 C=C) 480

chloromethane 170

bromomethane 210 each additional C=C +30

iodomethane 260

UV & Visible Spectrophotometry

Instrumentation

UV visible

Light source Hydrogen discharge lamp Tungsten-halogen lamp

Cuvette QUARTZ glass

Detectors photomultiplier photomultiplier

UV & Visible Spectrophotometry

Applications

◦ Analysis of unknowns using Beer’s Law calibration curve

◦ Absorbance vs. time graphs for kinetics

◦ Single-point calibration for an equilibrium constant determination

◦ Spectrophotometric titrations – a way to follow a reaction if at least one substance is colored – sudden or sharp change in absorbance at equivalence point

UV & Visible Spectrophotometry

Atoms in a molecule are constantly in motion◦ There are two main vibrational modes:

Stretching - (symmetrical/asymmetrical) change in bond length - high frequency

Bending - (scissoring/stretch/rocking/twisting) change in bond angle - low freq.

◦ The rotation and vibration of bonds occur in specific frequencies Every type of bond has a natural frequency of vibration, depending

on the mass of bonded atoms (lighter atoms vibrate at higher frequencies) the stiffness of bond (stiffer bonds vibrate at higher frequencies) the force constant of bond (electronegativity) the geometry of atoms in molecule

The same bond in different compounds has a slightly different vibration frequencies.

Functional groups have characteristic stretching frequencies.

IR-Spectroscopy

IR region◦ The part of electromagnetic radiation between the visible and microwave regions 0.8

m to 50 m (12,500 cm-1-200 cm-1).

◦ Most interested region in Infrared Spectroscopy is between 2.5m-25 m (4,000cm-1-400cm-1), which corresponds to vibrational frequency of molecules

Interaction of IR with molecules◦ Only molecules containing covalent bonds with dipole moments are infrared

sensitive

◦ Only the infrared radiation with the frequencies matching the natural vibrational frequencies of a bond (the energy states of a molecule are quantitised) is absorbed

◦ Absorption of infrared radiation by a molecule rises the energy state of the molecule increasing the amplitude of the molecular rotation & vibration of the covalent bonds

Rotation - Less than 100 cm-1 (not included in normal Infrared Spectroscopy)

Vibration - 10,000 cm-1 to 100 cm-1

◦ The energy changes through infrared radiation absorption is in the range of 8-40 KJ/mol

IR-Spectroscopy

Use of Infra-Red spectroscopy

◦ IR spectroscopy can be used to distinguish one compound from another. No two molecules of different structure will have exactly the same natural

frequency of vibration, each will have a unique infrared absorption spectrum.

A fingerprinting type of IR spectral library can be established to distinguish a compounds or to detect the presence of certain functional groups in a molecule.

◦ Obtaining structural information about a molecule Absorption of IR energy by organic compounds will occur in a manner

characteristic of the types of bonds and atoms in the functional groups present in the compound

Practically, examining each region (wave number) of the IR spectrum allows one identifying the functional groups that are present and assignment of structure when combined with molecular formula information.

◦ The known structure information is summarized in the Correlation Chart

IR-Spectroscopy

Atomic absorption/emission spectroscopes involve e-’s changing energy states

Most useful in quantitative analysis of elements, especially metals

These spectroscopes are usually carried out in optical means, involving

conversion of compounds/elements to gaseous atoms by atomisation. Atomization is the most critical step in flame spectroscopy. Often limits the precision of these methods.

excitation of electrons of atoms through heating or X-ray bombardment

UV/vis absorption, emission or fluorescence of atomic species in vapor is measured

Instrument easy to tune and operate

Sample preparation is simple (often involving only dissolution in an acid)

Atomic Absorption/Emission Spectroscopy

Source: R. Thomas, “Choosing the Right Trace Element Technique,” Today’s Chemist at Work, Oct. 1999, 42.

Atomic Absorption Spectrometer (AA)

Source

Sample

P P0

Chopper

Wavelength Selector

Detector Signal ProcessorReadout

Type Method of Atomization

Radiation Source

atomic (flame) sample solution aspirated Hollow cathode into a flame lamp (HCL)

atomic (nonflame) sample solution HCL

evaporated & ignited

x-ray absorption none required x-ray tube

Atomic Emission Spectrometer (AES)

Source

Sample

P Wavelength Selector

Detector Signal ProcessorReadout

Type Method of Atomization Radiation Source

arc sample heated in an electric arc sample

spark sample excited in a high voltage spark sample

argon plasma sample heated in an argon plasma sample

flame sample solution aspirated into a flame sample

x-ray emission none required; sample

bombarded w/ e- sample

Atomic Fluorescence Spectrometer (AFS)

Source

Sample

P P0

Chopper

90o

Wavelength Selector

Detector Signal ProcessorReadout

Type Method of Atomization

Radiation Source

atomic (flame) sample solution aspirated into a flame sample

atomic (nonflame) sample solution sample

evaporated & ignited

x-ray fluorescence none required sample