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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
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