ULTRAVIOLET SPECTROSCOPY TERM PAPER 12

8/7/2019 ULTRAVIOLET SPECTROSCOPY TERM PAPER 12

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!!!.TERM PAPER OF CHEMISTRY.!!!

.NAME UPKAR SINGH LODHI

.ROLL NUMBER- RB6005B57

.SECTION - B6005

.REG. NUMBER-11011735

.TOPIC- ULTRAVIOLET SPECTROSCOPY.

.SUBMITTED TO Dr. NISHA SAXENA.


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Acknowledgement

I would like to thank all those who encouraged me to do this project . I thanks

to Mrs. NISHA SAXENA who helped me a lot in editing the contents of this

report by making necessary conditions. I am also extremely thankful to my

friends in providing me with the latest knowledge regarding the report.

Their immense help and suggestions for improving the con tents of the report

are highly appreciable. I also thanks to my parents and brother for theirpatience and support extended to me all times.

I also gratefully acknowledge the valuable contribution of many academics for

the editing and finalization of this report. The contribution of the publication

department in bringing out this report is also duly acknowledged.


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ULTRAVIOLET SPECTROSCOPY

INTRODUCTION

Spectroscopy was originally the study of the

interaction between radiation and matter as a

function of wavelength (). Historically,

spectroscopy referred to the use of visible

lightdispersed according to its wavelength,

e.g. by prism. Later the concept was expanded

greatly to comprise any measurement of a

quantity as a function of either wavelength of

frequency. Thus, it also can refer to a

response to an alternating field or varying

frequency (). A further extension of the

scope of the definition added energy(E) as a

variable, once the very close relationship E =

h for photonwas realized (h is theplank

constant). A plot of the response as a function

of wavelengthor more commonly

frequencyis referred to as aspectrum; see

alsospectral linewidth.

Spectrometry is the spectroscopic technique

used to assess the concentration or amount of

a given chemical (atomic, molecular, or ionic)

species. In this case, the instrument that

performs such measurements is

aspectrometer, spectrophotometer, or

spectrograph.

Spectroscopy/spectrometry is often used in

physical and analytical chemistry for the

identification of substances through the

spectrum emitted from or absorbed by them.

Spectroscopy/spectrometry is also heavily

used in astronomy and remote sensing. Most

large telescope have spectrometers, whichare used either to measure the chemical

composition and physical properties of

astronomical objects or to measure their

velocities from the Doppler shift of their

spectral lines.

Classificationofmethods

1.Nature of excitataion measured

2.Measurement process

1. Natureofexcitationmeasured

The type of spectroscopy depends on the

physical quantity measured. Normally, the

quantity that is measured is an intensity, of

energy either absorbed or produced.

y Electromagnetic spectroscopy

involves interactions of matter with

electromagnetic, such as light.y Electron specroscopy involves

interactions with electron beams.

Auger spectroscopy involves inducing

the Auger effect with an electron

beam. In this case the measurement

typically involves the kinetic energy of

the electron as variable.

y Acoustic spectroscopy involves the

frequency of sound.

y Dielectric spectroscopy involves the

frequency of an external electrical

fieldy Mechanical spectroscopy involves the

frequency of an external mechanical

stress, e.g. a torsion applied to a piece

of material.


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2. Measurement process

Most spectroscopic methods are

differentiated as either atomic or molecular

based on whether or not they apply to atoms

or molecules. Along with that distinction, they

can be classified on the nature of their

interaction:

y Absorption spectroscopy uses the

range of the electromagnetic spectra

in which a substance absorbs. This

includes atomic absorption

spectroscopy and various molecular

techniques, such as infrared,

ultraviolet-visible and microwave

spectroscopy

y Emission spectroscopyuses the range

of electromagnetic spectra in which asubstance radiates (emits). The

substance first must absorb energy.

This energy can be from a variety of

sources, which determines the name

of the subsequent emission, like

luminescence. Molecular

luminescence techniques include

spectrofluorimetry.

y Scattering spectroscopy measures the

amount of light that a substance

scatters at certain wavelengths,

incident angles, and polarizationangles. One of the most useful

applications of light scattering

spectroscopy is Raman spectroscopy

COMMON TYPE OF

SPECTROSCOPY

Absorption

Absorption spectroscopy is a technique in

which the power of a beam of light measured

before and after interaction with a sample is

compared. Specific absorption techniques

tend to be referred to by the wavelength of

radiation measured such as ultraviolet,

infrared or microwave absorption

spectroscopy. Absorption occurs when the

energy of the photons matches the energy

difference between two states of the

material.

X-ray

When X-rays of sufficient frequency (energy)

interact with a substance, inner shell

electrons in the atom are excited to outer

empty orbitals, or they may be removedcompletely, ionizing the atom. The inner shell

"hole" will then be filled by electrons from

outer orbitals. The energy available in this de-

excitation process is emitted as radiation

(fluorescence) or will remove other less-

bound electrons from the atom (Auger effect).

The absorption or emission frequencies

(energies) are characteristic of the specific


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atom. In addition, for a specific atom, small

frequency (energy) variations that are

characteristic of the chemical bonding occur.

With a suitable apparatus, these characteristic

X-ray frequencies or Auger electron energies

can be measured.X-ray absortion and

emission spectroscopy is used in chemistryand material sciences to determine elemental

composition and chemical bonding.

X-ray crystallography is a scattering process;

crystalline materials scatter X-rays at well-

defined angles. If the wavelength of the

incident X-rays is known, this allows

calculation of the distances between planes of

atoms within the crystal. The intensities of the

scattered X-rays give information about the

atomic positions and allow the arrangement

of the atoms within the crystal structure to becalculated. However, the X-ray light is then

not dispersed according to its wavelength,

which is set at a given value, and X-ray

diffraction is thus not a spectroscopy.

Flametechnique

Liquid solution samples are aspirated into a

burner or nebulizer/burner combination,

desolvated, atomized, and sometimes excitedto a higher energy electronic state. The use of

a flame during analysis requires fuel and

oxidant, typically in the form of gases.

Common fuel gases used are acetylene

(ethyne) or hydrogen. Common oxidant gasesused are oxygen, air, or nitrous oxide. These

methods are often capable of analysing

metallic element analytes in thepart per

million, billion, or possibly lower

concentration ranges. Light detectors areneeded to detect light with the analysis

information coming from the flame.

y Atomic Emission Spectroscopy - This

method uses flame excitation; atoms

are excited from the heat of the flame

to emit light. This method commonly

uses a total consumption burner with

a round burning outlet. A higher

temperature flame than atomic

absorption spectroscopy (AA) is

typically used to produce excitation of

analyte atoms. Since analyte atoms

are excited by the heat of the flame,

no special elemental lamps to shine

into the flame are needed. A high

resolution polychromator can be usedto produce an emission intensity vs.

wavelenght spectrum over a range of

wavelengths showing multiple

element excitation lines, meaning

multiple elements can be detected in

one run. Alternatively, a

monochromator can be set at one

wavelength to concentrate on

analysis of a single element at a

certain emission line. Plasma emission

spectroscopy is a more modern

version of this method. See flameemission spectroscopy for more

details.

y Atomic absorption

spectroscopy(often called AA) - This

method commonly uses a pre-burner

nebulizer (or nebulizing chamber) to

create a sample mist and a slot-

shaped burner that gives a longer

pathlength flame. The temperature ofthe flame is low enough that the

flame itself does not excite sample

atoms from their ground state. The

nebulizer and flame are used to

desolvate and atomize the sample,

but the excitation of the analyte

atoms is done by the use of lamps

shining through the flame at various

wavelengths for each type of analyte.

In AA, the amount of light absorbed

after going through the flame

determines the amount of analyte inthe sample. A graphite furnace for

heating the sample to desolvate and

atomize is commonly used for greater

sensitivity. The graphite furnace

method can also analyze some solid

or slurry samples. Because of its good

sensitivity and selectivity, it is still a

commonly used method of analysis


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for certain trace elements in aqueous

(and other liquid) samples.

y Atomic Fluorescence Spectroscopy -

This method commonly uses a burnerwith a round burning outlet. The

flame is used to solvate and atomize

the sample, but a lamp shines light at

a specific wavelength into the flame

to excite the analyte atoms in the

flame. The atoms of certain elements

can thenfluoresce emitting light in a

different direction. The intensity of

this fluorescing light is used for

quantifying the amount of analyte

element in the sample. A graphite

furnace can also be used for atomicfluorescence spectroscopy. This

method is not as commonly used as

atomic absorption or plasma emission

spectroscopy.

Plasma Emission Spectroscopy In some ways

similar to flame atomic emission

spectroscopy, it has largely replaced it.

y Direct-current plasma (DCP)

A direct-current plasma (DCP) is created by anelectrical discharge between two electrodes.

A plasma support gas is necessary, and Ar is

common. Samples can be deposited on one of

the electrodes, or if conducting can make up

one electrode.

y Glow discharge-optical emission

spectrometry (GD-OES)

y Inductively coupled plasma-atomic

emission spectrometry(ICP-AES)

y laser induced breakdown

Spectroscopy(LIBS), also called Laser-

induced plasma spectrometry (LIPS)

y Microwave-induced plasma (MIP)

Sparkorarc (emission)spectroscopy - is used

for the analysis of metallic elements in solid

samples. For non-conductive materials, a

sample is ground with graphite powder to

make it conductive. In traditional arc

spectroscopy methods, a sample of the solid

was commonly ground up and destroyed

during analysis. An electric arc or spark is

passed through the sample, heating thesample to a high temperature to excite the

atoms in it. The excited analyte atoms glow,

emitting light at various wavelengths that

could be detected by common spectroscopic

methods. Since the conditions producing the

arc emission typically are not controlled

quantitatively, the analysis for the elements is

qualitative.Nowadays, the spark sources with

controlled discharges under an argon

atmosphere allow that this method can be

considered eminently quantitative, and its use

is widely expanded worldwide throughproduction control laboratories of foundries

and steel mills.

Visible

Many atoms emit or absorb visible light. In

order to obtain a fine line spectrum, the

atoms must be in a gas phase. This means that

the substance has to be vaporised. The

spectrum is studied in absorption or emission.

Visible absorption spectroscopy is often

combined with UV absorption spectroscopy in

UV/Vis spectroscopy. Although this form may

be uncommon as the human eye is a similar

indicator, it still proves useful when

distinguishing colours.


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Ultraviolet

All atoms absorb in the Ultraviolet (UV) region

because these photons are energetic enough

to excite outer electrons. If the frequency is

high enough, photoionization takes place. UV

spectroscopy is also used in quantifying

protein and DNA concentration as well as the

ratio of protein to DNA concentration in a

solution. Several amino acids usually found in

protein, such as tryptophan, absorb light in

the 280 nm range and DNA absorbs light in

the 260 nm range. For this reason, the ratio of

260/280 nm absorbance is a good general

indicator of the relative purity of a solution in

terms of these two macromolecules.

Reasonable estimates of protein or DNA

concentration can also be made this way

using Beer's law.

Infrared

Infrared spectroscopy offers the possibility to

measure different types of inters atomic bond

vibrations at different frequencies. Especially

in organic chemistry the analysis of IR

absorption spectra shows what type of bonds

is present in the sample. It is also animportant method for analysing polymers and

constituents like fillers, pigments and

plasticizers.

ULTRAVIOLET SPECTROSCOPY

1. Background

An obvious difference between certain

compounds is their colour. Thus, Quinone is

yellow; chlorophyll is green; the 2,4-

dinitrophenylhydrazone derivatives of

aldehydes and ketones range in colour from

bright yellow to deep red, depending on

double bond conjugation; and aspirin is

colourless. In this respect the human eye is

functioning as a spectrometer analyzing the

light reflected from the surface of a solid or

passing through a liquid. Although we see

sunlight (or white light) as uniform or

homogeneous in color, it is actually composed

of a broad range of radiation wavelengths in

the ultraviolet (UV), visible and infrared (IR)

portions of the spectrum. As shown on the

right, the component colours of the visible

portion can be separated by passing sunlight

through a prism, which acts to bend the light

in differing degrees according to wavelength.

Electromagnetic radiation such as visible light

is commonly treated as a wave phenomenon,

characterized by a wavelength or frequency.

Wavelength is defined on the left below, as

the distance between adjacent peaks (or

troughs), and may be designated in meters,

centimeters or nanometers (10-9

meters).

Frequency is the number of wave cycles that

travel past a fixed point per unit of time, and

is usually given in cycles per second, or hertz

(Hz). Visible wavelengths cover a range from

approximately 400 to 800 nm. The longest

visible wavelength is red and the shortest is

violet. Other common colors of the spectrum,

in order of decreasing wavelength, may be

remembered by the mnemonic: ROY G BIV.

The wavelengths of what we perceive asparticular colors in the visible portion of the

spectrum are displayed and listed below. In

horizontal diagrams, such as the one on the

bottom left, wavelength will increase on

moving from left to right.


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y Violet: 400 - 420 nmy Indigo: 420 - 440 nm

y Blue: 440 - 490 nm

y Green: 490 - 570 nm

y Yellow: 570 - 585 nm

y Orange: 585 - 620 nm

y Red: 620 - 780 nm

When white light passes through or is

reflected by a colored substance, a

characteristic portion of the mixed

wavelengths is absorbed. The remaining light

will then assume the complementary color to

the wavelength(s) absorbed. This relationship

is demonstrated by the color wheel shown on

the right. Here, complementary colors are

diametrically opposite each other. Thus,

absorption of 420-430 nm light renders a

substance yellow, and absorption of 500-520

nm light makes it red.Green is unique in that

it can be created by absoption close to 400

nm as well as absorption near 800 nm.

Early humans valued colored pigments, and

used them for decorative purposes. Many of

these were inorganic minerals, but several

important organic dyes were also known.

These included the crimson pigment, kermesic

acid, the blue dye, indigo, and the yellowsaffron pigment, crocetin. A rare dibromo-

indigo derivative, punicin, was used to color

the robes of the royal and wealthy. The deep

orange hydrocarbon carotene is widely

distributed in plants, but is not sufficiently

stable to be used as permanent pigment,

other than for food colouring. A common

feature of all these colored compounds,

displayed below, is a system of extensively

conjugated pi-electrons.


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2. Th

El

to

n

ti

Spectu

The vs

e s

ec

c

s

es

s

he

s

ec

.

s

he

hat s

s us cannot be

seen, but can be detected by dedicated

sensing instruments. This e ectromagnetics ectrum ranges from veryshort wave engths

(incuding gamma and x-rays) to very long

wavelengths (including microwaves and

broadcast radio waves). The following chart

dis lays many of the important regions

of thisspectrum, and demonstrates the

inverse relationship between

wavelength and fre

uency (shown in the

top e

uation below thechart).

Theenergy associated with a given segment of

the spectrum is proportional to its fre

uency.

The bottom e

uation describes this

relationship, which provides theenergycarried

by a photon of a given wavelength of radiation.

3. UV-Visible Abso ption Spect

To understand why some compounds arecolored and others are not, and to determine

the relationship of conjugation to color, we

must make accurate measurements of light

absorption at different wavelengths in and

near the visible part of the spectrum.

Commercial optical spectrometers enable

such experiments to beconducted with ease,

and usually survey both the near ultraviolet

and visible portions of thespectrum.

The visible region of the spectrum comprises

photon energies of 36 to 72 kcal/mole, and

the near ultraviolet region, out to 200 nm,

extends this energy range to 143 kcal/mole.Ultraviolet radiation having wavelengths less

than 200 nm is difficult to handle, and is

seldom used as a routine tool for structural

analysis.

The energies noted above are sufficient to

promote or excite a molecular electron to a

higher energy orbital. Conse

uently,

absorption spectroscopy carried out in this

region is sometimes called "electronic

spectroscopy". A diagram showing thevarious

kinds of electronic excitation that may occur

in organic molecules is shown on the left. Of

the six transitions outlined, only the two

lowest energy ones (left-most, colored blue)

are achieved by the energies available in the

200 to 800 nm spectrum. As a rule,

energetically favoured electron promotion

will be from the highest occupied molecular

orbital (HOMO) to the lowest unoccupied

molecular orbital (LUMO), and the resulting

species iscalled an excited state.

When sample molecules areexposed to light

having an energy that matches a possible

electronic transition within the molecule,

some of the light energy will be absorbed as

the electron is promoted to a higher energy

orbital. An optical spectrometer records the


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wavelengths at which absorption occurs,

together with the degree of absorption at

each wavelength. The resulting spectrum is

presented as a graph of absorbance (A) versus

wavelength, as in the isoprene spectrum

shown below. Since isoprene is colorless, it

does not absorb in the visible part of thespectrum and this region is not displayed on

the graph. Absorbance usually ranges from 0

(no absorption) to 2 (99% absorption), and is

precisely defined in context with

spectrometer operation.

Because the absorbance of a sample will be

proportional to the number of absorbing

molecules in the spectrometer light beam

(e.g. their molar concentration in the sample

tube), it is necessary to correct the

absorbance value for this and otheroperational factors if the spectra of different

compounds are to be compared in a

meaningful way. The corrected absorption

value is called "molar absorptivity", and is

particularly useful when comparing the

spectra of different compounds and

determining the relative strength of light

absorbing functions (chromophores). Molar

absorptivity () is defined as:

Molar

Absorptivity, =

A / cl

(whereA= absorbance,c = sample

concentrationinmoles/liter &l =lengthoflight paththroughthe

sampleincm.)

If the isoprene spectrum on the right was

obtained from a dilute hexane solution (c = 4

* 10-5

moles per liter) in a 1 cm sample

cuvette, a simple calculation using the above

formula indicates a molar absorptivity of

20,000 at the maximum absorption

wavelength. Indeed the entire vertical

absorbance scale may be changed to a molar

absorptivity scale once this information about

the sample is in hand. Clicking on the

spectrum will display this change in units.

From the chart above it should be clear that

the only molecular moieties likely to absorb

light in the 200 to 800 nm region are pi-

electron functions and hetero atoms having

non-bonding valence-shell electron pairs.Such light absorbing groups are referred to as

chromophores. A list of some simple

chromophores and their light absorption

characteristics is provided on the left above.

The oxygen non-bonding electrons in alcohols

and ethers do not give rise to absorption

above 160 nm. Consequently, pure alcohol

and ether solvents may be used for

spectroscopic studies.

The presence of chromophores in a molecule

is best documented by UV-Visible

spectroscopy, but the failure of mostinstruments to provide absorption data for

wavelengths below 200 nm makes the

detection of isolated chromophores

problematic. Fortunately, conjugation

generally moves the absorption maxima to

longer wavelengths, as in the case of

isoprene, so conjugation becomes the major

structural feature identified by this technique.

Molar absorptivities may be very large for

strongly absorbing chromophores (>10,000)

and very small if absorption is weak (10 to

100). The magnitude of reflects both the sizeof the chromophore and the probability that

light of a given wavelength will be absorbed

when it strikes the chromophore.


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4. The! "

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nce ofConju% $

tion

A comparison of the absorption spectrum of

1-pentene, max = 178 nm, with that of

isoprene (above) clearly demonstrates the

importance of chromophore conjugation.

Further evidence of this effect is shownbelow. The spectrum on the left illustrates

that conjugation of double and triple bonds

also shifts the absorption maximum to longer

wavelengths. From the polyene spectra

displayed in thecenter diagram, it isclear that

each additional double bond in the

conjugated pi-electron system shifts the

absorption maximum about 30 nm in the

same direction. Also, the molar absorptivity

() roughly doubles with each new conjugated

double bond. Spectroscopists use the terms

defined in the table on the right whendescribing shifts in absorption. Thus,

extending conjugation generally results in

bathochromic and hyperchromic shifts in

absorption.

The appearance of several absorption peaks

or shoulders for a given chromophore is

common for highlyconjugated systems, and is

often solvent dependent. This fine structure

reflects not only the different conformations

such systems may assume, but also electronic

transitions between the different vibrational

energy levels possible for each electronicstate.Vibrational finestructure of this kind is

most pronounced in vapour phase spectra,

and is increasingly broadened and obscured in

solution as the solvent is changed from

hexane to methanol.


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To understand why conjugation should cause

bathochromic shifts in the absorption maxima

of chromophores, we need to look at the

relative energy levels of the pi-orbitals. When

two double bonds are conjugated, the four p-

atomic orbitals combine to generate four pi-molecular orbitals (two are bonding and two

are antibonding). This was described earlier in

the section concerning diene chemistry. In a

similar manner, the three double bonds of a

conjugated triene create six pi-molecular

orbitals, half bonding and half antibonding. The

energetically most favourable__> *

excitation occurs from the highest energy

bonding pi-orbital (HOMO) to the lowest

energy antibonding pi-orbital (LUMO).

The following diagram illustrates this excitation

for an isolated double bond (only two pi-

orbitals) and, on clicking the diagram, for a

conjugated diene and triene. In each case the

HOMO is colored blue and the LUMO is colored

magenta. Increased conjugation brings the

HOMO and LUMO orbitals closer together. The

energy (E) required to effect the electron

promotion is therefore less, and the

wavelength that provides this energy is

increased correspondingly (remember = h

c/E).

Examples of __> * Excitation

Many other kinds of conjugated pi-electron

systems act as chromophores and absorb light

in the 200 to 800 nm region. These include

unsaturated aldehydes and ketones and

aromatic ring compounds. A few examples are

displayed below. The spectrum of the

unsaturated ketone (on the left) illustrates the

advantage of a logarithmic display of molar

absorptivity. The __> * absorption located at

242 nm is very strong, with an = 18,000. The

weak n __> * absorption near 300 nm has an

= 100.

Benzene exhibits very strong light absorption

near 180 nm ( > 65,000) , weaker absorption

at 200 nm ( = 8,000) and a group of much

weaker bands at 254 nm ( = 240). Only the

last group of absorptions are completely


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displayed because of the 200 nm cut-off

characteristic of most spectrophotometers.

The added conjugation in naphthalene,

anthracene and tetracene causes

bathochromic shifts of these absorption bands,

as displayed in the chart on the left below. Allthe absorptions do not shift by the same

amount, so for anthracene (green shaded box)

and tetracene (blue shaded box) the weak

absorption is obscured by stronger bands that

have experienced a greater red shift. As might

be expected from their spectra, naphthalene

and anthracene are colorless, but tetracene is

orange.

The spectrum of the bicyclic diene (above

right) shows some vibrational fine structure,

but in general is similar in appearance to that

of isoprene, shown above. Closer inspection

discloses that the absorption maximum of the

more highly substituted diene has moved to a

longer wavelength by about 15 nm. This"substituent effect" is general for dienes and

trienes, and is even more pronounced for

enone chromophores.

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ULTRAVIOLET SPECTROSCOPY TERM PAPER 12