ultraviolet spectroscopy term paper 12
TRANSCRIPT
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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! "
po#t
$
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.