Spectroscopy 1
Spectroscopy 1
Spectroscopy 1
b
I – Intensity of transmitted light
- initial intensity of incident
light b – thickness (path –length)
k – linear absorption co-efficient
ln(I/ )=-
/
kb ln(
I )=kb
Changing
to common
logarithms
we get,
Second law – Beer’s law
It states that, the amount of light absorbed by a material is proportional to
the number of
Absorbing
molecules(concentration) Again it
can be represented –
2.303 log( /I) = k’c
K’=absortivity constant
c= concentration
K and k’ merge
together = a
Log = /I = a b c
a = k & k’
b = thickness
C = concentration
This combined law states that the amount of light absorbed is
proportional to the Concentration of the absorbing substance & to the
thickness of the absorbing material
(path – length)
The quantity /I it is absorbance (O.D – optical density)
The two terms are mathematically commutable i.e., one can be calculated from
the
other
A=log - log I
= 100%
Log 100 =
2
=2-log
I Or
O.D is dirctly proportional to the concentration if path is
constant So if we know the
Unknown(sample) O.Dvalue of unknown
of the O.D concentration can beof std
x concentration
= of the
calculated Concentration
O.D of the std
Terms describing UV absorptions
1. Chromophores: functional groups that
give electronic transitions.
2. Auxochromes: substituents with unshared
pair e's
like OH, NH, SH ..., when attached to π
chromophore they generally move the absorption
max. to longer λ.
3. Bathochromic shift: shift to longer λ, also called
red shift.
4. Hysochromic shift: shift to shorter λ, also called
blue shift.
5. Hyperchromism: increase in ε of a band.
6. Hypochromism: decrease in ε of a band.
UV-VISIBLE
Spectroscopy :
Uv-vis spectroscopy is also known as
electronic spectroscopy. In which the amount
of light absorbed at each wavelength of Uv
and visible regions of electromagnetic
spectrum is measured. This absorption of
electromagnetic radiations by the molecules
leads to molecular excitation.
ELECTRONIC
SPECTROSCOPY:
• Ultraviolet (UV) and visible
(VIS) spectroscopy
• This is the earliest method of
molecular spectroscopy.
• A phenomenon of interaction of
molecules with ultraviolet and visible
lights.
• Absorption of photon results in
electronic transition of a molecule, and
electrons are promoted from ground
state to higher electronic states.
• The first discovery of electromagnetic waves other than
light came in 1800, when William Herschel
discovered infrared light. He was studying the
temperature of different colors by moving a
thermometer through light split by a prism.
• The types of electromagnetic radiation are
broadly classified into the following classes
• Gamma radiation
• X-ray radiation
• Ultraviolet radiation
• Visible radiation
• Infrared radiation
• Terahertz radiation
• Microwave radiation
• Radio waves
• Ultraviolet: 190~400nm
• Violet: 400 - 420
nm
• Indigo: 420 - 440
• nm
Green: 490 - 570 nm
•• Blue: 440 -- 585
Yellow: 570 490 nm
nm
• Orange: 585 - 620 nm
• Red: 620 - 780
nm
ULTRAVIOLET
RAYS
Shorter wavelength and higher frequency than
visible light Carry than visible light
ELECTRONIC
TRANSITIONS:
There are three types of electronic
transition which can be considered;
• Transitions involving p, s, and n
electrons
• Transitions involving charge-
transfer electrons
• Transitions involving d and f
electrons
ABSORBING SPECIES
CONTAINING P, S,
AND N ELECTRONS:
• Absorption of ultraviolet and visible
radiation in organic molecules is
restricted to certain functional
groups (chromophores) that contain
valence electrons of low excitation
energy.
Transitions
• An electron in a bonding s orbital is excited to
the corresponding antibonding orbital. The
energy required is large. For example, methane
(which has only C-H bonds, and can only
undergo transitions) shows an
absorbance maximum at 125 nm. Absorption
maxima due to transitions are not
seen in typical UV-VIS spectra (200 - 700
nm)
n Transitions
• Saturated compounds containing atoms with
lone pairs (non-bonding electrons) are
capable of n transitions. These
transitions usually need less energy than
transitions. They can be initiated by light
whose wavelength is in the range 150 - 250
nm. The number of organic functional groups
with
n peaks in the UV region is small.
n and Transitions
• Most absorption spectroscopy of organic
compounds is based on transitions of n or
electrons to the excited state.
• These transitions fall in an experimentally
convenient region of the spectrum (200 -
700 nm). These transitions need an
unsaturated group in the molecule to
provide the electrons.
INSTRUMENTA
TION
Light source:
UV - Hydrogen lamp ( hydrogen stored under
pressure) , Deuterium lamp and Xenon lamp-
it is not regularly used becos of unstability and
also the radiation of UV causes the generation
of ozone by ionization of the oxygen
molecule.
VIS – Tungston filament lamp , Tungston
halogen lamp and carbon arc lamp.
Waveselectors are mainly either filters or
monochrmators.
Filters : Gelatin filters are made using a
layer of gelatin coloured with organic dyes
that are sealed between glassplates. This
filters resolve polychromatic light into a
relatively wide band width of about 40 nm
and these are commonly used in
colorimeters since they have low
transmittance i.e. 5 – 20 %.
Monochromators :
Consists of an entrance slit which admits the polychromatic light
from the source.
A collimating device – lens or mirror which helps in reflecting
the polychromatic light to the dispersion device.
A wavelength resolving device - prism or grating.
A focussing lens or mirror
Exit slit
Sample holder/ containers :
Cuvettes – Quarts or fused silica , ordinary glass is known to absorb
uv
rad.
for IR – samples are ground with potassium bromide and pressed
into a pellet, if aqueous solution silver chloride is coated inside the
cell. While preparing samples selection of solvents is imp. , becos
they do
absorb light.
INSTRUMENTATION:Single
and Double Beam
Spectrometer
• Single-Beam: There is only one light beam
or optical path from the source through to
the detector.
• Double-Beam: The light from the source,
after passing through the monochromator, is
split into two separate beams-one for the
sample and the other for the reference.
VISIBLE
LIGHT
• Shorter wavelength and higher frequency than infrared
rays.
• Electromagnetic waves we can see
•Longest wavelength= red light
Shortest wavelength= violet (purple)
light
Single beam spectrophotometer
A single beam of radiation pass
through a single cell, the reference cell is
used to set the absorbance scale at zero for
the wavelength to be studied. It is then
replaced by sample cell to determine the
absorbance of the sample at that
wavelength . This was the earliest design
and is still use in both teaching and
industrial labs.
Double beam spectrophotometer