UV-VIS
SPECTROSCOPY
(Ultraviolet and Visible Spectroscopy )
•Violet: 400 - 420 nm
•Indigo: 420 - 440 nm
•Blue: 440 - 490 nm
•Green: 490 - 570 nm
•Yellow: 570 - 585 nm
•Orange: 585 - 620 nm
•Red: 620 - 780 nm
Color and light absorption –The chromophore concept
•Compounds that absorb light
of wavelength between 400
and 800 nm ( visible light )
appear colored to the human
eye , the precise color being
a function of which wavelength
the compound subtract from the
white light(complementary color)
Very many compounds have strong ultraviolet absorption bands
the shoulder of which may tail in to the visible spectrum,
absorbing violet from white light spectrum leaves the
complementary colors , which appear yellow \ orange to the
human eye , for these reasons yellow and orange are the most
common colors among the organic compounds .
Chromophores :Originally the term chromophor was applied to
the system responsible for imparting color to a compound
( the derivation from Greek , chromophorous , a color carrier )
The term has been retained within an extended interpretation to
imply “ any functional group that absorbs electromagnetic
radiation , whether or not a ‘color . Thus the carbonyl group
is a chromophore. ( list of simple organic chromophores )
Auxochrome : is a group that could enhance the color
imparting properties of a chromophore without being
itself a chromophore , Eg : -OR , -NH2 , -NR2
The synergist effect of auxochromes is coupled with their
ability to extend the conjugation of a chromophore by sharing
of non bonding electrons.
Chromophore Example Excitation λmax, nm ε Solvent
π __
> π*
C=C Ethene 171 15,000 hexane
π __
> π*
C≡C 1-Hexyne 180 10,000 hexane
n __
> π*
290 15 hexane
C=O Ethanol π __
> π*
180 10,000 hexane
n __
> π*
275 17 ethanol
N=O Nitromethane π __
> π*
200 5,000 ethanol
Methyl n __
> σ*
C-X X=Br 205 200 hexane
bromide n __
> σ*
X=I 255 360 hexane
Methyl Iodide
The strength of electronic spectroscopy lies in its ability to
measure the extent of multiple bond or aromatic conjugation
with in the molecules . The non bonding electron on
Oxygen , Nitrogen and Sulfur may also be involved in
extending the conjugation of multiple bond systems.
Electron spectroscopy can in general differentiate
conjugated dienes from non conjugated dienes ,
conjugated dienes from conjugated trienes ,α β unsaturated
ketones from their β γ analoges etc
The absorption of ultraviolet \ visible radiation by a molecule
leads to transition among the electronic energy levels of the
molecule and this reason it called as electronic spectroscopy
Historically ,routine ultraviolet spectrometer were developed
before infrared , NMR or Mass Spectrometer and we find
now that some of the ultraviolet correlation that were previously
useful have long been superseded by the later developed
techniques , so that the application of ultraviolet spectroscopy
to structural investigation may appear disappointingly restricted .
IR is the method of choice for the detection of nitrile groups
or carbonyl groups
NMR reveal for more information about the nature of
substitutes on the benzene than the electronic spectroscopy
Theory of Electronic spectroscopy
Orbital involved in the electronic transition
Eg . Alkane , Alkenes , Saturated aliphatic Ketones
Many factors influence the relative energies of molecular
orbital and a knowledge of these factors is the essence of an
understanding of electronic spectroscopy
• Solvent effect
• Substitution effect
• Conjugation .
Laws of Light Absorption
BEER’S AND LAMBERT’S Law
* Beer’s law relates the absorption to the concentration of
absorbing species
• Lambert’s law relates the total absorption to the optical path
length
* Log ( I0\ I ) = ε c l
Molar (where A= absorbance, c = sample concentration in moles/liter
& l = length of light path through the sample in cm.)
Absorptivity,
ε = A/ c l
The molar absorptivity , ε, is constant for a particular compound
at a given wavelength , and is commonly expressed as εmax -
the molar absorptivity at an absorption band maximum .
Instrumentation and Sampling
The Ultraviolet –Visible Spectrometer
* Dispersive ( monochromator ) , a detector
* Photodiode array ( grating ) , array of detector
* Fourier Transform Instrument ( mirrors )
Photomultiplier detectors
Depends upon the photo emissive property of compounds like
Be0
•Sample and reference cells
•Glass absorbs strongly under 300 nm
• Matched silica
Solvents and solutions
Solvent shifts may occur , and comparisons between spectra
should only be made with this realization
Solvents for electronic spectroscopy
solvent lower wave length limit / nm
water 205
ethanol 210
hexane 210
cyclohexane 210
methanol 210
diethyl ether 210
acetonitrile 210
THF 220
mdc 235
Chloroform 245
carbon tetra chloride 265
benzene 280
Solvent Effect
The position and intensity of an absorption band may shift when
the spectrum is recorded in different solvents
•conjugated dienes and aromatic hydrocarbons experience very
little solvent shift .
•αβ-Unsaturated carbonyl compound shows two different shifts
the π → π * band moves to longer wavelength ( red shift ) ,
while n → π * band moves to shorter wavelength (blue shift)
Terminologies for Absorption shifts
Descriptive
Nature of Shift
Term
To Longer Wavelength Bathochromic
To Shorter Wavelength Hypsochromic
To Greater
Hyperchromic
Absorbance
To Lower Absorbance Hypochromic
Effect of Conjugation
Application of Electronic spectroscopy -
Conjugated dienes, trienes and polyenes
For dienes and trienes the position of the most intense band can
be correlated in most instances with the substituents present by
making use of empirical relationship , called Woodward rules
Base value 215 nm , since the two double bonds are
heteroannular : there are 4 alkyl substituents ( the ring residue
a,b ,c and the methyl group d ). Adding 4 X 5 nm : the double
bond in ring A is exocyclic to ring B , adding 5 nm: the total is
215+20+5 = 240 nm .This is with in the 2 nm observed value
Application of Electronic spectroscopy -
αβ Unsaturated carbonyl compounds
For αβ unsaturated carbonyl compounds : the original data is
complied by Woodward, the many extension and verifications
being added over the years.
The method of use is similar to that for conjugated diene , the
only two variants being 1) the distinction between α and β
substituents and 2) the important of solvent shifts
Note that the Woodward rule calculate the position of the intense
π → π * transition , not the weak n → π * transition
For the compound I the base value is 215 nm : for one α -alkyl
group add 10 nm , for β –alkyl group add 12 nm : the total is
215+10+12=237 nm
For compound II the base value 215 nm : for one α -alkyl
group add 10 nm , for β –alkyl group add 2 X 12 nm , for a
double bond exocyclic to two rings add 10 nm : the total is
215+10+24+10=259 nm
Naphthalene , anthracene , tetracene
Thank you
