ULTRAVIOLET/VISIBLE

SPECTROSCOPY
By: POSKY
• The ultraviolet region of the electromagnetic spectrum is divided
into two parts:
 The near ultraviolet region – 400 – 200 nm
 The vacuum ultraviolet region – 200 – 10 nm

• Electromagnetic radiation in the wavelength range of 400 – 800 nm

is referred to as the VISIBLE REGION.
• Absorption of ultraviolet or visible radiation by a molecule causes an
electron to be excited from the lower molecular orbital (ground
state) to a higher molecular orbital (excited state)
• For simple organic molecules which have no conjugated pi-bonds, for
example ethene, UV radiation of wavelength below 200 nm is needed
for electronic transition to occur. UV radiation below 200 nm are
strongly absorbed by oxygen in the air. Therefore a more
sophisticated instrument in which oxygen is completely excluded is
required.
• Inspite of the fact that UV and visible spectroscopy is limited to
conjugated systems it does give a very useful information for
structural determination

UV SPECTROMETER
Compartments
• radiation source – deuterium lamp (ultraviolet) or tungsten halogen
lamp (visible)
• monochromator – grafting and prisms to disperse light
• photometer - detects the amount of photons that is absorbed by
sending signal to a galvanometer or digital display
• sample compartment – cuvettes (glass, plastic or quartz) for holding
the samples
• detector – measures the intensity of the transmitted light
• recorder – displays the signal in the form of a spectrum
 Preparation of sample for uv spectrometry
• Compound is usually in the gaseous or liquid solution
• In the gaseous form the sample is injected into cells made of quartz
equipped with gas outlets and inlets and have variable length
• The compound is accurately weighed and dissolved in an
appropriate solvent. The solution is then made up to the mark in a
volumetric flask. Aliquots of the solution are taken and then diluted
to a concentration that will give a reasonable absorption on the UV
spectrometer.
• In practice the UV spectrum of a compound is obtained by
irradiating the sample with UV light of continuously changing
wavelength. When the wavelength of the radiation corresponds to
the energy required to excite the electron from the lower to higher
energy level, the radiation is absorbed. This is displaced on a chart
that plots absorbance (A) versus wavelength
• The solvents chosen should not react with the compound and
should also not absorb in the region. The solvent should also be pure
•   The UV Spectrum
T=

A = log = log

A = Ɛ.c.l = = log Beer – Lamberts Law

Where

A – Absorbance, T = Transmittance, Ɛ = molar absorptivity

c = concentration of the solute, l = path length of the sample
•   Electronic Transitions
• On irradiation of a sample with UV radiation, electrons are promoted
from the highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO).
• Common electronic transitions;
 δ
 n
 π

antibonding
antibonding

nonbonding n

bonding π
bonding δ
 UV Spectroscopy terms
• Chromophore – presence of groups with π – bonds .
• Auxochrome – An atom or group of atoms with non-bonding
electrons and which when attached to a chromophore alters the
wavelength and the intensity of absorption
• Bathochromic shift (red shift) – shift of an absorption band to longer
wavelength due to substituents or solvents
• Hypsochromic shift (blue shift) - shift of an absorption band to a
shorter wavelength due to substituents or solvents
• Hyperchromic effect – an increase in absorption intensity
• Hypochromic effect – a decrease in absorption intensity
•  Compounds containing only sigma bonds
Saturated hydrocarbons contain only δ bonds. The only transition that
occur involves δ transitions. The energy needed for this
excitation is in the order of 185 kcal/mol. This energy can be obtained
only in the far UV region and thus saturated hydrocarbons do not absorb
in the near UV region

• Saturated compounds containing non bonding electrons

Saturated compounds that contain heteroatoms such as O, N, S or
halogens have non bonding electrons (n) thus there are two different
types of transitions
n

Energy require for these transitions are high and therefore most of such
compounds do not absorb in the near UV region
•   Compounds containing π – electrons
Compounds containing π and non bonding electrons can undergo three
kinds of transitions
n
π
n

Ethylenic Chromophore
• An isolated carbon-carbon double bond shows an intense
absorption band at 165 nm and is due to π .A
second band occurs at 193 nm which is due to the elevation
of the two π electrons to
• Introduction of alkyl group onto the ethylene group moves
the absorption to longer wavelength
• As the number of alkyl groups increases the bathochromic shift also
increases
• The attachment of a heteroatom having nonbonding electrons
brings about a bathochromic shift
• Cycloalkenes absorb in the same region as those of acyclic alkenes.
Changes in the size of the ring has no effect on the wavelength of
absorption
• If a compound has more than one carbon-carbon double bond which
are non conjugated and are separated by at least one methylene
group then the compound absorbs in the same region as that with a
single C = C. However the intensity of the absorption increases with
the number of carbon-carbon double bonds.
• An allene shows a strong absorption band at 170 nm
 Conjugated Dienes
• In a conjugated diene there is overlap between the two pi-bonds and
this increases the wavelength at which the compound absorbs in the
UV
• The longer the chain of conjugation, the longer the wavelength of
absorption, i.e. there is bathochromic shift as the number of
conjugated carbon-carbon double bonds increases. Example whereas
the wavelength of 1,3-butadiene is 217 nm that of 1,3,5-hexatriene is
253 nm
• Homoannular diene absorbs at a higher wavelength than
heteroannular diene
• a trans isomer absorbs at a higher wavelength than the cis isomer
• The introduction of more alkyl groups into a conjugated system
increases the wavelength at which UV absorption occurs
•   Fieser Woodward Rules
• Extensive examination of the UV absorption spectra of conjugated
systems led Fieser and Woodward to come out with rules for
calculation values for conjugated systems

Rules for Diene Absorption

Base value for heteroannular diene 214

Base value for homoannular diene 253
Increaments for:
Double bond extending conjugation +30
Alkyl substituent +5
exocyclic double bond +5
Polar groups: OAc +0
OR +6
SR +30
Cl, Br +5
NR +60
Solvent correction +0
calc = Total
••  Calculate the of the following structures
observed for i, ii and iii are 235 nm, 262 nm and 275 nm
respectively

Research work – Look out for the demerits with Fieser-

Woodward Rules
•   Fieser – Kuhn Rules
In the Fieser – Kuhn rules both the and values are related to the
number of conjugated double bonds, as well as the structural units in
the molecule by the following equation:
= 114 + 5M + n(48 – 1.7n) – 16.5 – 10

= (1.74 ×)n

Where
n = number of conjugated double bonds
M = number o falkyl or alkyl-like subsituents on the conjugated systems
= number of rings with endocyclic double bonds in a conjugated system
= number of rings with exocyclic double bonds
•  Calculate and for the following compounds