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CH437 CLASS 18
ULTRAVIOLET-VISIBLE SPECTROSCOPY 1
Synopsis. Introduction: energy transitions and absorption spectra. Selection rules. Beer-Lambert
Law. Solvents for UV-visible spectroscopy. Chromophores.
Introduction
Energy transitions in UV-visible spectroscopy are electronic, involving 125-650
kJ/mol of energy (much higher than those needed for vibrational transitions in IR
spectroscopy or for nuclear spin transitions in NMR spectroscopy). The range of
energies above roughly correspond to absorbed wavelengths of 190-800 nm:
that is, in the ultraviolet-visible region of the electromagnetic spectrum. The major
electronic transitions in UV-visible absorption spectroscopy are HOMO  LUMO
transitions, as shown below.
A common molecular orbital energy level diagram for organic molecules is shown
below, where the most common HOMOLUMO transitions are *, n* and
*, but * and n* are also possible.
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These are most commonly associated with the following organic molecules:

Alkanes

C=O

Alkenes, alkynes, aromatics, C=O, azo compounds, etc
n

Oxygen, nitrogen, sulfur and halogen compounds
n

C=O
In practice, for molecules there is a series of vibrational energy levels between
each of the electronic energy levels: molecules can undergo vibrational and
electronic transition at the same time, as illustrated below.



LUMO
E



HOMO
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It can be seen that there is a considerable number of transition possibilities, over
a range of wavelengths. This gives the characteristically broad bands (covering
several nm) seen in typical UV-visible spectra, as illustrated in the examples
below. If the absorbing substance of reasonable polarity is dissolved in a nonprotic apolar solvent, it is often possible to see vibrational fine structure, as in
example (a), since there is minimal interaction between the solute and solvent.
However in polar protic solvents, the interactions (e.g. hydrogen bonded and
dipole-dipole interactions) are considerable and very little fine structure is seen,
the band being an envelope of the spectrum in (a), but with max at a somewhat
different value.
Selection Rules
Not all possible transitions are actually allowed. There are selection rules that
quantum theory imposes on electronic transitions (as with vibrational and nuclear
spin transitions) to determine which are allowed and which are forbidden. In
practice, certain forbidden transitions are observed, but the absorption intensity is
always low compared with those of allowed transitions. The selection rules are
based upon orbital symmetry: this symmetry is broken by vibrational components
of the transition, thus allowing “forbidden” transitions to occur (although with low
probability).
The most important forbidden transition is n*.
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The Beer-Lambert Law
This law relates the intensity of light absorption (the absorbance) to the
concentration of absorbing substance
Absorbance is defined as: A = log (Io/I) = cl, as illustrated below.
The corresponding expression for Amax (the value of A at max) is
Amax = log(Io/I) = maxcl
 is the molar absorptivity, a constant that is characteristic of the particular
substance (in specified solvent and at specified temperature) at a particular
wavelength and hence of the particular
-system present.
max is the
corresponding constant for max. For conjugated dienes, max ~10,000 – 25,000
(l/mol/cm) and consequently conjugated substances can be identified by their
(allowed)   * 
max
values and max values. Similarly, max values for (the
forbidden) n* transitions are between 0 and 1000. Also, max is useful in
analytical chemistry and biochemistry for the determination of concentrations of
known substances.
Solvents for UV-Visible spectroscopy
The important spectroscopic constants max and max, associated with a molecule
that absorbs UV-visible light are solvent-dependent, so that when quoting these
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constants the solvent should always be stated. A polar solvent forms hydrogen
bonds more readily with the ground state (e.g. the HOMO) of polar molecules,
thus stabilizing this state with respect to the excited state (e.g. the LUMO) as for
carbonyl n* transitions:
This shifts max to lower values (toward the UV). An example of this is found in
the UV-visible spectra of carbonyl compounds, as typified by solvent shifts for the
n* transition of acetone, below.
Solvent
H2O
CH3OH
C2H5OH
CHCl3
C6H14
max/nm
265
270
272
277
279
For less polar compounds, polar solvents may form hydrogen bonds more easily
with the LUMO than with the HOMO (as for * transitions). In which case, the
opposite effect occurs, max is shifted to higher wavelengths in polar solvents.
A second important feature of solvent is its own absorption in the UVvisible region: a good solvent should not absorb in the same region as the
compound under investigation.
Usually solvents that do not contain conjugated systems are best for UV-visible
spectroscopy, since their max values will be low wavelength (well below 200 nm if
possible – well away from the absorption regions of most molecules, particularly
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conjugated or aromatic molecules). The table below shows some UV
spectroscopy solvents and their “cutoff points” – minimum regions of
transparency.
Solvent
Cutoff /nm
Solvent
Cutoff /nm
Acetonitrile
190
n-Hexane
201
Chloroform
240
Methanol
205
Cyclohexane
195
Isooctane
195
1,4-Dioxane
215
Water
190
95% Ethanol
205
Trimethyl
210
phosphate
Chromophores, Molecular Orbitals and Transitions
A chemical entity (group of atoms) that absorbs (UV-visible) radiation is known
as a chromophore. The exact energy (and hence max) of absorption depends
on the molecular orbital energies, which in turn depend upon the structure of the
chromophore. It is usually very difficult to give accurate prediction purely from
theory how absorption changes as the chromophore structure changes. Instead,
empirical rules (Class 20) are much more conveniently used for certain kinds of
conjugated chromophores. For now, the major chromophores will be considered
in terms of MO orbital transitions, as outlined below.
* Transitions
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+
C_
_ C
+C
+
C
_
*
C-C
_
 C-C
These transitions are the only ones possible in alkanes, where there is lack of
both –bonding and atoms with non-bonded electrons. They are of such high
energy that they require absorption of UV radiation of very short wavelength:
shorter than those that are readily accessible using ordinary spectrometers.
n* Transitions
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C_
+
_
+N
C-N
_
N
C
*
n
+
_ C
+
N _
 C-N
These are also rather high energy transitions, but most of them are within the
range of typical spectrometers, although below the cutoff point of most solvents.
They are found in alcohols, ethers, amines and sulfur compounds and have
typical ranges of 175-200 nm (alcohols and ethers) and 200-220 nm (thiols and
sulfides).
* Transitions
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_
+
*
C
_
C
+
+
C

C
_
With unsaturated compounds (alkenes, alkynes, aromatics and carbonyl
compounds), * transitions become possible. Likewise, these transitions are
also of rather high energy (max typically ~170-175 nm for non-conjugated
alkenes and alkynes), but their max values are sensitive to conjugation and to
substitution, as will be seen in Classes 19 and 20.
n* Transitions
_
+
C
+
*
O
_
+
C
_
O
+
C
C
n(py)

_
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Unsaturated molecules that contain oxygen, nitrogen or sulfur may undergo
n* as well as * transitions. The most important of these molecules are
carbonyl compounds, whose transitions are also rather sensitive to substitution
(see Class 19). Typical carbonyl compounds undergo n* transitions around
280-290 nm with very low intensity (max ~ 15), being forbidden transitions. They
also have * transitions at about 190 nm (max ~ 1000).
A list of absorptions of simple isolated chromophores is given below
It will be seen that many of these chromophores absorb in the region 160-210
nm. However, attachment of substituent groups in place of hydrogen in a
chromophore structure changes the position and intensity of the absorption band
due to the principal (or basic) chromophore. These substituents are known as
auxochromes and have four different effects on absorptions:
Bathochromic shift (red shift) – shift to longer wavelength (lower energy).
Hypsochromic shift (blue shift) – shift to shorter wavelength (higher
energy).
Hyperchromic shift – increase in intensity of absorption.
Hypochromic shift – decrease in intensity.
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