Molecular Electronic Spectroscopy
Electronic changes have been occurs in this region, since the
energy limits equal to 8000cm-1frequency (ranged from 750nm
into less 200nm). Its classified into three different region (A, B,
and C), since the visible is C-region, that's ranged from 750nm
into400nm and the near UV-region (B) is ranged from 400nm
into 200nm.the last region-A is the vacuumed UV-region that’s
ranged from 200nm and less than (oxygen absorbed). The
energy value limits reach the outer shell electrons of chemical
species and promotion to higher energy levels.
Assumption:
Nuclei remain frozen (R unchanged) during the transition.
1- Vertical transitions between potential energy curves.
2- There is no selection rule governing the allowed
vibrational changes accompanying an electronic transition.
3- Instead, the probability of undertaking a v” → v’ transition
is governed by Franck‐Condon factors.
Electronics spectroscopy studies give up information about
electronics structures of molecules, electronics energies at
different molecular levels, and the know lodgment of emission
and absorption spectra for colored chemical compounds.
A large change in energy are coming from electronic transitions
also rearrangements of electrons that’s produced change in
electrostatic power on nuclei. At the same time of electronic
transition there are vibrational and rotational; so can be analysis
the vibrational structure from electronic transition spectra. The
following figure shows the diagram of electronic transitions.
Figure 1. Molecular energy levels (electronic transitions).
If sample spectra at gas phase, but at solid phase, liquid phase
there are confused in spectral lines as one band width broad
peaks and cannot be analysis. Therefore the spectra in gas phase
are very complexes than other phases. In some time absorption
energy is so enough to dissociation the molecules.
E total =E electronic + E vibration + E rotation
All molecules have electronic spectra because the change
in electronic distribution produced the change in dipole moment
.Alternative in rotational spectra the molecules must have
electrical dipole moment ,in vibration spectra the molecules
must have change in the electrical dipole moment through the
movements.
All semi diatomic molecules can be have electronic spectra
also within rotation &vibration like N2 ,H2 ,Br2 .from this
spectra can be drive constants and bond frequency as shows in
dissociation bond of diatomic molecule figure 2.
Zero point energy (Z.P.E) is the energy of molecules at ground
state. Z.P.E=1/2h𝝼= De-Dº ///De= spectroscopic dissociation
energy////Dº=bond dissociation energy
Figure 2.potential energy curve for dissociation bond of
diatomic molecule.
According to the theory of molecular orbital, there are
different types of transitions occurs due electron promotion from
lowering energy orbital levels into higher molecular orbital
energy levels, or transitions can be represented by transition
from atomic character orbital into higher energy molecular
orbital.
UV-Visible spectra of Chemical compounds
This is mainly a study of molecules and their electronic
transitions. Molar Absorptivity (ε) ranges from 0 to 105 for use
in absorbance measurements. Transitions with ε < 103 are
considered to be of low intensity. In organic molecules, most
bonding electrons are excited by λ < 185 nm (Vacuumed-UV).
Recall that E(eV) = 1239 / λ (nm)
Most functional groups have lone pairs whose energies
place them in the near UV and visible range. These groups are
called “chromophores”, although this is a bit odd, since all
molecules are “chromophores” under the right conditions.
The main electronic transitions are four only some of them
are forbidden and the other are allowed transition due the lowest
value of energy, that’s occurs in organic, heterorganic, and
inorganic compounds. The transitions occurs due the absorption
of radiation energy light, this transitions depends on the way of
bonding between atoms in the chemical species. In some case
they found single type of bonding (saturated hydrocarbons) the
only significance transition is sigma to sigma transitions. In
other species they found one or more than hetero atoms (like
oxygen, nitrogen, and phosphor) that’s have electrons in atomic
orbital character (nonbonding molecular orbital).therefore pi to
pi, none to pi and pi to sigma transition may occurs.
The main types are described in figure 3.
Figure 3. Electronic transitions diagram.
The following diagram represents the formaldehyde CH2O
molecule transitions.
--- 𝞂*CH
--- 𝞂*CH
--- 𝞂*CO
--- 𝞹*CO
Energy increasing
---------------------------------------------------levels
Non bonding
--- ---nO
---𝞹CO
---𝞂CO
--- 𝞂CH
--- 𝞂CH
n→σ* Transitions
Still rather high in energy, with λ between 150 and 250 nm.
These tend to be relatively weak absorbers.
Not many molecules with n→σ* transitions in UV/vis region
Molecule
λmax
εmax
H2O
(partial Rydberg) 167
1480
CH3OH
184
150
CH3C
l 17
3 200
CH3I
258
365
(CH3)2S
229
140
(CH3)2O
184
2520
CH3NH2
215
600
(CH3)3N
227
900
Solvents can interact with the analyst molecules and shift
absorbance peaks and intensities.
Red Shift (Bathochromic) – Peaks shift to longer
wavelength.
Blue Shift (Hypsochromic) – Peaks shift to shorter
wavelength.
n→π* generally blue shifted by solvent; solvation of and
hydrogen bonding to the lone pair. Large shifts (up to 30 nm).
Both n→π* and π→π* red shifted; attractive polarization
forces, increase with increasing solvent polarity. Small shifts
(less than 5 nm).
Reasons of Shifting of electronic transitions
They found different reasons for the remove the transition
into up normal positions:A- Effect of substituted groups on the original spectrum.
Alternative the functional groups (provided with hetero
atoms) with unsaturated center will remove or shifted the
absorption into red region (longer wave length- red shift).
hν
Θ
C--C=N+=
-C=C--N=
Alternative of two unsaturated centers (double bonds) causes
the delocalization of electron alone the axis of molecule, were
the absorption of π⇒π* will remove into red shift (low
frequency) with low energy value.
B- Effect of solvent
In some of polar solvent, transitions are shifted into red
region due interaction of polar excited groups with polar
solvents.
hν
=C=C=polar solvent⇒ solvent side Θ--+… ΘC-C+… Θ--+…solvent
side
But in some interaction between the molecules and solvents,
they give up more stability (localization) of electrons, therefore
absorption position alter to shorter wave length (blue shift) with
highly energy value (hydrogen bonding). The transition didn't
occur unless additional energy value to break down the
hydrogen bond and transition in blue region.
The calculation of the hydrogen bonding strength in compounds
can be doing thereby measuring the transition into two different
solvent polarities. Taking the difference between them in energy
value as in following example: n⇾π* of carbonyl group in
acetone.
In Hexane 279nm (506.3 kJ mol-1)
In Water 264.5nm (527.2 kJ mol-1)
Energy difference equal to20.9 kJ mol-1
Solvents can also induce significant changes in the intensity of
peaks.
Hyperchromic – Increase in absorption intensity.
Hypochromic – Decrease in absorption intensity.
C- Effect of complementary groups
(Charge transfer spectrum)
When they found more than one substituted group into
aromatic ring with different effect (one of them is electron
donating and the other is electron drawing) in Para
position for each others, these groups can be called
complementary groups.
H2N--
--NO2 ↔ H2N+==
=NO2=
Red shift (λmax=375nm, ℰ=1600m2 mol-1
The transition is called Charge transfer, and the band is Charge
transfer band, this phenomenon is due the resonance exchange
effect. If the two substituted groups into the aromatic ring have
the same electronic effect (both of them drawing or donating)
the resultant spectrum as individual spectrum of one group and
there aren’t complementary groups. The same thing occurs if
they are not in Para position from each others.
D- Effect of hydrogen bonded (inter &intra).
This reason is come out from hydrogen bond formation in two
different case, the first is inter hydrogen bonding formation in
the same chemical species moiety like in salicylic acid ,the
second case is intra hydrogen bonding formation between two
chemical species moiety in bulk solution like ethanol and
acetone. Both types will alter the transition into blue shift
(shorter wave length).
Spectra of inorganic compounds
Importance are significance only in transition metals were those
metals with partially filled d-orbital and also Lanthanides with
Actinides (have partially filled f-orbital), those ions have finite
colors due absorption bands in IR & visible region and in some
time reach the UV-region. There are two types of molecules
around the metal ions:-A-Negative ions (CN- ,-OH , Cl- ) Bpolar molecules (NH3, H2O)
Application of UV-Visible spectroscopy
They found different useable applications are:A-Chemical analysis (qualitative and quantitative)
1. Calibration Curve
2. Standard Addition (matrix effects/complex sample)
3. Internal Standard (compensate for random and systematic
errors; difficult).
B-Spectrum of charge transfer.
C-Stoichiometry of complexes.
They found three spectrometry methods of metal-ligand ratio
determination.
1-Mole ratio (one component is constant concentration and the
other is continuous change).
2-Jobe method (the total molar concentration is constant for all
component in solutions).
D-kinetic of chemical reactions.
The absorbance for reactant or products is changing within the
time of reaction according to their concentrations.
E-Equilibrium of Acid –Base
The equilibrium constant of acids-base can be determine
according to their relative absorbance, therefore can be
determine the dissociation degree of acids and bases.