Application of
Ultraviolet/Visible
Molecular
Absorption Spectroscopy
Absorption - Relaxation
Absorbing Species
M + h -----> M* -----> M + heat
Types of Transitions
Three types of transitions
1. , , and n electrons
2. d & f electrons
3. charge transfer electrons
Types of Transitions
Electron distribution in sigma and pi
molecular orbitals
Types of Transitions
Types of molecular orbitals in
formaldehyde
Absorbing Species Containing
, , and n Electrons
Electronic molecular energy levels
Anti-bonding
Anti-bonding
n
n Nonbonding
Bonding
Bonding
Types of Transitions
-----> * vacuum UV
UV Spectra
Ultraviolet
spectra for
typical organic
compounds
Visible Spectra
Ultraviolet
absorption spectra
for 1,2,4,5-tetrazine
(a.) in the vapor
phase, (b.) in
hexane solution, and
(c.) in aqueous
solution
Types of Transitions
n -----> *
normally below
200 nm
Some Examples of
Absorption Due to
n -> * transitions
Types of Transitions
n -----> * 200-700 nm
low (10 -> 100)
Types of Transitions
Absorption
Characteristics of
Some Common
Chromophores
Types of Transitions
Effect of Multichromophores on Absorption
Types of Transitions
Absorption Characteristics of Aromatic Compounds
Types of Transitions
2. d & f electrons
3. charge transfer electrons
Absorption by Elements of the
First and Second Transition Series
Ligand Field Theory
Electron-density distribution in the five d-orbitals
Absorption by Elements of the
First and Second Transition Series
Ligand Field Theory
"Effect of ligand field on
d-orbital energies."
Absorption by Elements of the
First and Second Transition Series
Ligand Field
Theory
Absorption
spectra of some
transition-metal
ions
Absorption by Elements of the
First and Second Transition Series
Ligand Field Theory
Effect of Ligands on
Absorption Maxima
Associated with d ->
d transitions
Charge-Transfer Absorption
max > 10,000
Solvents for UV-Visible Regions
Analysis of Mixtures of
Absorbing Substances
Selection of Wavelength
Absorption spectrum
of a two-component
mixture
Analysis of Mixtures of
Absorbing Substances
Solution of Binary Mixture
"Schematic representation of the
absorption spectra of solutions containing
(1) c1 moles per liter of substance 1
(2) c2 moles per liter of substance 2
(3) c1 moles per liter of substance 1 and c2
moles per liter of substance 2."
Solution of Binary Mixture
Wavelength 1
Am,1 = a1,1*b*c1 + a2,1*b*c2
Am,1 = (a1,1*c1 + a2,1*c2)*b
Wavelength 2
Am,2 = a1,2*b*c1 + a2,2*b*c2
Am,2 = (a1,2*c1 + a2,2*c2)*b
Solution of Binary Mixture
let A1 = Am,1 A2 = Am,2
D1 = a1,1 D2 = a1,2
E1 = a2,1 E2 = a2,2
then A1 = (D1*c1 + E1*c2)*b
A2 = (D2*c1 + E2*c2)*b
Solution of Binary Mixture
solve for c2
A2/b = (D2*c1 + E2*c2)
A2/b - D2*c1 = E2*c2
E2*c2 = A2/b - D2*c1
c2 = (A2/(b*E2) - (D2*c1)/E2
Solution of Binary Mixture
then
A1 = (D1*c1 + E1((A2/(b*E2)-(D2*c1)/E2))*b
A1/b = (D1*c1 + E1((A2/(b*E2)(D2*c1)/E2))
A1/b = (c1(D1 - D2*(E1/E2))+(E1/E2)*(A2/b))
A1/b - (A2/b)*(E1/E2) = c1(D1-D2*(E1/E2))
Solution of Binary Mixture
thus
(A1/b - (A2/b)*(E1/E2))
c1 = -------------------------------
(D1 - D2*(E1/E2))
and
C2 = (A2/(E2*b) - (D2*c1)/E2
Method of Standard Addition
Al c s Vs
cx = ---------------
(A2 - Al )Vx
HP8452a Spectrometer
"Time-Based Scan Reports"
"Time-Based Scan Reports"
Solution of Binary Mixture
Photometric Titrations
Typical photometric titration curves. Molar absorptivities of the
substance titrated, the product, and the titrant are given by
s, p, and t, respectively
Photo-acoustic Spectroscopy
Block diagram
of a single-
beam photo-
acoustic
spectrometer
with digital
data
processing
Photo-acoustic Spectroscopy
Photo-acoustic spectra
of smears of blood and
blood components
