Granger Chap 6
Chapter 6: Molecular UV-Vis Spectroscopy
UV-Vis Spectroscopy Overview
Definition: Technique used to determine the electronic nature (quantum) of molecules.
Quality Determination: Electronic nature can be qualitatively deduced.
Chromophores and Auxochromes
Chromophore: A structural feature or functional group that absorbs radiation.
Auxochrome: A functional group with one or more lone pairs of electrons that enhances the chromophore's absorption properties.
Chromogen: Molecule that absorbs UV-Vis radiation.
Electrons in Chromophores
Main Transitions: Related to electron transitions within the chromophore.
Auxochromes as Electron Donors: Includes functional groups like:
Hydroxyl groups
Amino groups
Carboxylic acid groups
Sulfonic acid groups
Impact: While they don't absorb radiation themselves, they enhance the absorption of the chromophore.
Page 2: Wavelength and Color Representation
Color Perception:
Absorbing red photons will make material appear blue.
Absorbing blue photons will make material appear red.
Wavelength Range: Covers 195 nm to 900 nm;
UVA: 315 – 400 nm
UVB: 280 – 315 nm
UVC: 100 – 280 nm
Page 3: Instrument Design
Components of UV-Vis Spectroscopy Instrument:
Source: Tungsten lamp, Deuterium lamp.
Monochromator: Separates light into different wavelengths.
Detector: Measures the intensity of transmitted light.
Signal Processor: Processes the detected signal.
Readout Device: Displays the results.
Spectrophotometer Example
Single Wavelength Spectrophotometer: Basic design illustrated.
Components: Lens, Slit, Grating, 1024-element diode array.
Page 4: Beer’s Law Introduction
Beer’s Law: Describes the relationship between absorbance and concentration.
Absorbance (A) = log10(P0/P)
Where P0 is initial radiant power, and P is power after passing through the sample.
Page 5: Derivation of Beer’s Law
Formula Elements:
A = Absorbance (unitless)
T = Transmittance (unitless)
ε = Molar absorptivity (L/mole·cm)
b = Path length (cm)
c = Concentration (mol/L)
Correlation: Relates absorbance to concentration via molar absorptivity.
Page 6: Example Problems
Example 1: Guanosine absorption at 275 nm, ε275 = 8400 M-1cm-1, A275 = 0.70. Calculate concentration.
Example 2: Calculate absorption coefficient with 4 g/L solution at 50% transmittance & 2 cm pathlength.
Example 3: Effect of concentration change to 8 g/L on transmission.
Additional Problem: Absorption coefficient for glycogen-iodine complex at given parameters.
Page 7: Quantum Considerations
Transitions in UV-Vis:
Sigma to Sigma Antibonding (σ)
Pi to Pi Antibonding (π)
Nonbonding to Pi Antibonding (n)
Molecular Orbital Theory (MO Theory): Used for understanding transitions.
Page 8: Structure Influences on UV-Vis Absorption
Saturated vs Unsaturated Molecules: Unsaturated hydrocarbons likely have detectable transitions.
Aromatic Molecules: Characterized by conjugated pi systems with unique photophysical properties.
Bathochromic Shift: Observed when energy gap decreases.
Page 9: Effects of Heteroatoms
Heteroatoms: Can influence absorption spectra through their lone pairs.
Must be bound to pi-system for significant effect.
Franck-Condon Principle: Relates to the coupling of electronic transitions in different states, influenced by overlap integrals.
Page 10: Solvent Effects
Solvent Impact: Background noise and potential perturbation of electronic structure due to dipolar relaxation.
Types of Solvents: Polar vs Nonpolar, affecting electron stabilization.
Page 11: Metal Complexes
Characteristics: Transition metal complexes are typically colored due to electronic transitions between d orbitals.
Ligand Interaction: Ligand field theory explains how external factors influence d orbital splitting.
Lewis Acid-Base interaction: Metal acts as a Lewis acid while ligand acts as a Lewis base.
Page 12: Spectrochemical Series
Ligand Influence: Arranges ligands based on their effect on d orbital splitting.
Types of Complexes: Focus on octahedral and tetrahedral complexes.
Page 13: Octahedral and Tetrahedral Complexes
Octahedral Crystal Field Splitting Diagram: Understanding energy differences in d orbitals under different geometries.
Tetrahedral Complexes: Different energy patterns compared to octahedral complexes.
Page 14: Color Perception Via Metal Complexes
Color Determination: Based on the absorbance spectrum, predicting the color of metal complex solutions.
Page 15: Selection Rules for Electronic Transitions
Conditions for Transitions: Must fulfill specific selection rules for electronic transitions to occur effectively.