Presentation on theme: "UV / visible Spectroscopy l Introduction l Identification of organic species l Quantitation of inorganic species Colorimetric analysis."— Presentation transcript:
UV / visible Spectroscopy l Introduction l Identification of organic species l Quantitation of inorganic species Colorimetric analysis
UV / visible Spectroscopy l The origin of the analytical signal l Excitation of an atom or molecule by ultraviolet or visible radiation. l 190 - 900nm
UV / visible Spectroscopy l The radiation which is absorbed has an energy which exactly matches the energy difference between the ground state and the excited state. l These absorptions correspond to electronic transitions.
l Electronic transitions involve the promotion of electrons from an occupied orbital to an unoccupied orbital. l Energy differences of 125 - 650 kJ/mole.
UV / visible Spectroscopy l Beer-Lambert Law A = log(I O /I) = cl
UV / visible Spectroscopy A = log(I O /I) = cl v A = Absorbance (optical density) v I O = Intensity of light on the sample cell v I = Intensity of light leaving the sample cell v c = molar concentration of solute v l = length of sample cell (cm) = molar absorptivity (molar extinction coefficient)
UV / visible Spectroscopy l The Beer-Lambert Law is rigorously obeyed when a single species is present at relatively low concentrations.
UV / visible Spectroscopy l The Beer-Lambert Law is not obeyed: v High concentrations v Solute and solvent form complexes v Thermal equilibria exist between the ground state and the excited state v Fluorescent compounds are present in solution
UV / visible Spectroscopy l The size of the absorbing system and the probability that the transition will take place control the absorptivity ( ). l Values above 10 4 are termed high intensity absorptions. l Values below 1000 indicate low intensity absorptions which are forbidden transitions.
UV / visible Spectroscopy l Organic Spectroscopy l Transitions between MOLECULAR ORBITALS
UV / visible Spectroscopy l Highest occupied molecular orbital HOMO l Lowest unoccupied molecular orbital LUMO
l Not all transitions are observed l There are restrictions called Selection Rules l This results in Forbidden Transitions
UV / visible Spectroscopy l The characteristic energy of a transition and the wavelength of radiation absorbed are properties of a group of atoms rather than of electrons themselves. l The group of atoms producing such an absorption is called a CHROMOPHORE
ALWAYS use in conjunction with nmr and infrared spectra.
UV / visible Spectroscopy l As structural changes occur in a chromophore it is difficult to predict exact energy and intensity changes. l Use empirical rules. Woodward-Fieser Rules for dienes Woodward’s Rules for enones
UV / visible Spectroscopy 1. Bathochromic shift (red shift) v lower energy, longer wavelength v CONJUGATION. 2. Hypsochromic shift (blue shift) v higher energy, shorter wavelength. 3. Hyperchromic effect v increase in intensity 4. Hypochromic effect v decrease in intensity