Applications of UV VIS Molecular Spectroscopy Chapter 14 Molar absorptivities 8 7 x 10 19 P A A capture cross section of molecule in cm2 10 15 cm2 molecule P allowed transition probability 0 1 1 104 to 105 M 1cm 1 P 0 01 10 3 forbidden transitions Molecular Absorption Absorption of light is a two step process Absorption n h absorption 10 8 to 10 9 sec Relaxation n heat relaxation process n M A B C photochemical decomposition n h emission of fluorescence and phosphorescence Visible Absorption Spectra Environment has a HUGE influence Vapor electronic vibrational rotational transitions hexane electronic vibrational rotational transitions aq electronic vibrational rotational transitions smooth The absorption of UV visible radiation generally results from excitation of bonding electrons Can identify functional groups in a molecule Can be used for quantitative and qualitative analysis Molecular orbital is the nonlocalized fields between atoms that are occupied by bonding electrons when two atom orbitals combine either a low energy bonding molecular orbital or a high energy antibonding molecular orbital results Sigma orbital The molecular orbital associated with single bonds in organic compounds Pi orbital The molecular orbital associated with parallel overlap of atomic P orbital n electrons No bonding electrons MO theory to examine the simple diatomic molecule H2 The two 2p atomic orbitals overlap Interaction in end on Interaction in parallel http www youtube com watch feature player embedded v VfBcfYR1VQo UV VIS electronic transitions Typical UV Absorption Spectra Organic molecules n 100 3000 1000 10 000 Inorganic ions Transi1ons in the d electrons nonbonding electrons low Charge transfer complexes Typically large 10 000 Absorption by organic compounds Absorption of saturated organic compounds LOW Absorption of unsaturated organic compounds Unsaturated functional group orbital Effects of Multiple Chromophores Rule of thumb for conjugation Transitions for organic molecules transition in vacuum UV single bonds n saturated compounds with non bonding electrons 150 250 nm 100 3000 not strong n requires unsaturated functional groups eq double bonds provide orbitals most commonly used energy good range for UV Vis 200 700 nm n 10 100 1000 10 000 Absorption by inorganic compounds What about inorganics Common anions n nitrate 313 nm carbonate 217 nm Most transition metal ions absorb in the UV Vis region For the first and second transition metal series the absorption process results from transitions of 3d and 4d electrons The bands are often broad The position of the maxima are strongly influenced by the chemical environment Depend on its oxidation state the nature of the ligand bonded to it In the lanthanide and actinide series the absorption process results from electronic transitions of 4f and 5f electrons first and second transition metal series lanthanide and actinide series Charge transfer absorption Charge transfer absorption A charge transfer complex consists of an electron donor group bonded to an electron acceptor When this product absorbs radiation an electron from the donor is transferred to an orbital that is largely associated with the acceptor 1 Large molar absorptivity max 10 000 2 Many organic and inorganic complexes charge transfer complex Metal ions involved Organic compounds Qualitative applications of UV Vis absorption spectroscopy Now solvents are your container Solvents effect They need to be transparent and do not erase the fine structure arising from the vibrational effects Polar solvents generally tend to cause this problem Same solvent must be used when comparing absorption spectra for identification purpose The effect of slit width 1 nm 5 nm 10 nm 20 nm Definitions CHOMOPHORE A covalently unsaturated group responsible for electronic absorption for example C C C O and NO2 AUXOCHROME A saturated group with nonbonded electrons which when attached to a chromophore alters both the wavelength and the intensity of the absorption BATHOCHROMIC SHIFT The shift of absorption to a longer wavelength due to substitution or solvent effect a red shift HYPSOCHROMIC SHIFT The shift of absorption to a shorter wavelength due to substitution or solvent effect a blue shift HYPERCHROMIC EFFECT An increase in absorption intensity HYPOCHROMIC EFFECT A decrease in absorption intensity Bathochromic shift Red shift A change of medium Increasing polarity of solvent then increase the attractive polarization forces between solvent and absorber thus decreases the energy of the unexcited and excited states with the later greater Substitution an auxochrome is attached to a carboncarbon double bond Hypsochromic shift Blue shift n A change of medium Increasing polarity of solvent better solvation of electron pairs n level has lower E For example Acetone max 279 nm in hexane max 264 5 nm in water Substitution an auxochrome is attached to double bonds where n electrons are available e g C O Undergraduate Instrumental Analysis Sixth Edition By James W Robinson Eileen M Skelly Frame George M Frame II Hyperchromic effect Hypochromic effect Hyperchromic effect leading to increased absorption intensity Hypochromic effect leading to decreased absorption intensity Quantitative applications of UV Vis absorption spectroscopy why Wide applicability organic and inorganic Low detection limit High selectivity Good accuracy Ease and convenience of data process Applications Absorbing species Non absorbing species derivation The relationship between A and C Standard additional method Analysis of Mixtures Titrations Complex ions the method of continuous variations Mole Ratio method Kinetic methods Stopped flow kinetic The relationship between A and C Standard Addition Method Very useful when sample is in a complex matrix Measurement done to the sample spiked sample A standard addition analysis using only two increments of sample A1cs Vs cx A2 A1 Vx The relationship between A and C Solving for a mixtures Both species M and N absorb di erently Select two wavelengths Perform 4 calibra1on curves Calculate 4 absorp1vity coe cients 1 2 N1 2 Measure the absorbance at both wavelengths A1 1bcM N1bcN A2 2bcM N2bcN Titrations Titration curve Spectrophotometry is used to determine the nal point Instrumentation Photometric Titra1ons Complex ions Continuous various Method Used to calculate stoichiometry ra1os Vtotal and Ntotal are constant The mole ra1o of reactants varies systema1cally Complex ions Mole Ratio Method Concentra1on of