Chemistry 311: Instrumentation Analysis

Transcription

1 : Text: Ch. 9,10, 11 Rouessac (~1 weeks) Introduction to UV/Visible Molecular Spectroscopy Beers law and limitations MO Theory and Absorption etc Luminescence IR Techniques Winter 2011 Page 1 Molecular Absorption Spectroscopy: Page 2 1

2 Molecular Absorption Spectroscopy: Page 2 Beer s Law: A = -log T = log P 0 /P t = εbc However, this never realized as scattering and other losses also reduce beam. Losses can be accounted for by using solvent, ie., Beer s Law: For multiple absorbing species A = A 1 + A 2 +A 3... = ε 1 bc 1 + ε 1 bc 1 + ε 1 bc

3 Limitations of Beer s Law: Real Limitations High Concentrations (>0.01 M) Analytes intact and alter properties Results in non-linear calibrations Salts and other electrolytes also a factor ε is dependant on the refractive index of the medium and the refractive index is dependant on the solution composition. Not a significant problem for concentrations less than 0.01M Apparent Limitations Apparent Chemical Limitations When analyte chemically reacts (or associates) with solvent Apparent instrument limitations Beer s law applies to monochromatic radiation Molecular species produce broad bands Stray Light can also cause a deviation from linear Beer s law 3

4 Limitations of Beer s Law: Stray Radiation Causes a similar effect to Polychromatic Review of Basic MO Theory: 4

5 Review of Basic MO Theory: Review of Basic MO Theory: 5

6 Transition Types and Properties: σ σ* Transitions: These require significant energy. CH 4 : σ C-H σ* C-H; λ = 125 nm C 2 H 6 : σ C-H σ* C-H; λ = 135 nm Spectral range difficult to observe. ΔE =hν significant n σ* Transitions: λ range 150 nm to 250 nm ε small 100 to 3000 L cm -1 mol -1 strongly effected by solvent (shorter λ) Most effected by bond type n π* Transitions: with π π* Transitions represent the most applicable to organic molecules spectrally convenient range 200 to 700 nm Required unsaturated functional group referred to as chromophore ε small 10 to 100 L cm -1 mol -1 effected by solvent (shorter λ - a blue shift or hypochromic shift) hypochromic shift due to solvation stabilization of n e- (up to 30nm shift) Transition Types and Properties: π π* Transitions: Most commonly utilized transitions (200 to 700 nm) ε large 1000 to 20,000 L cm -1 mol -1 solvent can produce a longer λ - a red shift or bathochromic shift (~5nm) bathochromic shift due to a stabilization of both π and π* (excited more) Absorption by Inorganic Species: Many inorganic anions contain n electrons and/or π bonds, thus, n π* and π π* transitions are also important with Metals transitions involving d (and f) electrons important d orbital transitions common and important Crystal-Field theory (Ligand field) Charge-transfer absorptions also very important (very large ε) Page 2 6

7 Page 2 Crystal-Field Theory (Ligand-Field Theory): d - orbital orientation 7

8 Crystal-Field Theory (Ligand-Field Theory): Ligand effect on energy Additional Considerations: Solvents: Can absorb radiation to produce overlapping peaks (spectral window), shift λ maxim by interaction with the analyte. In general polar solvents have a more significant effect and tend to smooth fine structure. Nonabsorbing Analytes: can be measured by addition of a reagent that produces a complex or other chromatophoric species. λ selection: Normally chosen near maximum where curve is as flat as possible. This region is also less sensitive to slight λ deviations. Absorption is effected by solvent, ph, temperature, [electrolyte], and interfering substances. Therefore, these must be known and/or reproducible. Beer s law should never be assumed and multipoint calibration usually required. Standard Addition: method of choice for systems with a complex matrix. 8

9 Molecular Luminescence: Involves the measurement of emission from electronically excited species, normally produced by adsorption of radiation (Photoluminescence) or chemical reaction (Chemical Luminescence). Three main types: Fluorescence: Refers to emission occurring immediately after absorption of excitation wavelength. Normally does not involve a change in electron spin. The observed λ is always equal (resonance fluorescence) or longer than the absorbed λ. Phosphorescence: Refers to emission occurring a set time after absorption of excitation wavelength. Involve a change in electron spin. The observed λ is always longer than the absorbed λ. Chemical Luminescence: The measurement of emission resulting from a chemical reaction. The λ observed is not normally produced by the analyte but instead by a reaction product. Theory of Photoluminescence: 9

10 Rates of Absorption and Emission: Absorption rate to sec Fluorescent rates inversely proportional to molar absorptivities. Since ε is an indication of absorption probability, emission probability linked. ε = 10 3 to 10 5 excited state lifetime 10-7 to 10-9 sec ε = 10 to 10 3 excited state lifetime 10-6 to 10-5 sec ε for singlet triplet very weak 10-4 to 10 sec: Phosphorescence Relaxation / Deactivation Processes, rates vary by type Vibrational relaxation or less. Energy transferred by collision. Significantly enhanced in condensed phases such as liquids. In solution, fluorescence is often from lowest vibrational state of excited state. Internal Conversion: Transfer from initial excited state to a lower energy excited state. These occur without the emission of radiation. External Conversion: Transfer of energy to solvent. Also called, Collisional Quenching reduced by low T and high viscosity Intersystem Crossing: Conversion with a change of multiplicity Phosphorescence: emission after intersystem crossing Dissociation: Electron transferred to unstable vibrational state Predisposition: Intersystem crossing to unstable vibrational state 10

11 Key Variables effecting Fluorescence: Quantum Yield (Quantum Efficiency, φ): the ratio of molecules that luminesce to the total number of excited molecules. where: k f = rate of fluorescence; k i = rate of Intersystem crossing k ec = rate of external conversion; k ic = rate of internal conversion k pd = rate of predissociation; k d = rate of dissociation Quantum Efficiency and transition type: σ * σ fluorescence transitions rare as at 200 nm (140 kcal/mol) these are energetic enough for k pd and k d to dominate π* n and π* π most common, with π* π having greatest φ Since π* π has larger ε k f is larger for π* π than π* n Also energy difference between π* and triplet states large so that k i is much smaller 11

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14 Inorganic Species: Direct fluorescence - form chelate For species that quench, monitor decrease Transition metals have greater deactivation mechanisms that limit fluorescence most methods apply to non-transition metals Non-transition metals usually form colorless chelates. Typical Examples for Inorganic Analysis: 14

15 Chemiluminescence: Produced when a chemical reaction yields an electronically excited species that emits light as it relaxes to it s ground state. A + B C* + D C* C + hν I CL = φ CL (dc/dt) = φ EX φ EM (dc/dt) Where: I CL = Intensity of Chemical Luminescence; φ CL = quantum yield of CL dc/dt = rate of C formation; φ EX = states per C reacted; φ EM = photons per C This technique is extremely sensitive as little or no background. Very specific, very simple apparatus required (vial and photomultiplier). However, not a very common occurrence Bioluminescence: Chemiluminescence from biological systems (fire fly) 15