chemistry 311: topic 2 - atomic spectroscopy · 2006-10-12 · chemistry 311: topic 2 - atomic...

Chemistry 311: Topic 2 - Atomic Spectroscopy

Introduction to UV/Visible Molecular Spectroscopy (CH 13/14) o Beers law and limitations o MO Theory and Absorption o etc

Luminescence (CH 15)

IR Techniques (CH 16/17)


Molecular Absorption Spectroscopy:(See Skoog, Table 13-1, Review Terms)


A = log ≈ log Psolvent

Psolution

P0

Pt

Beer’s Law: A = -log T = log P0/Pt = εbc However, this never realized as scattering and other losses also reduce beam (See Skoog, Figure 13-1) Losses can be accounted for by using solvent, ie.,

Beer’s Law:

For multiple absorbing species o A = A1 + A2 +A3.... = ε1bc1 + ε1bc1 + ε1bc1 + ...


Limitations of Beer’s Law: Real Limitations

o High Concentrations (>0.01 M) Analytes intact and alter properties Results in non-linear calibrations

o Salts and other electrolytes also a factor o 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 o Apparent Chemical Limitations

When analyte chemically reacts (or associates) with solvent o Apparent instrument limitations

Beer’s law applies to monochromatic radiation • Molecular species produce “broad” bands • (See Skoog Figures 13-4, 13-5)

Stray Light can also cause a deviation from linear Beer’s law See Figure 13-9


Limitations of Beer’s Law: Beer’s law applies to monochromatic radiation

o For two wavelengths λ’ and λ”

Equivalent to : o Am = log(P0’ + P0”) - log(P0’10-ε’bc + P0” -ε”bc) o If ε’ = ε” then: Am = log(P0’) - log(Pt’) = ε’bc

Am = log (P0’ + P0”)

(Pt’ + Pt”) Am = log

(P0’ + P0”)

(P0’10-ε’bc + P0” -ε”bc)


Limitations of Beer’s Law: Stray Radiation Causes a similar effect to Polychromatic

A’ = log (P0 + Ps)

(Pt + Ps)


Review of Basic MO Theory:


Review of Basic MO Theory: H 1s and C 2sp2 σ bond (2x) and σ* bond (2x) O 2sp2 and C 2sp2 σ bond and σ* bond O 2px and C 2 px π bond and π* bond O 2sp2 n lone pair (2x) _ _σ* _ _π* _↑_ 2sp2 C 2px_↑_ _↑_2pxO, _↑↓_ _↑↓_2sp2 H 1s _↑_ _↑↓_π Note:

Another C 2sp2 and O 2sp2 interact for σ & σ* bond Another C 2sp2 and H s interact for σ & σ* bond _↑↓_σ

C = OH

H

••••


Transition Types and Properties: σ → σ* Transitions:

These require significant energy. CH4: σ C-H → σ* C-H; λ = 125 nm C2H6: σ C-H → σ* C-H; λ = 135 nm Spectral range difficult to observe. ∆E =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)


Crystal-Field Theory (Ligand-Field Theory): d - orbital orientation


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.


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: (see Skoog, Figure 15-1)


Rates of Absorption and Emission: Absorption rate 10-14 to 10-15 sec

Fluorescent rates inversely proportional to molar absorptivities. Since ε is an indication of absorption probability, emission probability linked.

o ε = 103 to 105 → excited state lifetime 10-7 to 10-9 sec o ε = 10 to 103 → excited state lifetime 10-6 to 10-5 sec o ε for singlet → triplet very weak → 10-4 to 10 sec: Phosphorescence

Relaxation / Deactivation Processes, rates vary by type

o Vibrational relaxation 10-12 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.

o Internal Conversion: Transfer from initial excited state to a lower energy excited state. These occur without the emission of radiation.

o External Conversion: Transfer of energy to solvent. Also called, Collisional Quenching reduced by low T and high viscosity

o Intersystem Crossing: Conversion with a change of multiplicity o Phosphorescence: emission after intersystem crossing o Dissociation: Electron transferred to unstable vibrational state o Predisposition: Intersystem crossing to unstable vibrational state


Key Variables effecting Fluorescence:

Quantum Yield (Quantum Efficiency, φ): the ratio of molecules that luminesce to the total number of excited molecules.

where: kf = rate of fluorescence; ki = rate of Intersystem crossing

kec = rate of external conversion; kic = rate of internal conversion kpd = rate of predissociation; kd = rate of dissociation

Quantum Efficiency and transition type:

o σ * → σ fluorescence transitions rare as at 200 nm (140 kcal/mol) these are energetic enough for kpd and kd to dominate

o π* → n and π* → π most common, with π* → π having greatest φ o Since π* → π has larger ε ∴ kf is larger for π* → π than π* → n o Also energy difference between π* and triplet states large so that ki is

much smaller Key Variables effecting Fluorescence:

φ = kf

kf + ki + kec + kic + kpd + kd


Molecular Structure: o Low energy, intense π → π* transitions occur in aromatic systems, ∴

these are the most common. o Simple heterocyclic rings, pyridine, furan, thiophene, and pyrrole do not

fluoresce as n → π transition lowest and leads to triplet state conversion o Fused heterocyclic rings do fluoresce o Rigid aromatic rings fluoresce much stronger than non-fused ie.

biphenyl, biphenylene, and fluorene (More rigid lower internal conversion)

Temperature and Solvent Effects

o High temperature facilitates deactivation and thus inhibits fluorescence o lower viscosity solvents enhance collisions and thus deactivation o Heavy atom solvents (Cl, Br, etc.) facilitate intersystem crossing.

pH effect o For molecules that have acidic or basic characteristics pH huge effect

Dissolved Oxygen o dissolved oxygen significantly reduces fluorescence. photochemical

oxidation or paramagnetic promotion of intersystem crossing


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:


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

ICL = φCL (dC/dt) = φEX φEM (dC/dt) Where: ICL = Intensity of Chemical Luminescence; φCL = quantum yield of CL dC/dt = rate of C formation; φEX = states per C reacted; φEM = 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)

chemistry 311: topic 2 - atomic spectroscopy · 2006-10-12 · chemistry 311: topic 2 - atomic...

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