CH 415 Chapter 14: UV-Vis Molecular Spectroscopy

UV-Vis Molecular Spectroscopy

• Used to study molecules and their electronic transitions

• Principle: The energy absorbed corresponds to the amount necessary to promote an electron from one orbital to another.

• Commonly used to determine the concentration of an absorbing species in solution (Quantitative Analysis) using Beer’s Law (A=log10 I0/I = elc) (e in Lmol^-1cm^-1, l in cm, c in mol/L

Molar absorptiviti’s (e)

e = 8.7 × 10^19 PA

A - capture cross section of molecule in cm² (about 10^-15 cm²/molecule

P - (allowed) transition probability, 0.1 - 1 (molar absorptivity 10^4 - 10^5 M^-1cm^-1

P< 0.01 (e < 10^-3) = forbidden transitions

Molecular Absorption

Absorption of light is a two step process:

1. Absorption M + hv → M* (absorption 10^-8 - 10^-9 sec)

2. Relaxation: M* → M + heat (relaxation process)

M* → A+B+C (photochemical decomposition)

M* → M + hv (emission of fluorescence and phosphorescence)

Visible Absorption Spectra

• Environment has a HUGE influence 

• Vapor: electronic/vibrational/rotational transitions

• (hexane): electronic transitions

• (aq): electron transitions (smooth)

* As intermolecular interaction increases, vibrational and rotational character decreases.

• the absorption of UV-Vis radiation generally results from excitation of bonding electrons

• Can identify functional groups in a molecule

• can be used for quantitive and qualitative analysis

Molecular orbital

the non-localized fields b/t atoms that are occupied by bonding electrons (when two atom orbitals combine, either a low-energy bonding molecular orbital or a high energy anti bonding 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 diatomic molecule H2

Subtractive combination - they subtract to get node, an anti bonding region

Addidtive combination - they add to get bonding region

End-on Interaction vs. In parallel interaction

UV/VIS electronic transitions

• molecules have quantized energy levels

• bonding orbitals are lower in energy than anti-bonding orbitals

• non-bonding orbitals contains lone pair of electrons

• as light absorbs electrons “jump” from bonding or non-bonding orbital to the anti-bonding orbitals

sigma* (normally empty)

Pi* (normally empty)

n(non-bonding)(contain lone pairs)

pi(bonding)(contain normal bonding pairs of electrons)

sigma(bonding)(contain normal bonding pairs of electrons)

3 types of transitions (arrange in terms of energy):

1. Excitation associated with single bond e- → vacuum UV (<185 nm) (difficult to access)

2. Most application of absorption spectroscopy → n→pi* or pi→pi* (200-700nm)(both require unsaturated functional group to provide pi orbitals - chromophores)

3. e for n→ pi* are usually low: 10-100 M^-1cm^-1

4. e for pi→pi* are usually higher: 1000-15000 M^-1cm^-1

1. sigma → sigma* TdeltaE large (lambda < 150 nm) e=10-10,000 L/mol*cm

2. n→sigma* (halogens, N,O,S) deltaE smaller (lambda = 150-250 nm) e=200-2000 L/mol*cm

3. pi→pi* n→pi* deltaE small (lambda = 200-700nm) e=10-10,000 L/mol*cm (ideal for UV-Vis spectrometry of organic chromophore)

(1 has the largest change in energy, 2 in middle, and 3 has smallest change in energy)

Typical UV Absorption Spectra

• organic molecules: n→sigma* (e = 100-3000)

pi→pi* (e = 1000-10,000)

• inorganic ions: transitions in the d electrons (non-bonding e-s)(low)

• Charge-transfer complexes (typically large; e>10,000)

Absorption by saturated organic compounds- have low e(max)

Absorption of unsaturated organic compounds - have high e(max) ; transition type: pi→pi* (for alkene 13,000; for alkyne 10,000)

Unsaturated functional group →pi orbital

Effects of Multiple Chromophores

Olefin - lambda (max) - 184; e(max)=10,000

Diolefin (unconjugated) - lambda (max) -185; e(max) = 20,000

Diolefin (conjugated) - lambda (max) - 217; e(max) = 21,000

Spectral Shift

• Red shift of lambda max w/ increasing conjugation (CH2=CHCH2CH2CH2CH2 (lambda (max) = 185 nm)

• CH2=CHCH=CH2 (lambda (max)=217)

• Red shift of lambda max with # of rings (benzene lambda max = 204 nm, Naphthalene lambda (max) - 286 nm

3 Electronic Transitions for organic molecules

1. sigma → sigma* transition in vacuum UV (single bonds)

2. n→sigma* saturated compounds with non-bonding e-s

- lambda = 150-250 nm; e=10-3000 (not strong)

3. n→pi*, pi→pi* requires unsaturated functional groups (e.g. double bonds, provide pi orbitals) most commonly used, energy good range for UV/Vis

- lambda about 200-700 nm

- n→pi* : e=10-100

pi→pi* : e =1000-10,000

Electronic Transitions for Inorganic compounds

• common anions n→pi* 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(Ni2+,Cr2O72-,Cu2+,Co2+)

-the bands are often broad

-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(Ho3+,Er3+,Pr3+,Sm3+)

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 donor is transferred to an orbital that is largely associated with the acceptor.

1. Large molar absorptivity (e(max)>10,000

2. Many organic and inorganic complexes

• metal ions involved, organic compounds (Fe(SCN)2+, Fe(then)32+, Starch-I5-)

Qualitative Applications of UV-Vis Absorption Spectroscopy

1. Solvents Effect - they need to be transparent and do not erase the fine structure arising from the vibrational effects

-Polar solvents (heptane, water) tend to cause this problem, same solvent must be used when comparing absorption spectra for identification purpose.

2. Slit Width Effect - More narrower slit width (1 nm) →higher absorbance; More wider slit width (20 nm) → lower absorbance

3. Bathochromic Shift - the shift of absorption to a longer wavelength due to substitution or solvent effect ( a red shift)

2. Hypsochromic Shift - the shift of absorption to a shorter wavelength due to substitution or solvent effect (a blue shift)

3. Hyperchromic Effect - an increase in absorption intensity.

4. Hypochromic Effect.- a decrease in absorption intensity

5. Chromophore - a covalently unsaturated group responsible for electronic absorption (ex. C=C, C=O, NO2)

6. Autochrome - a saturated group with non bonded e-s that when attached to a chromophore, alters both the wavelength and intensity of absorption

Bathochromic Shift (red shift pi→pi*)

• a change of medium - increasing polarity of solvent, then increase the attractive polarization forces b/t solvent and absorber, thus decreasing the energy of the unexcited and excited states with later greater.

• substitution: an autochrome is attached to a C=C

Hypsochromic Shift (a blue shift n-pi*)

• a change of medium - increasing polarity of solvent → better salvation of e- pairs (n level has lower E)

Ex. Acetone: lambda(max) of 279 nm (in hexane) is 264.5 nm (in water)

• Substitution: an aux-chrome is attached to double bonds where n electrons are available (ex. C=O)

Hyperchromic Effect vs. Hypochromic Effect

Hyperchromic - leading to increased absorption intensity

Hypochromic - leading to decreased absorption intensity

Absorption Shifts

Bathochromic - to longer wavelength

Hypsochromic - to shorter wavelength

Hyperchromic - to greater absorbance

Hypochromic - to lower absorbance

Quantitative Applications of UV-VIS Absorption Spectroscopy

Advantages:

• 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)

• relationship b/t A and C (standard additional method, analysis of mixtures)

• Titrations

• Complex ions (method of continuous variations, Mole-Ratio method)

• Kinetic methods (stopped-flow kinetic)

Relationship b/t A and C (standard addition method)

• very useful when sample is in a complex matrix

• measurement done to sample+spiked sample

• a standard-addition analysis using only two increments of sample

Cx = A1CsVs / (A2-A1)Vx

Solvig for a mixtures

• both species (M and N) absorb differently

• select two wavelengths

• perform 4 calibration curves

• calculate 4 absorptivity coefficients: eM1,eM2,eN1,eN2

• measure absorbance at both wavelengths

A1 = eM1bCM + eN1bCN

A2 = eM2bCM + eN2bCN

Titrations

Titration Curve

• spectrophotometry used to determine the final point (source→filter→cell on magnetic stirrer→detector)

Photometric titrations:

a. colorless analyte and regent → colored product

b. colorless analyte and colored reagent → colorless product

c. colored analyte and colorless reagent → colorless product

d. colored analyte and reagent → colorless product

Complex Ions

Continuous-Various Method:

• used to calculate stoichiometric ratios

• Vtotal and Ntotal are constant

• the mole ratio of reactants varies systematically

Mole-Ratio Method:

• concentration of one reactant held constant, while other is varied

• two straight lines of different slopes that intersect at a mole ratio corresponding to the combined ratio in the complex

Kinetic Methods

Enzymatic reactions:

• enzyme used to catalyze the rxns

• determination of [E]:[S] » Km

• determination of [E]:[S'] « Km

Kinetic monitoring techniques - stopped-flow:

• a stopped-flow instrument can mix solutions in a couple of milliseconds and measure the dynamics of interacting proteins

Single vs. Double Mixing Stopped-Flow

A (yellow) +2B (light blue) →3C (red) (+9.67, +0.05)