Instrumental Analysis Exam 3B: Application of UV-Vis Molecular Absorption Spec
What is a charge-transfer complex?
A charge-transfer complex consists of an electron-donor group bonded to an electron acceptor. These complexes can be organic or inorganic
What is an auxochrome?
A functional group that does not itself absorb in the UV region, but which has at least one pair of n electrons and has the effect of shifting chromophore peaks to longer wavelengths (red shift) as well as increasing their intensities.
Find ε for indicator species HIn and In⁻, given b (1.00cm), [HIn], A₄₈₅, A₆₂₅.
Beer's law: A=εbc Since b=1.00, ε=A/c for each species at each wavelength.
What is the difference between a singlet and a triplet?
Both involve the promotion of one electron from n→π*. In a singlet, the electrons maintain opposing spins. In a triplet, the spins are parallel, causing instability.
Why do conjugated double bonds cause a red shift (to longer wavelengths)?
Conjugation provides the electrons with much larger bonding areas (areas of overlap) in which to move around, lowering the energy and increasing λmax with each conjugation
Find Ka for the indicator HIn, given A₄₈₅, A₆₂₅ of a buffer solution (pH=5.00) containing the indicator.
Ka=[H⁺][In⁻]/[HIn] Write equations for A at each respective wavelength x, given b=1.00 cm, and using previously calculated ε for each species, then solve for [HIn] and [In⁻]: Ax= ε[HIn] + ε[In⁻] [H⁺]= 1.00 x 10⁻(pH)
Which chromophores have a higher ε between alkenes and alkynes?
Molar absorptivity is due to double bonding, so alkenes have higher ε
What are chromophores?
Molecules capable of absorbing UV-Vis radiation and which contain a covalently unsaturated functional group responsible for electronic absorption (for example, C=C, C=O, and NO₂).
What are n electrons?
Non-bonding electrons
What are the three possible relaxation mechanisms for an excited molecule?
Non-radiative: M*→M+heat Photochemical decomposition: M*→A+B+C Radiation emission: M*→M+hv (fluorescence or phosphorescence)
What ε results from forbidden transitions?
P< 0.01 forbidden transitions ⟶ (ε < 10⁻³M⁻¹ cm⁻¹)
What is the range of ε from allowable transitions?
P> 0.01-1 allowable transitions ⟶ (ε ~10⁻⁴ to 10⁻⁵M⁻¹ cm⁻¹)
How does slit width (effective bandwidth) affect absorption spectra?
Peak heights and separation are distorted at wider bandwidths. Narrow bandwidths give better resolution (sharper peaks).
When do n→σ* transitions occur?
Saturated compounds (containing heteroatoms w/lone e⁻ pairs) (ex.) CH₃OH etc: λ=150-250nm
What variables can influence the absorption spectrum of a substance?
Solvent pH of solution Temp [Electrolyte] Interfering species
What are the E₂ Band and B Band?
The UV spectra of aromatic hydrocarbons are characterized by three sets of bands that originate from π→π* transitions. The E₂ and B Bands are the weaker and weakest, respectively, of the three bands.
How does the excited state of a charge-transfer complex differ from that of an organic chromophore?
The excited state of a charge-transfer complex is the product of a kind of internal oxidation-reduction process. This behavior differs from that of an organic chromophore in which the excited electron is in a molecular orbital shared by two or more atoms.
What is a pi (π) orbital?
The molecular orbital associated with parallel overlap of atomic P orbital
What is a sigma (σ) orbital?
The molecular orbital associated with single bonds in organic compounds
What considerations are important when choosing a solvent?
Transparency Effects on the spectral fine signature from vibrational effects (seen well in gas-phase spectra)
What is charge-transfer absorption?
When a charge-transfer complex product absorbs radiation, an electron from the donor (a ligand) is transferred to an orbital that is largely associated with the acceptor (metal with empty orbitals).
How does the addition of unconjugated double bonds affect λmax and ε?
When db's are more than one single bond apart (unconjugated): ∙ε are relatively additive (hyperchromic shift) ∙λ is relatively constant
How does the environment affect the visible absorption spectra for an analyte?
With ↑ intermolecular interactions, there is ↓ vibrational and rotational character
What is molar absorptivity (ε)?
a measurement of how strongly a chemical species absorbs light at a given wavelength. It is an intrinsic property of the species; the actual absorbance of a material, of a sample also depends on the pathlength and the concentration of the species, according to the Beer-Lambert law,
When choosing chromophores, what criteria is used?
always want to select the highest ε possible
When ions undergo solvation by water in aqueous solution, why isn't it always written? revisit Slide 63 to understand changing solvent conditions (pH?) to change the hydration trend
as a rule, the solvated ion compound is written only when the water is deprotonated, like with Cr₂O₇²⁻. When the water molecules solvating an ion are not deprotonated, the ion is written by itself (i.e. Ni²⁺)
Why is it a rule that the same solvent must be used when comparing absorption spectra for identification purposes?
bc positions of absorption maxima are influenced by the nature of the solvent
Why is the electronic spectra of organic molecules containing chromophores usually complex?
because the superposition of vibrational transitions on the electronic transitions leads to an intricate combination of overlapping lines. The result is a broad band of absorption that often appears to be continuous.
How can confirmation of the presence of an aromatic amine or a phenolic structure may be obtained?
by comparing the effects of pH on the spectra of solutions containing the sample with published λmax values for phenol and aniline (and their respective ions).
How does the addition of conjugated double bonds affect λmax and ε?
conjugated db's increase λmax (red shift) along with additive ε
Hypochromic effect
decrease in absorption intensity
What types of transitions can be observed on the visible absorption spectrum of a vapor?
electronic vibrational rotational
Why are σ→σ* transitions rare?
excitation associated with single bond electrons require a lot of energy Large ∆E (λ<185nm)
What does absorption of UV-Vis radiation generally result from?
excitation of *BONDING* electrons
What the usual ε values for n→σ* transitions?
fairly low: 100-3000 M⁻¹cm⁻¹
What the usual ε values for π→π* transitions?
higher: 1000-15000 M⁻¹cm⁻¹
When do σ→σ* transitions occur?
in the high energy vacuum UV range (ex.) CH₄: λ=125nm
Hyperchromic effect
increase in absorption intensity
How is absorption spectra affected by addition of aromatic rings?
just like with conjugation, there is a red shift of λmax with increasing number of aromatic rings (ex.) benzene (E₂) λmax=204nm naphthalene (E₂) λmax=286nm
What causes the blue shift in carbonyl groups as solvent polarity increases?
lone pairs on the carbonyl oxygen form H-bonds with the polar solvent
What the usual ε values for n→π* transitions?
low: 10-100 M⁻¹cm⁻¹
What does large molar absorptivity value tell us about the electronic transition?
molar absorptivity indicates the probability of electron transfer, and a high value means that the transition occurs easily (high probability)
What function can n electrons perform?
n electrons can be useful to change polarity of a molecule and/or form H-bonds, for example
What solvent are more likely to result in spectra similar to gas-phase spectra?
nonpolar solvent such as hydrocarbons
What transitions cause a blue shift?
n→π* (slide 41)
What electronic transitions are most generally used for absorption spectroscopy?
n→π* or π→π* have small ΔE (200-700nm) Both require molecules with an unsaturated functional group to provide π orbitals (chromophores)
What types of transitions can be observed on the visible absorption spectrum of a non-polar solvent?
only electronic transitions
Solvent effects (polar v np) on absorption; blue-shift?
slide 40
red shift?
slide 41 find explanation
What are the cutoff wavelengths for solvents?
the approximate wavelength below which the solvent cannot be used bc of absorption (lower wavelength limit) −these wavelengths depend stongly on the purity of the solvent
What are molecular orbitals?
the nonlocalized fields between atoms that are occupied by bonding electrons. (when two atomic orbitals combine, either a low-energy bonding molecular orbital or a high energy anti-bonding molecular orbital results.)
How does transition probability relate to the lifetime of an excited molecule?
the shorter the lifetime in a particular state, the lower the probability for that state
How does a charge-transfer absorption spectra appear?
the transfer involves actual *movement* of electrons (rather than just intraorbital transition) and results in a defined peak and very large molar absorptivity (ε max > 10,000)
What types of transitions can be observed on the visible absorption spectrum of a polar solvent?
there will be a single smooth absorption band bc of the strong intermolecular forces
How are electronic transition energies calculated?
using the Schrodinger equation
What solvent should be used whenever possible?
water, bc it does not absorb above 190nm
What is a characteristic UV-Vis spectra for aromatic rings?
weak absorption band around 260nm, corresponding to the long-wavelength B Bands, with indication of vibrational fine structure appearing as a series of sharp peaks due to the superposition of vibrational transitions on the basic electronic transitions
What is a characteristic UV-Vis spectra for carbonyl groups?
weak absorption band in 280-290nm region with blue shift as solvent polarity increases
Can functional groups be detected using UV-Vis absorption spec?
yes, but not definitively. Other techniques must be used for confirmation.
What is the lifetime of a molecule excited by light absorption (M*)? M+hv→→M*
~10⁻⁸ to 10⁻⁹sec
How is molar absorptivity (ε) calculated?
ε=8.7x10¹⁹(P)(A), where P= prob. of electronic transition (0-1) A=cross section target area (~10⁻¹⁵cm²/molecule)
What are the most easily occurring electronic transitions?
π→π*
What are molecular electronic transitions possible for UV-Vis absorption?
σ→σ* π→π* n→σ* n→π*
What does a larger ε mean in terms of sensitivity?
↑ε means ↑sensitivity
Find A₄₈₅, A₆₂₅ of a solution that had a concentration of xM in the indicator before it was buffered (pH=6.00).
−Find the ratio (R) of [In⁻]/[HIn] in the buffer solution using previously calculated Ka and given pH=6.00: R=[In⁻]/[HIn]= Ka/[H⁺], where [H⁺]=1.00x10⁻⁶ −Solve for [HIn] and [In⁻] using the given initial concentration (x) and the ratio (R): [In⁻] + [HIn] = x R[HIn] + [HIn]= x =>[HIn]= x/(R+1) and [In⁻]= x - [HIn] −Find A₄₈₅, A₆₂₅: A₄₈₅,₆₂₅= (ε b [HIn]) + (ε b [In⁻])
Bathochromic shift
−shift of absorption to a longer wavelength due to substitution or solvent effect (red shift) −(π→π*) Slide 43
Hypsochromic shift
−shift of absorption to a shorter wavelength due to substitution or solvent effect (blue shift) −(n→π*) Slide 44
What spectral characteristics are seen from ions in the 1st and 2nd transition metal series? See Co/Cr lab and slide 63
∙Absorption from transitions of 3d and 4d e⁻ ∙broad bands of visible radiation at one of their oxidation states produces color ∙position of λmax strongly influenced by chemical environment ∙energies of the d-orbitals depend on nature of bound ligands
How can UV-Vis be used for inorganic analysis to detect metals?
∙metals form complexes bc they have empty orbitals in which to receive e- ∙ligands with lone e- will easily complex with metal and there will be a red shift
What spectral characteristics are seen from ions in the lanthanide and actinide series?
∙transition of 4f and 5f e⁻ occur, shielded from external influences by electrons that occupy orbitals with larger principal quantum numbers. As a result, the bands tend to be narrow and unaffected by the species bonded by the outer electrons
