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Applications of UV/Vi Molecular Absorption Spectrometry. I. Absorbing Species. • Absorption of light is a two step process: Absorption M + h n M* Relaxation M* M + heat. • The heat evolved (very minute) does not affect the system temperature.
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Applications of UV/Vi Molecular Absorption Spectrometry I. Absorbing Species • Absorption of light is a two step process: Absorption M + hn M* Relaxation M* M + heat • The heat evolved (very minute) does not affect the system temperature. • There are other modes of relaxation that cause deviations in Beer’s Law. A) Photodecomposition B) Fluorescence C) Phosphorescence
Absorbing species containing s, p, and n electrons (organic compounds). Antibonding Bonding Formaldehyde
Observed Electronic Transitions •ss * and n s * are high energy, short wavelength transitions. - ss *l < 185 nm (Vacuum UV) - n s *l = 150 – 250 nm (mainly vac. UV) - Very difficult to measure • n p *and pp * 200 – 700 nm • The most important and useful • transitions in molecular UV • spectroscopy. - Molar absorptivities (e): n p * 10 – 100 L cm-1 mol-1 pp * 1000 – 10,000
s* s* s* s* p* p* p* p* hv n hv n p p p p s s s s ethylene absorbs at longer wavelengths: max = 165 nm, = 10,000 The n to π* transition is at even lower wavelengths but is not as strong as π to π* transitions. It is said to be “forbidden.” Example: Acetone n- max =188 nm ; = 1860 n max = 279 nm ; = 15
Factors that change transition energies 1. Solvent Effect • As polarity increases, λ↓ for n π* (shift to shorter λ, Blue shift) • As polarity increases, λ ↑ for π π* (Shift to longer λ, Red shift) • As polarity increases, fine structure ↓ (Fine structure due to vibrational modes) Absorption spectra for tetrazine
In heptane In methanol UV-VIS spectra of 4-methyl-3-penten-2-one in methanol (left) and heptane (right). The ~320 nm absorption is the n π* transition, the ~240 nm is mainly π π*
2. Organic Chromophores Molecules having unsaturated bonds or free nonbonding electrons that can absorb radiation of relatively low energy are called chromophores. Examples include alkenes, alkynes, ketones, aldehydes, phenyl and other aromatic species, etc. 2.a. Effect of Conjugation of Chromophores As conjugation is increased in a molecule, more delocalization (stability) of the π electrons results. The effect of this delocalization is to decrease the π* molecular orbital. The result is a decrease in transition energy from π- π* and thus a red or bathochromic shift.The molar absorptivity will increase in this case and better quantitative analysis will be achieved.
• Conjugation causes delocalization of p electrons stabilizing p*, therefore shifting absorbance to longer wavelength (lower energy).
Highly conjugated molecules are colored Lycopene β-Carotene
