Chem 454

Visible/UV Absorption Spectra

Molecular Spectra

• can't usually get molecules to emit
• thermal, bombardment produces molecular damage

Samples

• Gases-- often high resolution, details
• Liquids-- usually dilute solutions
  • relatively broad bands (50-100 nm)
  • little detailed information about the species
  • hard to distinguish individual species, lots of overlap
• Solids-- often somewhat narrower lines again
  • powers relatively useless (scattering)
  • crystals, glasses
  • scattering is a problem if you measure transmitted light (since you assume all losses are due to absorption)
  • reflections spectra-- can get information on a sample that scatters light (absorbed light isn't scattered)
  • photoacoustic spectra-- (absorbed light causes the signal)

• Valence Electrons
• Electronic Transitions
• Molecular Orbitals, Molecular Energy Levels
  • Superimpose small energy details (vib, rot'n)
  • now many transitions, close in energy

Gases

• may resolve actual electronic/vibrational states
  • (I₂ vapor shows this)
  • may actually see 10-100 separate lines in a given spectral band
• with very careful work, can see rotational levels too
  • not with monochromator (BW too large)
  • with narrow tunable lasers
• a typical gas sample is very dilute
• at STP conc is 1 mole/22.4 l = 50 mmolar
• more likely to be 10-100 torr (1/70- 1/7 atm)
• 7 to 0.7 mMolar

Solutions

• significant line broadening
  • solvent interactions
  • variable effect, variable energies
  • slightly different absorption band for each molecule
  • net: reduced absorption, wider band
• enough that we are unlikely to see individual vibrations
  • but all show up, yielding 50-100 nm wide bands
  • most molecular detail is averaged away
• concentrations:
  • rarely "neat" or pure
  • e is typically 100- 20,000
  • cell is typically 1 cm
  • Abs 0.1-0.7 desirable
  • at 0.5 Abs, 1000 e, c= 0.5 mMolar
  • at mol wt of 100, that's 50mg/liter= 50 ppm

Absorption, typically uses Lambert Beer Law

• Abs = -log₁₀( I/Io)
• A = e c l
• by convention, c in mol/liter, l= length in cm
• note that Abs, as a log, has no units
  • people who are uncomfortable with unitless quantities often express absorbance in AU or Absorbance Units-- a bit silly since 1 AU = 1 (no units.)
  • to be fair, this is often done to provide a clearer identity of a number (AU basically means this is an absorbance value.)
• e has units cm-1 M-1
• Molar Extinction Coefficient, e = (Greek Epsilon)
• Visible Absorption
  • by transitional metal complexes
    • metal ion-water typically has e= 100-1000 (you see the color at 0.1 M but a 0.001 M solution looks virtually colorless.)
    • simple spectrophotometric analysis involves reactions to produce complexes with much larger extinction coefficients (like Fe + 1,10 phenathrolene.) These may have e=1000-5000.
  • species that absorb very strongly (like dyes) often involve pi electrons and e up to 25,000 is common.

• the standard cuvet (cuvette) has 1 cm path length
  • typically quartz, but also glass and selected plastics
  • optical quality windows are used
• longer cells (to 10 cm) are available for weak absorbing species or very dilute solutions.
  • (For example, species with very low solubility)
  • few cell compartments can handle a cell longer than 10-15 cm.
  • there are multipass cells that effectively transform a 10 cm zone into, say, 1000 cm or 10 m path length
    • one problem-- 100 reflections off mirrors
    • if 5% reflection, cell alone only transmits 0.95¹⁰⁰ = 0.6% transmitting