free radicals basics

Detection
1. Free radical scavenger
  1. Addition = inhibition
2. EPR spectroscopy
  1. up/down orbital energy is different in a magnetic field
    1. exposed to microwave radiation with energy = energy difference between orbitals
      1. Unpaired electron (spin up or down) will resonate between two orbitals
        microwave absorbance with a frequency corresponding to the energy gap created by the magnetic field
        microwave frequency expressed as a fraction of magnetic field strength
        Resonance frequency depends on the nature of the free radical when magnetic field strength is fixed
  2. Electron Paramagnetic Resonance or Electron Spin Resonance
  3. Easiest in condensed phase ( [FR] is higher)
3. Difficult because FRs are short lived, especially reactive ones - low concentration
Chlorine and Bromine
1. Cl has 7 electrons
  1. Cl2 --> 2Cl*
    1. Initiated by UV light
  2. (Bromine behaves the same way)
Oxygen
1. Diatomic oxygen (O2)
  1. 12 electrons, 2 unpaired in pi*x and pi*y orbitals
    1. paramagnetic (but does not behave as a FR)
    2. 3 peaks on EPR spectrum
      1. 'triplet oxygen'
  2. Excitation by UV
    1. one pi* orbital is full, the other empty. no unpaired electrons.
      1. 'singlet oxygen'
      2. higher energy than triplet oxygen but transition back is slow. strongly forbidden.
      3. reactive. t1/2 in gas about one hour
2. Ozone (O3)
  1. not paramagnetic
    1. Resonant structure
      1. Split by UV. O3 --> O2 + O*
  2. not a free radical
  3. less stable than O2
3. Peroxides O2 (2-)
  1. acid form: hydrogen peroxide. H2O2
    1. can accept two electrons to form 2OH(-)
      1. strongly exothermic
  2. alkyl peroxides RO2H
  3. Not FRs but strong oxidisers
4. Hydroxyl radicals OH*(-)
  1. H2O --> H* + OH*
    1. normally when a FR attacks H2O in the atmosphere
  2. Highly reactive
  3. Fenton reaction
5. Peroxy radicals ROO*
  1. Formation
    1. ROOH --> ROO* + H*
    2. R* + O2 --> ROO*
      1. O2 has two unpaired electrons. One joins with R and the other remains free
6. O (2-)
  1. 6 electrons
    1. requires 2 extra electrons for a stable octet
    2. acquires 1 electron to become an oxygen radical anion *O-
    3. 2 unpaired electrons (biradical)
      1. highly reactive
Nitrogen compounds
1. nitrous oxide N2O
  1. not a free radical
  2. N=N=O
  3. very stable
2. nitric oxide NO*
  1. free radical
    1. Similar to triplet oxygen but only one of the pi* orbitals is occupied
    2. fairly reactive
      1. half life of seconds (biological) to hours (atmospheric)
  2. signalling molecule
    1. important in tropospheric FR reactions
3. Nitrogen dioxide NO2
  1. structure resemble ozone
    1. O=N(+)-O(-)
    2. one less electron so one electron is unpaired
      1. paramagnetic
      2. strong EPR signal
      3. Not very reactive - not considered a FR although it can behave as one
        acts as a free radical scavenger - its concentration is high because of its stability
        as a FR: NO2 + *OH --> HNO3
  2. low energy photon (around 400nm) capable of exciting an electron
    1. reddish brown
    2. NO2 -->NO* + O**
    3. NO* + O3 --> NO2 + O2
      1. main route by which NO2 is produced in the troposphere
        brown smog
Kinetics
1. 3 stages
  1. Initiation
    1. reactive FR created
    2. Often photochemical
      1. usually follow 1st order kinetics
        dependent on the amount of light in the spectral region absorbed by the initiator
        usually UV
        dependent on altitude
  2. Propagation
    1. Free radical + ordinary molecule --> free radical
    2. majority are linear reactions. (one FR produced per FR)
      1. branched chains lead to explosions
    3. usually fast
  3. Termination
    1. FR -> unreactive species
    2. often when two FRs react together
    3. Usually second order kinetics
      1. rate depends on [free radical]^2 or [product] if 2 different FRs react
        [FR] is low so termination rates are low
        stable FR like NO can reach higher concentrations = good terminator
2. Dependent on: number of conversion steps initiated per unit time & the length of each chain
  1. Ratio of initiation to termination
  2. number of propagation steps per initiation
    1. typically 100s to 1000s in atmosphere

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