Energy Level Diagrams and Primary Processes

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Energy Level Diagrams and Primary Processes
2
Primary Processes

• One molecule is excited into an electronically
  excited state by absorption of a photon, it can
  undergo a number of different primary processes.
• Photochemical processes are those in which the
  excited species dissociates, isomerizes,
  rearranges, or react with another molecule.
• Photophysical processes include radiative
  transitions in which the excited molecule emits
  light in the form of fluorescence or
  phosphorescence and returns to the ground state,
  and intramolecular non-radiative transitions in
  which some or all of the energy of the absorbed
  photon is ultimately converted to heat.

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Primary Processes

• Fluorescence is defined as the emission of light
  due to transition between states of like
  multiplicity, that is e.g., S1 ? S0 h?. This
  is an allowed transition and hence the lifetime
  of the upper state with respect to fluorescence
  is usually short (10-6 -10-9 s), e.g., lifetime
  of HO radical in the excited state A2 state is 07
  microsecond.
• Phosphorescence is defined as the emission of
  light due to a transition between different spin
  multiplicities.
• Intersystem crossing (ISC) is the intramolecular
  crossing from one state to another of different
  multiplicity without the emission of radiation.

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Primary and Overall Quantum

• ?i number of excited molecules proceeding by
  process I/total number of photons absorbed.
• For chemical processes, two kinds of quantum
  yields are usually defined, primary ?, and an
  overall quantum yield, ?.
• HCHO h? ? (a) H HCO
• ? (b) H2 CO
• Reaction (a) is a significant source of HO2 and
  ultimately HO in troposphere.

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Primary and Overall Quantum

• ?a number of H atoms (or HCO radicals)
  formed/number of photons absorbed by HCHO
• Experimentally we often measure the yields of
  stable products rather than of atoms and free
  radicals and hence
• ?A number of molecules of product A
  formed/number of photons absorbed by reactant
• ?H2 number of molecules of H2 formed/number of
  photons absorbed by HCHO
• ?CO number of molecules of CO formed/number of
  photons absorbed by HCHO

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Primary and Overall Quantum

• It is important to note that CO and H2 production
  in photolyses of pure H2CO vapor arises not only
  directly from the primary molecular detachment
  but also from the subsequent secondary reactions
  of the reactive species H and HCHO formed.
• H HCO ? H2 CO
• 2H ( M) ? H2
• 2HCO ? 2 CO H2
• ? CO HCHO

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Primary and Overall Quantum

• The magnitude of primary and overall product
  quantum yields gives valuable indication of the
  relative importance of photophysical and
  photochemical processes.
• If ? ltlt 1(small primary photochemical quantum
  yields) indicate that photophysical processes
  must be important.
• Of the overall product quantum yield is greater
  than 1 (? gt1) usually indicate that a chain
  reaction is occurring.

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Primary and Overall Quantum

• In the photolysis of Cl2 and H2, ?HCl can be as
  high as 1 million.
• Cl2 h? ? 2Cl
• Cl H2 ? HCl H
• H Cl2 ? HCl H
• Quantum yields for photophysical processes are
  defined in an analogous way
• ?f total of excited molecules that
  fluoresce/total number of photons absorbed

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Primary and Overall Quantum

• The sum of all photophysical and photochemical
  primary quantum yields must be unity ??i 1
• Einstein-Stark law (second law of photochemistry)
  which states that the absorption of light by a
  molecule is a one quantum process for low to
  moderate light intensities, e.g., 1013-1015
  quanta s-1. It is based on the fact that the
  probability that an electronically excited
  molecule absorbing a second quantum of light
  during its short liftime of ? 10-8 s is very
  small. For high quantum flux, this law does not
  apply.
• ? (?f ?P ?deactivation ?a ?b ) 1.0

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Intermolecular non-radiative processes

• Collisional deactivation and energy transfer
• At atmospheric pressure or in the liquid state,
  the excited molecule can undergo many collisions
  with ground state molecules leading to several
  pathways for collisional deactivation.
• Energy transfer from A to the collision partner
  can occur in which the excitation energy appears
  as access vibrational, rotational, and/or
  electronic energy of molecule B (conservation of
  spin angular momentum)
• SO2(3B1) O2(3?g-) ? SO2(1A1) O2 (1?g, 1?g)

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Types of primary photochemical processes

• Photodissociation It can produce atoms and free
  radicals which either directly or via secondary
  thermal reactions, are the source of free
  radicals such as HO, HO2, and RO2.
• NO2 (X2 A1) h? (290 lt ? lt 430 nm) ? NO (X2?)
  (O3P)
• This process is spin-allowed.
• Another example is spin-allowed dissociation of
  ozone
• O3(1B2) h? (? lt 320 nm) ? O(1D) O2(1?g)

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Types of primary photochemical processes

• Interamolecular rearrangement
• E.g., photolysis of o-nitrobenzaldehyde in the
  vapor, solution, or vapor phase
• NO2?(C(O)H) h? ? NO?(C(O)OH)
• Photoisomerization
• Cis and trans isomerization
• CH3-C(O) CHCH(CH3) (trans) h? ? CH3-C(O)
• CHCH(CH3) (cis)

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Types of primary photochemical processes

• Photodimerization
• Anthracene dimerization
• ??? h? ? ??????
• This occurs in competition with photooxidation
  when air is present.
• Hydrogen abstraction
• ?-C(O)- ? CH3CH(OH)CH3 h? ? ?-C(OH)- ?
• CH3C.(OH)CH3

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Types of primary photochemical processes

• Photosensitized reactions
• An electronically excited molecule can transfer
  its energy to a second species which then
  undergoes a photochemical process even though it
  was not itself directly excited.
• Sens(S0) h? ? Sens(S1)
• Sens(S1) (intersystem crossing) ? Sens(T1)
• Sens(T1) Acceptor(S0) (energy transfer) ?
• Sens(S0) Acceptor(T1)
• Acceptor(T1) ? Products
• E.g., polycyclic aromatic hydrocarbons on
  combustion generated particles.

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Next lecture advanced reading

• Atkinson, R.,1985. Chemical Reviews, 85(1), p.
  195

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Photochemistry
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Photochemistry

• Grotthus-Draper Law
• Only the light which is absorbed by a molecule
  can be effective in producing photochemical
  changes in the molecule.
• Beer-Lambert Law
• Log (I0/I) ?Cl

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Photochemistry

• Energy levels and molecular absorption Spectra
• Etotal Ev Er Ee
•  
• Ev h?vib (v 1/2) and ?v ? 1
• For purely vibrational transition, the selection
  rule for absorption of light requires that there
  be a changing dipole moment during the vibration.
  This oscillating dipole moment produces an
  electric filed that can interact with the
  oscillating electric and magnetic fields of
  electromagnetic radiation.

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Photochemistry

• Rotational Transitions
• If a molecule has a permanent dipole moment, its
  rotation in space produces an oscillating
  electric field, this can interact with
  electromagnetic radiation, resulting in light
  absorption. 
• For a rigid rotor
• Er B(J) (J1) cm-1
• B h/8?2Ic
• where I ?r2 and ? -1 (MA-1 MB-1)
• For idealized case ?Er 2BJ
• ?J ? 1

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Photochemistry

• Vibration-Rotation Transition
• Molecules vibrate and rotate simultaneously. The
  total change in energy is the sum of the changes
  in vibrational and rotational energy.
• Selection rules ?v ? 1, ?J ? 1

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Photochemistry

• Electronic Transition
• The electronic states of a diatomic molecules are
  described by several molecular quantum numbers
  such as ?, S, and ?.
• Selection rule not as clear cut as in the case of
  vibration and rotation but in case of many of
  tropospherically interesting molecules
• ?? 0, ? 1 and ?S 0
• A molecule can undergo an electronic transition,
  along with simultaneous vibrational and
  rotational transitions. In this case there is no
  restriction on ?v.

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Reactions

• Reaction of HO radicals with selected organic
  compounds
• O3 h?? O(1D) O2
• O(1D) H2O ? 2 HO
• HO is so-called the cleanser or detergent of the
  atmosphere during the daytime.
• Other important primary reactions of
  hydrocarbons photolysis, NO3 (at night), O3 (for
  unsaturated compounds), Cl and Br in marine
  boundary layer.

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Reactions

• Main Hydrocarbon groups
• Alkanes
• Alkenes
• Alkynes
• Aromatics and substituted aromatics
• Alcohols and ethers
• Further reading
• R. Atkinson, Atmospheric Environment, Vol. 24A,
  No. 1, pp 1-41, 1990.

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Reactions

• HO Reactions Initial step for chemicals resulted
  from primary emissions
• HO RH ? H2O R.
• (alkanes ethane, propane, n-and pentanes, )
•  HO gtCClt ? gtC.-C(HO)lt
• (ethene, propene, )
• HO CH3-? ? H2O .CH2-?
• ( M)? addition to the ring
• (benzene, toluene, xylenes, )
• HO RCHO ? H2O RCO
• (formaldehyde, acetaldehyde, )
• HO R-C?C-R (M) ? HOCRC.R
• (acetylene, propyne, 1-butyne, ..)
• HO ROH ? H2O RO.
• (methanol, ethanol, to a lesser extent C3 and C4
  alcohols)

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Reactions

• HO CH3OC(CH3)3 ? .CH2OC(CH3)
• ?H2O CH3OC(CH3)2C.H2
• (dimethyl ether, diethyl ether, and methyl
  tert-butyl ether)
• Carboxylic acids formic acid, 50 days life
  time, major removal mechanisms wet or dry
  deposition
• (formic and acetic acids) 
• Hydroperoxides measured components are methyl
  hydroperoxides (CH3OOH) and tert-butyl
  hydroperoxide (CH3)3OOH). Major removal by
  photolysis and HO
• HO CH3OOH ? H2O CH3OO.
• HO CH3OOH ? H2O C.H2OOH

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Secondary Reactions

• R. O2 ? RO2
• (R. O2 ? (RO2) ? HO2 alkene
• (M) ? RO2
• At elevated temperatures, H-abstraction channel
  from peroxy radical has been observed, however,
  it is formed from the activated RO2 (Slagle et
  al., 1984., Kinetics of the reaction of ethyl
  radicals with molecular oxygen from 294 to 1002
  K., J. Phys. Chem., 88, 215-227).

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Secondary Reactions

• And at high pressure limit peroxy radical
  formation is the sole reaction process.
• For the alkyl, hydroxy alkyl (other than alpha),
  benzyl, and methyl-substituted benzyl, acetyl,
  and allyl radicals to date the reaction with O2
  proceeds via addition to from a peroxy radical
  with a room temperature rate constants of ? 10-12
  cm3 molecule -1 s-1.
• Niki et al., 1981, Chem. Phys. Lett., 80,
  499-503.

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Reactions

• Exceptions to the Rule
• Formyl radical
• HCO O2 ? HO2 CO
• (k 6 ? 10-12 cm3 molecule-1 s-1 independent of
  temperature over range 295-500 K, Veyret and
  Lesclaux, 1981, J. Phys. Chem., 85, 1918-1922)
• Simple ?-hydroxy radicals, .CH2OH,
• .CH2OH O2 ? HCHO HO2
• .CH2OH O2 ?(.OOCH2OH ? HOOCH2O.)
• ? HCHO HO2
• (k298 9.0 ? 10-12 cm3 molecule-1 s-1) 
• Formation of phenyl due to HO addition to the
  aromatic ring and ring cleavage

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Reactions

• Subsequent Reactions of RO2
• RO2 RO2 ? 2 RO. O2 (a)
•  
• (not accessible for tertiary RO2s)
•  
• RO2 RO2 ? ROH R'R'CO O2 (b)
• RO2 RO2 ? RROORR O2 (c)
•  
• The overall rate k ka kb kc

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Reactions

• Example
• R1 R2 CH3
•  
• R1R2CH.O2 R1R2CH.O2 ? 2 R1R2CHO O2
•  
• R1R2CH.O2 R1R2CH.O2 ? R1R2CHOH R1R2CO
• O2
•  
• R1R2CH.O2 R1R2CH.O2 ? R1R2CHOOCHR1R2
• O2

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Reactions

• IUPAC Recommendations
• k(primary RO2 primary RO2) 1 ? 10-13 cm3
  molecule-1 s-1
• independent of temperature
• k(secondary RO2 secondary RO2) 1.6 ? 10-12 e
  -2200/T cm3 molecule-1 s-1 
• k(tertiary RO2 tertiary RO2) 1.7 ? 10-10
  e-4775/T cm3 molecule-1 s-1
• with ka/k ? kb/k 0.5 for self reactions of
  primary and secondary RO2 radicals, and
  approximately independent of temperature.

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Reactions

• Other reactions of RO2s
• RO2 NO ? RO NO2
• (M)? RONO2
•  
• Via ROONO ? RO--N ?RONO2 ? (M) RONO2
• \
• O-O
• For acetyl peroxy radical (CH3CO3) or acyl peroxy
  radicals reactions with NO
• RCO3 NO ? RCO2 NO2
• RCO2 ?R. CO2
• K 5.1 ? 10-12 e200/T cm3 molecule-1 s-1
• RO2 HO2 ? ROOH O2
• For R CH3 Atkinson et al., 1989 k 1.7 ? 10-13
  e1000/T
• RO2 NO2 (M) ? RO2NO2
• RCO3 NO2 (M) ? RC(O)O2NO2