College of Saint Mary    Fall 2008    Physical Chemistry, CHM-450/451

Instructor: Dr. Peter Ilich    My name _____________________________

Final project: PHOTOCHEMISTRY & PHOTOBIOLOGY

(1)    Define a photochemical reaction

• Quantum yield
  • Examples:
• Conditions for a photochemical reaction:
  • Excitation energy must be resonant (equal to) or larger than a chemical bond energy
  • Provide numerical examples,
    • E.g. two stacked thymine (letter T in the genetic code) make a bond at 325 nm or higher energy radiation; show what should be the T-T bond energy
    • Provide a comparison with known light sources

(2)    Earth atmosphere (EA) photochemistry

• COMMENT: This is an increasingly important area with several subjects presented in our textbook
• Definitely show the O₃ shielding effect (e.g. show the absorption maximum and compare this wavelength to an average chemical bond)
• Show the basic photochemistry of CFC compounds, Ch. 19.5.
• Select another example of EA photochemistry, from the textbook or another source.

(3)    Photosynthesis:

• COMMENT: This is a large subject and it is easy to get lost in it or bogged down around some sub-topic. 
• Remember -- this is a Physical Chemistry and not an English report; use structural formulas, photon wavelengths & energies (10^-18 to 10^-19 J per particle) and chemical bond energies (250 to 1000 kJ/mol); make quantitative comparisons rather than long narratives.
• Again, no matter how long or short you choose to dedicate to subject CLEARLY define or describe its essence so (1) a 14-year old would understand it and (2) a physicist would not find major points of objections.  I know -- this is a tall order -- but well worth striving toward.

(4)    Vision:

• With photosynthesis this is probably the most deserving topic
• SUGGESTION:  Use a simple model, e.g. represent the retinal C-C and C=C conjugated chain as a particle (I'll leave it to you to figure out its length) and fill it up with the as many electrons (a pair per each level) as there are C-atoms in the conjugated chain (NOT all C atoms!).
• Compare this rudimentary model calculation with the known peak energy output of the sun that triggers the rhodopsin photochemistry
• I have quite a bit of material on this subject so if you need help email me -- I'd be willing to share it a reasonable fee.

(5)    Biological effects of (electromagnetic) radiation:

• Make a rough scheme that shows what kind (energy, rather) of radiation can cause what damage (the best comparison is with the energy of a diatomic pair split into two radicals; I can help you with these data; alternatively, you can use the Gaussian 03W program to calculate these numbers -- this should be easy and fun).
• Now you may choose your example (Google it out) of "photodynamic therapy": again, stick to the PHYSICS rather than story.
• Now the Thymine  - Thymine interaction becomes important again; find the EM wavelength and energy of sun-bathing beds and compare it with energy needed to cause a chemical change.
• I want each of you to find the (approximate) radiation energy of your cell phones and compare it -- in numbers  -- with an "average" chemical bond energy and comment on what kind damage a cell phone can inflict on you.

(6)    References, notes & comments: 

• Do not try to go without references, even for every picture you pick from Google Image; this is a good practice that can potentially save you from a lot of legal problems in your future carrier; besides, it is an act of basic fairness to recognize someone else's work

(7)    Our final exam is scheduled for Thursday, Dec 18, 2008, 10:00 am to noon.

(8)     Email me questions, comments.  I would prefer to see the draft of your presentations before you actually present it. (It may also help you with the final grade.)