What happens when a molecule absorbs light?
• Electronic transition (singlet & triplet)
• Heat loss
Example of formaldehyde
• in ground state:
◦ 2 pairs of non- bonding e- on oxygen
◦ sigma bond and pi bond (from 2p atomiy
orbitals of C and O) between C and O
How can formaldehyde absorb light?
• lowest unoccupied orbital: C=O pi antibonding
• highest occupied orbital: nonbonding O orbital
• lowest-energy electronic transition
◦ 2 possibilities, depending on spin quantum
number in excited state
What happens to formaldehyde when it absorbs light?
• Singlet state
◦ opposed spins
◦ S =1lowest-energy excited singlet state
• Triplet state
◦ parallel spins
◦ T = 1owest-energy excited triplet state
• in general T 1as lower energy than S 1 Why is formaldehyde colorless?
• if 397 nm absorbed: green-yellow colour
• probability of transition between singlet and triplet states (n(S0)→
π*(T1)) extremely small
• singlet to singlet transition (n(S0)→π*(S1)) much more probable (UV
Vibrational and rotational states of formaldehyde
• each of the 4 atoms can along 3 axes in space
◦ 4x3=12 different ways
◦ 3 translations in x, y and z directions
◦ 3 rotations about x-, y- and z-axes
◦ 6 vibrations
• degrees of vibrational modes for linear molecules: 3n-5
• degrees of freedom for nonlinear molecules: 3n-6
Molecules that absorb UV or visible radiation
• Organic compounds with unsaturated bonds (i.e., double & triple
◦ conjugation shifts the absorption maximum to longer
• organic compounds with S, Br, I
• inorganic ions and molecules
◦ ions of lanthanides & actinides have absorption spectra with
Do you remember the lesson?
Which conditions would you select for the determination of Fe?
~575nm Conditions for the determination of Fe
a) select intermediate wavelength
b) select wavelength corresponding to first maximum
c) select wavelength corresponding to highest maximum
d) select wavelength corresponding to highest maximum and adjust Fe concentration to get