• photolysis, cleavage of a molecule via light (absorption of light).
• slide 3: light is both a wave and a particle. UV is between 200-400. visible is
400-700. benzene molecule is half a nanometer. relatively speaking, large
wavelengths on molecular scales. include speed of light. 2.998 x 10^8 meters
• slide 4: vis region is the section that's perceived by the human eye. UV region
runs from 200 to 400 but there other subdivisions of UV region: UVA, UVB,
UVC. UVC absorbed by nitrogen and oxygen, ionosphere, outside of the ozone
layer. get photolysis.
• as wavelength increases, degree of penetration increases, longer the
wavelength, the more likely it will reach your skin.
• BDE: amount of energy require dot homolyze a bond, to dissociate a bond
homiletically. cut X-X bond, single bond holding the 2 atoms together to
generate 2 radical specifies. increases k/cal per mol fgoing from left to right,
increase bond disccoisation energies. if you look at OH bond in water around
120 k/cal per mol, a photon that has a wavelength around 290-300 in the UV
fro braking hydrogen oxygen bond, photon contains enough energy to break
homiletically the oxygen hydrogen bond in water. when molecule absorbs
the photon, change in energy, delta E is the different between he final state
and the initial state
• . use wave equation to replace the frequency. frequency of a wave is equal to
speed of wave divided by wavelength, becomes Hc/lamba where H is planks
constant. 1 joule is 4.184 calories. PUT IN CHEAT SHEET
• this energy =, get tiny number (10^-19 joules). this formally is the energy for
one photon. unit is either joule or k/joule. generally if taking about molecules,
work with practical units which is per mole. avagandros number, planks
equation for a mole of protons. better, practical numbers.
• only variable is the wavelength, get to equations on slide 6 - put on cheat
• not a linear relaitonshiop, reciprocal
• slide 7: at longer wavelengths, small energies, those energies arenough to
provoke vibrational. high energy, short wave lengths, permanent changes in
the molecule - bond cleavage. 200n- electronic transitions with possibility of
breaking a bond in between the two extremes. might induce permanent
damage or might not in molecule depending on what the molecule is.
• slide 8: absorption of a UV vis photon - enough energy to induce promotion
of energy from HOMO to LUMO of a particular molecule valence electron
transitions. molecule in ground state - all electrons are paired and filled up orbintals from bottom to top - lowest energy electronic config. absorb a
photon - promote electron from homo to lump - excited state. standard
abbrev. is hv form planks equation.
• slide 9: ex DTBP - like many peroxides, when DTBP absorbs energy either
from heat or light, form excited state of peroxide. with excess energy, can
induce homolysis of bond, homoylze weakest bond in molecule , weakest
bond in peroxide is the O-O bond. excess energy is used to break the O-O
bond. generates two tertbutoxyl radicals. energy requirements? energy of
photon has to be enough energy to break the O-O bond. BDE of O-O single
bond is about 40 k/cal per mol. wavelength of light that corresponds to 40
k/cal per mole. planks equation is about 700 nm. zero deocmopositin, why?
absorption at 700 is zero, all the photons sending to solution are just passing
through - not being absorbed at 700, it's transparent. all the photons were
wasted. because they were never absorbed, can never generated excited
state therefore cannot decompose. solution? if you choose 300 instead of 700
- will induce decamp, but you're using a photon that contains more energy
than required. satisfied 2nd condition but overkilling energy requirements.
means you could induce secondary runs or side products because you're
putting more energy than required
◦ criteria 1: photon has to contain enough energy i.e. wave length must
be enough to satisfy energetic requirements.
◦ criteria 2: photon must be absorbed
• slide 10: few exceptions which is relevant is the example of oxygen. HOMO
contained of two degenerate molecular orbitals, when you fill up orbitals, get
two electrons left over in ground state, so ground state is paramagnetic.
overall magnetic moment because there are wo unpaired electrons so it will
be attracted to a magnetic field. don't change spin of electron when you
• slide 11: when molecules are spin paired, called a singlet ground state
because you have a diamagnetic molecule. promote one of the electrons. on
left lowest energy config. because all the electrons are spin pared, it's the
singlet ground state, symbol is S0 (molecule A^1). absorb a photon, promote
one electron without changing spin, promote from HOMO to LUMO, i,e, first
singlet excited state. called S1 (1^A*). some molecules are capable of taking
the promoted electron and flipping its spin. flip the spin of promoted
electron.excited state where the pains are parallel - paramagnetic excited
state which would be the 1st triplet excited state (T1). if you have the spin
flip, molecule would still be in the excited state (3^A*)
• slide 12: S0 is the ground state, lowest possible electron config for molecule 0
s0 is lower energy than s1 because eyou have to absorb light to make s1.
could imagine than in order to flip spin of electron in the LUMO, imagine that molecule has to use a tiny quntatity of its energy to flip the spin. got some
extra energy from photo and pay for spin flip by using small prtion of excess
energy. results in T1 having a slightly lower energy than S1. real explanation
is with Hund's rule - filling orbitals with parallel spins first. according to the
rule, one with max multiplicity, config of lowest energy - favourable config,
better to have spin parallel than spin pair. because T1 has spin paired,
slightly lower energy than S1. generally try of any excited state. could say
than Tn is always slightly lower than Sn.
• slide 13: multiplicity of the state. ex. diamagnetic molecule - means all
electrons are spin paired. if this is the case, can calculate the overall spin
(sum of individual spin quantum numbers) then take absolute value - overall
spin of molecule. for diamagnetic molecule - corresponding spi