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CHM3122 (7)


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University of Ottawa
Kathy Focsaneanu

• 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 per scond. • 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 sheet. • 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 promote it • 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
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