1、,A Drivers Guide to Photochemistry: Roads (ie. Surfaces), Crossings and Intersections,(A Discussion of MMP+ 6.1-6.6),Tracy Morkin November 26, 2002.,Our Road Map,Where have we been?,Where are we going next?,photophysical properties(ie. R = P) - state energy diagramsFranck-Condon Principle - similar
2、nuclear geometriesBorn-Oppenheimer Approximation,photochemical reactions(ie. R P) - state correlation diagramsthe consequences of differentnuclear geometriesBorn-Oppenheimer Approximationmay break down,How do we know with path R will take?,Then, we address the theory:1. Conical intersections and Fro
3、ntier MO Theory (start today)2. Stereochemical consequences of orbital symmetrey (Tues. Dec. 3)3. Conservation of Energy and Spin (Wednesday Dec. 4)4. Prof. Robbs visit (Thursday Dec. 5),We use exemplars of chromophores:,Potential Energy Curves vs. Potential Energy Surfaces,similar nuclear geometry
4、between ground and excited state,significantly different nuclear geometries between R, I and P.,taken from Ch. 3,from Prof. Robbs website,r,centre of mass,Potential Energy and Force,Force acting on the particle at r:,r = potential energy curve at a given geometry,Single Point on an Energy Surface,Im
5、portance of geometry on 1. energy barriers on excited and ground state surfaces2. energy minima on excited and ground state surfaces3. touching and intersecting points of surfaces4. avoided crossings that create minima5. barrier-free and adiabatic reactions,Influence of Collisons and Vibrations on a
6、n Energy Surface,Collisions are a reservoir of continuousenergy (0.6 kcal/mol per impact)Collisons can add or remove energy froma systemExample: solution-phase vs. gas phaselifetimes - few collisions in the gas phase,Ground State vs. Photochemical Reactions,Multiple Surfaces,Ground State (Thermal) R
7、eactions,Photochemical Reactions,Single Surface,How does a particle on the excited surface return to the ground state? FUNNELS!,4 Topologies for Funnels: 2-D,Extended surface touching,Extended surface matching,Surface Crossing,Equilibrated Surface Minimum,R,R,R*,R*,R*,R,R,R*,I,I*,I,P*,P*,P,P,4 Topol
8、ogies of Funnels: 3-D,Extended surface touching,Extended surface matching,Conical Intersection,Avoided Crossing,Non-Crossing Rule and Avoided Crossings,Surface Crossing,R,Avoided Crossing,Two energy curves with a common geometry, energy and nuclear positions.,When the two states are the same, there
9、will be a mixing to produce 2 adiabatic surfaces.,Born-Oppenheimer Approx. applies,Conical Intersections,Born-Oppenheimer Approx. breaks down!,associated with FAST motions - there is no TIMEfor Y* to respond to nuclear motion and mixing doesNOT occur.the surface crossing is maintained!,Consequences
10、of Conical Intersections:energy gap is 0, so the probability of the transition is 100%limited only by vibrational relaxation, so the timescale is on the order of femto-or picosecondsno “jump” between surfaces, the reaction can appear concerted and stereochem.can be conserved,Avoided Crossings vs. Co
11、nical Intersections,AC,CI,Avoided Crossings,Conical Intersection,point can wander in energy minimumfinds a trajectory that depends on nuclear motion,- point enters cone with initialgeometry and is affected by: a) gradient of energy change asa function of nuclear motion b) direction of nuclear motion
12、that is best mix of Y* and Y - the excited state equivalent of a concertedreaction,Diradicaloid Geometries,Diradicaloids - correspond surface touchings, conical intersections or avoided crossings- serve as funnels- possibility of zwitterionic structures,s Bond Stretch:,p Bond Twist:,Energy Diagram,N
13、ote point of intersection - Could be: Touching surfaces Avoided crossing Conical intersection,diradicaloids are short-lived due to their (nearly) degenerate orbitals andthe rate-determining step is often the primary photochemical reaction (ex. bond cleavage).,s-Bond Stretching: Dissociation of H2,St
14、retching the s bond produces a diradaloid geometry On the g.s. surface, S0, all geometries are stable except at large nuclear distanceswhich produce 1D Along the T1 surface, all geometries are unstable and minimum activation is needed toProduce 3D 4. Along S1 and S2 the bond is unstable and have sha
15、llow minima; cleavage produces Z states,p Bond Twisting: Ethylene,twist,1,2-diradical,Consequences of Twisting,twisting about the C-C bond of an electronically excited ethylene relieves e-erepulsion form the p* e.*twisting lowers the energy of all the excited statesenergies of S2, S1 and T1 decrease
16、 as a function of twisting: electronic excitation has effectively broken the p bond and the bonding is more like a single C-C bondS0 increases because the p bond is being broken,Minima (funnels) in S2, S1 and T1 surfaces at 90 geometry Avoided crossing at Z2 and D1 S0 and T1 touch at 90, but not extended as in H2 example In S2 and S1, get zwitterionic behavior once twist starts In T1, get diradical behavior at all geometries,
