Chapter 6: A Qualitative Theory of Molecular Organic Photochemistry December 12, 2002 Larisa Mikelsons

Simplified schematic of the 2 lowest singlet surfaces for a concerted pericyclic reaction: 4N e- concerted disrotatory pericyclic reactions are generally photochemically allowed 4N + 2 e- concerted disrotatory photoreactions are generally photochemically forbidden Concerted pericyclic reactions which are g.s. forbidden are generally e.s. allowed in S 1 due to a miminum which corresponds to a diradicaloid Pericyclic reactions which are g.s. allowed are generally e.s. forbidden in S 1 because of a barrier to conversion to product structure and the lack of a suitable surface crossing from S 1 to S 0 4N or 4N + 2 = # of e-s involved in bond making or bond breaking

6.11 Typical State Correlation Diagrams for Nonconcerted Photoreactions Most photochemical reactions are not concerted and involve reactive intermediates (D, Z) along *R  I Use H-abstraction reaction of the n,  * state of ketones as an exemplar for *R  I reactions Assumption: the strictly planar approach represents the reaction coordinate (gives the best frontier orbital interaction between n O and  XH ; orbitals can be readily classified)  XH  p X and n O   OH correlations are assumed to be avoided State symmetries can be deduced from the orbital correlation diagram Orbital correlation diagram for H-abstraction by formaldehyde:

First Order state correlation diagram for coplanar H-abstraction: S 1 and T 1 correlate with the lowest states of the product and coplanar H-abstraction is symmetry-allowed. S 2 and T 2 correlate with zwitterionic forms of the product (which have very high energies) and coplanar H-abstraction is symmetry-forbidden. Destruction of the perfect coplanar geometry results in a weakly avoided crossing between the S 1  1 D and S 0  Z 1 surfaces. The T 1  3 D and S 0  Z 1 remains.

6.13 State Correlation Diagrams for  -Cleavage of Ketones Orbital symmetries for the  -cleavage of acetone: Two diradicaloid geometries possible: a)a bent acyl which is s b) a linear acyl fragment with  s and  a orbitals. The (  s,p) state is S and the (  a,p) state is A

Orbital correlation diagrams for  -cleavage of acetone: Zero Order state correlation diagram for  -cleavage of acetone: For symmetry-allowed  -cleavage of n,  * states to yield a linear acyl fragment, S 1 (n,  *) and T 1 (n,  *) correlate with 1 D(  a,p c ) and 3 D(  a,p c ). For symmetry-forbidden  -cleavage of n,  * states to yield a bent acyl fragment, S 1 (n,  *) and T 1 (n,  *) correlate with D*(sp 2,  * CO ).

First Order correlation diagram for  -cleavage of acetone: Situation for cleavage to the linear fragment essentially the same Occurrence of weakly avoided crossings for cleavage to the bent acyl fragment

6.14 A Standard Set of Plausible Primary Photoreactions for ,  * and n,  * States Plausible primary photochemical reactions that are initiated in S 1 ( ,  *) T 1 ( ,  *) 1. Concerted pericyclic reactions1. Hydrogen atom or e- abstractions 2. Reactions characteristic of carbonium2. Addition to unsaturated bonds ions and of carbonanions3. Homolytic fragmentations 3. Cis-trans isomerization4. Rearrangement to a more stable carbon centered radical Zwitterionic and/or concertedDiradicaloid and non-concerted

The photochemistry of n,  * states is completely diradicaloid to a good approximation. The plausible primary processes are: n-Orbital Initiated  *-Orbital InitiatedAtom abstractionRadical addition Electron abstractionElectron donation  -Cleavage  -Cleavage Electrophilic characteristicsNucleophilic characteristics Like an alkoxy radical (RO)Like a ketyl radical (R 2 COH)Sensitive to steric factors influencing substrate’s approach in molecular substrate’s approach above and plane and near “edges” of carbonyl O below the “faces” of CO

6.15 Intersystem Crossing in Radical Pairs and Diradicals S 1  1 I processes: No spin prohibition to 1 I  P so 1 I = 1 RP, 1 D 1 I undergo either recombination or disproportionation which are both extremely rapid Reactions may be stereospecific T 1  3 I processes: Spin prohibition to 3 I  P due to 3 I  1 I If ISC is slow relative to diffusional separation of the radical pair then free radical formation occurs If ISC is slow relative to rotation about C-C bonds then loss of stereochemistry results in any intramolecular reactions of the diradical Rate of ISC in RP and D determined by spin-orbit interactions and possibly by very weak magnetic interactions with nuclear spins and laboratory magnetic fields

6.16 Magnetic Energy Diagrams Including the Electron Exchange Interaction Important situations of Zeeman splitting and exchange splitting: Condition I  Solvent separated spin correlated geminate pairs and extended biradicals Condition II  Molecular triplets, spin correlated pairs in a solvent cage and small biradicals

6.17 Magnetic Interactions and Magnetic Couplings Magnetic couplings that are important for ISC: spin-orbit coupling, electron-nuclear hyperfine coupling and Zeeman coupling. Due to electric or to magnetic dipoles interactingDue to overlap of wavefunctions Interaction  [(  1  2 )/r 3 ](3cos 2  - 1) (overlap integral)Interaction   e  p |  (0)| 2 Rate  (strength of the interaction) 2  1 / r 6 Interaction distance independent

Magnetic Coupling Mechanisms Spin Hamiltonian operator H  used to classify the magnetic coupling mechanism of an electron spin, S 1, with other magnetic moments Zeeman Coupling: external coupling of electron spin to the magnetic moment of an applied laboratory field. H Z = g  e H 0 S 1 Dipole-dipole Coupling: internal coupling of electron spin to the magnetic moment of another electron spin, S 2. H DP = D e S 1 S 2 Hyperfine Coupling: internal coupling of electron spin to the magnetic moment of a nuclear spin, I. H HF = aS 1 I Spin-orbit Coupling: internal coupling of electron spin to the magnetic moment due to orbital motion of the electron, L. H SO =  S 1 L Spin-lattice Coupling: coupling of electron spin to the oscillating magnetic fields resulting from molecular motions of the environment. H SL  S 1  L Spin-photon Coupling: coupling of electron spin to the oscillating magnetic field associated with an electromagnetic field. H h  S 1  h

6.18 Coupling Involving Two Correlated Spins T +  S and T -  S Transitions Initially S 1 and S 2 are correlated in T + H i (electron or nuclear spin) couples with S 2 and causes T +  S ISC Zero field: three T sublevels strongly mixed and radiationless T +  S ISC plausible (assuming J = 0) High field: radiationless T +  S ISC not plausible but radiative transition is plausible if J is very small Vector diagram for T -  S transition similar to that of T +  S transition

T 0  S Transitions Initially S 1 and S 2 are correlated in T 0 H i (electron or nuclear spin) couples with S 1 and causes T 0  S ISC Zero field: three T sublevels strongly mixed and radiationless T 0  S ISC plausible (assuming J = 0) High field: radiationless T 0  S ISC plausible if J= 0

6.19 ISC in Radical Pairs and Diradicals. Exemplar Systems Simplified paradigm of a photochemical process proceeding through an n,  * triplet electronically excited state, T 1 : Paradigm for the  -cleavage reaction of ketones:

Surface energy diagram displaying the spin and molecular dynamic features of a dynamic radical pair: Energy surface description uses the exemplar of the stretching and breaking of a C-C single bond. ISC step S 1  T 1 occurs “vertically” when the exchange interaction is very large When J is large it controls the correlated precessional motion of the two electron spins so only a strong interaction can induce ISC When the RP is not in contact J decreases and torques can cause the electron spin to be rephased or flipped

Visualization of Spin Dynamics. ISC in Geminate RPs in Zero Field Distance dependence of spin correlated radical pairs: ISC not plausible during bond breaking since breaking takes s while spin precession is of the order of s. In region 2 J is large so the spins are strongly correlated and ISC is implausible in the contact pair. In region 3 J is comparable to available magnetic interactions for the pair so the spins are weakly correlated and ISC is plausible. In region 4 J = 0 and neither the phase nor the orientation of the spin on one center influences the phase or orientation of the spin at the other center.

6.20 Energy Surfaces as Reaction Maps or Graphs Orbital interactions and state correlation diagrams supply the basic elements of a qualitative theory of photoreactions. The possible products may be deduced from state correlation diagram maps. The probable products may be deduced from consideration of a) symmetry-imposed barriers b) minima which facilitate pathways from an excited surface to the ground state