B. Breit. D. A. Evans Introduction to Photochemistry Chem 206 Excited State Geometry of Dienes Photochemistry of carbonyl compounds Photochemical excitation of carbonyl group:n→π Allylmethylene diradical TI 75-80 kcal/mol n→ pyramidal trans-trans not observed dipole: 2.34D dipole: 1.56D Explain why trans-trans isomer is not observed in this photosiomerization 十++ +++ ++++ ground n-I n-丌π-丌丌-π aglet trip 人(mb(A Note the diradical character of the excited states photochemically excited carbonyl compounds undergo many of the reactions typical of Rationalize the course of the sensitized transformationB. Breit, D. A. Evans Introduction to Photochemistry Chem 206 Excited State Geometry of Dienes C C H C H R C R H H light C H R C C H H R H Allylmethylene diradical C C H C R H C H H R light cis-cis C C H C H R C H H R cis-trans C C H C H R C R H H trans-trans not observed Explain why trans-trans isomer is not observed in this photosiomerization C H Me C C H H H H C C H C H Me C H H H light Me Me H Me H direct H2C Me Ph Ph hu Ph Ph sensitized hu direct Ph Ph Rationalize the course of the sensitized transformation O H H hn Photochemistry of Carbonyl Compounds Photochemical excitation of carbonyl group: n ® p* * planar pyramidal dipole: 2.34D dipole: 1.56D p * p n ground state n - p * singlet n - p * triplet p - p * singlet p - p * triplet Note the diradical character of the excited states. Photochemically excited carbonyl compounds undergo many of the reactions typical of radical species. n ® p* S 1 T 1 C O H H 80–85 kcal/mol 75–80 kcal/mol C O C O