Molecular Orbitals 1.76 The following two compounds are isomers; that is, they are different compounds with the same molecular formula. We discuss this type of isomerism in Chapter 5 Cl Cl Cl Cl (a) Why are these different molecules that do not interconvert? (b) Absorption of light by a double bond in a molecule excites one electron from a molecular orbital to a Tr\" molecular orbital. Explain how this absorption can lead to interconversion of the two isomers. Light Solution (a) The cis and trans isomers do interconvert , inthe presence of catalyst only . The catalyst may be a chemical or light enrgy or heat energy . The interconvertion of the cis to trans or trans to cis needs energy and necessarily a catalyst . (b) The cis-trans isomerization of alkenes occurs under irradiation with light that they absorb. Isomerization takes place through a singlet excited state. Isomerization can also be effected via triplet state. Triplet excited state is attained either by intersystem crossing or by using a photo sensitizer that can transfer triplet energy to the compd to be isomerized. A typical sensitizer is C6H5COC6H5 which by irradiation , is excited from ground state Bo to a long –lived singlet Bs* which readily undergoes intersystem crossing to lower lying triplet Bt* this triplet can be transferred to the molecule to be isomerized. .

1. Molecular Orbitals 1.76 The following two compounds are isomers; that is, they are different
compounds with the same molecular formula. We discuss this type of isomerism in Chapter 5 Cl
Cl Cl Cl (a) Why are these different molecules that do not interconvert? (b) Absorption of light
by a double bond in a molecule excites one electron from a molecular orbital to a Tr" molecular
orbital. Explain how this absorption can lead to interconversion of the two isomers. Light
Solution
(a) The cis and trans isomers do interconvert , inthe presence of catalyst only . The catalyst may
be a chemical or light enrgy or heat energy . The interconvertion of the cis to trans or trans to cis
needs energy and necessarily a catalyst .
(b) The cis-trans isomerization of alkenes occurs under irradiation with light that they absorb.
Isomerization takes place through a singlet excited state. Isomerization can also be effected via
triplet state. Triplet excited state is attained either by intersystem crossing or by using a photo
sensitizer that can transfer triplet energy to the compd to be isomerized. A typical sensitizer is
C6H5COC6H5 which by irradiation , is excited from ground state Bo to a long –lived singlet
Bs* which readily undergoes intersystem crossing to lower lying triplet Bt* this triplet can be
transferred to the molecule to be isomerized.