Question

a) Explain what is meant by the Franck-Condon principle (2 marks) b) A molecule is excited from the ground state to a higher electronic state. Describe the vibrational structure you would expect to observe and how it arises (4 marks) c) Explain using molecular orbital theory why N2 is expected to be a stable molecule whereas Be2 is expected to be an unstable molecule (4 marks)

Answer 1

1) Frank condon principle is principle in spectroscopy.The intensities of the molecular spectral band due to electronic transition are consistent with the assumption that relatively large mass of atomic nuclei in the molecule prevents appreciable change in the configuration during such transition

In other words rearrangement of electrons is much faster than motion of nuclei.Therefore nuclear configuration does not change significantly during energy absorption process.Thus the absorption spectra of molecule is characterised by vertical excitation energy.
Franck-Condon Principle: Vibronic Transitions E1 Eo Internuclear Distance Energy

3)

2pz 2P 2p *2Px "2py 111 2pz 2Px 2py 2s 2s 2s N2 N

In case ofdinitrogen bond order is =3

ps 2s -1 2s Be2 Be Be ENERGY

In case of Be2 bond order is=0

So no bond exist between two atoms of Be.

Molecule of Be does not exist.But N2 molecule has bond order=3.Therefore N2 is stable