Question

The allowed UV symmetry transitions for PF5 (a D3h molecule) are A1g, B1g, and B2g. Explain why these three are allowed.

Answer 1

There are actully two main slection rules for electronic transmissions,

1) spin selection rule:- delta S=0 i.e transition to same state ( both spins having same sign) is forbidden.

2) laporte selection rule:- for any molecule posessing a centre of inversion, transition between two "g" states or two "u"states are forbidden. threre should be a change in parity. i.e g-u or u-g transitions are allowed and g-g or u-u are forbidden.

the structure of PF5 is trigonal bipyramidal and has D3h symmetry. and has an inversion centre,the possible transitions are pi-pi* and n-pi* transitions. the allowed symmetry trnsitions are A1g,B1g and B2g. because,

1) In case of molecules without centre of inversion, the electronic transition should give symmetric irreducible representations Ag for allowed trasitions

2)there is a term called transition moment and transition moment integral. transition moment is usually denoted as the electric dipole moment associated with initial and final states,if it is zero then the transition is forbidden.

3) if transition moment integral is zero then the transition are not allowed.

here only A1g,B1g nad B2g yeild non zero transition moment and transition moment integral.