Electronic Spectroscopy

Question 1

Born-Oppenheimer approximation

Answer 1

Can consider electronic, vibrational and rotational parts separately therefore have separate electronic, vibrational and rotational selection rules

Question 2

Spin selection rule

Answer 2

ΔS = 0, conservation of angular momentum

Question 3

Symmetry selection rule

Answer 3

Allowed transition of either Tx, Ty or Tz span the symmetry representation of the ground state term symbol

Question 4

Linear-bent selection rule

Answer 4

When change in electronic configuration causes a change in point group use the selection rules for the point group of lower symmetry

Question 5

Franck-Condon Principle

Answer 5

Nuclei dont move during electronic transition = Born-Oppenheimer approx, nuclear conformation readjusts after nuclear transition, motion of nuclei continues from same initial geometry in new electronic state, momentum of nuclei must be conserved and bond length conserved over electronic transition

Question 6

FC principle with similar bond character in both states

Answer 6

Re = Re’, good overlap between v”=0 and v’=0,1 and no overlap between v”=0 and highly excited v’

Question 7

FC with weaker bond in excited state potential

Answer 7

Re doesn’t = Re’, poor overlap between v”=0 and v’=0, good overlap between v”=0 and continuum beyond dissociation limit possible

Question 8

Dissociative excited state potential

Answer 8

No vibrational levels in excited state, upon excitation molecule dissociated

Question 9

Rotational selection rues

Answer 9

Σ-Σ transitions, Δj = +1 (R-branch), Δj = -1, (P-branch)
Π-Σ or Π-Π transitions, Δj = -1, 0, +1 (P, Q, R branches)

Question 10

Photophysical processes

Answer 10

Fluorescence, non-radiative relaxation, inter system crossing, phosphorescence and internal conversion

Question 11

Photochemical processes

Answer 11

Photodissociation, intermolecular reaction/rearrangement and Bimolecular reaction

Question 12

Gas phase collision timescale

Answer 12

Translational/rotational transferred every collision
Vibrational transferred every 100-1000 collisions
Electronic highly dependent
Collision rate in gas phase at 1 atm and 298K = 1x10^10 s-1

Question 13

Gas phase low P colllision rate

Answer 13

Collision timescale longer than fluorescence lifetime

Question 14

Solution-phase energy transfer

Answer 14

No free rotation
Solvent caging/diffusion rather than translation
Vibrational transferred every 100-1000 collisions
Electronic highly dependent
Collision rate in solution phase at 298K = 1x10^12 s-1

Question 15

Radiationless decay

Answer 15

Energy transfer to solvent/surroundings, timescale = 1x10^-10 s-1

Question 16

Relaxed fluorescence

Answer 16

Seen when rate for radiationless decay is faster than that for fluorescence

Question 17

Internal conversion

Answer 17

Polyatomic extra degree of freedom means states of same symmetry and spin can intersect, transition between states of same symmetry and spin

Question 18

Inter system crossing

Answer 18

Transfer between different spin states

Question 19

Phosphorescence

Answer 19

Only seen if ISC possible, technically spin forbidden but molecule has nowhere else to go

Question 20

Quenching

Answer 20

Collision with another molecule that causes electronic transfer from one state to another, energy taken up by nuclear motion of subject and nuclear motion and/or electronic states of quencher, Bimolecular