SPECTROSCOPY: UV-VIS

Question 1

Where does colour of compounds come from

Answer 1

Excitation of an electron to an excited state from a ground state is when this light is absorbed. Colour/light released when electron comes back to ground state

Question 2

What is the beer lambert equation

Answer 2

A = ecl

A = absorbance
e = molar absorption coefficient (M-1 cm-1)
c= concentration (M)
l = path length (cm)

Question 3

What is the spin rule

Answer 3

Transitions are allowed when the spin does not change. Spin of whole complex if more than one electron is moving

Question 4

What is the Laporte rule

Answer 4

In a centrosymmetric molecule, the only allowed transitions are those accomplished by a change in parity.

Transitions are allowed between different orbitals (d to p) and forbidden when in the same orbital (d to d) in a centrosymmetric complex (octahedral)

Can be relaxed due to JT distortion etc.

Question 5

Outline the characteristics of an octahedral UV-VIS

Answer 5

d-d transitions: weak laporte forbidden (e <1000)

charge transfer: strong, spin and laporte allow (1000< e <10000)

intraligand transitions (pi to pi*): very strong, spin and laporte allowes (e > 5000)

Question 6

Outline the characteristics of a tetrahedral UV-VIS

Answer 6

Much higher intensities than octahedral due to laporte acceptance (e = 50-800 compared to e = 0.1-100)

lambda values at longer wavelengths due to smaller delta tet splitting than delta oct

Question 7

What are the differences between octahedral and tetrahedral UV-VIS

Answer 7

Less intensity of d-d transitions in octahedral due to Laporte

CFSE is large of oct giving a smaller wavelength

Question 8

For what complexes to 3 band systems typically occur

Answer 8

Weak field d2, 3, 7, 8.