UV/Vis-Spectroscopy

= investigation of electronic transitions within a molecule

Absorption

Ground state

Excited state

hn

Emission

Electronic transitionEnergy

n

n ~hcc

hhEEE ga

Ea

Eg

Conversion factors

1 eV = 8066 cm–1 = 96.5 kJ•mol–1

1 eV = 1.602•10–19 J

UV/Vis spectra of the [M(H2O)6]n+ cations

general observation:

d1, d4, d6, d9 (one band)

d2, d3, d7, d8 (three bands)

d5 (several sharp, relatively weak

bands) (only high-spin compounds)

d0, d10: no absorption bands

Intensities of absorption bands

emax (extinction coefficient), dimension M–1cm–1

Lambert-Beer law: emax = E / (c·l)

range: emax = 0 to > 105 M–1cm–1

e

emax

max

400 nm = 25000 cm-1

200 nm = 50000 cm-1

SiO2 cuvette

c

n

n

1~

l = diameter

of cuvette

Selection rules*

1. Spin selection rule S = 0 or MS = 0

(Transition between same spin states

allowed: singlet -> singlet, triplet -> triplet, others are

forbidden: singlet -> triplet, doublet -> singlet, etc.)

emax < 1 M‒1cm‒1

spin multiplicity MS = 2S+1

S = Ss = n/2 (total spin quantum

number)

[Mn(H2O)6]2+

hn

Pauli-Principlenot obeyed

S = 5/2 S = 5/2 S = 3/2

S = 1, forbidden

* were developed for metal atoms and ions (where they are rigorously obeyed), not complexes

.. only one electron is involved in any transition

emax = 1-10 M‒1cm‒1

In the case of spin orbit coupling (as is the case for trans.-metal complexes),

the spin-selection rule is partially lifted

(=> weak, so-called inter-combination bands arise with e = 0.01 – 1.0 M‒1cm‒1)

(example: 4A2g→2Eg- transition, in the case of CrIII, l.s.-CoIII, or MnII)

an e ~ 0.01 M‒1cm‒1 is hardly detectable

[Co(H2O)6]2+

hn

Pauli Prinzipleobeyed

S = 3/2 S = 3/2

S = 0, allowed

[Cr(NH3)6]3+

Spin selection rule

2. d-d-transitions are forbidden

Transitions that are allowed must involve an overall change in

orbital angular momentum of one unit, i.e. L = +1 or -1.

Transitions within the same sub-level are forbidden

allowed: s p, p d

forbidden: d d, p p

Mixing d, p and s functions can lead to partial lifting of the rule

(this explains, why d-d-transitions are observed at all, as all

MO‘s have also some s and p character)

Orbital selection rule L = 1

Laporte rule: parity must change

allowed: g u, u g

forbidden: g g, u u

parity: g(even), u(uneven); index refers to symmetry behaviour of the wave

function (orbital, state) with respect to an inversion operation about origin

This rule is a specific variant of the symmetry selection rule (will be explained later)

Laporte selection rule (only for systems with inversion

symmetry)

g, even:

s and d orbitals

s-, n- and p* bonds

u, uneven:

p and f orbitals

s-, n- and p* bonds

LaPorte rule: parity must change (holds for systems with inversion symmetry)

allowed: g u, u g

forbidden: g g, u u

parity: g(even), u(uneven); index refers to symmetry behaviour of the wave

function (orbital, state) with respect to an inversion operation about origin

for octahedral complexes, all d-d transitions are forbidden (d-orbitals are „g“)

intensities of bands in non-centrosymmetric molecules generally higher

(since the Laporte ban is lifted)

cis/trans-[CoCl2(NH3)4]Cl (cis-complex

has more intense absorption bands)

tetrahedral complexes are more intensely

colored than octahedral ones

Examples

see next page

Laporte rule / An Example

Half width at half height > 3000 cm–1

Franck-Condon-Principle

Fine structure (vibrations) not resolved

Form of the bands

• Transitions are vertical

• The electronic transitions will be from the ground electronic to a vibrationally

excited electronic state (no→nn‘). As M-L bonds are constantly vibrating, light strikes

the molecules in various vibrational positions. Thus the bands are broad.

• Transitions that are forbidden by the spin selection rule (but which are observed

very weakly as the result of spin-orbit coupling) are much narrower. The corresponding

lines of the terms (e.g. the 6A1g and the 4A1g terms for Mn2+) are almost

parallel to each other, and thus do not vary much with o.

Other effects that are of interest, but of the scope of this lecture:

band splitting due to symmetry reduction, band polarisation, dichroism,

see the text books.

Form of the bands

Summary: three rules: spin S=0, Laporte (ug; gu), orbit l = 1

typical values for emax:

[Mn(H2O)6]2+ spin forbidden, Laporte forbidden, emax < 1 M‒1cm‒1

[Co(H2O)6]2+ spin allowed, Laporte forbidden, emax = 1-10 M‒1cm‒1

[CoCl4]2– spin allowed, emax = 600 M‒1cm‒1

all are l forbidden (d→d)

Bands with larger intensity in general due to

charge-transfer transitions (e.g. MnO4‒, LMCT)

or p-p* transitions within the ligand

Rule of thumb:

charge transfer transitions have emax > 103 M‒1cm‒1

* were developed for metal atoms and ions (where they are rigorously obeyed),

for complexes, these rules are not so strictly obeyed; i.e. vibronic coupling, heavy atom effect; s.p.d-mix

Ligands are unsymmetric

KMnO4

Exercise

How many d-d transitons do you expect for an octahedral Scandium(II)

complex? Are these transitions spin allowed?

Sc2+-Ion, d1, => one band;

d→d transitions are forbidden (orbit selection rule);

the absorption is spin allowed,

the absorption is Laporte forbidden (T2g→Eg); parity does not change

e is expected to be 1-10 M–1cm–1 (this is experimentally observed)

[Sc(H2O)6]2+, d1

hn

S = 1/2 S = 1/2

S = 0, spin allowed

doublet doublet2T2g 2Eg

Exercise

Sc2+ is instable in solution

Sc2+ is stable only in solid state: CsScCl3, CsScBr3, CsScI3(prepared by reduction of Cs3ScIII

2X9 with Sc metal)

e.g. Inorg. Chem. 1981, 20, 2627-2631 (no UV/vis data reported)

Ti3+ complexes have e ~ 10-50 M–1cm–1 as predicted

Jahn-Teller Theorem

Any non-linear molecular system in a degenerate electronic state will

Be unstable and will undergo distortion to form a system of lower symmetry

and lower energy thereby removing the degeneracy

Oh

D4h

D4h

Oh

D4d

x2-y2

z2

xy

xz, yz

free Ion

octahedralcrystal field

z elongatedoctahedron

'

2/3

1/3

(two long, 4 short)

D4d

x2-y2

z2

xy

2/3

1/3

xz, yz

z compressedoctahedron

(two short, 4 long)

'

' >