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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)
'
' >