UV-Vis spectroscopy

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UV-Vis spectroscopy
UV-Vis spectroscopy
Dr. Davide Ferri
Paul Scherrer Institut
056 310 27 81
[email protected]
The electromagnetic spectrum
VISIBLE
1015-1016 Hz
ULTRAVIOLET
1016-1017 Hz
200
source: Andor.com
UV-vis spectroscopy
Use of ultraviolet and visible radiation
Electron excitation to excited electronic level (electronic transitions)
Identifies functional groups (-(C=C)n-, -C=O, -C=N, etc.)
Access to molecular structure and oxidation state
pros
economic
non-invasive (fiber optics!)
versatile (e.g. solid, liquid, gas)
extremely sensitive (concentration)
fast acquisition (but S/N!)
IR
Raman
NMR
XAFS
UV-Vis
EPR
0
200
400
600
800
Number of publications
1000
cons
no atomic resolution
broad signals (spectral resolution, multiple overlapping components)
1200
Electronic transitions
hν
Organic molecule
Organic molecule
anti-bonding
σ*
anti-bonding
empty
π*
lone pairs
e
occupied
n
e
e
e
π
e
bonding
e
bonding
e
σ
n→π* n→σ* π→π* σ→σ*
E= hν
high e- jump → high E
high E → high ν
e
n→π* n→σ* π→π* σ→σ*
λ= c/ν
high ν → low λ
Electronic transitions
anti-bonding
σ*
π*
e
n
e
π
e
bonding
e
σ
n→π* n→σ* π→π* σ→σ*
σ→σ*
high E, low λ (<200 nm)
n→σ*
150-250 nm, weak
n→π*
200-700 nm, weak
Condition to absorb light
(200-800 nm):
π and/or n orbitals
π→π*
200-700 nm, intense
CHROMOPHORE
The UV spectrum
λmax
1.0
σ*
n
e
π
σ
π→π*
0.8
absorbance
π*
217 nm
0.6
0.4
0.2
no visible light absorption
0.0
200
energy
signal envelope
E*
220
240
280
300
wavelength (nm)
vibrational
electronic levels
rotational
electronic levels
E0
260
How many signals do you
expect from CH3-CH=O?
The UV spectrum
Conjugation effect
λmax
delocalisation
λ
ν
171
217
258
C6H8
C4H6
C2H4
π*
e
π
e
e
E
The UV spectrum
Conjugation effect: β-carotene
absorbance
white light
300
360
420
wavelength (nm)
480
540
The UV spectrum
Complementary colours
650 - 780
580 - 595
500 - 560
380 - 435
435 - 480
480 - 490
If a colour is absorbed by white light, what the eye detects
by mixing all other wavelengths is its complementary
colour
Inorganic compounds
UV-vis spectra of transition metal complexes originate from
Electronic d-d transitions
dσ
eg
degenerate
d-orbitals
∆
+ ligand
TM
t2g
TM
dπ
…
Inorganic compounds
dx2-y2
Crystal field theory (CFT) - electrostatic model
same electronic structure of central ion as in isolated ion
perturbation only by negative charges of ligand
dx2-y2, dz2
dxy, dxz, dyz
∆
dz2
atom in
spherical field
∆
dx2-y2
∆
dxy
dx2-y2, dz2
dxy
gaseous atom
dxy, dxz, dyz
dyz, dxz
∆ = crystal field splitting
tetrahedric
field
octahedric
field
tetragonal
field
dz2
square planar
field
Inorganic compounds
d-d transitions: Cu(H2O)62+
eg
degenerate
d-orbitals
∆
+ 6H2O
light
Cu2+
t2g
Cu(H2O)62+
Yellow light is absorbed and the Cu2+ solution is coloured in blue (ca. 800 nm)
The greater ∆, the greater the E needed to promote the e-, and the shorter λ
∆ depends on the nature of ligand, ∆NH3 > ∆H2O
Inorganic compounds
TM(H2O)6n+
elec. config. TM
gas complex
t2g1
t2g1
t2g3
t2g3eg1
t2g3eg2
Ti(H2O)63+
Ti(H2O)63+
Cr(H2O)63+
Cr(H2O)62+
Mn(H2O)62+
t2g6eg3
Cu(H2O)62+
Ni2+
absorbance
3d1
3d2
3d3
3d4
3d5
3d6
3d7
3d8
3d9
Fe2+
Cu2+
Co2+
Cr3+
Ti3+
400
500
V4+
600
700
800
wavelengths (nm)
d-d transitions: εmax = 1 - 100 Lmol-1cm-1, weak
900
1000
Inorganic compounds
d-d transitions: factors governing magnitude of ∆
Oxidation state of metal ion
∆ increases with increasing ionic charge on metal ion
Nature of metal ion
∆ increases in the order 3d < 4d < 5d
Number and geometry of ligands
∆ for tetrahedral complexes is larger than for
octahedral ones
Nature of ligands
spectrochemical series
I- < Br- < S2- < SCN- < Cl- < NO3- < N3- < F- < OH- <
C2O42- < H2O < NCS- < CH3CN < py < NH3 < en <
bipy < phen < NO2- < PPh3 < CN- < CO
Inorganic compounds
UV-vis spectra of transition metal complexes originate from
Electronic d-d transitions
eg
degenerate
d-orbitals
∆
+ ligand
TM
t2g
TM
Charge transfer
Inorganic compounds
Charge transfer complex
no selection rules → intense colours (ε=50‘000 Lmol-1cm-1,
strong)
Association of 2 or more molecules in which a fraction of
electronic charge is transferred between the molecular
entities. The resulting electrostatic attraction provides a
stabilizing force for the molecular complex
Electron donor: source molecule
Electron acceptor: receiving species
CT much weaker than covalent forces
Ligand field theory (LFT), based on MO
Metal-to-ligand transfer (MLCT)
Ligand-to-metal transfer (LMCT)
Inorganic compounds
Ligand field theory (LFT)
involves AO of metal and ligand, therefore MO
what CFT indicates as possible electronic transitions (t2g→eg)
are now: πd→σdz2* or πd→ σdx2-y2*
πpx*, πpy*, πpz*
σp*
4p
σs *
4s
σd*
eg
∆
t2g
3d
πdxy, πdxz, πdxy
σd
AOL
AOTM
∆ = crystal field splitting
2s
σp
σs
MO(TML6n+)
Inorganic compounds
Ligand field theory (LFT)
LMCT
ligand with high energy lone pair
or, metal with low lying empty orbitals
high oxidation state (laso d0)
M-L strengthened
4σ
2π∗
2π∗
O
1π
MLCT
ligands with low lying π* orbitals (CO, CN-, SCN-)
low oxidation state (high energy d orbitals)
M-L strengthened, π bond of L weakened
2π∗
1π
3σ
C
5σ
2π∗
Metal
back donation!!!
CO adsorption on
precious metals
Band gap
■ Analysis of semiconductors
Photocatalysis
A+
TiO2, 3.2 eV
reduction
A+ 1e- A+
energy
CB
A
e-
band
gap
VB
+
B
oxidation
B+ h+ B-
B-
Band gap
DIRECT
INDIRECT
energy
energy
CB
CB
phonon
VB
k
How to measure
inflection point
energy at exp. increase
intercept energy axis
VB
k
Metal colloids
■ Analysis of metals
■ Localized plasmon resonance
When
wavelength larger than metal particle
What
collective excitations of conduction electrons (plasmons)
limit: ca. 20-30 nm
λ position depends on nature of metal
Au
size
size
Instrumentation
Dispersive instruments
Measurement geometry:
- transmission
- diffuse reflectance
double beam spectrometer
single beam spectrometer
In situ instrumentation
Diffuse reflectance (DRUV)
Fiber optics
to detector
gas outlet
- 20% of light is collected
- gas flows, pressure, vacuum
- time resolution (CCD camera)
[spectra colleted at once]
- coupling to reactors
- no NIR (no optical fiber > 1100 nm)
- long term reproducibility (single beam)
- Limited high temperature (ca. 600°C)
- long meas. time
- spectral collection (λ after λ)
→ different parts of spectrum do not represent same reaction time!!!
Weckhuysen, Chem. Commun. (2002) 97
In situ instrumentation
Integration sphere
White coated integration sphere
(MgO, BaSO4, Spectralon®)
integration
sphere
- > 95% light is collected
- high reflectivity
- wide range of λ
- only homemade cells
for example, for cat. synthesis
Weckhuysen, Chem. Commun. (2002) 97
Examples
Determination of oxidation state: 0.1 wt% Crn+/Al2O3
Cr6+ (250, 370 nm)
reduction in CO atmosphere
Cr3+/Cr2+
Weckhuysen et al., Catal. Today 49 (1999) 441
Examples
Determination of oxidation state: 0.1 wt% Crn+/Al2O3
calibration
distribution of Crn+
Cr3+
100
Cr6+
%
Cr6+
Cr5+
Cr3+
Cr2+
50
deconvolution
0
A
B
C
A: calc. 550°C
B: red. 200°C
C: red. 300°C
D: red. 400°C
E: red. 600°C
F: re-calc. 550°C
Weckhuysen et al., Catal. Today 49 (1999) 441
D
E
F
Examples
UV-vis probe in a pilot-scale reactor: propane dehydrogenation
GC
10 vol% C3H8, 90 vol% N2, 5000 ml/min
20 wt% Cr3+/6+Ox/Al2O3
Weckhuysen et al., Chem. Commun. 49 (2013) 1518
Examples
UV-vis probe in a pilot-scale reactor
average intensity 600-700 nm
bottom UV-vis probe
dehydrogenation
top UV-vis
probe
bottom UV-vis
probe
bottom UV-vis
probe
regeneration
top UV-vis
probe
Coke formation fast on top section of reactor
Coke is combusted fast in top section of reactor
Weckhuysen et al., Chem. Commun. 49 (2013) 1518

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