Transcript MS PowerPoint - Catalysis Eprints database

What is Operando Spectroscopy?
Spectroscopic characterization of catalysts
under realistic reaction conditions with
simultaneous real-time online analysis of
reaction products
SPECTROSCOPIC METHODS
IN CATALYSIS
COVERAGE
Application of UV-Visible spectroscopy
Application of Infra red spectroscopy
Application of Resonance spectroscopy
ELECTRONIC SPECTROSCOPY
Using light absorption for changing the charge distribution about
a molecule
This is a lot of energy - often can break bonds
Types of electronic transitions
ORGANICS: Involving p, s, n electrons
Saturated compounds s s (<150 nm), n s (<250 nm) Deep
UV
Double bonds/unsaturated systems
p p , n p transitions : UV and visible (200-700 nm)
Inorganics: Additionally, transitions between d orbitals split
by presence of
ligand field. Usually in visible
d-d transition
Charge transfer transition Electron moves between ligand
and metal.
One must act as donor and other as acceptor
Light will be resonant with electronic energy gap at
equilibrium nuclear geometry
Electronic Spectra
At equilibrium, molecule is in ground electronic state
→ lowest energy electronic state and typically in v=0.
• Transitions to higher lying electronic states are
accompanied by changes in v, J.
• Excitation is accompanied by vibrational excitation,
feels restoring force in excited state.
Franck-Condon principle
vertical transitions
Electrons respond much faster than nuclear
motion, therefore an excitation proceeds
without a change to the nuclear geometry.
Light will be resonant with electronic energy
gap at equilibrium nuclear geometry.
Table . The electronic spectral data of the complexes recorded in ethanol
[frequency (cm-1)/ εmax (mol-1 dm3 cm-1)].
Complex
ν(104 cm-1)/
ε
μeff/μB
π→π*
π→π*
π*→d
d→d*
d→d*
[Mn(C5H7
O2)3]
4.90
40.8/175 36.4/24000
00
30.7/9500
24.8/950 17.5/100
[Mn(C5H7
O2)2L1]
4.76
41.6/310 30.2/12000
00
30.2/17500
24.3/100 17.6/70
0
[Mn(C5H7
O2)2L2]
4.81
42.0/225 35.9/14500
00
30.3/11000
24.8/150 17.5/170
0
Selection Rules for Electronic Spectra of Transition Metal Complexes.
The Selection Rules governing transitions between electronic energy levels of transition metal
complexes are:
ΔS = 0 The Spin Rule
Δl = +/- 1 The Orbital Rule (Laporte)
The first rule says that allowed transitions must involve the promotion of electrons without a
change in their spin.
The second rule says that if the molecule has a centre of symmetry, transitions within a given
set of p or d orbitals (i.e. those which only involve a redistribution of electrons within a given
subshell) are forbidden.
Relaxation of the Rules can occur through:
a) Spin-Orbit coupling - this gives rise to weak spin forbidden bands
b) Vibronic coupling - an octahedral complex may have allowed vibrations where the molecule
is asymmetric.
Absorption of light at that moment is then possible.
c) π-acceptor and π-donor ligands can mix with the d-orbitals so transitions are no longer
purely d-d.
Types of transition
Charge transfer, either ligand to metal or metal to
ligand. These are often extremely intense and are
generally found in the UV but they may have a tail
into the visible.
d-d, these can occur in both the UV and visible
region but since they are forbidden transitions
have small intensities.
Expected Values
The expected values should be compared to the following rough guide.
For M2+ complexes, expect Δ = 7500 - 12500 cm-1 or λ = 800 - 1350 nm.
For M3+ complexes, expect Δ= 14000 - 25000 cm-1 or λ = 400 - 720 nm.
For a typical spin-allowed but Laporte (orbitally) forbidden transition in
an octahedral complex, expect ε < 10 m2mol-1.
Extinction coefficients for tetrahedral complexes are expected to be
around 50-100 times larger than for octrahedral complexes.
B for first-row transition metal free ions is around 1000 cm-1. Depending
on the position of the ligand in the nephelauxetic series, this can be
reduced to as low as 60% in the complex.
Expected intensities of electronic transitions
Expected intensities of electronic transitions
Transition type
Example
Typical value of ε m2 mol-1
Spin forbidden,
Laporte forbidden
[Mn(H2O)6]2+
0.1
Spin allowed (octahedral
complex),
Laporte forbidden
[Ti(H2O)6]3+
1
Spin allowed (tetrahedral
complex),
Laporte partially allowed
by d-p mixing
[CoCl4]2-
50
Spin allowed,
Laporte allowed
e.g. charge transfer bands
[TiCl6]2- or MnO4-
1000
Basics of Light, EM Spectrum, and X-rays
• Light can take on many forms. Radio waves, microwaves,
infrared, visible, ultraviolet, X-ray and gamma radiation are
all different forms of light.
• The energy of the photon tells what kind of light it is. Radio
waves are composed of low energy photons. Optical
photons--the only photons perceived by the human eye--are
a million times more energetic than the typical radio photon.
The energies of X-ray photons range from hundreds to
thousands of times higher than that of optical photons.
• Very low temperatures (hundreds of degrees below zero
Celsius) produce low energy radio and microwave photons,
whereas cool bodies like ours (about 30 degrees Celsius)
produce infrared radiation. Very high temperatures (millions
of degrees Celsius) produce X-rays.
The absorption of UV or visible radiation corresponds
to the excitation of outer electrons. There are three
types of electronic transition which can be
considered;
Transitions involving p, s, and n electrons
Transitions involving charge-transfer electrons
Transitions involving d and f electrons
When an atom or molecule absorbs energy, electrons
are promoted from their ground state to an excited
state. In a molecule, the atoms can rotate and vibrate
with respect to each other. These vibrations and
rotations also have discrete energy levels, which can
be considered as being packed on top of each
electronic level.
.
Absorbing species containing p, s, and n electrons
Absorption of ultraviolet and visible radiation in organic molecules is
restricted to certain functional groups (chromophores) that contain valence
electrons of low excitation energy. The spectrum of a molecule containing
these chromophores is complex. This is because the superposition of
rotational and vibrational transitions on the electronic transitions gives a
combination of overlapping lines. This appears as a continuous absorption
band.
Possible electronic transitions of p, s, and n electrons are;
Sigma to sigma* Transitions
An electron in a bonding sigma orbital is excited to the corresponding antibonding
orbital. The energy required is large. For example, methane (which has only C-H
bonds, and can only undergo sigma to sigma* transitions) shows an absorbance
maximum at 125 nm. Absorption maxima due to sigma to sigma* transitions are not
seen in typical UV-Vis. spectra (200 - 700 nm)
n to sigma* Transitions
Saturated compounds containing atoms with lone pairs (non-bonding electrons) are
capable of n to sigma* transitions. These transitions usually need less energy than
sigma to sigma * transitions. They can be initiated by light whose wavelength is in
the range 150 - 250 nm. The number of organic functional groups with n to sigma*
peaks in the UV region is small.
N to pi* and pi to pi* Transitions
Most absorption spectroscopy of organic compounds is based on transitions of n or
pi electrons to the pi* excited state. This is because the absorption peaks for these
transitions fall in an experimentally convenient region of the spectrum (200 - 700
nm). These transitions need an unsaturated group in the molecule to provide the pi
electrons.
Molar absorbtivities from n ® pi* transitions are relatively low, and range from 10
to100 L mol-1 cm-1 . pi ® pi* transitions normally give molar absorbtivities between
1000 and 10,000 L mol-1 cm-1 .
The solvent in which the absorbing species is dissolved also has an effect
on the spectrum of the species. Peaks resulting from n to pi* transitions
are shifted to shorter wavelengths (blue shift) with increasing solvent
polarity. This arises from increased solvation of the lone pair, which
lowers the energy of the n orbital. Often (but not always), the reverse
(i.e. red shift) is seen for pi to pi* transitions. This is caused by attractive
polarisation forces between the solvent and the absorber, which lower the
energy levels of both the excited and unexcited states. This effect is
greater for the excited state, and so the energy difference between the
excited and unexcited states is slightly reduced - resulting in a small red
shift. This effect also influences n to pi* transitions but is overshadowed
by the blue shift resulting from solvation of lone pairs.
Charge - Transfer Absorption
Many inorganic species show charge-transfer absorption and are
called charge-transfer complexes. For a complex to demonstrate
charge-transfer behaviour, one of its components must have
electron donating properties and another component must be able
to accept electrons. Absorption of radiation then involves the
transfer of an electron from the donor to an orbital associated with
the acceptor.
Molar absorbtivities from charge-transfer absorption are large
(greater that 10,000 L mol-1 cm-1).