spectroscopy presention
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04/18/2023 2
Importance of In-situ Spectroscopy : Comparison with
conventional offline analysis
Theory of FTIR
Instrumentation and sampling : ATR FTIR Spectroscopy
How to interpret Infra-Red spectra
Case study – Convergent synthesis of alkenes by cross
coupling of carbenes
Outline
3
In Situ FTIR Spectroscopy: MotivationMonitor reactions In Situ under Actual Reaction Conditions
Effectively monitor reactions that have components sensitive to oxygen water vapor or temperature.
Eliminate or reduce the need for offline analysisComprehensive concentration information in real-time
Follow instantaneous changes in reactants, intermediates and products providing a molecular video of reaction chemistry
Eliminate time delay associated with offline analytical methods
Study reaction kinetics , Elucidate Mechanism and Pathway Real-time reaction kinetics information Monitor changes in functional groups provides in-depth
understanding of reaction mechanism and pathway
In-situ Spectroscopy : Data Acquisition
ATR raw data
0 100 200 300 400
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
Co
nc
en
tra
tio
n (
mo
l/L)
Time (min)
concentration of phenol concentration of cyclohexanol concentration of cyclohexanone
180
200
220
240
260
280P
res
su
re (
ps
i)
UV-Vis probe
Raman probe
ATR-IR 300 C, 35 bar max.
4
OH
O
OH
+ 2H2
+ H2
+ 3H2+ H2- H2O
ATR analyzed data
Kinetic Analysis: Off-line technique
Diels Alder Reaction Large number of separate
experiments required Concentrations are distorted from
synthetically relevant conditions
Drawbacks
Kinetic Analysis : In-situ Technique
[1]0 [2]0 [e] = [2]0 – [1]0
= [2] – [1]Exp. A 0.2M 0.6 M 0.4 MExp. B 0.2M 0.4M 0.2M
JACS, 2007,129,15100.
Diels Alder Reaction
Kinetic Analysis : In-situ TechniqueRearranged rate equation Rate equation
Two experiments via in-situ technique
Two SETS of experiments via classical offline analysis technique
8
• Chemical bonds between the atoms within the molecules vibrate. • A diatomic molecule is considered as a two spherical masses (m1
& m2) connected with a spring with a given force constant k. • Applied Freq. = Natural Freq. of Vibration • Absorption of IR radiation takes place and a peak is observed.
Working Principle of IR Spectroscopy
1ν
2π μ
k
c
Working Principle of IR Spectroscopy
The Harmonic Oscillator
Can Model the fundamental Vibrations between the atoms using Hook’s Law.
The potential energy curve of such an oscillator is parabolic in shape and symmetrical about the equilibrium bond length.
V
+A
Displacement
-A 0
-A
+A
0
Δn = ±1
only certain energy levels exist :
1 ;
2
1 ;
2
1
2
nE n hv E h
k vv v
c
kv
c
Transition moment Integral 0 ; if =const
( ') and ( ) are orthogonal
M
21.
2V k x
Morse Potential Model Bond dissociate if stretched
beyond a certain distance
Bond length cannot be less
than minimum inter nuclear
distance
In the case of the Anharmonic
Oscillator, the vibrational
transitions no longer only obey
the selection rule n = 1.
Vibrational transitions with n
= 2, 3, ... are also possible A more accurate description of the vibrational energies is given by the anharmonic oscillator (also called Morse potential) with energy of
Modes of vibrations
R H bending
R
RHH
R
RHH
R
RHH
R
RHH
R
RHH
R H
antisymmetric
symmetric
rockingscissoring
in-planebending
stretching
Region Origin of the absorption
NIR Overtones and combination bands of fundamental molecular vibrations
MIR fundamental molecular vibrations
FIR molecular rotations
1
2
kv
c
Most organic functional groups exhibit MIR absorption spectrum High Molar absoptivities Ideal for Qualitative and Quantitative analysis
04/18/2023 12
Working principle of Interferometer
0,x n
( 1/ 2)x n
Moving mirror
Zero path diff
Moving mirror
Fixed mirror
IR light source
Photodetector
Beam Splitter
Constructive interference
Destructive Interference
Power signal
04/18/2023 13
Fourier Transform of an Interferogram
Optical Path Difference Wavenumbers cm-1
Rela
tive
Inte
nsity
Optical Path Difference Optical Path Difference
Sign
al fr
om
phot
o de
tect
or
Sign
al fr
om
phot
o de
tect
or As we move the mirror the optical path difference changes
By taking the fourier transform we can identity the frequencies present in the interferogram
Interferogram IR Spectrum
04/18/2023 15
Absorbance / TransmittanceLambert Beer Law
A = (absorptivity coefficient) x b (path length) x c (concentration)
16
ATR Sampling : Method of Measurement
ReactIR 45m FTIR Spectrometer
ATR Probe
Experimental Apparatus
17
• The infrared radiation interacts with the sample through series of standing waves called evanescent waves.
• An evanescent wave is produced each time the infrared beam is reflected from the inside surface of the crystal
• The evanescent waves penetrate the sample at each reflection point.
ATR Sampling : Method of Measurement
At the reflection point
2-3 µm
04/18/2023 18
Multiple Reflection
Enhanced sensitivity makes it Ideal for analyzing samples when the component of interest is low.
70% Iso propyl alcohol
04/18/2023 19
How to Read / Interpret an IR Spectra ??
Fingerprint RegionFunctional Group Region
1. Which is the material under test ? (identifying unknown material)
2. What Functional Groups are Present in the
material under test ?
04/18/2023 21
Case Study : Convergent synthesis of Alkenes by Cross Coupling of Carbenes
Carbenoid
Hansen, J. H., Parr, B. T., Pelphrey, P., Jin, Q., Autschbach, J. and Davies, H. M. L.; Angew. Chem. Int. Ed., 50: 2544–2548
22
Convergent synthesis of Alkenes by Cross Coupling of Carbenes
Hansen, J. H., Parr, B. T., Pelphrey, P., Jin, Q., Autschbach, J. and Davies, H. M. L.; Angew. Chem. Int. Ed., 50: 2544–2548
Acceptor carbenoid (2) homodimerization is very favorableDonor/acceptor carbenoid (1a) homodimerization is not favorableWhat factors control the cross coupling and why does it work so
well?
Donor/Acceptor + Acceptor
23
In-situ Analysis
(1a) decomposes faster than the (2) and then preferentially reacts with other diazo compound.
Reaction monitored by the presence of C=N stretch bands
Hansen, J. H., Parr, B. T., Pelphrey, P., Jin, Q., Autschbach, J. and Davies, H. M. L.; Angew. Chem. Int. Ed., 50: 2544–2548
Acceptor Donor/Acceptor
24
Cross Coupling of Carbene In Equimolar mixture of (1a) and (2), the
consumption of (2) is 1.5 times faster than (1a) All of 2 is consumed within 7s Once 2 is consumed the decomposition of 1a is
significantly low. Homodimerization of (2) does not go to
completion
Hansen, J. H., Parr, B. T., Pelphrey, P., Jin, Q., Autschbach, J. and Davies, H. M. L.; Angew. Chem. Int. Ed., 50: 2544–2548
Acceptor Donor/Acceptor
25
Mechanism for Cross coupling of Carbene
Hansen, J. H., Parr, B. T., Pelphrey, P., Jin, Q., Autschbach, J. and Davies, H. M. L.; Angew. Chem. Int. Ed., 50: 2544–2548
Product cross dimerization rely on:
A significant decomposition
rate difference between the
two diazo compounds
A preference for trapping of
the initial carbenoid with
the other diazo compound.
Key findings:
04/18/2023 26
SummaryProvides a full kinetic analysis from a
minimum number of reaction progress experiments
Elucidation of mechanism and pathway
Detection of reactive intermediatesOptimize reactions and develop safe ways
to work with carbenoid precursors
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