FTIR Data

Molecular Spectroscopy

of Acetylene

Dairong Liu

[email protected]

SES 4340

OFFICE HOURS BY APPOINTMENT

1

Introduction

 Spectroscopy is the study of

interactions between electromagnetic

radiation and matter

 𝐸 = ℎ𝜈 = ℎ𝑐 ǁ𝜈 = ℎ𝑐

𝜆

 Molecules get excited vibrationally by

absorbing Infrared (IR) radiations

 In IR Spectroscopy molecular vibrations

are observed

 IR radiation was discovered by William

Herschel in 1800

2

InfraRed Light

3

 IR wavelength ranges from approx. 14000 – 40 cm-1 (between Red and Microwave)

 IR activates Vibrational states

 Stretching modes - NIR

 Bending modes - FIR

 3 types of IR:

 Near IR (NIR) : 14000-4000 cm-1

 Mid IR (MIR) : 4000-400 cm-1

 Far IR (FIR) : 400-40 cm-1

Uses of IR Light

 Science

 Medicine

 Industry

 Telescopes

4

More Uses

 Motion Sensors

 Thermal Cameras

 Night vision Goggles

 Short range wireless communication

5

Our Application

 Observe the bending mode of acetylene and its deuterium derivative

 Observe rotational transitions and nuclear spin effects

 Determine the rotation constant, vibration-rotation interaction constant,

moment of inertia, and bond lengths

6

Classical Mass on a Spring

 Atoms bonded together act as masses on spring

 Bond is not rigid

 Vibration frequency depends on mass

 EM absorption occurs when light frequency matches vibration frequency

 Absorption causes a change in vibrational state which is detected by spectrometer

7

Hooke’s Law in Harmonic oscillator

 Vibration frequency

depends on mass and

stiffness of the spring

 Classical description has

continuous energy

function

 Quantum energy levels

are not continuous

8

Quantum Mechanical Harmonic Oscillator

 𝐸𝑣𝑖𝑏 = ℎ𝜈𝑣𝑖𝑏 = ℎ𝑐 ǁ𝜈𝑣𝑖𝑏

 Energy of vibration is proportional to frequency

 Absorption occurs when vibration frequency matches light frequency

 Molecular vibrations have the same frequency as IR light

 Quantum probability shows that vibrational energy can be outside of allowed region

9

Anharmonic Effects

 Molecules cannot vibrate harmonically

 Morse function describes anharmonic

oscillation

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Conditions to Obtain IR Spectrum

 Selection Rule: Dipole moment

of bond must change during

vibration

 Applied frequency = Vibration

frequency

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Rigid Rotor

For quantum number, J

We have 𝐸𝑟 = ℎ2

8𝜋2𝐼𝑒 𝐽 𝐽 + 1

If B = ℎ

8𝜋2𝐼𝑒

Then 𝐸𝑟 = 𝐵𝐽 𝐽 + 1 h 12

Rotational Spectroscopy

 Molecules classified by inertial

axes

 3 axes: Ia, Ib, Ic

 For intertia, typically 𝐼𝑐 > 𝐼𝑏 > 𝐼𝑎

 Linear Molecules: 𝐼𝑐 = 𝐼𝑏 > 𝐼𝑎

 The inertia is now the sum of the

distance between each mass and

the center of mass of the rotor

multiplied by the square of the

distance between them

 𝐼𝑒 = σ 𝑚𝑟2

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a

b

c

Conditions of Rotational Spectroscopy

 Selection Rules

 Molecule must have a dipole moment

 ΔJ = ±1

 ΔMJ = 0, ±1

 Spectra are symmetric

 ǁ𝜈 = 𝐸

ℎ𝑐 =

ℎ

8𝜋2𝐼𝑐 𝐽 𝐽 + 1 = 𝐵𝐽(𝐽 + 1)

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Rotational vibrational spectroscopy

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Which one will have

higher energy?

Which EM adsorption

will have a higher

wave number?

Vibration-Rotation Coupling

 Rotation happens at lower energy than

vibration

 There are many rotational energy levels

between two vibrational energy levels

 Rotational spectra can be obtained with IR

spectroscopy

 Vibrational state dependence

 𝐵𝑣 = 𝐵𝑒 − 𝛼 𝑣 + 1

2

 Rotational constant, B

 Vibration-Rotation interaction constant, α

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P branch

ΔJ= -1

R branch

ΔJ= +1

Rotational-Vibrational Spectroscopy

 Transitions with ΔJ = 1 are R-

branch

 Transitions with ΔJ = -1 are

P-branch

 𝜈𝑅 𝐽 = 𝜈𝑣𝑖𝑏 + 𝐵0 + 𝐵1 ( )

𝐽 + 1 − 𝐵0 − 𝐵1 𝐽 + 1 2

 𝜈𝑃 𝐽 = 𝜈𝑣𝑖𝑏 + 𝐵0 + 𝐵1 𝐽 − 𝐵0 − 𝐵1 𝐽2

17

Fourier Transform Infrared Spectrometer

 Michelson Interferometer

invented 1881

 Peter Fellgett obtained first

IR spectrum with FTIR 1949

 FTIR commercially available

1960

18

Fourier Transform Infrared Spectrometry

 FT converts time domain to

frequency domain

 Cooley-Tuckey invented algorithm

for fast FT 1966

 Fast and sensitive

 Scan all frequencies at once

19

Predicting the FTIR Spectrum

 Number of normal modes

Non linear molecule: 3N - 6

Linear molecule: 3N - 5

 Example: H2O has 3 IR modes

 symmetric O-H stretching

 asymmetric O-H stretching

 O-H bending

 3 IR bands are seen in the spectrum for water

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Experiment Overview

 Goal: Determine the bond lengths of the C-H and C≡C bonds

 Simple steps:

 Synthesize sample

 Analyze with FTIR Spectrometer

 Interpret Data

 Clean Up

21

Acetylene (C2H2)

 Isolate and remove IR cell for background scan (prep ice bath for reaction

flask)

 Isolate manifold from reaction flask and CT

 Inject Water (Deionized)

 Open valve to reaction flask after 30 seconds

 Isolate and remove IR cell when pressure reaches ~100 Torr for sample scan

 Isolate CT from Mech Pump and open valve from manifold to CT 22

 Attach reaction flask with

CaC2, IR cell, Cold Trap

(CT), and Mechanical Pump

Hose

 Pump down manifold

 Check for leaks

 TA Add liquid N2

Di-deutero-acetylene (C2D2)

 Attach reaction flask with CaC2, IR cell, Cold Finger (CF), Cold Trap, Mech pump hose

 Pump down manifold

 Check for leaks

 T.A. add liquid N2 to CT

 Isolate and remove IR cell for background scan (prep water bath for reaction flask)

 Reattach IR cell and cool CF

 Isolate manifold from CT

 Inject D2O leaving reaction flask valves open (Pressure does not rise much)

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Di-deutero-acetylene (C2D2) Cont.

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 After bubbling stops, isolate CT from mech pump and expose manifold to CT

 Pump down manifold when pressure is stable

 Isolate rxn flask when pressure is stable

 Isolate manifold from CT

 Remove dewar from CF

 Isolate and remove IR cell for sample scan when pressure reaches ~200 Torr

 Isolate CT from mech pump and expose manifold to CT

Reminders

 Glass manifold – handle with care

 Do not use too much CaC2

 Make sure there is no water in the flask before adding CaC2

 Follow manual instructions carefully

25

Clean Up

 Pump down manifolds

 Isolate CT from manifold and mech

pump

 Remove CT and place in bucket in

fume hood

 Open valve to vent CT

 Leave CT in hood

 Rinse CF with water in fume hood

 Place CF in oven

 Remove reaction flasks

 Rinse with water in fume hood

 Pour 1st wash in FTIR waste

 Use 0.1M HCl and sonicator to

clean flasks

 Pour washings down sink

 Place flasks in oven

 IF FLASKS CONTAIN RESIDUE FOR

THE NEXT ROTATION, POINTS WILL

BE DEDUCTED FROM REPORTS

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Actual Spectra

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𝜈0 = 1328.18 𝑐𝑚−1

𝑅 0 ≈ 1330 𝑐𝑚−1

𝑃 1 ≈ 1326 𝑐𝑚−1

𝜈0 = 1041.49 𝑐𝑚−1

𝑅 0 ≈ 1043 𝑐𝑚−1

𝑃 1 ≈ 1039 𝑐𝑚−1

Key point: be careful with the

wavenumbers!

Data Analysis

 Identify P branch and R branch in spectrum

 Label peaks with J values

 Plot 1

2 [𝜈𝑅 𝐽 − 𝜈𝑃 𝐽 ] against 2𝐽 + 1

 Plot 1

2 [𝜈𝑅 𝐽 − 𝜈𝑃 𝐽 + 2 ] against 2𝐽 + 3

 𝐵𝑣 = 𝐵𝑒 − 𝛼𝑒(𝑣 + 1

2 )

 Calculate 𝛼𝑒, 𝐵𝑒, 𝐼𝑒, 𝑟𝑒𝐶𝐶, 𝑟𝑒𝐶𝐻

 Tabulate 𝜈0, 𝐵𝑒, 𝛼𝑒, 𝐼𝑒, 𝑟𝑒𝐶𝐶, 𝑟𝑒𝐶𝐻

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Acetylene Plots

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1

2 𝜈𝑅 𝐽 − 𝜈𝑃 𝐽 = 𝐵1(2𝐽 + 1)

1

2 𝜈𝑅 𝐽 − 𝜈𝑃 𝐽 + 2 = 𝐵0(2𝐽 + 3)

Intercept should be set to zero. It can be done in origin while taking

linear fit.

Calculations

 𝐵𝑣 = 𝐵𝑒 − 𝛼𝑒 𝑣 + 1

2

 𝜈𝑅 𝐽 − 𝜈𝑃 𝐽 = 2𝐵1 2𝐽 + 1

 𝜈𝑅 𝐽 − 𝜈𝑃 𝐽 + 2 = 2𝐵0 2𝐽 + 3

 𝐵𝑒 = ℎ

8𝜋2𝑐𝐼𝑒

 𝐼𝑒 = σ 𝑚𝑅2

 𝑅𝑐 = 1

2 𝑟𝐶𝐶

 𝑅𝐻 = 1

2 𝑟𝐶𝐶 + 𝑟𝐶𝐻

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Here we have 𝐼𝑒 = 2𝑚𝑅𝐶 2 + 2𝑚𝑅𝐷

2

And 𝐼𝑒 ′ = 2𝑚𝑅𝐶

2 + 2𝑚𝑅𝐻 2

Then we assume: 𝑅𝐻= 𝑅𝐷

Error Propagation

 Use partial derivatives for error propagation

 𝜎𝛼𝑒 2 = (

𝜕𝛼𝑒

𝜕𝐵0 )2𝜎𝐵0

2 + ( 𝜕𝛼𝑒

𝜕𝐵1 )2𝜎𝐵1

2

 𝜎𝐵𝑒 2 = (

𝜕𝐵𝑒

𝜕𝐵0 )2𝜎𝐵0

2 + ( 𝜕𝐵𝑒

𝜕𝛼𝑒 )2𝜎𝛼𝑒

2 or 𝜎𝐵𝑒 2 = (

𝜕𝐵𝑒

𝜕𝐵1 )2𝜎𝐵1

2 + ( 𝜕𝐵𝑒

𝜕𝛼𝑒 )2𝜎𝛼𝑒

2

 𝜎𝐼𝑒 2 = (

𝜕𝐼𝑒

𝜕𝐵𝑒 )2𝜎𝐵𝑒

2

 𝜎𝑟𝐶𝐶 2 = (

𝜕𝑟𝐶𝐶

𝜕𝐼𝑒,𝐻 )2𝜎𝐼𝑒,𝐻

2 + ( 𝜕𝑟𝐶𝐶

𝜕𝐼𝑒,𝐷 )2𝜎𝐼𝑒,𝐷

2

 𝜎𝑟𝐶𝐻 2 = (

𝜕𝑟𝐶𝐻

𝜕𝐼𝑒,𝐻 )2𝜎𝐼𝑒,𝐻

2 + ( 𝜕𝑟𝐶𝐻

𝜕𝐼𝑒,𝐷 )2𝜎𝐼𝑒,𝐷

2 + ( 𝜕𝑟𝐶𝐻

𝜕𝑟𝐶𝐶 )2𝜎𝑟𝐶𝐶

2

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Lab Report – Title Page/Abstract

 Title page should be simple in design

 Contains experiment title centered on page in a reasonable font size

 Contains author info (Name, Partner name, TA, Date) in regular font used for

report body

 Abstract should be short

 Short paragraph summarizing the experiment

 Includes: goal of experiment, theoretical models used, results of experiment,

comparison to Tidwell et al

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Lab Report – Data/Calculations

 Data should be presented in tables with appropriate titles and significant

figures

 Data in tables should include error values

 Graphs should appropriately formatted with descriptive title

 Graphs should be 2 or more per page (only 4 graphs in this experiment)

 Graphs cannot be generated in Excel, use Origin or other software

 Linear regression data must be legible

 Use appropriate scientific notation when necessary (in MS Word 𝑚 × 10𝑛

should be used instead of mEn)

 Use MS Word equation editor tool to show calculations

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Lab Report - Discussion

 Should explain the results of the experiments

 Compare your results with Tidwell et al.

 Are nuclear spin effects detected in your spectra?

 Are the results expected or not?

 Why did the experiment work/fail?

 How does the experiment connect to the theory?

 Remember sentence structure and word choice

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Lab Report – Conclusion

 Short summary of experiment

 Includes obtained results, experimental success or failure

 One paragraph

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Lab Report – References

 ACS style citations

 Use UIC Library RefWorks if necessary

 References must be cited internally and externally

 Should have more references than just the lab manual

 This presentation can be used as reference because it is not peer-reviewed

 References can be any journal articles, textbooks, national databases

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Don’t forget the

basics!