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Chemistry 2000 Slide Set 6:Vibrational spectroscopy of polyatomic molecules
Marc R. Roussel
January 14, 2020
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 1 / 29
Solution-phase IR spectroscopy
Example: IR spectrum of liquid ethanol
Source: Spectral Database of Organic Compounds, http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi, Jan.
16, 2013
Note: The wavenumber axis often runs backward, as shown here.
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 2 / 29
Solution-phase IR spectroscopy
Infrared spectroscopy and the identification of compounds
One important application of spectroscopy (in general) is for theidentification of unknown compounds.
Certain bonds in organic molecules are associated with characteristicIR bands in specific spectral regions:Bond Spectral region/cm−1
C H 2800–3000
C C
H
(including aromatic CH) 3000–3200
O−H (non-hydrogen-bonded) 3500–3700 (sharp)O−H (hydrogen-bonded) 3200–3500 (broad)
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 3 / 29
Solution-phase IR spectroscopy
Example: The IR spectrum of ethanol
C−H stretches
hydrogen−bonded OH
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 4 / 29
Solution-phase IR spectroscopy
Alkene and alkyne carbon-carbon bond stretches
Bond Spectral region/cm−1
C=C 1640–1675 (sometimes)C ––– C 1950–2300 (sometimes)
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 5 / 29
Solution-phase IR spectroscopy
Example: IR spectrum of liquid cis-3-hexene
CH3
C C
HH
C=
C s
tretc
halkane CH
C
alkene CH
H2
CH2
CH3
Spectrum source: Spectral Database of Organic Compounds,
http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi, Jan. 20, 2013
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 6 / 29
Solution-phase IR spectroscopy
Example: IR spectrum of liquid trans-3-hexene
C=
C s
tret
ch m
issi
ng
3
C C
H
H
alkane CH Calkene CH H2
CH2
CH3
CH
Spectrum source: Spectral Database of Organic Compounds,
http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi, Jan. 20, 2013
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 7 / 29
Solution-phase IR spectroscopy
The fingerprint region of the spectrum
The region from 900 to 1300 cm−1 is called the fingerprint region ofthe IR spectrum.
In this region, we typically find many peaks arising from variouslow-energy stretching and bending motions of the molecules.
Very difficult to assign peaks in this region but they are very differenteven for closely related compounds
Used for confirmation that a particular (known) compound has beenisolated
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 8 / 29
Solution-phase IR spectroscopy
Example: Fingerprint regions ofcis- and trans-3-hexene compared
−1wavenumber (cm )
trans
1200
Tra
nsm
itta
nce (
%)
cis
1200
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 9 / 29
Theory of IR spectroscopy
Review: Molecular dipole moments
A bond dipole is a slight separation of charge between twonon-identical atoms connected by a bond.
The size of the bond dipole is proportional to the amount of chargeseparation and to the bond length.
The dipole moment of a molecule is the vector sum of the bonddipoles.
A polar molecule has a non-zero dipole moment.
Examples: CO2, H2O
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 10 / 29
Theory of IR spectroscopy
Normal modes
Except in diatomics, molecular vibrations generally involve motions ofseveral atoms, i.e. more than one bond is deformed at a time.
The vibrational modes must conserve overall molecular momentum.
We can choose vibrational modes that are independent motions,called normal modes.
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 11 / 29
Theory of IR spectroscopy
Number of normal modes
A molecule made up of N atoms can move in 3N different ways (onedirection of motion per atom per Cartesian axis).
3 of these motions are associated with the translational motion of themolecule as a whole.
A nonlinear molecule has 3 modes associated with rotation of themolecule as a whole.
The remaining 3N − 6 modes of a nonlinear molecule are the normalmodes of vibration.
A linear molecule only has 2 rotational modes.
The remaining 3N − 5 modes of a linear molecule are the vibrationalnormal modes.
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 12 / 29
Theory of IR spectroscopy
Normal modes of H2O
N = 3 atoms, nonlinear molecule
=⇒ 3 normal modes
O
H HH
O
H H
O
H
Symmetric stretch Asymmetric stretch Bend
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 13 / 29
Theory of IR spectroscopy
Normal modes of CO2
N = 3 atoms, linear molecule
=⇒ 4 normal modes
O C O OCO
Symmetric stretch Asymmetric stretch
O C O
Bend (×2)
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 14 / 29
Theory of IR spectroscopy
Selection rule
A selection rule is a rule that tells us when a particular kind ofspectroscopic event can occur.
In IR absorption spectroscopy, the key selection rule is that the dipolemoment of the molecule has to change during the vibration.
A normal mode that can absorb an IR photon is said to beIR active.
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 15 / 29
Theory of IR spectroscopy
Normal modes of CO2 in IR spectroscopy
Which of these modes are IR active?
O C O OCO
Symmetric stretch Asymmetric stretch
O C O
Bend (×2)
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 16 / 29
Theory of IR spectroscopy
IR spectrum of CO2
2020-01-14, 1(15 PM
Page 1 of 2file:///Users/roussel/Desktop/124-38-9-IR.webarchive
NIST Chemistry WebBook (https://webbook.nist.gov/chemistry)
CARBON DIOXIDEINFRARED SPECTRUM
Wavenumber (cm-1)
Tran
smita
nce
100020003000
0.4
0.8
bend
asymmetricstretch
combinationbands
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 17 / 29
Theory of IR spectroscopy
Normal modes of H2O in IR spectroscopy
Which of these modes are IR active?
O
H HH
O
H H
O
H
Symmetric stretch Asymmetric stretch Bend
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 18 / 29
The greenhouse effect
Application: Earth’s heat balance
Energy from the Sun mostly arrives at the Earth in the form of visiblelight.Note that the atmosphere is essentially transparent at opticalwavelengths.
The Earth reflects some of that energy (esp. snow and ice at poles),but absorbs a lot of it.Averaged over the whole planet, about 30% of the light coming in isjust reflected back to space.
The planet radiates mostly in the infrared (blackbody radiation).
The atmosphere contains many gases that absorb in the infrared, sosome of the radiation from the Earth is absorbed in the atmosphere,but then what happens to the energy captured by the atmosphere?
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 19 / 29
The greenhouse effect
Application: Earth’s heat balanceGreenhouse gases
When a gaseous molecule becomes vibrationally excited by absorbinginfrared radiation, the excess vibrational energy can be converted totranslational kinetic energy during collisions.
Energy is constantly redistributed in collisions and otherenergy-transfer processes.
A gas at temperature T also emits “blackbody” radiation.
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 20 / 29
The greenhouse effect
Application: Earth’s heat balanceBlackbody curves
0 200 400 600 800 1000 1200 1400 1600 1800 2000
CO2 bend
Emission
intensity
/cm-1
T = 320 KT = 288 KT = 220 K
ν∼
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 21 / 29
The greenhouse effect
Application: Earth’s heat balanceGreenhouse gases
N2, O2 and Ar, the major components of the atmosphere, don’tabsorb in the IR. (Why?)
The next two most common components of the atmosphere, waterand carbon dioxide do absorb in the IR.
Gases that absorb in the IR are called greenhouse gases.
The atmospheric water content is set by the balance of evaporationand precipitation, which depends on the atmospheric temperature. Itis a responding variable.
We worry a lot about CO2 because we are adding a lot of it to theatmosphere, which affects energy transfer through the atmosphere.
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 22 / 29
The greenhouse effect
Application: Earth’s heat balancePhotons reabsorbed vs lost to space
At lower altitudes, photons emitted at wavelengths that CO2 canabsorb travel only a short distance (a few meters) before they are infact absorbed by a CO2 molecule.Similar statements could be made about other greenhouse gases intheir respective absorption ranges.
Absorption of IR photons slows the migration of heat through theatmosphere.
Near the top of the atmosphere, where the pressure of CO2 is low,there is a much larger probability that a photon emitted toward spacewill actually escape without being reabsorbed.
Important fact: At those altitudes, the atmosphere is a lot cooler.
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 23 / 29
The greenhouse effect
Application: Earth’s heat balanceCO2 concentration and temperature vs altitude
0
50
100
150
200
250
300
350
400
450
0 5 10 15 20 210
220
230
240
250
260
270
280
290
[CO2]/ppm
T/K
h/km
[CO2]T
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 24 / 29
The greenhouse effect
Application: Earth’s heat balanceEarth emission spectrum (taken over North Africa)
Hanel et al., J. Geophys. Res. 77, 2829 (1972)
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 25 / 29
The greenhouse effect
Application: Earth’s heat balanceInterpretation of Earth’s emission spectrum
Emission from CO2 comes from high in the atmosphere (where it’scool and there isn’t much CO2 to block the outgoing IR photons).
Emission from water comes from lower down (where it’s not quite ascool, since condensation of water prevents it from getting too high inthe atmosphere).
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 26 / 29
The greenhouse effect
Application: Earth’s heat balanceInterpretation of Earth’s emission spectrum
Incoming
visible light
reflected from
surface
"greenhouse" photons
trapped
solar radiation
"no
n−
gre
en
ho
use"
IR p
ho
ton
s
"
photons
"CO"water"
photons
2
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 27 / 29
The greenhouse effect
The wrap-up
Greenhouse gases like CO2 slow the escape of heat from theatmosphere to space.
A simple analogy is that the atmosphere acts like a blanket.
This is not inherently a bad thing. The planet would be a lot colder(average surface temperature of about 255 K, or −18 ◦C) if therewere no greenhouse effect.
Adding greenhouse gases to the atmosphere is analogous to makingthe blanket denser, resulting in a higher temperature under theblanket.
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 28 / 29
The greenhouse effect
Application: Earth’s heat balanceCarbon dioxide
From 1959 to 2019, the CO2 concentration in the atmospheremeasured at the Mauna Loa observatory has risen from an annualaverage value of 316 ppm to 411 ppm, an increase of 30%.
The rate of increase in the CO2 concentration is also rising, fromabout 0.6 ppm y−1 in the early 1960s to about 2.6 ppm y−1 now.
Warming induced by greenhouse gas emissions is a self-reinforcingproblem:
It increases the amount of water vapor in the atmosphere.On average, less of the planet is covered with ice.Melting permafrost releases methane, a very powerful greenhouse gas.. . .
There is no escaping the physics: adding greenhouse gases to theatmosphere heats up the planet.
Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 29 / 29