Chemistry 2000 Slide Set 6: Vibrational spectroscopy of polyatomic - - PowerPoint PPT Presentation

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 the identification of unknown compounds. Certain bonds in organic molecules are associated with characteristic IR 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

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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 H H

C=C stretch alkane 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 stretch missing

3

C C H H

alkane CH

C

alkene CH

H 2 CH 2 CH 3 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 of the IR spectrum. In this region, we typically find many peaks arising from various low-energy stretching and bending motions of the molecules. Very difficult to assign peaks in this region but they are very different even for closely related compounds Used for confirmation that a particular (known) compound has been isolated

Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 8 / 29

Solution-phase IR spectroscopy

Example: Fingerprint regions of cis- and trans-3-hexene compared

−1

wavenumber (cm ) trans

1200

Transmittance (%) 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 two non-identical atoms connected by a bond. The size of the bond dipole is proportional to the amount of charge separation and to the bond length. The dipole moment of a molecule is the vector sum of the bond dipoles. 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 of several 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 (one direction of motion per atom per Cartesian axis). 3 of these motions are associated with the translational motion of the molecule as a whole. A nonlinear molecule has 3 modes associated with rotation of the molecule as a whole. The remaining 3N − 6 modes of a nonlinear molecule are the normal modes of vibration. A linear molecule only has 2 rotational modes. The remaining 3N − 5 modes of a linear molecule are the vibrational normal 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 H H O H H O H

Symmetric stretch Asymmetric stretch Bend

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Theory of IR spectroscopy

Normal modes of CO2

N = 3 atoms, linear molecule = ⇒ 4 normal modes

O C O O C O

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 of spectroscopic event can occur. In IR absorption spectroscopy, the key selection rule is that the dipole moment of the molecule has to change during the vibration. A normal mode that can absorb an IR photon is said to be IR 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 O C O

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

NIST Chemistry WebBook (https://webbook.nist.gov/chemistry) CARBON DIOXIDE INFRARED SPECTRUM Wavenumber (cm-1) Transmitance 1000 2000 3000 0.4 0.8 bend asymmetric stretch combination bands 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 H H 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 visible light. Note that the atmosphere is essentially transparent at optical wavelengths. 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 is just reflected back to space. The planet radiates mostly in the infrared (blackbody radiation). The atmosphere contains many gases that absorb in the infrared, so some 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 balance

Greenhouse gases

When a gaseous molecule becomes vibrationally excited by absorbing infrared radiation, the excess vibrational energy can be converted to translational kinetic energy during collisions. Energy is constantly redistributed in collisions and other energy-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 balance

Blackbody curves

200 400 600 800 1000 1200 1400 1600 1800 2000 CO2 bend Emission intensity /cm-1 T = 320 K T = 288 K T = 220 K

ν

∼

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The greenhouse effect

Application: Earth’s heat balance

Greenhouse gases

N2, O2 and Ar, the major components of the atmosphere, don’t absorb in the IR. (Why?) The next two most common components of the atmosphere, water and 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 evaporation and precipitation, which depends on the atmospheric temperature. It is a responding variable. We worry a lot about CO2 because we are adding a lot of it to the atmosphere, 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 balance

Photons reabsorbed vs lost to space

At lower altitudes, photons emitted at wavelengths that CO2 can absorb travel only a short distance (a few meters) before they are in fact absorbed by a CO2 molecule. Similar statements could be made about other greenhouse gases in their respective absorption ranges. Absorption of IR photons slows the migration of heat through the atmosphere. Near the top of the atmosphere, where the pressure of CO2 is low, there is a much larger probability that a photon emitted toward space will 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 balance

CO2 concentration and temperature vs altitude

50 100 150 200 250 300 350 400 450 5 10 15 20 210 220 230 240 250 260 270 280 290 [CO2]/ppm T/K h/km [CO2] T

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The greenhouse effect

Application: Earth’s heat balance

Earth 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 balance

Interpretation of Earth’s emission spectrum

Emission from CO2 comes from high in the atmosphere (where it’s cool and there isn’t much CO2 to block the outgoing IR photons). Emission from water comes from lower down (where it’s not quite as cool, since condensation of water prevents it from getting too high in the atmosphere).

Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 26 / 29

The greenhouse effect

Application: Earth’s heat balance

Interpretation of Earth’s emission spectrum

Incoming visible light reflected from surface "greenhouse" photons trapped solar radiation " n

• n

− g r e e n h

• u

s e " I R p h

• t
• n

s " photons "CO "water" photons

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The greenhouse effect

The wrap-up

Greenhouse gases like CO2 slow the escape of heat from the atmosphere 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 there were no greenhouse effect. Adding greenhouse gases to the atmosphere is analogous to making the blanket denser, resulting in a higher temperature under the blanket.

Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 28 / 29

The greenhouse effect

Application: Earth’s heat balance

Carbon dioxide

From 1959 to 2019, the CO2 concentration in the atmosphere measured at the Mauna Loa observatory has risen from an annual average value of 316 ppm to 411 ppm, an increase of 30%. The rate of increase in the CO2 concentration is also rising, from about 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-reinforcing problem:

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 the atmosphere heats up the planet.

Marc R. Roussel Spectroscopy of polyatomic molecules January 14, 2020 29 / 29