17.2 Infrared spectroscopy Flashcards

1
Q

What happens when bonds absorb infrared radiation

A

Makes them bend or stretch more

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2
Q

What is a stretch

A

A rhythmic movement along the line between the atoms so that the distance between the two atomic centres increases and decreases

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3
Q

What is a bend

A

Results in change in bond angle

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4
Q

What does the amount that a bond stretches or bends depend on

A

The mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms
The strength of the bond - stronger bonds vibrate faster than weaker bonds.

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5
Q

What is the scale called wavenumber

A

Proportional to frequency

Frequency values of a bond

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6
Q

The vibrations of most bonds are observed in the IR wavenumber range of…

A

…200cm^-1 to 4000cm^-1

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7
Q

What is some Infrared radiation re-emitted from the Earth as

A

Longer-wavelength Infrared radiation

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8
Q

The greenhouse effect and longer wavelength IR radiation

A

Greenhouse gases absorb this as it has the same frequency as the natural frequency of their bonds
Vibrating bonds in these molecules re-emit this energy as radiation which increases the temperature of the atmosphere close to the Earth’s surface

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9
Q

What are the three most abundant greenhouse hases

A

Water vapour
Carbon dioxide
Methane

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10
Q

Infrares spectroscopy to identify functional groups in organic molecules

A
  1. Sample under investigation placed inside IR spectrometer
  2. Beam of IR in range 200-4000cm^-1 passed through sample
  3. Molecule absorbs some IR frequencies and emerging beam of radiation analysed to identify frequencies that have been absorbed by sample
  4. IR spectrometer usually connected to computer that plots graph of transmittance against wavenumber
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11
Q

What is the fingerprint region of a spectrum

A

Contains unique peaks which can be used to identify particular molecule under investigation
Using computer software or comparing to booklets of published spectra

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12
Q

WHAT DO YOU NEED TO KNOW HOW TO IDENTIFY (FUNCTIONAL GROUPS) FROM AN IR SPECTRUM IN THE EXAM

A

O-H groups in alcohols
C=O group in aldehydes, ketones and carboxylic acids
COOH group in carboxylic acids

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13
Q

What region do all organic compounds produce a characteristic peak at from the presence of C-H bonds

A

2850 to 3100cm^-1

OFTEN CONFUSED WITH O-H peak in alcohols

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14
Q

What is the range for the absorbance peak in the IR spectrum of an alcohol caused by the O-H bond

A

3200-3600cm^-1

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15
Q

What is the range for the absorbance peak in the IR spectrum of an alcohol caused by the C-O bond

A

1000-1300cm^-1

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16
Q

What is the range for the absorbance peak in the IR spectrum of an aldehyde or ketone caused by the C=O bond

A

1630-1820cm^-1

Usually around 1700cm^-1

17
Q

What is the range for the absorbance peak in the IR spectrum of a carboxylic acid caused by the C=O bond

A

1630-1820cm^-1

Usually around 1700cm^-1

18
Q

What is the range for the broad peak in the IR spectrum of a carboxylic acid caused by the O-H group

A

2500-3330cm^-1

19
Q

Why isn’t the C-O peak always reliable in the IR spectrum of a carboxylic acid

A

Because it is in the fingerprint region

20
Q

How can pollutants be identified

A

By their IR spectral fingerprints
Remote sensors analyse IR spectra of vehicle emissions to detect and measure carbon monoxide, carbon dioxide and hydrocarbons

21
Q

How do IR-based breathalysers work

A

Pass a beam of IR radiation through captured breath in sample chamber and detect IR absorbance of compounds in breath
Bonds present in ethanol are detected
More IR absorbed, higher reading, more ethanol in breath

22
Q

What does a typical sequence for identification include

A

Elemental analysis
Mass spectrometry
Infrared spectroscopy
Once you have empirical formula and molecular mass, can determine molecular formula

23
Q

What is elemental analysis

A

Use of percentage composition data to determine empirical formula

24
Q

What is mass spectrometry

A

Use of the molecular ion peak from mass spectrum to determine molecular mass; use of fragment ions to identify sections of molecule

25
Q

What is infrared spectroscopy

A

Use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule