M4 Spectroscopy Flashcards
1
Q
How does IR radiation cause covalent bonds to vibrate more and absorb energy?
A
- Atoms in molecules are joined by covalent bonds. These bonds possess energy and vibrate naturally about a central point, vibrations increasing with temperature.
- The atoms in molecules are therefore in constant motion. The bonds can absorb IR radiation, which makes them bend or stretch more.
- One type of vibration increases/decreases the distance between the two atoms, the other vibration is a bend, changing the bond angle.
- Any bond can only absorb the radiation that has the same frequency as the natural frequency of the bond. (must have a permanent dipole)
2
Q
Describe infrared spectroscopy
A
- Infrared spectroscopy is a technique used to identify compounds based on changes in vibrations of atoms when they absorb IR of certain frequencies.
- A spectrophotometer irradiates the same with IR radiation and then detects the intensity of IR radiation absorbed by the molecule.
- IR energy is only absorbed if a molecule has a permanent dipole that changes as it vibrates (symmetrical molecules are therefore IR inactive)
- The resonance frequency is the specific frequency at which the bonds will vibrate. Rather than displaying frequency, an IR spectrum shows a unit called a wave number.
- Wavenumber is the reciprocal of the wavelength and has units of cm^-1
- Characteristic absorptions can be matched to specific bonds in molecules, enabling chemists to determine the functional groups present.
3
Q
What does the amount that a bond stretches or bends depend on?
A
- Mass of the atoms in the bond (heavier atoms vibrate more slowly than lighter atoms)
- Strength of the bond (stronger bonds vibrate faster than weaker bonds)
4
Q
Describe infrared radiation and atmospheric gases
A
- Much of the Sun’s visible light and IR radiation is unaffected by atmospheric gases. This radiation passes through the atmosphere to the Earth’s surface, where most is absorbed.
- However, some is re-emitted from the Earth’s surface in the form of longer-wavelength IR radiation.
- Water, carbon dioxide and methane absorb this longer-wavelength IR radiation, because it has the same frequency of their bonds.
- Eventually the vibrating bonds in these molecules re-emit this energy as radiation that increases the temperature of the atmosphere close to the Earth’s surface, leading to global warming.
5
Q
What characteristic peak do all organic compounds have?
A
A peak between 2850-3100 cm^-1 from the presence of C-H bonds.
6
Q
Describe the infrared spectrum of an alcohol
A
- Broad absórbanme peak 3200-3690 caused by O—H bond in an alcohol.
- Peak between 1000-1300 caused by the C—O bond.
7
Q
Describe the infrared spectrum of an aldehyde or ketone
A
- The IR spectrum of an aldehyde or ketone has a key absorbance peak within 1630-1820 caused by the C=O.
ketone = C=O
aldehyde = C=O
/
H
8
Q
Describe the infrared spectrum of a carboxylic acid
A
- Key absorbance peak within 1630-1820 caused by the C=O bond.
- Broad peak at 2500-3330 caused by O-H group in carboxylic acid.
- Also a peak at the 1000-1300 representing the C—O bond.
9
Q
What are the applications of infrared spectroscopy?
A
- IR-based breathalysers pass a beam of IR radiation through the captured breath in the sample chambre and detect the IR absorbance of the compounds in the breath.
- The more IR radiation absorbed, the higher the reading, and the more ethanol in the breath.
- Many pollutants can be identified by their IR spectral fingerprints. Remote sensors analyse the IR spectra of vehicle emissions to detect and measure carbon monoxide, carbon dioxide and hydrocarbons in busy town centres or by motorways to monitor localised pollution.
10
Q
Describe mass spectrometry
A
- An organic compound is placed in the mass spectrometer, it looses an electron and forms a positive ion (the molecular ion).
- The mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound.
eg. CH3CH2CH2OH —> CH3CH2CH2OH+ + e-
The molecular ion M+ is the positive ion formed when a molecule looses an electron.
11
Q
How do you find the molecular mass from a mass spectrum?
A
- The molecular ion peak (M+ peak) has to be located.
- The molecular ion peak is the clear peak at the highest m/z value on the right-hand side of the spectrum.
- There is usually a very small peak one unit after the M+ peak. This is referred to as the M + 1 peak. The M + 1 peak exists because 1.1% of carbon is present as the carbon-13 isotope.
12
Q
What is fragmentation?
A
- In the mass spectrometer some molecular ions break down into smaller pieces known as fragments in a process called fragmentation.
- The other peaks in a mass spectrum are caused by fragment ions, formed from the breakdown of the molecular ion.
- The simplest fragmentation breaks a molecular ion into two species - a positively charged fragment ion and a radical.
- Any positive ions formed will be detected by the mass spectrometer, but the uncharged radicals are not detected.
13
Q
How can fragmentation peaks be used to identify an organic molecule?
A
- The mass spectrum of of each compound is unique, as molecules will all fragment in slightly different ways depending on their structures.
- Mass spectra can identify molecules, even if two molecules may have the same molecular mass and molecular ion peak, the fragment ions in the spectrum will be different.
14
Q
What is the typical sequence for identification?
A
- Elemental analysis - use of percentage composition data to determine empirical formula
- Mass spectrometry - use of molecular ion peak from a mass spectrum to determine the molecular mass; use of fragment ions to identify sections of a molecule.
- Infrared spectroscopy - use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule.
