Mass Spectra and IR - Topic 7 Flashcards
What is the molecular ion peak in a mass spectrum?
The molecular ion peak is the peak with the largest m/z ratio and corresponds to the complete molecule. It is equal to the Mr of the molecule and is also known as the parent ion.
What happens when molecules are passed through a mass spectrometer?
The mass spectrometer detects the molecular ion and its fragments formed by fragmentation.
what is fragmentation
when a molecule passes through a mass spectrum and its covalent bonds break forming its ion and a free radical
- several peaks in mass spec occur due to fragmentation
What does the peak with the highest m/z value represent in a mass spectrum?
The peak with the highest m/z value represents the molecular ion, which is the unfragmented molecule. The mass/charge ratio is equal to the Mr of the molecule.
What causes covalent bonds to vibrate in IR spectroscopy?
Absorption of infrared (IR) radiation at characteristic frequencies causes covalent bonds to vibrate.
What does the IR region above 1500 cm⁻¹ used for
functional group identification.
What does the IR region below 1500 cm⁻¹ indicate?
t is known as the fingerprint region, which is complex and unique to each molecule.
Why is the fingerprint region (below 1500 cm⁻¹) hard to interpret?
It contains many overlapping signals, making functional group identification difficult.
Why do some molecules absorb infrared (IR) radiation and others do not?
- Molecules which change their polarity as they vibrate can absorb infrared radiation. E.g. C-H, C=O, O-H
- Molecules such as H2, O2 and N2 cannot change their polarity as they vibrate so can absorb infrared radiation and don’t register on an infra red spectrum
what is the link between IR absorption and greenhouse gases
Molecules like H₂O, CO₂, CH₄, and NO absorb IR radiation and act as greenhouse gases. Molecules like O₂ and N₂ do not absorb IR, so they are not greenhouse gases.
what does the frequency of infrared radiation absorbed by a covalent bond depend on
- the atoms either side of the bond
- the position of the bond in the molecule
