CHAPTER 17 - SPECTROSCOPY Flashcards
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What happens when an organic compound is placed in a mass spectrometer
Loses electron and forms a positive ion - the molecular ion
(pg 252)
What does a mass spectrometer do?
Detect the mass-to-charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound
How do you find the molecular ion peak (M+ peak) on a mass spectrum
The clearest peak at the highest m/z value on the right hand side
What is the M+1 peak and why does it happen
A small unit after the M+ peak
Due to 1.1% of carbon present as Carbon-13 isotope
What is fragmentation
The breakdown of molecular ions down into smaller pieces known a fragments or fragment ions
What are the other peaks on the mass spectrum caused by, not M+ or M+1 peaks
Fragment ions
What does a simple fragmentation of a molecular ion form
Forms a fragment ion (charged and detected by spectrum)
Radical (uncharged and not detected by spectrum)
Explain why there sometimes is a peak in the spectrum after the molecular ion peak
The M+1 peak arises because there is a small percentage of Carbon-13 in the sample and this adds one to the molecular mass of the molecular ion
What are the two types of vibration
Stretch and Bend
What is a stretch
A rhythmic movement along the line between atoms so that the distance between the atomic centres increase and decrease
What is a bend
Change in bond angle
What does the amount of bond stretch or bend depend on
The mass of the atom - Heavier atoms vibrate slower than lighter atoms
The strength of the bond - stronger bonds vibrate faster than weaker bonds
What impact does Infra-red (IR) have upon bonds
Absorbed by bonds
makes them bend or stretch more
What is wavenumber
The scale used in IR spectroscopy, used for convenient scale
How are greenhouse gases affected by Infra-red radiation
Absorb longer-wavelength IR radiation, as it has the same frequency as the natural frequency of their bonds
Vibrating bonds in these molecules re-emit this energy as radiation that increases the temperature of atmosphere close to earth’s surface, leading to global warming
What is IR spectroscopy used for
Identification of functional groups present in organic molecules
What is the method for Infra-red Spectroscopy
Sample under investigation is placed inside an IR spectrometer
A beam if IR radiation in the range 200-4000 cm-1 is passed through the sample
The molecule absorbs some of the IR frequencies and the emerging bean of radiation is analysed to identify the frequencies absorbed by the sample
The IR spectrometer is usually connected to a computer that plots a graph of transmittance against wavenumber
(pg 258, 259 and 260)
What is the fingerprint region
Below 1500cm-1, which contains unique peaks to identify the particular molecule under investigation
What uses are there for IR spectroscopy
Identification of Pollutants
Remote sensors for vehicle emissions
Breathalysers
What are the 3 techniques used for an identification sequence and what do each do
Elemental analysis -
use of percentage composition data to determine the empirical formula
Mass Spectrometry -
Use of the 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 on a molecule
