17) Spectroscopy Flashcards
Define molecular ion M+
the positive ion formed in mass spectrometry when a molecule loses an electron
The mass spectrometer detects the _ of the molecular ion
mass-to-charge ratio (m/z)
Define molecular ion peak (M+ peak)
the clear peak at the highest m/z value on the right-hand side of the mass spectrum, which gives the molecular mass of the compound
Define M+1 peak
very small peak one unit after the M+ peak which exists because 1.1% of carbon is present as the carbon-13 isotope
Define fragmentation
the process in mass spectrometry that causes a positive ion to split into smaller pieces: a positive fragment ion (appears as other peaks on the spectrum) and an uncharged radical (not detected)
What are axis of mass spectra?
relative intensity (%) m/z
Different fragmentation patterns can help to…
identify an organic molecule
What is the effect of covalent bonds absorbing infrared radiation?
bonds bend or stretch more
What does the amount a bond stretches/bends depend upon?
the mass of the atoms in a bond (heavier atoms vibrate more slowly)
the strength of the bond (stronger bonds vibrate faster)
Any particular bond can only absorb radiation that has…?
the same frequency as the natural frequency of the bond
What is wavenumber proportional to?
frequency
What happens to the sun’s visible and IR radiation after it passes through the atmosphere to the Earth’s surface?
most of it is absorbed however, some is re-emitted from the Earth’s surface in the form of longer-wavelength IR radiation which is absorbed by greenhouse gases
Name 3 greenhouse gases
water vapour, carbon dioxide and methane
What happens to greenhouse gases after they have absorbed longer-wavelength IR radiation?
eventually vibrating bonds re-emit this energy as radiation which increases the temperature of the atmosphere close to the Earth’s surface, leading to global warming
Define infrared spectroscopy
an instrumentation method of analysis that identifies bonds from absorption of the infrared radiation of different wavelengths
means of identifying functional groups present
Describe the 3 steps of infrared spectroscopy
1) sample placed in IR spectrometer and a beam of IR radiation (200-400cm-1) is passed through the sample
2) molecule absorbs some IR frequencies and emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample
3) IR spectrometer connected to computer that plots a graph of transmittance against wavenumber
Each peak is observed at a wavenumber, which can be related to…?
a particular bond in the molecule
Define fingerprint region
an area of an infrared spectrum below 1500cm-1 that gives a characteristic pattern for different compounds
Why do all organic compounds produce a characteristic peak between 2850 and 3100cm-1?
due to the presence of C-H bonds
How would you recognise an alcohol from IR spectra?
absorbance peak within the range of 3200-3600cm-1 caused by the O-H bond
How would you recognise an aldehyde/ketone from IR spectra?
absorbance peak within the range of 1630-1820cm-1 caused by the C=O bond
How would you recognise a carboxylic acid from IR spectra?
absorbance peak within the range of 1630-1820cm-1 caused by the C=O bond
AND broad peak at 2500-3330cm-1 caused by the O-H group
GIve 2 applications of infrared spectroscopy
remote sensors to identify and monitor localised pollution
IR-based breathalysers (the more IR radiation absorbed, the higher the signal, the more ethanol in the breath)
In combined techniques, what can we find out by elemental analysis?
percentage composition data -> empirical formula
In combined techniques, what can we find out by mass spectrometry?
M+ peak -> molecular mass
fragment ions -> identify sections of a molecule
In combined techniques, what can we find out by infrared spectroscopy?
absorption peaks -> identify bonds + functional groups present
