Mass Spectrometry and Infrared Spectrometry Flashcards
All molecules absorb….
infrared radiation
What does the amount of vibration of the bond depend on
- bond strength
- bond length
- the mass of each atom involved in the bond
What do most bonds vibrate at
they vibrate at a frequency between 300-4000 which is the infrared part of the electromagnetic spectrum
What does absorbed infrared radiation do to the bonds
it makes the covalent bonds vibrate more with either a stretching or bending motion
What are the ways in which the bond vibrates
they either bend or stretch
- draw this
How can the absorbed energies of the bond be displayed
it is displayed on an infrared spectrum
What does the infrared spectrum help us determine
the infrared spectrum helps us determine details about a compounds chemical structure, it indicates the presence of functional groups in the compound under investigation
How does the infrared spectrometer work
A beam of infrared radiation is passed through a sample of the material under investigation, the beam contains a full range of frequencies in the infrared region
the molecules absorb some of the frequencies and the emerging beam is analysed to plot a graph of transmittance against frequency - this is the infrared spectrum of the molecule
- graph is plotted by a computer that analyses the radiation which passes through completely uninterrupted, and compare this with the radiation that has been passed through the material being investigated
How is the frequency of the infrared radiation measured
using wave numbers with the units of cm-1
Draw an infrared spectrum graph
DRAW IT
Describe the infrared spectrum graph
- absorption is shown by peaks, this is where the radiation has been absorbed
- these parts indicate the bonds present in the substance
Why do most organic compounds produce a peak at about 3000cm-1
due to absorption by C-H bonds - do not get this confused with O-H peak
How do you identify aldehydes and ketones
- peak at 1680-1750cm-1 this represents a C=O bond in both aldehydes if this peak is present the substance contains a C=O bond
- no O-H peak which makes it different from a carboxylic acid
How do you identify alcohols
peak at 3200-3500 represents an O-H group, if this peak is present then the substance analysed contains a O-H bond as part of the alcohol group
How do you identify carboxylic acids
- has to have a broad peak at 2500-3300 cm-1 which indicates the presence of the O-H group is a carboxylic acid
- strong sharp peak at 1680-1750 cm-1, this represents the C=O group in a carboxylic acid
What is a molecular ion
this is the positive ion formed in mass spectrometry when a molecule loses an electron
What happens when an organic compound is driven through a mass spectrometer
- it is vapourised and driven through the mass spectrometer - some molecules lose an electron each and are ionised, the resulting ion is called the molecular ion
- sample then undergoes fragmentation
- acceleration of ions
- deflection of ions
Describe concepts about molecular ions
- the mass of the lost electron is negligible
- the molecular mass ion has a molecular mass equal to the relative molecular mass of the compound
- molecular ions can be detected an analysed
Describe the breaking of molecular bonds by fragmentation
excess energy from the ionisation process can be transferred to the molecular ion making it vibrate
- this causes the bonds to weaken and the molecular ion can split into pieces by fragmentation
- this means that the original molecules break up into positive fragment ions and other neutral species
WRITE A FRAGMENTATION REACTION
PG 220
What is detected in a mass spectrometer
- molecular ions
- fragment ions
What produces the peak with the highest mass change (M/Z) value in the mass spectrum
the molecular ion
- other peaks are as a result of fragmentation
What does high resolution mass spectrometry do
- it produces a spectrum with extreme precise values for the molecular ion, making accurate molecular mass determination easy
What is mass spectrometry used for
- it is used to determine the structure of an unknown compound and give it its precise identity
