Chapter 17 - Spectroscopy Flashcards
(36 cards)
When an organic compound is placed inside the mass spectrometer, what happens?
It loses an electron and forms a positive ion, which is called the molecular ion
How to find the molecular ion?
It will be the largest peak furthest to the right
What is the M+1 peak?
A very small peak after the molecular ion on the spectrometer
Why is the M+1 peak formed?
Isotopes - normally due to the carbon-13 isotope
What is fragmentation?
The process in which the molecular ion is broken down into smaller fragments
What are the smaller peaks on a spectrometer caused by?
Fragments
What two things are produced when a molecular ion is fragmented?
A positively charged ion and a free radical
Out of the two things produced by fragmentation, which will be detected by the spectrometer
The positively charged ion
What does the unit m/z show?
It is a ration of mass to charge
How is it possible to tell two compounds with the same Mr apart using mass spectrometry?
They have different structures so won’t produce the same fragments, which will be seen on the spectrum
How to combine spectrometry techniques to work out a compound?
1) Use the composition to work out the molecular mass of the compound
2) Work out what functional groups are in the compound from its infrared spectrum
3) Use the mass spectrum to work out the structure of the molecule
What is the m/z value of CH3+ ion?
15
What is the m/z value of a C2H5+ ion?
29
What is the m/z value of a C3H7+ ion?
43
What is the m/z value of a C4H9+ ion?
57
How to calculate the number of carbon atoms in a compound using the spectrum?
(Height of M+1 peak/height of M peak) x 100
What are the two ways in which a bond can move?
Bend or stretch
What happens when a bond absorbs infrared radiation?
Making it bend or stretch
What happens when a stretch occurs?
Rhythmic movement along the line between the atoms so that the distance between the two atomic centres increase and decrease
What does the amount that a bond stretches or bends depend upon?
- The mass of the atom in the bond (heavier atoms vibrate more slowly than light atoms)
- The strength of the bond (Stronger bonds vibrate faster than weaker bonds)
Will stronger bonds vibrate more or less?
More
When will a bond absorb infrared radiation?
Radiation can be absorbed when it has the same frequency as natural frequency of the bond
What can infrared spectroscopy be used for?
Identifies the functional groups present in organic molecules
The process of infrared spectroscopy
1) The sample under investigation is placed inside in IR spectrometer
2) A beam of IR Radiation in the range 200–4,000 cm^-1is passed through the sample
3) molecules absorb some of the IR frequencies, the emerging beam radiation is analysed to identify the frequency that Have been absorbed by the sample
4) the IR spectrometer is usually connected to a computer that plots a graph of transmittance against the wavenumber
