Module 4 (chapter 17) - spectroscopy Flashcards
1
Q
mass spectra
A
used to identify the molecular mass of an organic compound to gain further information about its structure
2
Q
what happens when an organic compound is placed in a mass spec
A
- it loses an electron to form a positive ion (the molecular ion M+)
- the mass spec detects the mass to charge ratio (m/z) of the molecular ion which gives the molecular mass of the compound
3
Q
how to find molecular mass from a mass spectrum
A
- the molecular ion peak is located at the highest m.z value on the right hand side of the mass spectrum
- you will usually see a very small peak one unit after the M+ peak referred to as the M+1 peak
- this exists as 1.1% of carbon is present as the carbon 13 isotope
- e.g. propanol has a molecular mass of 60 but a small proportion will have a mass of 61 due to the presence of a carbon 13 atom)
4
Q
fragmentation
A
- some molecular ions break down into smaller fragments in a process called fragmentation
- the other peaks in the mass spec are caused by fragment ions caused by the breakdown of molecular ions
- this process is unique and molecules will fragment in a slightly different way depending on their structure
5
Q
simplest fragmentation process
A
breaks a molecular ion into two species
- a positively charged fragment ion and a radical
- any positive ions form will be detected by the mass spec but the uncharged radical won’t
6
Q
infrared radiation and covalent bonds
A
- atoms in molecules are joined by covalent bonds which possess energy and vibrate naturally about a central point
- the amount of vibrations increases with temperature. the atoms in molecules are therefore in constant motion
- these bonds can absorb infrared radiation which makes them bend and stretch more
- any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond
- the frequency values are very large so chemists use wavenumber whig is proportional to frequency
- the wavelength of of most IR bon vibrations is between 200 and 400cm-1
7
Q
types of vibrations
A
- a stretch is a rhythmic movement along the line between atoms so that the distance between the two atoms so that the distance between the two atomic centres increases and decreases.
- a bend results in a change in bond angle
8
Q
what does the amount of bending and stretching depend on
A
- the mass of an atom in the bond, heavier atoms vibrate more slowly than lighter atoms
- the strength of the bond, stronger bonds vibrate faster than weaker bonds.
9
Q
infrared radiation and atmospheric gases
A
- much of the sun’s visible IR radiation is relatively unaffected by atmospheric gases. this radiation passes through the atmosphere to the earth’s surface where most of it is absorbed.
- yet some is re-emitted from the earth’s surface in the form of longer wavelength IR radiation
- water vapour, carbon dioxide and methane absorb this longer wavelength IR radiation as it has the same frequency as the natural frequency of their bonds
- eventually this is re -emitted as radiation that increases the employment of there atmosphere close to the earth’s surface leading to global warming
10
Q
infrared spectroscopy
A
infrared spectroscopy is used as a means of identifying the functional groups present in organic molecules
-each dip known as a ‘peak’ is observed as a wavenumber that can be related to a particular bond in a molecule
11
Q
how does infrared spectroscopy work
A
- the sample under investigation is placed inside the IR spectrometer
- a beam of IR radiation in the range 200-400cm-1 is passed through the sample
- the molecule absorbs some of the IR frequencies and the emerging beam of radiation is analysed to identify the frequencies that have been absorbed by the sample
- the IR spectrometer is usually connected to a compute that plots a graph of transmittance against wave number
12
Q
fingerprint region
A
- below 1500 cm-1
- there are a number of unique peaks which can be used to identify the particular molecule under investigation
- this can be through computer software of by physically comparing the spectrum to booklets of published spectra.
- difficult to clearly identify peaks in this region
13
Q
where should all organic compounds produce a peak
A
between 2850 and 3100 due to the presence of the C-H bond
14
Q
applications of IR spectroscopy
A
many pollutants can be identified by their IR spectral fingerprints
- e.g. remote sensors analyse the IR spectra of vehicle emissions to detect and measure carbon monoxide, dioxide and hydrocarbons in busy areas indicating localised pollution
- IR based breathalysers pass a beam of IR radiation through the captured breath in the sample chamber and detect the characteristic bonds present in ethanol. the more IR radiation present the higher the reading and so the more ethanol in breath
15
Q
sequence od identification
A
- elemental analysis (use of compositional 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 determine sections of a molecule)
- infrared spectroscopy (use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in a molecule)
