Infra red radiation and molecules When a molecule is exposed to electromagnetic radiation covalent bonds will absorb energy in the infrared region of the spectrum. Since each particular type of bond has its own natural vibrational frequency we can use these analytically. The amount of vibration depends on: the bond strength the bond length the mass of each atom involved in the bond.
For IR radiation to be absorbed the vibrations must cause a change in the dipole moment of the molecule. Symmetrical bonds in e.g. O 2 and N 2 do not absorb radiation and are not IR active. Since most molecules have several different types of bond molecules will cause many absorptions. The same bond may have many different vibrational modes.
Recording a Spectrum An infra red beam is split into 2 and passes through a sample and a reference cell containing solvent only. The solvent absorptions are thus removed from the sample and only the absorptions due to the test molecule are seen. N.B. Frequency is measured using WAVENUMBER which is actually 1/wavelength with the units cm -1. This makes the numbers in the spectrum easy to handle. Absorptions are diagnostic of bonds present.
Even though energy is absorbed and the spectrum consists of several troughs these are still described as peaks (since they represent increasing amounts of energy absorbed). Most organic compounds produce a peak at around 3000 cm -1 due to absorption by C-H bonds which is therefore NOT used diagnostically. However (see table p.168) O-H bonds and C=O bonds can be used diagnostically for alcohols, aldehydes, ketones and carboxylic acids. Very broad absorptions are due to the large amounts of energy absorbed by very polar bonds like O-H.
When interpreting simple spectra: Concentrate on major absorptions only. Remember that absence of an absorption is diagnostic and popular in exam questions. Do not expect to make absolute identifications from IR spectra only.
IR Analysis and Breath Testing The legal limit for blood alcohol in the UK is 80mg of ethanol per 100cm 3 of blood. Initial breath testing using a breathalyser involves the use of potassium dichromate absorbed onto crystals. Exhaled breath passes through the crystals and ethanol is oxidised, causing the crystals to go green as the dichromate is reduced to Cr 3+ ions. A positive breath test is a cue for an IR test back at the police station. A broad absorption at 3340 cm -1 and a sharp absorption at 2950 cm -1 can be used by the computer to calculate the % of ethanol present.
Simple Mass Spectrometer Early developments in mass spectrometry listed on p. 170 in the text book are NOT required by the syllabus, but make interesting reading, nor are the workings of the machines. The key is that POSITIVE ions are created and accelerated then deflected (by a magnet) round a bend which serves to focus the beam of ions since heavy and light ones will hit the sides. To ensure that all ions are detected the magnetic field is varied. The ions are detected and recorded according to their mass to charge ratio (written m/z). For AS only ions with a single + charge are considered so m/z = A r The use of mass spectrometers to determine relative isotopic masses are required.
Using the Mass Spectrum of Neon to work out RAM Neon shows 3 isotopes with m/z (mass number) 20,21 and 22. Peak heights show relative abundances as: 90.5, 0.3 and 9.2 respectively. To calculate the RAM remember: RAM = (20 x 90.5)+(21 x 0.3)+(22x9.2) 100 = =20.19 (or 20.2 to 3sf) Use the mass spectrum for magnesium on the following slide to calculate the RAM for magnesium.
Syllabus requires that you know that mass spec is used as a method for identifying elements e.g. in the Mars space probe and in monitoring levels of environmental pollution such as lead. Also mentioned: pesticides in the food chain. food-contaminants.html?sec=health&spon=&pagewanted=all
Mass Spectrometry and Organic Molecules The value of mass spectroscopy with isotopes of elements is limited. However it is widely used as a routine analytical tool in organic chemistry because the beam of high energy electrons (or whatever the ionising source is) disrupts the covalent bonds within the molecule and fragments it. There is usually a fragment made which represents the MOLECULAR ION where a single electron has been knocked off the molecule leaving it positively charged with the same mass since the mass of an electron is negligible. Energy can cause the molecular ion to vibrate strongly enough to weaken the bonds and cause fragmentation.
Using Fragmentation Patterns to Confirm Molecular Structures The mass spectrum of ethanol has many peaks due to the fragmentation of the molecular ion. These can often be explained from a knowledge of the structure of the molecule under test or used diagnostically to identify unknown or new molecules. Examples include: m/z 15 CH 3 + fragment m/z 17 OH + fragment m/z 29 C 2 H 5 + fragment m/z 31 CH 2 OH + fragment m/z 46 M + C 2 H 5 OH + the molecular ion. Can you suggest fragments for peaks at m/z 45, 43, 27?
Using Fragmentation Patterns to Confirm Molecular Structures(2) Look at the mass spectra for Pentane and 2-Methyl pentane. Can you identify the key fragments which unambiguously identify which of thesespectra is which? Identify the M + for A and B. Any ideas for possible identification?
Using Fragmentation Patterns to Confirm Molecular Structures(3) Identify the fragments responsible for the peaks A,B and C for pent-1-ene as follows. Identify the molecular ion peak in the spectrum of the unknown alkane. It is a 2-methyl…… branched chain. Draw structures for the fragment ions represented by the peaks with m/z values of 43, 57 and 71.
Combined Techniques It is worth remembering that no one single analytical technique is ever used alone. Mass Spectroscopy is invariably carried out in sequence with Gas Chromatography. IR and NMR are also used with the other 2. The job of a modern analytical chemist is not a lot different to the antics of the various CSI scientists on the TV. Any new substance is routinely fingerprinted and records of the various spectra stored on databases to enable swift analysis when required.