Topic 11: Part 2 - Spectroscopy Flashcards
Define spectroscopy.
The study of the way light (electromagnetic radiation) and matter interact.
What are the three main techniques used in spectroscopy?
1) Nuclear Magnetic Resonance (NMR) spectroscopy
2) Infra-red spectroscopy
3) Mass spectrometry (NON-SPECTROSCOPIC TECHNIQUE)
Give the formula for the index of hydrogen deficiency (IHD).
IHD = 0.5 x [(2C) + 2 - H - X + N]
What is the purpose of mass spectrometry?
To determine the relative atomic mass of the molecule.
Describe mass spectrometry.
A vaporised organic sample is passed into an ionisation chamber of a mass spectrometer and bombarded with a stream of electrons. This produces molecular ions (M+). The molecular ions are then accelerated through magnetic and/or electric fields. Some of the molecular ions break down into fragments. This produces a fragmentation pattern which gives useful information about the structure of the molecule.
How is the presence of bromine detected in mass spectrometry?
In nature, two isotopes of Bromine (⁷⁹Br and ⁸¹Br) exist in almost equal amounts. So two molecular ion peaks of equal height will be produced.
How is the presence of chlorine detected in mass spectrometry?
There will be two molecular ion peaks produced. However, the peak corresponding to ³⁵Cl will be three times larger than that of ³⁷Cl.
What is the purpose of infra-red spectroscopy?
To provide information about the different types of chemical bonds/functional groups present and NOT present within the molecule.
Describe infra-red spectroscopy.
A beam of infra-red light is passed through an organic sample. The molecules absorb some of this energy from the infra-red light. This causes the bonds between the atoms to vibrate (stretch and bend). This information is displayed on a graph.
A bond will only interact with infra-red light if it is polar (difference in electronegativity). For non-polar bonds to absorb energy in the infra-red region it must be attached to asymmetrical fragments on either side. This can induce a dipole movement which allows the bond to absorb infra-red light and vibrate.
What type of nuclei are used in proton nuclear magnetic resonance (NMR) spectroscopy?
Nuclei with either an odd mass number or an odd atomic number as they possess spin. A nucleus without spin cannot be detected by NMR spectroscopy.
Describe proton nuclear magnetic resonance (NMR) spectroscopy.
To be detected by NMR spectroscopy the nuclei must have an odd mass or atomic number. This means they possess spin. The sample is spun in the field of a large electromagnet and a radio-frequency field is applied. An NMR spectrum is the plot of the induced voltage against the sweep of the field.
*All samples are measured against a reference compound – Tetramethylsilane (TMS).
What four factors should you consider when interpretating the H NMR spectra?
1) The number of absorption signals. This tells us the number of different proton environments.
2) The position of the signals (chemical shift). This shows the type of proton environments.
3) The area under the signals (integration). This shows the number of protons in each environment.
4) The splitting pattern of the signals. This shows the number of protons on the adjacent/neighbouring atoms.
With what does splitting occur on the H NMR spectra?
With chemically different H’s. It does not happen with chemically similar H’s or H atoms on OH groups.
Why is tetramethylsilane used as a reference compound in H NMR?
Because it absorbs upfield and away from most of the other protons. It is volatile so it is easily removed from the sample and it is unreactive so will not interfere.