Chapter 20- Analytical techniques Flashcards
What can analytical techniques be used for?
Structure determination, mass determination
What is the index of hydrogen deficiency?
It’s the degree of unsaturation. It determines how many rings, double bonds, and triple bonds are present in the compound. It does not give the exact number of rings or double or triple bonds though.
Index of hydrogen deficiency=
(H(reference) - H(molecule))/2
What is the reference compound for the index of hydrogen deficiency?
A alkane with the same number of carbon atoms as the other molecule. The molecular formula is CnH(2n+2)
What are the key stages of mass spectrometry?
Stage 1: Ionisation. The atom or molecule is ionised by knocking one or more electrons off to give a positive ion. Most mass spectrometers work with positive ions.
Stage 2: Acceleration. The ions are accelerated so that they all have the same kinetic energy.
Stage 3: Deflection. The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.
Stage 4: Detection. The beam of ions passing through the machine is detected electrically.
How is the relative atomic mass worked out, if the mass spectrum is given in relative abundance?
Suppose you had 123 typical atoms of boron. 23 of these are 10B and 100 are 11B. The total mass of these would be (23 x 10) + (100 x 11) = 1330. The average mass of these 123 atoms would be 1330 / 123 = 10.8 (to 3 significant figures).
How is the relative atomic mass worked out, if the mass spectrum is given in percentage abundance?
Suppose you had 100 atoms of zirconium. 51.5 of these are 90 Zr, 11.2 are 91 Zr, 17.1 are 92 Zr, 17.4 are 94 Zr, and 2.8 are 96 Zr. The total mass of these 100 typical atoms would be: (51.5 x 90) + (11.2 x 91) + (17.1 x 92) + (17.4 x 94) + (2.8 x 96) = 9131.8. The average mass of these 100 atoms would be 9131.8 / 100 = 91.3 (to 3 significant figures).
What is the molecular ion?
It’s the ion of the molecule being analysed by mass spec. It is also a radical cation because it’s got one unpaired electron and has a positive charge.
When molecules interact with infrared radiation, what happens to the covalent bonds?
They get excited to higher vibrational energy levels
What does m/z stand for?
Mass/charge
It’s the mass to charge ratio of a molecule
What is infrared spectroscopy?
IR spectroscopy is the study of absorption of IR radiation by molecules, by measuring the frequencies of IR radiation absorbed by a substance
What is the radiation in the vibrational IR region measured in?
It’s measured in reciprocal cm (cm^-1) and is referred to as its wavenumber
What does a bond have to be in order to be infrared active?
The bond has to be polar; the more polar the bond, the stronger its absorption of IR radiation
Describe the theory behind IR spectroscopy
Molecules aren’t rigid – the bonds within them can move (vibrate). If these vibrations cause a change in the dipole moment of the molecule, it is said to be IR active. This can be measured in IR spectroscopy
What’s used as the reference material in NMR spectroscopy?
Tetramethylsilane (TMS)
What signal is assigned to TMS in both H and C NMR?
𝛿=0
What does NMR stand for?
Nuclear Magnetic Resonance
What form of electromagnetic radiation is used in NMR spectroscopy?
Radio waves are used in NMR spectroscopy
How many distinct carbon signals are expected in the 13C-NMR spectrum of 3-methylpentan-3-ol?
Even though there’s 7 carbons, there’s some symmetry in the molecule. Therefore, there should be 4 signals in the spectrum
Describe the theory behind NMR spectroscopy
It’s based on the fact that some nuclei have spin. These nuclei either have an odd mass number or an odd atomic number. Because the external magnetic field is in part generated by the movement of electrons in and around atoms close to a specific nucleus, the exact local field experienced by that nucleus will vary depending on where it is in a molecule and what arrangement of neighbouring atoms is present.
Why is NMR useful?
The ability to detect and measure signals from such different situations makes NMR an extremely powerful diagnostic tool for investigating the structure of molecules.
Why is mass spectrometry useful?
It’s used in the determination of relative atomic masses (Ar), relative molecular masses (Mr), and molecular structure
What are the two types of mass spectrometry?
Low resolution- gives M/Z in whole numbers, so you can’t differentiate between molecules with the same mass to 1 d.p.
High resolution- normally measured to 4 d.p., giving better precision to measurements
