Topic 21: Measurement and Analysis

Question 1

Fundamental equation of electromagnetic radiation

Answer 1

E= hc/λ
Where: E= energy of radiation, c= velocity, h= planks constant, v= frequency, λ= wavelength

Question 2

Nuclear Magnetic Resonance (NMR) spectroscopy radiation type:

Answer 2

Radio waves (10^-3 m), We can see: how neighboring atoms of certain nuclei in a molecule are connected together, as well as how many atoms of these type are present in different locations in the molecule

Question 3

Infra-red spectroscopy radiation type:

Answer 3

Infra-red (10^-5 m), We can see: The functional groups which are present in a molecule

Question 4

UV-visible spectroscopy radiation type:

Answer 4

Ultra-violet (10^-8 m), We can see: Conjugated systems (ie alternating single and double bonds) in organic molecules as well as the metal-ligand interactions in transition metal complexes

Question 5

X-ray crystallography radiation type:

Answer 5

X-rays (10^-10 m), We can see: how all the atoms in a molecule are connected in a three-dimensional arrangement

Question 6

Mass spectrometry radiation type:

Answer 6

Non-spectroscopic technique, We can see: The mass to charge ratio of the molecular ion (ie the molecular weight) and the fragmentation pattern which may be related to the structure of the molecular ion.

Question 7

What is NMR spectroscopy

Answer 7

Radio waves can cause nuclei in some atoms to change magnetic orientation

Question 8

Structure determination from spectroscopic data strategy

Answer 8

Empirical formula (simplest whole number ratio of the atoms in a molecule) –> Mass spectrum (molecular ion/molar mass-molecular formula) –> Index of Hydrogen Deficiency (IHD = 0.5(2C+2-H-X+N) –> Infrared (identifies functional groups) –> 1H NMR (identifies the number of H’s bonded to other atoms)

Question 9

Index of hydrogen deficiency: IHD = 0.5(2C+2-H-X+N)
Where X= Halogens
N= Nitrogen
H= Hydrogen
C= Carbon
(O and S atoms do not contribute to the IHD)

Answer 9

Determination of the number of multiple bonds/rings in a molecule. It can be seen that: for every two hydrogen atoms fewer than in the alkane with the same number of carbon atoms, there is one double bond or ring present (double bond equivalent).
The number of double bond equivalents is sometimes called the degree of unsaturation of the IHD)

Question 10

Mass spectrometry & how it works

Answer 10

Does NOT use electromagnetic radiation, compounds are ionised, then broken into fragments
The ion of heaviest mass (molecular ion) corresponds to the molar mass.
In a mass spectrometer, ions are bombarded with a high energy electron which can remove an electron from the molecules, this leaves the molecules with a positive charge, these are then accelerated through magnetic and/or electric fields which, by deflecting the relative to their mass, give information that allows their molecular mass to be calculated.

Question 11

Reading spectra:

Answer 11

• The x-axis is the mass/charge ratio (m/z), where z represents the charge of the ion
• The y-axis is the intensity of the peak provided, the taller the peak is then the more common the corresponding ion.
• The molecular ion will typically be the ion with the highest m/z ratio
• Other peaks present are due to fragmentation of the molecular ion, fragmentation often occurs by the breaking of a carbon-carbon single bond