4.12 Structure Determination Flashcards
What does the mass spectrometer do?
The mass spectrometer analyses gaseous samples and compounds, providing information about relative molecular mass
What form of mass spec can be used to determine a molecular formula?
High-resolution mass spectroscopy
What are the four stages of mass spec?
Ionisation, acceleration, deflection, and detection
How is the positive ion formed in mass spec?
A high-energy electron gun knocks off an electron to produce the cation
How is the cation accelerated?
Using an electric field
How is the cation deflected?
Using a variable magnetic field
How is the cation detected?
Using electric plates
What is the base peak on the mass spectra?
The base peak is the tallest peak, the one with the greatest abundance
What is the molecular ion peak?
The molecular ion peak is on the far right, it is caused by the produced cation
Why is the cation a free radical?
Because it has a lone pair of electrons
What is the equation for mass spec?
M(g) + e- ==> M+(g) + 2e-
What is the equation for the splitting cation?
M+* ==> X+ + Y*
What is infrared spectroscopy used for?
It is used to provide information about the types of bonds present in a molecule
How is IR spectroscopy possible?
different bonds absorb different frequencies of IR radiation
What is the fingerprint region?
It is the part of the spectrum between 1500cm and 400cm, it’s pattern is unique to each compound
what does n.m.r. spectroscopy provide?
It provides information about the position of certain atoms in a molecule
What does n.m.r. spec rely on?
It relies on a property of atomic nuclei called nuclear spin
What property must an atom have to have nuclear spin?
The nuclei of the atom must have an odd mass number
What can be said about a nucleus in the spin state opposed to the magnetic field?
A nucleus in the spin state opposed to the field is at a higher energy than a nucleus aligned with it
What state must a sample be to produce an n.m.r. spectrum?
The sample must be a liquid or solvent
What properties must the solvent be for the sample to be dissolved in it?
It must be proton free so that it does not affect the spectrum, EG CCl4
What is the first stage of n.m.r. spectroscopy?
The sample is put in a cylindrical tube and lowered into the spectrometer
What happens after the sample is in the spectrometer?
It is spun on its axis between the poles of a very powerful electromagnet. The applied field is varied
Why is the n.m.r. spectrum produced?
Each atom in the sample absorbs a different frequency of electromagnetic radiation
Why is a reference sample added to the sample?
It is difficult to measure the precise resonance frequency of H or C so the difference between the resonance frequencies of the sample and the reference are measured
What is the reference sample used?
TMS - Si(CH4)3
Why is chemical shift calculated?
Chemical shift is calculated to allow results from different spectrometers to be compared
What does chemical shift values between molecules depend on?
The chemical environment
What is the definition of a chemical environment?
It involves any atoms covalently bonded to it, and neighbouring atoms or groups of atoms
What are equivalent atoms?
Atoms in the same chemical environment
What are non-equivalent atoms?
Atoms in different chemical environments
What is the resonance frequency of an atom affected by?
It is affected by the local electron density
Why does electron density affect chemical shift?
Electrons oppose the magnetic field, reducing its effect on the resonating nucleus and causing it to be shielded
What is the effect of electronegative atoms?
Chemical shift increases in atoms that are covalently bonded to electronegative atoms such as oxygen as the become deshieded
What does the area under each peak represent in an H n.m.r. spectra? This is the same as the height of peaks in an integrated spectra
The area under each peak is proportional to the number of atoms in the same chemical environment
What can be worked out from the height of integrated spectra peaks?
The relative number of atoms in each chemical environment
What are 3 reasons why TMS is chosen as the reference standard?
- The H and C nuclei are highly shielded due to the electronegativity of silicon being low, meaning the peaks will be upfield
- All hydrogen and carbon atoms are equivalent, producing sharp peaks
- TMS is cheap, non-toxic and does not react with the sample
What does high-resolution proton n.m.r. give?
Gives fine detail about the relative position of each hydrogen atom in a compound, due to spin-spin coupling
When are protons said to be coupled?
When they are in different neighbouring chemical environments
What is the effect of spin-spin coupling?
A single peak is split into a cluster of peaks in high res n.m.r. spectra
What rule is used to work out the splitting pattern for a peak corresponding to a chemical environment?
n + 1 rule
What splitting pattern is produced by CH?
Doublet
What splitting pattern is produced by CH2?
Triplet
What splitting pattern is produced by CH3?
Quartet
