Spectroscopy and chromatography - Topic 19 Flashcards
different types of radiation
1) infrared in analysis
2) microwaves for heating
3) radio waves in NMR
4) ultraviolet in initiation of reactions
Effect of infrared in analysis on molecules
infrared energy causes bonds to vibrate. This can be used to identify the types
of bond in a molecule
Effect microwaves for heating on molecules
certain molecules absorb the microwaves causing them to rotate
Effect on radiowaves in NMR on molecules
can cause the hydrogen nucleus to change its spin state. This can give us
information about the arrangements of hydrogens in a molecule.
Effect ultraviolet in initiation of reaction on molecules
UV energy can break bonds such as the Cl-Cl bond or C-Cl bond
What is NMR spectroscopy used for
to determine the structure of the molecules
what is mass spectrometry used for
to find the relative molecular mass of a compound
what does the molecular ion (M+) weak tell us
the relative molecular mass of the original molecule
what do high resolution mass spectrometers do
identify different molecules with the same molecular mass rounded to the near whole number
Explain why high resolution mass spectrometers are more accurate than standard low resolution mass spectrometers
- they measure the relative mass to several decimal places unlike low resolution which is only able to measure relative mass to the nearest whole number
What makes a nucleus detectable by NMR?
1) If a atomic nucleus has an odd number of nucleons (protons and neutrons) then it has a nuclear spin
2) the nuclear spin creates a weak magnetic field
3) NMR detects how these magnetic fields are affected by larger external magnetic field
why can hydrogen and 13^C be detected by NMR
- hydrogen has 1 proton so does have a nuclear spin
- 13^C has 7 neutrons so does have a nuclear spin
- hence these can create a weak magnetic field which NMR can detect how larger external magnetic fields can affect them
explain how does NMR spectroscopy work
1) nuclei spin in random directions however when an external magnetic field is then applied they align in 2 directions
2) the nuclei spins either in the direction of the external magnetic field or against it
3) those that spin in the direction of the magnetic filed have a lower energy
4) NMR fires out radio waves and at a specific frequency the nuclei that are aligned with the magnetic field absorb the energy and flip to the higher energy level
5) those with a higher energy can drop to lower energy and emit radio waves
6) initially there are more nuclei aligned with the magnetic field so more energy is absorbed and emitted
7) NMR measured the amount of energy absorbed
what affects the energy absorbed by the nuclei in NMR
The chemical environment around a nucleus affects the energy it absorbs. For example:
- Shielding - surrounding electrons reduces the magnetic field felt by the nucleus, so it absorbs less energy.
- Nearby atoms or groups, such as electronegative elements - can withdraw electron density, reducing shielding. This causes the nucleus to feel a stronger magnetic field and absorb more energy.
why is the energy absorbed dependent on the environment of the nuclei
Different environments change how much shielding a nucleus has, which affects how strongly it feels the magnetic field. This causes it to absorb different amounts of energy at different frequencies.
what is required for an atom to be in the same environment
it has to be bonded to an atom or group of atoms that are identical
how many carbon environments are there in 2- bromopropane
2 carbon environments
1) in the two CH3 groups attached to the middle carbon
2) the two outer carbons are both bonded to CHBr(CH3)
how many carbon environments in 1-bromopropane
3 carbon enviroments
1) one carbon (CH2Br) is bonded to CH2CH3
2) the middle carbon is bonded to CH2Br(CH3)
3) one carbon (CH3) is bonded to CH2(CH2Br)
how many hydrogen environments are there in bromoethane
2 hydrogen environment
1) one hydrogen (CH3) is bonded to CH2Br
2) one hydrogen (CH2Br) is bonded to CH3
how many hydrogen environments are there in 1,2bromoethane
1 hydrogen environment
each hydrogen are in the same environment as they are equal distance from bromine atoms
what is TMS (tetramethylsilane)
a chemical used as a standard when looking at chemical shift in NMR spectra
why do we use TMS
- as nuclei absorb different amounts of energy at different frequencies and it is difficult to measured the magnitude of these without a standard chemical to measure against.
- the standard we use is TMS
describe and draw structure of TMS
- TMS has 12 hydrogen in the same environments
- it is inert, non toxic and volatile so easy to remove from sample
Structure of TMS 29:06 https://www.youtube.com/watch?v=zjjR4iD9s8A&t=1553s
what type of peak does TMS produce
a large single peak away from all the sample peaks
what is a chemical shift (δ)
the difference between the TMS peak and the peaks produced by a substance under test, measured in ppm (parts per million)
what causes a peak to be produced at δ=0
TMS will be produced at δ = 0 as TMS is used to calibrate NMR machines when analysing samples
two types of NMR
13^C NMR
High resolution 1^HNMR
what does 13^C NMR spectroscopy tell us
how many different carbon environments there are in a sample being tested
how does 13^C NMR work
the number of peaks on a 13^C NMR (excluding the one at δ = 0) tells us the number of different carbon environments
how is the position of the peak affected in a 13^C NMR
- The position of the peak increases when a carbon atom is less shielded.
- This occurs when electronegative atoms withdraw electron density from the carbon, reducing shielding.
- This means it must absorb more energy to flip its spin, resulting in a higher chemical shift.
- so the peak appears further to the left (downfield) on the NMR spectrum.
how do we predict the number of peaks there would be in cyclic compounds
- look for symmetry by drawing a line of symmetry
- look for the number of different carbon environments to predict number of peaks
What are the issues with using the data booklet to identify different carbon environments from their peaks
- a peak at 190 will suggest a carbonyl group however we cannot be sure id this is an aldehyde or ketone
- there are overlaps at peak 60 so we dont know whether at peak 60 there is an amine, alcohol, ester or ethene
what does 1^HNMR do
it tells us how many different hydrogen environments there ate and how many hydrogens in each environment in the sample
what dos the peaks on 1^HNMR tell us
the number of different hydrogen environments
what does the number above the peaks on 1^HNMR tell us
the ratio of the areas under the peaks which corresponds to the number of hydrogens bonded to the carbon atom
what is a splitting pattern
peaks that split into smaller peaks
how can the splitting pattern help us determine the structure of a molecule on a 1^HNMR spectrum
the number of smaller peaks corresponds to the number of hydrogen atoms on the adjacent carbon plus one which is also known as spin spin coupling
all the types of splitting pattens and their corresponding hydrogen atoms on the adjacent carbon
singlet peak - 0 hydrogens on adjacent carbon
doublet peak - 1 hydrogens on adjacent carbon
triplet peak - 2 hydrogens on adjacent carbon
quartet peak - 3 hydrogens on adjacent carbon
how to find the height ratio when we have split peaks in a 1^HNMR spectroscopy
measure the vertical parts of the integration trace which will correspond to the ratio
what is elemental analysis
a method where we can determine the structure of the sample under test by determining the percentage composition or mass of elements that make up a compound
what does thin layer chromatography (TLC) do
it allows us to separate and identify compounds
parts of the TLC
- glass lid - prevents solvent from evaporating
- mobile phase - a liquid solvent
- stationary phase - solid silicon dixoide or aluminium oxide
steps of TLC
1) place the plate in a solvent - the base line must be above the solvent level
2) leave until solvent has moved up to near the top of the plate
3) remove the plate and mark the solvent front and allow to dry
how does TLC work
- some chemical spots dissolve in the solvent whilst others may not and stick to the stationary phase, leaving a chromatogram
- the number of spots tells you how many chemicals make up the mixture
- we can then identify the chemicals using the positions on the chromatogram using Rf values
what is gas chromatography used for
to seperate volatile liquids and identify
parts of gas chromatography
- very thin column that is wound up inside an oven to save space
- the column is lined with a viscous liquid that acts as the stationary phase
how does gas chromatography work
1) the sample is injected into the machine and carried by an inert gas which is the mobile phase
2) each substance takes a different amount of time to travel through the column and reach the detector
3) the length of time it takes is called the retention time
why does the retention time vary in gas chromatography
- some molecules spend more time stuck to the stationary phase and some spent more time travelling in the mobile phase
how to identify the substance and its quantity in gas chromatography
- by using a gas spectra
- each peak represents a different substance and each substance has a different retention time
- compare retention times with a library of known retention times to identify the substance
- the area under the peak tells us the amount of the substance, the larger the area the more substance
when is high performance liquid chromatography (HPLC) used
where it may not be suitable to use gas chromatography and where the sample has a hight boiling point or decomposes on heat
parts of HPLC
- stationary phase - includes small solid particles of silica or hydrocarbons in a column
- mobile phase - a solvent of methanol and water
steps of HPLC
1) HPLC solvent is pressurised using a pump to which the sample under test is added and pushed through the column
2) each substance takes a different amount of time to travel through the column and reach the detector as some parts will spend more time sticking to the stationary phase so will take longer to come through the column
3) after the sample leaves the column it is separated and enters the detector part, where UV light is shone at the sample and the absorptivity is measured, this produces a chromatogram
how to identify a substance using HPLC
- a spectra is produced and each peak represents a different substance and each substance has a different retention time
- compare retention times with a library of known retention times to identify the substance
what is gas chromatography - mass spectrometry (GC-MS) used for
to identify the substances in a mixture
how does gas chromatography - mass spectrometry (GC-MS) identify substances
1) the mixture of substances is fed through a GC or HPLC machine and this seperates them
2) the detector of the GC or HPLC is replaced try a mass spectrometer
3) the separated compounds run through an MS machine which produces a spectra to identify the individual substances
4) the substances can be positively identified as the mass spectra produced are compared with a library of spectra store on a computer
What causes covalent bonds in a molecule to vibrate when exposed to infrared radiation?
Certain bonds in a molecule absorb infrared radiation at characteristic frequencies, causing the covalent bonds to vibrate.
What does the infrared spectrum above 1500 cm⁻¹ identify?
for “Functional group identification.”
How do modern breathalysers use infrared spectroscopy?
Modern breathalysers measure ethanol in the breath by analyzing the infrared absorption spectrum.
How does a computer use infrared spectra for compound identification?
A computer compares the IR spectra against a database of known pure compounds to identify the compound.
