NMR And Chromatography Flashcards
What do spectroscopic methods involve?
Making observations of how molecules are affected when subjected to electromagnetic radiation
In nuclear magnetic resonance (NMR) spectroscopy what part of the electromagnetic spectrum is used?
The radio frequency region
In nuclear magnetic resonance (NMR) spectroscopy what part of the electromagnetic spectrum is used?
The radio frequency region
The nuclei of some atoms have a property known as nuclear spin. What do nuclei that posses spin have and what does this enable them to act as?
They have their own magnetic field and can be considered to behave as if they are small bar magnets
In an external magnetic field what can the nuclei with magnetic spin do?
They can either allign themselves with the external field (lower energy state) or opposite to it (higher energy state)
If a molecule in a strong magnetic field is irradiated with radio frequency electromagmetic waves what can the nuclei of some of the atoms do and what is this called?
They can absorb the radiowaves and move from the low-energy to the high-energy state. These absorptions are called resonances and they occur at different energies depending upon the surrounding environment of the atom
What can the different resonaunce energies be used to do?
Identify atoms, count them and work out their positions in the molecule relative to other atoms. It is this behaviour that leads to the production of rhe NMR spectrum.
When carrying out NMR spectroscopy do we keep the external magnetic field constant or the radio frequency?
The radio frequency and change the external magnetic field
Why is the hydrogen atom particularly useful for investigation by NMR and what is this type of NMR called?
Becayse of the large number of hydrogen atoms in almost all organic compounds. This is referred to as proton NMR
What does it mean if nuclei have different chemical environments and what will this cause?
They have different neighbouring atoms so will absorb a radio frequency at slightly different external field strengths. The different environments are said to cause a chemical shift of the absorption.
How can an NMR spectrum be produced?
If the value of the external field is recorded as the different resonances occur the spectrum can be produced with the horizontal scale as the chemical shift scale with units ppm and the vertical scale the strength of absorption
Why will the 1H nuclei in bromoethane produce two different resonances?
Because they have two distinct chemical environemts
How are chemical shifts in ppm related to the electron density near the resonating nucleus?
If the electron density around the resonating nucleus is low (as a result of bonding to an electronegative group of atoms or a delocalised system) the nucleus is said to be deshielded and the resonance occurs at a higher ppm value and vice versa
In NMR spectrum why are samples investigated in dilute solution?
It seperates the sample molecules from each other, preventing them from interacting and causing very complex absorptions
What solvents are used in NMR spectroscopy when investigating 1H atoms and why?
Tetrachloromethane (CCl4) or deuterated trichloromethane (CDCl3) are usually used because they are powerful solvents for organic compounds and do not contain 1H atoms. This means that they do not resonate and so do not interfere with the 1H NMR spectrum oc the sample
During the production of an NMR spectrum what is mixed with the sample to add a reference point to which the NMR spectrometer is tuned and how does this tuning take place?
Tetramethylsilane (TMS). The magnetic field is adjusted until the 1H nuclei of TMS resonate: this is given a chemical-shift value of 0
Why is TMS used?
- silicon has a very low electronegativity. As a result the hydrogen nuclei in TMS resonate at a field strength well above that of any 1H nuclei in common organic molecules
- it gives one strong sharp, easily detected absorption because the absorption is caused by the combined effects of 12 equivalent 1H atoms
- TMS is non toxic and cheap
- it does not react with the sample
- tms is easily seperated from the sample molecule because it has a low boiling point
What are equivalent atoms?
Those in identical chemical environments. Usually due to symmetry within the molecule
Why do chemical shifts vary from machine to machine?
Because the value of the external magnetic field that causes a particular proton to resonate depends on the radio frequency that the NMR spectrum uses
What helps to avoid the problem of the chemical shifts varying from machine to machine?
The use of a reference compound (TMS) and a scale for chemical shift without units (the delta scale)
Due to the fact that chemical shifts are so small what do we do?
Multiply the actual differences by 10^6 and quote the units of the scale as parts per million (ppm)
Why would a low resolution NMR spectrum of ethanol show three different absorptions?
Because the ethanol molecule contains three sets of 1H atoms, each in a different chemical environment
The three absorptions of ethanol can be thought of as signal A (CH3) B (CH2) and C (OH). What is the order of decreasing chemical shifts and why?
Signal C has the largest chemical shift as the 1H atom is directly bonded to an electronegative atom (OH)
Signal B has a larger chemical shift than A because the CH2 group is nearer to the electronegative oxygen atom than the CH3 group is
What are the strengths of absorption proportional to?
The number of equivalent 1H atoms causing the absorption
How are the strength of absorptions measured?
By the area under each absorption peak
What is the integrated spectrum?
The areas of absorption that are meausred electronically and superimposed digitally or graphically on the main spectrum
When examined with a high resolution mass spectrometer what happens to the signals in ethanol?
The baisic signals are split into groups of signals. The CH3 signal is split into three producing what is called a triplet, the CH2 signal splits in four forming a quartet and the OH signal is not split and appears as a singlet
What is a coupling effect?
The splitting of the absorptions that is caused by the influence of 1H atoms bonded to neighbouring atoms
When a particular 1H nucleus resonates in an applied magnetic field, what is the actual magnetic field that acts on it?
The sum of the applied field and the fields from all it’s neighbours
What does it mean that 1H atoms are coupled?
They are linked through space by the interactions of their neighbours
The magnetic field from a neighbouring 1H nucleus on the next carbon along the chain may act with or against the applied magnetic field. There is a 50:50 chance of either. What does this mean in a CH3?
That the three equivalent 1H nuclei of a CH3 group will resonate either at a slightly lower applied field or at a slightly higher applied field. The end result is that the original resonance is split into a doublet in a ratio of 1:1
For two equivalent neighbouring 1H atoms what could their magnetic fields be? And what does this mean in regard to CH3?
Both with the applied field, both against it or one with while the other is against
This means that the three equivalent 1H nuclei if the CH3 geoup are coupled to the two neighbours in three different ways. The result is a triplet. The intensities that make up the triplet are in the ratios 1:2:1. These are the relative probabilities of the two neighbours fields adding with, cancelling out or adding against the applied field
Between which H atoms does coupling only occur with and what does it not occur with?
It only occurs between H atoms on adjacent carbon atoms (including H atoms of two different adjacent carbon atoms). Equivalent H atoms do not couple with eachother
What is the n+1 rule?
The NMR absorption of a proton that has n equivalent neighbouring protons is split into n + 1 peaks
What can we use instead of 12C nuclei in NMR?
13C nuclei
Why are 13C spectra much easier to analyse than 1H spectra?
Because this isotope had a low natural abundance so the probability of two 13C atoms being bonded together is very low. Therefore coupling between two adjacent 13C atoms is not observed and 13C spectra only consist of a series of singlet resonances
The resonances of which carbon atoms occur at higher chemical shifts?
The resonances of carbon atoms attached to electronegative elements and those of aromantic and alkene carbon atoms
What does the 13C spectrum of ethanol consist of?
Two resonances with the resonance at the higher chemical shift assugned to the carbon atom attached to the electronegative oxygen atom
What should be the first step in analysing a 23C spectrum?
Counting the number of 13C resonances
What are some different types of chromatography?
Paper chromatography, thin-layer chromatography (TCL), column chromatography, gas-liquid chromatography (GLC) and high-performance liquid chromatography (HPLC)
What is chromatography?
A technique that enables the separation of mixtures
What principle does all chromatography make use of?
That components in a mixture when dissolved in a fluid will flow through another material (the stationary phase) at varying rates
What does seperation in chromatography depend on?
How the components interact with the stationary phase (their retention) and how soluble they are in the fluid
What was the foundation for a chromatographic techniques.
Column chromatography
Who developed column chromatography, when and why?
Mikhail Tsvet devoped it in the early 1900s to seperate plant pigments
What happens in column chromatography?
An inert solud callee the stationary phase (usually powdered silica gel or alumina) is placed in the column with a liquid solvent called the mobile phase. The sample mixture, dissolved in the solvent phase is introduced at the top of the column and the column is keot topped up eith fresh solvent (eulent) as the sample flows through the column. The components with the greatest attraction to the stationary phase take the longest time to flow through the column. If the components are coloured you can use their Rf value to identify them. If the components are colourless then other techniques can be used (e.g. fluorescence under UV radiation) to show their position in the column
How does thin-layer chromatography take place?
Used a layer of silica on glass, aluminium or plastic as the stationary phase. A small spot of mixture is added near to the bottom of the TCL plate and the plate is dipped in a solvent (the mobile phase). As the solvent moves up the TLC plate, the components of a mixture interact with the stationary phase differently and move up at varying rates. The TLC plate is removed when the solvent has nearly reached the top of the plate and the compounds are visualised using fluorescence under UV radiation or by a chemical staining technique (e.g. using ninhydrin)
Then the retention factor will be worked out for each component in the mixture
How do you work out the Rf of a component in a mixture?
Distance moved by compound/ distance moved by solvent
What happens in gas-liquid chromatography?
- The mobile phase is an inert gas such as dry nitrogen or helium and the stationary phase is usually ditomaceous earth coated with a non-volatile liquid.
- The stationary phase is packed into a long coiled tube and placed in a heated oven.
- For GLC to be used, the sample must either be a gas or a volatile liquid at the temperature of the oven.
- The sample is injected into the oven through a self-sealing disc, and the vapour formed is carried through the stationary phase using the inert-mobile phase.
- the temperature of the stationary phase can be varied to optimize the seperation of the mixture in the sample.
- The time taked to pass through the column is called the retention time. This depends on the nature of the solute, the volatility of the solute and the attraction between the stationary and mobile phases. The retention time can be compared with standard reference values to identify the compound.
- as the compounds of the mixture leave the column they can be detected by either thermal conductivity or, more commonly flame ionisation. In flame ionisation the outlet gas is mixed with hydrogen and air and burned to produce ions. These ions allow a current to be transmitted which is then converted to a signal on a chart recorder
- the relative sized (areas) of the peaks are related to how much of each compound is present in the mixture
