Flashcards
NMR
Cards in this deck (46)
- 1The nucleus of some atoms have and overallnuclear spin as they have an odd number of protons and/or neutrons (e.g. 1H, 13C, 19F, 31P), although many atoms do not (e.g. 12C).
A nucleus with spin generates a small magnetic field. - 2When a nucleus with spin is placed in a magnetic field, the small magnetic field generated by the nuclear spin cab be.....?aligned with or against the main magnetic field.
- 3There is a small difference in energy between these two alignments that corresponds to the energy of radio-waves. Consequently, if radio-waves are passed through the substance......?some frequencies of radio-waves will be absorbed.
These radio-waves cause the nuclei to flip from one spin direction to the other – it ‘resonates’ between the two energy levels - 4The radio wave frequency is specific to theenvironment the nucleus is in (what other atoms it is surrounded by.
- 5NMR is especially useful to organic chemists as they can find out aboutthe H atoms (through 1H NMR) and the C atoms (through 13C NMR).
- 613Carbon NMRSince carbon is the element central to organic chemistry, 13C-NMR plays an important role in determining the structure of unknown organic molecules and the study of organic reactions and processes.
13C chemical shifts commonly occur between 0 -200 ppm. - 7the13C-NMR spectrum of an organic compound provides information concerning1. the number of different types of carbon atoms in the molecule
2. what the carbon atoms are - 8Eg EthanolEthanol has two carbon environments – as there are two carbons in ethanol which have different bonding, one carbon is connected to 3 hydrogens and 1 carbon, the other carbon is connected to two hydrogens, an oxygen and a carbon.
Oxygen is a lot more electronegative than hydrogen so the carbon which is bonded to the oxygen will be experiencing some ‘deshielding’, this is where the electrons in the bond are pulled away from the atom, exposing the nuclei spin more - 9Eg PentanePentane has a line of symmetry, so the carbons on either end have exactly the same bonding as each other and the carbons one in from the end are exactly the same as each other, so they are experiencing the same environment – we say they are equivalent, and will therefore only show one peak for each different environment
- 10Position of 13C NMR Peakswhat the carbon is bonded to (or near to) has an effect of where the peak is in the NMR spectra.
The closer the carbon is to a more electronegative atom, the less electrons it has to ‘shield’ its nucleus from the magnetic field. This ‘deshielding’ effect causes a bigger energy gap, and therefore a higher chemical shift (ppm), moving the peak more to the left of the spectra (downfield). - 11Number of signals - equivalent H atomsIn a spectrum, there is one signal for each set of equivalent H atoms, with the area of each signal being proportional to the number of equivalent H atoms it represents.
- 12Relative intensity of signals – number of equivalent H atomsIn 1H NMR (but not 13C NMR) the area of the signal is proportional to the number of H atoms it represents.
It is not the height of the signal that matters but the overall area that the signal covers.
( the same area and so represent the same number of H ) - 13There are a number of ways in which the relative size (area) of the signals can be shownrelative intensity of the signals from which the simplest whole number ratio can be calculated.
3:1:2 - 14Sometimes an “integration trace” is drawn on the spectrum. What do the heights show?The relative height of these traces gives the relative number of hydrogen atoms represented by each signal.
- 15Solvents-1H NMR spectra are nearly always recorded using a solution of the sample.
-If the solvent contains any 1H atoms, then these will appear in the spectrum (and may hide signals due to H atoms in the sample).
-Consequently solvents are used that contain no 1H atoms. Often these solvent contain deuterium (D or 2H) atoms instead of 1H atoms as D atoms have no nuclear spin.
-Examples of commonly used solvent include: CCl4 and deuterated solvents such as CDCl3, C6D6, etc. - 16CalibrationTo calibrate the spectrum, a small quantity of tetramethylsilane (TMS) is added to samples as this produces a single signal providing an internal standard to which other peaks are compared.
- 17Why is TMS used?-it gives a signal that is further right than most of the signals from organic compounds (due to the low electronegativity of Si);
-it only gives one signal as all twelve H atoms are chemically equivalent;
-it is non-toxic;
-it is inert;
-it has a low boiling point (26°C) and so can be easily removed from the sample afterwards. - 18Chemical shift-The spectra are recorded on a scale known as the chemical shift (δ), which is how far the signal is away from the signal for TMS is parts per million.
-Most signals are between 0 and 10 ppm away from the signal for TMS, and the scale is usually shown as below on the final spectrum. - 19what does the chemical shift depend on?The chemical shift depends on what other atoms/groups are near the H. The closer the H is to electronegative atoms (e.g. O, Cl), the greater the shift. Also the more electronegative atoms near, the greater the shift
- 20(chemical shift )δ =δ =
Field for TMS - Field measured
------------------------------------ x 10^6
Field for TMS - 21The chemical shift is sensitive totemperature, nature of the solvent and the concentration.
- 22The stronger the hydrogen bonding thelarger the chemical shift.
- 23High resolution NMR - Spin couplingThe amount of energy, which is required to cause a proton to resonate, depends on its molecular environment and is thus affected by neighbouring protons.
The neighbouring protons can either be aligned with or against the applied external magnetic field. If they are aligned with the field they will exert less of a shielding effect than if they are aligned in the opposite direction. This has the effect of splitting the resonance peak of the adjacent proton - 24The protons within the CH3 group as a whole can have.......... possible orientations what does this mean?4
the CHO proton will experience four slightly differing magnetic fields and will absorb at four differing frequencies giving rise to four signals centred on the expected chemical shift. The signals are only seen on high-resolution NMR spectra. - 25The number of peaks can be determined fromthe adjacent group number of protons +1
- 26The number of lines a signal is split into shows what?The number of lines a signal is split into gives information about neighbouring H atoms - whether they are inequivalent, and if they are, how many H atoms there are.
- 27Usually only H atoms on the neighbouring C atom cause what?splittings (couplings).
The signal for H atoms is only split by inequivalent Hs – so if the H atoms on the next atom are equivalent they do not cause splittings. - 28The number of lines =The number of lines = 1 + the number of inequivalent H atoms on adjacent C atoms
(the “n + 1” rule) - 29If there are more than 3 neighbouring inequivalent H atoms, then it can be classed asa multiplet
- 30It should be noted that the H atom of the OH group in alcohols rarely causes splittings (couplings), or is split (coupled) itself. why?the proton is hydrogen bonded and can exchange between the molecule and water, sometimes referred to as a labile proton. Sometimes the H atom of an OH group appears as a broad hump.
- 31Triplet & quartet3:2
CH3 –CH2 –
(ethyl group)
The triplet and quartet do not have to be next to each other in the spectrum.
The atom joined to the other side of the CH2 cannot have an H’s on (unless they do not do not couple, e.g. OH group or an equivalent CH2 as in butane). - 32Triplet & triplet2:2
– CH2 – CH2 –
The two triplets do not have to be next to each other in the spectrum.
The two CH2 groups must be inequivalent (otherwise they would produce one signal not two!).
The atoms joined either side of the CH2s cannot have any H’s on them. - 33Singlet3
CH3 –
(methyl group)
The atom joined to the CH3 cannot have any H’s on it. - 34How many orientations can the proton within the CHO group have?2
- 35How would you expect the CH3 protons trace to be split?into 2
- 36Why would the intensities of these peaks be the same?Equal chance of spin of CHO proton being with or against the external field
- 37What does integrated trace show?Area under peak shows relative numbers of protons in each environment
- 38CH2 protons are more de-shielded byadjacent electronegative O atom
- 39Why do you think the effect is referred to as spin coupling?Trace of protons in one environment affect/are coupled to the spin of protons in the adjacent environment.
- 40Summary of nmr. Number of peaksNumber of different H environments
- 41Summary of nmr. Integrated traceArea under peak = no. of protons
- 42Summary of nmr. Chemical shiftShift left (downfield)
with more electronegative atom neighbour - 43Summary of nmr. Peak splittingn protons split adjacent peak into n+1
- 44Summary of nmr. Labile protonDue to H bonding – they do not cause splitting nor are split themselves
- 45Medical Resonance Imagingprinciples of nmr have been developed and provide a non-invasive diagnostic technique which allows visualisation of soft tissues.
More recent developments of magnetic resonance spectroscopy allow the levels of different metabolites within the body to be measured.
functional MRI scans - 46Nmr spectroscopy is a valuable tool in the pharmaceutical industry.It is used to check the purity of compounds.
It is a non-destructive method that is faster and only requires a small, (20mg.) sample size compared to other methods.
It can be used to establish that only one enantiomer is present which is most important for pharmaceutical purposes.
Solid state nmr can be used to study the compound of interest in the tablet formulation.
Information
- Description
- A level Chemistry Flashcards on NMR , created by megan trent on 02/11/2018.
- Created by
- megan trent
- Views
- 0
- Rating