Flashcards

NMR spectroscopy.

megan trent English 37 cards

Cards in this deck (37)

1TheoryEvery isotope of every element has a ground state nuclear spin quantum number I.
Each nucleon (proton or neutron) has an intrinsic spin, like electrons, and the total nuclear spin is governed by the number of unpaired protons and neutrons in the nucleus
Unpaired nucleons give rise to a magnetic moment, and it is this magnetic moment that NMR is concerned with.
2Theory (dont need to learn equation)
3Parameters of some observable magnetic nuclei
4TheoryTake for example a proton where I = ½.
I is quantised, where discrete values of angular momentum mI may be observed.
The number of energy states is governed by 2I+1 rule.
In the absence of a magnetic field these levels are all degenerate.
5Theory (need to remember Bo - it means the magnetic field strength) h is a constant y is also a constant so dont need to worry delta energy is directly proportional to the magnetic field strength so frequency is also directly proportional tp magnetic field strength advantages of having a stronger magnetic field is better sensitivity amand better spectral resolution (so sharp points)
6TheoryWe can label the energy levels according to the orientation of the nuclear magnetic moment.
When the nuclei are placed in a magnetic field they will align either with or opposed to the field.
If γ is positive, then the nuclear magnetic moment aligned with the field (parallel), will have slightly lower energy than those aligned against the field antiparallel).
See the following diagram showing the energy levels of a proton,
7Theory
8Theory.It can be seen that we have energy levels that are separated by a discrete energy interval.
Transitions can be promoted by the application of the appropriate energy, which in this case is in the MHz region.
9Theory.
10Chemical ShiftThe electrons spinning around the atom will have a small magnetic field that will influence the nucleus.
A secondary magnetic field is induced by the spinning electrons that opposes the applied field.
The nucleus therefore feels the effect of Beffective rather than Bo.
11Chemical Shift σ means the electron density
12Chemical ShiftThis environment differs between different chemical species.
- e.g H protons in a methyl group will have different resonance than those in an amino group. And protons in Toluene are different from those in acetic acid etc…
This shift in resonance frequency due to the chemical environment is called the chemical shift.
13Chemical Shift ppm - parts per million
14Sample PreparationSamples are dissolved, in the relevant, high purity deuterated solvent. Typically 5 – 20 mg/ml
It is important to produce homogeneous solutions free of undissolved particles, that would be detrimental to spectral resolution. Filter solutions if necessary.
15Deuterated SolventsNMR spectra are usually measured using solutions of the substance being investigated. It is important that the solvent itself doesn't contain any simple hydrogen atoms, because they would produce confusing peaks in the spectrum
The simple way of avoiding this, is to use a solvent in which any ordinary hydrogen atoms are replaced by its isotope, deuterium - Typical deuterated solvents include , CDCl3, C6D6, D2O, DMSO d6, and CD3OD.
Deuterium atoms have sufficiently different magnetic properties from ordinary hydrogen that they don't produce peaks in the area of the spectrum that we are looking at
16Sampling timeDependant on sample concentration and the natural abundance of the nuclei under observation
An improvement to the signal to noise ratio can be made by increasing the number of scans. This is known as the ensemble average
17Sampling time
18Relaxation delay
19Equivalent Hydrogen atomsHydrogen atoms attached to the same carbon atom are said to be equivalent. Chemically equivalent protons have the same chemical shift.
But hydrogen atoms on neighbouring carbon atoms can also be equivalent if they are in exactly the same environment.
20Interpretation of 1H NMR Single Pulse Spectra The important aspects involved when analysing 1H NMR spectra are:1. chemical shift. A range of chemical shifts can be observed in a NMR spectrum due to the magnetic environment experienced by each nucleus. These differences are caused by the variation in electron density in the neighbourhood of each nucleus.
2. 1H-1H splitting patterns. The rule that governs splitting patterns is: a nucleus, equally coupled to n others, will give rise to a signal with n+1 lines. For example a proton can be observed as a doublet, triplet or quartet whenever it is coupled equally to either one, two, or three protons.
21Interpretation of 1H NMR Single Pulse Spectra3. Integration. The area under each set of peaks is proportional to the number of protons in resonance at that frequency. Therefore sets of peaks from a methyl (CH3) group coupled to a methylene (CH2) group would give an integral ratio of 3:2
4. The coupling constant (J). For a set of split peaks (multiplet), the separation between the lines is measured in Hertz and is known as the coupling constant J. Additionally for a 1H-1H coupling in for example CH3-CH2 , both sets of multiplets will have the same coupling constant. J values are widely different -from 1Hz to 18Hz.
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23Application of measuring the coupling constant (J).
24Figure 1. Typical 1H Chemical Shifts
251H Splitting patterns
261H NMR spectrum
2713C Decoupled NMR Experiment13C NMR experiments can be run 1H decoupled (i.e. showing no interactions with protons) in order to simplify the spectrum. ( NOTE, to gain information regarding JCH the spectrum would have to be run, proton coupled. Spectra can be very complicated for larger molecules)
The decoupled spectrum will show one band for every chemically different 13C atom in the molecule.
2813C Decoupled NMR ExperimentA range of chemical shifts can be observed in the 13C NMR spectrum just as in the 1H spectrum, except the 13C NMR spectral range runs from typically 0-220 ppm. See Figure 2 for a list of common chemical shifts with respect to the environment of the atom under observation.
Signals cannot be simply quantified, by integration, as in the 1H experiment.
29Figure 2. Typical 13C Chemical Shifts
3013C NMR spectrum
31Chemical Shift.
32Chemical Shift. D2O shake.
33Exam Questions – Analytical Techniques CHEM 1039 MAY 2010 PAPER (Andy Mendham)
34Exam Questions – Analytical Techniques CHEM 1039 MAY 2009 PAPER
35Determine the structural formula for the compounds whose 1H NMR spectra are given. B) C3H6O2.
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