Transcript NMR Spectroscopy SL and HL
Chapter 11: Measurement and Data Processing Title: Lesson 3 Spectroscopic identification of organic compounds (NMR) (SL and HL)
Learning Objectives:
– – Describe how mass spectrometry (MS), proton nuclear magnetic resonance spectroscopy ( 1 H NMR), and infrared spectroscopy (IR) are techniques that can be used to help identify the structure of compounds Describe high resolution NMR spectroscopy (HL ONLY)
Nuclear magnetic resonance spectroscopy (NMR)
Technique for finding the
structure and shape of molecules
– a combo of nuclear physics and chemistry The nuclei of atoms with an odd number of protons or mass e.g. 1 H, 13 C, 19 F, and 31 P
spin
and behave like
tiny bar magnets
If placed in an external magnetic field,
applied field some of these nuclei will line up with an
, and, if they have sufficient energy,
some will line up against it
This arrangement leads to two nuclear energy levels The energy needed for the nuclei to reverse their spin and change their orientation in a magnetic field can be provided by
radio waves
Planck’s Equation
How does it work?
A sample is placed in an
electromagnet
Field strength is varied until the radio waves have the exact frequency to make the
opposite direction nuclei flip over and spin the
This is called spectrum
resonance
be detected electronically and recorded in the form of a and can NMR Spectroscopy Video - RSC NMR spectroscopy is non-invasive as the sample can be recovered unchanged after the experiment
How does NMR spectroscopy work?
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Hydrogen nuclei in different chemical environments have different chemical shifts…
As electrons shield the nucleus from the full effects of the external magnetic field,
differences in electron distribution produce different energy separations between two spin energy levels
Nuclei in
different chemical environments produce different signals
in the spectrum…
1 H NMR
is particularly useful (found in all organic molecules) Signals are measured against a
standard signal produced by 12 hydrogen nuclei in tetramethylsilane (TMS)
The position of the NMR signal relative to this standard is called
the chemical shift
of the proton NMR Video - Part 1 Hydrogen in particular environments have characteristic chemical shifts E.g. Ethanol has 3 different proton or hydrogen environments (colour coded in diagram) Complete list is given in section 27 of the IB data booklet
Proton NMR spectroscopy
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Interpreting
1
H NMR spectra
NMR Video - Integration NMR Video - Chemical Shift
1 H NMR spectrum for ethanal Spectrum trace shows:
A peak at 9.7 for C
H
O A peak at 2.1 for C
H 3
Note: Area under the C H 3 peak is three times larger than that under the C H O peak, this tells us the relative number of protons in each environment
The integrated gives this information more directly, as it goes up in steps which are proportional to the number of protons
Integration and the number of hydrogens
The height of the peaks in an NMR spectrum does not give us any useful information.
However, the
area under the peaks
on a 1 H NMR spectrum is proportional to the number of hydrogen atoms causing the signal. The ratio of the areas under the peaks tells you the ratio of 1 H atoms in each environment.
2 1 3
The spectrum can be
integrated
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Interpreting 1 H NMR spectra activity – (Number of Environments)
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Solutions
Combining the information
The analyst may have to use data from other sources along with techniques such as IR spectroscopy in order to give a complete structure of the molecule… (d) Infrared Spectroscopy (a) Empirical formula (b) Molecular formula (c) Mass Spectrometry (e) NMR Spectrometry
Uses of NMR spectroscopy
NMR spectroscopy uses the same technology as
magnetic resonance imaging
(MRI). This is an important non-invasive method of gaining information about internal structures in the body used in diagnostic medicine and scientific research.
NMR spectroscopy is also used in the pharmaceutical industry to check the purity of compounds.
15 of 45 Often, a combination of mass spectrometry, infrared spectroscopy and NMR spectroscopy is used in modern analysis to elucidate the structure of organic molecules.
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Spectroscopic identification of organic compounds Further NMR spectroscopy (HL)
Tetramethylsilane (TMS) as the reference NMR signals are measured against a standard produced by the 12 hydrogen nuclei in tetramethlysilane (TMS) All the hydrogen nuclei are in the same environment, so one signal is recorded
Because silicon has a lower electronegativity than carbon, TMS absorbs radio waves in a different region from that absorbed by hydrogen nuclei attached only to carbon
The standard signal will not overlap with any signals under investigation Chemical shift (
δ
) of a proton in a molecule is defined as: You won’t need to use this in calculations… V = frequency of radio waves absorbed by the protons in the sample V 0 = frequency of radio waves absorbed by the protons in the TMS
The absolute frequency of the signal depends on the strength of the magnetic field.
However, the chemical shift – relative to the standard – stays the same.
TMS is inert and soluble in most organic solvents. Low boiling point mean it can easily be removed from the sample.
High resolution
1
H NMR Spectroscopy
NMR Video - Spin-Spin Coupling NMR Video - Spin-Spin Coupling Part 2 NMR spectrum does not generally consist of a series of single peaks shown in the low resolution spectra shown earlier High resolution NMR reveals that the single peaks are split or resolved into a group of smaller parts Splitting of peaks happens because the effective magnetic field, experienced by particular nuclei, is modified by the magnetic field produced by neighbouring protons.
This is known as
spin-spin coupling
.
Magnetic field experienced by the proton in the methyl group depends on the spin of the proton attached to the carbon atom of the carbonyl group (C H O) Ethanal – low and hi res spectrum
Magnetic field of the C H 3 group depends on the proton spin of the C H O group… ‘With’ field The local magnetic field is increased when the magnetic field of the C H O proton is aligned with the external field and decreased against it when aligned As the energy separation between the two spin states of a proton depends on the local magnetic field… There are two possible values for energy difference between the
two nuclear energy levels
for the
CH 3 protons
‘Against’ field
Δ E a Δ E n
Instead of one signal corresponding to one energy difference, Δ E,
two signals corresponding to Δ E a and Δ E n are produced
Each line corresponds to a different spin on the neighbouring proton Both equally likely, so lines are of equal intensity 2 peaks (because of 2 different alignments of CHO proton) 2 peaks for C H 3 groups due to the possible combinations of alignments for the proton in C H O ‘With’ ‘Against’
4 peaks (because of different alignments of CH 3 protons) The low-resolution peak corresponding to the C H O proton is split due to the different magnetic fields produced by the combinations of spin for the three
protons of the neighbouring methyl group
Possible alignments for 3 protons in CH 3 group
Important Note: The lack of splitting with -OH groups
Unless the alcohol is absolutely free of any water, the hydrogen on the -OH group and any hydrogens on the next door carbon don't interact to produce any splitting. The -OH peak is a singlet and you don't have to worry about its effect on the next door hydrogens.
NOTE: Singlets also apply to amine groups…
• •
4 peaks in the signal for C H O due to the C H 3 group
2 possible orientations for each proton, so total of 2 3 combos possible.
Four different local magnetic fields Four signals with relative intensities 1,3,3,1
Spin coupling
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Splitting patterns produced from different numbers of neighbouring protons can be deduced by Pascal’s triangle:
NOTE:
Protons bonded to the same atom do not interact with one another (they behave as a group) Protons on non-adjacent carbon atoms do not interact with one another Nice Website About NMR Theory - Chem Guide
Splitting pattern activity
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1
H NMR chemical shift assignment
The chemical shift values of peaks on an 1 H NMR spectrum give information about the likely types of proton environment in a compound.
Type of proton δ/ppm
0.7
–1.2
2.1
–2.6
9.0
–10.0
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H NMR chemical shift assignment activity
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Solutions
X-ray diffraction
We see the world because light shines on objects and we observe the light that is scattered from them We cannot examine individual atoms and molecules this way because the wavelength of visible light is too long to interact effectively with matter so small Inter-atomic distances are of the order of 10 -9 m which corresponds to wavelengths of X rays
When X rays pass through a crystalline solid, they are scattered in an orderly way by the electrons in the substance Scattered waves interact with each other to cause a diffraction pattern
‘In phase waves’:
Peaks are still aligned Waves interfere constructively They reinforce each other, producing a resultant wave with larger amplitude than the original
‘Out of phase waves’:
Peaks misaligned by 180 o Waves interfere destructively Peak of one wave aligned with the trough of another Cancel each other out completely if the amplitude is the same (complete
destructive interference)
When X rays shine on a crystal, they are reflected in consecutive planes Scattered waves interfere as they travel different distances through a crystal The X rays may be at different phases (depending on wavelength) as they hit the detector
What affects the diffraction pattern?
Depends on the relationship between:
Angle of incidence ( θ ) The wavelength of the incident X ray ( λ ) The distance between atoms and their relative orientations (d) X Rays Diffraction Video X rays of a single or small range of wavelengths are used to ensure a simple correspondence between the diffraction pattern and the crystal structure Sample must be in solid state to give ordered diffraction patterns that can be interpreted
The electron density map
A map of electron density in a solid can be determined directly from the X ray diffraction pattern Contour lines connect points with the same electron density Identity of the atoms can be determined as the pattern in electron densities are related to an element’s electron configuration Note: Hydrogen atoms only have 1 electron, so are not visible due to low electron density!
Anthracene