Organic Chemistry 4.2 - ExpDetStruc - IR Spectra and NMR Flashcards
What can infrared spectroscopy be used to identify?
functional groups in organic compounds
What happens when IR radiation is absorbed by an organic compound?
the bonds in the molecule vibrate (‘‘stretch and bend’’)
When IR radiation is ___ by an organic compound, the bonds within the molecule ___ (___ and ___)
absorbed, vibrate (stretch and bend)
The wavelengths of IR radiation absorbed depend on the ___ of the ___, and the types of ___ involved in it.
strength, bond, atoms
Which two things determine the wavelengths of IR radiation absorbed?
bond strength and types of atoms involved
Describe what happens in IR spectroscopy. (2)
IR radiation is passed through a sample of the organic compound, and then onto a detector which measures the intensity of different wavelengths of transmitted radiation
IR spectroscopy involves passing __ radiation through a sample of an ___ compound, and then onto a ___ which measures the ___ of different wavelengths of ___ radiation.
IR, organic, detector, intensity, transmitted
The absorbance of IR radiation is measured in…
wavenumber (cm^-1)
How do you calculate wavenumber?
wavenumber = 1 / wavelength
Page 15 of the data booklet shows ‘characteristic ___ by particular vibrations’ - which is just the ranges (measured in ___) of radiation absorbed by particular ___ ___.
absorbtions, wavenumber/cm-1, functional groups
The absorbtion region below ___cm^-1 on an IR spectra is known as the ___ region, because it is unique to each ___ ___.
1400, fingerprint, organic compound
NUCLEAR MAGNETIC RESONANCE SPECTRA
Proton NMR (aka…) can give information about the different ___ ___ of ___ atoms in an ___ molecule, and how many there are in each ___.
chemical environments, hydrogen, organic, environment
What are different proton environments defined by?
*ie what makes protein environments different?
the different atoms/groups attached to the carbon with the given proton
Different proton environments are defined by the different ___/___ attached to the ___ with the ___.
atoms/groups, carbon, proton
There is a table on page 17 of the data booklet that provides value ranges for each ___ ___ in different environments.
chemical shift
How are different chemical environments represented on a 1H NMR spectra?
by a peak - each peak represents a different environment
How is the number of protons in a given environment represented…
1. On a low-res spectra?
2. On a high-res spectra?
- the height of the peak
- the area under the peak // Integration Curve
How many proton environments are in a molecule of…
1. Ethanol?
2. Butan-1-ol?
3. Propane?
4. Pentan-3-one?
- 3
- 5
- 2
- 3
If a molecule is asymmetrical, then each carbon will have two different ___ attached.
groups
Take butan-1-ol (CH3 CH2 CH2 OH)
The first carbon has one C2H5O group attached,
the second carbon has one ___ group and one ___ group attached.
And so on.
A symmetrical molecule will begin to have repeat environments past the ___ mark.
this is not exam language
CH3, CH2OH.
halfway
The height of a line representing one hydrogen atom in an OH group will be ___ the height of the hydrogen atoms in a CH2 group, and ___ the height of the hydrogen atoms in a CH3 group.
one half,
one third
1H nuclei behave like tiny ___ in a strong ___ ___.
magnets, magnetic field
Some 1H nuclei ___ with the field and have lower energy.
The rest ___ ___ the field and have higher energy.
align.
align against
1H nuclei that align with the magnetic field have (higher/lower) energies than those that align ___ it.
lower, against
The lower energy ___ state can flip to a ___ energy spin state if…
spin, higher, sufficient energy is absorbed
This energy must be from the ‘___ frequency’ region of the EMS.
radio
As the nuclei fall back down to the ___ energy spin ___, radiation is ___.
lower, state, emitted
This emitted radiation is ___, and plotted on a ___.
detected, spectrum
On a 1H NMR spectrum, the chemical shift is related to the ___ of the 1H atom.
environment
What is chemical shift, δ, measured in?
ppm
How does 1H NMR work? (4)
A magnetic field is applied to an organic compound, which makes the 1H nuclei act like magnets. Some align their spin with the field and have lower energy and others disalign with the field and have higher energy. When radio waves are absorbed by the lower energy 1H nuclei, their spin flips to the higher energy spin state. When they fall back down, radiation is emitted, and detected, and plotted on a spectra
Which substance is used as the standard reference in NMR spectroscopy?
tetramethylsilane (TMS)
Tetramethylsilane is the standard ___ substance, which gives a chemical shift value of…
reference, 0 ppm
Why is tetramethylsilane called a standard reference substance?
because it has a chemical shift value of 0 ppm (negative control basically)
Why is high resolution NMR better than low resolution NMR?
it tells you everything that low res does - and more.
High resolution uses higher ___ ___ than low resolution NMR.
radio frequencies
What feature can high resolution spectra have that low res can’t?
multiplets
What is a multiplet?
a peak that is split into multiple peaks
How are multiplets formed?
when a 1H atom interacts with neighbouring carbon atoms
The number of 1H atoms on ___ ___ atoms determines the number of ___ within a multiplet.
neighbouring carbon, peaks
What is the relationship between number of 1H atoms on neighbouring carbons and the number of peaks in a multiplet?
number of peaks in a multiplet is one larger than the number of protons on neighbouring carbons (‘n+1 rule’)
Imagine a molecule of ethanal
How many peaks would be in the multiplet that represents the 1H atom on the first carbon?
four
(because 3 neighbouring hydrogens + 1 = 4)
Imagine a propane molecule
How many peaks for one of the middle carbon’s hydrogens?
seven
(because 2x CH3 = 6, + 1 = 7)
The second hydrogen on the middle carbon is irrelevant, because it’s not neighbouring
NMR KEY POINTS
1. Number of peaks =
2. Ratio of height / area under peaks =
3. Chemical shifts (ppm) give important info about…
4. High res spectra have clusters called ___, which give info about…
5. ___ res spectras tell you everything that ___ res spectra do, and more.
- Number of different environments
- ratio of number of hydrogens in each environment
- the type of environment the proton is in
- multiplets, the number of hydrogens on neighbouring carbons (no of multiplets is one more than the number of neighbouring Hs)
- High, low
High res NMR spectra have an ‘integration curve’ which is used to infer the…
the ratio between numbers of hydrogen atoms in each environment (low res have height, high res has area of peak/height difference of curve)
