Organic 15: NMR Flashcards
What is NMR used for?
Finding the structures of molecules
Generally, how does NMR work?
A magnetic field is applied to a sample surrounded by a source of radio waves and a radio receiver
This generates an energy change in the nuclei of atoms which can be detected
Electromagnetic energy is emitted which can be interpreted by a computer
What are the two types of NMR?
Carbon-13 NMR
Proton NMR
Why is carbon-13 used instead of carbon-12 in NMR?
Because carbon-13 has a nuclear spin and carbon-12 does not
Not all carbon-13 atoms resonate at the exact same magnetic field strength, they feel it differently depending on the functional group they are part of
What determines the chemical shift in carbon-13 NMR?
The greater the electron density around a carbon-13 atom, the smaller the magnetic field felt by the nucleus, and the lower the frequency at which it resonates
What is chemical shift measured in?
Parts per million (ppm)
Varies depending on atomic environment
What determines the size of the chemical shift?
Less shielded functional groups = higher chemical shift (i.e. lower electron density)
What is tetramethylsilane?
A compound used as a standard to compare the chemical shift of other atoms
All twelve hydrogen atoms have identical environments, so the chemical shift of each is zero
TMS is a liquid and is added to samples before the NMR spectra is run to calibrate the spectrum
Used as it is inert, non-toxic, and easy to remove from the sample
What solvents are used in carbon-13 NMR?
CDCl3 (deuterated chloroform)
Used as it is relatively inert
How does proton NMR work?
Each hydrogen atom is attached to a different functional group, so feels the magnetic field differently
The greater the electron density around the hydrogen atom, the smaller the chemical shift
Why are the chemical shift values for proton NMR smaller than those of carbon NMR?
Protons have a stronger response to magnetic fields than carbon atoms
They are also more shielded if surrounded by electron-rich areas
What determines the chemical shift in proton NMR?
The further away a hydrogen atom is from an electronegative atom, the smaller the chemical shift
What does the area under the peak on proton NMR represent?
It is proportional to the number of hydrogen atoms of each type
What is an integration trace?
The area of the peak is related to the number of hydrogen atom producing it
This can be difficult to evaluate by eye, so the instrument produces a line called an integration trace
The relative heights of the steps of this trace give the relative number of each type of hydrogen
What is spin-spin coupling?
The magnetic field applied to samples is affected by the natural magnetic field of the hydrogen atoms on the neighbouring carbon atom, causing splitting
