7. NMR Flashcards
why does the magnetic field around a proton have spin on it?
because it has a charge
what kind of interaction occurs when a proton is placed in an external magnetic field?
there is a quantized interaction.
look at image properly
what does the external magnetic field do?
it puts protons into a higher energy state which make them align against the external magnetic field
what are the two spin states that protons can be in and how do they change states?
alpha state - lower energy
beta state - high energy
can change when a proton absorbs energy
the proton is in resonance wit the applied mangetic field - this is why its NMR
why is the strength of the magnetic field linked closely to the instensity of the NMR signal?
becuase of the amount of energy that is required to absorb to gain energy.
high external magentic field = lots of absorbing energy to change spin states
what is chemical shift?
The resonant frequency of a nucleus relative to a standard in a magnetic field.
what is the physical origin of chemical shift in NMR
TMS is a very shielded molecule. therefore has a low frequency on NMR (0). TMS therefore is our reference for NMR. The shielding constant becamse the origin of the chemical shift
what is the NMR experiment?
a very high and homogenous external magentic field is generated from supercoducting magnets
short pulses from the radio frequency (RF) from the RF coil makes a net magnetisation of the sample at 90 degrees of the external magnetic field
what happens after the radio frequency pulses have stopped the net magnetisation (M) three points
the net magnetisation:
- decays back to the original position
- rotates about the direction of the external magentic field
- makes an electrical current in the coil (this is measured)
what would happen to the chemical shift if CI is replaced by CF in NMR?
chemical shift would increase
what occurs to the electron density, shielding and chemical shift when an electronegative ion/atom is used?
- decreased electron density around H
- decreased shielding
- chemical shift increases
what is the coupling constant?
its the measurement of interactions between a pair of protons. this is when there are 2 hydrogens that are not chemically equivalent but are in close proximity that they effect each other peaks
what is the coupling constant controlled by? 1
the geomtetry of the oribatls between the coupling nuclei
how do we know what the intensity of the peaks will be with coupled protons?
depends on how many protons there is. its n+1 where n is the number of protons and +1 for the amount of lines. use the binomial expression of pascals triagle
what happens if theres ethanol in water? why dont they lead to splitting?
in an H2O solution, the H is exchanged with OH faster than the timescale of the NMR, this means coupling is not detected. if water was removed coupling could be detected
what changes in the net magentisation in the NMR experiment? 3 things
the net magentisation decays back to the original position
rotates around B0
induces an electrical current
what is the difference between spin - spin and spin - lattice relaxation?
spin - spin: transfer of energy to surroundings
spin lattice: recovering as energy is transferred from one equivalent nucelus to another
