NMR Flashcards
WHAT CAN NMR be used to hel do
find the structures of very complex molecules
what is applies
a magnetic field is applied to a sample which is surrounded by a source of radio waves and a reciever
what does thi sgenerate
an energy change in the nuclei of the atom in the sample which is detected
what happens to the energy
electromagnetic energy is emitted which can be interpreted by a computer
what the difference between carbon 12 and 13
carbon 12 has no nucleas spin and carbon 13 does
what is the difference between the carbon 13 in a compound
they will not all resonante with the same frequency when a magnetic field is applied
why do they resonate diferently
the carbon atoms have different functional groups so feel the magnetic field differently as the nuclei is shielded by the electrons
what happens to the nuclei with more electrons round them
better shielded so smaller magnetic field felt by the nucleas and lower the resonance frequency
what does the nmr show
graph of energy absorbed against the resonant frequency
what is the chemical shift
the difference in resonating nucleas of the nucleas ti tms and that of the measured one
what is tms and what it chemical shift value
tetramethyl silane with a chemical shift value of zero
what happens when oxygen is connected to the carbon
the oxygen is electronegative so draws electrons twoards itself deshielding the carbon nucleas so it resonates more
why is tms used
it is inert, not toxic and easy to remove from the sample
why is proton nmr easier
the h1 nucleas being examine nearly all exist in this state compared to the carbon 13
what feels the magnetic field
the hydrogen which is attached to different functional groups that feel the magnetic field
what happens to nuclei with more electron around the hydrogen
more shielding from the magnetic field so smaller chemical shift
how does the chemical shift value compare to that of carbon
it is usaually smaller than that of carbon
what happens if all the hydrogen in identical enviroments
you only get one chemical shift value
what is proportional in a nmr for hydrogen
the area under the peak is proprtional to the number of hydrogen atoms of each type
what does integration trace allow us to do
identify the area of the peak and their ratio to other peaks so we can identify how many hydrogen are present in the same enviroment
what happens if you zoom into a nmr
they are split into particular patterns known as spin spin ocupling patterbs
whay does this occur
the applied magnetic field can be felt by any hydrogen atom on the neighbouring carbon atom
what does it give us information on
the number of hydrogens on the adjacent carbon n+1
what are nmr usually run in
in solutiom
what must we ensure about the solvent
must not contain any hydrogen
why cant it contain any hydrogen
as the signal from the hydrogen atom in the solution would swamp the signals from the sample
what solvent is commonly use
tetrachloromethane which contains no hydrogen or dueterium which contain h3
