15E.5 Flashcards
what is nuclear magnetic resonance spectroscopy
it is a technique used to find the structures of organic compounds. it depends on the ability of a nuclei to resonate in magnetic field
nucleons have
opposite spin
the nucleons with an even number there spin will
cancel out
the nucleons with an odd number there spin will
most of the spin will cancel out other than the odd ones, causing the nucleus to have a residual spin
residual spins cause
a magnetic field, in which the nuclei can be imagined as a tiny magnet
when magnetic nuclei is affected by external magnetic field what will happen
the nuclei will either line up, or oppose the magnetic field
as the spins of the nuclei ‘flip’ between lining up and opposing the external magnetic field
they (the nuclei) can absorb electromagnetic radiation as there is an energy difference between the two states (opposing and lining)
the nuclei of the two adjacent atoms in a molecule can
influence each other as well as there electron can have an effect, this means that the electromagnetic radiation is absorbed differently by different atoms in a molecule, and these are detected as signals at different frequencies
for an NMR graph the vertical axis is labelled as
absorption (the absorption of radio frequency energy) (no units)
for an NMR graph the horizontal axis is labelled as
chemical shift ( sometimes delta ) and measured in ppm (parts per million
for a atom to show up in the NMR it must have a
odd number of nucleons so a residual spin
what is chemical shift (of a proton or a group of protons)
it is a number (measured in ppm) that indicates the behavior of the proton in a magnetic field compared with tetramethylsilane. it can be used to identify the chemical environment of the carbon atoms or of the hydrogen atoms (protons) attached to it
what is a 13C NMR
the use of NMR to detect 13C nuclei within the molecules of a substance, in order to determine the structure
what is a proton NMR
the use of NMR to detect 1H nuclei within the molecules of a substance, in order to determine the structure
the organic compounds that are being analyzed by NMR need to be
dissolved in solvent
why cant we use all solvents for NMR
because most solvents contain carbon and hydrogen atoms that will produce a signal in the NMR graph interfering with the signals being produced by the compound being analyzed
what solvent do we use in NMR
CDCl3 and D stands for deuterium
why do we use CDCl3 as a solvent in NMR
because Deuterium is made from 2H isotope of hydrogen so it has one proton and one neutron so there spin cancels out meaning they wont have a produce a signal that would interfere with the compounds signal, and the carbon only produces one signal that can be easily removed from the spectrum
what is tetramethylsilane TMS used for
to be a reference standard that give a prominent signal since carbon and hydrogen atoms in different chemicals environments will give different signals
how does TMS look like
page 143
what is silane
SiH4
why is TMS suitable
- it has 12 hydrogen atoms, all joined in the same way in a symmetrical argument , so it produces a single strong signal
- its chemically unreactive, so it wont react with most organic compounds
each 13C and 1H atom in a compound form
a signal at a characteristic chemical shift, depending on the other atoms attached to it
What is TMS chemical shift
0 and all other atoms have values related to this
chemical shift can be thought of as
resonant frequency of an atom
