Apuntes
Nuclear Magnetic Resonance
Contenido del apunte (3)
Página 1
Página 2
Principle of NMR
Atomic nuclei with odd atomic mass or number have a spin property= having a magnetic field
They combine in two ways with or against the applied field. The two states has slightly different energy levels
Electromagnetic radiation which corresponds with the gap between the two states=flip form low energy state to a high energy one The size of the energy gap determines the frequency of the radiation required.
Each element require a particular frequency,bu they all lie in the radio part of the spectrum
Moving form high energy to low energy state releases frequency
Proton NMR
Detector receives all the signal of the hydrogen present in the molecule
Fourier transformation is used to distinguish hydrogen atoms that are belonging to different molecule environment
PARTS OF AN NMR INSTRUMENT
Magnet: a superconducting magnet housed in a cylindrical casing
Has 50 amperes that flow continuously, no resistance by the coil yet at 4K
; 4 Kelvin with a jacket of liquid helium, moving close to the walls of the cylinder is the nitrogen jacket at 77 K
The sample is in the center of the cylinder called a probe which contains the coils that sends and receives the radiofrequency signals
All the hydrogen are replaced with deuterium so that the signals would not interfere in the sample. Tetramethysilane is often added as a reference contains 12 hydrogen atoms taken as the zero of the spectrum
The sample sits around nitrogen. must be in the center of the magnetic field when put in the instrument= uniform magnetic field
autochanger=part of the instrument where the sample is placed.
Upon puttting it inthe isntrument, the instrmnt will apply several pulses of radiowaves the peak form the far right, is the reference or TMS, 0ppm The distance on the lefet of the horizontal axis is called chemical shift measured relative ot he peak of the hydrogen atoms relative to TMS. The area under each peak is proportional to the number of each type of proton
Spin-spin splitting is caused by the magnetic field of adjacent protons
n+1 rule=number of peaks
For C13, the oens that are going to change is the frequency of the signal