W3 NMR Spectroscopy Flashcards
transitions in UV-Vis, IR and NMR
UV-Vis: high energy needed, high frequency and short wavelength
IR: less energy needed, lower frequency and longer wavelength
NMR: even less energy needed, even lower frequency and longer wavelength
which direction does magnetic field always go
north to south
definition of magnetogyric ratio
constant for each type of nucleus
higher ratio > ‘better magnet’ > generates higher magnetic field when it spins
definition of Larmor frequency
the energy supplied due to photons, that exactly matches the energy difference between two levels > absorption of energy
what type of nuclei is not observable by NMR
nuclei that have even number of protons and neutrons > do not behave as magnets
what is the magnetic ration of a 1H proton
267.513 (10^6 rad s-1 T-1)
why do we see so many peaks in an NMR spectrum if proton absorb only at one frequency
magnetic field that we apply is constant but magnetic field in sample experience by each proton is different due to their environment > different frequencies > different peaks
factors that affect frequency of absorption
- electronegativity: proton nearer to electronegative atom > more desuhielded > experience stronger magnetic field > higher frequency
- pi systems: external magnetic field make electrons circulate faster > produce secondary magnetic field directed against external magnetic field > centre of ring will have less magnetic field while outside have more magnetic field
why is there a need for a reference in NMR
resonance frequency and effectiveness of shielding proportional to magnitude of magnetic field > frequency observed based on external magnetic field > different for different labs
frequency (x-axis) has to be expressed in a way that is magnetic field independent
what is parts per million
when calculating chemical shift, shift is very small > multiply by 10^6 ppm
why is TMS/DSS used as a reference
protons of both are all equivalent and very shielded > protons produce a single absorption band that acts as a reference
both have chemical shifts of 0 ppm
definition of chemical shift
the difference in magnetic field experienced by nuclei of a molecule compared to a standard reference
what happens when we increase external magnetic field
difference between energy levels increase
smaller difference > more probability nuclei will come back down and release energy after going up > no net energy absorption > lower absorption > weaker peaks
when an NMR machine is labelled 400MHz for eg, what does it mean
the frequency of precession of the proton when they are exposed to the magnetic field of that spectrometer
it is also the frequency that corresponds to the energy required to jump from one level to another
