NMR Spectroscopy Flashcards
(16 cards)
what creates a magnetic field
when the hydrogen nucleus spins
what causes splits in energy in protons
when a strong magnetic field is applied
half spin with the field and half against
explain the magnetic fields with even and odd numbers of protons/neutrons
even no. if protons and neutrons = magnetic field cancels
odd no. of protons OR neutrons = magnetic moment
how to flip proton between states
supply exact amount of energy
energy needed depends on external magnetic field
energy absorb depends on nucleus/enviroment/field strength
explain the functions of the spectromer
- sample is placed in a cyndrical tube between a powerful magnetic field energy
- sample wrapped in probe coil connected to radio frequency generator and receiver
- sample dissolved in solvent that doesnt affect NMR signals
how does the solvent used in the spectrometer not affect signals
it doesnt contain H atoms or does but in the form of deuterium which is not NMR active as it has an even no. of nucleons therefore doesnt spin
explain properties of low resolution nmr spectra
- each peak = different environment
- no. against peak = relative area under peak
- ratio of area under = ratio of H atoms
- position of peaks show the groups Hydrogens are in
explain properties of high resolution nmr spectra
- shows spin-spin splitting
what is TMS and why is it used
tetramethylsilane is used as a reference compound as:
- their electrons in CH are closer to H atoms than most compounds
- H nuclei are most shielded from magnetic field
- 12 H atoms are chemically equivalent
- gives a strong single peak for comparison
what is meant by chemically equivalent
H atoms are the same distance from each other, in the same environment and all appear at the same chemical shift
state different spin-spin splittings
- 1 peak = SINGLET = next to C w/ no H
- 2 peaks = DOUBLET = next to CH
- 3 peaks = TRIPLET = next to CH2
- 4 peaks = QUARTET = next to CH3
why are there different hydrogen environments
non-equivalent hydrogen atoms (resonate at different frequencies)
what does the no. of peaks show
how many protons are adjacent to a particular proton
no. of adjacent protons = n
no. of peaks = n+1
what reduces size of magnetic field
amount of electrons
factors that affect shielding of nucleus
- electron density: high electron density = nucleus shielded (low chemical shift … upfield)
- electronegativity: **more electronegative* = less energy needed for resonance (H feels more of field as its deshielded … downfield)
