6.3.2 Spectroscopy Flashcards
what does NMR spectroscopy stand for
nuclear magnetic resonance
what does NMR spectroscopy detect
- the nuclear spins of the C13 and H1 isotopes
- have an odd number of nucleons (protons and neutrons)
- H1 is just a proton, so proton spectroscopy
give an overview of how an NMR spectrometer works
- nucleus absorbs energy from strong magnetic field
- and radio frequency radiation
- radio frequency radiation has less energy than IR radiation, so needs a strong magnetic field to be able to detect it
- this makes the nucleus rapidly flip between 2 spin states (the resonance)
- same technique used in MRI machines
what is chemical shift, and on what scale is it measured
the frequency shift of the electrons surrounding the atomic radius
- measured on a chemical shift scale δ
- units ppm (parts per million)
what is TMS
- tetramethylsilane
- (CH3)4Si
- used as the standard reference chemical
- all chemical shifts are measured against this
- and has a δ value of 0 ppm
- (all referenced against TMS at δ=0ppm)
what is the amount of chemical shift dependent on
- determined by the chemical environment
- means that absorption peaks at different chemical shifts for certain compounds
why is NMR useful
can find the C/H arrangement of a compound without carrying out chemical tests, or needing to destroy the compound
what needs to happen to a sample before NMR spectroscopy can take place
must be dissolved in a solvent first
- (and whole sample is placed inside the NMR spectrometer with TMS too)
why is using a solvent in NMR tricky
- most common solvents contain C/H too
- so will produce C13 and H1 signals
what type of solvent is used in NMR spectroscopy
- a deuterated solvent:
- the H1 atoms are replaced with H2/D deuterium atoms
- so no signals are produced in the frequency range of H1/C13
- most commonly use CDCl3 (deuterated trichloromethane)
- still produced a peak in the C13 NMR spectrum, but is filtered out before displaying
what information can C13 NMR tell you
- the number of different carbon environments present (via the number of peaks)
- the type of carbon environments present (via the chemical shifts)
what do chemical shift values in C13 NMR tell you
- the type of chemical environments
- start with C-C, to C-EN atom, to C=C/ring, to C=O
why may a C13 NMR produce speak outside the range
- different solvent used
- different concentrations used
- different substituents used
what is the chemical shift of a C atom determined by
- the position of the atom within the molecule
- so C-atoms bonded to different atoms/groups will have different environments and absorb at different chemical shifts
- and if 2 carbon atoms are positioned symmetrically within a molecule, they will be equivalent and have the SAME chemical environment ( so absorb radiation at the same shift and contribute the same peak)
how many chemical environments does propanal have
3
- one next to CH2COH group
- one between CH3 and CHO group
- one next to CH2H3 group
how many chemical environments does propanone have
2
- one as a part of C=O
- 2 symmetrical carbons, in a CH3 group and next to a C=O-CH3
what would the C13 NMR of propanal look like
- 3 peaks for 3 environments
- one at C=O
- and 2 in the C-C range (one shifted more to the left as closer to the EN O atom)
what would the C13 NMR of propanone look like
- 2 peaks for the 2 environments
- one at C=O
- and one at C-C (relatively proportional, but doesn’t really matter)
what are key notes of C13 NMR
- symmetry is present based on the group, and what it is bonded too
- even if it doesn’t look symmetrical, remember how it would look in the 3D structure, and that bonds can rotate
- remember to be careful as to which C you are looking at, IN BOLD
- even if you are comparing isomers, may still be able to distinguish between due to the number of peaks and chemical shifts of the NMR
what 4 things does proton NMR tell you
1) the number of different proton environments (via the number of peaks)
2) the types of proton environments (via chemical shift)
3) the relative numbers of each type of proton (via the integration traces/ratio numbers of the relative peak areas)
4) the number of non-equivalent protons adjacent to the given proton (from spin-spin splitting patterns)
why may the values of proton NMR go outside the given range
- concentration
- solvent
- substituents
how do protons produce the same and different peaks on NMR
- protons with the same chemical environment absorb at the same chemical shift value [are equivalent]
- and increase the size of the peak
- protons of different types have different chemical environments and absorb at different chemical shifts [non-equivalent]
how do you tell whether protons are equivalent or not in proton NMR
- look for planes of symmetry
how many proton environments are there in butanoic acid
- 4
- as even the 2 CH2 groups are bonded to 2 different groups, and there are no lines of symmetry
