NMR spec
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what does a nucleus with spin generate (eg 1H and 13C)
a small magnetic field so when a nucleus with spin is placed in a magnetic field, the small magnetic field generated by the nuclear spin can be aligned with or against the magnetic field
what does the small difference in energy between the alignments of the big and small magnetic field alignments in NMR mean?
corresponds to the energy of the radiowaves consequently, if radiowaves are passed through the substance, some frequencies of radiowaves are absorbed to the flip the nucleus from one spin direction to another
what is NMR useful in?
medicine and MRI machines are a type of NMR machine
how is NMR done? ie what happens to sample?
the sample is usually dissolved in a solvent in a long thin tube and placed inside the machine where it is bombarded with radiowaves in a strong magnetic field (the stronger the magnetic field the better the resolution of the signals)
what solvent is used in NMR spec and why?
solvents have to have no 1H atoms because they would appear in the spectrum often the solvent used contains deuterium (D or 2H) as D atoms have no nuclear spin CDCl3 or CCl4 used these are also volatile and can be evaporated off easily
what is used to calibrate the spectrum and what does it provide?
small quantity of tetramethylsilane (TMS) is added as this produces a signal providing an internal standard to which other peaks are compared (it appears at 0ppm)
why is TMS used as a reference standard?
-it only gives one signal at 0ppm -it is non-toxic -it is inert -it has a low boiling point so can easily be evaporated off (volatile) -it gives a signal which is further right than most of the signals from organic compounds
in 1H NMR, what does the chemical shift value depend on?
what atoms/groups are near the H the closer the H is to electronegative atoms, e.g. O and Cl, the greater the shift
what does each signal represent on 1H NMR?
each set of equivalent H atoms
what does the area of each signal represent in 1H NMR?
it is proportional to the number of equivalent H atoms it represents it isn’t the height that matters but the overall area the signal covers
what does the number of lines a signal is split into represent in 1H NMR?
gives info about neighbouring H atoms- whether they are equivalent and how many H atoms the signal for H atoms is only split by inequivalent Hs- so if the H atoms on the next atom are equivalent they do not cause splitting
what is the equation for number of peaks in proton NMR?
number of inequivalent H atoms on adjacent C atoms + 1
what is the order for the number of peaks in proton NMR?
singlet doublet triplet quartet multiplet (don’t need to be able to interpret)
what should be noted about the H atom of an OH group in proton NMR?
rarely causes splitting and sometimes appears as a broad hump
what does the chemical shift value give us information about?
the chemical environments of the H atoms in the molecule
what does a triplet and quartet usually suggest is present?
ethyl group (CH3-CH2) and the atom joined to the other side of the CH2 cant have H’s on
what does two triplets usually represent?
CH2-CH2 not necessarily next to each other the two CH2 groups must be inequivalent otherwise they would just produce one signal atoms joined on either side of the CH2’s cannot have any H’s on
what does a doublet and quartet indicate?
CH3-CH atom joined to CH3 cannot have any H’s on it
what does a singlet indicate?
a CH3 on the end of the structure and what it is attached to can’t have any H’s OR an OH on an alcohol or carboxylic acid
what are the two key differences between 1H NMR and C13 NMR?
in C13 there is no splitting in C13 there is no link between the area of signal and number of C13 atoms
the chemical shift value for TMS is 0ppm, explain the role of electron shielding in determining this reference point?
-shielding is the effect where electrons around a nucleus reduce the magnetic field that it experiences. -the protons in TMS are strongly shielded -this causes the signal to be upfield
explain, in terms of interactions between protons, why the signals in the HNMR spectrum of chloromethane are split into multiplets?
the signal is split by the magnetic field/ spin of protons on an adjacent carbon atom this is called spin-spin splitting (coupling)
describe why different protons within the same molecule often require different frequencies of radiation to achieve resonance?
electrons surrounding a nucleus shield it from an external magnetic field the degree of this shielding depends on the chemical environment so different frequencies are needed for resonance.
explain why CHCl3 is an unsuitable solvent in HNMR and suggest a more appropriate solvent?
-it contains 1H atoms/ protons -these would produce their own peak in the NMR spectrum -which could obstruct/ hide results from the sample -suitable solvent includes CCl4 as no 1H atoms/ protons