NMR Flashcards
chemical shifts are explicitly determined by what
the degree to which a proton is shielded/deshielded by the surrounding nuclei
the more shielded a proton is
the lower its δ
what desheilds the proton
if a proton is attached to an electron withdrawing system such as an aromatic, a field opposing the applied magnetic field is created, which deshields the proton.
what is the effect of deshielding the proton
effect of increasing the δ value of the proton adjacent to an aromatic function, i.e. it will resonate at higher frequency (also known as lower field), i.e. at a higher chemical shift value.
frequency emitted by nuclear depends on what
its chemical environment
chemical shift values are..
very sensitive; methyl group protons have chemical shifts of 0.80-1.4 ppm, whereas aromatic protons are observed at δ ~ 7-8 ppm
frequencies
differ for each nucleus, unless they are chemically equivalent and in identical molecular environments i.e. tetramethylsilane, TMS, or its water-soluble analogue, tetra- deuterated trimethylsilylpropionate (TSP)
resonance frequencies are converted to what
chemical shifts
what do chemical shift values allow
results from different experiments to be readily compared – can be thought of as a ‘normalisation’
chemical shift definition
the resonant frequency of a sample compared to that of a reference (tetramethylsilane usually used as has a δ (chemical shift) values of 0.00 ppm)
absorbed frequency measured in
Hz
spectrometer frequency measured in
MHz meaning there is a million fold difference in frequencies here hence the term ‘parts per million (PPM)’
chemical shift (reported as parts per million shift downfield from the TMS standard) equation
chemical shift = ((frequency of absorbed electromagnetic radiation by sample nucleus in Hz)-(frequency of absorbed electromagnetic radiation by TMS standard in HZ))/Spectrometer frequency in MHz(this accounts for magnetic strength)
examples of chemical shifts
look at ppt
1H spectrum of methylacetate
- The 1H NMR spectrum of methylacetate has two signals
- Signal at ~ 3.7 ppm deshielded more in view of electronegative groups adjacent (-O-CH3 function)
- Signal at ~ 2.1 ppm attributable to H3C-CO- function.
advantages spin spin coupling of ethyl acetate
Look on ppt to see graphs
• The spin of protons adjacent to a particular proton split the original proton signal
• The blue protons from the r.h.s.
-CH3 function on the ester are split three ways by the two red protons:
• For option 1, there are two equivalent alignments, where the effects of the red protons cancel each other out and don’t perturb the δ value of the original signal. This leads to the central line of the blue triplet with a 2 x (double) intensity
• Options 2 and 3 result in an equal and opposite split, hence the triplet observed
• The red protons are split into a quartet by the alcoholic ester terminal-CH3 protons
splittings all follow what?
pascals triangle (on ppt)
complications spin spin coupling of ethyl acetate
- The l.h.s. methyl function protons remain the ‘expected’ signal we observed for methyl acetate, since they are isolated from adjacent (r.h.s.) protons by the ester group
Pascal’s triangle
determines the splitting pattern of each peak.
In general, n-equivalent neighboring hydrogens will split a 1H signal into an (n + 1) Pascal pattern.
“neighbouring” – no more than three bonds away
look at ppt
N must be what to give rise to a pascal splitting pattern
equivalent neighbouring hydrogens
if the neighbouring hydrogen s are not equivalent then..
then you will see a complex pattern (known as a complex multiplet).
splitting pattern of OH
do not split neighboring hydrogen signals nor is it split. It is normally a broad singlet of relative integration 1 between 1 – 5.5 ppm (variable) when spectra are acquired in deuterated organic solvents.
why don’t we see the OH function when spectra of alcohol are acquired in aqueous solution
because of exchange with NMR-inactive deuterium in 10% (v/v) D2O added.
spin spin coupling
focusing on one group of magnetically-equivalent 1H nuclei in a particular functional group – Imagine that you are sitting on the carbon atom involved, i.e. a –CH3, -CH2 or –CH function.
- All you need do now is count the number of adjacent nuclei!
Examples of spin spin coupling
look at ppt.
NMR applications- quantitative drug analysis
Drugs can be quickly quantified by measuring suitable protons such as those in methyl groups – usually against the intense signal of those in t-butanol using the following equation: look at ppt
