1H NMR Spectra Flashcards
chemical shifts are determined by what
the degree to which a proton is shielded/deshielded by the surrounding nuclei
the more shielded a proton…
the lower its δ
how is a proton deshielded
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 effect does deshielding a proton have
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 values.
Exact frequency emitted by a nucleus depends on what
its chemical environment
methyl group protons have chemical shifts of what
0.80-1.4ppm
aromatic protons have chemical shifts of what
δ ~ 7-8 ppm
frequencies differ for each nucleus unless what
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, δ
what are chemical shifts defined as
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)
see powerpoint for
chemical shift equation
see powerpoint for
examples of chemical shifts and spectrums
what is an advantage of NMR
spin-spin coupling
what is spin-spin coupling
The spin of protons adjacent to a particular proton split the original proton signal
see powerpoint for
spin-spin coupling example
splitting in spin-spin coupling follow what tradition
Pascal’s Triangle
see powerpoint for
multiplets and pascals triangle table
what is a neighbouring molecule classed as
no more than three bonds away
n-equivalent neighbouring hydrogens will split a 1H signal into what
(n + 1) Pascal pattern
what happens if the neighbouring protons are not equivalent
n must be equivalent neighbouring hydrogens to give rise to a Pascal splitting pattern. If the neighbouring protons are not equivalent, then you will see a complex pattern (known as a complex multiplet).
splitting of alcohol hydrogens
alcohol hydrogens (–OH) usually 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.
what happens when spectra of alcohols are acquired in aqueous solution?
we do not see the –OH function proton signal because of exchange with NMR-inactive deuterium in 10% (v/v) D2O added.
n+1 rule
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!
see powerpoint for
example of n+1 rule
give an application of NMR
quantitative drug analysis
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 an equation (see powerpoint for equation)
see powerpoint for
NMR case study
