Experiment 6.03 Computational NMR
PTCL Undergraduate Practical Course
This collection of exercises uses computer simulations to explore the fundamental features of
NMR that make it a uniquely powerful technique for probing the properties and behaviour of
To run an experiment, click on its name (in blue). Consult the
printed notes for this experiment for instructions
on how to interact with the applets.
1. Singlet spectrum
Free induction decay and spectrum of an NMR singlet
Here you will see the FID (left) and the corresponding spectrum (right) of an imaginary
compound with a single magnetic nucleus, say a proton (1H). The FID is in the "time domain"
(seconds) and the spectrum is in the "frequency domain" (Hertz). The general form of the FID
is a damped oscillation; for its analytic form see the laboratory instructions.
2. Doublet spectrum
Free induction decay and spectrum of an NMR doublet
This exercise is similar to the previous one, but instead of a single line we see a
"doublet", i.e. two lines of equal amplitude separated by a frequency J. This is the kind
of spectrum one might see for a nucleus coupled to an adjacent proton (whose resonance is
not shown). J would then be the spin-spin (or scalar) coupling constant.
3. Chemical shifts
Identification of compounds from chemical shifts and
In simple cases, the structure of a molecule can easily be deduced from the integrals and
chemical shifts [Ch2] of the lines in its 1H NMR spectrum, provided the molecular formula
is known [p11].
4. Multiplet patterns
Multiplet patterns arising from spin-spin coupling
The interaction between nuclear spins (spin-spin coupling) in molecules in solution produces
characteristic patterns in their NMR spectra called multiplets (doublets, triplets etc.) [Ch 3].
In this exercise you are given spectra of a nucleus A coupled to n equivalent X nuclei,
whose spectra are not shown. You have to determine n and I, the spin angular
momentum quantum number of the X nuclei [pp 24, 29].
5. Spin-spin coupling
Conformational analysis using spin-spin coupling constants
The "three bond" couplings between protons on adjacent carbon atoms, e.g. in the
H-C-C-H fragment, depend on the dihedral angle between the two H-C-C planes. In particular,
trans (Q = 180 ) and gauche (Q = 60 ) interactions have significantly different spin spin
coupling constants (Jt and Jg respectively). The spectrum you are asked to interpret in this
exercise is that of the a and two b protons in a conformationally mobile amino acid or peptide.
6. Chemical exchange
Rates of conformational equilibrium from linewidths
The NMR spectra of molecules participating in chemical or conformational equilibria show effects
that depend on the rate of the forward and backward processes and the difference in resonance
frequencies of the nuclei involved [Ch 4]. This exercise deals with a classic example of such
"chemical exchange" effects: the internal rotation of the NO group in dimethylnitrosamine
7. Spin lattice relaxation
Spin-lattice relaxation times by inversion recovery
When the populations of the energy levels involved in NMR transitions are disturbed they
return to equilibrium (the Boltzmann distribution) by a process called spin-lattice relaxation,
in which energy is exchanged between the spins and their surroundings, the "lattice"
[pp 56-7]. This process is often exponential, and has a time constant called the spin-lattice
relaxation time, T1. The value of T1 for a nucleus is determined by the structure of the molecule
(e.g. the proximity to the relaxing nucleus of other magnetic nuclei), and by the rate of rotation
of the molecule in solution. The experiment most commonly used to measure T1 values is called the
inversion recovery method [p 79]. In this exercise you "perform" a series of inversion
recovery experiments on 1,3 dinitrobenzene to determine the relaxation times of its three 1H
8. Strong coupling
Spectra of strongly coupled spins
The NMR spectrum of a pair of coupled protons with different chemical shifts comprises two
doublets - but the intensities of these four NMR lines are only equal, and the doublets are only
centred at the two chemical shift positions, when the difference in resonance frequencies of the
two spins is much larger than their spin spin coupling.
9. More multiplet patterns
Spectrum of a nucleus coupled to 3 spin-1/2 nuclei
We saw examples in exercise 4 of the multiplet patterns that can arise from the coupling to groups
of equivalent nuclei. Here you can see the patterns for a nucleus coupled to up to 3 inequivalent
spin - half nuclei (e.g. protons), whose NMR signals are not shown. The variables are the three
Starting with two of the couplings equal to zero, see how the spectrum changes when a second and
then a third coupling is introduced. For different values of the three coupling constants you
should be able to see a doublet, a triplet, a quartet, a doublet of doublets, a doublet of triplets,
a triplet of doublets, a doublet of doublets of doublets and so on.
You can get some rather peculiar patters when some of the coupling constants have integral ratios,
or when the sum of two of them equals the third.
Updated February 3, 2009.