Spin Locking: Total Correlation
Spectroscopy (TOCSY)NMRApplication Note 8COSY

would expect to see a cross peak between hydrogen nuclei

In an NMR experiment, magnetisations perpendicular to thein the following pairs of positions, (A, B), (B,C) and (C,D).

static magnetic field B0 will rotate about the B0 field at its

In contrast, the TOCSY spectrum of these hydrogens would

characteristic Larmor frequency, often referred to as chemical

show cross peaks between all pairs of nuclei (see Figure 1).

shift precession. In addition to the chemical shift precession
the magnetisations will also evolve under the influence of themutual coupling between the spins, the scalar or J-coupling.
The chemical shift evolution accounts for the position of aparticular resonance in the spectrum, while the J-couplingis the source of the peak splitting patterns. When theJ-couplings are small in comparison to the difference between
the resonance frequencies of the coupled spins, the evolution
is dominated by the chemical shift. This may be true because
the B0 field is high or because the pair of spins are separated
in the molecule by several bonds producing a small coupling.
In some experiments it is desirable to suppress the chemical
shift evolution and allow the spin system to evolve under theJ-coupling only. This is often referred to as isotropic mixing.
The suppression of the chemical shift evolution can beachieved by applying a strong RF pulse along a chosen
direction keeping the magnetisations locked in alignment
with the field. This is referred to as spin locking and the RF
field as the spin lock field. There are a number of methods
exploiting composite pulses designed to provide a spin lockwhere imperfections in the RF-pulse can be continuallyrefocused to provide a clean spin lock. The two most common
are the MLEV-17 [Bax, A and Davis, DG, Journal of Magnetic
Resonance 65, 355-360, 1985] and DIPSI [Shaka, AJ, Lee, CJ,and Pines, A, Journal of Magnetic Resonance 77(2), 274-293,1988].
TOCSY
Total correlation spectroscopy TOCSY, is a homonuclear 2D
experiment similar to COSY (see application note 7) in which
the J-coupling between two hydrogen nuclei manifests asa cross peak in the spectrum. Unlike COSY, however, thedetection of the coupled spins is not limited to nearest
neighbours. The TOCSY experiment exploits the isotropic
mixing condition during spin locking to produce cross peaks
between all hydrogen nuclei that form part of an unbroken
chain of coupled spins.

Figure 1: Comparison of 2D COSY and 2D TOCSY spectra for ahypothetical molecule in which hydrogen A is coupled to hydrogen B,
which is coupled to C, which in turn, is coupled to D. Lines are drawn
to connect the peaks below the diagonal, illustrating the throughbond connectivity shown by each spectrum; heavier lines indicate
couplings shown in both spectra, lighter lines indicate connectivity
shown in the TOCSY spectrum but not the COSY.

To further illustrate this concept, we consider the molecule
trans-2-hexenoic acid shown in figure 2 (top). The 1D 1H
NMR spectrum (figure 2 bottom) of this molecule shows six
resonances, five which have been labelled A to E corresponding
to hydrogens attached to the carbon backbone of the moleculeand a sixth which corresponds to the –OH of the carboxylic
acid group. The resonances A to E all show peak splitting
corresponding to the manner in which they are coupled to theneighbouring hydrogens. The COSY spectrum (figure 3) reveals
the coupling between A,B and B,C and C,D and D,E. It also
shows the coupling between C and E, which might be expected
because of the double bond between D and E resulting in astronger coupling between C and E than D and B for instance.
The TOCSY spectrum (figure 4) shows the coupling between all
pairs of hydrogen nuclei confirming the fact that resonances Ato E comprise a single unbroken chain of coupled spins. If wecompare figure 3 to the TOCSY spectrum of ethyl crotonate
in figure 5 we can see that the ethyl crotonate comprises twoseparate chains of spins, the ethyl group, resonances A andC and the crotonyl group, resonances B, D and E. The lackof cross peaks between the two sets of spins confirms thepresence of a nucle which effectively breaks the J-coupling
chain. In this case theConsider a chain of four hydrogens, labelled A, B, C andoxygen from theD, where hydrogen A is coupled to B which is coupled to C

ester coupling.

which, in turn, is coupled to D. In a COSY experiment weEOB

CH3

HODCA

Figure 4:. Spin lock was implemented using MLEV-17

Figure 2: The structure (top) and 1D 1H spectrum (bottom) oftrans-2-hexenoic acid. Hydrogen positions on the carbon backbone
are labelled A to E to identify the appropriate resonance in thespectrum. The unlabelled singlet ay 12.5 ppm corresponds to the–OH group of the carboxylic acid.

Figure 5: TOCSY spectrumof ethyl crotonate.

Figure 3: COSY spectrum of trans-2-hexenoic acidvisit nmr.oxinst.com/x–pulse for more information or email magres@oxinst.com

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