Homonuclear 2D NMR at 60 MHz

An NMR spectrum is produced by performing a Discrete

NMR
Application Note 7

The COSY experiment is demonstrated using ethyl crotonate.

Fourier Transform (DFT) on a series of time domain data points,Omeasured with a particular gap between each point. It does
not matter to the DFT whether the points have been collected
in “real time” or if the series of points have been indirectlyconstructed. This fact is the basis for two dimensional (2D) NMR
experiments where one dimension is collected in the usual directmanner and a second dimension is constructed in a stepwisemanner.

H 3COCH3

Figure 2: Molecular structure of ethyl crotonate showing groups thatshare mutual J-coupling. The blue circles are the ethyl group and thered circles are the crotonate group, the peaks related to these groups
are highlighted in figure 4.

Figure 1: Schematic representation of a 2D NMR experiment

The blocks associated with a 2D NMR experiment are shownin figure 1. In the first step the nuclei are excited with RFpulses to generate a non-equilibrium state. This state isallowed to evolve for a time t1 in step two, before being
subjected to further RF manipulation in step three. Finally anNMR signal is recorded. This process is repeated N times withthe value of t1 being incremented each step, such that thefinal data is an array of N-nmr signals differing only throughthe effect of the evolution in step 2. This array can be Fouriertransformed with respected to the time t2 and t1 to produce
a 2D spectrum.
COSY
Correlation Spectroscopy, or COSY as it is commonly referred
to, is an NMR experiment that correlates the chemical shiftsof spins that share a mutual J-coupling. The J-coupling is theinteraction between nuclei that is mediated through chemicalbonds. In general this coupling gets weaker as two hydrogen
nuclei are separated by an increasing numbers of bonds. Asa result, the 1H COSY spectrum correlates hydrogen nuclei
on adjacent carbons or, in the case of multiple bond carbons,the next nearest carbons. The method is most commonlyused to determine the underlying structure of the carbon
back bone in an organic molecule. A COSY spectrum should
be symmetrical about a diagonal running from the top right tobottom left (figure 3).

Figure 3: Full COSY spectrum for ethyl crotonate. All peaks in the1D spectrum show a diagonal peak in the COSY and the cross peaks(off diagonal peaks) show coupling between the particular chemicals
shifts.

All signals that appear in the 1D spectrum will show a peakalong this diagonal. The cross-peaks (off diagonal peaks) show
which hydrogens share a J-coupling through the correlation
between the two chemical shifts. In figure 4 the correlation
between the -CH3 and -CH2- 1H chemical shifts in the ethylgroup, are shown in the right hand plot. The left hand plotshows the correlation network between the two olefinic (–CH=)hydrogens and the terminal –CH3 of the crotonate group. In thisportion of the molecule all hydrogens are coupled and thus crosspeaks are seen between all three chemical shifts.
Figure 4: Expanded regions of the COSY spectrum showing theethyl region of the spectrum (below right) and the crotonate region(below). The diagonal peaks are highlighted as dashedcircles and the cross peaks are highlighted with full circles.

(a) Full COSY spectrum.

(b) Crotonate correlations from COSY spectrum.

(c) Ethyl group correlations from COSY spectrum.

visit nmr.oxinst.com/x–pulse for more information or email magres@oxinst.com

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