BIOINORGANIC CHEMISTRY A Short Course Second Edition

NUCLEAR MAGNETIC RESONANCE 113

timet 1 the magnetization separates into two pairs of vectors due to the chemi-
cal shift difference, with the separation in each pair being due to the coupling
interaction. The second 90 ° pulse swings the magnetization into the yz plane,
where it starts to precess around thez axis. If the vectors were in the right half
of thexy plane, they are swung into the − z direction; if they were in the left
half of thexy plane, they are swung into the + z direction. Inversion of the
components causes transfer of polarization, and the signal of either doublet
will be a function oft 1. For each value of t 1 the FID will contain the four fre-
quencies of the two AX doublets distorted in both intensity and phase. The
FIDs are collected at a number oft 1 values, and then they are transformed to
yield a stack of spectra with different degrees of distortion. These are then
transformed at each frequency to obtain a two - dimensional (2D) map. Where
there is no correlation between spins, the frequency is the same in both dimen-
sions. These signals appear on a diagonal plot, which is the normal 1D spec-
trum. Where there is a correlation, the chemical shift and coupling constant
mix in each resonance and a signal (peak) appears off the diagonal. The off -
diagonal peaks (cross peaks) appear at the intersection of the A and X chemi-
cal shift resonances. Instantaneously, one can identify two peaks in the 1D
spectrum that correlate to each other, that exhibit spin – spin coupling, and that
are therefore located next to each other.
The heteronuclear COSY experiment investigates the connectivities
imparted by coupling paths between two different nuclei, most commonly
those between^1 H and^13 C. The experiment is facilitated by the fact that one -
bond CH coupling constants are much larger than the two or three bond (CCH
or CCCH) coupling constants. Decoupling of protons from^13 C is accomplished
in the usual way by broad - band decoupling during the t 2 period while accu-
mulating the^13 C FID. Other, more complex pulse sequences are capable of
decoupling the proton resonances as well. See Figure 3.16 for a simplifi ed

Figure 3.16 Pulse sequence used to produce a^1 H –^13 C heteronuclear COSY spectrum.
(Adapted with permission of Nelson Thornes Ltd. from Figure 8.9 of reference 21 .)

t 1 /2 t 1 /2 Decouple

during t 2

proton

frequency

carbon

frequency

(x)

90° pulse 90° pulse

90° pulse

(x)

180° pulse

FID

collected

t 2

Δ 1 Δ 2