QMGreensite_merged

Section 3.4.4), then the rf pulses in the spin echo sequence can be applied
toonly oneof the scalar coupled spins (theIspins in the following
example). For example, the spin echo sequence can be applied selectively
to the proton spins in an^1 H–^15 N scalar coupled spin system. Again,I
spin chemical shift is refocused and can be ignored. As before, following
the 90xpulse on theIspin, evolution occurs as follows:

Iy^2 JISIzSzt)
IycosðJIStÞþ 2 IxSzsinðJIStÞ


!

Ix

IycosðJIStÞþ 2 IxSzsinðJIStÞ: ½ 2 : 280 Š

Only theIyterm is inverted by the 180xpulse; theSspin is unaffected.
The second delay generates

IycosðJIStÞþ 2 IxSzsinðJIStÞ

)

2 JISIzSzt

Iycos^2 ðJIStÞ
 2 IxSzsinðJIStÞcosðJIStÞ

þ 2 IxSzcosðJIStÞsinðJIStÞþIysin^2 ðJIStÞ, ½ 2 : 281 Š

which reduces to

IycosðJIStÞþ 2 IxSzsinðJIStÞ )

2 JISIzSzt

Iy: ½ 2 : 282 Š

So, for the heteronuclear spin echo,

Iy )


t
Ix
t

Iy ½ 2 : 283 Š

andboththe chemical shift and the heteronuclear coupling are refocused.
In essence, theSspins have been decoupled from theIspins by use of the
echo sequence.
The scalar coupling interaction evolves over the entire duration of
the spin echo sequence in ahomonuclear spin echobecause the 180 8
echo pulse affects both theIand theSspins equally. By analogy, the
heteronuclear scalar coupling interaction evolves over the duration of a
spin echo sequence if 1808 pulses are applied to both the I and the S spins
simultaneously(usually using two rf transmitter channels). The overall
effect of the –t– 180xðÞI,S –t– pulse sequence on initial
Iymagnetiza-
tion is given by

Iy )

t
Ix,Sx
t

Iycosð 2 JIStÞ
 2 IxSzsinð 2 JIStÞ: ½ 2 : 284 Š

2.7.7.2 Insensitive Nuclei Enhanced by Polarization Transfer Pulse
sequence elements can be combined to produce more complex sequences

96 CHAPTER 2 THEORETICALDESCRIPTION OFNMR SPECTROSCOPY