QMGreensite_merged

designed to perform specific tasks. An important experiment that takes
advantage of the basic schemes is the INEPT (Insensitive Nuclei
Enhanced by Polarization Transfer) sequence ( 17 ). The INEPT sequence
is a crucial component of many multidimensional NMR experiments.
The aim of the INEPT sequence is to transfer magnetization from a
sensitive nucleus with a high magnetogyric ratio (usually protons) to
a less sensitive nucleus with a lower magnetogyric ratio (e.g., nitrogen
or carbon) by means of the scalar coupling interaction. By doing this, the
detected signal from the heteronucleus will be increased. The INEPT
sequence is written:

Ispin: 90 x
t
 180 x
t
 90 y,

Sspin: 180 x 90 x
acquire:

½ 2 : 285 Š

Up to the final pair of 90 8 pulses, the sequence is a spin echo in which
both spins have been affected by 180xpulses, so that chemical shift is
refocused during the echo, but scalar coupling evolves fully. Beginning
with equilibrium magnetizationKIIz, in whichKI¼h!I/(4kBT) [2.126],

KIIz )



2 Ix
t
ðIxþSxÞ
t K

IfIycosð^2 JIStÞ
^2 IxSzsinð^2 JIStÞg: ½^2 :^286 Š

A90ypulse is applied to theIspin, and a 90xpulse is applied to the
Sspin,

KIfIycosð 2 JIStÞ
 2 IxSzsinð 2 JIStÞg

)

 2 ðÞIyþSx

KIfIycosð 2 JIStÞ
 2 IzSysinð 2 JIStÞg: ½ 2 : 287 Š

If the delay t¼1/(4JIS), then the final signal is given by

[h!I/(4kBT)]2IzSy. The antiphase I spin magnetization has been
transferred to theSspin. The antiphase term is scaled by a factor of
h!I/(4kBT). This is an important advantage over simply recording a spec-
trum on theSspin following the application of a simple 90 8 pulse. In this
case, after a 90xpulse is applied to theSspin, the observable magnetization
is given by
[h!S/(4kBT)]Sy, which is an in-phase doublet. The intensity
ratio between the INEPT and conventional experiment is given by

INEPT

Conventional

¼

(^) I
(^) S
½ 2 : 288 Š
The advantage of performing the INEPT experiment becomes enormous
as the magnetogyric ratio of theSspin decreases. INEPT procedures
are used with great effect in multidimensional heteronuclear NMR
2.7 PRODUCTOPERATORFORMALISM 97