experiments. An additional advantage of the INEPT experiment,
sometimes overlooked, is that the repetition rate of the experiment is
set by the relaxation time constants of theIspin rather than theSspin.
Typically, theIspin is proton, and the time constants can be notably
shorter than the relaxation time constants are for theS spins (see
Chapter 5).
2.7.7.3 Refocused INEPT NMR spectroscopy is a relatively
insensitive technique because, as has been noted in Chapter 1
(Section 1.1), the differences in populations between stationary states
of a nuclear spin are very small numbers. Maximizing the sensitivity of
NMR experiments consequently is a major concern. The amplitudes
of the resonance signals in a scalar coupled heteronuclear spin system
can be increased dramatically bydecouplingthe spins involved in the
scalar coupling interaction. Decoupling reduces the effect of the scalar
coupling constant with the result that the signal normally observed as a
multiplet is collapsed into a singlet resonance at the Larmor frequency
of the unperturbed spin. Sensitivity is increased because the amplitude of
the singlet is given by the sum of the amplitudes of the multiplet
components. As will be discussed in Chapter 3 (Section 3.5), decoupling
can be achieved by the application of a suitable rf field on one of the
spins in a heteronuclear scalar coupled spin system.
As the following example indicates, increased sensitivity is not
obtained necessarily by application of a decoupling field to an arbitrary
coherence. If a decoupling field is applied to theIspins during detection
of theSspins following the INEPT sequence introduced in the previous
section, the resonance signal disappears completely. Following the
INEPT sequence, the density operator is proportional to 2IzSy.
Decoupling prevents evolution of this operator into observable
in-phase single-quantumSxcoherence under the influence of the scalar
coupling Hamiltonian (Section 2.7.4). Viewed another way, the 2IzSy
operator represents an antiphase doublet, in which one multiplet
component of the doublet is positive and the other component is
negative. Collapsing the doublet by decoupling theIspins results in the
mutual cancellation of the doublet components of opposite sign.
Constructive interference between multiplet components is obtained
only if the decoupling field is applied to an in-phase operator (with
respect to the decoupled spin).
Re-examination of the INEPT experiment indicates that the
antiphase coherence 2IzSycan be converted into in-phase coherence by
an appropriate extension to the INEPT pulse sequence. The ensuing
98 CHAPTER 2 THEORETICALDESCRIPTION OFNMR SPECTROSCOPY