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Two-Dimensional (2D) NMR
Spectroscopy
INTRODUCTION
The developments in pulse technology since the 1980s form the basis of two-dimensional
(2D) NMR spectroscopy [110, 114]. The two important parameters that are obtained
from the NMR spectra are chemical shift and the spin-spin coupling constant. These
parameters sometimes cannot be obtained from plex spectra. Therefore, different
measurement techniques are needed. With help from the new techniques discovered
by chemists, it is now possible to simplify the interpretation of NMR spectra of very
complex molecules. In 1991, the Nobel Prize was given to the Swiss chemist Ernst for
his studies within this subject (the second Nobel Prize to be won within the field of
NMR). Let us first answer the question 'What are the new facilities brought by 2D NMR
spectroscopy' and move onward then to the theory of 2D spectroscopy and spectrum
analysis.
(1) The spectral parameters needed for spectrum interpretation are chemical shifts and
spin-spin coupling constants as mentioned above. From time to time, it is possible
that these parameters cannot be extracted directly from the spectra even in the
measurements with higher ic field instruments, /-resolved spectroscopy
offers a new way to resolve even highly overlapped signals into readily interpretable
signals to observe chemical shifts and coupling constants on different axes.
(2) One of the useful methods applied in the structure determination plex
molecules is the double resonance experiment (see Chapter 7). The double resonance
experiment determines the coupling relation between the protons in a given mole•
cule. This technique can be easily applied to well-resolved systems. However, it is
difficult to obtain information plex systems. Through the use of homonuclear
correlation spectroscopy (COSY), one can easily obtain information concerning
the spin-coupled systems.
(3) The measurement of carbon nuclei re