Other Instrumental Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 328

Spin-Spin Coupling

Thus far, we have not explained what caused the spilt peaks as shown in
Figure 12.11. Signals in an NMR spectrum can be a single peak (a singlet) or can be
split into two (a doublet), three (a triplet), four (a quartet), or even more peaks.
The number of peaks into which a particular proton is split is called its multiplicity.
The splitting makes interpretation of an NMR spectrum a little harder, but it provides
more structural information about the chemical bonding. We just learned that
chemical shift is derived from the density variation of electrons that circulate the
nucleus. In a significant contrast, signal splitting of a nucleus is caused by a nearby
spin-active nucleus. These splittings of signals are called spin-spin coupling, which
are derived from bond interactions between two spin-active nuclei.
The number of splitting peaks (multiplicity of signal) in 1 H-NMR spectroscopy
can be predicted by the nþ1 rule, which states that the NMR signal of a nucleus
coupled to n equivalent hydrogens will be split into a multiplet with (nþ1) lines.
For example, 1,1,2-trichloroethane (Cl 2 CHCH 2 Cl) has two types of H (hence two
groups of signals). The less shielded proton in Cl 2 CH (downfield) will have
2þ1 ¼ 3 split peaks (triplet) because two equivalent H atoms are present nearby in
CH 2 Cl group. Conversely, the more shielded protons in CH 2 Cl (upfield) will have
1þ1 ¼ 2 split peaks because there is only one H atom nearby.
It is, thus, important to keep in mind that it is not the number of protons giving
rise to a signal that determines the number of split peaks (multiplicity), instead,
it is the number of protons bonded to the immediately adjacent carbons that
determines the multiplicity. Also keep in mind that equivalent protons do not
split each other's signal. Thus, unlike 1,1,2-trichloroethane, 1,2-dichloroethane
(ClCH 2 CH 2 Cl) shows only a singlet because four protons in this molecule are
chemically equivalent.
In addition to the number of split peaks, the spacing between the lines of
splitting peaks is also important in relating NMR spectrum to chemical structure.
The spacing in the frequency unit (Hz) is called the coupling constant (denoted by
J), which determines the extent of coupling between two nuclei. Since coupling is
caused solely by the internal molecular forces, the magnitudes of J are not dependent
on the operating frequency of the NMR spectrometers. This characteristic coupling
constants (J) can then be used to infer the number and type of bonds that connect the
coupled protons as well as the geometric relationship of protons. For example, the
trans-3-chloropropenoic acid (ClCH

CHCOOH) has a coupling constant of 14 Hz,
whereas the cis-3-chloropropenoic acid has a coupling constant of 9 Hz.
The above discussions are largely based on 1 H-NMR. Since many of these
principles are applicable to 13 C-NMR, it is essential for us to summarize only
the unique features of 13 C-NMR. First, the signals in 13 C-NMR are not normally
split by neighboring carbons and the interpretation of 13 C-NMR spectra is easier
than 1 H-NMR spectra. The reason for the lack of splitting is because the probability
of two

13 C carbons being next

to each other in a molecule is very small

13 C-NMR is less sensitive because of the low

(1 : 1%1 : 1%¼ 0 : 0121%). Second,

13 C and its gyromagnetic constant (Table 12.1). To alleviate this

abundance of