106 INSTRUMENTAL METHODS
The number of nuclei in each letter category is indicated by a subscript. Using
the proton as an example, CH 3 CH 2 Cl (chloroethane) is an example of an A 3 X 2
system while CH 2 CHCl (vinyl chloride) is an example of an ABX system. When
chemical shifts differences are large, coupling between protons on adjacent
atoms will follow the simplen + 1 multiplicity rule for the number of peaks in
a multiplet (the general rule is 2 nI + 1, where I is the nuclear spin). This is named
a fi rst - order pattern. The ABX system is almost fi rst - order, but A a M b X x or
Aa B b C c systems exhibit complex spin - coupling multiplet patterns.
An example of spin – spin coupling between the^195 Pt nucleus ( I = 1/2, abun-
dance = 33.8%) and the proton (^1 H, I = 1/2, abundance = 99.985%) is shown
schematically in Figure 3.13 for the complex trans - MeBrPt(PMe 2 R) 2 (where
R is a 2,4 - dimethoxyphenyl group).^21 The two major methyl proton resonances
are indicated and connected to the responsible peaks. The height of the central
downfi eld methyl resonance indicates that it corresponds to the protons of
four methyl groups attached to phosphorus, and thence to the magnetically
inactive platinum nucleus (66.2% abundant). The two smaller satellite peaks
at one - quarter intensity on either side of the major downfi eld peak originate
from the same methyl protons coupled to the magnetically active^195 Pt nucleus.
The longer coupling path from^195 Pt through^31 P to^1 H results in a weaker,
smaller coupling constant (a so - called^3 J coupling) when compared to the
upfi eld pattern for methyl protons of the methyl group directly attached to
platinum. The 1 : 4 : 1 pattern for the upfi eld peak again indicates that the cou-
pling corresponds to the 33.8% abundant platinum nucleus (^195 Pt). The upfi eld
resonance corresponds to the protons of the methyl group directly attached
to the platinum atom, and thus the satellite peaks exhibit an appreciably
stronger coupling and consequently a largerJ value.
A more complete discussion of spin – spin coupling may be found in Chapter
3 of reference 21 and many instrumental chemistry texts.^2
3.4.4 Techniques of Spectral Integration and Spin – Spin Decoupling
Two other important NMR concepts and related NMR techniques are intro-
duced in Figure 3.13. The fi rst of these is that the size of the peak is propor-
tional to the number of protons resonating at a given frequency. One can
Figure 3.13 1 H spectrum of the complex trans - MeBrPt(PMe 2 R) 2. (Adapted with per-
mission of Nelson Thornes Ltd. from Figure 3.13 of reference 21 .)
PMe 2 R
RMe 2 P MeBr
Pt