BIOINORGANIC CHEMISTRY A Short Course Second Edition

NUCLEAR MAGNETIC RESONANCE 107

integrate the area under the different peaks in a spectrum containing a number

of proton resonances and determine how many protons resonate at each given

frequency. Frequently, this can lead to confi rmation of a proposed molecular

structure. The second concept might have occurred to the reader when con-

sidering the phosphorus nucleus in the compound.^31 P nuclei are 100% abun-

dant and should couple to both the magnetically active platinum and hydrogen

nuclei. This fact should complicate the spin – spin couplings in Figure 3.13. This

has not taken place because the effect of the^31 P spin - 1/2 nucleus has been

removed by a technique called double irradiation or spin - decoupling. In this

technique an NMR sample is irradiated at a resonant frequency for one par-

ticular nucleus (in this case the^31 P nucleus). This irradiation causes the orienta-

tion of the nucleus to become indeterminate, and the resonance of adjacent

nuclei will not show splitting due to spin – spin coupling with the irradiated

nucleus. Thus the spectrum is simplifi ed. This technique is applied in many dif-

ferent ways in NMR spectroscopy. For instance, heteronuclear spin - decoupling

is essential for^13 C NMR spectroscopy. The large number of^13 C –^1 H couplings

in an organic molecule or ligand would make the^13 C spectrum extremely

complex if the spins were not decoupled. However, because many protons in

different electronic environments in organic molecules or ligands result in

many resonances, a range of frequencies must be used for spin - decoupling. This

has the favorable result of increasing the intensity of^13 C nuclei (1.108% abun-

dant) and the unfavorable result of not allowing integration of peaks to count

numbers of carbon atoms of specifi c types. Spin – spin coupling is discussed

3.4.6 The Nuclear Overhauser Effect (NOE),

3.4.5 Nuclear Magnetic Relaxation,

If one perturbs a physical system from equilibrium and then removes the

perturbing infl uence, the system will return to its original equilibrium condi-

tion. This does not happen instantaneously but occurs over some time, accord-

ing to the equation

()()expnn nn

t

T

−=−et e

−

(^0) ( ) (3.24)
where ( n − ne ) t is the displacement from equilibrium, ne , at time t , ( n − ne ) 0 is the
displacement from equilibrium, ne , at time zero, and T is the relaxation time.
Two types of relaxation processes are known with possibly different relax-
ation times. These are known as T 1 and T 2. Looking at Figure 3.11 , one can
imagine a 180 ° pulse that inverts the magnetization. Following the end of the
pulse, relaxation processes begin to return magnetization to its initial state.
This process is called T 1 or longitudinal relaxation because it takes place in
the direction ofB 0. If one uses a 90 ° B 1 pulse, the magnetization is moved to
thexy plane ( Mxy ) as in the rotating frame fi gure, Figure 3.11. This transverse
magnetization rotates at the nuclear Larmor frequency; and because some