Page 489 - Carrahers_Polymer_Chemistry,_Eighth_Edition

452 Carraher’s Polymer Chemistry

isotope has a distinct magnetogyric ratio that varies a little with the particular chemical environ-
ment in which they are placed.
While NMR has been a strong characterization tool for polymers for many years, it has increased
in its usefulness because of continually improved instrumentation and techniques. When a nucleus
is subjected to a magnetic field, two phenomenon are observed—Zeeman splitting and nuclear

precession. Zeeman splitting creates 2I + 1 magnetic energy states, where I is the spin quantum
number. When the atomic mass and atomic number are even numbers, I = 0 so that these nuclei are
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unable to have multiple energy levels when exposed to a magnetic fi eld. Thus, C, which has both
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an even atomic number and atomic mass is NMR inactive, whereas C, which has an uneven atomic
mass, is NMR active. Nuclear precession is the motion of a spinning body whose axis of rotation
changes orientation. The precessional frequency is equal to the magnetic field strength times the

magnetogyric ratio.

In a magnetic field, NMR-active nuclei can be aligned with the magnetic field (low-energy state)

or aligned against the field (high-energy state). At room temperature, there are slightly more nuclei

in the lower-energy state than in the higher-energy state. As magnetic energy is supplied that cor-
responds to the energy gap, quantum level, between the low- and high-energy states, some nuclei
in the low-energy state move to the high-energy state resulting in an absorption of energy, which is
recorded as a NMR spectra. The difference between the two energy states is related to the strength
of the external magnet. Better spectra are obtained when instruments with larger magnetic fi elds are
employed.
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Because of the small but consistent concentrations of C present in all organic compounds, it is
necessary to use more sophisticated NMR spectroscopy for determining the effect of neighboring
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electrons on these nuclei. However, C-NMR spectroscopy is a valuable tool for investigating poly-
mer structure.
Following is a short description of some of the current techniques.
Nuclear Overhauser Effect—The nuclear Overhauser effect (NOE) only occurs between nuclei
that share a dipole coupling, that is, their nuclei are so close that their magnetic dipoles interact.
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Techniques that use NOE enhance C spectra and allow spacial relationships of protons to be
determined.
Two-Dimension NMR—Basically, the two-dimensional NMR techniques of nuclear Overhauser
effect spectroscopy (NOESY) and correlation spectroscopy (COSY), depend on the observation
that spins on different protons interact with one another. Protons that are attached to adjacent atoms
can be directly spin-coupled and thus can be studied using the COSY method. This technique
allows assignment of certain NMR frequencies by tracking from one atom to another. The NOESY
approach is based on the observation that two protons closer than about 0.5 nm perturb one anoth-
er’s spins even if they are not closely coupled in the primary structure. This allows spacial geometry
to be determined for certain molecules.
The use of actively shielded magnetic fi eld gradients has made the use of pulsed fi eld gradients

possible. The use of pulsed field gradients reduces experiment time, minimizes artifacts, and allows
for further solvent suppression.

In pulsed NMR, the magnetic field is turned on for the time necessary to rotate the magnetization
o
o

vector into a plane called the 90 rotation or 90 pulse. The field is turned off and the magnetiza-
tion vector rotates at a nuclear precession frequency relative to the coil. This induces a NMR signal
that decays with time as the system returns to equilibrium. This signal is called the free induction
decay (FID).
After a sample is excited, the spin loses excess energy through interactions with the surroundings
eventually returning to its equilibrium state. This process is exponential and is called spin-lattice
relaxation. The decay is characterized by an exponential time constant.
Two-dimensional experiments allow the more precise determination of coupling relationships.
Such experiments are carried out by collecting a series of FID spectra. The time between the pulses

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