NMR method

The two main classes of NMR experiment applied to the study of the radiation chemistry of polymers are concerned with measurement of either the chemical structure of the polymer after irradiation, or the changes in the physical properties brought about by cross-linking or scission. In Sections 3-5 of this review the use of NMR to identify changes in chemical structure is discussed. The changes in the NMR line shape and relaxation times induced by radiation, and the use of these methods to determine radiochemical yields, are discussed in Section 2. 

When an insulating material is subjected to an applied electric field, charge separation and molecular rearrangement occur within the material, causing the phenomenon of polarization. The magnitude of the polarization is measured by a property of the material called the dielectric constant. This macroscopic 

Dielectric relaxation, as the name implies, is concerned with the time, frequency, and/or temperature dependence of the dielectric constant. Since the magnitude of the dielectric constant is related to the molecular structure, its dependence on time, frequency, and/or temperature generally reflects molecular motion. In the case of homogeneous polymers, the dielectric relaxation technique may therefore be used as a probe for the study of transitions and relaxations in a manner analogous to that already discussed for mechanical relaxation. In this chapter we are concerned with the application of dielectric relaxation to amorphous polymers, and we attempt to point out differences between the dielectric and mechanical relaxation techniques. 

In a manner now familiar, we start by treating dielectric relaxation phenomenologically, that is, in macroscopic terms, where the existence of molecules is ignored. In Section 2, we extend our development by incorporating molecular considerations. Applications of these ideas to polymers are treated in Sections 3 through 4. 

For the purposes of this discussion, we need only be concerned with electrical circuits that contain capacitances, C, and resistances, R. The resistance is the dissipative element, formally analogous to the dashpot in the mechanical model case. It is defined by Ohm s law  

Co is the capacitance and is measured in farads. If the plates are separated by a dielectric, that is, by an insulating material (rather than a vacuum), the capacitor will accept more charge at the same potential due to polarization of the dielectric. Under these conditions, the capacitance becomes 

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More generally, note that the applieation of almost any multiple pulse sequenee, where at least two pulses are separated by a time eomparable to the reeiproeal of the eoupling eonstants present, will lead to exehanges of intensity between multiplets. These exehanges are the physieal method by whieh eoupled spins are eorrelated in 2D NMR methods sueh as eorrelation speetroseopy (COSY) [21]. 

Martin G E and Zekster A S 1988 Two-dimensionai NMR Methods for Estabiishing Moiecuiar Connectivity (Weinheim VCH) Contains a wealth of practical experience. 

NMR is an important teclnhque for the study of flow and diflfiision, since the measurement may be made highly sensitive to motion without in any way influencing the motion under study. In analogy to many non-NMR-methods, mass transport can be visualized by imaging the distribution of magnetic tracers as a fiinction of time. Tracers may include paramagnetic contrast agents which, in particular, reduce the transverse... 

Orrell K G, Sik V and Stephenson D 1990 Quantitative investigations of moieouiar stereodynamios by ID and 2D NMR methods Prog. Nucl. Magn. Reson. Specfrosc. 22 141-208... 

A second 2D NMR method called HETCOR (heteronuclear chemical shift correlation) is a type of COSY in which the two frequency axes are the chemical shifts for different nuclei usually H and With HETCOR it is possible to relate a peak m a C spectrum to the H signal of the protons attached to that carbon As we did with COSY we 11 use 2 hexanone to illustrate the technique... 

Carbon-13 nmr. Carbon-13 [14762-74-4] nmr (1,2,11) has been available routinely since the invention of the pulsed ft/nmr spectrometer in the early 1970s. The difficulties of studying carbon by nmr methods is that the most abundant isotope, has a spin, /, of 0, and thus cannot be observed by nmr. However, has 7 = 1/2 and spin properties similar to H. The natural abundance of is only 1.1% of the total carbon the magnetogyric ratio of is 0.25 that of H. Together, these effects make the nucleus ca 1/5700 times as sensitive as H. The interpretation of experiments involves measurements of chemical shifts, integrations, andy-coupling information however, these last two are harder to determine accurately and are less important to identification of connectivity than in H nmr. 

Density. Density of LLDPE is measured by flotation in density gradient columns according to ASTM D1505-85. The most often used Hquid system is 2-propanol—water, which provides a density range of 0.79—1.00 g/cm. This technique is simple but requires over 50 hours for a precise measurement. The correlation between density (d) and crystallinity (CR) is given hy Ijd = CRj + (1 — Ci ) / d, where the density of the crystalline phase, ify, is 1.00 g/cm and the density of the amorphous phase, is 0.852—0.862 g/cm. Ultrasonic methods (Tecrad Company) and soHd-state nmr methods (Auburn International, Rheometrics) have been developed for crystallinity and density measurements of LLDPE resins both in pelletized and granular forms. 

Analytical and test methods for the characterization of polyethylene and PP are also used for PB, PMP, and polymers of other higher a-olefins. The C-nmr method as well as k and Raman spectroscopic methods are all used to study the chemical stmcture and stereoregularity of polyolefin resins. In industry, polyolefin stereoregularity is usually estimated by the solvent—extraction method similar to that used for isotactic PP. Intrinsic viscosity measurements of dilute solutions in decahn and tetraHn at elevated temperatures can provide the basis for the molecular weight estimation of PB and PMP with the Mark-Houwiok equation, [rj] = KM. The constants K and d for several polyolefins are given in Table 8. 

The hydroxyl number can be deterrnined in a number of ways such as acetylation, phthalation, reaction with phenyl isocyanate, and ir and nmr methods. An imidazole-catalyzed phthalation has been used to measure the hydroxyl number for a number of commercial polyether polyols and compared (favorably) to ASTM D2849 (uncatalyzed phthalation) (99). The uncatalyzed method requires two hours at 98°C compared to 15 minutes at the same temperature. 

Hydroxyl number and molecular weight are normally determined by end-group analysis, by titration with acetic, phthaUc, or pyromellitic anhydride (264). Eor lower molecular weights (higher hydroxyl numbers), E- and C-nmr methods have been developed (265). Molecular weight deterrninations based on coUigative properties, eg, vapor-phase osmometry, or on molecular size, eg, size exclusion chromatography, are less useful because they do not measure the hydroxyl content. 

The barriers to rotation in a series of (V,(V-dimethylformamidine derivatives of pyrido-[2,3-d]- and -[3,4-d]-pyridazines have been measured by an NMR method <78JHC1105), and NMR studies have also been used in a study of tautomerism in the latter system <75BSF702). 

In order to examine whether this sequence gave a fold similar to the template, the corresponding peptide was synthesized and its structure experimentally determined by NMR methods. The result is shown in Figure 17.15 and compared to the design target whose main chain conformation is identical to that of the Zif 268 template. The folds are remarkably similar even though there are some differences in the loop region between the two p strands. The core of the molecule, which comprises seven hydrophobic side chains, is well-ordered whereas the termini are disordered. The root mean square deviation of the main chain atoms are 2.0 A for residues 3 to 26 and 1.0 A for residues 8 to 26. 

To obtain the secondary and tertiary stmcture, which requires detailed information about the arrangement of atoms within a protein, the main method so far has been x-ray crystallography. In recent years NMR methods have been developed to obtain three-dimensional models of small protein molecules, and NMR is becoming increasingly useful as it is further developed. 

The situation is different for other examples—for example, the peptide hormone glucagon and a small peptide, metallothionein, which binds seven cadmium or zinc atoms. Here large discrepancies were found between the structures determined by x-ray diffraction and NMR methods. The differences in the case of glucagon can be attributed to genuine conformational variability under different experimental conditions, whereas the disagreement in the metallothionein case was later shown to be due to an incorrectly determined x-ray structure. A re-examination of the x-ray data of metallothionein gave a structure very similar to that determined by NMR. 

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

Table 2.14 summarizes the steps by which molecular structures can be determined using the NMR methods discussed thus far to determine the skeleton structure, relative configuration and conformation of a specific compound. 

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