Frank A. Bovey

Calculation of Nuclear Magnetic Resonance Spectra of Aromatic Hydrocarbons

The free electron model of Pauling is used to calculate the magnetic field around a benzene ring which is rotating rapidly about all axes in an external magnetic field. It is assumed that the π electrons precess in two circular paths, one on each side of the ring, equal in radius to the C–C distance in the benzene ring. The separation of these rings is taken as 1.28 A, which gives a calculated value for the nuclear magnetic resonance shielding value for benzene protons equal to the observed value. The field thus calculated is employed to predict shielding values for other aromatic compounds. Agreement with experiment is in general good, but there are some exceptions.

On the Stereoregularity of Vinyl Polymer Chains

We explore quantitatively the kind and extent of information concerning the distribution of configurational sequences in vinyl (and related) polymer chains obtainable from data such as presently available high‐resolution NMR spectroscopy measurements. After finding the number of observationally distinguishable types of sequences, containing n monomer units [n(ads)], we obtain the necessary consistency relations which must hold between the observed proportions of n(ads), explicitly for all n≤5. Geometrically, the possible regions accessible to the n(ad) proportions form certain simplexes in a space of D(n) dimensions. The extreme points always correspond to repeating polymers. The computed dimensionality number D(n) plays a central role in fixing the number of free parameters of the polymer propagation model determinable by an n(ad) analysis. We discuss in some detail certain Markovian and the non‐Markovian Coleman—Fox propagation model in relation to n(ad) analysis. We introduce the concept of a completel...

NMR Study of Rotational Barriers and Conformational Preferences. I. Cyclohexyl Fluoride

The equatorial:axial ratio and rate of conformational isomerization of cyclohexyl fluoride has been measured by observation of the 19F resonance as a function of temperature (—87.6° to +29.5°) for a 25 vol.% solution in CCl3F. Rate information is readily obtained over a wide temperature range, since the equatorial and axial fluorines differ in chemical shift by 20.5 ppm; the equatorial and axial α protons differ in chemical shift by only 0.46 ppm.Interpretation of the spectra in terms of isomerization rates is complicated by the following: (a) the equatorial and axial conformers are present in unequal proportions; (b) their ratio changes with temperature; and (c) 19F–H coupling produces fine structure and/or peak broadening. The general expression for spectral line shape resulting from exchange between two unequal, chemically shifted spin populations is plotted (using the IBM 7090 computer), chemical shift and conformer ratio data being obtained from the low temperature 19F spectra. The observed spectra f...

NMR Study of Rotational Barriers and Conformational Preferences. II. d11‐Cyclohexane

The rate of conformational isomerization of cyclohexane has been measured over the temperature range —75.0 to —47.0° by observation of the NMR spectrum of C6HD11, which is much simpler than that of cyclohexane itself and is amenable to exact kinetic treatment. The line broadening arising from D—H coupling was eliminated by double resonance. It is found that ΔF‡ is equal to ΔH‡ within experimental error for the chair‐to‐chair reaction and is 10.5 kcal (206.0°K) (10.2 kcal for the chair‐to‐boat process); ΔS‡ for the chair‐to‐chair process is thus about zero and for the chair‐to‐boat process is about +1.4 eu.

Polymer NMR Spectroscopy. X. The Use of H1–H1 Spin Decoupling in the Elucidation of Polymer Structure

High‐resolution NMR spectroscopy of vinyl polymers in solution is an effective means of determination of the stereochemical configuration of their chains. In polymers having only a single α substituent, however, spin coupling of α and β protons complicates the interpretation of the spectrum considerably and also makes it difficult to observe those stereoisomeric sequences which are present as minor components. Decoupling of the α‐ and β‐proton spins has been found to be helpful in the interpretation of the spectra of polyvinyl chloride, polyvinyl fluoride, and polyvinyl methyl ether. It is not helpful for polyvinyl acetate in carbon tetrachloride, since here the chemical shift of the α and β protons is not sensitive to stereochemical configuration.

4 – The Solid-State NMR of Polymers

Most uses for polymers are in the solid state, and understanding the properties of solid polymers has been an important factor driving the development of solid-state NMR methods. Solid-state NMR has been used for polymer characterization because it can provide information about polymers over a wide range of length scales. On the most detailed level is the information about chain conformation that can be extracted from the chemical shifts. Unlike solutions, where there is a large amount of molecular motion, the chains are more rigid in crystalline polymers and amorphous polymers below their glass transition temperatures, so the chemical shifts reflect the actual chain conformation rather than the average chain structure. NMR has also proved useful for the study of multiphase solid polymers because polymers in different environments have different molecular dynamics and different NMR relaxation times. These differences can be exploited such that the NMR signals from a particular phase of the material can be observed. In this way it is possible to selectively observe the crystalline, amorphous, interfacial, and rubbery materials.

The dipolar and exchange contributions to the spin-lattice relaxation of the imino protons in poly(rA)· poly(rU)

Abstract Selective, semiselective, and biselective excitation has been used to study the spin-lattice relaxation of the hydrogen-bonded imino protons in the sonicated, double-stranded RNA polymer poly(rA)· poly(rU). The spin-lattice relaxation of the imino protons has contributions from both dipolar interactions and exchange with the solvent, and the relative contribution of each to the observed rate depends on temperature. When exchange is slow compared to the dipolar contribution, the two components can be resolved by measuring the relaxation rate with and without prior inversion of the solvent peak. When exchange is fast, the contributions can be resolved by comparing the initial selective relaxation rate with the rate of saturation transfer when the solvent peak is inverted. This allows the exchange and dipolar contributions to be separated at temperatures well below the duplex melting temperatures and provides an easy way to study the mechanism of proton exchange with the solvent.

3 – THE SOLUTION CHARACTERIZATION OF POLYMERS

One of the main uses of NMR in polymer science is materials characterization, which provides the link between the synthesis of new materials and the structure-property relationships. In many polymers the thermal, mechanical, optical, and electronic properties depend on the chain microstructure. The level to which the microstructure can be elucidated depends on the resolution and the methods available for establishing the resonance assignments. The resolution depends on a number of factors, including the nuclei under observation, chain dynamics, concentration, temperature, and solvent. The resolution is best when observing a nuclei with a wide range in chemical shifts. However, the sensitivity for such nuclei is not as good as that for protons so high-solution concentrations, often on the order of 10-30 wt%, are required. Many polymers are soluble to this degree and this does not present a serious experimental limitation.

Structure of Chains by Solution NMR Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is a most effective and significant method for observing the structure and dynamics of polymer chains both in solution and in the solid state. In the solid state, where the motions of the chains are relatively slow, the resonances are broad owing to the local dipolar field at each observed nucleus. This phenomenon of dipolar broadening (see Section 17.2.3.1) tends to abolish all structural information. In solution, where chain motion is fast (Chapter 18), this effect is averaged nearly to zero and one may obtain detailed structural information. This chapter deals with solution NMR spectroscopy. Corresponding information may be obtained — mainly by 13C spectroscopy — for the solid state by ‘magic angle’ NMR (Chapter 19), although with substantially decreased resolution.

Solid-state thermochromism and piezochromism in the polysilylenes

The polysilylenes exhibit complex solid-state structures and electronic properties. In this report the solid-state conformational structures of symmetrically substituted poly(di-n-alkylsilylenes) are described. Comparisons between the different structures and their respective UV absorption characteristics demonstrate that the relationship is complex and aspects of molecular geometry beyond chain conformation must be considered. The thermochromic and piezochromic behavior of several of these polysilylenes is discussed. The all-trans conformation of the silicon backbone is associated with both phenomena. The properties of several polysilylene copolymers have also been investigated. We find that some of the copolymers form a well-ordered structure and exhibit absorption characteristics similar to the crystalline phases of the corresponding homopolymers.