Takeshi Yamanobe

13C NMR chemical shift and crystal structure of paraffins and polyethylene as studied by solid state NMR

Abstract High-resolution 13C NMR spectra of cyclic paraffins, n-paraffins and polyethylene in the solid state have been measured by the cross polarization—magic angle spinning technique. It is shown that the 13C chemical shift of the main peak for the trans-zigzag inner methylene carbons of paraffins with the orthorhombic form appears at about 1 ppm further upfield than that with the triclinic form. This difference was theoretically verified to be caused by a local difference in intermolecular interactions in the orthorhombic nad triclinic forms, based on the tight-binding molecular orbital calculation within the CNDO/2 framework.

Chain conformation of polyethylene in the solid state as studied by 13C cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopy

High-resolution 13C nuclear magnetic resonance (n.m.r.) spectra of melt-quenched polyethylene and polyethylene single crystals are measured by the cross-polarization/magic angle spinning technique. Melt-quenched polyethylene and polyethylene single crystals have four small upfield peaks, a shoulder on the main peak and three small peaks, respectively. Based on the 13C n.m.r. resonance lines of cyclic paraffin C64H128 reported previously, it is concluded that the main peak and the three upfield peaks arise from the trans zigzag structure region and the folded structure region, respectively. From these peak intensities, it is estimated that the stem length of polyethylene single crystals is approximately 125 A. Taking into account an error in the estimation of the small peak intensities, the calculated stem length of 125 A is consistent with the crystal thickness (120–150 A) observed directly by electron microscopy. It can be concluded, therefore, that polyethylene single crystals mainly contain sharply folded structure. Melt-quenched polyethylene may contain sharply folded structure to some extent in addition to loose loops.

13C NMR chemical shift and electronic structure of an infinite polymer chain as studied by tight‐binding theory within the CNDO/2 framework: Polyethylene and cis and trans polyacetylenes

Formulas for the calculation of the 13C NMR chemical shift of infinite polymer chains were derived by the tight‐binding theory within the CNDO/2 framework incorporated with the sum‐over‐state method. This formalism was applied to the calculation of the 13C NMR chemical shift tensor of polyethylene and cis and trans polyacetylenes. The calculated results were found to agree with the experimental data.

Polyethylene structure in the solid state as studied by variable-temperature 13C CP/MAS n.m.r. spectroscopy

Abstract High-resolution 13 C n.m.r. spectra of melt-quenched polyethylene sample were measured within the temperature range of - 120 to 90°C by means of variable temperature/magic angle spinning technique. Based on these results, the temperature change of polyethylene structure in the solid state was discussed.

Conformation and molecular packing of n-alkyl side chains protruding from α-helical poly (l-glutamates) as studied by 13C CP/MAS NMR spectroscopy

Abstract 13 C NMR spectra of a series of poly( l -glutamates) with n -alkyl side chains of various lengths ( n = (number of carbon atoms in the alkyl group) = 8, 12, 14, 16 and 18) were recorded by the cross polarization/magic angle spinning method, in order to elucidate conformational features and molecular packing through the observation of 13 13 C chemical shifts. From these experimental results, it was found that the main chain of the poly(γ- n -alkyl l -glutamates) takes the α-helical conformation irrespective of side chain length, and the n -alkyl side chains can participate in the crystallization if they are long enough. In these side chain crystallites, all- trans zigzag conformation of alkyl chain was clarified and the type of molecular packing was presumed based on the reference data of n -alkanes. Further, we discuss the conformation of n -alkyl side chains of poly( n -stearyl l -glutamate) in isotropic solution and liquid crystalline solution.

Conformational behaviour of poly (β-aspartate) with n-alkyl side chains in the solid state as studied by 13C-CP/MAS-NMR spectroscopy

Abstract 13 C cross-polarization/magic-angle-spinning NMR experiments were carried out for poly (β-aspartate) with long n-octadecyl side chains as a function of temperature, in order to elucidate the conformational feature in the solid state. From the experimental results it was found that the conformational change in the main chain from the right-handed α-helix to the left-handed α-helix occurs within the temperature range 23.5–100°C, while long n-octadecyl side chains take on an all- trans zig-zag conformation in the crystalline state at room temperature and are in a mobile state above 40°C. It is suggested that such a conformational change of the main chain leads to the prevention of the formation of thermotropic liquid crystals in the above temperature range. The main-chain and side-chain conformations of poly (β-aspartate) with n-alkyl side chains of various lengths at room temperature are discussed.

Carbon-13 NMR chemical shift and electronic structure of polypeptide as studied by tight-binding MO theory: poly (β-benzyl L-aspartate) with the right-handle α-helix and left-handed α-helix forms

Abstract An attempt was made to calculate 13 C NMR chemical shifts of poly(β-benzyl L-aspartate) having the right-handed α-helix (α R -helix) and left-handed α-helix (α L -helix) forms by a tight-binding MO sum-over-states theory within the extended Huckel framework, in order to examine whether or not the conformation-dependent 13 C chemical shifts previously determined by the cross polarization-magic angle spinning technique are reproduced by a change of electronic structure of the polymer. It is found that the relative displacements of the observed C α , C β and carbonyl 13 C chemical shifts between the α R - and α L -helices are reproduced qualitatively by the calculation.

13C.N.M.R. chemical shift and crystal structures of cyclic paraffins of long chain lengths

Abstract High resolution 13 C.N.M.R. spectra of cyclic paraffins of long chain lengths up to the carbon number 200 in the crystalline state have been measured by the cross polarization-magic angle spinning technique. It is found that 13 C chemical shift of the main peak for the trans zigzag methylene carbons of cyclic paraffins having the carbon number from 36 to 80 (triclinic form) appears at about 1 p.p.m. further downfield than those having the carbon number from 128 to 200 (orthorhombic form). Such a difference of about 1 p.p.m. is caused by a local change in intermolecular interactions which result in going from the orthorhombic to the triclinic form.

13C NMR Chemical shifts and electronic structure of cis and trans polycetylenes as studied by tight-binding theory within the INDO/S framework

Abstract A tight binding sum-over-states theory has been used to calculate the 13 C NMR chemical shifts of polyacetylenes based on the INDO/S method. The calculations are used to elucidate the values of the isotropic 13 C NMR chemical shift and the components of the 13 C NMR shielding tensor as determined by CP/MAS NMR in the solid state. The calculated results are found to agree with the experimental data more quantitatively than those based on the CNDO/2 method reported previously.

13C NMR chemical shift and electronic structure of polyoxymethylene in the solid state

Abstract High-resolution 13 C NMR spectra of polyoxymethylene (POM) in the solid state have been measured in order to obtain a relationship between the conformation and 13 C NMR chemical shift tensor (δ 11 , δ 22 and δ 33 ) and its isotropic average. It was found that the 13 C isotropic chemical shift of POM in the crystalline region appears upfield with respect to that in the noncrystalline region and that the width Δδ ( = δ 11 - δ 33 ) in the crystalline region is much larger than that in the noncrystalline region. These experimental findings can be reasonably explained by a theoretical calculation for an infinite POM chain based on a tight-binding molecular orbital calculation within the CNDO/2 framework.