William M. Ritchey

Suppression of signals from the probe in bloch decay spectra

An approach to suppressing spurious resonances due to the probe in NMR is presented which utilizes DEPTH (I) pulse sequences to select homogeneous regions of the radiofrequency field. This method is well suited to eliminating background resonances from the probe in Bloch decay spectra in solids and the glass resonance in solution 29Si NMR. Bloch decay spectra obtained in probes that are designed primarily for cross-polarization magic-angle sample spinning, CP/MAS, studies often contain resonances from materials which are close to the coil. These materials are needed for, among other reasons, insulating the coil and holding it in place, as stators and bearings in the MAS assembly, as insulation in quarter wave stubs, and for thermal insulation where variable temperature capabilities are desired. The actual materials, Teflon, Kel-F, boron nitride, Macor, and aluminum oxide, are generally chosen since they contain no protons and, therefore, do not yield a cross-polarization spectrum. In Bloch decay spectra, however, these materials do display broad background signals which makes the observation of dilute samples (e.g., surface species) difficult. Since these extraneous signals originate from well outside the coil, they are in a region of inhomogeneous and lower RF field than the sample of interest; therefore pulse sequences which are currently used in surface coil studies to selectively excite regions of homogeneous RF fields may be employed to suppress these signals. A similar case exists for 29Si NMR of liquids in which the glass resonance from Dewars, the sample tube and the insert on which the coil is wound, interferes. Figure I shows for various pulse widths the Bloch decay spectra of methylamine adsorbed onto a high-surface-area eta-alumina. The eta-alumina is a Bayerite with a surface area of 400 m*/g, donated by Alcoa. It was treated in vucuo for 2 h at 400°C with subsequent adsorption of methylamine at room temperature in a traditional gas manifold. The spectra were acquired on a Bruker MSL-400 with their broadband, variable temperature CP/MAS probe. The broad signal is from Teflon which is used to guide the spinner into the stator, as a thermal insulator which encloses the MAS spinning assembly, and as a dielectric in the transmission lines. It would be difficult to replace all of the Teflon in the probe without adding signals in the silicon, aluminum, or boron spectra for which the probe is also used. The sharp resonance at 24 ppm is assigned to methylamine species bound to Lewis acid sites. It is seen in the figure that the 90” pulse for the methylamine species is 4 ps, and that for the Teflon

High resolution solid state 13C n.m.r. of Canadian peats

Abstract Peat samples from three locations in Quebec, Canada, were characterized by 13 C CP/MAS n.m.r. spectroscopy. The n.m.r. analysis indicates that the peats contain a significant amount of unaltered plant components including cellulose, hemicellulose, lignin, waxes and resins. Integration of specific regions of the spectra can give semi-quantitative estimates of some of these plant components and, in limited cases, information about the degree of decomposition can be obtained from the 13 C CP/MAS n.m.r. spectrum. Dipolar dephasing experiments and difference spectroscopy were found to be valuable techniques for spectra assignments and enhancement of compositional differences.

Studies on cyclic polyols : Part IV. Nuclear magnetic resonance spectra and configurational assignments of polyhydroxycyclopentanes

Abstract The configurations of five cyclopentanetetrols and both 3,5-cyclopentenediols have been established by examination of their n.m.r. spectra. The configurational assignments are based to a large extent on the nature of the methylene signals (A 2 X 2 , ABX 2 or ABXY), and in one case the magnitude of the vicinal coupling constant is used as the basis of assigning the 1, sol4 2 ,3 structure in preference to the all- cis configuration. The present conclusions confirm earlier assignments based on purely chemical grounds. Spectral data for the ring protons of the tetrol tetrabenzoates, diols, epoxydiols and corresponding dibenzoates, a dibromoglycol and several O -isopropylidene derivatives of cyclopentanepolyols are presented. The internal chemical shifts between the A, B and X protons of the ABX 2 system observed in many of these compounds have been studied. The internal shifts δ x δ a and δ x δ b agree well with the internal shifts reported for the ABX system in two series of substituted bicyclo[2.2.1]heptenes. The linear variation of internal chemical shift with electronegativity of the substituent and the angular dependence of deshielding found in the bicycloheptenes is thus also found in the cyclopentanes. Unlike the case of the bicycloheptenes, which are rigid, the coupling constants J ax , J bx and J ab do not vary linearly with the electronegativity of the substituents, presumably because of the additional factor of variability of the dihedral angles in the various cyclopentanes, due to the greater flexibility of these molecules.

The Endo-Exo Isomerization of N-Phenyl-5-norbornene-2,3-dicarboximide

Abstract ENDO isomers of N-phenyl-5-norbornene-2,3-dicarboximide was converted to exo-isomers below retro-Diels-Alder temperature under various reaction conditions. The exo-endo yield ratio was studied by NMR and the 1H and 13C NMR chemical shifts of the isomers are reported. While no intermediate could be isolated using several different radical trapping agents, the results show that 7,5 proton chemical shift is a possible isomerization mechanism.

Solid State 13C NMR Studies of Polyoxymethylene: Isolation of Amorphous from the Crystalline Resonance and Solid State Conformations

Abstract The high resolution, CP/MAS 13C NMR Spectra of solid polyoxymethylene (POM) provide a single resonance peak. This pulse sequence does not resolve the crystalline and amorphous carbon resonances in this semicrystalline polymer. However, the resonances can be resolved using differences in relaxation behavior. The isolation of the amorphous carbon resonance from the crystalline carbon resonance is reported here with a modified CP/MAS experiment. The chemical shift differences of the amorphous crystalline carbons are related to the conformational differences of the polymer chains. For POM, the amorphous is at lower field than the crystalline, whereas, for polyethylene, the opposite occurs. These differences arise from the γ-gauche effect.

Investigation of the effect of gamma-radiation on high-density polyethylene by solid-state magic-angle 13C NMR spectroscopy

The structural and morphological changes occurring in high-density polyethylene when exposed to gamma-radiation have been investigated by high-resolution solid state 13C NMR. NMR measurements are used to study the effect of gamma-radiation on crystalline and amorphous components. In addition, resonance peaks due to direct cross-links have been detected. The changes in main chain unsaturation are also studied.

Molecular motions of liquid crystalline polymers with various spacer lengths in the glass and nematic states

A side-chain liquid crystalline polymer (LCP) based on the 4-hydroxy-4′-methoxy-α-methylstilbene mesogen attached through a flexible spacer of eight methylenic units to a poly(methyl acrylate) backbone is studied via solid-state 13C nuclear magnetic resonance (n.m.r.) spectroscopy. The molecular dynamics in the MHz frequency regime are characterized as a function of temperature by the spin—lattice relaxation time constant T1. Rotational correlation times (τcs) and activation energies (EaS) are calculated for motions at various local sites in the glass and nematic states. Rapid spinning of the methyl carbons occurs on the fast side of the T1 minimum, and the activation energy does not change at the phase transition. The motional activation energies of all three sites in the mesogen are nearly equal, indicating that the motion is collective. The activation energy decreases for the mesogen carbons in the nematic state by a factor of six, as the LCP is no longer frozen. In the glassy state the activation energy of the α spacer carbon is nearly equal to that of the mesogen, while the β spacer carbon is 30 times more mobile. The spacer begins to function as a flexible free spacer at the β position. Further flexibility is introduced at the γ and δ positions. Likewise, in the nematic state the mobility increases in the spacer from the α position inward. The activation energy of the backbone increases in the nematic state, with the backbone acting as a viscous drag to the motion of the mesogen. A comparison is made between the LCP with a spacer of eight methylenic units and an analogous LCP with a spacer of three methylenic units. In the glass state, Ea is larger for the motion of the mesogen, the α spacer carbon and the β spacer carbon, for the LCP with eight methylenic units.

I. Additive relations in alkylhalosilanes

Abstract 29 Si NMR chemical shifts of simple alkylhalosilanes have been shown to follow an additive relation with first and second order coefficients. Grant and Pauls method is extended to 29 Si NMR and 175 pieces of data are fit to 32 dependent variables with a correlation of 0.994 and a standard error of 7.67 ppm. The relative importance of the diamagnetic term in the shielding expression is discussed.

Lanthanide-induced proton, carbon, and phosphorus NMR shifts for a series of organophosphorus compounds

Abstract The lanthanide-induced proton, carbon-13, and phorphorus-31 NMR shifts for with Eu(DPM) 3 , Eu(FOD) 3 , Pr(DPM) 3 and Pr(FOD) 3 are reported. Proton shifts are dominated by the pseudo-contact effect for all ligands. Carbon shifts are dominated by the pseudo-contact effect with contact interactions found for the phosphine and phosphoryl compounds. Large 31 P contact shifts are found for the phosphoryl compounds and triethyl phosphine where direct phosphoryl-oxygen or phosphorus complexation occurs. Proton and 13 C results with europium and praseodymium reagents show evidence of different complexation mechanisms with triethyl phosphite. Large pseudo-contact phosphorus-31 shifts for triethyl phosphite indicate little or no direct phosphorus-lanthanide interaction.

Separation of Components in Crystalline and Amorphous Regions of Polyethylene by Solid State Carbon-13 Nmr Spectroscopy

Abstract The cross-polarization (CP) technique1–2, combined with magic angle sample spinning (MAS)3,4 and high power decoupling5, usually provides high resolution spectra of solids2,6–8. These high resolution 13C NMR spectra are in many cases sufficiently detailed to characterize the system9–17 In the case of polyethylene (PE), it was shown that distinct conformations in the crystalline regions lead to a narrow crystalline resonance and the non-crystalline regions appear as a broad shoulder on the upfield side. 14 Since it is often difficult to obtain detailed information from unresolved spectra, it is either desirable to resolve these peaks or isolate one resonance peak from another by alteration of the measurement. We report here the identification of different components in each of the crystalline and amorphous regions. This result is based on differences in the strength of the I3C-lH dipolar interactions, and these components are characterized by different dipolar dephasing time decay constants (TDD).