Robin K. Harris

29Si and 13C nuclear magnetic resonance studies of organosilicon chemistry. I. Trimethylsilyl compounds

Abstract Twelve trimethylsilyl compounds have been studied by 29 Si and 13 C NMR. Data for chemical shifts, coupling constants, and spin-lattice relaxation times are reported. Nuclear Overhauser effect measurements, made using the gated decoupling technique, enable the contributions of different mechanisms to the T 1 values to be partly evaluated. Influences of substituent electronegativity on the chemical shifts and coupling constants are discussed. A method used to measure the chemical shifts, which does not involve a reference compound in the sample solution, is described in detail.

UEAITR: A NEW COMPUTER PROGRAM FOR ANALYSIS OF NMR SPECTRA: ANALYSIS OF THE PROTON SPECTRUM OF TRIISOPROPYLPHOSPHINE.

Abstract A new computer program for the analysis of NMR spectra is reported. It adds iterative evaluation of input parameters to an earlier program (UEANMR II) which makes use of magnetic equivalence factoring to reduce the size of certain matrices. The present program, by iterating on line frequencies rather than energy levels, and by using magnetic equivalence factoring, offers advantages of speed and convenience over previously described computer programs for NMR analysis. The analysis of the proton spectrum of triisopropylphosphine is described as an example and the trend of 2 J H - P with increased alkyl substitution is found to be opposite to that of the trend of 2 J H - X for all other hetero-atoms, X , reported in the literature.

High resolution 13C n.m.r. spectra of solid isotactic polypropylene

Abstract The high-resolution 13 C n.m.r. spectra of three samples of solid isotactic polypropylene are reported. The spectra, obtained under conditions of proton dipolar-decoupling and fast magic-angle rotation and using cross-polarization, are of annealed and quenched samples of the α-crystalline form and of a sample of the β-crystalline form. Attention is drawn to the importance of knowing the proton relaxation characteristics in these experiments and some illustrative proton T 1 ϱ data are given. The 13 C n.m.r. spectrum of the annealed sample of the α-crystalline form shows well-resolved splittings of the methyl and methylene resonances in a 2:1 intensity ratio. These splittings are interpreted in terms of the crystal structure of the α-form as suggested by X-ray diffraction. Quenching the α-form causes significant changes in the spectrum including a loss of resolution of the splittings obtained from the annealed sample. The β-form shows broad symmetrical resonances for the methyl and methylene carbons. The chemical shifts and other spectral features are discussed in the light of the proposed crystal structures and the effects likely to be produced by quenching.

Choice of pulse spacings for accurate T1 and NOE measurements in NMR spectroscopy

Abstract Three pulse spacings in T 1 and NOE determinations are considered here; viz., acquisition times, pulse delays, and the recovery intervals in the inversion-recovery experiment. The aim is to achieve accuracies of better than ±3% in T 1 and NOE experiments without undue wastage of instrument time. Pulse delays ⩾5 T 1 are adequate for the Freeman-Hill modification of the inversion-recovery T 1 experiment; but longer pulse delays may be required in the NOE experiment, especially if the solutions are dilute. Suggested delays are ⩾6 T 1 for continuous decoupling and ⩾9 T 1 for gated decoupling. A wide variety of recovery intervals can be tolerated without a significant deterioration in the accuracy of T 1 , but a range of values up to 2 T 1 should be used for the best results.

Quantitative aspects of solid state 13C n.m.r. of coals and related materials

Abstract The quantitative aspects of cross-polarization (CP), which is used in conjunction with dipolar decoupling and magic-angle rotation to obtain high resolution 13 C n.m.r. spectra of coals, have been studied using a bituminous coal (82 wt% C, dmmf basis) and asphaltenes from an extract of the same coal. The condition for obtaining reliable quantitative data, that rotating frame 1 H relaxation times ( T 1 p ′ these govern the extent of CP) are much longer than the time required to polarize the carbons present (≈1 ms), was met for the asphaltenes. In contrast, about half the protons in the coal have T 1 p 5 of ≈ ⩽ 1 ms, these times being too short to allow CP of all the carbons. Although the aromaticities obtained for this coal were fairly constant (≈0.75) using (CP) contact times > 0.5 ms, the total peak intensity decreased markedly as the contact time was increased and was much less than that for the asphaltenes. These results indicate that not all the carbons in bituminous coals are observed by CP and, as a consequence, aromaticities reported in the literature for some bituminous coals appear to be low.

Silicon-29 NMR studies of aqueous silicate solutions: Part IV1. Tetraalkylammonium hydroxide solutions

Abstract Silicon-29 NMR spectra have been recorded for aqueous solutions of four tetraalkyl-ammonium silicates in the alkaline pH range. The assignment of the observed peaks is discussed in relation to previous work on alkali metal silicates and other published evidence. In the case of tetramethylammonium silicate solution, enrichment in 29Si yielded additional information. The results dictate the reassignment of the peak due to the cubic octamer given earlier for a potassium silicate solution.

Discriminatory experiments in high-resolution13C NMR of solid polymers

SummaryCross-polarization spectra of solid polymers can be misleading when the material exhibits proton spinlattice relaxation in the rotating frame which cannot be represented by a single exponential. Quantitative and qualitative demonstrations of the problem are given. The use of two experimental procedures for alleviating the difficulties is suggested, and examples for polyalkenes and for polyethylene terephthalate are illustrated.

Silicon-29 NMR studies of aqueous silicate solutions: Part II. Isotopic enrichment

Abstract Silicon-29 NMR spectra have been recorded for aqueous solutions of sodium silicate in the alkaline pH range using silica enriched in the 29 Si isotope. Work at low concentration shows that essentially only monomeric orthosilicate ions are present at ca. 0.01 M in silica. At higher concentrations, peaks which lack fine structure indicate species whose silicon nuclei are all equivalent. The experiments have assisted in the difficult process of assigning the spectra of silicate solutions.

N.M.R. spectra of the XnAA′ Xn′ type

Deceptively simple X spectra of the XnAA′Xn′ spin system with JXX′ = 0 are discussed. Two methods are suggested for obtaining coupling constant information other than |JAX + JAX′ | from such spectra. The methods are (a) computation of shapes of bands formed by overlapping lines, and (b) use of high r.f. powers in observing weak lines. The spectrum of tetramethyldiphosphine is examined in these ways and all the coupling constants obtained: |1 JPP | = 179·7 c/s, 2 JPH = ± 2·90 c/s, and 3 JPH = ± 11·25 c/s. These values are discussed briefly in relation to those for related molecules.

Slow magic-angle rotation 13C NMR studies of solid phosphonium iodides. The interplay of dipolar, shielding, and indirect coupling tensors

Abstract High-resolution 13C NMR spectra were recorded for solid [Me2Ph2P]+I− and [Me3PhP]+I− using high-power decoupling, cross-polarization, and magic-angle rotation. Spectra of the ipso carbon, obtained with the nonquaternary suppression (dipolar dephasing) pulse sequence, show an unequal distribution intensity of spinning sidebands for the two peaks split by the isotropic coupling constant 1JPC. This effect is traced to the joint influence of (P, C) dipolar coupling and 13C shielding anisotropy. The existence of scalar coupling enables magic-angle rotation to distinguish the two subspectra which are present. Analysis of the sideband patterns yields values of the NMR parameters. Under certain assumptions, including that the axis of greatest shielding is perpendicular to the PC bond, it is shown that 1JPC is positive. In contrast to traditional methods of determining signs, this conclusion does not rely on knowledge of signs of other coupling constants.