H. S. Gutowsky

Solid state aluminum-27 NMR studies of tridecameric Al-OxoHydroxy clusters in basic aluminum selenate, sulfate, and the mineral zunyite

Aluminum forms a number of tridecameric (Al,,) cluster compounds in which 02-, OH-, FP, and HZ0 are ligands. Examples are the basic aluminum salts (chloride, selenate, and sulfate, Refs. (1, 2)) and the m ineral zunyite (3). The basic aluminum salts are formed by partial hydrolysis of A1(H20)63’ by base, have commercial utility, and have recently been the subject of several studies using aluminum-27 NMR spectroscopy, both in solution (4, 5) and in the crystalline solid state (I, 6). The basic aluminum salts all contain a central AlO unit, surrounded by twelve six-coordinate Al atoms. Early solution NMR studies readily detected signals arising from AD4 in the tr idecamer (4), but only under certain conditions (at high temperature) could resonances attributable to the octahedral aluminums be observed, due presumably to their large quadrupole coupling constants (e2qQ/h). Similarly, detection of the octahedral aluminums in the basic chlorides and sulfates in the solid state has until now not been successful (6). In contrast, solid state NM studies using “mag ic-angle” sample spinning (MASS) (7) of the m ineral zunyite, a basic aluminum silicate having the structure [Ali,(OH, F)i6F2]Si502&1, have recently revealed quite narrow lines for both fourand six-coordinate aluminum (I). In this paper, we present 27A1 MASS, variable-angle sample-spinning (VASS) (8) and static NMR spectra of basic aluminum selenate, sulfate, and the m ineral zunyite, obtained at several magnetic field strengths, which has allowed us to determine e2qQ/h and isotropic chemical-shift values for both Al(4) and Al(6) sites in all three systems. We discuss the likely origins for the large differences in e2qQ/ h between Al(6) sites in zunyite (-2.5 MHz) and the basic aluminum selenate and sulfate (10 MHz). Availability of e*qQ/h (and asymmetry parameter, 7) information will be shown elsewhere to permit evaluation of rotational correlation times for the Ali3 clusters in solution, and thereby to permit estimation of oxygen-l 7 e’qQ/h values in such systems (9). The basic aluminum selenate and sulfate were obtained as Na[AI04 a Ali2(OH)24(OH2)12] (X0&.YH20 (X = Se or S and Y 13, Ref. (2)) from

On the temperature dependence of lanthanide-induced NMR shifts

The current theories of the origin of lanthanide-induced NMR shifts are examined and compared with one another and with experimental results in an attempt to understand the temperature dependence of the shifts. It is concluded that shifts arising from a pseudocontact mechanism should in general show a T−2 behavior as predicted by Bleaney. In most situations with a fixed molecular geometry, theory does not predict a substantial T−1 component of the shift; but in some instances a fortuitous combination of crystal field parameters can lead to additional T−n components (n ≥ 3) which cause the overall shift to appear to be a linear function of T−1 with a large intercept at T−1 = 0. Extension of Bleaneys method to the (positive) T−3 term shows it to be negligible compared to that in T−2 for systems with J = 32 and relatively small (∼10%) for larger J. In the absence of detailed experimental studies of crystal field parameters in solution, it is likely that for the majority of lanthanide adduct systems the most realistic description of the proton shifts is in terms of the well-established T−2 dependence.

Oxygen-17 and aluminium-27 nuclear magnetic resonance spectroscopic investigations of aluminium(III) hydrolysis products

We report the 17O and 27Al nuclear magnetic resonance (n.m.r.) spectra of the products of aluminium(III) hydrolysis by base. Our results indicate that combined use of solution (chemical shift, linewidth, and spin-lattice relaxation time) and solid-state (static, ‘magic-angle’ sample spinning, and cross-polarization ‘magic-angle’ sample spinning) n.m.r. techniques permits determination of nuclear quadrupolar coupling constants (e2qQ/h, in solution and solid state) for both 17O and 27Al nuclei, and in suitable cases, measurement of rotational correlation times in solution. Cross polarization allows differentiation between AlO4 and Al–O–H oxygens in solid-state n.m.r. spectra of tridecamer crystals. The 17O e2qQ/h value for AlO4 is in good agreement with predictions based on electronegativity considerations. The AlO4 oxygens in the tridecamer are essentially inert to substitution, while the others are quite labile. The rotational correlation time of the tridecamer in solution at 23 °C is ca. 1.3 × 10–10 s.

Effect of dielectric constant upon the magnetic anisotropy of the carbonyl group and its long-range chemical shifts

A quantum-chemical model has been used to calculate the paramagnetic contribution to the magnetic susceptibility of the carbonyl group in formaldehyde and acetone as a function of the solvents dielectric constant. The long-range nuclear shielding in the vicinity of the carbonyl group is calculated from the anisotropy in its magnetic susceptibility as a function of the dielectric constant. The shielding results are presented as a series of contour maps which serves as a basis for discussing the effects of solvents upon the proton chemical shifts observed in formaldehyde and acetone.

Oxygen-17 nuclear magnetic resonance spectroscopic studies of aqueous alkaline silicate solutions

High-field (11.75 T) oxygen-17 n.m.r. spectra have been obtained for a variety of aqueous silicate solutions isotopically enriched in the oxygen-17 nuclide. Oxygen-17 n.m.r. linewidths of the silicate anions are broad (Δv½ca. 3 to 20 p.p.m.) and the chemical shift range is relatively small (about 50 p.p.m.), often resulting in considerable spectral overlaps. Assignment of the observed resonances to known silicate structures is nevertheless possible in some cases by using suitable ‘model’ solutions containing silicate structures which have been previously characterized by 29Si n.m.r. spectroscopy and by making use of spectral simulations. In this manner, two overlapping spectral regions may be distinguished: a 35–55 p.p.m. range (from water) typical of non-bridging, or ‘terminal’ oxygen groups (Si–O– or Si–O–H, or both), and a 45–85 p.p.m. range, assigned to bridging groups (Si–O–Si). Contrary to recent Raman spectroscopic studies, 17O n.m.r. spectroscopy shows no evidence for any silicate species other than the monomeric silicate anion in very dilute solution (0.01 mol dm–3 in SiO2). Preliminary kinetic investigations indicate that in tetramethylammonium silicate solutions very different bulk water exchange rates exist for the two types of oxygen site present, with that for the non-bridging oxygen being much greater than that of the bridging oxygen. Concentrated alkali-metal silicate solutions yield complex 17O n.m.r. spectra with many overlapping signals.

The deuterium quadrupole coupling constants of di- and trihalomethanes

There have recently been published a number of papers reporting the deuterium quadrupole coupling constants (QCC) of small molecules dissolved in liquidcrystalline media (1-5). The results presented show, in many instances, wide variations in QCC values determined by different groups. These differences can be mainly attributed to the presence of different solvent-solute interactions in different liquid-crystal phases. Other causes of the range of values found arise from the difficulty in choosing appropriate bond lengths and making vibrational corrections to the dipolar couplings. Such uncertainties have led to some puzzling anomalies. For example, with the deuterohalomethanes, tribromomethane appears to have a considerably larger QCC than either trichloromethane or triiodomethane. These results have prompted us to determine the QCC values for deuterotrihalomethanes, C!HX3 (X = Cl, Br, or I), and dideuterodihalomethanes, C2H2X2, using solid-state deuterium quadrupole spin-echo techniques (6). Such measurements, at least for the di- and trihalomethanes, do not suffer so severely from uncertainties due to solvent effects, arising, for example, from the use of various liquid crystals containing aromatic and other more polar groups, or to uncertainties in bond lengths, vibrational averaging, and unknown electric field gradient asymmetry parameters. Spectra were obtained on a “home-built” spectrometer at 55.3 MHz using an 8.‘45 T, 3.0 in. bore Oxford Instruments (Qsney Mead, Oxford, U.K.) superconducting so’lenoid, a Nicolet (Madison, Wise.) 1180 computer and 1290 transient recorder, together with a variety of other digital and radiofrequency electronics. We also used a home-built solenoidal radiofrequency coil probe (sample volume -0.8 cm3), together with a quadrupole spin-echo sequence (6), for data acquisition. The 90” pulse width used was 3.3 ps. All samples (except [2H]triiodomethane) were obtained from Merck, Sharpe, and Dohme (Canada), and were used without further purification. [2H]Triiodomethane was prepared from [2H]trichloromethane and iodoethane, as reported in the literature (7). A typical experimental spectrum (displayed for the convenience of the reader as a “mirrored” on-resonance single-phase detected spectrum), together with its com

Chemical aspects of nuclear magnetic resonance

Abstract An unexpurgated historical synopsis is given of the authors role in the early applications of NMR to chemical problems, with emphasis upon the importance of chance, serendipity, ignorance and faculty clubs. Several research areas are discussed, including the broad lineshapes in solids and their motional narrowing, fluorine and proton chemical shifts, multiplets in liquids, chemical exchange, and an unsuccessful search for the ESR of conduction electrons. A brief account is given of some current work in which chemical exchange rates are being determined from the dependence upon resonance frequency of the coalescence temperature for several n,N-dimethylamides.

High-resolution solid-state oxygen-17 nuclear magnetic resonance spectroscopy of transition metal carbonyls

We have obtained the first high-resolution solid-state oxygen-17 n.m.r. spectra of a series of transition metal carbonyls and results indicate that the principal elements of the 17O chemical shift tensors may be determined, that the anisotropies are very large (Δδca. 600–700 p.p.m.) while the 17O quadrupole coupling constants are very small (ca. 1 MHz), and that magnetically nonequivalent C17O groups may be detected.

High-Field NMR of Membranes and Proteins

Deuterium NMR of specifically 2H-labelled phospholipids has been used to study interactions between lipids and proteins in model and biological membranes. Proteins decrease the order of the phospholipid hydrocarbon chains in both systems. Carbon-13 NMR spectra of lysozyme, taken in the presence of several denaturants, support a two state denaturation process.