Saul Meiboom

Nuclear Magnetic Resonance Study of the Protolysis of Trimethylammonium Ion in Aqueous Solution—Order of the Reaction with Respect to Solvent

It was found previously that the proton‐transfer reaction between trimethylammonium ion and trimethylamine in aqueous solution takes place via water. In the present investigation the number of water molecules involved in this reaction was determined. Proton exchange rates were determined from NMR measurements in trimethylamine—trimethylammonium chloride buffer solutions in O17‐enriched water. It is concluded that one water molecule is involved in the transfer reaction.In the appendix, theoretical equations are derived for the dependence of the observed spin‐echo decay rate on the 180° pulse rate for the case of fast exchange.

NUCLEAR MAGNETIC RESONANCE STUDY OF THE PROTON TRANSFER IN WATER

Measurements of the nuclear relaxation in water are reported. The transverse relaxation rate (1/T2) of the proton resonance is pH dependent. The effect is shown to be due to a spin‐spin splitting of the proton resonance by O17 (spin 5/2), which is only partially averaged out by proton exchange. The increase of relaxation rate is observable in natural water (0.037% O17), and becomes very appreciable in water enriched in O17. Additional information can be obtained by measuring relaxation rates in the presence of an rf field H1, using a method due to Solomon. A study of the width of the O17 resonance as a function of pH is in quantitative agreement with the results of the proton resonance. The observations provide a direct determination of the rate constants of the exchange reactions: H2O+H3O+→ lim k1H3O++H2O and H2O+HO−→ lim k2HO−+H2O. It is found that k1=(10.6±4)×109 liter mole−1 sec−1 and k2=(3.8±1.5)×109 liter mole−1 sec−1. The spin‐spin interaction between H and O17 in water is determined as 92±15 cps. In the Appendices, theoretical equations for the exchange contribution to the relaxation rate are derived.

Proton Relaxation in Dilute Solutions of Cobalt(II) and Nickel(II) Ions in Methanol and the Rate of Methanol Exchange of the Solvation Sphere

The nuclear relaxation of the hydroxyl and methyl protons in methanol containing low concentrations of Co++ and Ni++ ions has been investigated between —80°C and +80°C. This work complements previous studies of the NMR spectra of the solvation complexes in similar solutions. Over the temperature range studied, the relaxation due to the dissolved paramagnetic ions changes over two or three orders of magnitude. This dependence, as well as a corresponding temperature dependence of the chemical shift of the methanol peaks, can be quantitatively interpreted in terms of chemical exchange of methanol molecules between the coordination sphere of the paramagnetic ions and the bulk methanol. It is found that the dominant exchange process involves whole methanol molecules, i.e., proton transfer is relatively unimportant. The exchange rate is characterized by ΔH‡=13.8 kcal mole—1 and ΔS‡=+7.2 eu for Co++ ions; and ΔH‡=15.8 kcal per mole and ΔS‡=+8.0 eu for Ni++ ions. The hyperfine interaction between the paramagnetic...

Nuclear magnetic resonance studies of smectic liquid crystals

Nuclear magnetic resonance measurements in the smectic liquid crystalline phases of seven thermotropic compounds are reported. The compounds investigated are: terephthalbis(butylaniline); ethyl(methoxybenzylidene)amino] cinnamate; diheptyloxyazoxybenzene; octyloxybenzoic acid; dipentylazoxycinnamate; diethylazoxycinnamate; diethylazoxybenzoate. Proton spectra at 100 MHz of the neat liquid crystal molecules, as well as of simple probe molecules dissolved in small amounts in the liquid crystal, were studied in their dependence on temperature and on sample orientation with respect to the magnetic field. The results show that good alignment of the smectic phases can be obtained by cooling from the isotropic or the nematic phase in a magnetic field; once aligned, the smectic planes conserve their original orientation on rotation of the magnetic field. The results are consistent with the following models of the smectic phases: Smectic‐A is uniaxial and there is fast rotational and translational molecular diffus...

Nuclear Magnetic Resonance Study of the Solvation of Co++ in Methanol and Methanol—Water Mixtures

Proton magnetic resonance spectra of solutions of Co(ClO4)2 in acidified methanol, and in methanol—water mixtures are reported. At temperatures below about —50°C separate peaks for the bulk methanol and for the methanol bonded to Co++ are observed, and methanol exchange is too slow to contribute appreciably to the observed linewidths. Peak intensity measurements give a coordination number of six for Co++ with respect to methanol. From the temperature dependence of the peak frequencies, values of the hyperfine interaction between the bonded methanol protons and the Co++ ion were obtained. The results are +8.8×105 cps for the OH protons and +3.9×105 cps for the CH3 protons. The linewidths are shown to be predominately determined by electron—proton dipole—dipole interaction. The relevant correlation time is the electron relaxation time, which is approximately 5×10—13 sec at —60°C. This time decreases with increasing temperature, with an apparent activation energy of 0.9 kcal mole—1.In methanol—water solution...

Molecular Structure of Cyclobutane from Its Proton NMR in a Nematic Solvent

The protonnuclear magnetic resonancespectrum of cyclopropane has been measured in nematic p,p′‐di‐n‐hexyloxyazoxybenzene. The spectrum has been analyzed by determining the parameters of a spin Hamiltonian which permit its computer simulation. Several ways to determine the structure of cyclopropane from the elements of the spin Hamiltonian are employed, with all giving very similar geometric parameters. With the assumption of D 3h symmetry for cyclopropane, and a C–C bond length of 1.510 A determined by electron diffraction, we are able to fit the NMRspectrum with a structure having a C–H bond length of 1.123 A and an HCH angle of 114.4°, which are to be compared with the corresponding gas phase electron diffraction values of 1.089±0.003 A and 115.1±1°. The anisotropic motion corresponds to the molecular plane tending to be parallel to the applied magnetic field. For indirect spin—spin couplings we find J HH(cis)=+9.5±1 Hz, J HH(trans)=+5.5±1 Hz, and a positive sign for J CH, the indirect coupling between 13C and a directly bonded proton. The effects on the observed spectrum of J HH(gem) and J CH′, the indirect coupling of carbon 13 to a proton on another carbon, are too small to permit their determination. The shortness we find of the C–C bond relative to the C–H bond length is interpreted as a result of molecular vibrations. It appears that the analysis of NMR in nematic media provides an important method of precise molecular structure determination.

Nuclear Magnetic Resonance Spectroscopy of Bicyclobutane

The proton and carbon‐13 nuclear magnetic resonance spectra of bicyclobutane have been analyzed in detail. The use of information from NMR spectra of bicyclobutane in a nematic solvent permits an unambiguous assignment of all chemical shifts and spin–spin interactions to specific nuclei. Some of the assignments differ from previously published ones. A long‐range indirect spin–spin coupling of 5.9 Hz is found between the two exo protons, and an indirect coupling of 16.0 Hz is observed between the exo proton and the carbon‐13 to which the other exo proton is bonded.

Proton NMR in nematic liquid crystalline solvents: The use of deuterium decoupling

Nuclear magnetic resonance spectra of molecules dissolved in an oriented (nematic) solvent become exceedingly complex when the solute molecules contain more than about eight magnetic nuclei. In this paper a technique for greatly simplifying the proton spectra by partial deuterium substitution is described. The deuterium is decoupled by irradiation at a frequency corresponding to a double quantum deuteron transition. Experimental details are presented, as well as results obtained on partially deuterated cyclohexane. The behavior of the spectra as a function of deuterium irradiation intensity and frequency offset is shown in a series of experimental spectra, and compared with computer simulated spectra calculated with a theory presented in a companion paper. It is shown that the spectrum behavior under frequency offset of the irradiation can be a tool in the assignment of spectrum peaks.

Theory of proton NMR with deuteron decoupling in nematic liquid crystalline solvents

The theory required to interpret proton NMR with deuteron decoupling in liquid crystalline solvents is developed. The Bloom and Shoolery analysis of double resonance experiments, in which the spectra of a pair of nonidentical nuclei with isotropic coupling are interpreted in terms of quantization in an effective field in a rotating coordinate system, is extended to systems having anisotropic interactions such as direct dipolar and quadrupole couplings. Applications are made to model systems of one‐proton‐one‐deuteron and two‐protons‐two‐deuterons to facilitate interpretation of recent decoupling experiments, which are described in a companion paper, on partially deuterated cyclohexane in a nematic solvent.

Structure and bond shift kinetics of cyclooctatetraene studied by NMR in nematic solvents

A nuclear magnetic resonance study of cyclooctatetraene (COT) in a number of different nematic solvents covering a wide temperature range (− 35 to 170°C) is reported. Proton spectra for both normal COT and partially deuterated COT under conditions of deuterium decoupling were studied. From an analysis of the spectra in terms of direct dipolar interactions, it is concluded that the COT molecule has a symmetrical tub (D2d) conformation, and coordinates for the protons, within a scale factor, are derived. The spectrum undergoes profound changes with temperature, which are interpreted in terms of a bond shift process. The activation energy is found to be ΔE = 10.9 kcal mole−1 and the rate at 0 °C: 1/τ = 2.5 × 102 sec−1. In an appendix, symmetry considerations are used to identify those spectral peaks which are not affected by the bond shift process. In another appendix, the exchange broadening in a system consisting of two equivalent protons with direct dipolar interaction is treated quantitatively.