Zeev Luz

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.

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...

Electronic and nuclear relaxation in solutions of transition metal ions with spin S=3/2 and 5/2

Electronic relaxation in solutions of solvated Fe3+, Mn2+ (S=5/2) and Cr3+ (S=3/2) is controlled by modulation of the quadratic zero field splitting interaction. The modulation is caused by collisions of the hydrated complex with bulk solvent molecules. Theoretical expressions are derived for the electronic transversal and longitudinal relaxation times T 2e and T 1e and are used to interpret the E.S.R. data of these ions. The analysis yields results for the zero field splitting constants and for the mean lifetime between collisions. The latter are found to be in the range 4–9 × 10-12 s. The nuclear relaxation rate of the solvent nuclei are affected by the dipole-dipole and scalar interaction with the unpaired electrons of the paramagnetic ions. The usual equations for nuclear relaxation due to these interactions are modified to take into account the existence of several T 1e s and T 2e s. These equations are used to analyse proton relaxation data in aqueous solutions of Cr3+, Fe3+ and Mn2+. The E.S.R. and...

Proton Magnetic Resonance Shifts in Aqueous Solutions of Paramagnetic Metal Ions

The nuclear magnetic resonance of the water proton in aqueous solutions of paramagnetic metal ions is shifted by the isotropic hyperfine interaction AI·S with the electron spin of the metal. These shifts have been measured for solutions of Cr3+, Fe3+, Mn2+, Co2+, Ni2+, and Cu2+ between 0° and 100°C. Complete averaging of the shift due to rapid exchange of H2O in and out of the solvation shell occurred for Co2+, Cu2+, and Mn2+. However, the averaging of the shift was not complete for Cr3+, Fe3+ (and possibly Ni2+) and the kinetic parameters for the exchange process had to be considered in order to evaluate A.From the temperature dependence of the proton shifts in Cr3+ and Fe3+ solutions, we determined the pseudo first‐order rate constants k and the activation energies ΔE for the proton exchange between solvation shell and bulk. These values are: 8.7×104 sec−1 and 7.5 kcal mole−1 for Cr3+, 2.8×106 sec−1 and 12.1 kcal mole−1 for Fe3+. The proton exchange reaction proceeds via the acid dissociation of the hyd...

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...

Deuterium magnetic resonance of some polymorphic liquid crystals: The conformation of the aliphatic end chains

Preprmt LBL~6~76 ( •• ~ TWO-WEEK LOAN COpy This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Dioision, Ext. 5716

Proton Relaxation in Water

Measurements of the proton relaxation times in water as a function of pH were made. The transverse relaxation time T/sub 2/ was measured by the spin-echo method. The longitudinal relaxation time T/sub 1/ was measured by ihe 18O deg to 90 deg pulse scheme. The water used was doubly distilled, but not degassed. Its pH was adjusted by the addition of HCl in the acid range and NaOH in the basic range, and measured with a glass electrode. It appears that the observed dependence of T/sub 2/ on pH is not due to the presence of Na/sup +/ or Cl/sup -/ ions. The fact that T/su 2/ is appreciably shorter than T/sub 1/ shows that part of the relaxation is caused by a slow mechanism, i.e., one characterized by a correlation time longer than the Larmor period. It is proposed that in the central pH range, the periinent mechanism is the breaking and reforming of hydrogen bonds between water molecules. There is strong evidence that the hydrogen bonding causes a chemical shift of the proton resonance of the order of 100 cps (at 30 Mcps). (A.C.)

Quadrupole echo distortion as a tool for dynamic NMR: Application to molecular reorientation in solid trimethylamine

Deuterium NMR quadrupole echo spectra of trimethylamine‐d9 (TMA‐d9) were recorded in the solid phase from about −150 °C to the melting point (−117 °C). The spectra exhibit pronounced line shape variation with temperature which is interpreted in terms of two independent dynamic processes, (i) threefold methyl group reorientation about the respective C3 axes, and (ii) threefold whole molecule reorientation about the molecular C’3 axis. To derive quantitative kinetic parameters for these processes the experimental spectra are compared with simulated traces, placing particular emphasis on the effect of line shape distortion due to the delay time τ between the π/2 pulses in the quadrupole echo experiment. This approach improves the sensitivity of the fitting method and allows accurate determination of the kinetic parameters for both the C3 and C3 reorientation processes. The results are k(−150 °C)=3.0×104 s−1, k’(−150 °C)=0.1×104 s−1, ΔE=3.0±0.6 kcal/mol, and ΔE’=8.0±1.6 kcal/mol, respectively. In Appendix A a...

ESR and NMR in Aqueous and Methanol Solutions of Copper(II) Solvates. Temperature and Magnetic Field Dependence of Electron and Nuclear Spin Relaxation

ESR line‐shape measurements of Cu(ClO4)2 solutions in methanol and water at X‐ and Q‐band frequencies are reported. In both frequencies, above room temperature, the over‐all linewidth increases with increasing temperature, while at low temperatures the slopes level off and (at Q band) even change signs. The results are quantitatively interpreted in terms of two main electronic relaxation mechanisms, viz., modulation of the spin rotation and anisotropic g‐tensor interactions. From the magnetic parameters of the complexes determined in frozen solution and the field dependence of the linewidth, the correlation times are determined. It is found that in both solvents the anisotropic g tensor is modulated by the fast hopping of the complex distortion axis rather than by the molecular tumbling of the complex. Assuming a random jump of the distortion axis between the three octahedral principal directions, the kinetic parameters of this hopping process are calculated to be τi(25°C)=1.6× 10−11 sec−1, ΔEi=1.2 kcal/m...