No abstract

The rate constants of the intermolecular proton exchange in pure methanol, i.e. the reciprocal proton lifetimes, τ0−1, have been determined as a function of the temperature by total lineshape analysis of the 1H‐NMR spectra. Since CH3OH is an AB3 spin system of high order the quantum mechanical density matrix formalism was employed for the simulation of the spectra. The neglect of high order effects as well as the presence of impurities had led to inconsistencies in previous studies. For the first time, the primary kinetic isotope effects were determined indirectly by simulation of the 1H‐NMR spectra of CH3OD samples containing 1 vol‐% CH3OH. The results are given by for CH3OH, and for CH3OD, with a kinetic isotope effect of 3.2 ± 0.4 at 298 K. ‐ The data cannot be explained by a cyclic exchange mechanism. However, they can be quantitatively related to the autoprotolysis constant of methanol and to Grunwald's kinetic data on proton transfer in buffered methanol solutions. It is concluded that the proton lifetimes in pure methanol are determined by the natural amount of free solvated CH3OH2+ and CH3O− ions generated by autoprotolysis. The observed energy of activation is then the sum of two terms, namely the energy of activation of the proton jumps between the ions and a methanol molecule, and half the enthalpy of methanol selfdissociation. In the presence of acid or basic impurities the second term becomes negligible. We propose a method for the quantitative determination of these impurities in the 10−9 to 10−7 mol 1−1 range. From our results we derive an equilibrium isotrope effect of (KH/KD)298K = 6 ± 3 for the autoprotolysis of pure methanol, which has not been reported before. It is shown that the literature data on proton exchange in pure water and other protoc systems can be explained in a similar way. The mechanism of the neutralisation in water and methanol is discussed. For the neutralisation of solutions containing 1 mol of each ion it is found that the mean number of proton transfer steps necessary is 5.3 for water and 2.5 for methanol. These data correlate well with the molar solvent density.

ChemInform Abstract (space group: R3c; formation of discrete hydrogen-bonded cyclic trimers, in which the H-atoms move in a symmetrical, double minimum energy surface; dynamics of the annular prototropic tautomerism).

Nanostructured mesoporous silicates displaying hexagonally arranged channels, templated using a liquid crystal mesophase, were investigated using H‐2 and N‐15 NMR spectroscopy. It is shown that N‐15 MAS NMR spectroscopy allows to estimate the surface acidity of mesoporous materials by measuring the N‐H bond length via the N‐15 chemical shift. Further, H‐2 and N‐15 NMR spectroscopy identify significant differences between the structure of the surface area of HI, silica and MCM‐41. Static N‐15 NMR spectroscopy was used to differentiate between pyridine located in channels perpendicular to the magnetic field from, for example, pyridine located in channels orientated parallel to the field. This observation offers the possibility of separating the NMR signals and thus pursuiting the goal of “liquid phase single crystal NMR spectroscopy”.

The solid-state 'H MAS (magic-angle spinning), 2 H static, 15 N CP (cross polarization)-MAS and 15 N-'H dipolar CSA (chemical shielding anisotropy) NMR (nuclear magnetic resonance) spectra of two different modifications of Ca deuterated 15 N-polyglycine, namely PG I and PG II (-CO-CD 2 -15 NH-), are measured. The data from these spectra are compared to previous NMR, infrared, Raman and inelastic neutron scattering work. The deuteration of Ca eliminates the largest intramolecular 'H-'H dipolar coupling. The effect of the remaining (N)H-(N)H interaction (-5 kHz) is not negligible compared to the 15 N-'H coupling (about 10 kHz). Its effect on the dipolar CSA spectra, described as a two-spin system, is analyzed analytically and numerically and it is shown that those parts of the powder spectrum, which correspond to orientations with a strong dipolar 15 N-'H interaction, can be described as an effective two-spin system, permitting the measurement of the strength of the 15 N-'H dipolar interaction and the orientation of the dipolar vector with respect to the ' I N CSA frame. While in the PG 11 system the ' 5 N CSA tensor is collinear with the amide plane, in the PG I system the CSA tensor is tilted ca. 16° with respect to the (15,,15 22 ) CSA plane.