Volume 214, Issue 12 - 12

Precise temperature dependent H, D, 13 C and 14 N relaxation measurements on acetonitrile in liquid acetonitrile and acetonitrile water mixtures are presented. H and, for the first time, D induced nuclear Overhauser enhancements on 13 C are reported. Isotopic dilution experiments for the acetonitrile deuteron relaxation and relaxation measurements in the two phase region were performed. By a new method, the acetonitrile deuterium quadrupole coupling constant was determined to 162(6) kHz. An isotope effect of about 5% on the acetonitrile correlation time of the reorientation of the symmetry axis is observed. It decreases if the water content is increased. For the effective correlation time of the C-H/D vector the isotope effect is about 25%. This effect was separated into a contribution of 3% for only one molecule which is deuterated and 21% for the deuteration of the remaining molecules. The anisotropy of the reorientation of acetonitrile was calculated in the limit of rotational diffusion. In neat deuterated ammonia the reorientation about the symmetry axis is about ten times faster than the reorientation of the symmetry axis itself. Due to different activation energies the anisotropy increases with decreasing temperature. The isotope effect is also anisotropic and the rotational anisotropy in protonated acetonitrile is about fourteen. The anisotropy is higher in the binary mixtures. At the critical composition it is fourteen for the deuterated and twenty for the protonated sample. In the mixture of critical composition simultaneous relaxation measurements on both phases were possible. This allows a determination of the coexistence curve. No anomaly of the rates was observed on approaching the coexistence curve.

Concentration and temperature dependent measurements of the hydroxyl proton chemical shift and deuterium longitudinal relaxation time of binary mixtures of ethanol and carbon tetrachloride were made over a 60 degree temperature range. Concentrations ranged from 1.02 mole percent to neat ethanol. Ab initio calculations performed on thirteen different ethanol clusters (both cyclic and linear) show a linear correlation (R 2 = 0.99) between the deuterium quadrupole coupling parameter, χ D , and the hydroxyl proton chemical shift, δ H : χ D (kHz) = 291.48 - 14.96 δ H , where δ H is the proton chemical shift in ppm relative to TMS. From this correlation and experimental values of the chemical shift, values of χ D were obtained. The values for χ D range from 204 kHz for the neat liquid at 238 K to 284 kHz for infinite dilution in CCl 4 at 300 K. The χ D values along with the measured deuterium relaxation times were used to calculate the rotational correlation time, τ c . The correlation times range from 70 ps (238 K, 12.4 mole percent ethanol) to 1.3 ps (300 K, 1.0 mole percent). These correlation times and bulk viscosity data were used to calculate the average volume of the different ethanol supramolecular clusters present in the mixture. The average volumes range from 40 Å 3 (monomer) in very dilute solution to 325 Å 3 (cyclic pentamer) for the neat liquid.

The chemical shifts and relaxation times T 1 and T 2 of 3 He and 129 Xe nuclei of noble gas atoms as well as those of 7 Li and 133 Cs nuclei of isoelectronic Li + and Cs + ions have been measured in aqueous solutions, which contain paramagnetic Mn 2+ and Co 2+ ions. The magnetic relaxation of all studied nuclei in Co 2+ solutions proceeds via an electronic spin relaxation of these ions. The relaxation in Mn 2+ solutions is determined by the translational diffusion of the solute particles. The closest approach distances for collisions of paramagnetic ions with atoms of noble gases and alkali ions were estimated from relaxation data.

The reorientational motion of protonated and deuterated benzene in the neat liquid was investigated by 13 C NMR relaxation methods. Therefore, the dipolar (DD) spin-lattice relaxation rates were determined by measurement of the spin-lattice relaxation rates and the 13 C- 1 H/ 2 H nuclear Overhauser factors at a temperature of 293 K. The contribution of the chemical-shift anisotropy (CSA) mechanism to the spin-lattice relaxation rate was obtained at 293 K by measurements at different magnetic flux densities. From the NMR data the rotational diffusion constants for rotations about the C 6 axis ( R II ) and perpendicular to it ( R ⊥ ) were evaluated. The rotational anisotropies, defined as the ratio R II / R ⊥ , for the protonated and deuterated benzene were determined to be 3.0 and 1.7, respectively. While for R II a dynamic isotope effect, defined as the ratio of the rotational diffusion constants of the protonated to the deuterated benzene, of approximately 30% was found, unexpectedly a negative isotope effect of approximately -25% was observed for R ⊥ . Furthermore, the influence of C-H vibrations on the DD and CSA coupling constants and thus on the isotope effects was discussed.

Conformational preference of Ac-AzAla-NHMe has been investigated using ab initio calculations. Azaalanine in peptides prefer to adopt two conformational families including four conformations in which (φ,ψ) are (-90°, -13°), (90°, 13°), (-70°, 164°) and (70°, -164°). The stability at φ = ±90° is explained by repulsion of lone pairs on the nitrogens and hydrogen bonding between Ac-N and terminal N(Me)H. On the other hand, the angle ψ adopts only ~0° or 180° since the N(Me)-C(O) bond has double bond character. One conformational family in which the (φ,ψ) torsion angles are (-90°, -13°) or (90°, 13°), is similar to the i+2 position of typical β-I or β-II turns. The other conformation in which the (φ,ψ) torsion angles are (-70°, 164°) or (70°, -164°), is similar to the polyproline II structure appearing in collagen. We believe that these results are useful in designing constrained peptidomimetics for drug discovery and peptide engineering.

In the concept of ´´molecular traffic control´´, Derouane and Gabelica postulated an enhancement of the effective conversion rate in catalytic reactions if the reactant and product molecules avoid each other by preferentially choosing different channel systems on their diffusion path into and out of the catalyst particles. After two decades of controversial discussions, the feasibility of this concept has recently been demonstrated by considering a network of intersecting single-file channels, whose accessibility by the reactant and product molecules was mutually excluded. The present communication analyses the dependence of molecular distribution over the different elements of the channel system on the intrinsic reactivity and the pore filling factor. The obtained patterns are compared with those of a reference system where all channels are equally accessible by the reactant and product molecules. Special attention is given to the effect of particle self-blockages under molecular traffic control, which occurs for large intrinsic reactivities and pore filling factors. It is demonstrated that under these conditions the simulations end up in an immobilised state which is a function of the particular simulation run. Such a behaviour is not observable in the reference system.

The technique of time resolved frequency modulation (FM) spectroscopy has been shown to provide a very sensitive means to detect small radicals behind shock waves. Features of high temperature FM spectroscopy behind shock waves will be discussed and a general signal conversion procedure to carry out quantitative concentration measurements will be presented. Using a high modulation frequency, a high modulation index and high total optical power, singlet methylene radicals ( 1 CH 2 ) have been detected behind shock waves for the first time. For a pressure of 1 bar the narrow bandwidth absorption coefficient of 1 CH 2 at 590.707 nm (b 1 B 1 (0,14,0)4 04 ← ã 1 A 1 (0,0,0)4 14 ) was determined in the temperature range 1900-2800 K to be: α c /(cm 2 mol -1 ) = -6.16 × 10 5 + ((2.597 × 10 9 ) : (T/K)) - ((1.181 × 10 12 ) : (T/K) 2 ).