Until now, liquid-crystalline compounds with high negative dielectric anisotropy were usually realized by a lateral cyano group. A drawback to these cyano substituted liquid crystals, namely the considerable increase in the viscosity and the reduction of the thermodynamic stability of the mesophase, has been circumvented by preparing 2,3-difluorobenzene derivatives. A universal method to prepare a variety of classes of liquid-crystalline compounds containing the 2,3-difluorophenylene moiety has been developed. The new materials are characterized by high negative A& values of up to -6 , and viscosities comparable with the nonfluorinated compounds. The introduction of the two fluorine atoms also leads to an increase in K,3/K,i. They also suppress higher ordered smectic phases and transform SA into Sc phases. The new compound classes are promising materials for liquid crystal mixtures for various applications as electrically controlled birefringent, supertwisted nematic and ferroelectric liquid crystal displays.

Transient UV absorption spectra after UV laser excitation of S02 were recorded and analyzed with respect to collisional energy transfer. By use of previously determined calibration curves, the absorption-time signals were converted into average energy-number of collision profiles. Energy-dependent average energies transferred per collision (AE) were derived for 22 different collision partners. The temperature dependence of (A£j was determined over the range 300-1500 K by experiments in a C02 CW laser-heated reactor and in shock waves.

The UV absorption spectrum of SO2 molecules in the range 190-436 nm has been investigated under various excitation conditions. The dependence on the temperature was measured in shock-wave experiments over the range 800-4000 K. The results are compared with spectra recorded after laser excitation at 248 and 308 nm. The dependence of the absorption coefficient on the average excitation energy of the ensemble is determined for wavelengths which are suitable for observations in timeresolved collisional-energy-transfer experiments.

The thermal decomposition of SO, and of the primary dissociation product SO have been studied in shock waves by the uv absorption technique. The controversy about SO, dissociation data from uv absorption signals was resolved and attributed to the extensive overlap of SO, and SO uv absorption spectra. The derived rate coefficients

Proton magnetic resonance studies of 2'-deoxyadenosine (2'-dA), 3'-deoxyadenosine (3'-dA), 5'-deoxyadenosine (5'-dA) and 8-bromo-5'-deoxyadenosine (8-Br-5'-dA) have been carried out in the temperature range between -60 degrees and +40 degrees C for ND3 solutios, +40 degrees and +100 degrees C for D2O solutions, and finally +10 degrees and +60 degrees C for pyridine solutions. The analysis is based on the two-state S in equilibrium N model of the ribose moiety proposed by Altona and Sundaralingam. In all solvents, 2'-dA favours slightly the S state of the ribose and the g+ conformer of the exocyclic CH2OH group. However, 3'-dA prefers strongly the N state of the ribose and the g+ conformation. Both the S and N states of the ribose are equally favoured by 5'-DA and 8-Br-5'-dA. Evidence for the existence of an intramolecular hydrogen bond between 0(5') and N3 in purine (beta)-nucleosides is presented. It is also concluded that cordycepin (3'-dA) exists in solution mainly in the anti conformation of the base relative to the ribose.