Volume 44, Issue 7

The frequency dependence of the longitudinal proton spin relaxation time T 1 was measured by field-cycling and standard NMR techniques in the nematic, smectic and reentrant nematic mesophases of the reentrant liquid crystal 6OCB-8OCB. The results clearly demonstrate that, in contrast to conclusions reported in the literature, the underlying relaxation mechanisms are the same for the nematic and reentrant nematic order, but different in the nematic and smectic phase. In particular, at low frequencies the nematic and reentrant samples reveal a square-root dispersion profile (T 1 ,~ ν ½ ) characteristic for nematic director fluctuations, whereas the smectic samples show a dissimilar and only very narrow dispersion step. It can be attributed to a linear relaxation time increase (T 1 ~ v 1 ) expected for smectic-type order fluctuations. The experimental data are described and discussed in terms of four molecular processes, namely nematic or smectic order fluctuations, self-diffusion, rotational motions, and a resonant proton nitrogen coupling.

Mobile triplet excitons in A/TCPA have been detected from 1.2 to 300 K via ESR and zero-field ODMR. Their features such as linewidth and zero-field-splitting (zfs) parameters are discussed in terms of the temperature-dependent crystal structure. Between 1.2 and 4.2 K the excitons are caged by barriers. When raising the temperature a thermally activated hopping takes place between two triplet bands separated by 40 cm −1 , having different zfs parameters | D A | = 2126.5, |E A | = 241.0, | D B | = 2112.5, | E B | = 238.0 MHz and different zfs tensor orientations. Above 100 K an increase of the librations of the anthracene molecules broadens the homogeneous excitonic lines. The homogeneous linewidth is measured by the free induction decay. The CT character of the triplet state is estimated as c 2 CT <̳4%.

The electrical resistivity of GdBrH x (0.67 <̳ x <̳ 1.0) and GdBrH 2 , has been measured as a function of temperature and environment (H 2 , vacuum). A two-electrode dc method and a contactless microwave technique were used. Both methods indicate a metal-to-semiconductor transition for the reaction GdBrH x GdBrH 2 .

The nonlinear transport behavior during low-temperature avalanche breakdown of extrinsic germanium is associated with the self-generated formation of spatio-temporal current structures. Very close to the critical phase transition between different conducting states, the underlying physical relaxation processes develop on relatively slow macroscopic time scales in the ms range. We have evaluated the slowing down of the characteristic time constants from independent measurements of the system response behavior to external pulsed excitations.

The absorption spectra of carrier free 14 C 16 O 2 in the spectral range of 2290-2150 cm −1 by a medium resolution FTIR spectrometer, and the spectral region of 2258 − 2229 cm −1 with a high resolution tunable diode laser spectrometer have been recorded. Spectroscopic constants were calculated from the 00 0 1 − 00 0 0 and 01 1 1 −01 1 0 transitions.

The infrared absorption lines of a mixture of 14 C 2 H 2 and 12 C 14 CH 2 around 725 cm −1 have been recorded with a high resolution (0.04 cm −1 ) FTIR spectrometer. Rotational structures of P, R and Q branches of the ν 1 5 bands were analyzed and molecular constants ν 0 , B′, B″, D′ and D″ calculated.

The paper discusses the interrelationship between statical chaos and dynamical initial value problems. It is pointed out that approximate homoclinic and heteroclinic solitons can be perturbed to produce spacial asymptotic chaos in some buckled structural elastic systems which constitute strictly speaking boundary value problems.

High sensitivity and resolution of our microwave Fourier transform (MWFT) spectrometer and careful analysis of the measured transients enabled us to determine very accurate spin-rotation and spin-spin interaction constants of methane. In addition we were able to increase the accuracy of the existing centrifugal distortion constants.

The 14 N hyperfine structure in the rotational spectra of cyclopropylamine and cyclopropyl cyanide has been reinvestigated by microwave Fourier transform spectroscopy. The observed quadrupole coupling constants in units of MHz are: X aa = 2.3338(18), X bb = 1.7874(20), X cc = −4.1209(20) for cyclopropylamine and X aa = −3.4536(35), X bb = 1.7468(51), X cc = 1.7068(51) for cyclopropyl cyanide.

The crystal structures of glycyl-l-alanine hydrobromide monohydrate, 1, and glycyl-l-alanine hydroiodide monohydrate, 2, were determined and the temperature dependence of the 127 I NQR frequencies was investigated in the temperature range 77 <̳T/K <̳ 370. The 127 I NQR frequencies are strongly influenced by hydrogen bonds and this is proved by the frequency shift of the H ⇄ D exchange. By deuteration the nuclear quadrupole coupling constant e 2 qQh −1 ( 127 I) of 2 is shifted downwards 2.72 MHz at room temperature. The title compounds are isotype, and at room temperature they crystallize monoclinic with the space group C 2 2 − P2 1 with two molecules in the unit cell. The lattice constants for 1 are a = 1068.7 pm, b = 614.1 pm, c = 762.0 pm, and β= 108.55° and for 2 are a = 1093.3 pm, 6 = 637.1 pm, c = 770.9 pm, and β= 107.29°.

The 14 N nuclear quadrupole hyperfine structure in the rotational spectra of three isotopic species of methyl azide, CH 3 14 N 3 , CH 3 15 N 14 N 2 , and CH 3 14 N 2 15 N, has been resolved using microwave Fourier transform spectroscopy. The quadrupole coupling constants of 14 N at all three positions have been evaluated and are compared with those from an ab initio calculation in the literature. Since the spectra of the substituted species have been obtained for the first time, they have provided new structural information: the rotational constants are consistent with a structure in which the NNN chain is slightly bent.

For the first time carbohydrate derivatives were studied as dopants for calamitic smectic phases. Fast switching ferroelectric cells could be realized, and electroclinic effects were observed. By inversion of configuration at a chiral center of these compounds switching times, solubility, and helical twisting power vary by orders of magnitude. The studied derivatives showed modest solubility properties.

The ground state rotational spectrum of 2-isocyano-propane is assigned. The rotational constants and the quartic centrifugal distortion constants are determined by Microwave Fourier Transform (MWFT) Spectroscopy. The analysis of the Stark effect leads to a total dipole moment of 4.055(1) D.

The strong diabaticity of the 2 Σ + g excited states of He + 2 has been computationally verified in the region of the well known avoided crossing between the 1 2 Σ + g and the 2 2 Σ + g adiabatic states. A large transition rate between these two states has been found, which is of the order of 10 15 sec -1 .

The rotational Zeeman effect of 1,2,4-trifluorobenzene has been studied for 8 low-J rotational transitions in magnetic fields between 1.9 and 2.4 Tesla. The observed susceptibility anisotropics and molecular g-values are: (2χ aa− χ bb− χcc) = 37.85(69) • 10 −6 erg G −2 mole −1 , (2χbb−χcc−χaa) = 56.85(54) • 10 −6 erg G −2 mole −1 , g aa = −0.0393(3), g bb = −0.0277(3), and gcc = 0.0042(2). The Zeeman parameters have been used to derive the molecular electric quadrupole moments and vibronic ground state expectation values for the electronic second moments. The observed out-of-plane quadrupole moment is discussed with reference to an additivity scheme proposed earlier. The observed out-of-plane component of the molecular magnetic susceptibility tensor is in excellent agreement with the value predicted earlier from the CNDO/2-π-electron density alternation at the ring atoms.