Volume 36, Issue 7

The internal energetic transfer processes of an E - b and T - t Jahn-Teller system are con­ sidered. The first system allows an exact description, whereas in the case of the second one ap­ proximations must be introduced in the region of intermediate electron-phonon coupling strengths. The occupation probabilities of the states of the uncoupled systems change with time by virtue of the electron-phonon interaction. The energies are directly related to these occupation probabilities. The resonant nature of the T - t system is reflected in the internal dynamics of the latter.

In evaluating the hfs coupling constant A from levelcrossing signals in excited J = 1 states with 7=1/2 where the hfs splitting is smaller than the radiation width, an independent determi­ nation of the lifetime of the state under study is essential. This is demonstrated in the 6s7p 1Pi-state of171 Yb(7 = 1/2). The results are: r(7p *Pi) = 8.9(3) nsec and |^(7p !Pi) | =9(2) MHz.

Fluorene mixed crystals have been investigated by sensitized fluorescence measurements in order to prove the importance of dipole-dipole interactions for singlet energy transfer. Fluorene: dibenzofuran (F:DBF) and fluorene:benz[f]indan (F:BI) are used. In these systems the optical transition dipole moments of host and guest molecules are perpendicular and parallel to one another, respectively. For these systems the host-guest energy transfer rate has been determined as a function of the guest concentration from the quantum ratio of guest and host emission (F:BI) and from fluorescence decay time measurements (F:DBF). The latter verified the particularly low transfer rate in F : DBF found in previous steady state investigations. The measured transfer rates at 1.3 K could be explained quantitatively taking into account the relative orientations of the host and guest transition dipoles and the oscillator strengths of the optical transitions. An increase of the transfer rate with increasing temperature between 1.3 K and 100 K in the system F: BI could be related to a change from energy transfer within perturbed states to excitonic band states of the fluorene crystal.

The fluorescence, So -> Si-and So -> S2-absorption spectra of naphthalene in n-pentane at 77 K were measured. The Franck-Condon-Factors are evaluated using equilibrium geometries and normal coordinates of the molecule in the ground (So), in first (Si) and secound (S2) excited singlet states which were calculated by the QCFF/PI-method and by taking the Dushinskii-Efekt into acount. These results were compared with experimentally determined frequencies and estimated relative intensities.

flu e n c e o f A d s o r p tio n u p o n S o u n d A t te n u a tio n in a T u b e * Sound waves propagating in a tube filled with gas are attenuated because of absorption in bulk, because of viscothermal effects, and possibly because the gas is adsorbed upon the wall. The latter effect is shown to depend upon a thermal relaxation time, the near-equilibrium desorption rate, and a phenomenological coefficient containing the sticking probability. In case of multilayer adsorption, especially upon metals, this data could be deduced from measurements of sound attenuation. An early experiment by Maurer is discussed in this light.

o lta g e -In d u c e d C h o le s te r ic S tr u c tu r e -T r a n s f o r m a tio n in T h i n L a y e rs Measurements of various threshold voltages which occur in thin layers of cholesteric liquid crystals with positive dielectric anisotropy under homeotropic wall-alignment conditions are presented. Characteristic times are determined for some of the structure-changing processes which take place under application of electric-field changes. The measurements have been performed for various pitch-to-thickness ratios, and are expressed in empirical formulae.

The relationship between the surface structure, film thickness and the photoelectrochemical behaviour (photocurrent, corrosion and stability) of n-Ti02-films produced by controlled thermal oxidation of titanium foils is carefully investigated. X-ray surveys, optical and scanning electron microscopical observations were performed to elucidate the surface structure. It is established that a special etching procedure of Ti-metal foils, followed by controlled oxidation at 600 to 700 °C for 10 to 30 min in the presence of sufficient oxygen results in n-Ti02 layers with a defined surface structure and high photoactivity. As charge transfer through the Schottky barrier is decisive for the photoefficiency, the formation kinetics of the oxide layer is the most influencing factor.

Although the procedure to calculate the ligand field energies of terms of a many d-electron configuration in cubic symmetry is known, we present a novel semiquantitative method for the splitting of d-electron terms in cubic symmetry. The method is based on quantitative calculations of one-electron orbitals of a given angular momentum. S p littin g o f T e r m s B e y o n d a D -T e r m in a n O c ta h e d r a l L ig a n d F ield

A detailed description of the overall kinetics of photochemical reactions has to deal with photo-physical activation and de-activation rates as well as with true photochemical rates. Based on the hypothesis that for photoreactions involving the lowest excited triplet state the chemical reaction rates of the individual triplet zero-field levels have different values, a method is presented for the evaluation of these rates from bulk measurements under steady state illumination conditions. The complications arising from the detection of solid state reactions are discussed, and a simple solution is given, illustrated by a numerical example.

le c tro n S p in R e s o n a n c e o f M a n g a n e se N i t r a t e in D ie th y la c e ta m id e -W a te r S o lu tio n At small water concentrations, ESR spectra at room temperature exhibit distinguishable con­ tributions which are ascribed to the following ionic species: (I) a small fraction of residual manga­ nese ions with mixed amide-water solvation, (II) amide solvated ions (having altered hyperfine splitting and gr-factor due to nitrate in an outer coordination sphere, so that they may be con­ sidered loose ion pairs), (III) tight ion pair species. Presumably, (II) and (III) are not well defined "chemical" species but constituents of fluctuating aggregations.

e m ie m p iric a l C a lc u la tio n s o f V a le n c e F o rc e C o n s ta n ts in C 2X 4 , C3X4 a n d C 4X 4 C o m p o u n d s Valence force constants were calculated for C2X4, C3X4 and C4X4 (X = H, F, Cl, Br) with a point charge model using CNDO data. The magnitude of the valence force constants is discussed in terms of the polarization of the static electron distribution.

u r th e o re tis c h e n I n te r p r e ta t io n d e r A n is o tr o p ie d e r 13C -N M R -c h e m is d ie n V e rs c h ie b u n g e n des E th y le n s u n d B en zen s a u f h a lb e m p iris c h e r G r u n d la g e On the Semiempirical Theoretical Interpretation of the Anisotropy of 13C n.m.r. Chemical Shifts of Ethylene and Benzene The paramagnetic contribution of the 13C screening constants and their anisotropics are cal­ culated by using localized orbitals and the CNDO/2 method in the molecular orbital description. A comparison of the average energy method for calculating of shielding tensors with the sum-over-states method is performed. It is concluded that the average excitation energy is higher for the screening perpendicular to the molecular plane.

The conjugative interaction of the C=N bond with the cyclopropylring is studied using UV-spectrophotometric technique. The observed small bathochromic shift in the n-n* bands, relative to the spectra of similar compounds but with no cyclopropylring, is attributed to the interaction of the internal Walsh MO with the nonbonded MO of the Nitrogen atom (Zlint-n interaction) in the bisected conformation of the molecule. The observed bathochromic shifts of the 71-71* bands are explained in terms of the A-n interaction. The gas phase spectra of the azomethine derivatives show vibronic structuring of the band which when analyzed yeld wave number differences be­ tween the successive vibronic peaks, of the magnitude 720—860 cm-1. They are attributed to to the wave number differences between the successive C—N=C deformed ion levels in the excited state. I n te r a c tio n o f th e C = N B o n d w ith th e C y c lo p r o p y l R in g

Two independent symmetry coordinate sets for P4S3 are developed, starting from the "method of fragments". A simple, approximate force field is expressed in terms of the two sets of symmetry coordinates, and the corresponding compliance matrices are given. The invariance of compliants is demonstrated. The potential energy distribution (PED) is discussed. An example is shown where the PED terms are clearly inadequate for the description of normal modes. A general warning against the interpretation of the PED in terms of such descriptions for cage-like structures seems to be warranted. V ib r a tio n s o f a C a g e -lik e M o le c u le , P 4S3 : S o m e T h e o r e tic a l A sp e c ts

The lifetimes for the 4d95p configuration have been determined by zero field level crossing technique. The re­ sults are (in nsec):

The hfs of the 4d 9 5p 1 P 1 0 -and 3 D 1 0 -states of 105 Pd(I = 5/2) has been investigated by level-crossing spec­ troscopy. From the measurements we get the following hfs coupling constants: A ( 3 D 1 0 ) = - 302 (6) MHz and A ( 1 P 1 0 ) = -220 (6) MHz.

A simple and effective method is described to calculate the ground state energy of H~ starting with the Hartree Fock wavefunction. The expectation values of the opera­ tors 〈r 1 • r 2 〉, 〈r 1 n + r 2 n 〉 and 〈p 1 • p 2 〉 can be estimated easily with the present procedure. Oscillator strength sums S(k) for k= -1,0, 1 are also evaluated.