Second-order nonlinear susceptibilities of surfaces

Systematic studies were carried out with the goal to further develop optical Second-Harmonic Generation (SHG) as a tool for surface and interface analysis. For this purpose hexadecane thiol, HS(CH₂)₁₅CH₃, on polycrystalline Au films served as a model system to study the spectral behavior of the susceptibility of the gold surface and its variation as a function of coverage. Experiments have been performed at different coverages, with different polarization combinations of the incident and detected light, different angles of incidence and different wavelengths ranging from z=615 nm to 660 nm. In addition to the intensity of the generated second-harmonic light, its phase was measured. As a result, a complete determination of all independent elements of the susceptibility tensor became possible. The studies with different excitation wavelengths reveal the presence of resonances of the tensor elements and strikingly demonstrate the importance of phase measurements for an unambiguous interpretation of the data. The superposition of resonant contributions originating from different elements of the susceptibility tensor is discussed and gives rise to very distinct and peculiar spectral features.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Principal component and decomposition analysis of multicomponent mixtures of carcinogenic fluorophores

A series of binary and tertiary mixtures of polycyclic aromatic hydrocarbons is analyzed by using principal component and decomposition analysis of molecular fluorescence spectra. The results demonstrate the ability to determine the number and identity of species that are present. Failures caused by high correlation among spectra are considered.     

CNDO/2 and INDO calculations of a reaction pathway for the sigmatropic [1, 5] H-shift in cyclopentadiene

CNDO/2 and INDO calculations have been carried out in order to construct a suitable model for the activated complex during the reaction. In this reaction model the migrating hydrogen atom moves along an edge of the cyclopentadiene ring. An analysis of this situation suggests a partial electron transfer from the migrating hydrogen to the nascent cyclopentadienyl system. This charge transfer is discussed in terms of aromaticity. The calculated activation enthalpies are 10 kcal/mole (CNDO/2) and 17 kcal/mole (INDO), whereas the experimental value is ca. 24 kcal/mole [1].     

Die Bestimmung kleiner Fluorgehalte in Pflanzen

Zusammenfassung In der vorliegenden Arbeit wird eine Methode zur exakten Bestimmung des Fluorgehaltes in Pflanzen beschrieben. Das Verfahren stellt eine Kombination der Willard-Winter-Destillation mit einer von Belcher, Leonard u. West gefundenen und von uns verbesserten colorimetrischen Fluoridbestimmungsmethode dar. Diese positive Farbreaktion des Fluorions ist im Vergleich zu den bisher angewandten Ausbleichreaktionen (z.B. Thorium/Alizarinsulfonsäure- oder Zirkonium/Eriochromcyanin-Verfahren) erheblich spezifischer. Unter den Bedingungen der Pflanzenanalyse ist sie kaum störanfällig. Die aus über 100 Analysen ermittelte Standardabweichung des Verfahrens liegt unter den fixierten Analysen-bedingungen bei Fluorgehalten von 1·10^–3% F^– im Bereich von 4% (rel.).     

Photodissociation of hydrogen iodide on the surface of large argon clusters: the orientation of the librational wave function and the scattering from the cluster cage

A set of photodissociation experiments and simulations of hydrogen iodide (HI) on Arn clusters, with an average size n = 139, has been carried out for different laser polarizations. The doped clusters are prepared by a pick-up process. The HI molecule is then photodissociated by a UV laser pulse and the outgoing H fragment is ionized by resonance enhanced multiphoton ionization in a (2 + 1) excitation scheme within the same laser pulse at the wavelength of 243 nm. The measured time-of-flight spectra are transformed into hydrogen kinetic energy distributions. They exhibit a strong fraction of caged H atoms at zero-kinetic energy and peaks at the unperturbed cage exit for both spin-orbit channels nearly independent of the polarization. At this dissociation wavelength, the bare HI molecule exhibits a strict state separation, with a parallel transition to the spin-orbit excited state and perpendicular transitions to the ground state. The experimental results have been reproduced using molecular simulation techniques. Classical molecular dynamics was used to estimate the HI dopant distribution after the pick-up procedure. Subsequently, quasi-classical molecular dynamics (Wigner trajectories approach) has been applied for the photodissociation dynamics. The following main results have been obtained: (i) The HI dopant lands on the surface of the argon cluster during the pick-up process, (ii) zero-point energy plays a dominant role for the hydrogen orientation in the ground state of HI-Arn surface clusters, qualitatively changing the result of the photodissociation experiment upon increasing the number of argon atoms, and, finally, (iii) the scattering of hydrogen atoms from the cage which originate from different dissociation states seriously affects the experimentally measured kinetic energy distributions.     

Analysis of binary mixtures by computer decomposition of molecular fluorescence spectra

Qualitative analysis by computer decomposition of fluorescence spectra by means of the program SPECSOLV is discussed. Concentration studies of naphthacene in benzene and mixture analyses of single and binary solutions of anthracene, naphthacene, naphthalene,and rubrene are reported. The results demonstrate the ability to separate solvent and sample spectral contributions, and to assign component peaks in the case of mixtures. Semi-quantitative results are presented, and the feasibility of extending the study to computersearch systems based on component characterization of fluorescence spectra is discussed.     

An assignment of approximate internal energy distributions through substituent effects to ions produced by electron-impact

A probability distribution of internal energies is derived for aromatic molecular ions which can decompose to give only a single product whose relative intensity and appearance potential are each a function of Hammett sigma constants, with certain simplifying assumptions.     

Transport Properties of H+-Selective Membranes Containing Amine-Derivative Ionophores and Mobile Sites

Transport processes in plasticized PVC membranes containing the H⁺ chromoionophore ETH 5294 and additional tetraphenylborate-type mobile sites were analyzed by employing the potential-step method. Using chronoamperometry, impedance spectroscopy, and UV/VIS absorbance measurements, it is shown that the ionophore behaves as a carrier for protons, when the membranes are immersed between two aqueous solutions. It was possible to document the changes in the concentration profiles of both free ionophores and ionic species in some of the membranes. There is no evidence of proton hopping between the ionophores.     

Interactions of alkyltin salts with biological dithiols: dealkylation and induction of a regular beta-turn structure in peptides

Organotin compounds specifically target vicinal dithiols, thereby inhibiting the function of essential enzymes. Here, we present the NMR binding studies of trimethyltin (TMT) and dimethyltin (DMT) chlorides with a linear peptide (ILGCWCYLR) derived from the membrane protein stannin (SNN). We show that this peptide is able to dealkylate TMT and bind DMT, adopting a stable type-I beta-turn conformation. Both the NMR data and the calculated structures indicate that the two cysteines coordinate the tin atom in a distorted tetrahedral geometry. The molecular geometries and tin coordination state were confirmed using density functional theory (DFT). In addition, NMR spectral parameters back calculated from the DFT minimized structure compared well with experimental data. These results in conjunction with studies on peptide variants (i.e., C4S, C6S, and Y7F) demonstrate unequivocally the key role of biological dithiols in both the dealkylation and binding of organotin compounds. This peptide serves as a model system for alkyltin-protein interactions and gives new insights into the biological fate of alkyltin compounds.     

Molecular design of polymer coatings capable of photo-triggered stress relaxation via dynamic covalent bond exchange

Polymer coatings are frequently used to modify surface properties of inorganic substrates. However, the disparity in physical properties between polymer film and substrate often leads to residual stress development, which can be deleterious to the overall performance of coated materials. This work reports the molecular design of polymer films that dissipate stress upon irradiation with ultraviolet (UV) light. These polymers are synthesized by post-polymerization modification of the reactive polymer, poly(2-vinyl-4,4-dimethyl azlactone), to introduce dynamic crosslinks capable of light-initiated addition transfer fragmentation chemistry. Using a custom-built optical cantilever, contrasting film stress responses are observed between films containing dynamic bonds and analogous control films after UV light irradiation, which indicate successful stress relaxation. Further experiments demonstrate the complete relaxation of residual stress in dynamic films after an extended exposure, thereby generating a “stress-free” film. Films fabricated using this approach can be easily tailored to incorporate additional moieties to introduce desired surface properties for future application in a wide array of coatings.