Phenylpropargyl Radicals and Their Dimerization Products: An IR/UV Double Resonance Study

Two C9H7 isomers, 1-phenylpropargyl and 3-phenylpropargyl, have been studied by IR/UV double resonance spectroscopy in a free jet. The species are possible intermediates in the formation of soot and polyryclic aromatic hydrocarbons (PAH). The radicals are generated by flash pyrolysis from the corresponding bromides and ionized at 255-297 nm in a one color, two photon process. Mid-infrared radiation between 500 and 1800 cm(-1) is provided by a free electron laser (FEL). It is shown that the two radicals can be distinguished by their infrared spectra. In addition, we studied the dimerization products originating from the phenylpropargyl self-reaction. We utilize the fact that the pyrolysis tube can be considered to be a flow reactor permitting us to investigate the chemistry in such a thermal reactor. Dimerization of phenylpropargyl produces predominately species with m/z = 228 and 230. A surprisingly high selectivity has been found: The species with m/z = 230 is identified to be para-terphenyl, whereas m/z = 228 can be assigned to 1-phenylethynyl-naphthalene. The results allow to derive a mechanism for the dimerization of phenylpropargyl and suggest hitherto unexplored pathways to the formation of soot and PAH

A conformation-selective ir-uv study of the dipeptides ac-phe-ser-nh2 and ac-phe-cys-nh2: Probing the sh center dot center dot center dot o and oh center dot center dot center dot o hydrogen bond interactions

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Downconversion for the Er3+, Yb3+ couple in KPb2Cl5: a low-phonon frequency host

Downconversion of a single blue/green photon to two near-infrared photons offers a promising route to increase the efficiency of photovoltaic cells. Here we report on downconversion for the well-known upconversion couple (Er3+, Yb3+) doped into a host with low (∼200 cm−1) maximum phonon energy (KPb2Cl5). The intermediate energy level in both the upconversion and downconversion processes is the 4F7/2 level around 490 nm. While fast multi-phonon relaxation to the lower energy 2H11/2/4S3/2 levels is beneficial for upconversion, it prevents efficient downconversion. To reduce multi-phonon relaxation, a low-phonon energy host (KPb2Cl5) was doped with Er3+ and varying amounts of Yb3+ co-dopant. The results show that downconversion from the 4F7/2 level occurs, exciting two neighboring Yb3+ ions to the 2F5/2 level. The efficiency is however low due to multi-phonon relaxation from the 4F7/2 to the 4S3/2 level via the intermediate 2H11/2 level. Based on the results it is clear that efficient downconversion for the (Er3+, Yb3+) couple requires even lower phonon energy hosts (e.g. bromide host lattices). A Cl−–Yb3+ charge transfer absorption band is observed between 300 and 400 nm. Excitation in this band results in two broad emission bands centered around 430 and 700 nm at temperatures below 30 K, which are assigned to Cl−–Yb3+ charge transfer emission

Far-infrared spectra of the tryptamine A conformer by IR-UV ion gain spectroscopy

Contains fulltext : 166095.pdf (publisher's version ) (Open Access)We present far infrared spectra of the conformer A of tryptamine in the 200 to 500 cm-1 wavenumber range along with resonant photoionization spectra of the far-infrared excited conformer A of tryptamine. We show that single-far-infrared photon excited tryptamine has highly structured resonance enhanced multi-photon ionization spectra, revealing the mode composition of the S1-state. Upon multiple-far-infrared photon absorption, the resonance enhanced multi-photon ionization spectrum broadens allowing ion gain spectroscopy to be performed. In the ion gain spectrum we detect the fundamental far-infrared modes but also combination and overtone bands with high efficiency. The implications to dip spectroscopy using a free electron laser compared to more conventional light sources are discussed

Conformational Study of Z-Glu-OH and Z-Arg-OH: Dispersion Interactions versus Conventional Hydrogen Bonding

The gas-phase conformational preferences of the model dipeptides Z-Glu-OH and Z-Arg-OH have been studied in the low-temperature environment of a supersonic jet. IR-UV ion-dip spectra obtained using the free electron laser FELIX provide conformation-specific IR spectra, which in combination with density functional theory (DFT) allow us to determine the conformational structures of the peptides. Molecular dynamics modeling using simulated annealing generates a variety of low-energy structures, for which geometry optimization and frequency calculations are then performed using the B3LYP functional with the 6-311+G(d,p) basis set. By comparing experimental and theoretical IR spectra, three conformations for Z-Glu-OH and two for Z-Arg-OH have been identified. For three of the five structures, the dispersion interaction provides an important contribution to the stabilization, emphasizing the importance of these forces in small peptides. Therefore, dispersion-corrected DFT functionals (M05-2X and B97D) have also been employed in our theoretical analysis. Second-order Moller-Plesset perturbation theory (MP2) has been used as benchmark for the relative energies of the different conformational structures. Finally, we address the ongoing debate on the gas-phase structure of arginine by elucidating whether isolated arginine is canonical, tautomeric, or zwitterionic

Conformational study of z-glu-oh and z-arg-oh: Dispersion interactions versus conventional hydrogen bonding

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Exploring microsolvation of the anesthetic propofol

Propofol (2,6-diisopropylphenol) is a broadly used general anesthetic. By combining spectroscopic techniques such as 1- and 2-color REMPI, UV/UV hole burning, infrared ion-dip spectroscopy (IRIDS) obtained under cooled and isolated conditions with high-level ab initio calculations, detailed information on the molecular structure of propofol and on its interactions with water can be obtained. Four isomers are found for the bare propofol, while only three are detected for the monohydrated species and two for propofol center dot(H2O)(2). The isopropyl groups do not completely block the OH solvation site, but reduce considerably the strength of the hydrogen bonds between propofol and water. Such results may explain the high mobility of propofol in the GABA(A) active site, where it cannot form a strong hydrogen bond with the tyrosine residue