Electronic spectroscopy of transient species in plasma discharges

The work was focused on the spectroscopic study of carbon chain species as well as small boron clusters in the gas phase. The experimental apparatus was built before, however, in order to produce boron species. The source had to be modified as in this case a solid sample of precursor had to be used. Up to now the apparatus could be used to study the electronic transitions of variety of the species producing only by discharging a suitable gas precursor. The heating system built for this purpose worked flawlessly and the first gas-phase electronic spectrum of B3 could be obtained. This experiment showed that solid-state precursors can be also used for the production of the exotic species in the gas phase, which increases significantly the potential of the CRD experiment. The heated source can be promising for further studies of the bare carbon chains C6+, C7+ and C9+. Their spectra were already measured in the neon matrix and solid precursor (perchloronaphtalene) was used in this case. Since C10Cl8 worked very well in the matrix experiment, it seems reasonable to assume that in the CRD experiment this precursor could work as well. As mentioned before, cavity ringdown spectroscopy combined with a supersonic slit jet plasma have been applied to obtain gas-phase spectra of the variety of carbon chain species. They ranged from rather short ones like HC4H+, to relatively long such as HC10H+. Most of them are linear, however, spectra of the nonlinear carbon chains were also measured (C4H4+, C6H4+ and C8H4+). High sensitivity of the cavity ringdown method and low temperature attained in a jet, in many cases allowed to obtain and analyze rotationally resolved electronic spectra. One of the main results of the work was obtaining the electronic spectrum of the pentaacetylene cation. It turned out to be the largest polyacetylene cation measured in the gas phase so far. Astrophysical relevance of the HC10H+ was considered, however, the work showed that HC10H+ can not be a carrier of the DIBs. There was no match with the hitherto reported diffuse interstellar bands in the near infrared. Apart from carbon chain species, the spectrum of the cyclic B3 molecule was also measured. It was the first detection of this molecule in the gas phase. The analysis of the rotational structure of the spectrum allowed the geometry of the molecule to be inferred, which turned out to be the first structural information on cyclic boron trimer in the gas phase

Ligand spheres in asymmetric hetero Diels-Alder reactions catalyzed by Cu(II) box complexes: experiment and modeling

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The stereoselective hetero Diels–Alder reaction between ethyl glyoxylate and cyclohexadiene catalyzed by [Cu(II)t-Bu-(box)](OTf)2 was investigated. The reaction was performed step-by-step and the geometry of the Cu(II) complexes formed in the course of the catalysis was analysed by EPR spectroscopy, advanced pulsed EPR methods (ENDOR, and HYSCORE) and DFT calculations. Our results show that one triflate counterion is directly coordinated to Cu(II) during the catalytic process (axial position). This leads to penta-coordinated Cu(II) complexes. Solvent molecules are able to alter the geometry of the Cu(II) complexes although their coordination is weak. These findings provide an explanation for the solvent and counterion effects observed in many catalytic reactions

High resolution electronic spectroscopy of a non-linear carbon chain radical C6H4+.

The electronic spectrum of a member of a so-far-unstudied class of carbon chain radicals was observed:a nonlinear and noncyclic species. The spectrum was observed more or less accidentally around 604 nm when scanning for coincidences with diffuse interstellar band features in a hydrocarbon plasma. The observed spectrum has a clear rotational and K-type structure. Simulation of the spectrum allowed an accurate determination of the molecular constants of the carrier

DETECTION OF ATMOSPHERICALLY RELEVANT HYDROCARBONS BY DIODE LASER CAVITY RINGDOWN SPECTROSCOPY

Author Institution: Diagnostic Instrumentation and Analysis Laboratory, Mississippi State University, Starkville, MS 39759; Chemistry Department, State University of New York, College of Environmental Science and Forestry, Syracuse, NY 13210The first overtones of the asymmetric CH stretch of 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-1,3-butadiene and 1,3-butadiene were observed in gas phase. The absorption was monitored by means of continuous wave Cavity Ring Down Spectroscopy (cw CRDS) at room temperature. A near infrared diode laser was employed as a light source. The absorption cross-section was determined at 1651.5 nm. This study is a starting point for future studies of the peroxy radicals formed in the OH-initiated degradation of these atmospherically relevant compounds. The preliminary estimates of the absorption cross-section are 2.88 x $10^{-22}$ cm$^{2}$ for 2,3-dimethyl-1,3-butadiene and 7.22 x $10^{-22}$ cm$^{2}$ for isoprene, respectively. The NIR absorption cross-sections for isoprene will be calibrated using the known UV absorption cross-section

From Homoconjugated Push-Pull Chromophores to Donor-Acceptor-Substituted Spiro Systems by Thermal Rearrangement

Series of homoconjugated push-pull chromophores and donor-acceptor (D-A)-functionalized spiro compounds were synthesized, in which the electron-donating strength of the anilino donor groups was systematically varied. The structural and optoelectronic properties of the compounds were investigated by X-ray analysis, UV/Vis spectroscopy, electrochemistry, and computational analysis. The homoconjugated push-pull chromophores with a central bicyclo[4.2.0]octane scaffold were obtained in high yield by [2+2] cycloaddition of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) to N,N-dialkylanilino-or N,N-diarylanilino-substituted activated alkynes. The spirocyclic compounds were formed by thermal rearrangement of the homoconjugated adducts. They also can be prepared in a one-pot reaction starting from DDQ and anilino-substituted alkynes. Spiro products with N, N-diphenylanilino and N,N-diisopropylanilino groups were isolated in high yields whereas compounds with pyrrolidino, didodecylamino, and dimethylamino substituents gave poor yields, with formation of insoluble side products. It was shown by in situ trapping experiments with TCNE that cycloreversion is possible during the thermal rearrangement, thereby liberating DDQ. In the low-yielding transformations, DDQ oxidizes the anilino species present, presumably via an intermediate iminium ion pathway. Such a pathway is not available for the N, N-diphenylanilino derivative and, in the case of the N, N-diisopropylanilino derivative, would generate a strained iminium ion (A1,3 strain). The mechanism of the thermal rearrangement was investigated by EPR spectroscopy, which provides good evidence for a proposed biradical pathway starting with the homolytic cleavage of the most strained (CN)C-C(CN) bond between the fused four-and six-membered rings in the homoconjugated adducts

Oxorhenium(V) Complexes with Phenolate–Oxazoline Ligands: Influence of the Isomeric Form on the O‑Atom-Transfer Reactivity

The bidentate phenolate–oxazoline ligands 2-(2′-hydroxyphenyl)-2-oxazoline (<b>1a</b>, Hoz) and 2-(4′,4′-dimethyl-3′,4′-dihydrooxazol-2′-yl)­phenol (<b>1b</b>, Hdmoz) were used to synthesize two sets of oxorhenium­(V) complexes, namely, [ReOCl₂(L)­(PPh₃)] [L = oz (<b>2a</b>) and dmoz (<b>2b</b>)] and [ReOX­(L)₂] [X = Cl, L = oz (<b>3a</b> or <b>3a′</b>); X = Cl, L = dmoz (<b>3b</b>); X = OMe, L = dmoz (<b>4</b>)]. Complex <b>3a′</b> is a coordination isomer (<i>N</i>,<i>N</i>-cis isomer) with respect to the orientation of the phenolate–oxazoline ligands of the previously published complex <b>3a</b> (<i>N</i>,<i>N</i>-trans isomer). The reaction of <b>3a′</b> with silver triflate in acetonitrile led to the cationic compound [ReO­(oz)₂(NCCH₃)]­(OTf) ([<b>3a′</b>]­(OTf)). Compound <b>4</b> is a rarely observed isomer with a <i>trans</i>-ORe–OMe unit. Complexes <b>3a</b>, <b>3a′</b>, [<b>3a′</b>]­(OTf), and <b>4</b> were tested as catalysts in the reduction of a perchlorate salt with an organic sulfide as the O acceptor and found to be active, in contrast to <b>2a</b> and <b>2b</b>. A comparison of the two isomeric complexes <b>3a</b> and <b>3a′</b> showed significant differences in activity: 87% <b>3a</b> vs 16% <b>3a′</b> sulfoxide yield. When complex [<b>3a</b>′]­(OTf) was used, the yield was 57%. Density functional theory calculations circumstantiate all of the proposed intermediates with <i>N</i>,<i>N</i>-trans configurations to be lower in energy compared to the respective compounds with <i>N</i>,<i>N</i>-cis configurations. Also, no interconversions between <i>N</i>,<i>N</i>-trans and <i>N</i>,<i>N</i>-cis configurations are predicted, which is in accordance with experimental data. This is interesting because it contradicts previous mechanistic views. Kinetic analyses determined by UV–vis spectroscopy on the rate-determining oxidation steps of <b>3a</b>, <b>3a′</b>, and [<b>3a′</b>]­(OTf) proved the <i>N</i>,<i>N</i>-cis complexes <b>3a′</b> and [<b>3a′</b>]­(OTf) to be slower by a factor of ∼4

6,6-Dicyanopentafulvenes: Electronic Structure and Regioselectivity in [2 + 2] Cycloaddition–Retroelectrocyclization Reactions

We present an investigation of the electronic properties and reactivity behavior of electron-accepting 6,6-dicyanopentafulvenes (DCFs). The electron paramagnetic resonance (EPR) spectra of the radical anion of a tetrakis­(silylalkynyl) DCF, generated by Na metal reduction, show delocalization of both the charge and unpaired electron to the nitrogens of the cyano moieties and also, notably, to the silicon atoms of the four alkynyl moieties. By contrast, in the radical anion of the previously reported tetraphenyl DCF, coupling to the four phenyl rings is strongly attenuated. The data provide physical evidence for the different conjugation between the DCF core and the substituents in both systems. We also report the preparation of new fulvene-based push–pull chromophores via formal [2 + 2] cycloaddition–retroelectrocyclization reaction of DCFs with electron-rich alkynes. Alkynylated and phenylated DCFs show opposite regioselectivity of the cycloaddition, which can be explained by the differences in electronic communication between substituents and the DCF core as revealed in the EPR spectra of the radical anions