13 research outputs found

    Unimolecular Dissociation of 1‑Methylpyrene Cations: Why Are 1‑Methylenepyrene Cations Formed and Not a Tropylium-Containing Ion?

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    1-Methylpyrene radical cations undergo the loss of a hydrogen atom at internal energies above the first dissociation threshold. Imaging photoelectron photoion coincidence spectroscopy was employed in combination with RRKM modeling to determine a 0 K activation energy of 2.78 ± 0.25 eV and an entropy of activation of 6 ± 19 J K<sup>–1</sup> mol<sup>–1</sup> for this H-loss reaction. The ionization energy of 1-methylpyrene was measured by mass-selected threshold photoelectron spectroscopy to be 7.27 ± 0.01 eV. These values were found to be consistent with calculations at the CCSD/6-31G­(d)//B3-LYP/6-31G­(d) level of theory showing that the formation of the 1-methylenepyrene cation (resulting from H loss from the methyl group) is kinetically more favorable than the formation of a tropylium-containing product ion that is structurally analogous to the formation of the tropylium cation in H loss from ionized toluene. The shift away from a tropylium-containing structure was found to be due to the increased ring strain imposed on the C7 moiety when it is bound to three fused benzene rings. The RRKM results allow for the derivation of the Δ<sub>f</sub><i>H</i><sub>0</sub><sup>o</sup> (1-methylenepyrene cation) of 945 ± 31 kJ mol<sup>–1</sup>

    Unimolecular Dissociation of 1‑Methylpyrene Cations: Why Are 1‑Methylenepyrene Cations Formed and Not a Tropylium-Containing Ion?

    No full text
    1-Methylpyrene radical cations undergo the loss of a hydrogen atom at internal energies above the first dissociation threshold. Imaging photoelectron photoion coincidence spectroscopy was employed in combination with RRKM modeling to determine a 0 K activation energy of 2.78 ± 0.25 eV and an entropy of activation of 6 ± 19 J K<sup>–1</sup> mol<sup>–1</sup> for this H-loss reaction. The ionization energy of 1-methylpyrene was measured by mass-selected threshold photoelectron spectroscopy to be 7.27 ± 0.01 eV. These values were found to be consistent with calculations at the CCSD/6-31G­(d)//B3-LYP/6-31G­(d) level of theory showing that the formation of the 1-methylenepyrene cation (resulting from H loss from the methyl group) is kinetically more favorable than the formation of a tropylium-containing product ion that is structurally analogous to the formation of the tropylium cation in H loss from ionized toluene. The shift away from a tropylium-containing structure was found to be due to the increased ring strain imposed on the C7 moiety when it is bound to three fused benzene rings. The RRKM results allow for the derivation of the Δ<sub>f</sub><i>H</i><sub>0</sub><sup>o</sup> (1-methylenepyrene cation) of 945 ± 31 kJ mol<sup>–1</sup>

    Comparing Femtosecond Multiphoton Dissociative Ionization of Tetrathiafulvene with Imaging Photoelectron Photoion Coincidence Spectroscopy

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    In this paper we describe femtosecond photoionization and the imaging photoelectron photoion coincidence spectroscopy of tetrathiafulvene, TTF. Femtosecond photoionization of TTF results in the absorption of up to twelve 808 nm photons leading to ion internal energies up to 12.1 eV as deduced from the photoelectron spectrum. Within this internal energy a variety of dissociation channels are accessible. In order to disentangle the complex ionic dissociation, we utilized the imaging photoelectron photoion coincidence (iPEPICO) technique. Above the dissociation threshold, iPEPICO results show that the molecular ion (<i>m</i>/<i>z</i> = 204) dissociates into seven product ions, six of which compete in a 1.0 eV internal energy window and are formed with the same appearance energy. Ab initio calculations are reported on the possible fragment ion structures of five dissociation channels as well as trajectories showing the loss of C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>H<sub>2</sub>S from high internal energy TTF cations. A three-channel dissociation model is used to fit the PEPICO data in which two dissociation channels are treated as simple dissociations (one with a reverse barrier), while the rest involve a shared barrier. The two lower energy dissociation channels, <i>m</i>/<i>z</i> = 146 and the channel leading to <i>m</i>/<i>z</i> = 178, 171, 159, 140, and 127, have <i>E</i><sub>0</sub> values of 2.77 ± 0.10 and 2.38 ± 0.10 eV, respectively, and are characterized by Δ<i>S</i><sup>‡</sup><sub>600 K</sub> values of −9 ± 6 and 1 ± 6 J K<sup>–1</sup> mol<sup>–1</sup>, respectively. Competing with them at higher internal energy is the cleavage of the central bond to form the <i>m</i>/<i>z</i> = 102 fragment ion, with an <i>E</i><sub>0</sub> value of 3.65 ± 0.10 eV and Δ<i>S</i><sup>‡</sup><sub>600 K</sub> = 83 ± 10 J K<sup>–1</sup> mol<sup>–1</sup>

    Comparing Femtosecond Multiphoton Dissociative Ionization of Tetrathiafulvene with Imaging Photoelectron Photoion Coincidence Spectroscopy

    No full text
    In this paper we describe femtosecond photoionization and the imaging photoelectron photoion coincidence spectroscopy of tetrathiafulvene, TTF. Femtosecond photoionization of TTF results in the absorption of up to twelve 808 nm photons leading to ion internal energies up to 12.1 eV as deduced from the photoelectron spectrum. Within this internal energy a variety of dissociation channels are accessible. In order to disentangle the complex ionic dissociation, we utilized the imaging photoelectron photoion coincidence (iPEPICO) technique. Above the dissociation threshold, iPEPICO results show that the molecular ion (<i>m</i>/<i>z</i> = 204) dissociates into seven product ions, six of which compete in a 1.0 eV internal energy window and are formed with the same appearance energy. Ab initio calculations are reported on the possible fragment ion structures of five dissociation channels as well as trajectories showing the loss of C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>H<sub>2</sub>S from high internal energy TTF cations. A three-channel dissociation model is used to fit the PEPICO data in which two dissociation channels are treated as simple dissociations (one with a reverse barrier), while the rest involve a shared barrier. The two lower energy dissociation channels, <i>m</i>/<i>z</i> = 146 and the channel leading to <i>m</i>/<i>z</i> = 178, 171, 159, 140, and 127, have <i>E</i><sub>0</sub> values of 2.77 ± 0.10 and 2.38 ± 0.10 eV, respectively, and are characterized by Δ<i>S</i><sup>‡</sup><sub>600 K</sub> values of −9 ± 6 and 1 ± 6 J K<sup>–1</sup> mol<sup>–1</sup>, respectively. Competing with them at higher internal energy is the cleavage of the central bond to form the <i>m</i>/<i>z</i> = 102 fragment ion, with an <i>E</i><sub>0</sub> value of 3.65 ± 0.10 eV and Δ<i>S</i><sup>‡</sup><sub>600 K</sub> = 83 ± 10 J K<sup>–1</sup> mol<sup>–1</sup>

    On the Dissociation of the Naphthalene Radical Cation: New iPEPICO and Tandem Mass Spectrometry Results

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    The dissociation of the naphthalene radical cation has been reinvestigated here by a combination of tandem mass spectrometry and imaging photoelectron photoion coincidence spectroscopy (iPEPICO). Six reactions were explored: (R1) C<sub>10</sub>H<sub>8</sub><sup>•+</sup> → C<sub>10</sub>H<sub>7</sub><sup>+</sup> + H (<i>m</i>/<i>z</i> = 127); (R2) C<sub>10</sub>H<sub>8</sub><sup>•+</sup> → C<sub>8</sub>H<sub>6</sub><sup>•+</sup> + C<sub>2</sub>H<sub>2</sub> (<i>m</i>/<i>z</i> = 102); (R3) C<sub>10</sub>H<sub>8</sub><sup>•+</sup> → C<sub>6</sub>H<sub>6</sub><sup>•+</sup> + C<sub>4</sub>H<sub>2</sub> (<i>m</i>/<i>z</i> = 78); (R4) C<sub>10</sub>H<sub>8</sub><sup>•+</sup> → C<sub>10</sub>H<sub>6</sub><sup>•+</sup> + H<sub>2</sub> (<i>m</i>/<i>z</i> = 126); (R5) C<sub>10</sub>H<sub>7</sub><sup>+</sup> → C<sub>6</sub>H<sub>5</sub><sup>+</sup> + C<sub>4</sub>H<sub>2</sub> (<i>m</i>/<i>z</i> = 77); (R6) C<sub>10</sub>H<sub>7</sub><sup>+</sup> → C<sub>10</sub>H<sub>6</sub><sup>•+</sup> + H (<i>m</i>/<i>z</i> = 126). The <i>E</i><sub>0</sub> activation energies for the reactions deduced from the present measurements are (in eV) 4.20 ± 0.04 (R1), 4.12 ± 0.05 (R2), 4.27 ± 0.07 (R3), 4.72 ± 0.06 (R4), 3.69 ± 0.26 (R5), and 3.20 ± 0.13 (R6). The corresponding entropies of activation, Δ<i>S</i><sup>‡</sup><sub>1000K</sub>, derived in the present study are (in J K<sup>–1</sup> mol<sup>–1</sup>) 2 ± 2 (R1), 0 ± 2 (R2), 4 ± 4 (R3), 11 ± 4 (R4), 5 ± 15 (R5), and −19 ± 11 (R6). The derived <i>E</i><sub>0</sub> value, combined with the previously reported IE of naphthalene (8.1442 eV) results in an enthalpy of formation for the naphthyl cation, Δ<sub>f</sub><i>H</i>°<sub>0K</sub> = 1148 ± 14 kJ mol<sup>–1</sup>/Δ<sub>f</sub><i>H</i>°<sub>298K</sub> = 1123 ± 14 kJ mol<sup>–1</sup> (site of dehydrogenation unspecified), slightly lower than the previous estimate by Gotkis and co-workers. The derived <i>E</i><sub>0</sub> for the second H-loss leads to a Δ<sub>f</sub><i>H</i>° for ion <b>7</b>, the cycloprop­[<i>a</i>]­indene radical cation, of Δ<sub>f</sub><i>H</i>°<sub>0K</sub> =1457 ± 27 kJ mol<sup>–1</sup>/Δ<sub>f</sub><i>H</i>°<sub>298K</sub>(C<sub>10</sub>H<sub>6</sub><sup>+</sup>) = 1432 ± 27 kJ mol<sup>–1</sup>. Detailed comparisons are provided with values (experimental and theoretical) available in the literature

    Dissociation of the Anthracene Radical Cation: A Comparative Look at iPEPICO and Collision-Induced Dissociation Mass Spectrometry Results

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    The dissociation of the anthracene radical cation has been studied using two different methods: imaging photoelectron photoion coincidence spectrometry (iPEPCO) and atmospheric pressure chemical ionization–collision induced dissociation mass spectrometry (APCI-CID). Four reactions were investigated: (R1) C<sub>14</sub>H<sub>10</sub><sup>+•</sup> → C<sub>14</sub>H<sub>9</sub><sup>+</sup> + H, (R2) C<sub>14</sub>H<sub>9</sub><sup>+</sup> → C<sub>14</sub>H<sub>8</sub><sup>+•</sup> + H, (R3) C<sub>14</sub>H<sub>10</sub><sup>+•</sup> → C<sub>12</sub>H<sub>8</sub><sup>+•</sup> + C<sub>2</sub>H<sub>2</sub> and (R4) C<sub>14</sub>H<sub>10</sub><sup>+•</sup> → C<sub>10</sub>H<sub>8</sub><sup>+•</sup> + C<sub>4</sub>H<sub>2</sub>. An attempt was made to assign structures to each fragment ion, and although there is still room for debate whether for the C<sub>12</sub>H<sub>8</sub><sup>+•</sup> fragment ion is a cyclobuta­[<i>b</i>]­naphthalene or a biphenylene cation, our modeling results and calculations appear to suggest the more likely structure is cyclobuta­[<i>b</i>]­naphthalene. The results from the iPEPICO fitting of the dissociation of ionized anthracene are <i>E</i><sub>0</sub> = 4.28 ± 0.30 eV (R1), 2.71 ± 0.20 eV (R2), and 4.20 ± 0.30 eV (average of reaction R3) whereas the Δ<sup>‡</sup><i>S</i> values (in J K<sup>–1</sup> mol<sup>–1</sup>) are 12 ± 15 (R1), 0 ± 15 (R2), and either 7 ± 10 (using cyclobuta­[<i>b</i>]­naphthalene ion fragment in reaction R3) or 22 ± 10 (using the biphenylene ion fragment in reaction R3). Modeling of the APCI-CID breakdown diagrams required an estimate of the postcollision internal energy distribution, which was arbitrarily assumed to correspond to a Boltzmann distribution in this study. One goal of this work was to determine if this assumption yields satisfactory energetics in agreement with the more constrained and theoretically vetted iPEPICO results. In the end, it did, with the APCI-CID results being similar

    Newcomer et al. 2012 (Ecological Monographs) Organic C and Denitrification in Streams

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    The file “Newcomer et al. 2012 (Ecological Monographs) Organic C and Denitrification in Streams.xlsx” provides original data from the manuscript. The worksheets are named to correspond with the figures in the manuscript. “Figure 4” has field measurements of nitrate and DOC loads (g ha-1 day-1) and runoff (mm day-1). We measured discharge at Spring Branch and discharge was downloaded from USGS gaging stations at the other sites. “Figure 5” was created using discharge (cfs) downloaded from USGS gaging stations and dividing it by watershed area to get runoff (mm day-1). “Figure 6” has field measurements of mean C:N molar ratios for leaves, periphyton, grass, sediment, and stream particulate organic matter (POM). “Figure 7” has field measurements of 15N and 13C stable isotope signatures for leaves, periphyton, grass, sediment, and stream POM. “Figure 8” has laboratory measurements of denitrification potentials associated with glucose versus nitrate amendments. “Figure 9” has laboratory measurements of denitrification potentials associated with the use of leaves, periphyton, and grass as a carbon source

    Photodissociation of Pyrene Cations: Structure and Energetics from C<sub>16</sub>H<sub>10</sub><sup>+</sup> to C<sub>14</sub><sup>+</sup> and Almost Everything in Between

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    The unimolecular dissociation of the pyrene radical cation, C<sub>16</sub>H<sub>10</sub><sup>+•</sup>, has been explored using a combination of computational techniques and experimental approaches, such as multiple photon absorption in the cold ion trap Piège à Ions pour la Recherche et l’Etude de Nouvelles Espèces Astrochimiques (PIRENEA) and imaging photoelectron photoion coincidence spectrometry (iPEPICO). In total, 22 reactions, involving the fragmentation cascade (H, C<sub>2</sub>H<sub>2</sub>, and C<sub>4</sub>H<sub>2</sub> loss) from the pyrene radical cation down to the C<sub>14</sub><sup>+•</sup> fragment ion, have been studied using PIRENEA. Branching ratios have been measured for reactions from C<sub>16</sub>H<sub>10</sub><sup>+•</sup>, C<sub>16</sub>H<sub>8</sub><sup>+•</sup>, and C<sub>16</sub>H<sub>5</sub><sup>+</sup>. Density functional theory calculations of the fragmentation pathways observed experimentally and postulated theoretically lead to 17 unique structures. One important prediction is the opening of the pyrene ring system starting from the C<sub>16</sub>H<sub>4</sub><sup>+•</sup> radical. In the iPEPICO experiments, only two reactions could be studied, namely, R1 C<sub>16</sub>H<sub>10</sub><sup>+•</sup> → C<sub>16</sub>H<sub>9</sub><sup>+</sup> + H (<i>m</i>/<i>z</i> = 201) and R2 C<sub>16</sub>H<sub>9</sub><sup>+</sup> → C<sub>16</sub>H<sub>8</sub><sup>+•</sup> + H (<i>m</i>/<i>z</i> = 200). The activation energies for these reactions were determined to be 5.4 ± 1.2 and 3.3 ± 1.1 eV, respectively

    DataSheet1_Freshwater salinization syndrome limits management efforts to improve water quality.pdf

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    Freshwater Salinization Syndrome (FSS) refers to groups of biological, physical, and chemical impacts which commonly occur together in response to salinization. FSS can be assessed by the mobilization of chemical mixtures, termed “chemical cocktails”, in watersheds. Currently, we do not know if salinization and mobilization of chemical cocktails along streams can be mitigated or reversed using restoration and conservation strategies. We investigated 1) the formation of chemical cocktails temporally and spatially along streams experiencing different levels of restoration and riparian forest conservation and 2) the potential for attenuation of chemical cocktails and salt ions along flowpaths through conservation and restoration areas. We monitored high-frequency temporal and longitudinal changes in streamwater chemistry in response to different pollution events (i.e., road salt, stormwater runoff, wastewater effluent, and baseflow conditions) and several types of watershed management or conservation efforts in six urban watersheds in the Chesapeake Bay watershed. Principal component analysis (PCA) indicates that chemical cocktails which formed along flowpaths (i.e., permanent reaches of a stream) varied due to pollution events. In response to winter road salt applications, the chemical cocktails were enriched in salts and metals (e.g., Na+, Mn, and Cu). During most baseflow and stormflow conditions, chemical cocktails were less enriched in salt ions and trace metals. Downstream attenuation of salt ions occurred during baseflow and stormflow conditions along flowpaths through regional parks, stream-floodplain restorations, and a national park. Conversely, chemical mixtures of salt ions and metals, which formed in response to multiple road salt applications or prolonged road salt exposure, did not show patterns of rapid attenuation downstream. Multiple linear regression was used to investigate variables that influence changes in chemical cocktails along flowpaths. Attenuation and dilution of salt ions and chemical cocktails along stream flowpaths was significantly related to riparian forest buffer width, types of salt pollution, and distance downstream. Although salt ions and chemical cocktails can be attenuated and diluted in response to conservation and restoration efforts at lower concentration ranges, there can be limitations in attenuation during road salt events, particularly if storm drains bypass riparian buffers.</p

    Box and whisker plots of nitrate uptake velocity (ʋ<sub>f</sub>) in the buried and open reaches in Cincinnati, Ohio and Baltimore, Maryland, as reported in Beaulieu et al. [20] and Pennino et al. [21].

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    <p>Literature data were derived from a recent survey of 72 streams spanning several biomes and land-use conditions [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0132256#pone.0132256.ref016" target="_blank">16</a>]. Plots display 10<sup>th</sup>, 25<sup>th</sup>, 50<sup>th</sup>, 75<sup>th</sup>, and 90<sup>th</sup> percentiles and individual data points outside the 10<sup>th</sup> and 90<sup>th</sup> percentiles. Nitrate uptake velocity was 13 times greater in open than buried reaches (p<0.001, paired <i>t</i>-test).</p
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