Browsing by Subject "CHEMISTRY"

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  • Virjamo, Virpi; Fyhrquist, Pia; Koskinen, Akseli; Lavola, Anu; Nissinen, Katri; Julkunen-Tiitto, Riitta (2020)
    Knowledge about the defensive chemistry of coniferous trees has increased in recent years regarding a number of alkaloid compounds; in addition to phenolics and terpenes. Here, we show that Norway spruce (Picea abies (L.) H. Karst.), an important boreal zone tree species; accumulates 1,6-dehydropinidine (2-methyl-6-(2-propenyl)-1,6-piperideine) in its needles and bark. We reanalyzed previously published GC-MS data to obtain a full picture of 1,6-dehydropinidine in P. abies. 1,6-dehydropinidine appeared to especially accumulate in developing spring shoots. We used solid-phase partitioning to collect the alkaloid fraction of the sprouts and thin-layer chromatography to purify 1,6-dehydropinidine. The antibacterial properties of the 1,6-dehydropinidine fraction were tested using a broth microdilution method; with Streptococcus equi subsp. equi as a model organism. Based on our results 1,6-dehydropinidine is common in alkaloid extractions from P. abies (0.4 +/- 0.03 mg g(-1) dw in mature needles) and it is especially abundant in young spruce shoots (2.7 +/- 0.5 mg g(-1) dw). Moreover; 1,6-dehydropinidine extracted from P. abies sprouts showed mild antibacterial potential against Streptococcus equi subsp. equi (MIC 55 mu g mL(-1)). The antibacterial activity of a plant compound thought of as an intermediate rather than an end-product of biosynthesis calls for more detailed studies regarding the biological function of these coniferous alkaloids
  • Weller, Stefan; Schlindwein, Simon H.; Feil, Christoph M.; Kelemen, Zsolt; Buzsaki, Daniel; Nyulaszi, Laszlo; Isenberg, Stefan; Pietschnig, Rudolf; Nieger, Martin; Gudat, Dietrich (2019)
    Reactions of P-chloro-1,3,2-diazaphospha[3]ferrocenophanes with ECl3 (E = Al, Ga) under solvent-free conditions and with Na[Mn(CO)(5)] furnished salts featuring a ferrocenophane-based phosphenium cation and neutral phosphenium complexes, respectively. All products were characterized by spectroscopic studies. Single-crystal X-ray diffraction studies confirmed the ionic nature of the phosphenium tetrachloroaluminate and the structural analogy between the phosphenium complexes and Fischer-type carbene complexes. Distinct deviations in the conformation of the ansa bridge suggest electronic stabilization of the electrophilic phosphorus atom by phosphorus-nitrogen pi interactions in the free cation and by phosphorus-metal pi bonding in the complexes. The observation of short intermolecular contacts in the crystalline phosphenium salt and its chemical behavior toward donor solvents attest to the cation having an unusually high degree of Lewis acidity, which was confirmed by DFT studies and related to the presence of a rather large N-P-N angle. Computational studies indicate further that the free phosphenium cation exhibits a closed-shell electronic structure with a formal Fe(II) oxidation state and is thus a true analogue to ferrocenophane-based diaminotetrylenes.
  • Lu, Yiqun; Yan, Chao; Fu, Yueyun; Chen, Yan; Liu, Yiliang; Yang, Gan; Wang, Yuwei; Bianchi, Federico; Chu, Biwu; Zhou, Ying; Yin, Rujing; Baalbaki, Rima; Garmash, Olga; Deng, Chenjuan; Wang, Weigang; Liu, Yongchun; Petäjä, Tuukka; Kerminen, Veli-Matti; Jiang, Jingkun; Kulmala, Markku; Wang, Lin (2019)
    Gaseous sulfuric acid (H2SO4) is known as one of the key precursors for atmospheric new particle formation (NPF) processes, but its measurement remains challenging. Therefore, a proxy method that is able to derive gaseous sulfuric acid concentrations from parameters that can be measured relatively easily and accurately is highly desirable for the atmospheric chemistry community. Although such methods are available for clean atmospheric environments, a proxy that works well in a polluted atmosphere, such as that found in Chinese megacities, is yet to be developed. In this study, the gaseous sulfuric acid concentration was measured in February-March 2018, in urban Beijing using a nitrate based -long time-of-flight chemical ionization mass spectrometer (LToF-CIMS). A number of atmospheric parameters were recorded concurrently including the ultraviolet radiation B (UVB) intensity, the concentrations of O-3, NOx (sum of NO and NO2), SO2, and HONO, and aerosol particle number size distributions. A proxy for atmospheric daytime gaseous sulfuric acid concentration was derived via a statistical analysis method using the UVB intensity, [SO2], the condensation sink (CS), [O-3], and [HONO] (or [NOx]) as the predictor variables, where square brackets denote the concentrations of the corresponding species. In this proxy method, we considered the formation of gaseous sulfuric acid from reactions of SO2 and OH radicals during the daytime, and the loss of gaseous sulfuric acid due to its condensation onto the preexisting particles. In addition, we explored the formation of OH radicals from the conventional gas-phase photochemistry using O-3 as a proxy and from the photolysis of HONO using HONO (and subsequently NOx) as a proxy. Our results showed that the UVB intensity and [SO2] are dominant factors in the production of gaseous sulfuric acid, and that the simplest proxy could be constructed with the UVB intensity and [SO2] alone. When the OH radical production from both homogenously and heterogeneously formed precursors were considered, the relative errors were reduced by up to 20 %.
  • Lehtola, Susi (2020)
    Knowledge of the repulsive behavior of potential energy curves V (R) at R -> 0 is necessary for understanding and modeling irradiation processes of practical interest. V (R) is in principle straightforward to obtain from electronic structure calculations; however, commonly used numerical approaches for electronic structure calculations break down in the strongly repulsive region due to the closeness of the nuclei. In this work, we show by comparison to fully numerical reference values that a recently developed procedure [S. Lehtola, J. Chem. Phys. 151, 241102 (2019)] can be employed to enable accurate linear combination of atomic orbitals calculations of V (R) even at small R by a study of the seven nuclear reactions He-2 (sic) Be, HeNe (sic) Mg, Ne-2 (sic) Ca, HeAr (sic) Ca, MgAr (sic) Zn, Ar-2 (sic) Kr, and NeCa (sic) Zn.
  • Enders, Lukas; Casadio, David S.; Aikonen, Santeri; Lenarda, Anna; Wirtanen, Tom; Hu, Tao; Hietala, Sami; Ribeiro, Lucilia S.; Pereira, Manuel Fernando R.; Helaja, Juho (2021)
    A simple "reagent-free" thermal air treatment turns active carbon into a mildly oxidized material with increased quinoidic content that catalytically dehydrogenates saturated N-heterocycles to the corresponding aromatic compounds. Thermal decarboxylation improves the activity of the catalyst further, making it overall more efficient compared to other widely used carbocatalysts such as oxidized carbon nanotubes, graphene oxide and untreated active carbons. The substrate scope covers 1,2,3,4-tetrahydroquinolines (THQ), 1,2,3,4-tetrahydro-beta-carbolines and related N-heterocyclic structures. The developed protocol also successfully dehydrogenates 3-(cyclohexenyl)indoles to 3-aryl indoles, opening a concise transition metal-free approach to (hetero)biaryls as exemplified with the synthesis of the core structure of progesterone receptor antagonist. Hammett plots, deuterium KIE measurements and computations at DFT level suggest that bimolecular hydride transfer mechanism is more likely to operate between THQs and the o-quinoidic sites of the catalyst, than the addition-elimination hemiaminal route. Comparison of structural parameters and catalytic performance of various oxidized carbon materials, prepared by different oxidative and optional post treatments, revealed that quinoidic content and surface area correlate with the obtained yields, while carboxylic acid content has a clear inhibiting effect for the studied oxidative dehydrogenations (ODHs). The carbocatalyst itself can be prepared from inexpensive and environmentally benign starting materials and its catalytic activity can be enhanced by a simple thermal oxidation in air that produces no reagent waste. Furthermore, oxygen is used as terminal oxidant, and the carbocatalyst is recyclable at least six times without a notable loss of activity.
  • Liebmann, Jonathan; Sobanski, Nicolas; Schuladen, Jan; Karu, Einar; Hellen, Heidi; Hakola, Hannele; Zha, Qiaozhi; Ehn, Mikael; Riva, Matthieu; Heikkinen, Liine; Williams, Jonathan; Fischer, Horst; Lelieyeld, Jos; Crowley, John N. (2019)
    The formation of alkyl nitrates in various oxidation processes taking place throughout the diel cycle can represent an important sink of reactive nitrogen and mechanism for chain termination in atmospheric photo-oxidation cycles. The low-volatility alkyl nitrates (ANs) formed from biogenic volatile organic compounds (BVOCs), especially terpenoids, enhance rates of production and growth of secondary organic aerosol. Measurements of the NO3 reactivity and the mixing ratio of total alkyl nitrates (6 ANs) in the Finnish boreal forest enabled assessment of the relative importance of NO3-, O-3- and OH-initiated formation of alkyl nitrates from BVOCs in this environment. The high reactivity of the forest air towards NO3 resulted in reactions of the nitrate radical, with terpenes contributing substantially to formation of ANs not only during the night but also during daytime. Overall, night-time reactions of NO3 accounted for 49% of the local production rate of ANs, with contributions of 21 %, 18% and 12% for NO3, OH and O-3 during the day. The lifetimes of the gas-phase ANs formed in this environment were on the order of 2 h due to efficient uptake to aerosol (and dry deposition), resulting in the transfer of reactive nitrogen from anthropogenic sources to the forest ecosystem.
  • Hsu, Shih-Ying; Liu, Sheng-Yuan; Liu, Tie; Sahu, Dipen; Hirano, Naomi; Lee, Chin-Fei; Tatematsu, Ken'ichi; Kim, Gwanjeong; Juvela, Mika; Sanhueza, Patricio; He, Jinhua; Johnstone, Doug; Qin, Sheng-Li; Bronfman, Leonardo; Chen, Huei-Ru Vivien; Dutta, Somnath; Eden, David J.; Jhan, Kai-Syun; Kim, Kee-Tae; Kuan, Yi-Jehng; Kwon, Woojin; Lee, Chang Won; Lee, Jeong-Eun; Moraghan, Anthony; Rawlings, M. G.; Shang, Hsien; Soam, Archana; Thompson, M. A.; Traficante, Alessio; Wu, Yuefang; Yang, Yao-Lun; Zhang, Qizhou (2020)
    We report the detection of four new hot corino sources, G211.47-19.27S, G208.68-19.20N1, G210.49-19.79W, and G192.12-11.10, from a survey study of Planck Galactic Cold Clumps in the Orion Molecular Cloud Complex with the Atacama Compact Array. Three sources had been identified as low-mass Class 0 protostars in the Herschel Orion Protostar Survey. One source in the lambda Orionis region is first reported as a protostellar core. We have observed abundant complex organic molecules (COMs), primarily methanol but also other oxygen-bearing COMs (in G211.47-19.27S and G208.68-19.20N1) and the molecule of prebiotic interest NH2CHO (in G211.47-19.27S), signifying the presence of hot corinos. While our spatial resolution is not sufficient to resolve most of the molecular emission structure, the large line width and high rotational temperature of COMs suggest that they likely reside in the hotter and innermost region immediately surrounding the protostar. In G211.47-19.27S, the D/H ratio of methanol ([CH2DOH]/[CH3OH]) and the(12)C/C-13 ratio of methanol ([CH3OH]/[(CH3OH)-C-13]) are comparable to those of other hot corinos. Hydrocarbons and long-carbon-chain molecules such as c-C(3)H(2)and HCCCN are also detected in the four sources, likely tracing the outer and cooler molecular envelopes.
  • Kurten, Theo; Tiusanen, Kirsi; Roldin, Pontus; Rissanen, Matti; Luy, Jan-Niclas; Boy, Michael; Ehn, Mikael; Donahue, Neil (2016)
    COSMO-RS (conductor-like screening model for real solvents) and three different group-contribution methods were used to compute saturation (subcooled) liquid vapor pressures for 16 possible products of ozone-initiated alpha-pinene autoxidation, with elemental compositions C10H16O4-10 and C20H30O10-12. The saturation vapor pressures predicted by the different methods varied widely. COSMO-RS predicted relatively high saturation vapor pressures values in the range of 10(-6) to 10(-10) bar for the C10H16O4-10 "monomers", and 10(-11) to 10(-16) bar for the C20H30O10-12 "dimers". The group-contribution methods predicted significantly (up to 8 order of magnitude) lower saturation vapor pressures for most of the more highly oxidized monomers. For the differs, the COSMO-RS predictions were within the (wide) range spanned by the three group-contribution methods. The main reason for the discrepancies between the methods is likely that the group-contribution methods do not contain the necessary parameters to accurately treat autoxidation products containing multiple hydroperoxide, peroxy acid or peroxide functional groups, which form intramolecular hydrogen bonds with each other. While the COSMO-RS saturation vapor pressures for these systems may be overestimated, the results strongly indicate that despite their high O:C ratios, the volatilities of the autoxidation products of alpha-pinene (and possibly other atmospherically relevant alkenes) are not necessarily extremely low. In other words, while autoxidation products are able to, adsorb onto aerosol particles, their evaporation back into the gas phase cannot be assumed to be negligible, especially from the smallest nanometer-scale particles. Their observed effective contribution to aerosol particle growth may therefore involve rapid heterogeneous reactions (reactive uptake) rather than effectively irreversible physical absorption.
  • Yli-Juuti, Taina; Tikkanen, Olli-Pekka; Manninen, Hanna E.; Nieminen, Tuomo; Kulmala, Markku (2016)
    We analyzed nanoparticle growth during new-particle-formation events based on ten years of measurements carried out at a boreal forest site in Hyytiala, Finland, concentrating on the sub-3 nm particles and the role of sulfuric acid in their growth. Growth rates of 1.5-3 nm diameter particles were determined from ion spectrometer measurements and compared with parameterized sulfuric acid concentration and other atmospheric parameters. The calculated growth rates from sulfuric acid condensation were on average 7.4% of the observed growth rates and the two did not correlate. These suggest that neither sulfuric acid monomer condensation nor coagulation of small sulfuric acid clusters was the primary growth mechanism in these atmospheric conditions. Also no clear sign of organic condensation being the single main growth mechanism was seen. These observations are consistent with the hypothesis that several factors have comparative roles in the sub-3 nm growth.
  • Luecke, Ana-Luiza; Wiechmann, Sascha; Freese, Tyll; Nieger, Martin; Foeldes, Tamas; Papai, Imre; Gjikaj, Mimoza; Adam, Arnold; Schmidt, Andreas (2018)
    Unstable N-heterocyclic carbenes can be masked and stabilized as pseudo-cross-conjugated hetarenium-carboxylates which decarboxylate on warming. This study deals with the decarboxylation of carboxylates of mesoionic compounds to generate anionic N-heterocyclic carbenes. Lithium sydnone-4-carboxylates were therefore prepared via 4-bromosydnones by halogen-lithium exchange with nBuLi and subsequent treatment with carbon dioxide. Protonation gave the corresponding sydnone-4-carboxylic acids. Thermogravimetric measurements in addition to temperature dependent IR spectroscopy proved the decarboxylation of lithium sydnone-4-carboxylates and formation of the corresponding sydnone anions which can be represented as anionic N-heterocyclic carbenes. In DMSO-d6 solution, water favors the decarboxylation. Calculations have been performed to elucidate the mechanism of the decarboxylation in the absence and presence of water. (C) 2018 Elsevier Ltd. All rights reserved.
  • Zhang, Chao; Vehkamäki, Marko; Pietikäinen, Mika; Leskelä, Markku; Ritala, Mikko (2020)
    Novel area-selective molecular layer deposition (AS-MLD) of polyimide (PI) on Cu versus native SiO2 was studied. By use of 1,6-diaminohexane (DAH) and pyromellitic dianhydride (PMDA) as precursors, PI films can be selectively deposited on the Cu surface at 200-210 degrees C with a rate around 7.8 A/cycle while negligible growth takes place on SiO2. The selectivity was successfully demonstrated also on Cu/SiO2 patterns at 200 degrees C; after 180 MLD cycles, around 140 nm thick PI was deposited on Cu regions while
  • Fan, Xiaolong; Cai, Jing; Yan, Chao; Zhao, Jian; Guo, Yishuo; Li, Chang; Dällenbach, Kaspar; Zheng, Feixue; Lin, Zhuohui; Chu, Biwu; Wang, Yonghong; Dada, Lubna; Zha, Qiaozhi; Du, Wei; Kontkanen, Jenni; Kurten, Theo; Iyer, Siddhart; Kujansuu, Joni T.; Petäjä, Tuukka; Worsnop, Douglas R.; Kerminen, Veli-Matti; Liu, Yongchun; Bianchi, Federico; Tham, Yee Jun; Yao, Lei; Kulmala, Markku (2021)
    Gaseous hydrochloric (HCl) and hydrobromic acid (HBr) are vital halogen species that play essential roles in tropospheric physicochemical processes. Yet, the majority of the current studies on these halogen species were conducted in marine or coastal areas. Detection and source identification of HCl and HBr in inland urban areas remain scarce, thus limiting the full understanding of halogen chemistry and potential atmospheric impacts in the environments with limited influence from the marine sources. Here, both gaseous HCl and HBr were concurrently measured in urban Beijing, China, during winter and early spring of 2019. We observed significant HCl and HBr concentrations ranging from a minimum value at 1 x 10(8) molecules cm(-3) (4 ppt) and 4 x 10(7) molecules cm 3 (1 ppt) up to 6 x 10(9) molecules cm(-3) (222 ppt) and 1 x 10(9) molecules cm(-3) (37 ppt), respectively. The HCl and HBr concentrations are enhanced along with the increase of atmospheric temperature, UVB and levels of gaseous HNO3. Based on the air mass analysis and high correlations of HCl and HBr with the burning indicators (HCN and HCNO), gaseous HCl and HBr are found to be related to anthropogenic burning aerosols. The gas-particle partitioning may also play a dominant role in the elevated daytime HCl and HBr. During the daytime, the reactions of HCl and HBr with OH radicals lead to significant production of atomic Cl and Br, up to 2 x 10(4) molecules cm(-3) s(-1) and 8 x 10(4) molecules cm(-3) s(-1), respectively. The production rate of atomic Br (via HBr + OH) is 2-3 times higher than that of atomic Cl (via HCl + OH), highlighting the potential importance of bromine chemistry in the urban area. On polluted days, the production rates of atomic Cl and Br are faster than those on clean days. Furthermore, our observations of elevated HCl and HBr may suggest an important recycling pathway of halogen species in inland megacities and may provide a plausible explanation for the widespread halogen chemistry, which could affect the atmospheric oxidation in China.
  • Makela, Mikko K.; Bulatov, Evgeny; Malinen, Kiia; Talvitie, Juulia; Nieger, Martin; Melchionna, Michele; Lenarda, Anna; Hu, Tao; Wirtanen, Tom; Helaja, Juho (2021)
    Oxidized active carbon (oAC) catalyses the formation of polysubstituted quinolines from o-vinyl anilines and aldehydes. The reaction proceeds in a cascade manner through condensation, electrocyclization and dehydrogenation, and gives access to a wide range of quinolines with alkyl and/or aryl substituents as demonstrated with 40 examples. The metal-free catalytic procedure allows a heterogeneous protocol for the synthesis of various polysubstituted quinolines. The mechanistic studies imply that both the acid and quinoidic groups in oAC are integral for the catalytic manifold.
  • Kolsi, Laura E.; Yli-Kauhaluoma, Jari; Moreira, Vânia M. (2018)
    Bi(OTf)(3)center dot xH(2)O is a powerful catalyst for the dehydration of tertiary alcohols into alkenes in apolar solvents. The reaction proceeds smoothly and selectively, with amounts as low as 0.01 mol % catalyst, in yields up to 93%. Moreover, in polar solvents, Bi(OTf)(3)center dot xOH(2)O (0.1-1 mol %) selectively catalyzes the dimerization of the alcohols instead, forming new C-C bonds, in yields up to 96%. This mild, efficient, economic, and eco-friendly method is applicable across different chemical classes and amenable to several functional groups.
  • Booyens, Wanda; Van Zyl, Pieter G.; Beukes, Johan P.; Ruiz-Jimenez, Jose; Kopperi, Matias; Riekkola, Marja-Liisa; Vakkari, Ville; Josipovic, Miroslav; Kulmala, Markku; Laakso, Lauri (2019)
    Although atmospheric organic N compounds are considered to be important, especially in new particle formation and their contribution to brown carbon, these species are not that well understood. This can be partially attributed to their chemical complexity. Therefore, the aim of this study was to assess the characteristics of organic N compounds utilising comprehensive two-dimensional gas chromatography coupled with a time-of-flight mass spectrometer (GCxGC-TOFMS) in aerosol samples that were collected at a savanna-grassland background region and to determine the possible sources. 135 atmospheric organic N compounds were tentatively characterised and semi-quantified, which included amines, nitriles, amides, urea, pyridine derivatives, amino acids, nitro-and nitroso compounds, imines, cyanates and isocyanates, and azo compounds. Amines contributed to 51% of the semi-quantified concentrations, while nitriles, pyridine derivatives, and amides comprised 20%, 11%, and 8%, respectively, of the semi-quantified concentrations. Amines, nitriles, amides, and pyridine derivatives concentrations were higher during the dry season, which were attributed to meteorology and open biomass burning. Anthropogenic sources impacting air masses measured at Welgegund, as well as regional agricultural activities, were considered as the major sources of amines, while the regional influence of household combustion was most likely the main source of nitriles, amides, and pyridine derivatives. The other organic N species were most likely related to the influence of local and regional agricultural activities.
  • Gal-Or, Eran; Gershoni, Yaniv; Scotti, Gianmario; Nilsson, Sofia Märta Elisabeth; Saarinen, Jukka Kalle Samuel; Jokinen, Ville Petteri; Strachan, Clare Joanna; Boije af Gennäs, Per Gustav; Yli-Kauhaluoma, Jari Tapani; Kotiaho, Ahti Antti Tapio (2019)
    Additive manufacturing (3D printing) is a disruptive technology that is changing production systems globally. In addition, microfluidic devices are increasingly being used for chemical analysis and continuous production of chemicals. Printing of materials such as polymers and metals is already a reality, but additive manufacturing of glass for microfluidic systems has received minor attention. We characterize microfluidic devices (channel cross-section dimensions down to a scale of 100 mm) that have been produced by additive manufacturing of molten soda-lime glass in tens of minutes and report their mass spectrometric and Raman spectroscopic analysis examples. The functionality of a microfluidic glass microreactor is shown with online mass spectrometric analysis of linezolid synthesis. Additionally, the performance of a direct infusion device is demonstrated by mass spectrometric analysis of drugs. Finally, the excellent optical quality of the glass structures is demonstrated with in-line Raman spectroscopic measurements. Our results promise a bright future for additively manufactured glass microdevices in diverse fields of science.
  • Odeh, Issam; Arar, Sharif; Al-Hunaiti, Afnan; Sa'aydeh, Hiyam; Hammad, Ghada; Duplissy, Jonathan; Vuollekoski, Henri; Korpela, Antti; Petäjä, Tuukka; Kulmala, Markku; Hussein, Tareq (2017)
    The quality and chemical composition of urban dew collections with dust precipitates without pre-cleaning of the collecting surface WSF (white standard foil) were investigated for 16 out of 20 collected samples with collected volumes ranging from 22 to 230 ml. The collection period was from March to July 2015 at an urban area, Jubaiha, which is located in the northern part of the capital city Amman, Jordan. The obtained results indicated the predominance of Ca2+ and SO42- ions (ratio 2.2: 1) that originated from Saharan soil dust; where the collected samples were alkaline (mean pH = 7.35) with high mineralization (429.22 mg/L) exceeding the previously reported dew values in Amman-Jordan. A relocation of NaCl and to a less extent Mg2+ from sea to land by Saharan wind is indicated by the percent sea-salt fraction calculations (over 100 and 52, respectively). The collected samples exhibited high total organic carbon (TOC) values ranging from 11.86 to 74.60 mg/L, presence of particulate settled material with turbidity ranging from 20.10 to 520.00 NTU, and presence of undesired elements like boron (mean = 1.48 mg/L) that made it different in properties from other dew water collections at clean surfaces, and exceeding the standard limits for drinking water for these parameters set by Jordanian Drinking Water standards (JS286/2015)/WHO standard. The quality of this water is more close to that for raw or agricultural water but if it is meant to be used as potable source of water, at least sand and activated charcoal filters are needed to purify it.
  • Vegh, Russell B.; Bravaya, Ksenia B.; Bloch, Dmitry A.; Bommarius, Andreas S.; Tolbert, Laren M.; Verkhovsky, Michael; Krylov, Anna I.; Solntsev, Kyril M. (2014)
  • Hasan, Galib; Salo, Vili-Taneli; Valiev, Rashid; Kubecka, Jakub; Kurten, Theo (2020)
    Organic peroxy radicals (RO2) are key intermediates in the chemistry of the atmosphere. One of the main sink reactions of RO2 is the recombination reaction RO2 + R'O-2, which has three main channels (all with O-2 as a coproduct): (1) R-H=O + R'OH, (2) RO + R'O, and (3) ROOR'. The RO + R'O "alkoxy" channel promotes radical and oxidant recycling, while the ROOR' "dimer" channel leads to low-volatility products relevant to aerosol processes. The ROOR' channel has only recently been discovered to play a role in the gas phase. Recent computational studies indicate that all of these channels first go through an intermediate complex( 1)(RO center dot center dot center dot O-3(2)center dot center dot center dot OR'). Here, O-3(2) is very weakly bound and will likely evaporate from the system, giving a triplet cluster of two alkoxy radicals: (3)(RO center dot center dot center dot OR'). In this study, we systematically investigate the three reaction channels for an atmospherically representative set of RO + R'O radicals formed in the corresponding RO2+ R'O-2 reaction. First, we systematically sample the possible conformations of the RO center dot center dot center dot OR' clusters on the triplet potential energy surface. Next, we compute energetic parameters and attempt to estimate reaction rate coefficients for the three channels: evaporation/dissociation to RO + R'O, a hydrogen shift leading to the formation of R'(-H)=O + ROH, and "spin-flip" (intersystem crossing) leading to, or at least allowing, the formation of ROOR' dimers. While large uncertainties in the computed energetics prevent a quantitative comparison of reaction rates, all three channels were found to be very fast (with typical rates greater than 10 6 s(-1)). This qualitatively demonstrates that the computationally proposed novel RO2 + R'O-2 reaction mechanism is compatible with experimental data showing non-negligible branching ratios for all three channels, at least for sufficiently complex RO2.
  • Feil, Christoph M.; Hettich, Thomas D.; Beyer, Katharina; Sondermann, Christina; Schlindwein, Simon H.; Nieger, Martin; Gudat, Dietrich (2019)
    N-Heterocyclic phosphenium (NHP) and nitro-sonium (NO+) ligands are often viewed as isolobal analogues that share the capability to switch between different charge states and thus display redox "noninnocent" behavior. We report here on mixed complexes [(NHP)M(CO)(n)(NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)(3)(NO)] or PPN[Cr(CO)(4)-(NO)], respectively, and fully characterized (PPN = nitride-bistriphenylphosphonium cation). The structural and spectroscopic (IR, UV-vis) data support the presence of carbene-analogue NHP ligands with an overall positive charge state and pi-acceptor character. Even if the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that the NO units exhibit nitroxide (NO-) character. This assignment was validated by computational studies, which reveal also that the electronic structure of iron NHP/ NO complexes is closely akin to that of the Hieber anion, [Fe(CO)(3)(NO)](-). The electrophilic character of the NHP units is further reflected in the chemical behavior of the mixed complexes. Cyclic voltammetry and IR-SEC studies revealed that complex [(NHP)Fe(CO)(2) (NO)] (4) undergoes chemically reversible one-electron reduction. Computational studies indicate that the NHP unit in the resulting product carries significant radical character, and the reduction may thus be classified as predominantly ligand-centered. Reaction of 4 with sodium azide proceeded likewise under nucleophilic attack at phosphorus and decomplexation, while super hydride and methyl lithium reacted with all chromium and iron complexes via transfer of a hydride or methyl anion to the NHP unit to afford anionic phosphine complexes. Some of these species were isolated after cation exchange or trapped with electrophiles (H+, SnPh3(+)) to afford neutral complexes representing the products of a formal hydrogenation or hydrostannylation of the original M=P double bond.