25 research outputs found
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin
Observations of the Sun at Vacuum-Ultraviolet Wavelengths from Space. Part II: Results and Interpretations
Hydride donating abilities of the tetracoordinated boron hydrides
The hydride donating ability (HDA), determined as Gibbs free energy (ÎG°H â) for the reaction of Hâ dissociation, was assessed via the DFT/M06/6-311++G (d,p) calculations for 90 tetracoordinated borohydrides Li [L3B-H] taking into account the solvent effects via the optimization in MeCN and CH2Cl2 under SMD model. Obtained this way, the HDAMeCN values vary from 118.2 to 13.4 kcal/mol and correlate well with the Lewis acidity parameters (AN, HA and FA) of parent trigonal boranes (L3B). These data show numerically how the variation of the substituents at the boron atom allows the fine-tuning the BâH bond reactivity (reduction power) in the reactions involving hydride transfer as well as the selectivity of the reduction processes. The analysis of the data obtained shows that by varying the number of substituents and their nature, it is possible not only to change the properties of neutral trisubstituted boranes from highly electrophilic (represented by halogenide- and pseudohalogenide-boranes) to highly nucleophilic (exemplified by alkoxy-an amidoboranes), but also to repolarize the boron-bound hydrogen and make the proton transfer process more favourable than the hydride transfer. © 2018 Elsevier B.V
Hydride donating abilities of the tetracoordinated boron hydrides
The hydride donating ability (HDA), determined as Gibbs free energy (ÎG°H â) for the reaction of Hâ dissociation, was assessed via the DFT/M06/6-311++G (d,p) calculations for 90 tetracoordinated borohydrides Li [L3B-H] taking into account the solvent effects via the optimization in MeCN and CH2Cl2 under SMD model. Obtained this way, the HDAMeCN values vary from 118.2 to 13.4 kcal/mol and correlate well with the Lewis acidity parameters (AN, HA and FA) of parent trigonal boranes (L3B). These data show numerically how the variation of the substituents at the boron atom allows the fine-tuning the BâH bond reactivity (reduction power) in the reactions involving hydride transfer as well as the selectivity of the reduction processes. The analysis of the data obtained shows that by varying the number of substituents and their nature, it is possible not only to change the properties of neutral trisubstituted boranes from highly electrophilic (represented by halogenide- and pseudohalogenide-boranes) to highly nucleophilic (exemplified by alkoxy-an amidoboranes), but also to repolarize the boron-bound hydrogen and make the proton transfer process more favourable than the hydride transfer. © 2018 Elsevier B.V
Comprehensive Insight into the Hydrogen Bonding of Silanes
The interaction of a set of mono-, di- and trisubstituted silanes with OH proton donors of different strength was studied by variable temperature (VT) FTIR and NMR spectroscopies at 190â298 K. Two competing sites of proton donors coordination: SiH and Ï-density of phenyl ringsâare revealed for phenyl-containing silanes. The hydrogen bonds SiHâ
â
â
HO and OHâ
â
â
Ï(Ph) are of similar strength, but can be distinguished in the ÎœSiH range: the ÎœSiHâ
â
â
HO vibrations appear at lower frequencies while OHâ
â
â
Ï(Ph) complexes give SiâH vibrations shifted to higher frequency. The calculations showed the manifold picture of the noncovalent interactions in hydrogen-bonded complexes of phenylsilanes. As OHâ
â
â
HSi bonds are weak, the other noncovalent interactions compete in the stabilization of the intermolecular complexes. Still, the structural and electronic parameters of âpureâ DHB complexes of phenylsilanes are similar to those of Et3SiH. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei
Comprehensive Insight into the Hydrogen Bonding of Silanes
The interaction of a set of mono-, di- and trisubstituted silanes with OH proton donors of different strength was studied by variable temperature (VT) FTIR and NMR spectroscopies at 190â298 K. Two competing sites of proton donors coordination: SiH and Ï-density of phenyl ringsâare revealed for phenyl-containing silanes. The hydrogen bonds SiHâ
â
â
HO and OHâ
â
â
Ï(Ph) are of similar strength, but can be distinguished in the ÎœSiH range: the ÎœSiHâ
â
â
HO vibrations appear at lower frequencies while OHâ
â
â
Ï(Ph) complexes give SiâH vibrations shifted to higher frequency. The calculations showed the manifold picture of the noncovalent interactions in hydrogen-bonded complexes of phenylsilanes. As OHâ
â
â
HSi bonds are weak, the other noncovalent interactions compete in the stabilization of the intermolecular complexes. Still, the structural and electronic parameters of âpureâ DHB complexes of phenylsilanes are similar to those of Et3SiH. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei
Competition between the Hydride Ligands of Two Types in Proton Transfer to [{Îș3-P-CH3C(CH2CH2PPh2)3}RuH(η2-BH4)]
The interaction of the mixed hydridoâtetrahydridoborate ruthenium(II) complex [(Triphos)RuH(η2-BH4)] [1; Triphos = Îș3-P-CH3C(CH2CH2PPh2)3] with alcohols of variable acidic strength [MeOH, FCH2CH2OH (MFE), CF3CH2OH (TFE), (CF3)2CHOH (HFIP), and (CF3)3COH (PFTB)] was the subject of a combined computational (DFT) and spectroscopic (VT FTIR, NMR) study. The experimental spectra suggests that RuH···HO bond formation precedes the protonation of 1, and H2 evolution leads to the loss of boron and the formation of the dimetallic [{(Triphos)RuH}2(”,η2:η2-BH4)]+ cation. The experimentally determined basicity factor [Ej(RuH)] of the Ru-bound hydrido ligand of 1.43 is among the highest determined for ruthenium hydrides. Such high basicity leads to very easy proton transfer to the RuH ligand for strong alcohols (HFIP and PFTB). An alternative reaction pathway involving the migration of the bridging hydride (BHbr) to the ruthenium center is suggested for weaker proton donors (MeOH and TFE). © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Ammonia Borane Dehydrogenation Catalyzed by (Îș4-EP3)Co(H) [EP3 = E(CH2CH2PPh2)3; E = N, P] and H2 Evolution from Their Interaction with NH Acids
Two Co(I) hydrides containing the tripodal polyphosphine ligand EP3, (Îș4-EP3)Co(H) [E(CH2CH2PPh2)3; E = N (1), P (2)], have been exploited as ammonia borane (NH3BH3, AB) dehydrogenation catalysts in THF solution at T = 55 °C. The reaction has been analyzed experimentally through multinuclear (11B, 31P{1H}, 1H) NMR and IR spectroscopy, kinetic rate measurements, and kinetic isotope effect (KIE) determination with deuterated AB isotopologues. Both complexes are active in AB dehydrogenation, albeit with different rates and efficiency. While 1 releases 2 equiv of H2 per equivalent of AB in ca. 48 h, with concomitant borazine formation as the final "spent fuel", 2 produces 1 equiv of H2 only per equivalent of AB in the same reaction time, along with long-chain poly(aminoboranes) as insoluble byproducts. A DFT modeling of the first AB dehydrogenation step has been performed, at the M06//6-311++Gâ level of theory. The combination of the kinetic and computational data reveals that a simultaneous B-H/N-H activation occurs in the presence of 1, after a preliminary AB coordination to the metal center. In 2, no substrate coordination takes place, and the process is better defined as a sequential BH3/NH3 insertion process on the initially formed [Co]-NH2BH3 amidoborane complex. Finally, the reaction of 1 and 2 with NH-acids [AB and Me2NHBH3 (DMAB)] has been followed via VT-FTIR spectroscopy (in the -80 to +50 °C temperature range), with the aim of gaining a deeper experimental understanding of the dihydrogen bonding interactions that are at the origin of the observed H2 evolution. © 2017 American Chemical Society
Competition between the Hydride Ligands of Two Types in Proton Transfer to [{Îș3-P-CH3C(CH2CH2PPh2)3}RuH(η2-BH4)]
The interaction of the mixed hydridoâtetrahydridoborate ruthenium(II) complex [(Triphos)RuH(η2-BH4)] [1; Triphos = Îș3-P-CH3C(CH2CH2PPh2)3] with alcohols of variable acidic strength [MeOH, FCH2CH2OH (MFE), CF3CH2OH (TFE), (CF3)2CHOH (HFIP), and (CF3)3COH (PFTB)] was the subject of a combined computational (DFT) and spectroscopic (VT FTIR, NMR) study. The experimental spectra suggests that RuH···HO bond formation precedes the protonation of 1, and H2 evolution leads to the loss of boron and the formation of the dimetallic [{(Triphos)RuH}2(”,η2:η2-BH4)]+ cation. The experimentally determined basicity factor [Ej(RuH)] of the Ru-bound hydrido ligand of 1.43 is among the highest determined for ruthenium hydrides. Such high basicity leads to very easy proton transfer to the RuH ligand for strong alcohols (HFIP and PFTB). An alternative reaction pathway involving the migration of the bridging hydride (BHbr) to the ruthenium center is suggested for weaker proton donors (MeOH and TFE). © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei