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1H and 19F investigation of the reaction of B(C6F5)3 with water in toluene solutions
No full textTitrations of B(C6F5)3 (1) with water, in toluene-d8 solution, monitored by 19F and 1H NMR at 196 K, showed first the formation of the adduct [(C6F5)3B(OH2)] (2) and then its stepwise transformation into the two aqua species [(C6F5)3B(OH)]·H2O (3) and [(C6F5)3 B(OH2)]·2H2O (4) containing, respectively, one or two water molecules hydrogen-bonded to the protons of the B-bound water molecule. The NMR data show that in each titration step only two species were present in significant concentration: 1 and 2 up to 1 equiv, 2 and 3 between 1 and 2 equiv, 3 and 4 between 2 and 3 equiv. Above 3 equiv the solutions rapidly attained saturation and phase separation occurred (although there was evidence of interaction of 4 with more water molecules). Titrations at room temperature indicated an analogous stepwise course. Variable-temperature experiments demonstrated water exchange between the different aqua species and between the different water sites in the adducts 3 and 4 ("internal" or B-bound and "external" or H-bound). The rate of these processes increased with the amount of water bonded to B(C6F5)3. The exchange of B-bound water among the different B(C6F5)3 molecules (resulting in the 1 ⇔ 2 interconversion) caused the averaging of the 19F resonances of 1 and 2, above 273 K. Band shape analysis in the temperature range 235-312 K provided the kinetic constants, whose dependence on the concentration revealed a dissociative mechanism (ΔH‡ 67(2) kJ mol-1, ΔS‡ 58(7) J mol-1 K-1). For the adduct [(C6F5)3B(OH2)]·H2 O (3), four different dynamic processes have been recognized: (i) the exchange of H-bound water among different [(C6F5)3B(OH2)] adducts (the 2 ⇔ 3 exchange) or (ii) among different [(C6F5)3 B(OH2)]̇H2O adducts (the 3 ⇔ 4 exchange), (iii) the exchange between H-bound and B-bound water, (iv) the hopping of H-bound water between the two protons of B-bound water. This process was so fast that an averaged signal for the protons of internal water was observed even at 187 K. The rate of the process (i) increased with the concentration of 2, so that separate 19F and 1H signals for 2 and 3 were observed only in very dilute solutions at the lowest temperatures. Linear plots of the kinetic constants (estimated from 1H NMR spectra in the near fast exchange region, temperature range 188-214 K) vs the concentration of 2 allowed the estimation of the constant for the dissociative pathway (4 orders of magnitude faster than for the exchange of B-bound water) and for the bimolecular pathway [ΔH‡ 30(2) kJ mol-1, ΔS‡ 3(10) J mol-1 K-1]. Process (ii) was too fast on the NMR time scale to allow any kinetic investigation. Process (iii) caused the parallel broadening of both the 1H signals of 3 at T > 225 K, with a rate quite close to that of the dissociative exchange of water among different B(C6F5)3 molecules. The activation parameters (ΔH‡ 55(2) kJ mol-1, ΔS‡ 7(3) J mol-1 K-1, temperature range 233-273 K) allowed no discrimination between the exchange of an entire water molecule and the mere exchange of protons. Even small amounts of 4 accelerated process (iii), due to the occurrence of two much faster processes: the 3 ⇔ 4 exchange and the exchange between the protons of internal and external water in 4. The study of any kind of water mobility concerning the trihydrate 4 was prevented by the occurrence of proton exchange processes (so fast as to broaden the signals of internal and external water even at 188 K), possibly favored by the acidic dissociation of the protons of the B-bonded water molecule of 4
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Rhenium-Silver Bicyclic “Spiro” Hydrido-Carbonyl Clusters : NMR Investigation of Their Formation and Reversible Fragmentation
No full textThe reactions of the hydrido complexes [Re2H2(μ-H) (CO)8]- (1) and [Re4H2(μ-H) 3(CO)16]- (2) with 0.5 equivalents of AgOTf (OTf-=CF3SO3-) afforded rapidly and selectively the novel cluster anions [Ag(μ- H)4{Re 2(μ-H)(CO)8}2]- (3) and [Ag(μ-H)4{Re4(μ-H)3(CO) 16}2]- (4), respectively, formed by donor-ec-ceptor interactions between the rhenium hydrides and the Ag- cation. 1H and 109Ag NMR spectroscopies showed that each Ag atom is surrounded by four hydrido ligands, bridging Re-Ag interactions. The Re2AgRe2 cluster 3 therefore adopts the well known "bow-tie" geometry, constituted by two triangular clusters sharing the Ag vertex (spiro bis-cyclopropane). The Re4AgRe4 anion 4 has instead an unprecedented spiro bis-cyclopentane-like structure, which is stable in solution only at temperatures lower than 220 K. The NMR investigation of the behavior in solution of these complexes revealed different intra- and inter-molecular dynamic processes, involving reversible fragmentation of the cyclic structures, with elimination of neutral or anionic hydrido-carbonyl fragments and interconversion among the different spiro derivatives. In particular, at temperature higher than 220 K the onset of a dissociative equilibrium was observed, involving the egress of a Re2(μ-H) 2(CO)8 fragment from 4, with formation of the [Ag(μ- H)4{Re2(μ-H)(CO)8}{Re4(μ-H) 3(CO)16}]- anion 5, with a mixed spiro cyclopropaneecyclopentane-like structure, never previously observed. The fragmentation reaction was completely reversible on lowering the temperature. The mixed spiro derivative 5 was obtained (in mixture with the two symmetrical spiro derivatives 3 and 4) by either adding 1 equivalent of [Re 2(μ-H)2(CO)8] to 3, or by reacting 1 equivalent of Ag+ with a 1:1 mixture of the free hydrido complexes 1 and 2. Intermolecular exchanges between the Re4H5 fragments of 4 and 5, and between the Re2H3 fragments of 3 and 5 were revealed by 2D 1H EXSY NMR experiments, which also showed exchange between the spiro bis-cyclopropane 3 and the free hydrido complex 1. Furthermore, a higher binding ability towards Ag+ of 1 with respect to 2 was proved, since 1 was found to be able to displace 2 from the spiro bis-cyclopentane adduct 4 and from the mixed spiro cyclopropaneecyclopentane 5, leading to free hydrido complex 2 and the spiro bis-cyclopropane 3
1H, 19F, and 15N NMR study of the interaction between bis(pentafluorophenyl)borinic acid and nitrogen bases
Full text linkBis(pentafluorophenyl)borinic acid ((ArF)2BOH, ArF = C6F5, 1) is a molecule that, due to its multiple reactivity, can behave in different and not always straightforward ways in the presence of nucleophiles. It has been already shown its behaviour in dicloromethane solution in the presence of water, THF and methanol. Briefly, it exists as monomer and trimer (1m and 1t, see Chart 1), and the presence of nucleophiles strongly influence the thermodynamic and the kinetic of the monomer-trimer interconversion. Moreover, according to the nature of the nucleophile, 1 shows a camaleonic nature by forming several, sometimes unexpected, species.2,3 We have studied now the reaction of 1 in the presence of nitrogen bases, that can act as Lewis and Brønsted bases. Two nitrogen bases were studied, namely 1,8-bis(dimethylamino)naphthalene (DMAN), which can act as Brønsted base only, and pyridine, which is a good Lewis base but weak Brønsted base. First of all, the Brønsted base DMAN has been proved to be more efficient than oxygenated Lewis bases in catalyzing the trimerization process. Indeed it is enough a catalytic amount of base to cause the complete trimerization of 1. The so obtained trimeric deprotonated anion 2 (Chart 2) is unstable with respect to dearylation reactions, leading eventually (in the presence of more than 0.33 equiv of DMAN and at higher temperatures) to the tetrarylic species 3 (Chart 2). On the other hand, the behaviour of pyridine is quite different, since 0.33 equiv of base are requested to complete the trimerization of 1, and also in this case the obtained trimer is anionic. Moreover, the presence of 1 equivalent of pyridine does not cause the dearylation process but gives rise to the monomeric neutral 1:1 adduct 4 (Chart 2).
Both the identification and the characterization of the involved species were performed by multiparametric and multinuclear low temperature NMR spectroscopy
Solution structure, dynamics and speciation of perfluoroarylboranes through 1H, 11B and 19F NMR spectroscopy
No full textMultinuclear NMR spectroscopy has been used to study the solution properties of several perfluoroaryl borane derivatives. The information
obtained from all the NMR active isotopes present in these molecules made it possible to establish not only chemical identity, structure and
dynamics of their reaction products, but also the complexity of the solution speciation. In other words, multinuclear NMR helped in unravelling
the real forms in which they are present in solution, that in some cases can change dramatically according to even slight changes of the solution
conditions. Examples will be presented related to the chemistry of tris(pentafluorophenyl)borane, B(C6F5)3, and bis(pentafluorophenyl)borinic acid, B(C6F5)2OH
Solution conformation and dynamics of the ion pairs originating from the reaction of B(C6F5)(3) with bisindenyl dimethyl zirconium complexes
Full text linkThe two ion pairs [(4,7-Me(2)indenyl)(2)ZrMe](+)[MeB(C6F5)(3)](-) (1b) and [(indenyl)(2)ZrMe](+) [MeB(C6F5)(3)](-) (2b) have been generated in situ by reaction of stoichiometric B(C6F5)(3) with the corresponding dimethyl zirconocenes. It has been shown that molecular mechanics computations, guided by experimental H-1/H-1 NOE correlations, can provide information on the conformers present in solution. The dynamics of the ion pairs has also been investigated, showing the occurrence of both the processes previously characterized for this class of compounds, namely the B(C6F5)(3) migration between the two methyl groups and dissociation-recombination of the whole [MeB(C6F5)(3)](-) anion, the latter process being much faster than the first one (about three order of magnitude). Moreover, it has been shown that in certain conditions intermolecular processes can occur, which mimic the above-mentioned dissociative exchanges. In particular, the presence of species containing loosely bound [MeB(C6F5)(3)](-) anion fastens the exchange of this anion, while the presence of free B(C6F5)(3) accelerates its exchange between the two methyl sites
1H NMR Characterization of Organic and Inorganic Nanoparticles
Full text linkDrug delivery employing nano-object as liposomes, polymer conjugates, and nanoparticles suspended in solution is a subject of high current interest [1]. The characterization of the size and the surface functionalization of these nanoparticles is of primary importance. Microscopy techniques give information on deposited colloidal samples, after solvent evaporation, so that the correspondence with the nature of the species in solution is not granted. Dynamic Light Scattering (DLS), which is usually used to estimate the size of a colloidal sample in solution, can overestimate the radii of very small nanoparticles [2]. Diffusion NMR, and in particular Pulsed gradient spin-echo (PGSE) technique, has recently emerged as a valuable tool for colloids characterization [3], complementary to DLS from the point of view of the size evaluation, being highly reliable for the measurement of the smallest particles. Moreover, NMR provide information not only on the size, but also on the interaction between the capping ligands and the nanoparticle surface. In this work, we present the characterization through 1H PGSE NMR measurements of the size of spherical and rod-like TiO2/oleic acid nanoparticles and of conjugates between Re complexes and polyamidoamine nanoparticles. Moreover, by 1H NMR experiments the interaction between TiO2 and the capping oleic acid (OA) has been characterized.
[1] a) A. H. Faraji, P. Wipf, Bioorganic and Medicinal Chemistry 2009, 17, 2950-2962; b) S. M. Garg, A. V. Deshmukh, Nano Science and Nano Technology 2007, 1(2), 45-58; c) C. Khemtong, C. W. Kessinger, J. Gao, Chem. Commun. 2009, 3497-3510.
[2] C. Sanna, C. La Mesa, L. Mannina, P. Stano, S. Viel, A. Segre, Langmuir 2006, 22, 6031-6041.
[3] a) F. Ribot, V. Escax, C. Roiland, C. Sanchez, J. C. Martins, M. Biesemans, I. Verbruggen, R. Willem, Chem. Commun. 2005, 1019-1021; b) M. Valentini, A. Vaccaro, A. Rehor, A. Napoli, J. A. Hubbell, N. Tirelli, J. Am. Chem. Soc. 2004, 126, 2142-2147
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