68 research outputs found
Kinetic and Thermodynamic Preferences in Aryl vs Benzylic CâH Bond Activation with Cationic Pt(II) Complexes
Anhydrous cationic Pt(II) complexes [(NN)Pt(CH_3)(CF_3CD_2OD)]+ (1, NN = ArNC(Me)âC(Me)NAr), which are obtained by reaction of (NN)Pt(CH_3)_2 with B(C_6F_5)_3 in CF_3CD_2OD, activate CâH bonds of benzene and methylbenzenes, with enhanced reactivity compared to the previously prepared equilibrium mixtures with the (thermodynamically favored) aquo complexes. For methylbenzenes (toluene, p-xylene, mesitylene), activation at the aromatic and benzylic positions are kinetically competitive, but the product of the latter is strongly favored thermodynamically. This unusual trend is attributed to formation of η^3-benzyl structures, which can be observed spectroscopically for 1,4-diethylbenzene activation
Mechanism of CâH Bond Activation of Alkyl-Substituted Benzenes by Cationic Platinum(II) Complexes
While all methyl- and ethyl-substituted benzenes react with diimine Pt(II) methyl cations to give η^3-benzyl products, they do not all get there by the same pathway. For toluene and p-xylene, isotopic labeling shows that initial activation occurs at aryl positions with subsequent intermolecular conversion to the benzyl product. For ethylbenzene and 1,4-diethylbenzene, initial activation takes place exclusively at aryl CâH bonds, and conversion to the η^3-benzyl product takes place via intramolecular isomerization. Only in the most extreme case of steric crowding the reaction of a bulky diimine platinum methyl cation (Ar = Mes) with triethylbenzene does direct activation of the ethyl group become preferred to aryl activation
CâH Bond Activation by Dicationic Platinum(II) Complexes
Double protonolysis of diimine platinum dimethyls [(N-N)PtMe_2] (N-N â ArNC(Me)C(Me) â NAr) generates dicationic Pt(II) complexes that can activate a variety of CâH bonds, liberating 1 equiv of acid and forming organoplatinum species that are moderately stable to the resulting acidic conditions. Ethylbenzenes lead to η^3-benzyl complexes; mechanistic experiments suggest that η^3-benzyl product formation proceeds via CâH bond activation at the benzylic methylene position. In some cases Ï-arene complexes can be observed, but their role in the CâH activation process is not clear. Cyclohexane and 1-pentene react to give η^3-allyl complexes; allylbenzene gives a chelated phenyl-η^2-olefin structure, as determined by X-ray diffraction. No stable CâH activation products are obtained from methylbenzenes, benzene itself, or alkanes
Kinetic and Thermodynamic Preferences in Aryl vs Benzylic C-H Bond Activation with Cationic Pt(II) Complexes
From teaching physics to teaching children : beginning teachers learning from pupils
This paper discusses the development of beginning physics teachers' pedagogical content knowledge (PCK) in the context of teaching basic electricity during a one-year Professional Graduate Diploma in Education course (PGDE) and beyond. This longitudinal study used repeated semi-structured interviews over a period of four-and-a-half years. The interview schedule followed a line of development through the secondary school electrical syllabus in Scotland. Fifteen student teachers were interviewed during the PGDE year. Six of them were followed up at the end of the Induction Year (their first year as a newly qualified teacher), and again two-and-a-half years later. Thematic analysis of the interviews showed that before the beginning teachers had taught any classes, their initial focus was on how to transform their own subject matter knowledge (SMK) about electricity into forms that were accessible to pupils. As the beginning teachers gained experience working with classes, they gave vivid descriptions of interacting with particular pupils when teaching electricity which showed the development of their pedagogical knowledge. This played a significant role in the teachers' change of focus from teaching physics to teaching children as they transformed their SMK into forms that were accessible to pupils and developed their general pedagogical knowledge
In the dedicated pursuit of dedicated capital: restoring an indigenous investment ethic to British capitalism
Tony Blairâs landslide electoral victory on May 1 (New Labour Day?) presents the party in power with a rare, perhaps even unprecedented, opportunity to revitalise and modernise Britainâs ailing and antiquated manufacturing economy.* If it is to do so, it must remain true to its long-standing (indeed, historic) commitment to restore an indigenous investment ethic to British capitalism. In this paper we argue that this in turn requires that the party reject the very neo-liberal orthodoxies which it offered to the electorate as evidence of its competence, moderation and âmodernisationâ, which is has internalised, and which it apparently now views as circumscribing the parameters of the politically and economically possible
The 2dF-SDSS LRG and QSO Survey: The Star Formation Histories of Luminous Red Galaxies
We present a detailed investigation into the recent star formation histories
of 5,697 Luminous Red Galaxies (LRGs) based on the Hdelta (4101A) and [OII]
(3727A) lines. LRGs are luminous (L>3L*), galaxies which have been selected to
have photometric properties consistent with an old, passively evolving stellar
population. For this study we utilise LRGs from the recently completed 2dF-SDSS
LRG and QSO survey (2SLAQ). Equivalent widths of the Hdelta and [OII] lines are
measured and used to define three spectral types, those with only strong Hdelta
absorption (k+a), those with strong [OII] in emission (em) and those with both
(em+a). All other LRGs are considered to have passive star formation histories.
The vast majority of LRGs are found to be passive (~80 per cent), however
significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6
per cent) are identified. An investigation into the redshift dependence of the
fractions is also performed. A sample of SDSS MAIN galaxies with colours and
luminosities consistent with the 2SLAQ LRGs is selected to provide a low
redshift comparison. While the em and em+a fractions are consistent with the
low redshift SDSS sample, the fraction of k+a LRGs is found to increase
significantly with redshift. This result is interpreted as an indication of an
increasing amount of recent star formation activity in LRGs with redshift. By
considering the expected life time of the k+a phase, the number of LRGs which
will undergo a k+a phase can be estimated. A crude comparison of this estimate
with the predictions from semi-analytic models of galaxy formation shows that
the predicted level of k+a and em+a activity is not sufficient to reconcile the
predicted mass growth for massive early-types in a hierarchical merging
scenario.Comment: Accepted for publication in MNRAS, 13 pages, 10 figure
The UV-Optical Color Magnitude Diagram II: Physical Properties and Morphological Evolution On and Off of a Star-Forming Sequence
We use the UV-optical color magnitude diagram in combination with
spectroscopic and photometric measurements derived from the SDSS spectroscopic
sample to measure the distribution of galaxies in the local universe (z<0.25)
and their physical properties as a function of specific star formation rate
(SSFR) and stellar mass. Throughout this study our emphasis is on the
properties of galaxies on and off of a local "star-forming sequence." We
discuss how the physical characteristics of galaxies along this sequence are
related to scaling relations typically derived for galaxies of different
morphological types. We find, among other trends that our measure of the star
formation rate surface density is nearly constant along this sequence. We
discuss this result and implications for galaxies at higher redshift. For the
first time, we report on measurements of the local UV luminosity function
versus galaxy structural parameters as well as inclination. We also split our
sample into disk-dominated and bulge-dominated subsamples using the i-band
Sersic index and find that disk-dominated galaxies occupy a very tight locus in
SSFR vs. stellar mass space while bulge-dominated galaxies display a much
larger spread of SSFR at fixed stellar mass. A significant fraction of galaxies
with SSFR and SF surface density above those on the "star-forming sequence" are
bulge-dominated. We can use our derived distribution functions to ask whether a
significant fraction of these galaxies may be experiencing a final episode of
star formation (possibly induced by a merger or other burst), soon to be
quenched, by determining whether this population can explain the growth rate of
the non-star-forming galaxies on the "red sequence." (Abridged)Comment: 30 pages, 28 figures, scheduled to appear as part of the GALEX
Special Ap.J.Suppl., December, 2007 (29 papers
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