A
bstract
Next-generation neutrinoless double beta decay experiments aim for half-life sensitivities of ∼ 10
27
yr, requiring suppressing backgrounds to
<
1 count/tonne/yr. For this, any extra ...background rejection handle, beyond excellent energy resolution and the use of extremely radiopure materials, is of utmost importance. The NEXT experiment exploits differences in the spatial ionization patterns of double beta decay and single-electron events to discriminate signal from background. While the former display two Bragg peak dense ionization regions at the opposite ends of the track, the latter typically have only one such feature. Thus, comparing the energies at the track extremes provides an additional rejection tool. The unique combination of the topology-based background discrimination and excellent energy resolution (1% FWHM at the Q-value of the decay) is the distinguishing feature of NEXT. Previous studies demonstrated a topological background rejection factor of ∼ 5 when reconstructing electron-positron pairs in the
208
Tl 1.6 MeV double escape peak (with Compton events as background), recorded in the NEXT-White demonstrator at the Laboratorio Subterráneo de Canfranc, with 72% signal efficiency. This was recently improved through the use of a deep convolutional neural network to yield a background rejection factor of ∼ 10 with 65% signal efficiency. Here, we present a new reconstruction method, based on the Richardson-Lucy deconvolution algorithm, which allows reversing the blurring induced by electron diffusion and electroluminescence light production in the NEXT TPC. The new method yields highly refined 3D images of reconstructed events, and, as a result, significantly improves the topological background discrimination. When applied to real-data 1.6 MeV
e
−
e
+
pairs, it leads to a background rejection factor of 27 at 57% signal efficiency.
Computational calculations represent a very useful tool to study separately the occurrence of thermal and non-thermal effects of microwave irradiation through the determination of the thermodynamic ...and kinetic parameters of the reaction. In this paper, we approach the computational study of two previously reported cycloaddition reactions. All of the outcomes indicate the presence of a thermal effect alone for the microwave irradiation that produces changes in the regioselectivity or in the reaction mechanism.
Histone deacetylases (HDACs) play a key role in the regulation of gene expression and chromatin structure, and drugs targeting these enzymes might have an important impact in the treatment of human ...cancer. Herein, we report the characterization of (1H)-pyrroles as a new subfamily of HDAC inhibitors obtained by computational modeling of class-I human HDACs. From a functional standpoint, (1H)-pyrroles are powerful inductors of acetylation of histones H3 and H4, and restore the expression of growth-inhibitory genes. From a cellular view, these compounds cause a marked decrease in the viability of cancer cells in vitro and in vivo, associated with a cell-cycle arrest at G2/M and an inhibition of angiogenesis. Thus, (1H)-pyrroles emerge as a novel group of HDAC inhibitors with promising antitumoral features.
Density-functional theory studies on the 2 + 2 reaction between N-silylimines and ketenes to form the corresponding 2-azetidinones (β-lactams) help to clarify several aspects on the mechanism of the ...Staudinger reaction. This reaction has been studied experimentally by Panunzio et al. It is shown that the formation of the 2-azetidinone ring takes place via two consecutive reactions. The first reaction consists of the nucleophilic addition of the iminic nitrogen to the sp-hybridized carbon atom of the ketene, with simultaneous migration of the silyl group from the imine to the oxygen atom of the ketene. This leads to silyl enol intermediates, in good agreement with the experimental results. Formation of the N-silylated β-lactam takes place via a domino reaction consisting of a conrotatory thermal electrocyclization followed by a new silatropic rearrangement. It is also found that isomerization of the starting N-silylimine has a lower activation barrier than that associated with the formation of the C−N bond, which explains the stereochemical outcome experimentally observed. Further considerations on the asymmetric torquoelectronic effects involved in this reaction are also reported.
The mechanism of the aza-Wittig reaction between phosphazenes and aldehydes has been studied computationally, using DFT methods (B3LYP/6-31G* level), and experimentally. It has been found that the ...reaction consists of a tandem 2+2 cycloaddition−cycloreversion sequence in which π and σ orbitals as well as lone pairs are involved. Both 2+2 processes take place via thermally allowed supra-supra mechanisms. P-trimethyl-λ5-phosphazenes are predicted to be more reactive than their P-triphenyl analogues. The stereochemical outcome of the whole reaction depends only on the second step, because conformational changes in the intermediate 1,3,2-λ5-oxazaphosphazetidines have a much lower activation energy than the second 2+2 cycloreversion reaction. Preferential or exclusive formation of the corresponding (E)-imines is predicted.
The mechanism of the conversion of N-acyl-4-acyloxy-β-lactams into 1,3-oxazin-6-ones has been investigated using ab initio and density functional theories. It has been found that two pseudopericyclic ...reactions are involved in the whole process. The first key reaction is a retro-4-exo-dig cyclization instead of a thermal conrotatory electrocyclic ring opening. Magnetic characterization of the corresponding transition structure shows antiaromatic character, despite the low activation energy associated with this process. The second step is very exothermic and has no activation barrier. It corresponds to another pseudopericyclic reaction instead of a six-electron disrotatory electrocyclization. These results confirm that there is no correlation between aromaticity and pseudopericyclic reactions. In contrast, thermal-symmetry-allowed pericyclic reactions are always aromatic. Therefore, magnetic analysis of the corresponding transition structures constitutes a useful tool to distinguish between both kinds of processes.
If neutrinos are their own antiparticles the otherwise-forbidden nuclear reaction known as neutrinoless double beta decay can occur. The very long lifetime expected for these exceptional events makes ...its detection a daunting task. In order to conduct an almost background-free experiment, the NEXT collaboration is investigating novel synthetic molecular sensors that may capture the Ba dication produced in the decay of certain Xe isotopes in a high-pressure gas experiment. The use of such molecular detectors immobilized on surfaces must be explored in the ultra-dry environment of a xenon gas chamber. Here, using a combination of highly sensitive surface science techniques in ultra-high vacuum, we demonstrate the possibility of employing the so-called Fluorescent Bicolor Indicator as the molecular component of the sensor. We unravel the ion capture process for these molecular indicators immobilized on a surface and explain the origin of the emission fluorescence shift associated to the ion trapping.
Kinetic aspects of the synthesis of (2-methacryloxy)ethyloxy functionalized poly(l-lactide) macromonomers using aluminum mono- and trialkoxides carrying this functionality are presented. The results ...show the living character of the ring-opening polymerization independently of the reaction temperature, but there are some deviations of the classical first order with respect to the monomer concentration, particularly at 60 °C and for low monomer/aluminum catalyst ratios. The comparative study of aluminum trialkoxides with respect to the corresponding monoalkoxides makes clear that the polymerization initiated by trialkoxides is at least five times faster than that initiated by the monoalkoxide. This behavior is explained by taking into consideration semiempirical calculations based on the PM-protocol, which gives an idea of the reactivity of mono- and trialkoxides with good agreement between theoretical and experimental results.
More dead ends and detours Sierra, Miguel A; de la Torre, Maria C; Cossio, Fernando P ...
2013/01/01, 2013, 2013-07-11
eBook
Success comes in many forms and in synthesis it can be a failure that results in their ultimate successful solutions. This long-awaited sequel to "Dead Ends and Detours" retains the proven concept ...while featuring over 20 new case studies of failed strategies and their (successful) solutions in natural product total synthesis. Additionally, computational models are used to discuss the problem in much more detail and to provide readers with additional information not found in the primary literature. The topics range from classic synthetic reactions (e.g. Diels Alder reaction), metal-mediated coupling reactions, metathesis, and asymmetric catalysis to the importance of protecting and activating groups. This book will benefit not only graduate students in organic chemistry but also advanced researchers as they gain knowledge derived from the step-by-step analysis of mistakes made in the past and, thus be able to improve their own chemical reaction planning. With its coverage of the most commonly applied reaction types, the book perfectly complements its predecessor, which focuses on general aspects, such as reactivity and selectivity.
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