Can 2‐acylpyrroles form an intramolecular hydrogen bond?

The formation of intramolecular hydrogen bonding by certain N‐substituted 2‐acylpyrroles has been demonstrated by B3LYP/aug‐cc‐pVDZ calculations, the quantum theory of atoms in molecules, and the natural bond orbital method. Total electron energy densities H_BCP at the bond critical point of the H?O bond were applied to analyze the strength of these interactions. The relations between quantum theory of atoms in molecules, carbonyl stretching vibrational modes ν_{C = O}, and natural bond orbital parameters associated with the formation of the C–H?O interaction have been established. The short contacts were found experimentally in the crystal structure of a new 2‐acylpyrrole derivative 5‐chloro‐2‐oxopentyl‐1‐(5‐chloro‐2‐oxopentyl)pyrrolo‐2‐carboxylate. The influence of 2‐ and N‐substitution of 2‐acylpyrroles on C‐H?O interaction energy is discussed. It was found that the methylene group may act as a proton donor leading to a red‐shift or blue‐shift phenomenon of the ν_C–H stretching mode. Copyright © 2015 John Wiley & Sons, Ltd.     

Can tetra‐tert‐butylethylene be formed by ion–ion recombination or ion–molecule reaction of two di‐tert‐butylcarbene units?—a DFT study

The reaction channels of di‐tert‐butylcarbene ( 2 ), its radical anion, ( 3 ) and its radical cation ( 4 ) were investigated theoretically by using DFT/B3LYP with 6‐31+G(d) basis set and 6‐311+G(2d,p) for single point energy calculations. Conversion of the neutral carbene 2 to the charged species 3 and 4 results in significant geometric changes. In cation 4 two different types of C? (CH₃)₃ bonds are observed: one elongated sigma bond called “axial” with 1.61 Å and two normal sigma bonds with a bond length of 1.55 Å. Species 2 and 4 have an electron deficient carbon center; therefore, migration of CH₃ and H is observed from adjacent tert‐butyl groups with low activation energies in the range of 6–9 kcal/mol like similar Wagner–Meerwein rearrangements in the neopentyl‐cation system. Neutral carbene 2 shows C? H insertion to give a cyclopropane derivative with an activation energy of 6.1 kcal/mol in agreement with former calculations. Contrary to species 2 and 4 , the radical anion 3 has an electron rich carbon center which results in much higher calculated activation energies of 26.3 and 42.1 kcal/mol for H and CH₃ migrations, respectively. NBO charge distribution indicates that the hydrogen migrates as a proton. The central issue of this work is the question: how can tetra‐tert‐butylethylene ( 1 ) be prepared from reaction of either species 2 , 3 , or 4 as precursors? The ion–ion reaction between 3 and 4 to give alkene 1 with a calculated reaction enthalpy of 203.5 kcal/mol is extremely exothermic. This high energy decomposes alkene 1 after its formation into two molecules of carbene 2 spontaneously. Ion–molecule reaction of radical anion 3 with the neutral carbene 2 is a much better choice: via a proper oriented charge–transfer complex the radical anion of tetra‐tert‐butylethylene (11) is formed. The electron affinity of 1 was calculated to be negligible. Copyright © 2010 John Wiley & Sons, Ltd.     

Can nanoscale surface charge heterogeneity really explain colloid detachment from primary minima upon reduction of solution ionic strength?

This study theoretically examined colloid detachment from primary minima with ionic strength (IS) reduction on heterogeneous collector surfaces. The chemically and physically heterogeneous collector surfaces were modeled as a planar surface carrying nanoscale patches of different zeta potentials and nanoscale pillars/hemispheroids, respectively. The surface element integration technique was used to calculate interaction energies between colloid and collector surfaces. Two boundary conditions for the double-layer interaction energy were considered, namely constant surface potential (CSP), and linear superposition approximation (LSA). In contrast to prevailing opinions in the literature, our results show that colloids attached on the chemically heterogeneous surface cannot be detached by IS reduction under CSP condition due to an increase of the adhesive force/torque with decreasing IS. Detachment from chemically heterogeneous surfaces by IS reduction can occur under LSA condition only when the flow velocity is very high. In contrast, the presence of nanoscale physical heterogeneity can cause colloid detachment from primary minima by IS reduction under both CSP and LSA conditions at flow velocities commonly used in experimental studies because of a significant reduction in the adhesive forces/torques.     

M?ssbauer Spectroscopy: an elegant tool for the active sites identification and quantification in Pt-Sn-In based naphta reforming catalysts

The reaction of Ln(ClO₄)₃· nH₂O with triphenylphosphine oxide (TPPO) in methanol has led to the formation of [Ln(ClO₄)₂(tppo)₄]ClO₄·MeOH (Ln = Nd, Eu, Gd, Dy, Yb), in which the perchlorate anion acts as a symmetric bidentate. The emission spectra of Eu(III)-TPPO complexes, showing enhancement in the intensity due to the phenyl group, indicate an isotropic electron distribution for the nitrato complex [Eu(NO₃)₃(tppo)₂(EtOH)]. ¹⁵¹Eu and ¹⁵⁵Gd M?ssbauer spectra of the TPPO complexes also lead to the same conclusion.     

Self‐trapped N?H vibrational states in the polymorphs of glycine,L‐ and DL‐alanine

The polarized Raman spectra of the oriented single crystals of L ‐ and DL ‐alanine, α‐, β‐ and γ‐polymorphs of glycine have been studied at 3–300 K. Regularly spaced band packets have been observed in the spectral range of 2500–3000 cm⁻¹, with intensity decreasing noticeably on heating. These band packets were interpreted as the manifestations of the existence of N H self‐trapped states in these systems at low temperatures. The analysis of the polarized spectra has shown that the self‐trapping is observed exclusively for the NH stretching vibration of the amino groups, which is related to the NH···O hydrogen bonds along the head‐to‐tail chains of zwitterions in the crystal structures. The wavenumber of this NH stretching vibration, however, was proposed to depend not solely on the length of this NH···O hydrogen bond, but also on the lengths of all the other NH···O hydrogen bonds formed by the NH₃⁺ and the COO⁻ groups in the structure linking the head‐to‐tail chains with each other. The arguments in favor of the hypothesis that the self‐trapping in these systems can be mediated by zero‐point quantum motions, and not by lattice phonons, are considered. The unusually low wavenumber (2500 cm⁻¹) observed for the NH stretching vibration and indicating at the formation of a very strong NH···O bond is interpreted based on considering the effect of the crystalline environment on the formation and properties of the NH···O bonds in the head‐to‐tail chains of amino acid zwitterions. The results are interesting for understanding the factors determining the dynamics and structural instability of crystalline amino acids and also for biophysical chemistry, as the hydrogen bonded chains formed by amino acid zwitterions in the crystals can mimic the peptide chains. Copyright © 2009 John Wiley & Sons, Ltd.     

Relics in medieval altarpieces? Combining X‐ray tomographic,laminographic and phase‐contrast imaging to visualize thin organic objects in paintings

X‐ray radiography is a common tool in the study of old master paintings. Transmission imaging can visualize hidden paint layers as well as the structure of the panel or canvas. In some medieval altarpieces, relics seem to have been imbedded in the wooden carrier of paintings. These are most probably thin organic fibrous materials such as paper or textile, which in traditional radiography are shadowed by the more absorbing surrounding material. This paper studies the application potential of synchrotron‐based tomographic and laminographic imaging complemented with phase‐contrast imaging for detection of such relics. The techniques are applied to a dummy painting. The results demonstrate that by using these imaging methods it is possible to three‐dimensionally visualize hidden cavities in panels and detect thin fibrous low‐Z materials sandwiched between a high‐Z paint layer and a thick wooden panel.     

Assignment of pre‐edge peaks in K‐edge x‐ray absorption spectra of 3d transition metal compounds: electric dipole or quadrupole?

The characteristics of pre‐edge peaks in K‐edge x‐ray absorption near edge structure (XANES) spectra of 3d transition metals were reviewed from viewpoints of the selection rule, coordination number, number of d‐electrons, and symmetry of the coordination sphere. The contribution of the electric dipole and quadrupole transition to the peaks was discussed on the basis of the group theory, polarized spectra, and theoretical calculations. The pre‐edge peak intensity for T_d symmetry is larger than those for O_h symmetry for all 3d elements. The intense pre‐edge peak for tetrahedral species of 3d transition metals is not due to 1s–3d transition, but transition to the p component in d–p hybridized orbital. The mixing of metal 4p orbitals with the 3d orbitals depends strongly on the coordination symmetry, and the possibility is predictable by group theory. The transition of 1s electron to d orbitals is electric quadrupole component in any of the symmetries. The d–p hybridization does not occur with regular octahedral symmetry, and the weak pre‐edge peak consists of 1s–3d electric quadrupole transition. The pre‐edge peak intensity for a compound with a tetrahedral center changes as a function of the number of 3d electrons regardless of the kind of element; it is maximized at d⁰ and gradually decreases to zero at d¹⁰. The features of pre‐edge peaks in K‐edge XANES spectra for 4d elements and the L₁‐edge for 5d elements are analogous with those for 3d elements, but the pre‐edge peak is broadened due to the wide natural width of the core level. Copyright © 2008 John Wiley & Sons, Ltd.     

Intra‐ and intermolecular N?H···O hydrogen bonds in pyrrolyl derivatives of indane‐1,3‐dione – experimental and theoretical study

2‐(Pyrrol‐2‐ylmethylene)‐1,3‐indandione ( 4 ) and 2‐(pyrrol‐2‐ylmethylene)‐3‐dicyanomethylidene‐1‐indanone ( 5 ) were synthesized. Multinuclear and 2D‐NMR, IR, UV spectroscopic investigations as well as quantum chemical calculations showed the presence of strong hydrogen bonding in these molecules. For both molecules, the presence of two conformers, with and without H‐bond, was experimentally detected in the basic solvents (DMSO, acetone, pyridine) and the solvate complexes were theoretically calculated. Specific behavior of the intramolecular H‐bonded complexes different from that of the intermolecular H‐complexes is discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Can light signals travel faster than <Emphasis Type="Italic">c</Emphasis> in nontrivial vacua in flat space-time? Relativistic causality II

In this paper, we show that the Scharnhorst effect (vacuum with boundaries or a Casimir-type vacuum) cannot be used to generate signals showing measurable faster-than-c speeds. Furthermore, we aim to show that the Scharnhorst effect would violate special relativity, by allowing for a variable speed of light in a vacuum, unless one can specify a small invariant length scale. This invariant length scale would be agreed upon by all inertial observers. We hypothesize the approximate scale of the invariant length.     

Involvement of weak C?H···X hydrogen bonds in metal‐to‐semiconductor regime change in one‐dimensional organic conductors (o‐DMTTF)2X (X = Cl,Br, and I): combined IR and Raman studies

We report on the infrared (IR) and Raman studies of the three isostructural quasi‐one‐dimensional cation radical salts of 3,4‐dimethyl‐tetrathiafulvalene (o‐DMTTF)₂X (X = Cl, Br, and I), which all exhibit metallic properties at room temperature and undergo transitions to a semiconducting state in two steps: a soft metal‐to‐semiconductor regime change in the temperature region T_ρ = 5–200 K and then a sharp phase transition at about T_MI = 50 K. Polarized IR reflectance spectra (700–16 000 cm⁻¹) and Raman spectra (50–3500 cm⁻¹, excitation λ = 632.8 nm) of single crystals were measured as a function of temperature (T = 5–300 K) to assess the eventual formation of a charge‐ordered state below 50 K. Additionally, the temperature dependence of the IR absorption spectra of powdered crystals in KBr discs was also studied. The Raman spectra and especially the bands related to the CC stretching vibration of o‐DMTTF provide unambiguous evidence of uniform charge distribution on o‐DMTTF down to the lowest temperatures, without any modification below 50 K. However, the temperature dependence of Raman spectra indicates a regime change below about 200 K. Temperature dependence of both electronic dispersion and vibrational features observed in the IR spectra also clearly confirms the regime change below about 200 K and shows the involvement of C H···X hydrogen bonds in the electronic localization; some spectral changes can be also related with the phase transition at 50 K. Additionally, using density functional theory methods, the normal vibrational modes of the neutral o‐DMTTF⁰ and cationic o‐DMTTF⁺ species, as well as their theoretical IR and Raman spectra, were calculated. The theoretical data were compared with the experimental IR and Raman spectra of neutral o‐DMTTF molecule. Copyright © 2011 John Wiley & Sons, Ltd.     

Can the Λ(1405) resonance Be seen in neutron spectra from the K − + d reaction?

Neutron spectra from the low-energy reaction K ^? + d → π +Σ+n were calculated using coupled-channel Faddeev equations. The possibility of tracing the signature of the Λ(1405) resonance in the spectra was investigated for four different phenomenological models describing K? N-πΣ interactions. We found, that while in the direct spectra kinematic effects mask completely the peak corresponding to the resonance, the deviation spectrum method [J. Esmaili et al., Phys. Rev. C 83, 055207 (2011)] is able to eliminate kinematics and differentiate between different models of Λ(1405).     

What are the differences between Hantzsch ester and H2 as two‐hydrogen‐atom donors in acetonitrile on the thermodynamics of elementary steps?

Thermodynamic driving forces of eight possible elementary steps for Hantzsch ester and six possible elementary steps for H₂ to release two hydrogen atoms (2H^?) or ions (H^? and H⁺) in acetonitrile were examined using experimental method or the available related thermodynamic data for the first time, which can facilitate chemists to choose a suitable reducing agent between Hantzsch ester and H₂ to reduce a given organic unsaturated compound in acetonitrile and make a rational diagnosis on the detailed reaction mechanisms. The focus of this paper is to compare the differences between Hantzsch ester and H₂ to release two hydrogen atoms (or ions) on the thermodynamics of elementary steps in acetonitrile. Copyright © 2016 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy in living plant using triplex Au?Ag?C core–shell nanoparticles

This paper presents, for the first time, noninvasive imaging of a livingplant using biocompatible carbon‐encapsulated Au Ag nanoparticles (NPs) using micro‐Raman spectroscopy (MRS). A convenient and controllable hydrothermal synthetic route was developed to synthesize the layer‐by‐layer triplex Au Ag C core–shell NPs, which can incorporate the reporter molecule 4‐mercapto benzoic acid (4‐MBA). A unique approach was devised to deliver the carbon‐encapsulated surface‐enhanced Raman scattering (SERS) tags into the leaf of Nicotiana benthamiana. In vivo SERS mapping was subsequently performed to monitor the distribution of tags inside the leaf, which successfully avoided interference of autofluorescence from plant tissue. The imaging modality reported here and further the bio‐functionalized carbon‐encapsulated SERS NPshold significant potential as a strategy forbiochemical imaging in living plantsin a noninvasive and nontoxic manner, whichmight open up exciting opportunities for plant sciences. Copyright © 2010 John Wiley & Sons, Ltd.     

Can one have significant deviations from leptonic 3 $$\times $$ 3 unitarity in the framework of type I seesaw mechanism?

We address the question of deviations from \(3\times 3\) unitarity of the leptonic mixing matrix showing that, in the framework of type I seesaw mechanism, one may have significant deviations from unitarity that can be detected at the next round of experiments while some of the heavy neutrino masses are sufficiently low to become within experimental reach. For that purpose we introduce a specially useful parametrisation that enables to control all deviations of unitarity through a single \(3 \times 3\) matrix, which we denote by X and which connects the mixing of the light and heavy neutrinos in the context of type I seesaw. We show that there is no need for the Yukawa couplings to be extremely suppressed. We present specific examples where deviations from \(3\times 3\) unitarity are sufficiently small to conform to all the present stringent experimental bounds.     

Can contributions to B_s→l~+l~-γ from the magnetic-penguin operator with real photons in the standard model be neglected?

Using the Bs meson wave function extracted from non-leptonic Bs decays, we reevaluate the rare decays Bs→l+l- γ,(l=e,μ) in the Standard Model, including two kinds of contributions from the magnetic-penguin operator with virtual and real photons. We find that contributions to the exclusive decays from the magnetic-penguin operator b→sγ with real photons, which were regarded as negligible in the previous literature, are large and the branching ratios Bs→l+l-γ are enhanced by a factor of almost 2. With the predicted branching ratios of the order of 10-8, it is expected that these radiative dileptonic decays will be detected in LHC-b and B factories in the near future.