Extending the Domain‐Averaged Exchange‐Correlation Energies Within the Context of the MC‐QTAIM: Tracing Subtle Variations Induced by Isotope Substitution

Recently, it has been demonstrated that the domain‐averaged exchange‐correlation energies, V^xc, are capable of tracing the covalent character of atom–atom interactions unequivocally and thus pave the way for detailed bonding analysis within the context of the quantum theory of atoms in molecules (QTAIM) [M. García‐Revilla, E. Francisco, P. L. Popelier, A. Martín Pendás, ChemPhysChem 2013 , 14, 1211–1218]. Herein, the concept of V^xc is extended within the context of the newly developed multicomponent QTAIM (MC‐QTAIM). The extended version, , is capable of analyzing nonadiabatic wavefunctions and thus is sensitive to the mass of nuclei and can trace “locally” the subtle electronic variations induced by isotope substitution. To demonstrate this capability in practice, ab initio nonadiabatic wavefunctions for three isotopically substituted hydrogen cyanide molecules, in which the hydrogen nucleus was assumed to be a proton, deuterium, or tritium, were derived. The resulting wavefunctions were then used to compute and it emerged that for the hydrogen–carbon bond, the was distinct for each isotopic composition and varied in line with chemical expectations. Indeed, the introduction of paves the way for the investigation of vast numbers of structural and kinetic isotope effects within the context of the MC‐QTAIM.     

Reliability of Orientational Order Parameters Determined from Two‐dimensional X‐ray Diffraction Patterns: A Simulation Study

The orientational order parameter is one of the most important quantities to describe the degree of long‐range orientational ordering of liquid crystals. There are several approaches to experimentally measure this order parameter of liquid crystalline phases but every method includes substantial simplifications and assumptions. We present a simulation‐based approach to elucidate the reliability of the method of Davidson, Petermann and Levelut to measure via 2D X‐ray experiments. We have found that this method slightly underestimates by an absolute value of only 0.05 and thus provides reliable measures of by X‐ray diffraction.     

Electronic Spectroscopy of Methylcyanodiacetylene (CH3C5N)

The results of a study devoted to the electronic spectroscopy of gaseous, solid, and cryogenic matrix‐isolated methylcyanodiacetylene (CH₃C₅N) are reported. UV absorption and optical phosphorescence spectra of the compound are described here for the first time, and the corresponding vibronic assignments are proposed. UV absorption, studied directly or through the excitation of phosphorescence, revealed the ¹E‐‐ ¹A₁ system, very weak ¹A₂– ¹A₁ bands, and a strong, broad absorption feature, tentatively identified as ¹E– ¹A₁. Spectral measurements were assisted by quantum chemical calculations at the DFT and ab initio (coupled cluster) levels of theory.     

Fluorescence Dynamics in the Endoplasmic Reticulum of a Live Cell: Time‐Resolved Confocal Microscopy

Fluorescence dynamics in the endoplasmic reticulum (ER) of a live non‐cancer lung cell (WI38) and a lung cancer cell (A549) are studied by using time‐resolved confocal microscopy. To selectively study the organelle, ER, we have used an ER‐Tracker dye. From the emission maximum of the ER‐Tracker dye, polarity (i.e. dielectric constant, ?) in the ER region of the cells (≈500 nm in WI38 and ≈510 nm in A549) is estimated to be similar to that of chloroform ( =506 nm, ?≈5). The red shift by 10 nm in in the cancer cell (A549) suggests a slightly higher polarity compared to the non‐cancer cell (WI38). The fluorescence intensity of the ER‐Tracker dye exhibits prolonged intermittent oscillations on a timescale of 2–6 seconds for the cancer cell (A549). For the non‐cancer cell (WI38), such fluorescence oscillations are much less prominent. The marked fluorescence intensity oscillations in the cancer cell are attributed to enhanced calcium oscillations. The average solvent relaxation time (<τ_s>) of the ER region in the lung cancer cell (A549, 250±50 ps) is about four times faster than that in the non‐cancer cell (WI38, 1000±50 ps).     

Computational Design of New Heterofullerene‐Based Biomimetic α‐Carbonic Anhydrase Analogues

The biomimetic CO₂ hydration activity of Ru/Rh‐doped fullerenes was revealed by using density functional theory calculations. The mechanism of CO₂ hydration on the proposed heterofullerenes followed the mechanistic action of α‐carbonic anhydrases, and consisted of the adsorption and deprotonation of H₂O, CO₂ interaction with hydroxyl groups, CO₂ bending, and proton transfer to give the product. Free‐energy landscapes for the reaction showed the catalysts to be active for the reaction. H₂O adsorption over the catalysts was exergonic whereas CO₂ adsorption over the catalyst–OH complex was observed to be an endergonic process. Intramolecular proton transfer resulting in the final product, , was found to be the rate‐limiting step for the reaction on C₅₆N₃M (M=Ru/Rh), whereas H₂O dissociation was found to be the rate‐limiting step for the reaction on C₅₉M (M=Ru/Rh). C₅₆N₃M catalysts were found to be superior to C₅₉M catalysts for biomimetic CO₂ hydration, as indicated by the free‐energy landscapes and energy requirements.     

Mass‐Selected Circular Dichroism of Supersonic‐Beam‐Cooled [D4]‐(R)‐(+)‐3‐Methylcyclopentanone

UV spectroscopy and electronic circular dichroism (ECD) experiments on supersonic‐beam‐cooled deuterated (R)‐(+)‐3‐methylcyclopentanone ([D₄]‐(R)‐(+)‐3‐MCP) have been performed by using a laser mass spectrometer. The spectral resolution not only allowed excitation and CD measurements for single vibronic transitions but also for the rotational P, Q, and R branches of these transitions. The investigated transition showed the largest anisotropy factor ever observed for chiral molecules in the gas phase, which, due to residual saturation of the excited transition, represents only a lower limit for the real anisotropy factor. Furthermore, one‐color (1+1+1) and two‐color (1+1′) resonance‐enhanced multiphoton ionization (REMPI) measurements were performed and the effusive‐beam (room temperature) and supersonic‐beam results for [D₄]‐(R)‐(+)‐3‐MCP were compared. These results allowed a differentiation between single‐step ECD (comparable to conventional ECD) and cumulative ECD (only possible in multiphoton excitation) under supersonic‐beam conditions.     

Origin of the Director Tilt in the Lyotropic Smectic C* Analog Phase: Hydration Interactions and Solvent Variations

The origin and long‐range correlation of the director tilt in the recently discovered phase, which is the lyotropic analog of the thermotropic smectic C* (SmC*) liquid crystalline phase, are investigated. Polarized micro‐Raman spectroscopy reveals that the director tilt in the phase originates from a tilting of the aromatic 2‐phenylpyrimidine cores of the surfactant molecules. Optical measurements of the tilt angle show that its magnitude decreases with increasing solvent concentration, suggesting that the long‐range inter‐lamellar correlation of the tilt directions is reduced at increasing thickness of the solvent layers. The phase diagrams with four different solvents (water, formamide, N‐methylformamide, N,N‐dimethylformamide) are investigated, showing that the phase is only formed with those solvents that exhibit a dense network of hydrogen bonds. This observation suggests that these hydrogen bond networks play an essential role in the long‐range correlation of the director tilt between adjacent surfactant layers. To verify this assumption, mixtures with deuterated solvents are investigated, showing that the tilt angle in the phase is indeed reduced by this modification of the solvent′s hydrogen bond network.     

Study of the EFG tensor at 75As nuclei in Ge‐As‐Se chalcogenide glasses

Asymmetry parameters of the electric field gradient tensor at ⁷⁵As nuclei were determined for chalcogenide glassy semiconductors (CGS) of the Ge‐As‐Se system by comparing the experimental and simulated ⁷⁵As nuclear quadrupole resonance nutation interferograms. The electric field gradient asymmetry in CGS was analyzed, and it is believed that a structural change in these glassy semiconductors takes place at = 2.425. Electron paramagnetic resonance spectra of the Ge‐As‐Se system were obtained for the first time. A comparison was made between the results of analysis of the Ge‐As‐Se system by nuclear quadrupole resonance and electron paramagnetic resonance methods, and this allowed us to make the supposition that a structural phase transition occurs at = 2.4 from two‐dimensional to three‐dimensional CGS structure. Copyright © 2013 John Wiley & Sons, Ltd.     

The Threshold Photoelectron Spectrum of Fulvenone: A Reactive Ketene Derivative in Lignin Valorization

Unveiling reaction mechanisms by isomer-selective detection of reactive intermediates requires advanced spectroscopic knowledge. We study the photoionization of fulvenone (c-C₅H₄=C=O), a reactive ketene species relevant in catalytic pyrolysis of lignin, which was generated by pyrolysis of 2-methoxy acetophenone. The high-resolution threshold photoelectron spectrum (TPES) with vacuum ultraviolet synchrotron radiation revealed well-resolved vibrational transitions, assigned to ring deformation modes of the cyclopentadiene moiety. The adiabatic ionization energy was determined to be 8.25±0.01 eV and is assigned to the ²A₂← ¹A₁ transition. A broad and featureless band arising at 9 eV is associated with the ²B₁← ¹A₁ excitation. A conical intersection is responsible for the ultrafast relaxation of the fulvenone cation from the into the state resulting in a featureless and lifetime broadened band. These insights will increase the detection capabilities for fulvenone and thereby help to elucidate reaction mechanisms in lignin catalytic pyrolysis.     

An Efficient N‐Heterocyclic Carbene‐Ruthenium Complex: Application Towards the Synthesis of Polyesters and Polyamides

The ruthenium benzimidazolylidene‐based N‐heterocyclic carbene (NHC) complex 4 catalyzes the direct dehydrogenative condensation of primary alcohols into esters and primary alcohols in the presence of amines to the corresponding amides in high yields. This efficient new catalytic system shows a high selectivity towards the conversion of diols to polyesters and of a mixture of diols and diamines to polyamides. The only side product formed in this reaction is molecular hydrogen. Remarkable is the conversion of hydroxytelechelic polytetrahydrofuran ( = 1000 g mol⁻¹)—a polydispers starting material—into a hydrolytically degradable polyether with ester linkages ( = 32 600 g mol⁻¹) and, in the presence of aliphatic diamines, into a polyether with amide linkages in the back bone ( = 16 000 g mol⁻¹).

     

Determination of the “Privileged Structure” of 8-Hydroxyquinoline

An accurate semi-experimental equilibrium structure of 8-hydroxyquinoline (8-HQ) has been determined combining experiment and theory. The cm-wave rotational spectrum of 8-HQ was recorded in a pulsed supersonic jet using broadband dual-path reflection and narrowband Fabry-Perot-type resonator Fourier-transform microwave spectrometers. Accurate rotational and quartic centrifugal distortion constants and ¹⁴N quadrupole coupling constants are determined. Rotational constants of all ¹³C, ¹⁸O and ¹⁵N singly substituted isotopologues in natural abundance and those of a chemically synthesized OD isotopologue were used to obtain geometric parameters for all the heavy atoms and the hydroxyl hydrogen from a number of structure determination models. Theoretical approaches allowed for the determination of a semi-experimental equilibrium structure, in which computed rovibrational and electronic corrections were utilized to convert vibrational ground state constants into equilibrium constants. Despite the molecule having only a horizontal plane of symmetry and possessing 11 individual heavy atoms, microwave spectroscopy has allowed for a reliable and accurate structure determination. A mass dependent, structure was determined and proved to be equally reliable by comparison with the B3LYP-D3(BJ)/aVTZ equilibrium structure.     

Access to Base Adducts of Low‐Valent Organotin‐Hydride Compounds by Controlled,Stepwise Hydrogen Abstraction from a Tetravalent Organotin Trihydride

Hydrogen can be selectively removed from organotin trihydrides to generate the corresponding organohydrostannylene intermediates. Depending on the size of the substituent and the mode of generation, the intermediates undergo further reactions. Herein, we report on the formation of a variety of organotin hydrides with tin in the oxidation states Sn^II, Sn^I–Sn^III and Sn^III–Sn^III, all accessed by the controlled removal of hydrogen from the tetravalent Ar′Sn^IV trihydride (Ar′=2,6‐dimesitylphenyl, mesityl=2,4,6‐trimethylphenyl).     

CO2 and SnII Adducts of N‐Heterocyclic Carbenes as Delayed‐Action Catalysts for Polyurethane Synthesis

Catalytic rivals : Both CO₂‐protected tetrahydropyrimidin‐2‐ylidene‐based N‐heterocyclic carbenes (NHCs) and Sn^II‐1,3‐dimesitylimidazol‐2‐ylidene, as well as Sn^II‐1,3‐dimesitylimidazolin‐2‐ylidene complexes (example displayed), have been identified as truly latent catalysts for polyurethane (PUR) synthesis rivaling all existing systems both in activity and latency.

     

Modelling Photoionisation in Isocytosine: Potential Formation of Longer-Lived Excited State Cations in its Keto Form

Studying the effects of UV and VUV radiation on non-canonical DNA/RNA nucleobases allows us to compare how they release excess energy following absorption with respect to their canonical counterparts. This has attracted much research attention in recent years because of its likely influence on the origin of our genetic lexicon in prebiotic times. Here we present a CASSCF and XMS-CASPT2 theoretical study of the photoionisation of non-canonical pyrimidine nucleobase isocytosine in both its keto and enol tautomeric forms. We analyse their lowest energy cationic excited states including , and and compare these to the corresponding electronic states in cytosine. Investigating lower-energy decay pathways we find – unexpectedly - that keto-isocytosine⁺ presents a sizeable energy barrier potentially inhibiting decay to its cationic ground state, whereas enol-isocytosine⁺ features a barrierless and consequently ultrafast pathway analogous to the one previously found for the canonical (keto) form of cytosine⁺. Dynamic electron correlation reduces the energy barrier in the keto form substantially (by ∼1 eV) but it is nevertheless still present. We additionally compute the UV/Vis absorption signals of the structures encountered along these decay channels to provide spectroscopic fingerprints to assist future experiments in monitoring these intricate photo-processes.     

Carbon–sulfur chains: A high‐resolution infrared and quantum‐chemical study of C3S and SC7S

In the course of a 5 μm high‐resolution infrared study of laser ablation products from carbon–sulfur targets, the ν₁ vibrational mode region of linear C₃S has been studied continuously from 2046 to 2065 cm^?1. Besides the prominent vibrational fundamental, the region was found to feature the , and even hot bands, the latter two of which were observed for the first time. Owing to the high signal‐to‐noise ratio obtained, the ν₁ mode of S could also be observed in natural abundance for the first time at high spectral resolution in the infrared. At 2061 cm^?1, hidden inside the branch of the C₃S ν₁ fundamental mode, a weak new band is observed which exhibits very tight line spacing and stems from a heavy both carbon and sulfur containing carrier. On the basis of high‐level quantum‐chemical calculations of selected carbon–sulfur chains and other carbon‐rich cumulenes, this feature is attributed to the ν₅ vibrational fundamental of linear SC₇S, which stands for the first gas‐phase spectroscopic detection of this long cumulenic chain.     

Rate Coefficients for the Gas‐Phase Reaction of Ozone with C5 and C6 Unsaturated Aldehydes

Emissions of biogenic volatile organic compounds are higher than those from anthropogenic sources. In this work, we studied the kinetics of the reaction of three unsaturated aldehydes (trans‐2‐pentenal, trans‐2‐hexenal, and 2‐methyl‐2‐pentenal) with ozone in a rigid atmospheric simulation chamber coupled to an FTIR spectrometer at four different temperatures (273, 298, 333, and 353 K). Reaction rate constants (× 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹) at 298 K are 1.24 ± 0.06 for trans‐2‐pentenal (t‐2P), 1.37 ± 0.03 for trans‐2‐hexenal (t‐2H), and 1.58 ± 0.20) for 2‐methyl‐2‐pentenal (2M2P). The following Arrhenius expressions were deduced (cm³ molecule⁻¹ s⁻¹): The obtained data are presented and compared to those reported in the literature at room temperature, as well as to homologous alkenes. The atmospheric lifetimes with respect to ozone, derived from this study, are estimated to vary between 7 and 10 days.     

The Trifluoromethyl Anion: Evidence for [K(crypt‐222)]+

[K(crypt‐222)]⁺ ( 1 ) and [K(crypt‐222)]⁺ ( 3 ) are isostructural, displaying nearly identical unit cell parameters. The two structures are similar to the extent that the previously reported [K(crypt‐222)]⁺ model can be refined against the new data for [K(crypt‐222)]⁺ , with extra electron density being observed from the fourth fluorine atom of the . In agreement with experimental observations, theoretical calculations suggest that deprotonated [K(crypt‐222)]⁺ is highly unstable even at as low as 195 K. The previously considered 1:1 CHF ₃ clathrate of deprotonated [K(crypt‐222)]⁺ (crystallographically indistinguishable from 1 ) is ruled out on the basis of all available data.     

En Route to Stimuli‐Responsive Boron‐, Nitrogen‐, and Sulfur‐Doped Polycyclic Aromatic Hydrocarbons

Replacing both meso carbon atoms of the polycyclic aromatic hydrocarbon (PAH) bisanthene by boron atoms creates an efficient blue fluorophore with a strong electron‐accepting character. The corresponding meso‐B,S‐doped bisanthene exhibits a solvent‐dependent green‐to‐orange photoluminescence and undergoes a reversible reduction at E =?2.06 V (vs. FcH/FcH⁺). After oxidation of the sulfur atom, the resulting sulfoxide emits in the blue range of the spectrum, shows only negligible solvatochromism, and a reversible redox transition at E =?1.74 V. Several related B, N‐ and B, S‐containing PAHs have been prepared following the same modular synthetic procedure and are also described herein. In order to systematically compare their optoelectronic properties, all products have been investigated by cyclic voltammetry as well as UV/Vis absorption/emission spectroscopy.     

An Experimental Study of the Kinetics of the Reactions of Isopropyl,sec‐Butyl,and tert‐Butyl Radicals with Molecular Chlorine at Low Pressures (0.5–7.0 Torr) in the Temperature Range 190–480 K

In this work, we have measured the rate coefficients of the reactions of isopropyl (propan‐2‐yl), sec‐butyl (butan‐2‐yl), and tert‐butyl (2‐methylpropan‐2‐yl) radicals with molecular chlorine as a function of temperature (190–480 K). The experiments were done in a tubular laminar flow reactor coupled to a photoionization quadrupole mass spectrometer employing a gas‐discharge lamp for ionization. The radicals were homogeneously produced in the reactor by photolyzing suitable precursor molecules with 193‐nm pulsed exciplex laser radiation. The bimolecular rate coefficients were obtained by monitoring the radical decay signals in real time under pseudo–first‐order conditions. The rate coefficients of all three reactions showed negative temperature dependence. The bath gas used in the experiments was helium, and the rate coefficients appeared to be independent of the helium concentrations employed ([2.4–14] × 10¹⁶ cm^?3) for all three reactions. The rate coefficients of the reactions can be approximated in the studied temperature range by the following parameterizations: We estimate that the overall uncertainties of the measured rate coefficients are ±20%. We were able to observe 2‐chloropropane (i‐C₃H₇Cl) product for the i‐C₃H₇^● + Cl₂ reaction. No products were observed for the other two reactions, and the reasons for this are briefly discussed in the text.     

Microsecond Protein Dynamics from Combined Bloch-McConnell and Near-Rotary-Resonance R1p Relaxation-Dispersion MAS NMR

Studying protein dynamics on microsecond-to-millisecond (μs-ms) time scales can provide important insight into protein function. In magic-angle-spinning (MAS) NMR, μs dynamics can be visualized by rotating-frame relaxation dispersion experiments in different regimes of radio-frequency field strengths: at low RF field strength, isotropic-chemical-shift fluctuation leads to “Bloch-McConnell-type” relaxation dispersion, while when the RF field approaches rotary resonance conditions bond angle fluctuations manifest as increased rate constants (“Near-Rotary-Resonance Relaxation Dispersion”, NERRD). Here we explore the joint analysis of both regimes to gain comprehensive insight into motion in terms of geometric amplitudes, chemical-shift changes, populations and exchange kinetics. We use a numerical simulation procedure to illustrate these effects and the potential of extracting exchange parameters, and apply the methodology to the study of a previously described conformational exchange process in microcrystalline ubiquitin.