Low-lying Transitions Research Articles

On the Low-Energy Electromagnetic Dipole Modes in 151,153,155Sm Nuclei

The recently observed low-energy magnetic dipole (M1) and electric dipole (E1) excitations in deformed 151,153,155Sm are theoretically analysed. Rotational Invariant (RI-) and Translational-Galilean Invariant (TGI-) Quasiparticle Nuclear Model (QPNM) are used in the calculation of M1 and E1 properties, respectively. Both theories consider monopole pairing between nucleons, and the deformed Woods-Saxon potential is used as the mean-field potential. Pyatov's symmetry restoration procedure is applied in these models to eliminate the spurious modes from the intrinsic nuclear excitations. Model calculations show that although E1 transitions dominate the low-energy dipole spectra of 151,153,155Sm, many low-lying M1 transitions exist in these nuclei. It is shown that the most significant contribution to E1 and M1 excitation comes from ΔK=±1 transitions. The theory satisfactorily reproduces the gross features of low-lying dipole modes determined from the Oslo Method analysis of the experimental spectra.

Hunting for Sulfur-containing Molecules in Space: A Spectroscopic Characterization of Cyclopropenethione Based on High-level Ab Initio Calculations

Cyclopropenethione falls into the class of complex organic molecules but has not yet been observed in the interstellar medium or any circumstellar disks. However, its existence is very likely, and thus this study provides high-level ab initio predictions, which may serve as reference data for future observations or experimental work. Rovibrational configuration interaction theory based on multidimensional potential energy surfaces being obtained from explicitly correlated coupled-cluster calculations has been used to predict the fundamental vibrational modes, the microwave spectrum, and low-lying rovibrational transitions. Rotational constants as well as quartic and sextic centrifugal distortion constants were obtained from vibrational perturbation theory.

Anions Formed by Perchlorofluorobenzenes C6ClnF6-n (n = 0-6).

Anions formed by the perhalobenzene series C6ClnF6-n (n = 0-6) are studied computationally. All members of the series form both stable valence and stable nonvalence anions. At the geometry of the neutral parents, only nonvalence anions are bound, and the respective vertical electron affinities show values in the 20 to 60 meV range. Valence anions show distorted nonplanar structures, and one can distinguish two types of conformers. A-type conformers show puckered-ring structures and excess electrons delocalized over several C-Cl bonds [in the case of C6F6-, C-F bonds], while B-type conformers possess excess electrons essentially localized in a single C-Cl bond, which is accordingly strongly stretched and bent out of plane. For a specific anion, all conformers are close in energy (relative energies of less than 10 kJ/mol) and are connected by low-lying transition states. Accordingly, A-type and B-type conformers possess similar adiabatic electron affinities; however, their vertical detachment energies exhibit drastically different values, which should ease conformer distinction in photoelectron spectroscopy.

Novel Benzothiadiazole-based Donor-Acceptor Systems: Synthesis, Ultrafast Charge Transfer and Separation Dynamics.

Near-infrared (NIR) absorbing electron donor-acceptor (D-A) chromophores have been at the forefront of current energy research owing to their facile charge transfer (CT) characteristics, which are primitive for photovoltaic applications. Herein, we have designed and developed a new set of benzothiadiazole (BTD)-based tetracyanobutadiene (TCBD)/dicyanoquinodimethane (DCNQ)-embedded multimodular D-A systems (BTD1-BTD6) and investigated their inherent photo-electro-chemical responses for the first time having identical and mixed terminal donors of variable donicity. Apart from poor luminescence, the appearance of broad low-lying optical transitions extendable even in the NIR region (>1000 nm), particularly in the presence of the auxiliary acceptors, are indicative of underlying nonradiative excited state processes leading to robust intramolecular CT and subsequent charge separation (CS) processes in these D-A constructs. While electrochemical studies identify the moieties involved in these photo-events, orbital delocalization and consequent evidence for the low-energy CT transitions have been achieved from theoretical calculations. Finally, the spectral and temporal responses of different photoproducts are obtained from femtosecond transient absorption studies, which, coupled with spectroelectrochemical data, identify broad NIR signals as CS states of the compounds. All the systems are found to be susceptible to ultrafast (~ps) CT and CS before carrier recombination to the ground state, which is, however, significantly facilitated after incorporation of the secondary TCBD/DCNQ acceptors, leading to faster and thus efficient CT processes, particularly in polar solvents. These findings, including facile CT/CS and broad and intense panchromatic absorption over a wide window of the electromagnetic spectrum, are likely to expand the horizons of BTD-based multimodular CT systems to revolutionize the realm of solar energy conversion and associated photonic applications.

Direct probing of low-energy intra d-band transitions in gas-phase cobalt clusters

The interplay between constituent localized and itinerant electrons of metal clusters defines their physical and chemical properties. In turn, the electronic and geometrical structures are strongly entwined and exhibit strong size-dependent variations. Current understanding of low-energy excited states of metal clusters relies on stand-alone theoretical investigations and few comparisons with measured properties, since direct identification of low-lying states is lacking hitherto. Here, we report on the measurement of low-lying electronic transitions in cationic cobalt clusters using infrared photofragmentation spectroscopy. Broad and size-dependent absorption features were observed within 0.056 – 0.446 eV, well above the energies of the sharp absorption bands caused by cluster vibrations. Complementary time-dependent density functional theory calculations reproduce the main observed absorption features, providing direct evidence that they correspond to transitions between electronic states of mainly d-character, arising from the open d-shells of the Co atoms and the high spin multiplicity of the clusters.

Theoretical study of low-lying electronic states spectra and transition properties of BeS molecule

There are only limited theoretical results published on the transition properties of the X1Σ+, A1Π, B1Σ+, C1Δ, D1Σ−, E1Δ, a3Π, b3Σ+, and c3Δ states of BeS, and even some of the states have no theoretical and experimental reports on the relevant aspects of these transitions. Therefore, we derived these transition properties in this study. The potential energy curves of X1Σ+, A1Π, B1Σ+, C1Δ, D1Σ−, E1Δ, a3Π, b3Σ+, c3Δ, d3Σ−, and e3Σ− states of the BeS molecule with their Ω states and the transition dipole moments between them were calculated by the complete active space self-consistent field method, followed by the internally contracted multireference configuration interaction method. For the accurate computation of the transition properties, scalar relativistic corrections and core–valence correlation were considered. The radiative lifetimes were approximately 10−6 s for the A1Π and b3Σ+ states, 10−7 s for the E1Δ state, 10−7–10−8 s for the C1Δ and D1Σ− states, and 10−8 s for the B1Σ+, c3Δ, and e3Σ− states. The transition properties of the seven Ω states generated by the X1Σ+, A1Π, B1Σ+, and a3Π electronic states, including several electric dipole-forbidden transitions were researched. In particular, the B1Σ+ 0+–X1Σ+ 0+, B1Σ+ 0+–A1Π1, and A1Π1–X1Σ+ 0+ systems have strong transitions. In this paper, the radiative lifetimes of A1Π1, B1Σ+ 0+, a3Π0−, a3Π0+, and a3Π1 electronic states were also computed. For the A1Π1, B1Σ+ 0+, and a3Π1 states, the distribution of the radiation lifetime with the rotational angular momentum quantum number J at 0 ≤ υ ≤ 15 and J ≤ 70 was evaluated. It is expected that the outcome of the calculations in this paper can contribute some meaningful guidelines for conducting further theoretical and experimental investigations.

Insight into magnetically induced ring currents and photophysics of six-porphyrin nanorings.

The series of nanorings based on Zn-porphyrins and tetraoxa-isophlorins in different oxidation states (Q = 0, 2+, 4+, 6+) have been studied studied computationally at density functional theory level (DFT) using BHandHLYP functional combined with def2-SVP basis sets. Magnetically induced ring currents of nanorings have been calculated using the GIMIC method and the Ampère-Maxwell integration scheme. Ring current calculations show that neutral nanorings sustain equal diatropic and paratropic currents of 8 nA T-1, resulting in zero net ring current strengths. The charged nanorings sustain strong ring currents with tropicity depending on the oxidation state Q. Among the considered nanorings, the nanoring composed of 6 isophlorins c-Iso66+ is the most aromatic with a ring current of IGIMIC = 81.6 nA T-1. The structure c-P62+ with a ring current of IGIMIC = 54.9 nA T-1 can be considered as the most aromatic among the synthesized porphyrin nanorings. Spin-orbit coupling matrix elements, oscillator strengths, and excitation energies calculated at the CAM-B3LYP/def2-SVP level of theory were used to estimate rate constants for radiative and nonradiative processes. The algorithm based on X-H approximation were used to calculate the internal conversion rates (kIC). The main channel for the deactivation of the excitation energy in the studied nanorings is the process of internal conversion. The deactivation of excited energy occurs due to the vibrations of certain groups of C-H bonds in the nanorings. The nanoring c-Iso6 has magnetically allowed low-lying transitions that contributes significantly to the paratropic ring current, resulting in strong local antiaromaticity in the tetraoxa-isophlorin units.

Study of electronic structure and optical transition properties of low-lying excited states of AuB molecules based on configuration interaction method

High-level configuration interaction method including the spin-orbit coupling is used to investigate the low-lying excited electronic states of AuB that is not reported experimentally. The electronic structure in our work is preformed through the three steps stated below. First of all, Hartree-Fock method is performed to compute the singlet-configuration wavefunction as the initial guess. Next, we generate a multi-reference wavefunction by using the state-averaged complete active space self-consistent field (SACASSCF). Finally, the wavefunctions from CASSCF are utilized as reference, the exact energy point values are calculated by the explicitly correlated dynamic multi-reference configuration interaction method (MRCI). The Davidson correction (+Q) is put forward to solve the size-consistence problem caused by the MRCI method. To ensure the accuracy, the spin-orbit effect and correlation for inner shell electrons and valence shell electrons are considered in our calculation. The potential energy curves of 12 Λ-S electronic states are obtained. According to the explicit potential energy curves, we calculate the spectroscopic constants through solving radial Schrödinger equation numerically. We analyze the influence of electronic state configuration on the dipole moment by using the variation of dipole moment with nuclear distance. The spin-orbit matrix elements for parts of low-lying exciting states are computed, and the relation between spin-orbit coupling and predissociation is discussed. The predissociation is analyzed by using the obtained spin-orbit matrix elements of the 4 Λ-S states which spilt into 12 Ω states. It indicates that due to the absence of the intersections between the curves of spin-orbit matrix elements related with the 4 low-lying Λ-S states, the predissociation for these low-lying exciting states will not occur. Finally, the properties of optical transition between the ground Ω state <inline-formula><tex-math id="M3">\begin{document}$ {\rm A}^{1}{{{\Pi}}}_{1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231347_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231347_M3.png"/></alternatives></inline-formula> and first excited Ω state <inline-formula><tex-math id="M4">\begin{document}$ {{\mathrm{X}}}^{1}{{{\Sigma }}}_{{0}^{+}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231347_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231347_M4.png"/></alternatives></inline-formula> are discussed in laser-cooling filed by analyzing the Franck-Condon factors and radiative lifetime. And the transition dipole moment is also calculated. But our results reveal that the AuB is not an ideal candidate for laser-cooling. In conclusion, this work is helpful in deepening the understanding of AuB, especially the structures of electronic states, interaction between excited states, and optical transition properties. All the data presented in this paper are openly available at <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://www.doi.org/10.57760/sciencedb.j00213.00009">https://www.doi.org/10.57760/sciencedb.j00213.00009</ext-link>.

Ultrafast photogeneration of a metal-organic nitrene from 1,1'-diazidoferrocene.

Ferrocene and its derivatives have fascinated chemists for more than 70 years, not least due to the analogies with the properties of benzene. Despite these similarities, the obvious difference between benzene and ferrocene is the presence of an iron ion and hence the availability of d-orbitals for properties and reactivity. Phenylnitrene with its rich photochemistry can be considered an analogue of nitrenoferrocene. As with most organic and inorganic nitrenes, nitrenoferrocene can be obtained by irradiating the azide precursor. We study the photophysical and photochemical processes of dinitrogen release from 1,1'-diazidoferrocene to form 1-azido-1'-nitrenoferrocene with UV-pump-mid-IR-probe transient absorption spectroscopy and time-dependent density functional theory calculations including spin-orbit coupling. An intermediate with a bent azide moiety is identified that is pre-organised for dinitrogen release via a low-lying transition state. The photochemical decay paths on the singlet and triplet surfaces including the importance of spin-orbit coupling are discussed. We compare our findings with the processes discussed for photochemical dinitrogen activation and highlight implications for the photochemistry of azides more generally.

Introduction to neutron scattering

Neutron scattering is a very high-performance method for studying the structure and dynamics of condensed matter with similar approaches in wide ranges of space and time, matching dimensions in space from single atoms to macromolecules and in time from atomic vibrations over crystal phonons to low-lying transitions in the microwave range, and to motions of large molecular units. Concerning the number and depth of physical concepts, neutron scattering may be compared to modern nuclear magnetic resonance. Neutrons have contributed essential results to the understanding of atomic and molecular processes and are, in this respect, complementary to other materials science probes. Among others, three properties of thermal neutrons make them especially appropriate for such work: the neutron mass is similar to atomic masses, and both neutron energies and the wavelengths of the neutron material wave match typical values for condensed matter. A further important feature of neutron scattering, making it especially valuable in biochemistry and polymer sciences, is that hydrogen and deuterium atoms very significantly and specifically contribute to the signal in both diffraction and spectroscopy. Additionally, neutrons are scattered at the nuclei and directly reflect the nuclear structure and motions. Results from neutron scattering are of great general interest. This paper aims to provide an introduction for chemists on a level understandable also to students and researchers who are not going to become part of the neutron community and will not be involved in the experiments, but shall be able to understand the basic concepts of the method and its relevance to modern chemistry. The paper focuses on basic theory, typical experiments, and some examples demonstrating the applications. As for many modern experimental techniques, the interpretation of the results of neutron scattering is based on theoretical models and requires a significant mathematical overhead. Most results are only meaningful when compared with computer simulations. For understanding this, in this paper, the theory of scattering is developed, starting with intuitive models and presenting typical concepts such as the scattering triangle, energy and momentum transfer, and the relation of inelastic and elastic scattering to space- and time-dependent information. The interaction of neutrons with matter, scattering cross sections, beam attenuation, and coherent versus incoherent scattering are explained in detail. Two further typical concepts that are not generally familiar to scientists outside the community are the use of wave and particle equivalence, and of handling results as a scattering function that depends simultaneously on momentum and energy transfers. The possibility of obtaining neutron beams for scattering experiments at a few research centers around high-performance sources is explained, and experimentally relevant features of research reactors and spallation sources are mentioned. As neutron experiments always have to deal with small flux and extended beams and shielding, experimental conditions are very far away from laboratory methods where handling of samples and instruments is concerned. Experimental details are given for making experiments more understandable and familiarizing the reader with the method. Related to this are extended possibilities for handling samples in a large variety of different environments. In a further part of the manuscript, a variety of techniques and typical instruments are presented, together with some characteristic applications bringing alive the theory developed so far. This covers powder diffraction and structure of liquid water, triple-axis spectrometers and lattice phonons, backscattering spectrometry and rotational tunneling, time-of-flight spectrometry, and simultaneously probing the energy and shape of low lying vibrations and diffusion, filter spectrometer and vibrational spectroscopy without selection rules, small-angle neutron scattering and protein unfolding, as well as micelles, neutron spin echo spectroscopy, and polymer dynamics.

Reduced Absorption Due to Defect-Localized Interlayer Excitons in Transition-Metal Dichalcogenide-Graphene Heterostructures.

Associating atomic vacancies to excited-state transport phenomena in two-dimensional semiconductors demands a detailed understanding of the exciton transitions involved. We study the effect of such defects on the electronic and optical properties of WS2-graphene and MoS2-graphene van der Waals heterobilayers, employing many-body perturbation theory. We find that chalcogen defects and the graphene interface radically alter the optical properties of the transition-metal dichalcogenide in the heterobilayer, due to a combination of dielectric screening and the many-body nature of defect-induced intralayer and interlayer optical transitions. By analyzing the intrinsic radiative rates of the subgap excitonic features, we show that while defects introduce low-lying optical transitions, resulting in excitons with non-negligible oscillator strength, they decrease the optical response of the pristine-like transition-metal dichalcogenide intralayer excitons. Our findings relate excitonic features with interface design for defect engineering in photovoltaic and transport applications.

Nuclear ground state correlations

The paper discusses ground state correlations (GSC) in nuclei, which are especially clearly manifested when using the formalism of quantum Green’s functions (FG) and Feynman diagrams. In addition to one-particleone-hole GSC, which appear in the well known random phase approximation method (RPA), there exist and give a noticeable quantitative contribution THE GSCs resulting from the use of configurations that are more complex than in RPA. Namely, configurations containing phonons and configurations containing only three quasiparticles, respectively, the GSC-phon and GSC3. This last case of GSC3 is considered in detail in the present paper. It is shown that GSC3 make a significant quantitative contribution IN THE problems related to the explanation of some characteristics of low-lying excited states (phonons) and transitions between them in even-even nuclei.

Evaluation of Molecular Fragmentation in Polycyclic Aromatic Hydrocarbons by Time-of-Flight Secondary Ion Mass Spectrometry

In time-of-flight secondary ion mass spectrometry (TOF-SIMS), ionized molecules and molecular fragments (secondary ions) are generated in collisions of high-energy ions (primary ions) with a solid sample surface. Mass spectra of the emitted secondary ions are typically used to identify molecular species and to determine their spatial distribution on the sample surface. Here, we extend this application in a TOF-SIMS study of a series of polycyclic aromatic hydrocarbons (PAHs) where we focus on the fragmentation of these molecules, with the purpose of better understanding the fragmentation patterns of heavy aromatic molecules in petroleum. For all PAHs, the collision process generated (i) a series of smaller cation fragments and (ii) cations close in size to the original PAH (molecular cations). Stark differences are measured for various PAHs regarding the abundance of smaller fragments versus molecular cations. Observation of hydrogen-deficient (H-deficient) cation fragments indicates the formation of polyynes and allenes. For PAHs producing higher fractions of small cation fragments, these ions are surprisingly hydrogen rich (H-rich). The H/C ratio of fragments does not scale with the fraction of Clar sextet carbon, nor with energies of low-lying electronic transitions. Free radical cation fragments tend to be suppressed. For sufficiently large fragments, aromatic cations appear to be formed and include some free radical aromatics. There is ample production of molecular ions with loss of a single carbon atom or a methine group, which corresponds to the reduction of a 6-membered aromatic ring to a 5-membered ring. There is some enhancement of free radical molecular cations due to the corresponding formation of neutral polyynes. Fragment anions are also produced with a strong preference for very H-deficient carbon clusters, in some cases being the same as carbon cluster anions observed in space. Comparisons of PAH TOF-SIMS spectra with those of asphaltenes are discussed in detail.

Thermochemistry of the Smallest Hyperbolic Paraboloid Hydrocarbon: A High-Level Quantum Chemical Perspective

[5.5.5.5]hexaene is a [12]annulene ring with a symmetrically bound carbon atom in its center. This is the smallest hydrocarbon with a hyperbolic paraboloid shape. [5.5.5.5]hexaene and related hydrocarbons are important building blocks in organic and materials chemistry. For example, penta-graphene—a puckered 2D allotrope of carbon—is comprised of similar repeating subunits. Here, we investigate the thermochemical and kinetic properties of [5.5.5.5]hexaene at the CCSD(T) level by means of the G4 thermochemical protocol. We find that this system is energetically stable relative to its isomeric forms. For example, isomers containing a phenyl ring with one or more acetylenic side chains are higher in energy by ∆H298 = 17.5–51.4 kJ mol−1. [5.5.5.5]hexaene can undergo skeletal inversion via a completely planar transition structure; however, the activation energy for this process is ∆H‡298 = 249.2 kJ mol−1 at the G4 level. This demonstrates the high configurational stability of [5.5.5.5]hexaene towards skeletal inversion. [5.5.5.5]hexaene can also undergo a π-bond shift reaction which proceeds via a relatively low-lying transition structure with an activation energy of ∆H‡298 = 67.6 kJ mol−1. Therefore, this process is expected to proceed rapidly at room temperature.

Isotope shifts in cadmium as a sensitive probe for physics beyond the standard model

Isotope shifts (ISs) of atomic energy levels are sensitive probes of nuclear structure and new physics beyond the standard model. We present an analysis of the ISs of the cadmium atom (Cd I) and singly charged cadmium ion (Cd II). ISs of the 229 nm, 326 nm, 361 nm and 480 nm lines of Cd I are measured with a variety of techniques; buffer–gas-cooled beam spectroscopy, capturing atoms in a magneto-optic-trap, and optical pumping. IS constants for the D1 and D2 lines of Cd II are calculated with high accuracy by employing analytical response relativistic coupled-cluster theory in the singles, doubles and triples approximations. Combining the calculations for Cd II with experiments, we infer IS constants for all low-lying transitions in Cd I. We benchmark existing calculations via different many-body methods against these constants. Our calculations for Cd II enable nuclear charge radii of Cd isotopes to be extracted with unprecedented accuracy. The combination of our precise calculations and measurements shows that King plots for Cd I can improve the state-of-the-art sensitivity to a new heavy boson by up to two orders of magnitude.

Building surrogate models of nuclear density functional theory with Gaussian processes and autoencoders

From the lightest Hydrogen isotopes up to the recently synthesized Oganesson (Z = 118), it is estimated that as many as about 8,000 atomic nuclei could exist in nature. Most of these nuclei are too short-lived to be occurring on Earth, but they play an essential role in astrophysical events such as supernova explosions or neutron star mergers that are presumed to be at the origin of most heavy elements in the Universe. Understanding the structure, reactions, and decays of nuclei across the entire chart of nuclides is an enormous challenge because of the experimental difficulties in measuring properties of interest in such fleeting objects and the theoretical and computational issues of simulating strongly-interacting quantum many-body systems. Nuclear density functional theory (DFT) is a fully microscopic theoretical framework which has the potential of providing such a quantitatively accurate description of nuclear properties for every nucleus in the chart of nuclides. Thanks to high-performance computing facilities, it has already been successfully applied to predict nuclear masses, global patterns of radioactive decay like β or γ decay, and several aspects of the nuclear fission process such as, e.g., spontaneous fission half-lives. Yet, predictive simulations of nuclear spectroscopy—the low-lying excited states and transitions between them—or of nuclear fission, or the quantification of theoretical uncertainties and their propagation to basic or applied nuclear science applications, would require several orders of magnitude more calculations than currently possible. However, most of this computational effort would be spent into generating a suitable basis of DFT wavefunctions. Such a task could potentially be considerably accelerated by borrowing tools from the field of machine learning and artificial intelligence. In this paper, we review different approaches to applying supervised and unsupervised learning techniques to nuclear DFT.

Influence of strongly coupled plasma on the low-lying transitions of Be-like ions

Influence of strongly coupled plasma on the low-lying transitions of Be-like ions

Electronic and Vibrational Manifold of Tetracyanoethylene-Chloronaphthalene Charge Transfer Complex in Solution: Insights from TD-DFT and Ab Initio Molecular Dynamics.

The interplay between light absorption and the molecular environment has a central role in the observed photophysics of a wide range of photoinduced chemical and biological phenomena. The understanding of the interplay between vibrational and electronic transitions is the focus of this work, since it can provide a rationale to tune the optical properties of charge transfer (CT) materials used for technological applications. A clear description of these processes poses a nontrivial challenge from both the theoretical and experimental points of view, where the main issue is how to accurately describe and probe drastic changes in the electronic structure and the ultrafast molecular relaxation and dynamics. In this work we focused on the intermolecular CT reaction that occurs upon photon absorption in a π-stacked model system in dichloromethane solution, in which the 1-chloronaphthalene (1ClN) acts as the electron donor and tetracyanoethylene (TCNE) is the electron acceptor. Density functional theory calculations have been carried out to characterize both the ground-state properties and more importantly the low-lying CT electronic transition, and excellent agreement with recently available experimental results [MathiesR. A.; et al. J. Phys. Chem. A2018, 122 ( (14), ), 359429558802] was obtained. The minima of the ground state and first singlet excited state have been accurately characterized in terms of spatial arrangements and vibrational Raman frequencies, and the CT natures of the first two low-lying electronic transitions in the absorption spectra have been addressed and clarified too. Finally, by modeling the possible coordination sites of the TCNE electron acceptor with respect to monovalent ions (Na+, K+) in an implicit solution of acetonitrile, we find that TCNE can accommodate a counterion in two different arrangements, parallel and orthogonal to the C=C axis, leading to the formation of a contact ion pair. The nature of the counterion and its relative position entail structural modifications of the TCNE radical anion, mainly the central C=C and C≡N bonds, compared to the isolated case. An important red shift of the C=C stretching frequency was observed when the counterion is orthogonal to the double bond, to a greater extent for Na+. On the contrary, in the second case, where the counterion ion lies along the internuclear C=C axis, we find that K+ polarizes the electron density of the double bond more, resulting in a greater red shift than with Na+.

Structure of Xe126,128 studied in Coulomb excitation measurements

The electromagnetic properties of $^{126,128}\mathrm{Xe}$ were studied in subbarrier Coulomb excitation measurements performed at the National Superconducting Cyclotron Laboratory Re-accelerator facility, ReA3, at Michigan State University (MSU). $^{126}\mathrm{Xe}$ and $^{128}\mathrm{Xe}$ nuclei were accelerated to 3.74 and 3.81 MeV/nucleon, respectively, and were impinged on $^{196}\mathrm{Pt}$ and $^{208}\mathrm{Pb}$ targets. The $\ensuremath{\gamma}$ rays deexciting the populated low-lying states were detected in coincidence with the scattered nuclei using the JANUS setup. Transition and diagonal matrix elements for low-lying states and transitions in $^{126,128}\mathrm{Xe}$ were extracted from the experimental data using the gosia and gosia2 codes. The experimental results were compared with the theoretical calculations by the microscopic shell model and the Davydov-Filippov $\ensuremath{\gamma}$-rigid rotor model. The calculated results from the newly established shell model (called the PMMU model), which is based on the advanced Hartree-Fock Bogoliubov plus generator coordinate method (HFB $+$ gcm) for a large model space, agree well with the measurements in both nuclei, except for the second ${2}^{+}$ state. Interpretation for the experimentally determined nearly vanishing electric-quadrupole moment of this state remains a challenge for theory.

A combined computational and experimental study on the vibrational structure of ethynyl isothiocyanate, HCCNCS, a molecule with a Champagne bottle potential

The quasi-linear HCCNCS and DCCNCS molecules were studied by matrix-isolation infrared spectroscopy and vibrational configuration interaction (VCI) theory, the latter relying on the Watson Hamiltonian and an accurate potential energy surface including 4-mode coupling terms being obtained from explicitly correlated coupled cluster theory, CCSD(T)-F12a. Strong quartic force constants arising from a Champagne bottle potential render this molecule notoriously difficult to investigate. Anharmonic wavenumbers are provided for a number of low-lying vibrational transitions in the title compound and its deuterated isotopologue.