Opportunities for fundamental physics research with radioactive molecules.

Molecules containing short-lived, radioactive nuclei are uniquely positioned to enable a wide range of scientific discoveries in the areas of fundamental symmetries, astrophysics, nuclear structure, and chemistry. Recent advances in the ability to create, cool, and control complex molecules down to the quantum level, along with recent and upcoming advances in radioactive species production at several facilities around the world, create a compelling opportunity to coordinate and combine these efforts to bring precision measurement and control to molecules containing extreme nuclei. In this manuscript, we review the scientific case for studying radioactive molecules, discuss recent atomic, molecular, nuclear, astrophysical, and chemical advances which provide the foundation for their study, describe the facilities where these species are and will be produced, and provide an outlook for the future of this nascent field.

Enhancing the Potential of Fused Heterocycle-Based Triarylhydrazone Photoswitches.

Triarylhydrazones represent an attractive class of photochromic compounds offering many interesting features including high molar absorptivity, good addressability, and extraordinary thermal stability. In addition, unlike most other hydrazone-based photoswitches, they effectively absorb light above 365 nm. However, previously prepared triaryhydrazones suffer from low quantum yields of the Z→E photoisomerization. Here, we have designed a new subclass of naphthoyl-benzothiazole hydrazones that balance the most beneficial features of previously reported naphthoyl-quinoline and benzoyl-pyridine triarylhydrazones. These preserve the attractive absorption characteristics, exhibit higher thermal stability of the metastable form than the former and enhance the rate of the Z→E photoisomerization compared to the later, as a result of the weakening of the intramolecular hydrogen bonding between the hydrazone hydrogen and the benzothiazole moiety. Introducing the benzothiazole motif extends the tunability of the photochromic behaviour of hydrazone-based switches.

Enhancing the Potential of Fused Heterocycle-Based Triarylhydrazone Photoswitches.

Invited for the cover of this issue are Marek Cigán, Anna M. Grabarz and co-workers. The image depicts how a non-expert might imagine a "molecular photoswitch". Read the full text of the article at 10.1002/chem.202303509.

Chiral molecule candidates for trapped ion spectroscopy by ab initio calculations: From state preparation to parity violation.

Parity non-conservation (PNC) due to the weak interaction is predicted to give rise to enantiomer dependent vibrational constants in chiral molecules, but the phenomenon has so far eluded experimental observation. The enhanced sensitivity of molecules to physics beyond the Standard Model (BSM) has led to substantial advances in molecular precision spectroscopy, and these may be applied to PNC searches as well. Specifically, trapped molecular ion experiments leverage the universality of trapping charged particles to optimize the molecular ion species studied toward BSM searches, but in searches for PNC, only a few chiral molecular ion candidates have been proposed so far. Importantly, viable candidates need to be internally cold, and their internal state populations should be detectable with high quantum efficiency. To this end, we focus on molecular ions that can be created by near threshold resonant two-photon ionization and detected via state-selective photo-dissociation. Such candidates need to be stable in both charged and neutral chiral versions to be amenable to these methods. Here, we present a collection of suitable chiral molecular ion candidates we have found, including CHDBrI+ and CHCaBrI+, that fulfill these conditions according to our ab initio calculations. We find that organo-metallic species have low ionization energy as neutrals and relatively high dissociation thresholds. Finally, we compute the magnitude of the PNC values for vibrational transitions for some of these candidates. An experimental demonstration of state preparation and readout for these candidates will be an important milestone toward measuring PNC in chiral molecules for the first time.

Design of High-Performance Pyridine/Quinoline Hydrazone Photoswitches.

The design of P-type photoswitches with thermal stability of the metastable form of hundreds of years that would efficiently transform using excitation wavelengths above 350 nm remains a challenge in the field of photochromism. In this regard, we designed and synthesized an extended set of 13 pyridine/quinoline hydrazones and systematically investigated the structure-property relationships, defining their kinetic and photoswitching parameters. We show that the operational wavelengths of the pyridine hydrazone structural motif can be effectively shifted toward the visible region without simultaneous loss of their high thermal stability. Furthermore, we characterized the ground-state and excited-state potential energy surfaces with quantum-chemical calculations and ultrafast transient absorption spectroscopy, which allowed us to rationalize both the thermal and photochemical reaction mechanisms of the designed hydrazones. Whereas introducing an electron-withdrawing pyridyl moiety in benzoylpyridine hydrazones leads to thermal stabilities exceeding 200 years, extended π-conjugation in naphthoylquinoline hydrazones pushes the absorption maxima toward the visible spectral region. In either case, the compounds retain highly efficient photoswitching characteristics. Our findings open a route to the rational design of a new family of hydrazone-based P-type photoswitches with high application potential in photonics or photopharmacology.

DFT Functionals for Modeling of Polyethylene Chains Cross-Linked by Metal Atoms. DLPNO-CCSD(T) Benchmark Calculations.

Density functional theory (DFT) functionals for calculations of binding energies (BEs) of the polyethylene (PE) chains cross-linked by selected metal atoms (M) are benchmarked against DLPNO-CCSD(T) and DLPNO-CCSD(T1) data. PEX-M-PEX complexes as compared with plain parallel PEX···PEX chains with X = 3-9 carbon atoms are model species characterized by a cooperative effect of covalent C-M-C bonds and interchain dispersion interactions. The accuracy of DLPNO-CC methods was assessed by a comparison of BEs with the canonical CCSD(T) results for small PE3-M-PE3 complexes. Functionals for PEX···PEX and closed-shell PEX-M-PEX complexes (M = Be, Mg, Zn) were benchmarked against DLPNO-CCSD(T) BEs; open-shell complexes (M = Li, Ag, Au) were benchmarked against the DLPNO-CCSD(T1) method with iterative triples. Three dispersion corrections were combined with 25 DFT functionals for calculations of BEs with respect to PEX-M and PEX fragments employing def2-TZVPP and def2-QZVPP basis sets. Accuracy to within 5% for the closed-shell PEX-M-PEX complexes was achieved with five functionals. Less accurate are functionals for the open-shell PEX-M-PEX complexes; only two functionals deviate by less than 15% from DLPNO-CCSD(T1). Particularly problematic were PEX-Li-PEX complexes. A reasonable overall performance across all complexes in terms of the mean absolute percentage error is found for the range-separated hybrid functionals ωB97X-D3 and CAM-B3LYP/D3(BJ)-ABC.

Boronate Ester Bullvalenes.

Boronate ester bullvalenes are now accessible in two to four operationally simple steps. This unlocks late-stage diversification through Suzuki cross-coupling reactions to give mono-, di-, and trisubstituted bullvalenes. Moreover, a linchpin strategy enables preprogrammed installation of two different substituents. Analysis of solution phase isomer distributions and single-crystal X-ray structures reveals that isomer preference in the crystal lattice is due to general shape selectivity.

Material Size Dependence on Fundamental Constants.

Precise experimental setups for detection of variation of fundamental constants, scalar dark matter, or gravitational waves, such as laser interferometers, optical cavities, and resonant-mass detectors, are directly linked to measuring changes in material size. Here we present calculated and experiment-derived estimates for both α and µ dependence of lattice constants and bond lengths of selected solid-state materials and diatomic molecules that are needed for interpretation of such experiments.

Doping effects on the geometric and electronic structure of tin clusters.

Molecular beam electric deflection experiments on neutral single copper-doped tin clusters are presented at different cryogenic nozzle temperatures. The experimental cluster beam profiles SnNCu (N = 9-16) are compared with classical rotational dynamic simulations of globally optimized structures obtained by a genetic algorithm based on density functional theory. The formation of endohedral complexes with comparable geometry to manganese- and gold-doped tin is confirmed. Theoretical methods predict ionic structures of the type Cuδ-@SnNδ+ with electron transfer from the tin cage to the central copper dopant. This behaviour is discussed based on a molecular orbital picture particularly with respect to other transition metal tetrel complexes.

High accuracy theoretical investigations of CaF, SrF, and BaF and implications for laser-cooling.

The NL-eEDM collaboration is building an experimental setup to search for the permanent electric dipole moment of the electron in a slow beam of cold barium fluoride molecules [NL-eEDM Collaboration, Eur. Phys. J. D 72, 197 (2018)]. Knowledge of the molecular properties of BaF is thus needed to plan the measurements and, in particular, to determine the optimal laser-cooling scheme. Accurate and reliable theoretical predictions of these properties require the incorporation of both high-order correlation and relativistic effects in the calculations. In this work, theoretical investigations of the ground and lowest excited states of BaF and its lighter homologs, CaF and SrF, are carried out in the framework of the relativistic Fock-space coupled cluster and multireference configuration interaction methods. Using the calculated molecular properties, we determine the Franck-Condon factors (FCFs) for the A2Π1/2→X2Σ1/2 + transition, which was successfully used for cooling CaF and SrF and is now considered for BaF. For all three species, the FCFs are found to be highly diagonal. Calculations are also performed for the B2Σ1/2 +→X2Σ1/2 + transition recently exploited for laser-cooling of CaF; it is shown that this transition is not suitable for laser-cooling of BaF, due to the nondiagonal nature of the FCFs in this system. Special attention is given to the properties of the A'2Δ state, which in the case of BaF causes a leak channel, in contrast to CaF and SrF species where this state is energetically above the excited states used in laser-cooling. We also present the dipole moments of the ground and excited states of the three molecules and the transition dipole moments (TDMs) between the different states. Finally, using the calculated FCFs and TDMs, we determine that the A2Π1/2→X2Σ1/2 + transition is suitable for transverse cooling in BaF.

Synthesis and Analysis of Substituted Bullvalenes.

Herein we detail a practical synthesis of bullvalene and a variety of mono- and disubstituted analogues through cobalt-catalysed [6+2] cycloaddition of cyclooctatetraene to alkynes, followed by photochemical di-π-methane rearrangement. The application of isomer-network analysis, coupled with quantum-chemical calculations, provides a powerful automated tool for predicting the properties of bullvalene isomer networks.

Toward understanding the bonding character in complexes of coinage metals with lone-pair ligands. CCSD(T) and DFT computations.

We present CCSD(T) interaction energies and the bonding analysis for complexes of Cu, Ag, and Au with the lone-pair ligands H2O, OF2, OMe2, NH3, NF3, NMe3, H2S, SF2, SMe2, PH3, PF3, PCl3, and PMe3 (ML complexes). Both electron correlation and relativistic effects are crucial in the bonding of all complexes. AuPH3, AuPF3, and AuPCl3 (AuPX3) complexes exhibit particularly large relativistic effects, 30-46 kJ/mol. Upon neglecting relativistic effects, the Au-P bonds almost vanish aside from weak long-range van der Waals interactions. Highest binding energies are computed for complexes with Au, followed by Cu and Ag. For all coinage metals the strongest interactions are computed for PX3 ligands followed by SX2 and NX3 OX2 ligands. Upon methylation the interaction energy rises significantly. Metal-thiol complexes, particularly AuSCH3, form a separate class. Exceptional stability of gold complexes is due to large relativistic enhancement of the electron affinity of Au. Along with the electron affinity of a metal, we link the pattern of interaction energies in ML complexes with ionization potentials (IPs) of ligands. Strong interaction with P containing ligands is attributed to their lower IP and the lone pair → metal electron donation accompanied with the back-donation characteristic for P containing ligand. Energy data are accompanied with the natural bond orbital analysis. Computationally less demanding DFT computations with the PBE0 functional provide correct pattern of interaction energies when compared with benchmark CCSD(T) results.

Large vibrationally induced parity violation effects in CHDBrI.

The isotopically chiral molecular ion CHDBrI+ is identified as an exceptionally promising candidate for the detection of parity violation in vibrational transitions. The largest predicted parity-violating frequency shift reaches 1.8 Hz for the hydrogen wagging mode which has a sub-Hz natural line width and its vibrational frequency auspiciously lies in the available laser range. In stark contrast to this result, the parent neutral molecule is two orders of magnitude less sensitive to parity violation. The origin of this effect is analyzed and explained. Precision vibrational spectroscopy of CHDBrI+ is feasible as it is amenable to preparation at internally low temperatures and resistant to predissociation, promoting long interrogation times (Landau et al., J. Chem. Phys., 2023, 159, 114307). The intersection of these properties in this molecular ion places the first observation of parity violation in chiral molecules within reach.

Electron affinities of uracil: microsolvation effects and polarizable continuum model.

We present adiabatic electron affinities (AEAs) and the vertical detachment energies (VDEs) of the uracil molecule interacting with one to five water molecules. Credibility of MP2 and DFT/B3LYP calculations is supported by comparison with available benchmark CCSD(T) data. AEAs and VDEs obtained by MP2 and DFT/B3LYP methods copy trends of benchmark CCSD(T) results for the free uracil and uracil-water complexes in the gas phase being by 0.20 - 0.28 eV higher than CCSD(T) values depending on the particular structure of the complex. AEAs and VDEs from MP2 are underestimated by 0.09-0.15 eV. For the free uracil and uracil-(H(2)O)(n) (n = 1,2,3,5) complexes, we also consider the polarizable continuum model (PCM) and discuss the importance of the microsolvation when combined with PCM. AEAs and VDEs of uracil and uracil-water complexes enhance rapidly with increasing relative dielectric constant (ε) of the solvent. Highest AEAs and VDEs of the U(H(2)O)(5) complexes from B3LYP with ε = 78.4 are 2.03 and 2.81 eV, respectively, utilizing the correction from CCSD(T). Specific structural features of the microsolvated uracil-(H(2)O)(n) complexes and their anions are preserved also upon considering PCM in calculations of AEAs and VDEs.

Ambimodal Pericyclic Rearrangements of Dialkenyl-Bullvalenes Give Tetrahydro-1,8-ethenoheptalenes.

The fluxional structure of bullvalene is expanded by the discovery of a [5,5]-sigmatropic rearrangement of dialkenyl substituted derivatives. This gives rise to tetrahydro-1,8-ethenoheptalenes (THEH), representing the first examples of this tricyclic scaffold. Variation of the substitution pattern alters the product distribution, including one thermodynamically balanced between THEH and bullvalene isomers. DFT calculations are used to explore the thermodynamic landscape and reaction mechanism revealing a pretransition state bifurcation leading to a concerted ambimodal rearrangement pathway.

Toward Detection of the Molecular Parity Violation in Chiral Ru(acac)3 and Os(acac)3.

We present a theory-experiment investigation of the helically chiral compounds Ru(acac)3 and Os(acac)3 as candidates for next-generation experiments for detection of molecular parity violation (PV) in vibrational spectra. We used relativistic density functional theory calculations to identify optimal vibrational modes with expected PV effects exceeding by up to 2 orders of magnitude the projected instrumental sensitivity of the ultrahigh resolution experiment under construction at the Laboratoire de Physique des Lasers in Paris. Preliminary measurements of the vibrational spectrum of Ru(acac)3 carried out as the first steps toward the planned experiment are presented.

Stereomutation of Substituted Bullvalenes.

The stereomutation of substituted bullvalenes is an inevitable consequence of the valence isomerism that automerizes this unique fluxional hydrocarbon. The introduction of external stereogenicity in the substituents expands the reaction graphs and leads to a wealth of complex diastereochemical relationships. In this communication, we explore these possibilities and prepare a range of stereochemically rich substituted bullvalenes. This includes a series of disubstituted bullvalenes with two external stereocenters as a platform for fluxional, shape-diverse compound libraries. We also prepare a tethered bisbullvalene with central stereogenicity in the tether as an ensemble of 900 unique isomers that are completely stereomutable.

The electron affinity of astatine.

One of the most important properties influencing the chemical behavior of an element is the electron affinity (EA). Among the remaining elements with unknown EA is astatine, where one of its isotopes, 211At, is remarkably well suited for targeted radionuclide therapy of cancer. With the At- anion being involved in many aspects of current astatine labeling protocols, the knowledge of the electron affinity of this element is of prime importance. Here we report the measured value of the EA of astatine to be 2.41578(7) eV. This result is compared to state-of-the-art relativistic quantum mechanical calculations that incorporate both the Breit and the quantum electrodynamics (QED) corrections and the electron-electron correlation effects on the highest level that can be currently achieved for many-electron systems. The developed technique of laser-photodetachment spectroscopy of radioisotopes opens the path for future EA measurements of other radioelements such as polonium, and eventually super-heavy elements.

Network Analysis of Substituted Bullvalenes.

Substituted bullvalenes are dynamic shape-shifting molecules that exist within complex reaction networks. Herein, we report the synthesis of di- and trisubstituted bullvalenes and investigate their dynamic properties. Trisubstituted bullvalenes share a common major isomer which shows kinetic metastability. A survey of the thermodynamic and kinetic landscapes through computational analysis together with kinetic simulation provides a map of the internal dynamics of these systems.

Gold doping of tin clusters: exo- vs. endohedral complexes.

We present molecular beam electric deflection experiments on neutral gold-doped tin clusters. The experimental SnNAu (N = 6-16) cluster beam profiles are interpreted by means of classical trajectory simulations supplied, with cluster structures generated by a genetic algorithm based on density functional theory. The combined experimental and theoretical analysis confirms that at least nine tin atoms are necessary to form a cage that is capable of encapsulating a gold atom, with high symmetry only marginally distorted by the gold atom. Two-component DFT calculations reveal that for some clusters spin-orbit effects are necessary to properly describe these species. Partial charge analysis methods predict the presence of charge transfer effects from the tin host to the dopant, resulting in a negatively charged gold atom.