Ground State Isomers Research Articles

Density functional theory-based study on the structural, electronic and spectral properties of gas-phase PbMg n - (n = 2-12) clusters.

Gas-phase PbMg n- (n = 2-12) cluster structures were globally searched on their potential energy surfaces by means of the CALYPSO prediction software. Structural optimization and calculations of properties such as relative energy and electronic structure were then carried out by density functional theory for each size of low energy isomer. The structural, relative stability, natural charge population, natural electronic configuration and distribution of the strongest peaks of the infrared and Raman spectra of the low energy isomers of PbMg n- (n = 2-12) clusters were systematically investigated in the present work. It was shown that the PbMg7- cluster ground state isomer exhibits the highest stability, for which special electronic excitation and chemical bonding analyses were performed. It is reasonable to believe that this work enriches the structural, spectroscopic and other data of magnesium-based clusters and provides some theoretical basis for possible future experimental syntheses.

Characterization of Deformational Isomerization Potential and Interconversion Dynamics with Ultrafast X-ray Solution Scattering.

Dimeric complexes composed of d8 square planar metal centers and rigid bridging ligands provide model systems to understand the interplay between attractive dispersion forces and steric strain in order to assist the development of reliable methods to model metal dimer complexes more broadly. [Ir2 (dimen)4]2+ (dimen = para-diisocyanomenthane) presents a unique case study for such phenomena, as distortions of the optimal structure of a ligand with limited conformational flexibility counteract the attractive dispersive forces from the metal and ligand to yield a complex with two ground state deformational isomers. Here, we use ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) to reveal the nature of the equilibrium distribution and the exchange rate between the deformational isomers. The two ground state isomers have spectrally distinct electronic excitations that enable the selective excitation of one isomer or the other using a femtosecond duration pulse of visible light. We then track the dynamics of the nonequilibrium depletion of the electronic ground state population─often termed the ground state hole─with ultrafast XSS and OTAS, revealing a restoration of the ground state equilibrium in 2.3 ps. This combined experimental and theoretical study provides a critical test of various density functional approximations in the description of bridged d8-d8 metal complexes. The results show that density functional theory calculations can reproduce the primary experimental observations if dispersion interactions are added, and a hybrid functional, which includes exact exchange, is used.

Laser spectroscopy of triply charged 229Th isomer for a nuclear clock.

Thorium-229 (229Th) possesses an optical nuclear transition between the ground state (229gTh) and low-lying isomer (229mTh). A nuclear clock based on this nuclear-transition frequency is expected to surpass existing atomic clocks owing to its insusceptibility to surrounding fields1-5. In contrast to other charge states, triply charged 229Th (229Th3+) is the most suitable for highly accurate nuclear clocks because it has closed electronic transitions that enable laser cooling, laser-induced fluorescence detection and state preparation of ions1,6-8. Although laser spectroscopic studies of 229Th3+ in the nuclear ground state have been performed8, properties of 229mTh3+, including its nuclear decay lifetime that is essential to specify the intrinsic linewidth of the nuclear-clock transition, remain unknown. Here we report the trapping of 229mTh3+ continuously supplied by a 233U source and the determination of nuclear decay half-life of the isolated 229mTh3+ to be through nuclear-state-selective laser spectroscopy. Furthermore, by determining the hyperfine constants of 229mTh3+, we reduced the uncertainty of the sensitivity of the 229Th nuclear clock to variations in the fine-structure constant by a factor of four. These results offer key parameters for the 229Th3+ nuclear clock and its applications in the search for new physics.

Isomerization dynamics of a novel cis/trans‐only merocyanine

AbstractMerocyanines (MC) usually adopt ring opened zwitterionic structures that are interconvertible with their ring‐closed spiropyran photoisomers. By methylating the phenolate oxygen, and thereby blocking the ring‐closure reaction, a cis/trans‐only MC photoswitch was obtained, yielding a perfect candidate for a detailed examination of the cis/trans isomerization mechanism for this class of compounds. This photoswitch displays outstanding properties including excellent photoreaction quantum yields and photoswitching turnovers. Due to the central polymethine bridge of MC, in principle eight cis (C)/trans/(T) isomers are possible. Density Functional Theory (DFT) calculations revealed the CCT and TTT‐isomers of the studied compound as most stable cis and trans ground state isomers, respectively. UV/vis transient absorption studies combined with conical intersection computations with the complete active space self‐consistent field (CASSCF) method show that both trans/cis‐ and cis/trans‐photoisomerizations are initiated by a rotation of the central doubled bond fragment. A hot ground state species is then formed, which undergoes a second isomerization. Thus, the cis/trans reaction proceeds via a CCT‐CTT‐TTT sequence and the reverse reaction via TTT‐TCT‐CCT.

Spectroscopy and Photochemistry of Aluminum-Bearing Species in the Universe.

ConspectusMetal-bearing molecules impact the chemical and physical environment of many astronomical sources such as the circumstellar envelopes of large asymptotic giant branch and red supergiant stars, the interstellar medium, and planetary atmospheres (e.g., ablation of ∼20 tons per day into the Earth's upper atmosphere). In recent decades, the number of successfully detected metal-containing molecules has increased via rotational spectroscopic observations, which are driven by theoretical and experimental investigations. Following formation, the ultimate fate of each species (stabilization, dissociation, etc.) is determined by its electronic structure and electronic spectroscopic properties as it encounters the pervasive radiation fields in the vacuum of space. Studying these properties can evince the possibility of detection and predict the impact each molecule has on its surrounding environment. Aluminum, one of the most abundant elements and metals, is distributed throughout the universe as a constituent of gas-phase molecules (e.g., AlO, AlOH, AlCl, etc.) as well as condensed onto solid dust grains such as Al2O3. Free gas-phase aluminum-bearing molecules are synthesized by nonthermal equilibrium processes such as shocks and pulsations near the stellar photosphere or via the reaction of molecules on the surface of dust grains. Recent investigations in our research group utilizing quantum chemical methods, such as coupled cluster (CCSD(T) and CCSD(T)-F12) and multireference configuration interaction (MRCI) with large basis sets, have explored a wide breadth of spectroscopy and photochemistry of small (triatomic and tetratomic) aluminum-bearing molecules, including Al-H, Al-C, Al-N, Al-O, Al-Si, Al-P, and Al-S bonds, among others. The ground-state spectroscopy (rotational and vibrational) of various aluminum-bearing molecules is discussed in the context of experimental and observational detection potentials. These detection potentials depend on various factors, such as the magnitude of the permanent dipole moment (PDM) and the population of states yielding transition frequencies in detectable ranges. Many aluminum-bearing molecules possess large PDMs and may be prime candidates for astronomical and laboratory detection. Within this discussion, interesting aspects of the ground-state molecular orbital configuration of OAlNO are shown to lead to an uncommon triplet ground state. Additionally, the electronic absorption spectrum of the quasi-isoenergetic ground-state isomers of AlOSO is discussed as a sensitive method for detecting this species and differentiating between the two isomers. Finally, photochemical mechanisms key to the production of AlO and AlOH in low-density regions and the destruction of AlCO and AlOC are also discussed in order to understand the radiation-induced formation and destruction of these molecules.

Insights into the structural, electronic and spectral properties of gas-phase GaMgn+ (n = 2–12) clusters

A global potential energy surface search of the gas-phase GaMgn+ (n = 2–12) clusters was performed using CALYPSO, and they were comprehensively studied based on DFT. Specifically, the geometric growth of low-lying energy isomers, the relative stability of the ground state isomers, the electronic structure properties and the spectroscopic properties are fully studied. The results show that the structure of GaMgn+ (n = 3–12) clusters is based on tetrahedral and yurt-type patterns of growth. Calculations show that the ground state isomer of GaMg9+ is a relative special cluster due to it being the most locally stable. In addition, charge transfer studies show that all Ga atoms are electron receivers while most Mg atoms play the role of electron donor for GaMgn+ (n = 3–12) clusters. Electron configuration calculations confirm that the orbitals of the Mg and Ga atoms are sp hybridized in all the ground state isomers. Finally, to provide the corresponding data for possible future experimental validation, the present work also presents theoretical calculations of the IR, Raman and UV-Vis spectra of all the ground state isomers.

Monitoring the photochemistry of a formazan over 15 orders of magnitude in time.

2,3,5-triphenyltetrazolium chloride (TTC) may convert into phenyl-benzo[c]tetrazolocinnolium chloride (PTC) and 1,3,5-triphenylformazan (TPF) under irradiation with light. The latter reaction, albeit enzymatically rather than photochemically, is used in so-called TTC assays indicating cellular respiration and cell growth. In this paper, we address the photochemistry of TPF with time-resolved spectroscopy on various time scales. TPF is stabilized by an intramolecular hydrogen bond and switches photochemically via an E-Z isomerization around an N=N double bond into another TPF-stereoisomer, from which further isomerizations around the C=N double bond of the phenylhydrazone group are possible. We investigate the underlying processes by time-resolved spectroscopy in dependence on excitation wavelength and solvent environment, resolving several intermediates over a temporal range spanning 15 orders of magnitude (hundreds of femtoseconds to hundreds of seconds) along the reaction path. In a quantum-chemical analysis, we identify 16 stable ground-state isomers and discuss which ones are identified in the experimental data. We derive a detailed scheme how these species are thermally and photochemically interconnected and conclude that proton transfer processes are involved.

Rapid 3D roll-up of gas-phase planar gold clusters and affinity and alienation for Mg and Ge: A theoretical study of MgGeAun (n=1–12) clusters

SummaryA cluster is a special matter level above a single atom and between macroscopic and microscopic matter, and it is an important bridge to understanding the relationship between the structure and function of matter. Here, we perform a comprehensive theoretical study of 2D planar Aun (n = 1–12) clusters doped with both magnesium and germanium. Two interesting results are found, namely the rapid 3D "roll-up" structural growth of the GeMgAun (n = 1–12) cluster ground state isomers, and the relative "alienation" of the different sizes of the Aun (n = 1–12) cluster framework towards the Ge atom, and the relative "affinity" towards the Mg atom. This study will not only enrich the data on gold-based clusters but will also provide a simple and clear theoretical guide for the 3D structuring of planar clusters, i.e. the doping of different classes of "affinition" and "alienatation" atoms.

Systematic research on gallium atom-doped neutral small- and medium-sized gas-phase magnesium clusters: A DFT study of GaMgn (n=2-12) clusters.

Structure, stability, charge transfer, chemical bonding, and spectroscopic properties of Ga atom-doped neutral Mgn (n = 2-12) clusters have been systematically investigated by CALYPSO and density functional theory. All cluster structures are based on "tetrahedral" and "yurt-like" growth except for GaMg2. The ground state isomer of GaMg8 with high symmetry structure is predicted to be the best-fit candidate for the "magic" cluster because of its excellent stability. Natural bond orbital calculations reveal that Ga and Mg atoms play the role of electron acceptor and donor in all ground state isomers, while the orbitals in both Ga and Mg are sp-hybridized. Most importantly, chemical bonding studies based on atom-in-molecular theory have shown that the lowest-energy state of GaMg4 is so special, in that it has not only the critical size for the appearance of Mg-Mg covalent bonds, but also the only cluster that has both Ga-Mg covalent and non-covalent bonds. Finally, theoretical calculations of IR and Raman spectra of all ground state isomers indicate that the spectra of these clusters are observable in the low-frequency band, and thus they can be identified by spectroscopic experiments. Furthermore, the bond heterogeneity of the Ga-Mg in the GaMg4 ground state isomer has also been specifically investigated, including the fixed GaMg4 structure with Mg atoms added in different directions, as well as ab initio molecular dynamics sampling at different temperatures.

Computational probe for the geometrical structure and spectroscopic properties of Ga2Mgn+ (n = 1–11) clusters

Double gallium atoms doped cationic magnesium clusters, Ga2Mgn+ (n = 1–11), were systematically investigated through CALYPSO software with density functional theory (DFT). The structural growth of the ground state of Ga2Mgn+ (n = 1–11) clusters is two-dimensional structures (n = 1), tetrahedral-based (n = 2, 3), tent-like (n = 6, 7) and finally cake-like (n = 8, 9, 10, 11). It is found that Ga2Mg7+ cluster is the most stable one, and can therefore be identified as a “magic” cluster. Charge transfer studies showed that the charges are always transferred from magnesium atoms to gallium atoms. The geometric and electronic structure changes of the most stable Ga2Mg7+ ground state isomer in an external electric field were researched. In addition, the relationship between the bond nature of Ga2Mg7+ and its high stability had been studied through MOs and AdNDP. Finally, we obtained theoretical IR and Raman spectra of the average weights for each cluster at room temperature.

Astromers: Nuclear Isomers in Astrophysics

We develop a method to compute thermally-mediated transition rates between the ground state and long-lived isomers in nuclei. We also establish criteria delimiting a thermalization temperature above which a nucleus may be considered a single species and below which it must be treated as two separate species: a ground state species, and an astrophysical isomer ("astromer") species. Below the thermalization temperature, the destruction rates dominate the internal transition rates between the ground state and the isomer. If the destruction rates also differ greatly from one another, the nuclear levels fall out of or fail to reach thermal equilibrium. Without thermal equilibrium, there may not be a safe assumption about the distribution of occupation probability among the nuclear levels when computing nuclear reaction rates. In these conditions, the isomer has astrophysical consequences and should be treated a separate astromer species which evolves separately from the ground state in a nucleosynthesis network. We apply our transition rate methods and perform sensitivity studies on a few well-known astromers. We also study transitions in several other isomers of likely astrophysical interest.

Ultrafast Excited-State Dynamics of Tricarbocyanine Dyes Probed by Two-Dimensional Electronic Spectroscopy: Polar Solvation vs Photoisomerization.

Photophysical properties of tricarbocyanine dyes in various solvents have been widely investigated using a variety of spectroscopic tools. However, the presence of several ground-state isomers and interconversion between these isomers on an ultrafast timescale upon photoexcitation render unambiguous assignment of spectral features quite difficult. In this work, ultrafast excited-state dynamics of two tricarbocyanine dyes in two solvents, DNTTCI and IR140, in ethanol and ethylene glycol, are studied by two-dimensional electronic spectroscopy (2DES). We present a detailed discussion on design and calibration of the 2DES apparatus and on the method for data processing by phase-cycling. For DNTTCI we report a method to obtain solvation correlation function, the nature of which is found to be strongly dependent on the excitation frequencies; a blue-shifted spectrum at early time is observed and explained based on preferential emission from a subset among various isomers having overlapping spectral features. For IR140 in ethanol, four isomers with distinct spectral features are identified, and most importantly, three of these isomers were found to interconvert upon photoexcitation which completes within 100 fs and is explained based on a kinetic model of consecutive chemical reaction. Density functional theory calculations show the presence of several ground-state isomers for both these dyes. Through this work we demonstrate how 2DES can help us to decipher distinct excited-state photophysics in two carbocyanine dyes, polar solvation and photoisomerization, by resolving spectral congestion without sacrificing time resolution.

Nonadiabatic Dynamics of Charge-Transfer States Using the Anthracene-Tetracyanoethylene Complex as a Prototype.

Surface hopping quantum mechanical/molecular dynamics simulations have been performed for the tetracyanoethylene-anthracene complex to investigate the evolution of charge-transfer (CT) states after excitation into a locally excited (LE) state of anthracene. The scaled opposite-spin (SOS) second-order algebraic diagrammatic construction (SOS-ADC(2)) has been used to achieve a balanced description of LE and CT states. The calculations have been performed for two media, the gas phase and water (described by molecular mechanics, MM). The two dynamics variants show strongly different behaviors. Even though in both cases the conversion from the LE state to lower-lying CT states occurs with 100 fs, in the gas phase, the system remains in the excited state for longer than 2 ps, while in water, it returns to the ground state within 0.5 ps. Moreover, while in the gas phase the original neutral equilibrium structure should be recovered, in water, the ion-pair (IPr) CT state is strongly stabilized, creating a new competing ground-state isomer. Thus, we predict that the ground state of the complex in water should be composed of two species, the original neutral state and an IPr state. The existence of an IPr ground state in strongly polar environments opens interesting possibilities for the design of efficient charge-separating organic donor-acceptor interfaces.

Full-Dimensional Photodynamics of Bistable Proton Transfer Switches

Abstract Excited-state intramolecular proton transfers (ESIPT) are one of the fastest reactions in chemistry (<100 fs) which – among other features like high photostability – makes them an important reaction class for molecular switches. ESIPTs can be coupled with double bond rotation/isomerization, so that molecules can act as “molecular cranes”, facilitating long-range proton transfer. A versatile model system is 7-hydroxy-4-methylquinoline-8-carbaldehyde (HMQCA): it features two proton-accepting sites, two stable ground-state isomers and should allow for easy derivatization. There is also experimental and theoretical reference data available, however, only for static properties, e.g. ground-state IR spectra or potential energy surface scans. In this contribution we show the results of full-dimensional surface-hopping molecular dynamics (MD) of HMQCA after photo-excitation, employing semiempirical quantum mechanics coupled to floating-occupation configuration interaction. The results support the potential of HMQCA as prototype system for directed proton transport by ESIPT.

Theoretical study on the structural, electronic, and optical properties of BnCn (n = 1–13) clusters

We applied density functional theory (DFT) calculations to investigate the low-energy geometries and electronic characteristics of stoichiometric BnCn (n = 1–13) clusters. We performed harmonic vibration frequency analysis to ensure that the ground-state isomers are the real local minima. BnCn clusters tend to evolve from planar and annular structures to quasiplanar bowl structures to maintain the lowest structural energy as cluster size n increases. The clusters with even n have large HOMO–LUMO gaps and high stability. We used the time-dependent DFT (TDDFT) calculations to acquire the optical absorption spectra for the lowest-energy BnCn (n = 4, 6, 8, 10, 12) clusters. The clusters exhibit strong absorption in the ultraviolet region. With the increasement of n, the absorption of clusters, particularly that of the B8C8 cluster, intensifies in the visible region. Therefore, the clusters investigated in this work can be used to fabricate novel two-dimensional materials for visible-light absorption and have potential applications in various fields, such as catalysis.

Confining Photoacidity

Confining Photoacidity

Reaction mechanism between small-sized Ce clusters and water molecules II: an ab initio investigation on Cen (n = 1-3) + mH2O (m = 2-6).

Possible reactions between the products of the three independent reactions involving a small Ce cluster and a single water molecule, Cen + H2O (n = 1-3), and an additional H2O molecule are systematically investigated. The ground-state isomers of the final products and the reaction pathways involving multiple water molecules are predicted. We find that under either ambient or UV-irradiation conditions, all the reactions can entail low energy barriers. In addition, the final products of the reaction between Cen and more than two H2O molecules are also predicted through an extensive structural search. The calculated reaction energies suggest that although small-sized Ce clusters can react with more than two water molecules, the reactions with one or two water molecules are dominant. The electronic structures of all the ground-state isomers and the corresponding oxidation states of Ce atoms in these isomers are computed and determined via the natural bond orbital (NBO) method. The results indicate that a single Ce atom and a Ce2 cluster can react with a maximum of four and six water molecules, respectively, while a Ce3 cluster can react with more than six water molecules. This comprehensive study offers an improved understanding of the mechanism underlying the reactions between a single Ce atom or a small Ce cluster and two or more H2O molecules. Knowledge obtained from this study can be helpful for the development of high-performance Ce-doped or Ce-based catalysts.

Nuclear Data Sheets for A=170

Nuclear structure data pertaining to all nuclei with mass number A=170 have been evaluated, and incorporated into the ENSDF data file. This evaluation supersedes the previous evaluation (Coral M. Baglin), Nuclear Data Sheets 96, 611 (2002), (cutoff date 15 August 2002) and subsequent updates of the ENSDF file for Yb to Dy (cutoff 15 April 2010), Lu (cutoff 8 October 1998), Pt (cutoff 17 February 1999) and Ta, Os (cutoff 7 April 1998). Since the previous evaluation, 170Gd has been observed and the ground state T1/2 measured. New excited states have also been observed in the neutron rich 170Dy. The ground state T1/2 of 170Tb has been determined, however, there are currently still no excited states observed in this nucleus. There remains some uncertainty in the ground state and longer-lived isomer in 170Ir which could warrent further experimental investigation.

Insights of the role of shell closing model and NICS in the stability of NbGen (n = 7–18) clusters: a first-principles investigation

In the present report, the structures, energetics and electronic properties of neutral and cationic Nb-doped Gen (n = 7–18) clusters are systematically investigated under the first-principles density functional theory approach. The isomers in which the Nb atom is encapsulated inside a germanium cage are relatively stable compared to the exohedral surface doping. The thermodynamic stability and chemical activity of the ground-state isomers are analyzed through various energetic parameters. The results highlight the enhanced stability of the neutral NbGe12 hexagonal prism-like structure with D6h symmetry and cationic NbGe16 fullerene isomers. The negative nucleus-independent chemical shift can explain the enhanced stability of neutral NbGe12. However, the enhanced stability of cationic NbGe16 is explained by shell closing model associated with the quasi-spherical geometry with a sequence 1S21P61D101F61G122S22P6IF8IG62D10 following Hund’s rule. To understand the effect of hybridization on stability, we have calculated density of states (DOS) and projected DOS (PDOS). From PDOS, it is clear that Nb-p and Ge-s and p orbitals are mainly take part in hybridization; however, near below Fermi level, the dominating contribution comes from Nb-d orbitals. In addition, IR and Raman spectra of clusters are also calculated to explain their vibrational properties of the isomers. Specifically, IR spectrum of the clusters in the range of 12–16 shows the possible application of these clusters in the IR sensing device.

Properties of Highly Oriented Pyrolytic Graphite Supported TaSi16 Clusters: A Density Functional Investigation

Adsorption of the TaSi16 cluster on highly oriented pyrolytic graphite (HOPG) is studied using density functional methods. These calculations resolve some of the issues raised by the recent experiments of Shubita et al. (Shibuta, M.; Ohta, T.; Nakaya, M.; Tsunoyama, H.; Eguchi, T.; Nakajima, A. Chemical Characterization of an Alkali-Like Superatom Consisting of a Ta-Encapsulating Si16 Cage. J. Am. Chem. Soc. 2015, 137, 14015−14018) and provide additional insights into the system. For the first time, the ground-state structure of TaSi16 is obtained through a global search employing an evolutionary algorithm. A symmetric structure of the Si cage, together with a small charge transfer from Ta to the Si atoms, explains the observed core-level photoemission spectra. The ground-state isomer is deposited on HOPG at different points and in different random orientations. The cluster is found to be physisorbed on HOPG and retains the Ta-encapsulating cage structure, with little charge transfer between the cluster a...