Molecular Ring Research Articles

Two-dimensional Molecular Gas and Ongoing Star Formation around H ii Region Sh2-104

Abstract We performed a multi-wavelength study toward H ii region Sh2-104. New maps of 12CO J = 1 − 0 and 13CO J = 1 − 0 were obtained from the Purple Mountain Observatory 13.7 m radio telescope. Sh2-104 displays a double-ring structure. The outer ring with a radius of 4.4 pc is dominated by 12, 500 μm, 12CO J = 1 − 0, and 13CO J = 1 − 0 emission, while the inner ring with a radius of 2.9 pc is dominated by 22 μm and 21 cm emission. We did not detect CO emission inside the outer ring. The north–east portion of the outer ring is blueshifted, while the south–west portion is redshifted. The present observations have provided evidence that the collected outer ring around Sh2-104 is a two-dimensional structure. From the column density map constructed by the Hi-GAL survey data, we extract 21 clumps. About 90% of all the clumps will form low-mass stars. A power-law fit to the clumps yields . The selected YSOs are associated with the collected material on the edge of Sh2-104. The derived dynamical age of Sh2-104 is yr. Comparing the Sh2-104 dynamical age with the YSO timescale and the fragmentation time of the molecular ring, we further confirm that the collect-and-collapse process operates in this region, indicating positive feedback from a massive star for surrounding gas.

Low-field spin dynamics of Cr7Ni and Cr7Ni−Cu−Cr7Ni molecular rings as detected by μSR

Muon spin rotation measurements were used to investigate the spin dynamics of heterometallic ${\mathrm{Cr}}_{7}\mathrm{Ni}$ and ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ molecular clusters. In ${\mathrm{Cr}}_{7}\mathrm{Ni}$ the magnetic ions are arranged in a quasiplanar ring and interact via an antiferromagnetic exchange coupling constant $J$, while ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ is composed of two ${\mathrm{Cr}}_{7}\mathrm{Ni}$ linked by a bridging moiety containing one Cu ion, that induces an inter-ring ferromagnetic interaction ${J}^{\ensuremath{'}}\ensuremath{\ll}J$. The longitudinal muon relaxation rate $\ensuremath{\lambda}$ collected at low magnetic fields ${\ensuremath{\mu}}_{0}Hl0.15$ Tesla, shows that the two systems present differences in spin dynamics vs temperature. While both samples exhibit a main peak in the muon relaxation rate vs temperature, at $T\ensuremath{\sim}10$ K for ${\mathrm{Cr}}_{7}\mathrm{Ni}$ and $T\ensuremath{\sim}8$ K for ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$, the two compounds have distinct additional features: ${\mathrm{Cr}}_{7}\mathrm{Ni}$ shows a shoulder in $\ensuremath{\lambda}(T)$ for $Tl8$ K, while ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ shows a flattening of $\ensuremath{\lambda}(T)$ for $Tl2$ K down to temperatures as low as $T=20$ mK. The main peak of both systems is explained by a Bloembergen-Purcell-Pound (BPP)-like heuristic fitting model that takes into account of a distribution of electronic spin characteristic times for $Tg5$ K, while the shoulder presented by ${\mathrm{Cr}}_{7}\mathrm{Ni}$ can be reproduced by a BPP function that incorporates a single electronic characteristic time theoretically predicted to dominate for $Tl5$ K. The flattening of $\ensuremath{\lambda}(T)$ in ${\mathrm{Cr}}_{7}\mathrm{Ni}$-Cu-${\mathrm{Cr}}_{7}\mathrm{Ni}$ occurring at very low temperature can be tentatively attributed to field-dependent quantum effects and/or to an inelastic term in the spectral density of the electronic spin fluctuations.

Molecular Iodine-Mediated Ring Opening Reaction of Spiro-Epoxy Oxindoles with Electron Rich Arenes

Molecular Iodine-Mediated Ring Opening Reaction of Spiro-Epoxy Oxindoles with Electron Rich Arenes

Rotary and linear molecular motors driven by pulses of a chemical fuel.

Many biomolecular motors catalyze the hydrolysis of chemical fuels, such as adenosine triphosphate, and use the energy released to direct motion through information ratchet mechanisms. Here we describe chemically-driven artificial rotary and linear molecular motors that operate through a fundamentally different type of mechanism. The directional rotation of [2]- and [3]catenane rotary molecular motors and the transport of substrates away from equilibrium by a linear molecular pump are induced by acid-base oscillations. The changes simultaneously switch the binding site affinities and the labilities of barriers on the track, creating an energy ratchet. The linear and rotary molecular motors are driven by aliquots of a chemical fuel, trichloroacetic acid. A single fuel pulse generates 360° unidirectional rotation of up to 87% of crown ethers in a [2]catenane rotary motor.

Acid fuels the motion of a threaded ring

Molecular Machines A central goal in the construction of molecular-scale machines is the efficient achievement of one-way motion. Erbas-Cakmak et al. developed a class of machines that transmit pH changes into the two-stage guided motion of molecular rings threaded on a linear or cyclic axle. The design relies on temporary blocking groups and landing sites along the axle that toggle between active and passive states in response to acid or base. Trichloroacetic acid initiates the first stage of motion until it is decomposed by base in the solution, spurring the second phase. Science , this issue p. [340][1] [1]: /lookup/doi/10.1126/science.aao1377

Relation Between Ring Currents and Hydrogenation Enthalpies for Assessing the Degree of Aromaticity.

Magnetically induced ring-current strength susceptibilities and nucleus independent chemical shifts (NICS) have been studied for 15 single-ring aromatic, antiaromatic, and nonaromatic molecules. The current densities have been calculated at the density functional theory (DFT), Hartree-Fock (HF) theory, and second-order Møller-Plesset perturbation theory (MP2) levels using the gauge-including magnetically induced current method (GIMIC). The ring-current strength susceptibilities have been obtained by numerical integration of the current density flowing around the molecular ring. The calculated ring-current strength susceptibilities are almost independent of the level of theory. The relative degree of aromaticity deduced from the magnetic properties has been compared with the ones deduced from hydrogenation enthalpies that are considered to be proportional to aromatic stabilization energies (ASE). For the studied single-ring molecules, GIMIC, NICS, and ASE calculations yield similar trends. The study shows that there is a linear correlation between the magnetic and energetic criteria of aromaticity. The largest uncertainty originates from the accuracy of the energy data, because they are much more dependent on the employed computational level than the calculated magnetic properties. Thus, ring-current strength susceptibilities can be used for assessing the degree of aromaticity.

Acid- or photo-cleavable polyrotaxane: Subdivision of supramolecular main-chain type polyrotaxane structure induced by acidolysis or photolysis

A polyrotaxane with end moieties cleavable by acidic fission or photolytic cleavage was synthesized by end capping with (α,α-dimethyl-3,5-dimethoxybenzyloxy) carbonyl groups. The obtained polyrotaxane was subdivided into its molecular ring and axis components after treatment with neat trifluoroacetic acid at room temperature or irradiation with a high-pressure mercury lamp.

FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN). I. Project overview and initial results

Abstract The FUGIN project is one of legacy projects using a new multi-beam FOREST (four-beam receiver system on the 45 m telescope). This project aims to simultaneously investigate the distribution, kinematics, and physical properties of both diffuse and dense molecular gases in the Galaxy by observing 12CO, 13CO, and C18O J = 1–0 lines simultaneously. Mapping regions are parts of the first quadrant (10° ≤ l ≤ 50°, |b| ≤ 1°) and the third quadrant (198° ≤ l ≤ 236°, |b| ≤ 1°) of the Galaxy, where spiral arms, bar structure, and the molecular gas ring are included. This survey achieves the highest angular resolution to date (∼20″) for the Galactic plane survey in the CO J = 1–0 lines, which makes it possible to find dense clumps located farther away than the previous surveys. FUGIN will provide us an invaluable dataset for investigating the physics of the Galactic interstellar medium (ISM), particularly the evolution of interstellar gas covering galactic-scale structures to the internal structures of giant molecular clouds, such as small filaments/clumps/cores. We present an overview of the FUGIN project, the observation plan and initial results. These results reveal wide-field and detailed structures of molecular clouds, such as entangled filaments that have not been obvious in previous surveys, and large-scale kinematics of molecular gas, such as spiral arms.

Vibrational Spectroscopy and Proton Transfer Dynamics in Protonated Oxalate.

The dynamics and infrared spectroscopic signatures of proton transfer in protonated oxalate (p-Oxa) are studied using classical and quantum dynamics. The intermolecular interactions are described by a force field suitable to follow proton transfer. This allows to carry out multiple extended classical molecular dynamics (MD) and ring polymer MD simulations from which the infrared spectrum is determined. Simulations at 600 K sample the quantum mechanical ground state probability distribution and best reproduce the experimentally observed maximum absorption wavelength and part of the line shape. Comparison with the experimentally measured spectrum provides an estimate for the barrier height for proton transfer which can not be determined directly from experiment. A barrier of 4.2 kcal/mol is found to best reproduce the position and width of the infrared absorption of the transferring proton in p-Oxa and also leads to an infrared (IR) spectrum in good agreement with experiment for the deuterated species d-Oxa. A novel means to capture the two resonance forms of oxalate depending on the localization of the excess proton on either CO moiety is found to yield improved results for the spectroscopy in the framework region between 1000 and 2000 cm-1.

ALMA Images of the Orion Hot Core at 349 GHz

Abstract We present ALMA images of the dust and molecular line emission in the Orion Hot Core at 349 GHz. At 0.″2 angular resolution the images reveal multiple clumps in an arc ∼1″ east of Orion Source I, the protostar at the center of the Kleinmann–Low Nebula, and another chain of peaks from IRc7 toward the southwest. The molecular line images show narrow filamentary structures at velocities >10 km s−1 away from the heavily resolved ambient cloud velocity ∼5 km s−1. Many of these filaments trace the SiO outflow from Source I, and lie along the edges of the dust emission. Molecular line emission at excitation temperatures 300–2000 K, and velocities >10 km s−1 from the ambient cloud, suggest that the Hot Core may be heated in shocks by the outflow from Source I or from the Becklin–Neugebauer (BN)/SrcI explosion. The spectral line observations also reveal a remarkable molecular ring, ∼2″ south of SrcI, with a diameter ∼600 au. The ring is seen in high-excitation transitions of HC3N, HCN v2 = 1, and SO2. An impact of ejecta from the BN/SrcI explosion with a dense dust clump could result in the observed ring of shocked material.

Molecular Borromean Rings Based on Dihalogenated Ligands

Summary A series of Cp*Rh- and Cp*Ir-based molecular Borromean rings (BRs) have been prepared with dihalogenated ligands (fluoranilic acid, chloranilic acid, and bromianilic acid) and 4,4′-bipyridylacetylene. The halogen atoms play a crucial role in forming BRs. The strength of the interaction among the rings for different BRs is not the same and can be adjusted by changes in the halogen atoms on the precursor. We took advantage of this dissimilarity in the interactions to realize selective reversible conversion between BRs and a monomeric rectangle by the use of different p -dihalobenzenes. Subsequently, a stepwise separation method for p -dichlorobenzene, p -dibromobenzene, and p -diiodobenzene at ambient temperature was developed. The BRs used in the separation are recoverable and recyclable.

The 200-pc molecular cylinder in the Galactic Centre

Analysing the 3D structure of the molecular gas distribution in the central 200-pc region of the Galaxy, we show that the expanding molecular ring (EMR, also known as the parallelogram) and the central molecular zone (CMZ) exhibit quite different distributions and kinematics. The EMR composes a bipolar vertical cylinder with the total length as long as ∼170 pc and shows large non-circular velocities. On the contrary, the CMZ is distributed in a nearly rigid-body rotating ring and arms tightly concentrated near the galactic plane with full thickness less than ∼30 pc. Furthermore, the mass and density ratios of the EMR to CMZ are as small as ∼0.13 and 0.04, respectively. We discuss the origins of the EMR and CMZ based on the bar and explosion models. We suggest that the EMR's large vertical extent can be explained by a cylindrical shock wave model driven by an explosive activity in the Galactic Centre.

Hydrodynamic Simulations of the Central Molecular Zone with a Realistic Galactic Potential

Abstract We present hydrodynamic simulations of gas clouds inflowing from the disk to a few hundred parsec region of the Milky Way. A gravitational potential is generated to include realistic Galactic structures by using thousands of multipole expansions (MEs) that describe 6.4 million stellar particles of a self-consistent Galaxy simulation. We find that a hybrid ME model, with two different basis sets and a thick-disk correction, accurately reproduces the overall structures of the Milky Way. Through non-axisymmetric Galactic structures of an elongated bar and spiral arms, gas clouds in the disk inflow to the nuclear region and form a central molecular zone-like nuclear ring. We find that the size of the nuclear ring evolves into ∼ 240 pc at T ∼ 1500 Myr , regardless of the initial size. For most simulation runs, the rate of gas inflow to the nuclear region is equilibrated to ∼ 0.02 M ⊙ yr − 1 . The nuclear ring is off-centered, relative to the Galactic center, by the lopsided central mass distribution of the Galaxy model, and thus an asymmetric mass distribution of the nuclear ring arises accordingly. The vertical asymmetry of the Galaxy model also causes the nuclear ring to be tilted along the Galactic plane. During the first ∼100 Myr, the vertical frequency of the gas motion is twice that of the orbital frequency, thus the projected nuclear ring shows a twisted, ∞ -like shape.

Molecular structure and ring tunneling of phenyl formate as observed by microwave spectroscopy and quantum chemistry

Phenyl formate has been investigated by molecular jet Fourier-transform microwave spectroscopy in the frequency range from 2 to 26.5GHz. Quantum chemical calculations at the MP2/6-311++G(d,p) level of theory indicate that this molecule does not have a plane of symmetry at equilibrium, and that the phenyl ring performs a large amplitude tunneling motion from one side of the Cs configuration to the other. The tilt angle of the ring out of the H(CO)O plane is ±70° and the calculated tunneling barrier is only 28cm–1. The two lowest torsional states vt=0 and 1 are assigned in the experimental spectrum and fitted using the program SPFIT/SPCAT. The Coriolis splitting ΔE between these states is 46.2231(25) GHz, very close to the value of 48.24GHz calculated using a simple two-top torsional Hamiltonian of the formate group and the phenyl ring.

New Insights on Aromatic Pathways in Porphyrinoids

Electronic and spectroscopic properties of porphyrinoid macrocycles are, to a large extent, determined by the number of π-electrons participating in the conjugation network. Full control of the number of π-electrons implies full control of the aromatic or antiaromatic character of the molecule. To achieve this, knowledge about the electron mobility pathways within a molecule is essential. Experimentally, these pathways are not easily accessible. Therefore theoretical studies are necessary to complement experiments that aim at systematically synthesizing porphyrinoids with distinct properties. A possible way to achieve this is the systematic investigation of magnetically induced current densities to determine reliable aromatic pathways in molecules.[1] In my talk I will present an overview of different ways to systematically calculate and analyze the magnetically induced current density in porphyrin based molecules. The computational approach used in my group is the gauge including magnetically induced current density method (GIMIC).[2,3] GIMIC is a independent program that is used for the calculation of magnetically induced current densities using London orbitals.[2, 3] Numerical integration of the current flow around molecular rings and along selected chemical bonds can be used for determining current pathways and the degree of aromaticity of various molecules according to the magnetic criterion.[4,5] A new feature of the GIMIC program is the calculation of the anisotropy of the magnetically induced density using gauge including atomic orbitals.[6] Very recent results obtained for various porphyrinoid molecules will be highlighted illustrating the potential and challenges of the method.[7-9]

Structure and dynamics of the molecular gas in M 2–9: a follow-up study with ALMA

Context. M 2–9 is a young planetary nebula (PN) that shows the characteristics of its last ejections in unprecedented detail. These last ejections are thought to trigger the post−asymptotic giant branch evolution.Aims. To assemble an overall picture of how M 2–9 was shaped, we analyzed the characteristics of the different molecular gas components and their relation with the warmer parts of the nebula that are visible in the optical domain.Methods. 12 CO and 13 CO J = 3−2 line emission maps were obtained with the Atacama Large Millimeter/submillimeter Array with high angular-resolution and sensitivity. Results. Two equatorial rings are found to host most of the cold molecular gas in M 2–9, as has been described for previous 12 CO J = 2−1 emission observations. In addition, we have detected a double crown-shaped structure that is symmetric with respect to the main nebular axis, which is located 1.5′′ away from both sides of the equatorial plane. Their distribution and kinematics show a very close relationship with the inner molecular ring: both are part of the same small hourglass structure formed ~900 yr ago. Two clearly distinct ejections with a remarkable axial symmetry are found to have shaped the molecular gas distribution in M 2–9, in agreement with the ejection processes that were probably responsible for the optical lobes. For the first time, the physical conditions of the different molecular components in M 2–9 are comprehensively analyzed with a radiative transfer model. They are found to follow standard laws, like those obtained in other young PN, with densities and temperatures decreasing with radius and ballistic expansion. A total mass of ~5 × 10-3 M ⊙ was derived for the detected molecular component, the larger and older equatorial ring hosting most (~90%) of this gas.

Modulation of circular current and associated magnetic field in a molecular junction: A new approach

A new proposal is given to control local magnetic field in a molecular junction. In presence of finite bias a net circular current is established in the molecular ring which induces a magnetic field at its centre. Allowing a direct coupling between two electrodes, due to their close proximity, and changing its strength we can regulate circular current as well as magnetic field for a wide range, without disturbing any other physical parameters. We strongly believe that our proposal is quite robust compared to existing approaches of controlling local magnetic field and can be verified experimentally.

Spin and orbital angular momentum transfer into GanAsn nanocage: The change in induced magnetic field by tuning the light parameters and size of the molecule

In this study, we demonstrate that the direction of a photoinduced magnetic field in semiconductor nano-scale quantum dots (nanocages) can be changed by tuning the frequency and the light parameters. In the case of the spin angular momentum (SAM) transfer to ring-shaped GanAsn molecule, some electrons of occupied states are excited into the unoccupied states for a special laser frequency. As a result of the transition due to the circular polarized light, the excited electron which has a nonzero magnetic quantum number start to circulate around the molecular GaAs ring. The transfer of orbital angular momentum (OAM) due to the twisted light can induce similar effect on the molecular ring. In the case of twisted light, the electron can be excited different unoccupied states with higher magnetic quantum numbers. In the study, the optimization and electronic structure calculation have been performed by using density functional theory (DFT). The induced current and corresponding induced magnetic field can be controlled by the change in the frequency. Also, the type of the angular momentum of the light (SAM or OAM) can be used as a tool to manipulate the magnitude and direction of the magnetic moment.

RingDecomposerLib: An Open-Source Implementation of Unique Ring Families and Other Cycle Bases.

Many cheminformatics applications like aromaticity detection, SMARTS matching, or the calculation of atomic coordinates require a chemically meaningful perception of the molecular ring topology. The unique ring families (URFs) were recently introduced as a unique, polynomial, and chemically meaningful description of the ring topology. Here we present the first open-source implementation of the URF concept for ring perception. The C library RingDecomposerLib is easy to use, portable, well-documented, and thoroughly tested. Aside from the URFs, other related ring topology descriptions like the relevant cycles (RCs), relevant cycle prototypes (RCPs), and a smallest set of smallest rings (SSSR) can be calculated. We demonstrate the runtime efficiency of the RingDecomposerLib with computing time benchmarks for the complete PubChem Compound Database and thereby show the applicability in large-scale and interactive applications.

Stacking Interactions Induced Selective Conformation of Discrete Aromatic Arrays and Borromean Rings.

Herein, we describe how to utilize stacking interactions to achieve selective supramolecular transformation and molecular Borromean rings (BRs). By using a dinuclear naphthalenediimide (NDI)-based Cp*Rh acceptor and linear bipyridyl ligands, organometallic rectangles featuring dynamic behavior have been constructed. Unique discrete aromatic stacking arrays were formed by inducing pyrene units as guest molecules. The topology of the BRs was realized by the use of a strategically chosen ligand which was capable of participating in D-A interactions and hydrogen bonding, as evidenced from single-crystal X-ray analysis and computational studies. These self-assembly processes underline the advantages of dynamic bonding and π-π stacking interactions, and serve to illustrate a new approach to generating structurally and topologically nontrivial supramolecular architectures.