Search for gravitational-lensing signatures in the full third observing run of the LIGO-Virgo network

Gravitational lensing by massive objects along the line of sight to the source causes distortions of gravitational wave-signals; such distortions may reveal information about fundamental physics, cosmology and astrophysics. In this work, we have extended the search for lensing signatures to all binary black hole events from the third observing run of the LIGO--Virgo network. We search for repeated signals from strong lensing by 1) performing targeted searches for subthreshold signals, 2) calculating the degree of overlap amongst the intrinsic parameters and sky location of pairs of signals, 3) comparing the similarities of the spectrograms amongst pairs of signals, and 4) performing dual-signal Bayesian analysis that takes into account selection effects and astrophysical knowledge. We also search for distortions to the gravitational waveform caused by 1) frequency-independent phase shifts in strongly lensed images, and 2) frequency-dependent modulation of the amplitude and phase due to point masses. None of these searches yields significant evidence for lensing. Finally, we use the non-detection of gravitational-wave lensing to constrain the lensing rate based on the latest merger-rate estimates and the fraction of dark matter composed of compact objects

Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo

Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM

Inept N2 Activation of Tri-Nuclear Nickel Complex with Labile Sulfur Ligands Facilitates Selective N2H4 Formation in Electrocatalytic Conversion of N2

Conversion of N2 to the energy vector N2H4 under benign conditions is highly desirable. However, such N2 fixation processes are extremely rare. It has been recently reported that N2 to N2H4 conversion can be achieved electrochemically by using a trinuclear [Ni3(S2C3H6)4]2- complex (named as [Ni3S8]2-). There are hardly any precedents of Nitrogen Reduction Reaction (NRR) by molecular catalysts having Ni and the highly unusual selectivity for N2H4 over NH3 makes this electrochemical reduction unique. A systematic theoretical study employing calibrated Density Functional Techniques to unearth the mechanisms of NRR (4e-/4H+) and HER (2e-/2H+) was conducted for the aforementioned trinuclear Ni complex. Our findings unravel a curious case of ligand lability working in tandem with metal centers in facilitating this unprecedented electrocatalytic activity. Furthermore, it is shown that the poor N-N bond activation property of Ni is responsible for this unusual selectivity. Additionally, the HER mechanistic pathways have also been delineated in this report. The mechanistic intricacies thus unearthed in this study may assist in developing more efficient electrocatalysts for N2H4 production through NRR

Implicating the Role of Au-H Bonds in Photochemical N2 Fixation by Ruthenium Doped Gold Clusters

Dinitrogen fixation through the Nitrogen Reduction Reaction (NRR) under mild conditions without the use of sacrificial agents has its share of formidable hurdles. It has been shown recently that Ru-doped Au nanoclusters can reduce N2 molecule to NH3 only in the presence of UV-Vis light in aqueous medium. Herein, using theoretical techniques (Density Functional Theory) we shed light on the mechanistic avenues traversed for achieving this prodigious chemical feat. Our findings suggest that the bimetallic Au22Ru6 cluster successfully accomplishes the NRR process under ambient pressure and temperature conditions by the very virtue of its bifunctional nature. Contrary to the existing views, we find that NRR propagates through an alternative associative pathway, where the Ru dopant assists in N2 adsorption while the Au-H bonds formed from Au assisted water splitting is implicated for facilitating NRR

Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces: A Molecular Orbital Perspective

The question of quadruple bonding in C2 has emerged as a hot button issue, with opinions sharply divided between the practitioners of Valence Bond (VB) and Molecular Orbital (MO) theory. Here, we have systematically studied the Potential Energy Curves (PECs) of low lying high spin sigma states of C2, N2 and Be2 and HC≡CH using several MO based techniques such as CASSCF, RASSCF and MRCI. The analyses of the PECs for the 2S+1Σg/u (with 2S+1=1,3,5,7,9) states of C2 and comparisons with those of relevant dimers and the respective wavefunctions were conducted. We contend that unlike in the case of N2 and HC≡CH, the presence of a deep minimum in the 7Σ state of C2 and CN+ suggest a latent quadruple bonding nature in these two dimers. Hence, we have struck a reconciliatory note between the MO and VB approaches. The evidence provided by us can be experimentally verified, thus providing the window so that the narrative can move beyond theoretical conjectures

Theoretical Investigations on the Mechanistic Aspects of O₂ Activation by a Biomimetic Dinitrosyl Iron Complex

Though dinitrosyl-iron complexes (DNICs) are largely believed to act as NO carriers, several experiments on model DNICs have suggested that they can also act as nitrating agents in presence of dioxygen. Oxygen activation by DNICs has been implicated as a possible route for protein tyrosine nitration (PTN), which leads to neurodegenerative disorders. Herein using static and dynamic theoretical techniques we unravel a previously unknown dual state mechanistic paradigm for dioxygen activation of a biomimetic nitrating DNIC complex leading to phenolic nitration. Our computations reveal that the model DNIC, the ground electronic state of which is singlet, has a low-lying triplet state and an inherent singlet–triplet spin-crossover of DNICs can be triggered by fluxional changes in the bite angle of the two NO ligands. The presence of a low-lying triplet state in the DNIC affords an avenue for O2 activation other than a direct O2 activation by O2-induced spin-crossover of the singlet ground state. These two low-lying channels facilitate the formation of a peroxynitrite species. Nitration of phenolic substrates is facilitated by the release of NO2. The corresponding minimum energy crossing points (MECP) have been located. Along the reaction path, the changes in the electronic structure scenarios have been studied and interpreted. Our report also sheds light on the plausible mechanistic pathway of PTN by reactive species formed once O2 activation by DNICs have been achieved

Decoding Regioselective Reaction Mechanism of the Gentisic Acid Catalyzed by Gentisate 1,2-Dioxygenase Enzyme

Gentisate 1,2-dioxygenase (GDO), a ring-fission non-heme dioxygenase enzyme, displays a unique regioselective reaction of gentisic acid (GTQ) in the presence of molecular oxygen. GTQ is an important intermediate in the aerobic biodegradation pathways of recalcitrant polyaromatic hydrocarbons (PAHs) pollutants. Classical molecular dynamics simulations of wild-type GDO and its mutated variants (Asp174Glu and Asp174Ala) explored the presence of three active water molecules at the active site which plays pivotal roles in facilitating the oxidative cleavage of an aromatic C-C bond of the GTQ substrate. Three distinct reaction mechanisms using the QM/MM calculations decoded for the regioselective reaction of the GTQ catalyzed by GDO enzyme. The formation of the main product as a maleylpyruvate along with pathway A, which is the most favourable one. The first step for the conversion to an alkyl peroxo intermediate is a rate-determining step with an associated barrier of 21.4 kcal/mol at the uB3LYP-D3/def2-TZVP/OPLS level of theory on the quintet spin surface. Our study illustrates the crucial role of active water molecules in the stabilization of the O2 molecule, the O-O, and C-C bond cleavage steps and additionally uncovered the important anchor role of the Asp174 in the enzymatic cycle. Essentially, our findings paved a new route in the mechanism of degradation processes of PAHs pollutants by dioxygenase enzymes, and provide molecular insights to design iron-containing biomimetic catalysts