Minimum Geometry Research Articles

Simulation of the non-adiabatic dynamics of an enone-Lewis acid complex in an explicit solvent.

Unlocking the full potential of Lewis acid catalysis for photochemical transformations requires a comprehensive understanding of the ultrafast dynamics of substrate-Lewis acid complexes. In a previous article [Peschel et al., Angew. Chem. Int. Ed., 2021, 60, 10155], time-resolved spectroscopy supported by static calculations revealed that the Lewis acid remains attached during the relaxation of the model complex cyclohexenone-BF3. In contrast to the experimental observation, surface-hopping dynamics in the gas phase predicted ultrafast heterolytic dissociation. We attributed the discrepancy to missing solvent interactions. Thus, in this work, we present an interface between the SHARC and FermiONs++ program packages, which enables us to investigate the ultrafast dynamics of cyclohexenone-BF3 in an explicit solvent environment. Our simulations demonstrate that the solvent prevents the dissociation of the complex, leading to an intriguing dissociation-reassociation mechanism. Comparing the dynamics with and without triplet states highlights their role in the relaxation process and shows that the Lewis acid inhibits intersystem crossing. These findings provide a clear picture of the relaxation process, which may aid in designing future Lewis acid catalysts for photochemical applications. They underscore that an explicit solvent model is required to describe relaxation processes in weakly bound states, as energy transfer to the solvent is crucial for the system to reach its minimum geometries.

Energetics and spectroscopic studies of clusters and the temperature dependencies of the isomers: An approach based on a combined recipe of parallel tempering and quantum chemical methods.

A system associated with several number of weak interactions supports numerous number of stable structures within a narrow range of energy. Often, a deterministic search method fails to locate the global minimum geometry as well as important local minimum isomers for such systems. Therefore, in this work, the stochastic search technique, namely parallel tempering, has been executed on the quantum chemical surface of the system for -8 to generate global minimum as well as several number of local minimum isomers. IR spectrum can act as the fingerprint property for such system to be identified. Thus, IR spectroscopic features have also been included in this work. Vertical detachment energy has also been calculated to obtain clear information about number of water molecules in several spheres around the central anion. In addition, in a real experimental scenario, not only the global but also the local minimum isomers play an important role in determining the average value of a particular physically observable property. Therefore, the relative conformational populations have been determined for all the evaluated structures for the temperature range between 20K and 400K. Further to understand the phase change behavior, the configurational heat capacities have also been calculated for different sizes.

Protonation of histidine rings using quantum-mechanical methods.

Histidine can be protonated on either or both of the two N atoms of the imidazole moiety. Each of the three possible forms occurs as a result of the stereochemical environment of the histidine side chain. In an atomic model, comparing the possible protonation states in situ, looking at possible hydrogen bonding and metal coordination, it is possible to predict which is most likely to be correct. A more direct method is described that uses quantum-mechanical methods to calculate, also in situ, the minimum geometry and energy for comparison, and therefore to more accurately identify the most likely protonation state.

Ultrafast photochemistry and electron diffraction for cyclobutanone in the S2 state: Surface hopping with time-dependent density functional theory.

We simulate the photodynamics of gas-phase cyclobutanone excited to the S2 state using fewest switches surface hopping (FSSH) dynamics powered by time-dependent density functional theory (TDDFT). We predict a total photoproduct yield of 8%, with a C3:C2 product ratio of 0 trajectories to 8 trajectories. One primary S2 → S1 conical intersection is identified involving the compression of an α-carbon-carbon-hydrogen bond angle. Excited state lifetimes computed with respect to electronic state populations were found to be 3.96ps (S2 → S1) and 498fs (S1 → S0). We also generate time-resolved difference pair distribution functions (ΔPDFs) from our TDDFT-FSSH dynamics results in order to generate direct comparisons with ultrafast electron diffraction experiment observables. Global and target analysis of time-resolved ΔPDFs produced a distinct set of lifetimes: (i) a 0.548ps decay and (ii) a 1.69ps decay, both resembling the S2 minimum, as well as (iii) a long decay that resembles the S1 minimum geometry and the fully separated C2 products. Finally, we contextualize our results by considering the impact of the most likely sources of significant errors.

The anion photoelectron spectrum and diabatization of tetrazolyl.

The potential energy surface of tetrazolyl [cyclic (N4CH)] has a conical intersection seam between the two lowest-energy electronic states near the ground state minimum geometry. This work treats that molecule. The potential energy surfaces used in this study are based on a least-squares fitting procedure that includes abinitio energies, energy gradients, and derivative couplings described using polynomials up to fourth-order and abinitio data obtained from multireference configuration interaction wave functions. A five-electronic-state description was generated with a root mean square absolute energy error of 9.6cm-1, compared to 326.8cm-1 when only second-order terms were used. The time-independent multimode vibronic coupling in the KDC approximation was used to simulate and analyze the anion ultraviolet photoelectron spectrum of tetrazolide.

Study of six coordinated cobalt(III) oxophlorin with different axial ligands: Optimization of geometry and determining of energy and electronic configuration at various spin states by employing of B3LYP, BV86P and M06-2X methods

The six coordinated CoIIIoxophlorin have been studied with imidazole, pyridine and t-butylcyanide as axial ligands using B3LYP, BV86P and MO6-2X methods. Conversion between two isomers [(L)2CoIII(PO)][Formula: see text] and [(L)2CoIII(PO)][Formula: see text] can be occurred at various spin multiplicities, namely singlet, triplet and quintet states. L is employed to show axial ligand namely, imidazole or pyridine. London forces have a basic role in the stability of the mentioned complexes due to non-specific solvent effects. The latter fact has been obtained using the PCM model. Also, it is specified that minimum geometries have not been obtained for some parallel or perpendicular six-coordinated complexes with special axial ligands at a finite spin state. The B3LYP method indicates that one and three [Formula: see text]-electron can be found on the Co atom in every optimized complex at triplet and quintet spin states, respectively. Also, another [Formula: see text]-electron is placed on the ring of oxophlorin. This fact is obtained for different isomers of [(IM2(CoIII(PO)] and [(Py)2CoIII(PO)] at triplet and quintet spin states, while these complexes are optimized using B3LYP. Besides, results obtained from B3LYP show that the most stable state for six coordinated CoIIIoxophlorin with any axial ligand is the singlet state. Based on crystal field theory and molecular orbital theory, electron configuration and hybridization of cobalt in [(IM)2CoIII(PO)] at singlet state can be written as t2g6 eg0 and sp3d2, respectively. Former electronic configuration indicates a strong field with low spin for d orbitals of cobalt, and latter hybridization is expected for a metal with a coordination number of 6 in a complex with Oh symmetry. The results obtained are completely satisfied by NBO analysis.

Pushing The Extreme of Multicentre Bonding: Planar Pentacoordinate Hydride.

Planar hypercoordination has sparkled interest among the researchers from last few decades. Most of the elements in the Periodic Table have shown this remarkable structural feature. However, the smallest element, hydrogen, is missing in the list. No evidence is there in the literature. Herein, we introduce the first planar pentacoordinate hydrogen atom (ppH) in the global minimum geometry of Li5 H6 - cluster. Bonding analysis indicates that the central hydrogen atom is stabilized by multicentre bonding with five surrounding Li atoms. Natural charge analysis reveals that the central hydrogen is acting like a hydride which is strongly attracted by the positively charged surrounding lithium centres. The ppH structure is stabilized by strong electrostatic attraction as well as extensive multicentre bonding. Aromaticity has no role to play here. The cluster is dynamically stable and is expected to be detected in gas phase.

Pushing The Extreme of Multicentre Bonding: Planar Pentacoordinate Hydride

AbstractPlanar hypercoordination has sparkled interest among the researchers from last few decades. Most of the elements in the Periodic Table have shown this remarkable structural feature. However, the smallest element, hydrogen, is missing in the list. No evidence is there in the literature. Herein, we introduce the first planar pentacoordinate hydrogen atom (ppH) in the global minimum geometry of Li5H6− cluster. Bonding analysis indicates that the central hydrogen atom is stabilized by multicentre bonding with five surrounding Li atoms. Natural charge analysis reveals that the central hydrogen is acting like a hydride which is strongly attracted by the positively charged surrounding lithium centres. The ppH structure is stabilized by strong electrostatic attraction as well as extensive multicentre bonding. Aromaticity has no role to play here. The cluster is dynamically stable and is expected to be detected in gas phase.

Energetic and Spectroscopic Properties of Astrophysically Relevant MgC4H Radicals Using High-Level Ab Initio Calculations.

Considering the importance of magnesium-bearing hydrocarbon molecules (MgCnH; n = 2, 4, and 6) in the carbon-rich circumstellar envelopes (e.g., IRC+10216), a total of 28 constitutional isomers of MgC4H have been theoretically investigated using density functional theory (DFT) and coupled-cluster methods. The zero-point vibrational energy corrected relative energies at the ROCCSD(T)/cc-pCVTZ level of theory reveal that the linear isomer, 1-magnesapent-2,4-diyn-1-yl (1, 2Σ+), is the global minimum geometry on the MgC4H potential energy surface. The latter has been detected both in the laboratory and in the evolved carbon star, IRC+10216. The calculated spectroscopic data for 1 match well with the experimental observations (error ∼ 0.78%) which validates our theoretical methodology. Plausible isomerization processes happening among different isomers are examined using DFT and coupled-cluster methods. CASPT2 calculations have been performed for a few isomers exhibiting multireference characteristics. The second most stable isomer, 1-ethynyl-1λ3-magnesacycloprop-2-ene-2,3-diyl (2, 2A1, μ = 2.54 D), is 146 kJ mol-1 higher in energy than 1 and possibly the next promising candidate to be detected in the laboratory or in the interstellar medium in future.

On assessing the carbon capture performance of graphynes with particle swarm optimization.

Tackling climate change is one of the greatest challenges of current times and therefore the development of efficient technologies to limit anthropogenic emissions is of utmost urgency. Recent research towards this goal has alluded to the use of carbon-based solid sorbents for carbon capture. Graphynes (GYs), an interesting class of porous carbon membranes, have recently proven their potential as excellent membranes for gas adsorption and separation. Herein, we explored the CO2 and N2 adsorption characteristics and CO2/N2 selectivities of a class of GYs, namely γ-GY-1, γ-GY-2 and γ-GY-4. We investigated the putative global minimum geometries of adsorbed unary (n = 2-10) and binary (n : m; n, m ∈ [1, 8]) clusters of CO2 and N2 by employing a stochastic global optimization method called particle swarm optimization in conjunction with empirical intermolecular force field formulations. The intervening interactions are modeled using various pairwise potentials, including Lennard-Jones potential, improved Lennard-Jones potential, Buckingham potential and Coulombic potential. The binding energies for both unary and binary clusters are highest for adsorption on γ-GY-1, followed by γ-GY-2. The putative global minimum geometries suggested that N2 molecules preferred binding over the pore centres while CO2 molecules showed higher clustering propensity than any binding site preference. The predicted interaction energies suggested higher selectivity for CO2 over N2 for all the three γ-GYs.

Multi-material structural discrete variable topology optimization with minimum length scale control under mass constraint

Topology optimization of multi-material structure has a larger design space compared with single material optimization, and it also requires efficient material selection methods to provide references for designers. The multi-material additive manufacturing ensures the manufacturability of topology design of multi-material structures, but also puts forward new manufacturability requirements, such as the minimum size of multi-material and the connection interface. This paper focuses on the development of a topology optimization algorithm for multi-material structures under the total mass constraint based on discrete variables. Recursive multiphase materials interpolation (RMMI) scheme is utilized to establish the relationship between discrete design variables and element elastic modulus and density, and sequential approximate integer programming (SAIP) and canonical relaxation algorithms are constructed to update the discrete design variables. It is shown that this method can obtain multi-material topology design with clear interface display, and has the ability of free material coexistence and degradation, which indicates that the discrete variables have more advantages than the continuous variables in the RMMI scheme. Based on this, the material selection under mass constraints is discussed in depth, and it is pointed out that the traditional determination of material coexistence or degradation in design based on specific stiffness has limitations. Thus, the present work expands the necessary conditions of material coexistence. Then, a minimum size control method of multi-material design based on post-processing is proposed to solve the minimum size geometry constraint problem by SAIP. Numerical examples show that this method can not only precisely control the minimum size of each material phase (including void phase), but also resolve the problem of interface and boundary diffusion of the multi-material phase, and significantly improve the manufacturability of multi-material design. Finally, it is applied to optimization problems with complex design domain and irregular meshes which are closer to engineering design.

Alkali Metal Decorated Planar Tetracoordinate Hydrogen.

Despite the long list of planar tetracoordinate atoms, hydrogen is elusive. This is especially due to the inherent ability of hydrogen to form multicenter bonds with other centers. Herein, we introduce the first planar tetracoordinate hydrogen atom (ptH) in the global minimum geometry of a C2v symmetric Li4H4- cluster. Bonding analysis indicates that the central hydrogen atom is stabilized by multicenter bonding with four surrounding Li atoms. Natural charge analysis reveals that the central hydrogen is acting like a hydride, which is strongly attracted by the positively charged surrounding lithium centers. The ptH structure is stabilized by strong electrostatic attraction as well as extensive multicenter bonding. Aromaticity has no role to play here. The cluster is dynamically stable and is expected to be detected in the gas phase. Introduction of a heavier alkali metal such as sodium makes the planar C2v cluster a local minimum with slightly higher energy than the linear global minimum geometry.

Planar Tetracoordinate Hydrogen: Pushing the Limit of Multicentre Bonding

AbstractAmong the list of planar tetracoordinate atoms, the smallest element hydrogen is missing. No experimental and theoretical evidence have ever been put forwarded. Herein, we introduce the first planar tetracoordinate hydrogen atom (ptH) in the global minimum geometry of In4H+ cluster. Bonding analysis indicates that the central hydrogen atom is acting like a proton and significant charge transfer from the surrounding In4 framework results in a negative charge of the central hydrogen atom. The proposed global minimum geometry possesses σ‐aromaticity and the central hydrogen atom forms unusual multicentre bond with more than three centres.

Planar Tetracoordinate Hydrogen: Pushing the Limit of Multicentre Bonding.

Among the list of planar tetracoordinate atoms, the smallest element hydrogen is missing. No experimental and theoretical evidence have ever been put forwarded. Herein, we introduce the first planar tetracoordinate hydrogen atom (ptH) in the global minimum geometry of In4 H+ cluster. Bonding analysis indicates that the central hydrogen atom is acting like a proton and significant charge transfer from the surrounding In4 framework results in a negative charge of the central hydrogen atom. The proposed global minimum geometry possesses σ-aromaticity and the central hydrogen atom forms unusual multicentre bond with more than three centres.

The microwave spectrum of the sesamol (1,3-benzodioxol-5-ol) monomer and an analysis of its internal motion

The microwave spectrum of the sesamol (CH2O2C6H3OH) monomer has been collected using CP-FTMW spectroscopy in the 5.5–18.75 GHz region of the electromagnetic spectrum. Density functional theory calculations of the monomer have been performed. B3LYP|cc-pVTZ predicts the global minimum geometry to be s-trans-sesamol where the hydroxyl group is oriented 180° away from the dioxole ring. A second, local minimum, geometry for s-cis-sesamol was calculated to have the hydroxyl group oriented 0° towards the dioxole ring, and the zero-point energy difference between the two geometries is predicted to be 1.4 kJ/mol. Both unique conformers have been identified in the experimental spectrum. In accordance with the predicted dipole moments, only b-type transitions have been observed for s-trans-sesamol, while a- and b-type transitions have been assigned to s-cis-sesamol. The relative intensities in the observed spectrum indicate that the molecular beam contains a higher population of the s-trans-sesamol monomer. Satellite transitions were observed for both sesamol conformers. Treating the parent and satellite transitions as coupled vibrational states, the energy differences and Coriolis coupling constants were determined for each conformer. The magnitude of the coupling constants, the energy difference between the states, and a comparison to a similar state observed in 1,3-benzodioxole indicate that the excited states correspond with CH2 puckering of the dioxiole ring. The second moments of inertia and their contributions from the ring puckering have been analyzed for a series of functionalized molecules including piperonal, 1,3-benzodioxole and sesamol.

Evaluation of angled splitters as scour countermeasure at circular piers

Scour around bridge piers is local phenomenon may cause complete failure for the hydraulic structure. This study explored experimentally using splitter plates at specific angles installed upstream single circular bridge pier to improve the nearby flow field and minimize the local scour. Ninety runs were carried out considering 9 splitter models. Four splitter lengths and 3 vertex angles were used. The splitter lengths were between 0 and 1.5 times of pier diameter, each was tested for 3 vertex angles ranged between 0° and 30°. The tests were done under 9 different hydraulic conditions including 3 discharges of 75, 100, and 125 l/s and 3 tailwater depths of 12.5, 15, and 17.5 cm. The turbulent flow conditions were investigated by plotting velocity profiles at different sections. The bed configurations under clear water conditions were presented. The results of pier without splitter were used as reference. The study outcome that the local scour depth was decreased by the increase of splitter length and vertex angle. The near-bed flow velocity and the corresponding Froude No. downstream of the pier was minimized for 1.5D splitter length at 30° vertex angle and the minimum scour geometry was located. The effectiveness of splitters were remarkable by the decrease of flow discharge and increase of tailwater depth provided that the discontinuity of scour hole length in case of multi-circular piers. Regression analysis was employed to develop empirical formula for the estimation of maximum scour depth around circular pier with splitter.

Efficient Reverse Intersystem Crossing in Carbene‐Copper‐Amide TADF Emitters via an Intermediate Triplet State

AbstractThe mechanism behind reverse intersystem crossing (rISC) in metal‐based TADF emitters is still under debate. Thermal rISC necessitates small singlet/triplet energy gaps as realized in donor‐acceptor systems with charge‐transfer excited states. However, their associated spin‐orbit couplings are too small to account for effective rISC. Here, we report the first nonadiabatic dynamics simulation of the rISC process in a carbene‐copper(I)‐carbazolyl TADF emitter. Efficient rISC on a picosecond time scale is demonstrated for an initial triplet minimum geometry that exhibits a perpendicular orientation of the ligands. The dynamics involves an intermediate higher‐lying triplet state of metal‐to‐ligand charge transfer character (3MLCT), which enables large spin‐orbit couplings with the lowest singlet charge transfer state. The mechanism is completed in the S1 state, where the complex can return to a co‐planar coordination geometry that presents high fluorescence efficiency.

Efficient Reverse Intersystem Crossing in Carbene-Copper-Amide TADF Emitters via an Intermediate Triplet State.

The mechanism behind reverse intersystem crossing (rISC) in metal-based TADF emitters is still under debate. Thermal rISC necessitates small singlet/triplet energy gaps as realized in donor-acceptor systems with charge-transfer excited states. However, their associated spin-orbit couplings are too small to account for effective rISC. Here, we report the first nonadiabatic dynamics simulation of the rISC process in a carbene-copper(I)-carbazolyl TADF emitter. Efficient rISC on a picosecond time scale is demonstrated for an initial triplet minimum geometry that exhibits a perpendicular orientation of the ligands. The dynamics involves an intermediate higher-lying triplet state of metal-to-ligand charge transfer character (3 MLCT), which enables large spin-orbit couplings with the lowest singlet charge transfer state. The mechanism is completed in the S1 state, where the complex can return to a co-planar coordination geometry that presents high fluorescence efficiency.

Two Rings Around One Ball: Stability and Charge Localization of [1 : 1] and [2 : 1] Complex Ions of [10]CPP and C60/70 [ *

We investigate the gas-phase chemistry of noncovalent complexes of [10]cycloparaphenylene ([10]CPP) with C60 and C70 by means of atmospheric pressure photoionization and electrospray ionization mass spectrometry. The literature-known [1 : 1] complexes, namely [10]CPP⊃C60 and [10]CPP⊃C70 , are observed as radical cations and anions. Their stability and charge distribution are studied using energy-resolved collision-induced dissociation (ER-CID). These measurements reveal that complexes with a C70 core exhibit a greater stability and, on the other hand, that the radical cations are more stable than the respective radical anions. Regarding the charge distribution, in anionic complexes charges are exclusively located on C60 or C70 , while the charges reside on [10]CPP in the case of cationic complexes. [2 : 1] complexes of the ([10]CPP2 ⊃C60/70 )+ ⋅/- ⋅ type are observed for the first time as isolated solitary gas-phase species. Here, C60 -based [2 : 1] complexes are less stable than the respective C70 analogues. By virtue of the high stability of cationic [1 : 1] complexes, [2 : 1] complexes show a strongly reduced stability of the radical cations. DFT analyses of the minimum geometries as well as molecular dynamics calculations support the experimental data. Furthermore, our novel gas-phase [2 : 1] complexes are also found in 1,2-dichlorobenzene. Insights into the thermodynamic parameters of the binding process as well as the species distribution are derived from isothermal titration calorimetry (ITC) measurements.

Systematically derived thermodynamic properties for alkane oxidation

Key combustion properties, such as ignition delay time, show strong sensitivity to the thermochemistry of the main species in the standard radical oxidation pathway at low temperatures (600–1000 K). Significant uncertainties persist in current estimates of thermodynamic properties, particularly for the larger species of relevance to common practical fuels. In this article, we use advanced computational schemes to evaluate thermodynamic properties for the fuel, fuel radical, peroxy, hydroperoxy-alkyl radical, and hydroperoxide species for a set of 17 fuels containing up to 9 carbon atoms, and with various degrees of branching in the alkane backbone. The procedure, termed STAR-1D, combines conformer sampling to find the minimum geometry, B2PLYP-D3/cc-pVTZ harmonic frequency evaluations, and ωB97X-D/cc-pVTZ one-dimensional torsional mapping. It includes two physically-based scaling routines: a frequency-dependent scaling to B2PLYP-D3/cc-pVTZ anharmonic frequencies and a scaling of one-dimensional ωB97X-D/cc-pVTZ torsional profiles to reproduce the product of the B2PLYP-D3/cc-pVTZ frequencies in the harmonic limit. Substantive comparisons with existing experimental databases, together with careful examinations of key theoretical assumptions, are used to explore the accuracy of the predictions. These computationally intensive explorations of 195 species were facilitated by automated thermochemistry software. High accuracy 0 K heats of formation from a separate study are used with the STAR-1D computations to generate NASA polynomial representations. In addition to their intrinsic value, the present results also provide a reliable database for the optimization of group additivity or machine learning schemes for scaling to larger combustion systems. Towards this end a complementary extensive conformational analysis is carried out for the medium sized species and the thermodynamic properties of the lowest energy hydrogen-bonded and non-hydrogen bonded conformers are contrasted for larger hydroperoxy-alkyl radical species.