Substituent, Solvent, and Dispersion Effects on the Zwitterionic Character and Dimerization Thermochemistry of the Group 6 Fulvene Metal Tricarbonyl Complexes.

Dimerizations of fulvene metal tricarbonyl complexes of the type (C5H4CRR')M(CO)3 (R, R' = MeO, Me, H; M = Cr, Mo, W) to form a metal-metal bond and a new carbon-carbon bond, thereby giving binuclear cyclopentadienyl metal carbonyl derivatives, are predicted to be thermochemically favored but to have significant activation energies ranging from ΔE = 19 to 42 kcal/mol. However, the introduction of dimethylamino but not methoxy substituents onto the exocyclic carbon atom changes the situation drastically so that the monomers [C5H4CH(NMe2)]M(CO)3 and [C5H4C(NMe2)2]M(CO)3 become strongly thermochemically favored, lying ΔE = 43 kcal/mol (M = W) to 63 kcal/mol (M = Cr) below their corresponding dimers. In such dimethylamino-substituted (fulvene)M(CO)3 derivatives, the M-C distance to the exocyclic fulvene carbon is lengthened beyond the bonding distance to give a zwitterionic structure with a pentahapto fulvene ligand. Such M-C distances in (fulvene)M(CO)3 complexes, which have preferred zwitterionic structures, increase with increasing solvent polarity (i.e., dielectric constant) until a saturation point is reached.

Hyperfine structure of the NaCs b3Π2 state near the dissociation limit 3S1/2 + 6P3/2 observed with ultracold atomic photoassociation.

We report new observations of the hyperfine structure in three ro-vibrational levels of the b3Π2 state of NaCs near the dissociation limit 3S1/2 + 6P3/2. The experiment was done via photoassociation of ultracold atoms in a dual-species dark-spot magneto-optical trap, and the spectra were measured as atomic trap losses. The simulation of the hyperfine structure showed that the greater part of the observed structure belongs to almost isolated levels of the b3Π2 state, but there are other parts of mixed character where the contribution from the 1Σ symmetry dominates.

Comparative Study of the Thermal Stabilities of the Experimentally Known High-Valent Fe(IV) Compounds Fe(1-norbornyl)4 and Fe(cyclohexyl)4.

The high stability of the experimentally known homoleptic 1-norbornyl derivative (nor)4Fe of iron in the unusual +4 oxidation state is a consequence of the high reaction barriers of the singlet or triplet potential surfaces constrained by the global dispersion attraction and the great steric demands of the norbornyl groups. The much more limited stability of the corresponding cyclohexyl derivative (cx)4Fe may result from the conical intersection between the singlet potential surface and the quintet spin potential surface arising from the weaker dispersion attraction and the reduced steric effect of the cyclohexyl groups relative to the 1-norbornyl groups. In contrast, the high stability of the likewise experimentally known (cx)4M (M = Ru or Os) structures results from the larger ligand field splitting (Δ) of the d-orbital energies for the second and third-row transition metals ruthenium and osmium relative to that of the first-row transition metal iron. The cyclohexyl derivative (cx)4Fe is predicted to be reactive toward carbon monoxide to insert CO into up to two Fe-C bonds. However, the dispersion effect as well as the much larger size of the 1-norbornyl substituents prevents similar reactivity of (nor)4Fe with carbon monoxide.

Dispersion Effects in Stabilizing Organometallic Compounds: Tetra-1-norbornyl Derivatives of the First-Row Transition Metals as Exceptional Examples.

In 1972 Bower and Tennett first synthesized a series of tetra-1-norbornyl derivatives, (nor)4M, of the first-row transition metals from titanium to cobalt. These were found to be exceptionally stable for homoleptic metal alkyls containing only metal-carbon σ-bonds. The theoretical energies for the dissociation of 1-norbornyl ligands from these unusually high oxidation state organometallics through the reactions (nor)4M → (nor)3M + nor• and (nor)4M → (nor)2M + nor-nor indicate that dispersion effects play an important role in determining their exceptional stability. Thus, all of the (nor)4M (M = Ti to Cu) derivatives are viable with respect to 1-norbornyl radical dissociation when the London dispersion effect is considered. However, (nor)4Cu becomes disfavored if the dispersion correction is ignored. Thus, the stability of the (nor)4M molecules is seen to arise from the favorable combination of steric and dispersion force effects of the four 1-norbornyl groups tetrahedrally disposed around the metal atom and maximizing the dispersion attraction between them in a spherical hydrocarbon structure with a central metal atom. The tri-1-norbornyl derivatives (nor)3M appear be disfavored with respect to disproportionation into (nor)4M + (nor)2M. This is consistent with the experimental syntheses of the (nor)4M (M = Cr to Co) derivatives with the metal in the +4 oxidation by reactions with 1-norbornyllithium with metal halides in the +2 or +3 metal oxidation states. Both the OPBE method and the BPW91 method predict high-spin states for the d2 and d3 complexes (nor)4Cr and (nor)4Mn but low-spin states for (nor)4Fe and (nor)4Co, consistent with experiment.

Tris(butadiene) Metal Complexes of the First-Row Transition Metals versus Coupling of Butadiene to Eight- and Twelve-Carbon Hydrocarbon Chains.

The role that zerovalent nickel plays in catalyzing the trimerization of butadiene to 1,5,9-cyclododecatriene conveys interest in the properties of the tris(butadiene)metal complexes (C4H6)3M. In this connection the complexes (C4H6)3M (M = Ti-Ni) of the first-row transition metals have been investigated by density functional theory. The intermediate C12H18Ni which has been isolated in the nickel-catalyzed trimerization of butadiene but is too unstable for X-ray crystallography is suggested here to have an open-chain hexahapto η3,3-C12H18 ligand rather than the octahapto such ligand suggested by some investigators. The lowest energy (C4H6)3M structures of the other first-row transition metals from vanadium to cobalt are found to have related structures with open-chain C12H18 ligands having hapticities ranging from four to eight with hexahapto structures being most common. The nickel and cobalt (C12H18)M derivatives favor low-spin singlet and doublet spin states, respectively, whereas the manganese derivative (C12H18)Mn favors the high-spin sextet state corresponding to the half-filled d5 shell of Mn(II). A (C4H6)3Cr structure with three separate tetrahapto butadiene ligands analogous to the very stable (η4-C4H6)3M (M = Mo, W) with the favored 18-electron metal configuration is found to be a very high energy structure relative to isomers containing an open-chain C12H18 ligand.

Butadiene as a ligand in open sandwich compounds.

Theoretical methods show that the lowest energy bis(butadiene)metal structures (C4H6)2M (M = Ti to Ni) have a perpendicular relative orientation of the two butadiene ligands corresponding to a tetrahedral coordination of the central metal atom to the four C[double bond, length as m-dash]C double bonds of the butadiene ligands. Distribution of the metal d electrons in the resulting tetrahedral ligand field rationalizes the predicted spin states increasing monotonically from singlet to quartet from nickel to manganese and back from quartet to singlet from manganese to titanium.

[Studies on the Analytical Potential Energies for Partial Electronic States of Li(2) with Variational Algebraic Energy Consistent Method].

The full vibrational spectra especially those high-lying vibrational energies in the dissociation region of four specific electronic states 1(3)Δ(g), 33Σ(+)(g), 1(3)Σ-(g) and b(3)Π(u) have been obtained by using the improved variational algebraic method (VAM). The analytical potential energy functions (APEFs) of these electronic states are also determined with corresponding adjustable parameter λ by using the variational algebraic energy consistent method (VAECM) based on the VAM vibrational spectra. The full vibrational energies, vibrational spectroscopic constants, force constants f(n), and expansion coefficients a(n) of the VAECM potential are also tabulated for each electronic state in this study. The results show that the VAECM analytical potentials are superior to some other widely used analytical ones, and do not have the unphysical tiny barriers existing in the precious AECM potentials.

New potential energy surface features for the Li + HF → LiF + H reaction.

The existing potential energy surfaces for the Li + HF system have been challenged by the experiments of Loesch, Stienkemeier, and co-workers. Here a very accurate potential energy surface has been obtained with rather rigorous theoretical methods. Methods up to full CCSDT have been pursued with basis sets as large as core correlated quintuple ζ. Reported here are the reactants, products, two transition states, and three intermediate complexes for this reaction. These reveal one previously undiscovered equilibrium geometry. The stationary point relative energies are very sensitive to level of theory. The reaction has a classical endothermicity of 2.6 kcal mol(-1). The complex Li···HF in the entrance valley lies 6.1 kcal/mol below the reactants. The expected transition state Li···H···F is bent with an angle of 72.2° and lies 4.5 kcal/mol above the reactants. The latter predicted classical barrier should be no more than one kcal/mol above the exact barrier. Not one but two product complexes lie 1.6 and 2.2 kcal/mol above reactants, respectively. Between the two product complexes, a second transition state, very broad, is found. The vibrational frequencies and zero-point vibrational energies (ZPVE) of all stationary points are reported, and significantly affect the relative energies.

Comparative research of multichannel slab and discharge tube CO2 lasers.

The laser power and beam quality of the high-power CO(2) laser are very important for laser applications. The multichannel slab discharge CO(2) laser (MSDL) and the multichannel discharge tube CO(2) laser (MDTL) are two main lasers, which have different functions. Two lasers and laser beams are compared and studied quantitatively from the following factors: intensity distribution, M(2) factor, phase locking, misalignment, and output power. It is shown that MSDL could obtain the laser beam with high power when the misaligned angle is small, but the beam quality is poor in comparison with that of MDTL. MSDL is more sensitive to the misaligned angle than MDTL.

Theoretical study on the vibrational spectra and potential energy curves for SiC (X3Π) and SiS (X1Σ+) molecules.

In this work, the vibrational constants (ωe,ωexe) calculated by the variational algebraic method (VAM) and some other molecular constants (De,re,Be,αe) were used to construct the improved Hulburt-Hirschfelder (IHH) analytical potential energy function (APEF). Not only that, but the calculated VAM potential points are used as the 'true' energies to determine the value of the variational parameter λ which is the pivotal fitting parameter in the IHH potential. With limited experimental data, high-precision IHH potential can be achieved by combining the VAM and the IHH APEF. This combination of the VAM and the IHH APEF is referred to be VAIHH APEF, which is employed to study the vibrational energies and potential energy curves (PECs) of SiC (X3Π) and SiS (X1Σ+) molecules, yielding full vibrational spectra and spectroscopic constants. The calculational results indicate that the VAIHH APEFs of SiC (X3Π) and SiS (X1Σ+) molecules are in good agreement with the experimental RKR potential points. Accurate PECs of SiC (X3Π) and SiS (X1Σ+) molecules imply that the VAIHH APEF is of high quality.

[Vibrational levels and dissociation energies of diatomic systems using algebraic method].

The fixed order in the algebraic method (AM) suggested by Sun et al. is changed to be a flexible one in the vibrational energy expansion because the order of diatomic potential energy expansion may not be a constant. The AM with a flexible order was used to tackle the possible "butterfly effect" that may be encountered in spectroscopic computations, and to study the full vibrational levels {E(v)} and the dissociation energies D(e) for N2 - a'(1) sigma(u)(-), Li2(+) - 2 2sigma(g)(+), 4HeD(+) - X 1sigma(-) and 39K 85Rb- (2) 3sigma(+) electronic systems. The results reproduced all known experimental vibrational energies, and predicted correct dissociation energies and all unknown high-lying levels that may not be given if one uses original AM. The calculations showed that the modified AM can be extended to study the full vibrational spectra for many more diatomic systems.

[Studies on the Q-branch spectral lines of high-lying rovibrational transitions of diatomic system].

An analytical formula was proposed recently to predict the accurate P-branch spectral lines of rovibrational transitions for diatomic systems by taking multiple spectral differences. A similar analytical expression was suggested here to predict the Q-branch spectral lines of rovibrational transitions. This formula was applied to study the high-lying Q-branch emission spectra of the (4,1) and (3,1) bands of the A 1Π - X1 Σ+ system of IrN molecule using fifteen known accurate experimental transition data. The results show that not only the known experimental transition lines were reproduced but also the correct values of the unknown spectral lines were predicted.

Achieving vibrational energies of diatomic systems with high quality by machine learning improved DFT method.

When using ab initio methods to obtain high-quality quantum behavior of molecules, it often involves a lot of trial-and-error work in algorithm design and parameter selection, which requires enormous time and computational resource costs. In the study of vibrational energies of diatomic molecules, we found that starting from a low-precision DFT model and then correcting the errors using the high-dimensional function modeling capabilities of machine learning, one can considerably reduce the computational burden and improve the prediction accuracy. Data-driven machine learning is able to capture subtle physical information that is missing from DFT approaches. The results of 12C16O, 24MgO and Na35Cl show that, compared with CCSD(T)/cc-pV5Z calculation, this work improves the prediction accuracy by more than one order of magnitude, and reduces the computation cost by more than one order of magnitude.

A method for predicting the molar heat capacities of HBr and HCl gases based on the full set of molecular rovibrational energies.

A new method is presented for one to obtain the molar heat capacities of diatomic macroscopic gas with a full set of microscopic molecular rovibrational energies. Based on an accurate experimental vibrational energies subset of a diatomic electronic ground state, the full vibrational energies can be obtained by using the variational algebraic method (VAM), the potential energy curves (PECs) will be constructed by the Rydberg-Klein-Rees (RKR) method, the full set of rovibrational energies will be calculated by the LEVEL program, and then the partition functions and the molar heat capacities of macroscopic gas can be calculated with the help of the quantum statistical ensemble theory. Applying the method to the ground state HBr and HCl gases, it is found that the relative errors of the partition functions calculated in the temperature range of 300 âˆ¼ 6000 K are in excellent agreement with those obtained from TIPS database, and the calculated molar heat capacities are closer to the experimental values than those calculated by other methods without considering the energy levels of highly excited quantum states. The present method provides an effective new way for one to obtain the full set of molecular rovibrational energies and the molar heat capacities of macroscopic gas through the microscopic spectral information of a diatomic system.

A data- and model-driven strategy for the evaluation of the experimental transition lines: Theoretical prediction for the ground state of 12C16O.

An analytical formula that relates the molecular constants of the Herzberg expression and experimental transition lines is developed herein with a difference algebraic approach (DAA) model. Based on the data-driven strategy, the DAA model is able to deal with the tiny uncertainties that exhibit behind the experimental transition lines, which is applied to the P branch emission spectra of some first overtone bands of the ground electronic state of 12C16O. The relationship can be used to generate transition lines with sufficient accuracy, as evident from the high J of agreement with the HITRAN database, Velichko data, Goorvitch data and quantum-mechanical data. In addition, line intensities, absorption oscillator strengths and Einstein A coefficients of these lines, which are introduced to enhance the dataset and are in good agreement with those of other authors, are also reported to validate our results. These various comparative results show that the proposed data-driven strategy based on the DAA model is expecting to be a good algorithm that relies on relatively limited data for training.

Study on potential energy curves and ro-vibrational energies of DT, HT and T2 molecules.

Accurately monitoring and effectively controlling the tritium compounds based on their ro-vibrational energy structure are important issues in various nuclear systems. Because of their radioactivity, it is difficult to obtain the corresponding energies directly through experiments. In this paper, the potential energy curves and the corresponding ro-vibrational full spectrum of DT, HT and T2 systems are derived by ab initio methods. However, it is difficult to verify the reliability of the calculated results due to the lack of direct experimental support. Therefore, a data-driven reliability analysis method is proposed, which can confirm the reliability by extracting information from the relevant calculations and multiple experimental data (the vibrational level, rotational level, and molar heat capacity) of similar systems (HD, H2, D2). The results show that: 1) The potential energy curves obtained by the ab initio method can provide the full ro-vibrational energy spectrum with an accuracy of approximately 10 cm-1; 2) Macroscopic heat capacity information can be used to distinguish and calibrate the overall reliability of microscopic ro-vibrational energies; 3) For the isotopic energy level structure of hydrogen, the influence of isotopes is mainly mass effect.

Universal Precise Growth of 2D Transition-Metal Dichalcogenides in Vertical Direction.

Two-dimensional transition-metal dichalcogenides (TMDs) have been one of the hottest focus of materials due to the most beneficial electronic and optoelectronic properties. Up to now, one of the big challenges is the synthesis of large-area layer-number-controlled single-crystal films. However, the poor understanding of the growth mechanism seriously hampers the progress of the scalable production of TMDs with precisely tunable thickness at an atomic scale. Here, the growth mechanisms in the vertical direction were systemically studied based on the density functional theory (DFT) calculation and an advanced chemical vapor deposition (CVD) growth. As a result, the U-type relation of the TMD layer number to the ratio of metal/chalcogenide is confirmed by the capability of ultrafine tuning of the experimental conditions in the CVD growth. In addition, high-quality uniform monolayer, bilayer, trilayer, and multilayer TMDs in a large area (8 cm2) were efficiently synthesized by applying this modified CVD. Although bilayer TMDs can be obtained at both high and low ratios of metal/chalcogenide based on the suggested mechanism, they demonstrate significantly different optical and electronic transport properties. The modified CVD strategy and the proposed mechanism should be helpful for synthesizing and large-area thickness-controlled TMDs and understanding their growth mechanism and could be used in integrated electronics and optoelectronics.

Spectroscopy learning: A machine learning method for study diatomic vibrational spectra including dissociation behavior.

Molecular spectroscopy plays an important role in the study of physical and chemical phenomena at the atomic level. However, it is difficult to acquire accurate vibrational spectra directly in theory and experiment, especially these vibrational levels near the dissociation energy. In our previous study (Variational Algebraic Method), dissociation energy and low energy level data are employed to predict the ro-vibrational spectra of some diatomic system. In this work, we did the following: 1) We expand the method to a more rigorous combined model-driven and data-driven machine learning approach (Spectroscopy Learning Method). 2) Extracting information from a wide range of existing data can be used in this work, such as heat capacity. 3) Reliable vibrational spectra and dissociation energy can be predicted by using heat capacity and the reliability of this method is verified by the ground states of CO and Br2 system.

A joint data and model driven method for study diatomic vibrational spectra including dissociation behavior.

The details of quantum multi-body interactions are so rich and subtle which make it difficult to accurately model for some situations such as the behavior of diatomic long-range vibrations. In recent years, data-driven machine learning has made remarkable achievements in capturing complex relationships that are subtle. Combining the characteristics of these two fields, we propose a joint machine learning method to obtain reliable diatomic vibrational spectra including dissociation energy by using accessible heterogeneous micro/macro information such as low lying vibrational energy levels and heat capacity. Applications of this method to CO and Br2 in the ground state yield their state of the art of vibrational spectra including dissociation limit. The strategy introduced here is an exploration of combining the model-driven and data-driven method to cover subtle physical details that are difficult to study in a single way.

CuFe2O4/MoS2 Mixed-Dimensional Heterostructures with Improved Gas Sensing Response.

Mixed-dimensional (2D + nD, n = 0, 1, and 3) heterostructures opened up a new avenue for fundamental physics studies and applied nanodevice designs. Herein, a novel type-II staggered band alignment CuFe2O4/MoS2 mixed-dimensional heterostructures (MHs) that present a distinct enhanced (20-28%) acetone gas sensing response compared with pure CuFe2O4 nanotubes are reported. Based on the structural characterizations and DFT calculation results, the tentative mechanism for the improvement of gas sensing performance of the CuFe2O4/MoS2 MHs can be attributed to the synergic effect of type-II band alignment and the MoS2 active sites.