Accurate MRCI study of ground-state N2H2 potential energy surface

Many-body analysis and spectroscopic characterization of diazene oligomers: A theoretical study

The present study reports the many-body analysis and spectroscopic characterization of linear and cyclic diazene oligomers in gas and water solvent states. The oligomers of diazene from monomer to pentamer have been considered for the study. The spectroscopic studies such as geometrical parameters, infrared spectra, electronic absorption spectra, and natural transition orbitals (NTOs) were reported. Many-body analysis techniques have been implemented to study the interactions among the diazene oligomers. These calculations have been performed using exchange and correlation functional (B3LYP) and 6-311++G (d,p) basis set. The geometrical parameters and infrared modes of monomer diazene in the gas state are well-matched with the available experimental determinations at this level of theory. A significant change in vibrational modes of linear and cyclic diazene oligomers has been observed in the gas phase-to-water solvent state. The time-dependent density functional theory (TD-DFT) has been used to calculate the electronic absorption spectra of diazene oligomers. The Wavelength of electronic transitions, oscillator strength, and HOMO to LUMO gap has been reported. Many-body analysis shows that two-, three-, four-, and five-body energies have a remarkable contribution to the binding energy in addition to relaxation energies. All these calculations have been performed using Gaussian 16 program package.

Variational quantum eigensolver simulations with the multireference unitary coupled cluster ansatz: a case study of the C2v quasi-reaction pathway of beryllium insertion to H2 molecule

Variational quantum eigensolver (VQE)-based quantum chemical calculations have been extensively studied as a computational model using noisy intermediate-scale quantum devices. VQE uses a parametrized quantum circuit defined through an “ansatz”...

Quantum Algorithm for Full Configuration Interaction Calculations without Controlled Time Evolutions

A quantum phase estimation algorithm allows us to perform full configuration interaction (full-CI) calculations on quantum computers with polynomial costs against the system size under study, but it requires quantum simulation of the time evolution of the wave function conditional on an ancillary qubit, which makes the algorithm implementation on real quantum devices difficult. Here, we discuss an application of the Bayesian phase difference estimation algorithm that is free from controlled time evolution operations to the full-CI calculations.

Density Functional Theory Calculation on the Structural, Electronic, and Optical Properties of Fluorene-Based Azo Compounds

In the present work, a theoretical study was carried out to study the molecular structure, harmonic vibrational frequencies, normal force field calculations, and Raman scattering activities for fluorene π-conjugation spacer containing azo-based dye named trans- and cis-bis(9H-fluoren-2-yl)diazene (AzoFL) at density functional theory using B3LYP (Becke-3-Lee-Yang-Parr) functional and 6-31+G(d,p) basis set. The theoretical calculations have also been performed with fluorene and the trans- and cis-isomers of diazene, difluorodiazene by the same method DFT-B3LYP/6-31+G(d,p) and basis set. The present DFT calculation shows that the trans-AzoFL is more stable than the cis-AzoFL by 16.33 kcal/mol. We also report the results of new assignments of vibrational frequencies obtained on the basis of the present calculations. Time-dependent DFT (TD-DFT) and ZIndo calculations have been performed to study the UV-vis absorption behavior and frontier molecular orbitals for the above-mentioned compounds. The UV-vis spectrum from TD-DFT calculation shows the π-π* transition bands at λmax 423.53 nm (εmax 6.0 × 104 M-1 cm-1) and at λmax 359.45 nm (εmax 1.7 × 104 M-1 cm-1), respectively, for trans- and cis-AzoFL. Compared to parent trans-diazene (λmax 178.97 nm), a significant variation to longer wavelength (∼245 nm) is observed due to the incorporation of the fluorene (FL) ring into the -N=N- backbone. The co-planarity of the two FL rings with the longer N=N bond length compared to the unsubstituted parent diazene indicates the effective red shift due to the extended π-conjugation in trans-AzoFL. The nonplanarity of cis-AzoFL (48.1° tilted about the C-N bond relative to the planar N=N-C bond) reflects its ∼64 nm blue shift compared to that of trans-counterpart.

Quasi-Classical Trajectory Study of NH(3∑-) + NH(3∑-) Reactive Collisions

A full-dimension quasi-classical trajectory study of collisions between two NH radicals is presented. Interatomic interactions are represented by a previously reported global six-dimensional potential energy surface for singlet electronic state of the N₂H₂ system. This study suggests that the formation of N₂ from the collision of two NH radicals may occur via a one-step (NH + NH → N₂ + H + H) or two-step (NH + NH → N₂H + H → N₂ + H + H) microscopic reaction mechanism. A fast vibrational energy redistribution is observed in the four-body complex in the latter mechanism. Excitation functions are presented and discussed. A variant of the vibrational energy quantum mechanical threshold method was used to correct the zero-point energy leakage in the classical calculations. The influence of reactant's rotational and vibrational energy on reactivity was investigated by state-specific calculations with one or both reactants excited. Reaction rate constants for the ground and some rotationally excited states are presented using an Arrhenius-Kooij-like functional form.

Theoretical investigation of the isomerization pathways of diazenes: torsion vs. inversion

Diazenes are an important family of organic compounds used widely in synthetic and materials chemistry. These molecules have a planar geometry and exhibit cis-trans isomerization. The simplest of all these molecules - diazene (N2H2) - has been subjected to several experimental and theoretical studies. Two mechanisms have been proposed for the cis-trans isomerization of diazene, which are an in-plane inversion and an out-of-plane torsion. The activation energies for these pathways are similar and the competition between these two mechanisms has been discussed in the literature based on electronic structure theory calculations. Three decades ago, a classical dynamics investigation of diazene isomerization was carried out using a model Hamiltonian and it was indicated that the in-plane inversion is forbidden classically because of a centrifugal barrier and the out-of-plane torsion is the only isomerization pathway. In the present work, we investigated the cis-trans isomerization dynamics of diazene using ab initio classical trajectory simulations at the CASSCF(2,2)/aug-cc-pVDZ level of electronic structure theory. The simulation results confirmed the presence of the aforementioned centrifugal barrier for the inversion and torsion was the only observed pathway. The calculations were repeated for a similar system (difluorodiazene, N2F2) and again the centrifugal barrier prevented the inversion pathway.

Bias-Free Chemically Diverse Test Sets from Machine Learning

Current benchmarking methods in quantum chemistry rely on databases that are built using a chemist's intuition. It is not fully understood how diverse or representative these databases truly are. Multivariate statistical techniques like archetypal analysis and K-means clustering have previously been used to summarize large sets of nanoparticles however molecules are more diverse and not as easily characterized by descriptors. In this work, we compare three sets of descriptors based on the one-, two-, and three-dimensional structure of a molecule. Using data from the NIST Computational Chemistry Comparison and Benchmark Database and machine learning techniques, we demonstrate the functional relationship between these structural descriptors and the electronic energy of molecules. Archetypes and prototypes found with topological or Coulomb matrix descriptors can be used to identify smaller, statistically significant test sets that better capture the diversity of chemical space. We apply this same method to find a diverse subset of organic molecules to demonstrate how the methods can easily be reapplied to individual research projects. Finally, we use our bias-free test sets to assess the performance of density functional theory and quantum Monte Carlo methods.

Accurate ab initio based DMBE potential energy surface for the ground electronic state of N2H2

A global single-sheeted double many-body expansion potential energy surface is reported for the ground electronic state of N(2)H(2). Starting from an approximate cluster expansion of the molecular potential that utilizes previously reported functions of the same family for the triatomic fragments, four-body energy terms have been calibrated from extensive accurate ab initio data so as to reproduce the main features of the title system. The switching function formalism previously suggested for three-body systems [A. J. C. Varandas and L. Poveda, Theor. Chem. Acc. 116, 404 (2006)] has been generalized to approximate the true multisheeted nature of N(2)H(2) potential energy surface, thus allowing the correct behavior at the N((2)D) + NH(2)((2)A(")) and N((4)S) + NH(2)((4)A(")) dissociation limits. The resulting fully six-dimensional potential energy function reproduces the correct symmetry under permutation of identical atoms and predicts the main stationary points of the molecule in the valence and long-range regions in good agreement with available experimental and theoretical data on the diazene molecule.

Predicting Catalysis: Understanding Ammonia Synthesis from First-Principles Calculations

Here, we give a full account of a large collaborative effort toward an atomic-scale understanding of modern industrial ammonia production over ruthenium catalysts. We show that overall rates of ammonia production can be determined by applying various levels of theory (including transition state theory with or without tunneling corrections, and quantum dynamics) to a range of relevant elementary reaction steps, such as N(2) dissociation, H(2) dissociation, and hydrogenation of the intermediate reactants. A complete kinetic model based on the most relevant elementary steps can be established for any given point along an industrial reactor, and the kinetic results can be integrated over the catalyst bed to determine the industrial reactor yield. We find that, given the present uncertainties, the rate of ammonia production is well-determined directly from our atomic-scale calculations. Furthermore, our studies provide new insight into several related fields, for instance, gas-phase and electrochemical ammonia synthesis. The success of predicting the outcome of a catalytic reaction from first-principles calculations supports our point of view that, in the future, theory will be a fully integrated tool in the search for the next generation of catalysts.