Calculations Of Potential Energy Curves Research Articles

Photodissociation dynamics of methylamine in the blue edge of the A-band. II. The NH2 + CH3 channel.

The photodissociation dynamics leading to the C-N bond cleavage in methylamine (CH3NH2) are investigated upon photoexcitation in the blue edge of the first absorption A-band, in the 198-204nm range. Velocity map images of the generated methyl (CH3) fragment detected in specific vibrational modes, i.e., ν = 0, ν1 = 1, and ν2 = 1, through resonance enhanced multiphoton ionization, are presented along with the corresponding translational energy distributions and the angular analysis. The experimental results are complemented by high-level abinitio calculations of potential energy curves as a function of the C-N bond distance. While a similar single Boltzmann-type contribution is observed in all the translational energy distributions measured, the speed-dependent anisotropy parameter obtained through the angular analysis reveals the presence of two different mechanisms. Prompt dissociation through the conical intersection between the Ã1A' first excited state and the ground state located in the exit channel is, indeed, revealed as a minor channel. In contrast, slow dissociation on the ground state, presumably from frustrated N-H bond cleavage trajectories, constitutes the major reaction pathway leading to the methyl formation.

Imaging the photodissociation dynamics of internally excited ethyl radicals from high Rydberg states.

The site-specific hydrogen-atom elimination mechanism previously reported for photoexcited ethyl radicals (CH3CH2) [D. V. Chicharro et al., Chem. Sci., 2019, 10, 6494] is interrogated in the photodissociation of the ethyl isotopologues CD3CD2, CH3CD2 and CD3CH2 through the velocity map imaging (VMI) detection of the produced hydrogen- and deuterium-atoms. The radicals, generated in situ from photolysis of a precursor using the same laser pulse employed in their excitation to Rydberg states, decompose along the Cα-H/D and Cβ-H/D reaction coordinates through coexisting statistical and site-specific mechanisms. The experiments are carried out at two excitation wavelengths, 201 and 193 nm. The comparison between both sets of results provides accurate information regarding the primary role in the site-specific mechanism of the radical internal reservoir. Importantly, at 193 nm excitation, higher energy dissociation channels (not observed at 201 nm) producing low-recoil H/D-atoms become accessible. High-level ab initio calculations of potential energy curves and the corresponding non-adiabatic interactions allow us to rationalize the experimental results in terms of competitive non-adiabatic decomposition paths. Finally, the adiabatic behavior of the conical intersections in the face of several vibrational modes - the so-called vibrational promoting modes - is discussed.

Dissociative Recombination of K2+ and Rb2+ in Diode-Pumped Alkali Lasers: Potential Energy Curves and Atomic Products.

Dissociative recombination (DR) of K2+ and Rb2+ ions is one of the most important processes in K and Rb diode-pumped alkali lasers (DPALs) strongly affecting their power. We report on the calculations of potential energy curves of the K2+ and Rb2+ molecular ions and of the diabatic 1Σ+, 3Σ+, 1Δ, 3Δ, 1Π, 3Π, 1Φ, and 3Φ valence states of K2 and Rb2 that provide the routes for DR of the ions. These curves are required for subsequent calculations of DR rate constants. It was found that the most likely DR routes for K are along the dissociative states 61Σg+, 53Σu+, and 23Δu and for Rb only 61Σg+. The excited states of K atoms produced by DR are 42P and 52P. Most of the Rb atoms produced by DR are in the 62P excited state, while a small fraction of the atoms produced by the less likely routes along two states of Rb2 are in the 52P state. This conclusion contradicts the kinetic scheme for K and Rb DPAL proposed elsewhere, and thus the kinetic schemes of these DPALs should be modified according to the present results.

Systematic theoretical investigation of two novel molecules BtyC-1 and BtyC-2 based on ESIPT mechanism

Inexperiment, Song et al. have successfully synthesizedtwo novel molecules BtyC-1 and BtyC-2 and observedasingle and dual fluorescence peaks in these two molecules respectively. (Song et al. Tetrahedron Lett. 2019, 60, 1696–1701) However, they still lack a detailed and reasonable theoretical explanation. Then we wonder why these two similar structures behave so much differently? In this work, we focus on explaining the photochemical and photophysical properties of BtyC-1 and BtyC-2 by studying the excited state intramolecular proton transfer (ESIPT) mechanisms. Based on the optimized geometric configurations, the calculated infrared spectra indicate the intramolecular hydrogen bonding interactions are heightened in their excited states. The frontier molecular orbitals reflect the charge redistribution in photoinduced process, which explains that the driving force of ESIPT process is provided by enhanced hydrogen bonding interactions. In the meantime, the calculations of potential energy curves vividly explain the principle of the experimental dual fluorescence phenomenon. The analysis of Mulliken charges deepens the discussion of molecular structures on the potential energy barriers. Calculated absorption spectra via using density functional theory and emission spectra via using time-dependent density functional theory are consistent with the experimental data, which confirms the correctness of our calculation methods. The reduced density gradient isosurfaces help us distinguish the complex non-covalent bonds. Base on the above analyses, we conclude that there is no stable structure for BtyC-1 in excited state, which make it occur the ESIPT reaction spontaneously. BtyC-2 exists a stable normal structure in excited state. Its dual fluorescence signals are emitted by its normal and isomer structures, respectively.

Calculation of Potential Energy Curves for Ar*–He Collision Complex

The dependences of potential energies E(R) for 48 states of the Ar*–He collision complex on the interatomic distance R were obtained using multiconfigurational methods of quantum mechanics with consideration of spin–orbit coupling with active space (6, 7). The values E(R ≥ 10 A) are in good agreement, with an accuracy of 10–200 cm–1 (0.05–0.3%), with energies of Ar atom states.

Combining density-based dynamical correlation with a reduced-density-matrix strong-correlation description

A combined density and first-order reduced-density-matrix (1RDM) functional method is proposed for the calculation of potential energy curves (PECs) of molecular multibond dissociation. Its 1RDM functional part, a pair density functional, efficiently approximates the ab initio pair density of the complete active space self-consistent-field (CASSCF) method. The corresponding approximate on top pair density {\Pi} is employed to correct for double counting a correlation functional of density functional theory (DFT). The proposed ELS-DM{\Pi}DFT method with the extended L\"owdin-Shull (ELS) 1RDM functional with dispersion and multibond (DM) corrections augmented with the {\Pi}DFT functional closely reproduces PECs of multibond dissociation in the paradigmatic N_2 , H_2O, and H_2CO molecules calculated with the recently proposed CAS{\Pi}DFT (CASSCF augmented with a {\Pi} based scaled DFT correlation correction) method. Furthermore, with the additional M-correction, ELS-DM{\Pi}DFT+M reproduces well the benchmark PEC of the N_2 molecule by Lie and Clementi.

Spectroscopic properties of diatomic molecules CdI and CsCd: ab initio calculations

Ab initio methods, CASPT2 and CCSD(T), with the large contraction of the ANO-RCC basis set, were used for the calculations of potential energy curves (PECs) of CsI and CsCd. Spin-free PECs for the ground and low-lying excited states of both diatomics were investigated for doublet and quartet Λ–∑ states (22 for CdI and 13 for CsCd). Spin–orbit interaction was calculated using the CASPT2/RASSI method, leading to 6 Ω states of CdI and 8 Ω states of CsCd. Spectroscopic properties for the bound states were calculated from these data-equilibrium distance Re, harmonic frequency ωe, the first anharmonicity ωexe, rotational constant Be, excitation energy Te and dissociation energy De.

Ab initio diabatic and adiabatic calculations for francium hydride FrH

Explicit ab initio diabatic and adiabatic calculations of potential energy curves (PECs) of the states 1,3Σ+, 1,3Π, and 1,3Δ of francium hydride FrH have been carried out with several approaches.

Reproducing benchmark potential energy curves of molecular bond dissociation with small complete active space aided with density and density-matrix functional corrections.

Various effects of electron correlation accompany molecular bond dissociation, which makes the efficient calculation of potential energy curves a notoriously difficult problem. In an attempt to reliably reproduce both absolute energies and shapes of the benchmark dissociation curves, calculations with the combined CASΠDFT method are carried out for the prototype molecules H2, BH, F2, and N2. The complete active space (CAS) part of CASΠDFT accounts for long-range nondynamic correlation, while short-range dynamic correlation is accounted for with the corrected Lee-Yang-Parr correlation functional of density functional theory (DFT). The correction represents the suppression of dynamic correlation with nondynamic correlation, and it is a function of the ratio x(r) between the conditional and conventional densities obtained with the CAS on-top pair density Π(r). For the single-bonded molecules H2, BH, and F2, CASΠDFT succeeds in reproducing the shapes and absolute energies (for H2 and BH) of the benchmark curves, while for the triple-bonded N2 molecule, the addition to CASΠDFT of a multibond correction is required. It accounts for the middle-range dynamic correlation of the same-spin electrons in the (symmetrized) high-spin atomic electron configurations of the dissociating N2.

Site-specific hydrogen-atom elimination in photoexcited ethyl radical.

The photochemistry of the ethyl radical following excitation to the 3p Rydberg state is investigated in a joint experimental and theoretical study. Velocity map images for hydrogen atoms detected from photoexcited isotopologues CH3CH2, CH3CD2 and CD3CH2 at ∼201 nm, are discussed along with high-level ab initio electronic structure calculations of potential energy curves and non-adiabatic coupling matrix elements (NACME). A novel mechanism governed by a conical intersection allowing prompt site-specific hydrogen-atom elimination is presented and discussed. For this mechanism to occur, an initial ro-vibrational excitation is allocated to the radical permitting to access this reaction pathway and thus to control the ethyl photochemistry. While hydrogen-atom elimination from cold ethyl radicals occurs through internal conversion into lower electronic states followed by slow statistical dissociation, prompt site-specific Cα elimination into CH3CH + H, occurring through a fast non-adiabatic crossing to a valence bound state followed by dissociation through a conical intersection, is accessed by means of an initial ro-vibrational energy content into the radical. The role of a particularly effective vibrational promoting mode in this prompt photochemical reaction pathway is discussed.

Theoretical predication of a two-dimensional structure of non-layered ScC as an excellent electrode material for rechargeable Na-ion battery

Non-layered two-dimensional structures with the atomic thickness recently attract much attention. In this work, we predicated a two-dimensional structure of non-layered ScC, denoted as ScC sheet, and evaluated its potential as the electrode material of Na-ion battery. The stability of ScC sheet is determined by the cohesive energy, phonon spectrum and potential energy curve calculations. Interestingly, the proposed ScC sheet can exhibit many unexpected electrochemical properties. It is found that ScC sheet can possess strong Na adsorption capability. Both pristine and Na-interacted ScC sheets represent good electronic conductivity due to the metallic nature. The structural degradation of the ScC during the sodiation and desodiation processes will not be anticipated. Besides, our results reveal that the ScC sheet has a minimal diffusion barrier of 9 meV and the calculated theoretical capacity of ScC sheet can reach to 940 mAh·g−1 on the basic of the second layer adsorption. Thus, benefiting from these features, ScC sheet is expectable to be served as a promising electrode material for the Na-ion battery.

The spectrum of N2 from 4,500 to 15,700 cm−1 revisited with pgopher

Using a reference molecular atlas to ensure self-consistency of wavelength calibration is widespread practice. Boesch & Reiners (Astronomy & Astrophysics 582 A43 (2015)) reported a line list from a discharge of molecular nitrogen from 4500 to 11,000 cm−1 for this purpose. With a hollow-cathode discharge source, we have extended the experimental spectrum up to 15,700 cm−1, to include the range of Ti:sapphire lasers, since the density of N2 lines is greater than atomic atlases in common use. Recognizing that experimental conditions can vary, we have also analysed the spectra (comprising several B3Πg−A3Σu+, B′3Σu−−B3Πg, and W3Δu −B3ΠgN2 bands) with standard Hamiltonians, so that any part of the discharge spectrum in the range 4,500–15,700 cm−1 can be simulated. Parameters are given to do this with the spectral simulation and analysis package pgopher. (C. Western, J. Quant. Spectrosc. Rad. Transf., 186, 221 (2016)). The analysis also included high-level ab initio calculations of potential energy curves, transition moments and spin-orbit coupling constants and these were used in preparing the model, extending the potential range of applicability. The results are available in a variety of formats to suit possible applications, including the experimental spectrum in ASCII, a detailed line list with positions and Einstein A coefficients, and a PGOPHER input file to synthesize the spectrum at selectable temperature and resolution.

A MoO2 sheet as a promising electrode material: ultrafast Li-diffusion and astonishing Li-storage capacity

The potential of MoO2 crystal as an electrode material is reported, and nanostructural MoO2 systems, including nanoparticles, nanospheres, nanobelts and nanowires, were synthesized and proved to be advanced electrode materials. A two-dimensional (2D) geometric structure represents an extreme of surface-to-volume ratio, and thus is more suitable as an electrode material in general. Stimulated by the recent fabrication of 2D MoO2, we adopted an ab initio molecular dynamics simulation and density functional theory calculation to study the stability and electrochemical properties of a MoO2 sheet. Identified by a phonon dispersion curve and potential energy curve calculations, the MoO2 sheet proved to be dynamically and thermally stable. After lithiation, similar to most promising 2D structures, we found that a Li atom can strongly adsorb on a MoO2 sheet, and the lithiated MoO2 sheet presented excellent metallic properties. Note that, compared with most promising 2D structures, we unexpectedly revealed that the diffusion barrier of the Li atom on the MoO2 sheet was much lower and the storage capacity of the MoO2 sheet was much larger. The calculated energy barrier for the diffusion of Li on the MoO2 sheet was only 75 meV, and, due to multilayer adsorption, the theoretical capacity of the MoO2 sheet can reach up to 2513 mA h g−1. Benefiting from general properties, such as strong Li-binding and excellent conductivity, and unique phenomena, such as ultrafast diffusion capacity and astonishing storage capacity, we highlight a new promising electrode material for the Li-ion battery.

Force-field parameters for beryllium complexes in amorphous layers.

Unknown force-field parameters for metal organic beryllium complexes used in emitting and electron transporting layers of OLED structures are determined. These parameters can be used for the predictive atomistic simulations of the structure and properties of amorphous organic layers containing beryllium complexes. The parameters are found for the AMBER force field using a relaxed scan procedure and quantum-mechanical DFT calculations of potential energy curves for specific internal (angular) coordinates in a series of three Be complexes (Bebq2; Be(4-mpp)2; Bepp2). The obtained parameters are verified in calculations of some molecular and crystal structures available from either quantum-mechanical DFT calculations or experimental data. Graphical Abstract Beryllium complexes in amorphous layersᅟ.

Anomalously strong two-electron one-photon X-ray decay transitions in CO caused by avoided crossing.

The unique opportunity to study and control electron-nuclear quantum dynamics in coupled potentials offered by the resonant inelastic X-ray scattering (RIXS) technique is utilized to unravel an anomalously strong two-electron one-photon transition from core-excited to Rydberg final states in the CO molecule. High-resolution RIXS measurements of CO in the energy region of 12–14 eV are presented and analyzed by means of quantum simulations using the wave packet propagation formalism and ab initio calculations of potential energy curves and transition dipole moments. The very good overall agreement between the experimental results and the theoretical predictions allows an in-depth interpretation of the salient spectral features in terms of Coulomb mixing of “dark” with “bright” final states leading to an effective two-electron one-photon transition. The present work illustrates that the improved spectral resolution of RIXS spectra achievable today may call for more advanced theories than what has been used in the past.

Experimental and theoretical study of the electronic states and spectra of SbNa

Gas-phase emission spectra of the hitherto unknown free radical SbNa were measured in the NIR region with a Fourier-transform spectrometer. The emissions were observed from a fast-flow system in which antimony vapor in argon or neon carrier gas was passed through a microwave discharge and mixed with sodium vapor in an observation tube. Seven transitions from five low-lying excited states A12, A21, A30+, A40−, and B2 to the X10+ and/or X21 components of the X3Σ− ground state have been observed and analyzed.In parallel to the experiments, relativistic configuration interaction calculations of potential energy curves, vibrational constants, bond lengths, transition moments and radiative lifetimes were carried out to aid in the analysis of the experimental data.

Potential energy curves and generalized oscillator strength for doubly excited states of hydrogen molecule

We report calculations of potential energy curves for doubly excited states of the H2 molecule in the range at the multireference configuration interaction (MRCI) level based on multiconfigurational self-consistent field (MCSCF) orbitals. We focus on the Q2 states which lie between the second and third ionization thresholds of the molecular ion, i.e. the and states, respectively, and dissociate in the H(2l) + H(2l) channels. The MCSCF/MRCI approach allows us to successfully identify the molecular optically forbidden state which dissociates into two H(2s) atoms. Generalized oscillator strength as a function of transferred momentum for three doubly excited states is also presented.

Potential Energy Curves and Generalized Oscillator Strength for Doubly Excited States of the Hydrogen Molecule

SynopsisIn this work we report calculations of potential energy curves in the 1.2a.u. ≤ R ≤ 100a.u. range at the Multireference Configuration Interaction (MRCI) level for doubly excited states of the H2 molecule. The molecular state which dissociates into two H(2s) atoms is clearly identified. The Generalized Oscillator Strength as a function of transferred momentum for three doubly excited states is also presented.

Formation of molecular ions by radiative association of cold trapped atoms and ions

Radiative emission during cold collisions between trapped laser-cooled Rb atoms and alkaline-earth ions (Ca+, Sr+, Ba+) and Yb+, and between Li and Yb+, are studied theoretically, using accurate effective core potential based quantum chemistry calculations of potential energy curves and transition dipole moments of the related molecular ions. Radiative association of molecular ions is predicted to occur for all systems with a cross section two to ten times larger than the radiative charge transfer one. Partial and total rate constants are also calculated and compared to available experiments. Narrow shape resonances are expected, which could be detectable at low temperature with an experimental resolution at the limit of the present standards. Vibrational distributions are also calculated, showing that the final molecular ions are not created in their ground state level.

Helium-droplet-assisted preparation of cold RbSr molecules.

We present a combined experimental and theoretical study of the RbSr molecule. The experimental approach is based on the formation of RbSr molecules on helium nanodroplets. Utilizing two-photon ionization spectroscopy, an excitation spectrum ranging from 11,600 up to 23,000 cm(-1) was recorded. High level ab initio calculations of potential energy curves and transition dipole moments accompany the experiment and facilitate an assignment of transitions. We show that RbSr molecules desorb from the helium droplets upon excitation, which enables dispersed fluorescence spectroscopy of free RbSr. These spectra elucidate X(2)Σ(+) ground and excited state properties. Emission spectra originating from states corresponding to the Rb(5s(2)S) + Sr(5s5p(3)P) asymptote were recorded; spin-orbit coupling was included for the simulation. The results should provide a good basis for achieving the formation of this molecule in cold collisions, thus offering intriguing prospects for ultracold molecular physics.