Two-dimensional Potential Energy Surface Research Articles

Excited-state double proton transfer of 1,8-dihydroxy-2-naphthaldehyde: A MS-CASPT2//CASSCF study

Excited-state double proton transfer (ESDPT) is a controversial issue which has long been plagued with theoretical and experimental communities. Herein, we took 1,8-dihydroxy-2-naphthaldehyde (DHNA) as a prototype and used combined complete active space self-consistent field (CASSCF) and multi-state complete active-space second-order perturbation (MS-CASPT2) methods to investigate ES-DPT and excited-state deactivation pathways of DHNA. Three different tautomer minima of S1-ENOL, S1-KETO-1, and S1-KETO-2 and two crucial conical intersections of S1S0-KETO-1 and S1S0-KETO-2 in.and between the S0 and S1 states were obtained. S1-KETO-1 and S1-KETO-2 should take responsibility for experimentally observing dual-emission bands. In addition, two-dimensional potential energy surfaces (2D-PESs) and linear interpolated internal coordinate paths connecting relevant structures were calculated at the MS-CASPT2//CASSCF level and confirmed a stepwise ESDPT mechanism. Specifically, the first proton transfer from S1-ENOL to S1-KETO-1 is barrierless, whereas the second one from S1-KETO-1 to S1-KETO-2 demands a barrier of ca. 6.0 kcal/mol. The linear interpolated internal coordinate path connecting S1-KETO-1 (S1-KETO-2) and S1S0-KETO-1 (S1S0-KETO-2) is uphill with a barrier of ca. 12.0 kcal/mol, which will trap DHNA in the S1 state while therefore enabling dual-emission bands. On the other hand, the S1/S0 conical intersections would also prompt the S1 system to decay to the S0 state, which could be to certain extent suppressed by locking the rotation of the C5−C8−C9−O10 dihedral angle. These mechanistic insights are not only helpful for understanding ESDPT but also useful for designing novel molecular materials with excellent photoluminescent performances.

New de-excitation rates of the open shell dicarbon monoxide C2O(X3Σ−) in collision with He(1S)

Abstract In order to derive accurate physical conditions of the interstellar clouds (like TMC-1) from the rotational transitions of C2O, collisional rates are needed. Based on a new two-dimensional potential energy surface (2D-PES), we report a new rate coefficients calculation of C2O(X3Σ−) induced by collision with He. This PES was obtained from the explicitly correlated restricted open-shell coupled cluster with single, double and perturbative triple excitation (rccsd(t)-f12) ab initio approach associated with aug-cc-pVTZ basis sets. The C2O–He PES presents one minimum below its dissociation limit with a well depth of 42.376 cm−1. The close-coupling quantum time independent formalism was used to derive cross sections for E ≤ 370 cm−1 and N ≤ 10. De-excitation rate coefficients are obtained for thermal temperature below 80 K.

Experimental and Theoretical Examination of the Kinetic Isotope Effect in Cytochrome P450 Decarboxylase OleT.

Using a combination of experimental studies, theory, simulation, and modeling, we investigate the hydrogen atom transfer (HAT) reaction by the high-valent ferryl cytochrome P450 (CYP) intermediate known as Compound I, a species that is central to innumerable and important detoxification and biosynthetic reactions. The P450 decarboxylase known as OleT converts fatty acids, a sustainable biological feedstock, into terminal alkenes and thus is of high interest as a potential means to produce fungible biofuels. Previous experimental work has established the intermediacy of Compound I in the C─C scission reaction catalyzed by OleT and an unprecedented ability to monitor the HAT process in the presence of bound fatty acid substrates. Here, we leverage the kinetic simplicity of the OleT system to measure the activation barriers for CYP HAT and the temperature dependence of the substrate 2H kinetic isotope effect. Notably, neither measurement has been previously accessible for a CYP to date. Theoretical analysis alludes to the significance of substrate fatty acid coordination for generating the hydrogen donor/acceptor configurations that are most conducive for HAT to occur. The analysis of the two-dimensional potential energy surface, based on multireference electronic wave functions, illustrates the uncoupled character of the hydrogen motion. Quantum dynamics calculations along the hydrogen reaction path demonstrate that hydrogen tunneling is essential to qualitatively capture the experimental isotope effect, its temperature dependence, and appropriate activation energies. Overall, a more fundamental understanding of the OleT reaction coordinate contributes to the development of biomimetic catalysts for controlled C─H bond activation, an outstanding current challenge for (bio)synthetic chemistry.

On the Role of Molecular Polarizability in Positron Coupling to Vibrations in Homonuclear Diatomics

In a previous work [Poveda, Varella, and Mohallem (Poveda et al., Atoms, 2021, 9: 64) it was shown that the bell-like shape of the 0 → 1 vibrational excitation cross section of H2 as a function of the incoming positron energy, with its characteristic sharp onset at threshold, can be accounted for by a simple model which couples the positron to the vibrational mode of the molecule, throught the behavior of the target polarizabitity with the internuclear bond distance. The study, carried out via time-dependent wave-packet dynamics propagation, relies on a two-dimensional potential energy surface involving just the scattering (positron-target) and vibrational (target) coordinates. Here the model is extended to the full three-dimensional configuration space of the positron-diatomic complex, with the cross sections computed within a time-independent close-coupling approach. The present results confirm the previous findings, shedding light on the mechanisms through which a low-energy positron couples to the molecular vibrations.

Spectroscopic and theoretical study of the intramolecular π-type hydrogen bonding and conformations of 3-cyclohexen-1-ol

The conformations of 3-cyclohexen-1-ol (3CHOL) have been investigated by MP2/cc-pVTZ and CCSD/cc-pVTZ ab initio computations and by infrared spectroscopy. The six conformational minima correspond to specific values of the ring-twisting and OH internal rotation coordinates. The map of a two-dimensional potential energy surface (PES) that governs the conformational dynamics along the ring-twisting and the OH group internal rotation coordinates was generated. The relative energies of the conformational minima and the interconversion barriers for the ring twisting and internal rotations were computed. According to the MP2/cc-pVTZ computations, the conformer with intramolecular π-type hydrogen bonding has the lowest calculated energy by 108 cm−1. However, according to the CCSD/cc-pVTZ computations this conformer a tiny 10 cm−1 higher in energy than one without hydrogen bonding. The infrared spectrum in the OH stretching region agrees well with the predicted frequency differences between the conformers.

Neural network approach to diffusion of B and N adatoms on the Pt(111) surface

Artificial neural network boasts an outstanding class in terms of its performance and efficiency and is being spread widely in most fields of physics. We report our investigation of the diffusions of boron and nitrogen adatoms on the Pt(111) surface by performing molecular dynamics simulations equipped with machine-learned potentials. Platinum is commonly used as a substrate for the growth of hexagonal boron nitride (h-BN) thin films, and the diffusion of B and N atoms on the substrate, which are decomposed from the precursor molecules, plays important roles in the initial stages of h-BN growth. The two-dimensional potential energy surfaces and the trajectories of the B and N adatoms are consistent with the DFT calculation. The Arrhenius plots of the diffusion coefficients produce the diffusion barriers of the B and N adatoms on Pt(111), which agree well with the DFT barriers.

3-Phospholene and 3-phospholene-1-d1

High level theoretical CCSD/cc-pVTZ calculations were carried out on 3-phospholene and its d1 derivative. These confirmed the microwave conclusion that the five-membered ring is puckered in the same direction as the PH group. The puckering angle was calculated to be 15.7°. A one-dimensional potential energy function (PEF) for the ring-puckering motion was computed, and this confirmed that the PEF has a single minimum. A two-dimensional potential energy surface (PES) for the ring-puckering and P-H inversion motions was also computed, and this agreed very well with the published PES based on far-infrared spectra.

Over-barrier crossing in multidimensional tunneling

Studies of the dissociative chemisorption of N2 using two-dimensional empirical potential energy surfaces that differ by the position of the saddle point, all show high tunneling probabilities at energies well below the saddle-point energy of the reaction. This is highly unusual for a heavy-atom system and contradicted by one-dimensional analysis along the minimum-energy path. A mechanism of over-barrier crossing, related to the high-momentum tail of the vibrational wave function of N2, is demonstrated. We analyze the two-dimensional wave-packet propagation and compares to the probability of having vibrational momenta that exceeds the value required to overcome the relevant barrier.

Excited state hydrogen atom transfer pathways in 2,7-diazaindole – S1-3 (S = H2O and NH3) clusters

The photoinduced tautomerization reactions via hydrogen atom transfer in the excited electronic state (ESHT) have been computationally investigated in 2,7-diazaindole (27DAI) – (H2O)1-3 and 27DAI – (NH3)1-3 isolated clusters to understand the role of various solvent wires. Two competing ESHT reaction pathways originating from the N(1)–H group to the neighbouring N(7) (R(1H-Sn-7H)) and N(2) (R(1H-Sn-2H)) atoms were rigorously examined for each system. Both one- and two-dimensional potential energy surfaces have been calculated in the excited state to investigate the pathways. The R(1H-Sn-7H) was found to be the dominant route with reaction barriers ranging from 26–40 kJmol−1 for water clusters, and 14–26 kJmol−1 for ammonia clusters. The barrier heights for ammonia clusters were found to be nearly half of the that observed for the water systems. The lengthening of the solvent chain up to two molecules resulted in a drastic decrease in the barrier heights for R(1H-Sn-7H). The barriers of the competing reaction channel R(1H-Sn-2H) were found to be significantly higher (31–127 kJmol−1) but were observed to be decreasing with the lengthening of the solvent wire as in the R(1H-Sn-7H) pathway. In both the reactions, the angle strain present in the transition state structures was dependent upon the solvent chain's length and was most likely the governing factor for the barrier heights in each solvent cluster. The results have also affirmed that the ammonia molecule is a better candidate for hydrogen transfer than water because of its higher gas-phase basicity. The results delineated from this investigation can pave the way to unravel the excited-state hydrogen atom transfer pathways in novel N–H bearing molecules.

Vibrational Excitation Cross-Section by Positron Impact: A Wave-Packet Dynamics Study

The vibrational excitation cross-section of a diatomic molecule by positron impact is obtained using wave-packet propagation techniques. The dynamics study was carried on a two-dimensional potential energy surface, which couples a hydrogenlike harmonic oscillator to a positron via a spherically symmetric correlation polarization potential. The cross-section for the excitation of the first vibrational mode is in good agreement with previous reports. Our model suggests that a positron couples to the target vibration by responding instantly to an interaction potential, which depends on the target vibrational coordinate.

Electric-field-induced wave-packet dynamics and geometrical rearrangement of trilobite Rydberg molecules

We investigate the quantum dynamics of ultra-long-range trilobite molecules exposed to homogeneous electric fields. A trilobite molecule consists of a Rydberg atom and a ground-state atom, which is trapped at large internuclear distances in an oscillatory potential due to scattering of the Rydberg electron off the ground-state atom. Within the Born-Oppenheimer approximation, we derive an analytic expression for the two-dimensional adiabatic electronic potential energy surface in weak electric fields valid up to 500 V/m. This is used to unravel the molecular quantum dynamics employing the Multi-Configurational Time-Dependent Hartree method. Quenches of the electric field are performed to trigger the wave packet dynamics including the case of field inversion. Depending on the initial wave packet, we observe radial intra-well and inter-well oscillations as well as angular oscillations and rotations of the respective one-body probability densities. Opportunities to control the molecular configuration are identified, a specific example being the possibility to superimpose different molecular bond lengths by a series of periodic quenches of the electric field.

Spectroscopic and Theoretical Study of the Intramolecular π-Type Hydrogen Bonding and Conformations of 2-Cyclopenten-1-ol.

The conformations of 2-cyclopenten-1-ol (2CPOL) have been investigated by high-level theoretical computations and infrared spectroscopy. The six conformational minima correspond to specific values of the ring-puckering and OH internal rotation coordinates. The conformation with the lowest energy possesses intramolecular π-type hydrogen bonding. A second conformer with weaker hydrogen bonding has somewhat higher energy. Ab initio coupled-cluster theory with single and double excitations (CCSD) was used with the cc-pVTZ (triple-ζ) basis set to calculate the two-dimensional potential energy surface (PES) governing the conformational dynamics along the ring-puckering and internal rotation coordinates. The two conformers with the hydrogen bonding lie about 300 cm−1 (0.8 kcal/mole) lower in energy than the other four conformers. The lowest energy conformation has a calculated distance of 2.68 Å from the hydrogen atom on the OH group to the middle of the C=C double bond. For the other conformers, this distance is at least 0.3 Å longer. The infrared spectrum in the O-H stretching region agrees well with the predicted frequency differences between the conformers and shows the conformers with the hydrogen bonding to have the lowest values. The infrared spectra in other regions arise mostly from the two hydrogen-bonded species.

Cross-section investigation of MgOH (X 2 Σ + ) fine-structure excitation by helium

Inelastic fine rotational cross-sections of the magnesium monohydroxide (MgOH) molecule by collision with helium (He) atoms are investigated. We illustrate the two-dimensional potential energy surface (2D-PES) for the MgOH–He interacting system. The ab initio 2D-PES is computed using the restricted coupled cluster approach with single, double, and perturbative triple excitation connected to an augmented-correlation consistent-polarized valence quadruple zeta Gaussian basis set. One minimum located at (θ = 0°; R = 8.25 bohr) on the side of the hydrogen atom is observed. Inelastic cross-sections are investigated for the 13 first fine rotational levels for a total energy up to 100 cm−1. Propensity rules toward odd ΔN and ΔN = Δj are found.

Rotational excitation of C2H- anion in collision with H2.

The discovery of anions in the interstellar medium has shown that they are very reactive species. This gave them great importance in the modeling of the chemical and astrophysical evolution of the interstellar medium. The detection of the first anion C6H− followed by the other anions C4H−, C8H− and CN− in the interstellar medium has encouraged research on other detectable anions. The C2H− anion was observed for the first time in the circumstellar envelope of IRC+10216 and in TMC-1. In these cold and low-density regions, precise modeling of the chemical and physical conditions of the observed emission lines requires knowledge of the radiative and collisional excitation rates. We present here the first new two-dimensional Potential Energy Surface (PES) for C2H–H2 interaction. Rotational excitation of the anion by collision with para-H2(jH2 = 0) is investigated. The PES is obtained in the super-molecular approach based on a single and double excitation coupled cluster method with perturbative contributions from triple excitations (CCSD(T)). In all our calculations, all atoms were described using the augmented correlation-consistent triple zeta (aug-cc-pVTZ) basis sets and bond f unctions. Fully-quantum close-coupling calculations of inelastic integral cross sections are done on a grid of collision energies large enough to ensure converged state-to-state rate coefficients for the 16 first rotational levels of C2H− and for temperatures ranging from 5 to 120 K. For this collisional system, rate coefficients exhibit a strong propensity in favor of even Δj transitions.

Sodium isocyanide–Helium potential energy surface and astrophysical applications

In this work, we generated the two-dimensional potential energy surface (2D-PES) of the l-NaNC–He van der Waals interacting system. The electronic computations are done at the CCSD(T)-F12/aug-cc-pVTZ level. This 2D-PES exhibits a unique shallow minimum for helium approaching l-NaNC from the Na atom side. Afterward, we derived an analytical form for this ab initio 2D-PES, which is incorporated into quantum scattering code to compute the rotational (de-)excitation cross sections (rate coefficients) of l-NaNC by collision with He atom for total energies up to 2000 cm−1 (from 5 to 300 K). The propensity rules that favor Δj = − 1 transitions are less pronounced than in the case of t-NaCN isomer. Our data may be used for the determination of the abundances, in astrophysical media, of the candidate isomer l-NaNC with respect to those of the already detected t-NaCN isomer. We also performed comparison with the astrophysical relevant Al- and Mg-bearing (iso)cyanide systems interacting with He.

Low-temperature rate constants and radiative transfer for rotational de-excitation of C5S by collision with He

ABSTRACT The C5S molecule is the largest member of the series of sulphur-containing carbon chains CnS observed in space. Given the lack of data concerning this molecule, we computed rate coefficients of C5S(1Σ+) induced by collision with He. These rates are obtained for thermal temperature below 100 K by mean of a new two-dimensional potential energy surface (PES) calculated with the explicit correlated coupled cluster with single, double, and pertubative triple excitation (ccsd(t)-f12) ab initio approach and the aug-cc-pVTZ basis sets. The C5S–He PES presents three minimums of −59.726, −55.355, and −36.506 cm−1 below its dissociation limit. Using this PES, the integral cross-sections are performed in the close-coupling (CC) and coupled-state (CS) quantum time independent formalisms for $E_\mathrm{ c}\le 500 \, \mathrm{ cm}^{-1}$ and J ≤ 13 (for CC) and J ≤ 50 (for CS). By averaging these cross-sections we obtained the downward rate coefficients. The new collisional data are used to simulate the excitation of C5S in the circumstellar gas. We obtain the excitation and brightness temperatures of the four lines observed towards the IRC+10216 which confirms the necessity of using radiative transfer calculations to accurately determine C5S abundance since the local thermodynamic equilibrium conditions are not fulfilled. The new collisional data should help to estimate the abundance of C5S in several interstellar regions.

How fluxional reactants limit the accuracy/efficiency of infrequent metadynamics

In an infrequent metadynamics (iMetaD) simulation, a well-tempered metadynamics bias accumulates in the reactant basin, accelerating escapes to the product state. Like the earlier hyperdynamics strategy, iMetaD enables estimates of the unbiased escape rates. However, iMetaD applies the bias to visited locations in a collective variable (CV) space, not to the more specific visited locations in a full configuration space as done in hyperdynamics. This difference makes rate estimates from iMetaD sensitive to the choice of CVs, to parameters that control the bias deposition rate, and to the preparation of the initial state within the reactant basin. This paper uses an extremely simple discrete state model to illustrate complications that can arise in systems that exhibit fluxional transitions between sub-basins of the reactant state. Specifically, we show how the reactant-to-product escape time and relaxation times within the reactant basin(s) impose bounds on the admissible parameter choices for an iMetaD calculation. Predictions from the discrete state model are validated by iMetaD simulations on a corresponding two-dimensional potential energy surface.

Weak bonding of the hydrogen molecule by the S-state lanthanide ions Eu+, Yb+ and Lu+ from ab initio calculations

The complexes formed by the singly-charged S-state lanthanide ions Eu+, Yb+ and Lu+ with the hydrogen molecule are investigated using the ab initio coupled cluster CCSD(T) method with effective core potential and all-electron descriptions of lanthanides. Equilibrium structures, two-dimensional interaction potential energy surfaces and variational energies of the lowest rovibrational energy levels are obtained. Three complexes considered are qualitatively similar and exhibit weak bonding and perturbation of hydrogen fragment, as well as strong vibrational anharmonicity.

Quantum dynamical study of rotational excitation of CO+ (X2Σ+) by He collisions

The knowledge of accurate rate coefficients for collisional excitation of molecules by the abundant species like He, H2 and H are required for modeling the atmospheric conditions of interstellar medium. The present paper aims to study the transitions in the lowest twelve rotational levels in the ground vibrational state of CO+(X2Σ+) by collisions with helium atoms. We computed ab initio two-dimensional potential energy surface of 2Σ+/2A' symmetry for the He-CO+ van der Waals complex asymptotically correlating to He(1S) + CO+(X2Σ+) using restricted coupled-cluster method with perturbative treatment of triples employing augmented correlation consistent polarized valence quadruple-ζ basis set. Rovibrational bound states of He-CO+ complex for total angular momentum, J = 0 have been computed. The state-to-state rotational excitation cross sections are performed by close-coupling method up to center of mass collision energy of 2000 cm−1. The corresponding rate coefficients are obtained up to a temperature of 300 K.

The nature of intramolecular hydrogen bond in Naphthazarin

In this work, a feasible method has been introduced for investigating the proton tunneling frequency in Naphthazarin (NZ). A two-dimensional potential energy surface which couples OH stretching with in-plane bending modes has been computed for a firmed skeleton geometry, at the B3LYP/6-31G**, B3LYP/6-311G**, B3LYP/6-311++G** and MP2/6-31G** levels. This potential energy function is used to calculate the tunneling frequency, OH stretching, and in-plane bending frequencies. By calculations at the MP2/6-31G** level, the tunneling frequency of 15.3 and 0.9 cm−1 and the barrier height of 65.3 kJ/mol was obtained for light and deuterated NZ for the stepwise proton transfer, respectively. Also, the tunneling frequency of 0.8 and 0.0 cm−1 and the barrier height of 127.3 kJ/mol were obtained for the light and deuterated NZ for the concerted pathway, respectively. The obtained results indicate that both stepwise and concerted proton transfers pathways are probable.