Analytical Potential Energy Surface Research Articles

A molecular dynamics simulation of hydrogen atoms collisions on an H-preadsorbed silica surface

The interaction of hydrogen atoms and molecules with a silica surface is relevant for many research and technological areas. Here, the dynamics of hydrogen atoms colliding with an H-preadsorbed β-cristobalite (0 0 1) surface has been studied using a semiclassical collisional method in conjunction with a recently developed analytical potential energy surface based on density functional theory (DFT) calculations. The atomic recombination probability via an Eley–Rideal (E–R) mechanism, as well as the probabilities for other competitive surface processes, have been determined in a broad range of collision energies (0.04–3.0 eV) for off-normal (θv = 45°) and normal (θv = 0°) incidence and for two different surface temperatures (TS = 300 and 1000 K). H2,gas molecules form in roto-vibrational excited levels while the energy transferred to the solid surface is below 10% for all simulated conditions. Finally, the global atomic recombination coefficient (γE–R) and vibrational state resolved recombination coefficients (γ(v)) were calculated and compared with the available experimental values. The calculated collisional data are of interest in chemical kinetics studies and fluid dynamics simulations of silica surface processes in H-based low-temperature, low-pressure plasmas.

Coinage-metal mediated ring opening of cis-1,2-dimethoxycyclopropane: trends from the gold, copper, and silver Fischer carbene bond strength.

N-heterocyclic carbene (NHC) supported coinage metal cations proved to react in the gas phase with the electron-rich cis-1,2-dimethoxycyclopropane. Upon Collision Induced Dissociation (CID), several spectrometric fragment-ion signals were observed, one corresponding to the recovery of the bare cation IMes-M(+) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and the second to the methoxymethylidene metal complex IMes-M-[HCOCH3](+). The gold and copper complexes appear to stabilize the carbene sufficiently enough to promote the latter channel. On the contrary, the silver complex binds weakly to the methoxymethylidene moiety as observed by the predominance of the bare cation IMes-M(+) channel. Density Functional Theory (DFT) investigations of the Potential Energy Surface and Bond Energy Decomposition Analyses provided results that correlate well with the experimental data. In the case of the bare cation channel, two distinct reaction pathways were found: a straightforward decoordination of the cyclopropane and a cationic rearrangement of the three-membered ring into a dimethoxypropylene isomer before dissociation. However, for the abstraction of the methoxymethylidene moiety by the metal cation, only one pathway was found. In analogy to earlier studies by other groups, we found the trend Au > Cu > Ag for the metal-carbene bond strength.

Sliding Properties of MoS2 Layers: Load and Interlayer Orientation Effects

Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale.

Analytical Morse/long-range model potential and predicted infrared and microwave spectra for a symmetric top-atom dimer: a case study of CH₃F-He.

Four-dimensional ab initio intermolecular potential energy surfaces (PESs) for CH3F-He that explicitly incorporates dependence on the Q3 stretching normal mode of the CH3F molecule and are parametrically dependent on the other averaged intramolecular coordinates have been calculated. Analytical three-dimensional PESs for v3(CH3F) = 0 and 1 are obtained by least-squares fitting the vibrationally averaged potentials to the Morse/Long-Range potential function form. With the 3D PESs, we employ Lanczos algorithm to calculate rovibrational levels of the dimer system. Following some re-assignments, the predicted transition frequencies are in good agreement with experimental microwave data for ortho-CH3F, with the root-mean-square deviation of 0.042 cm(-1). We then provide the first prediction of the infrared and microwave spectra for the para-CH3F-He dimer. The calculated infrared band origin shifts associated with the ν3 fundamental of CH3F are 0.039 and 0.069 cm(-1) for para-CH3F-He and ortho-CH3F-He, respectively.

Quasi-classical trajectory study of the vibrational and translational effects on the O((3)P) + CD4 reaction.

The effects of vibrational excitation and translational energy, connected to mode selectivity and Polanyi's rules, are important issues in dynamics studies. To analyze these effects on the O((3)P) + CD4 reaction, an exhaustive dynamics study was performed using quasi-classical trajectory calculations on a full-dimensional analytical potential energy surface. The independent excitation of the C-D symmetric or asymmetric stretch modes leads to reactions with similar reaction cross sections and product scattering distributions, mode selectivity being discarded. Finally, translational energy raises the reactivity more effectively than an equal amount of energy in vibration, thus indicating that for this "central barrier" reaction it is not clear how to apply the venerable Polanyi's rules. The strong coupling between vibrational modes is responsible for this behavior, which seems to be the general tendency in polyatomic systems.

Theoretical kinetics study of the O(³P) + CH₄/CD₄ hydrogen abstraction reaction: the role of anharmonicity, recrossing effects, and quantum mechanical tunneling.

Using a recently developed full-dimensional accurate analytical potential energy surface [Gonzalez-Lavado, E., Corchado, J. C., and Espinosa-Garcia, J. J. Chem. Phys. 2014, 140, 064310], we investigate the thermal rate coefficients of the O((3)P) + CH4/CD4 reactions with ring polymer molecular dynamics (RPMD) and with variational transition-state theory with multidimensional tunneling corrections (VTST/MT). The results of the present calculations are compared with available experimental data for a wide temperature range 200-2500 K. In the classical high-temperature limit, the RPMD results match perfectly the experimental data, whereas VTST results are smaller by a factor of 2. We suggest that this discrepancy is due to the harmonic approximation used in the present VTST calculations, which leads to an overestimation of the variational effects. At low temperatures the tunneling plays an important role, which is captured by both methods, although they both overestimate the experimental values. The analysis of the kinetic isotope effects shows a discrepancy between both approaches, with the VTST values smaller by a factor about 2 at very low temperatures. Unfortunately, no experimental results are available to shed any light on this comparison, which keeps it as an open question.

Quantum chemical exploration of the intramolecular hydrogen bond interaction in 2-thiazol-2-yl-phenol and 2-benzothiazol-2-yl-phenol in the context of excited-state intramolecular proton transfer: A focus on the covalency in hydrogen bond

The present work demonstrates a computational exploration of the intramolecular H-bond (IMHB) interaction in two model heterocyclic compounds – 2-thiazol-2-yl-phenol (2T2YP) and 2-benzothiazol-2-yl-phenol (2B2YP) by meticulous application of various quantum chemical tools. Major emphasis is rendered on the analysis of IMHB interaction by calculation of electron density ρ(r) and Laplacian ∇2ρ(r) at the bond critical point using the Atoms-In-Molecule methodology. Topological features based on ρ(r) suggest that at equilibrium geometry the IMHB interaction develops certain characteristics typical of a covalent interaction. The interplay between aromaticity and Resonance-Assisted H-Bond (RAHB) has also been discussed using both geometrical and magnetic criteria. The occurrence of IMHB interaction in 2T2YP and 2B2YP has also been criticized under the provision of the Natural Bond Orbital (NBO) analysis. The ESIPT phenomenon in the molecular systems is also critically addressed on the lexicon of potential energy surface (PES) analysis.

A joint theoretical and experimental study of 1-acetylpiperazine: Conformational stability, infrared and Raman spectra

Infrared and Raman spectra of 1-acetylpiperazine (1-ap) have been recorded in the region of 4000–40cm−1. The conformational isomers, optimized geometric parameters, normal mode frequencies and corresponding vibrational assignments of 1-ap (C6H12N2O) have been examined by density functional theory (DFT), with the Becke-3–Lee–Yang–Parr (B3LYP) functional and the 6-31++G(d,p) basis set. Reliable conformational investigation and vibrational assignments have been performed by the potential energy surface (PES) and potential energy distribution (PED) analyses, respectively. Computations are carried out in both gas phase and solution using benzene and methanol. There is a good agreement between the theoretically predicted structural parameters and vibrational frequencies and those obtained experimentally. The normal chair conformation with equatorial substituents is not preferred due to the steric interaction.

The hydrogen abstraction reaction O(3P) + CH4: A new analytical potential energy surface based on fit toab initiocalculations

Based exclusively on high-level ab initio calculations, a new full-dimensional analytical potential energy surface (PES-2014) for the gas-phase reaction of hydrogen abstraction from methane by an oxygen atom is developed. The ab initio information employed in the fit includes properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, points on the reaction path, and points on the reaction swath, taking especial caution respecting the location and characterization of the intermediate complexes in the entrance and exit channels. By comparing with the reference results we show that the resulting PES-2014 reproduces reasonably well the whole set of ab initio data used in the fitting, obtained at the CCSD(T) = FULL/aug-cc-pVQZ//CCSD(T) = FC/cc-pVTZ single point level, which represents a severe test of the new surface. As a first application, on this analytical surface we perform an extensive dynamics study using quasi-classical trajectory calculations, comparing the results with recent experimental and theoretical data. The excitation function increases with energy (concave-up) reproducing experimental and theoretical information, although our values are somewhat larger. The OH rotovibrational distribution is cold in agreement with experiment. Finally, our results reproduce experimental backward scattering distribution, associated to a rebound mechanism. These results lend confidence to the accuracy of the new surface, which substantially improves the results obtained with our previous surface (PES-2000) for the same system.

Cu + NO scattering quantum dynamics

The interaction of copper atoms and nitric oxide radicals is described in terms of global, analytical potential energy surfaces derived from ab initio calculations published previously (Krishna and Marquardt, J. Chem. Phys. 136 (2012) 244303). These surfaces are used here for wave packet calculations within the Multi-Configurational Time Dependent Hartree (MCTDH) program to simulate the collision dynamics of Cu with NO. The feasibility of such calculations involving heavy elements is shown. Results indicate the existence of a rather long lived intermediate complex. Lifetimes of this complex as well as rotational distributions of scattered particles are discussed. At the highest level of theory used, vibrational term values for bound CuNO states agree well with available experimental data obtained from matrix isolation experiments.

Synthesis, molecular structure, FT-IR and XRD investigations of 2-(4-chlorophenyl)-2-oxoethyl 2-chlorobenzoate: A comparative DFT study

2-(4-Chlorophenyl)-2-oxoethyl 2-chlorobenzoate has been synthesized, its structural and vibrational properties have been reported using FT-IR and single-crystal X-ray diffraction (XRD) studies. The conformational analysis, optimized geometric parameters, normal mode frequencies and corresponding vibrational assignments of the synthesized compound (C15H10Cl2O3) have been examined by means of Becke-3–Lee–Yang–Parr (B3LYP) density functional theory (DFT) method together with 6-31++G(d,p) basis set. Furthermore, reliable conformational investigation and vibrational assignments have been made by the potential energy surface (PES) and potential energy distribution (PED) analyses, respectively. Calculations are performed with two possible conformations. The title compound crystallizes in orthorhombic space group Pbca with the unit cell dimensions a=12.312(5)Å, b=8.103(3)Å, c=27.565(11)Å, V=2750.0(19)Å3. B3LYP method provides satisfactory evidence for the prediction of vibrational wavenumbers and structural parameters.

Theoretical Kinetics Study of the F(2P) + NH3 Hydrogen Abstraction Reaction

The hydrogen abstraction reaction of fluorine with ammonia represents a true chemical challenge because it is very fast, is followed by secondary abstraction reactions, which are also extremely fast, and presents an experimental/theoretical controversy about rate coefficients. Using a previously developed full-dimensional analytical potential energy surface, we found that the F + NH3 → HF + NH2 system is a barrierless reaction with intermediate complexes in the entry and exit channels. In order to understand the reactivity of the title reaction, thermal rate coefficidents were calculated using two approaches: ring polymer molecular dynamics and quasi-classical trajectory calculations, and these were compared with available experimental data for the common temperature range 276-327 K. The theoretical results obtained show behavior practically independent of temperature, reproducing Walther-Wagner's experiment, but in contrast with Persky's more recent experiment. However, quantitatively, our results are 1 order of magnitude larger than those of Walther-Wagner and reasonably agree with the Persky at the lowest temperature, questioning so Walther-Wagner's older data. At present, the reason for this discrepancy is not clear, although we point out some possible reasons in the light of current theoretical calculations.

Dynamics of the O(3P) + CH4 hydrogen abstraction reaction at hyperthermal collision energies

Motivated by recent experiments on the title reaction at the high collision energy of 64 kcal mol(-1) reported by Minton et al., a detailed dynamics study was carried out using quasi-classical trajectory (QCT) calculations based on an analytical potential energy surface recently developed by our group, PES-2014. Our results reproduce the experimental evidence: most of the available energy appears as translational energy (80 ± 10%) and scattering distribution is forward, suggesting a stripping mechanism associated with high impact parameters. Of special interest is the triple (angle-velocity) differential cross section (a combination of translational and scattering distributions), which shows the same structure associated with the products. Agreement with experiment lends confidence to the new PES-2014 surface; this is encouraging, furthermore, because its fitting was made with thermal behaviour in mind, and higher energy areas were neither sampled nor weighted sufficiently.

Interpolation of Potential Energy Surfaces for Nonadiabatic Simulations of Biological Systems

Simulations of photophysical processes in biological systems are essential for understanding the role of molecular environment regarding the utilization of the light energy. However, such simulations are challenging in part due to the associated high computational costs, especially with the often adopted ab initio calculations of excited state potential energy information. Here, an application of a scheme for interpolating diabatic Hamiltonian matrix will be presented. We will show that this method is applicable for a sizeable molecular system (with 21 atoms) with a capability of describing the conical intersection properly and with an extensibility to condensed phase situations, by testing it with a model analytic potential energy surface of the green fluorescent protein chromophore. A procedure for using high-level quantum chemical calculations in the interpolation of diabatic Hamiltonian will also be presented together with the results of trajectory hopping simulations. Finally, we will discuss how this approach can be applied to computational studies of other photobiological systems.

Quantum Dynamical Study of the He + NeH+ Reaction on a New Analytical Potential Energy Surface

An analytical potential energy surface (PES) for the ground state of the [HeHNe](+) system has been constructed from a set of 19,605 ab initio data points, obtained from coupled cluster singles and doubles with perturbative triples correction calculations and the aug-cc-pVQZ basis set. The PES is based on the many-body expansion form proposed by Aguado and Paniagua (J. Chem. Phys. 1992, 96, 1265), and it has a root-mean-square error of 0.03 kcal/mol. The minimum energy pathways (MEPs) for different Ne-H-He angles are calculated, and it is found that the MEP for 180° (linear) goes through the deepest potential energy well. Preliminary quantum dynamical studies are performed for the He + NeH(+) (v = 0-2, j = 0-3) → HeH(+) + Ne reaction in the 0.0-0.5 eV collision energy range. Quantum calculations are carried out using a time-dependent wave packet method within the centrifugal sudden approximation. Reaction probabilities exhibit strong oscillatory behavior arising because of the metastable [HeHNe](+). Vibrational excitation has been found to enhance the reaction cross sections.

Structural, conformational, vibrational and quantum chemical analysis on 4-aminobenzohydrazide and 4-hydroxybenzohyrazide – A comparative study

Experimental and theoretical quantum chemical studies were carried out on 4-hydroxybenzohyrazide (4HBH) and 4-aminobenzohydrazide (4ABH) using FTIR and FT-Raman spectral data. The structural characteristics and vibrational spectroscopic analysis were carried performed by quantum chemical methods with the hybrid exchange-correlation functional B3LYP using 6-31G**, 6-311++G** and aug-cc-pVDZ basis sets. The most stable conformer of the title compounds have been determined from the analysis of potential energy surface. The stable molecular geometries, electronic and thermodynamic parameters, IR intensities, harmonic vibrational frequencies, depolarisation ratio and Raman intensities have been computed. Molecular electrostatic potential and frontier molecular orbitals were constructed to understand the electronic properties. The potential energy distributions (PEDs) were calculated to explain the mixing of fundamental modes. The theoretical geometrical parameters and the fundamental frequencies were compared with the experimental. The interactions of hydroxy and amino group substitutions on the characteristic vibrations of the ring and hydrazide group have been analysed.

Analytic Morse/long-range potential energy surfaces and predicted infrared spectra for CO–H2 dimer and frequency shifts of CO in (para-H2)N N = 1–20 clusters

A five-dimensional ab initio potential energy surface (PES) for CO-H2 that explicitly incorporates dependence on the stretch coordinate of the CO monomer has been calculated. Analytic four-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for vCO = 0 and 1 to the Morse/long-range potential function form. These fits to 30,206 points have root-mean-square (RMS) deviations of 0.087 and 0.082 cm(-1), and require only 196 parameters. The resulting vibrationally averaged PESs provide good representations of the experimental infrared data: for infrared transitions of para H2-CO and ortho H2-CO, the RMS discrepancies are only 0.007 and 0.023 cm(-1), which are almost in the same accuracy as those values of 0.010 and 0.018 cm(-1) obtained from full six-dimensional ab initio PESs of V12 [P. Jankowski, A. R. W. McKellar, and K. Szalewicz, Science 336, 1147 (2012)]. The calculated infrared band origin shift associated with the fundamental of CO is -0.179 cm(-1) for para H2-CO, which is the same value as that extrapolated experimental value, and slightly better than the value of -0.176 cm(-1) obtained from V12 PESs. With these potentials, the path integral Monte Carlo algorithm and a first order perturbation theory estimate are used to simulate the CO vibrational band origin frequency shifts of CO in (para H2)N-CO clusters for N = 1-20. The predicted vibrational frequency shifts are in excellent agreement with available experimental observations. Comparisons are also made between these model potentials.

Topology of conical/surface intersections among five low-lying electronic states of CO2: Multireference configuration interaction calculations

Multi-reference configuration interaction with single and double excitation method has been utilized to calculate the potential energy surfaces of the five low-lying electronic states (1)A1, (1)A2, (3)A2, (1)B2, and (3)B2 of carbon dioxide molecule. Topology of intersections among these five states has been fully analyzed and is associated with double-well potential energy structure for every electronic state. The analytical potential energy surfaces based on the reproducing kernel Hilbert space method have been utilized for illustrating topology of surface crossings. Double surface seam lines between (1)A1 and (3)B2 states have been found inside which the (3)B2 state is always lower in potential energy than the (1)A1 state, and thus it leads to an angle bias collision dynamics. Several conical∕surface intersections among these five low-lying states have been found to enrich dissociation pathways, and predissociation can even prefer bent-geometry channels. Especially, the dissociation of O((3)P) + CO can take place through the intersection between (3)B2 and (1)B2 states, and the intersection between (3)A2 and (1)B2 states.

Infrared and Raman spectra, conformational stability and vibrational assignment of 1-formylpiperazine

Infrared and Raman spectra of 1-formylpiperazine (1-fp) have been recorded in the region of 4000–50cm−1. The conformational analysis, optimized geometric parameters, normal mode frequencies and corresponding vibrational assignments of 1-fp (C5H10N2O) have been examined by means of Becke-3–Lee–Yang–Parr (B3LYP) density functional theory (DFT) method together with 6-31++G(d,p) basis set. Furthermore, reliable conformational investigation and vibrational assignments have been made by the potential energy surface (PES) and potential energy distribution (PED) analyses, respectively. Calculations are carried out with the possible six conformational isomers of 1-fp, both in gas phase and in solution using benzene and methanol. The experimental and theoretical results indicate that B3LYP method provides satisfactory evidence for the prediction vibrational wavenumbers, and the normal chair conformation with equatorial substituents is not preferred due to the steric interaction.

Electronic Structures and Spin Density Distributions of BrO2 and (HO)2BrO Radicals. Mechanisms for Avoidance of Hypervalency and for Spin Delocalization and Spin Polarization

The results are reported of an ab initio study of bromine dioxide BrO2, 1, and of the T-shaped trans- and cis-dihydroxides 2 and 3 of dihydrogen bromate (HO)2BrO. The thermochemistry has been explored of potential synthetic routes to (HO)2BrO involving water addition to BrO2, hydroxyl addition to bromous acid HOBrO, 4, protonation/reduction of bromic acid HOBrO2, 5, via tautomers 6-8 of protonated bromic acid, and by reduction/protonation of bromic acid via radical anion [HOBrO2](-), 9. The potential energy surface analyses were performed at the MP2(full)/6-311G* level (or better) and with the consideration of aqueous solvation at the SMD(MP2(full)/6-311G*) level (or better), and higher-level energies were computed at levels up to QCISD(full,T)/6-311++G(2df,2pd)//MP2. The addition of RO radical to bromous acid or bromite esters and the reduction of protonated bromic acid or protonated bromate esters are promising leads for possible synthetic exploration. Spin density distributions and molecular electrostatic potentials were computed at the QCISD(full)/6-311G*//MP2(full)/6-311G* level to characterize the electronic structures of 1-3. Both radicals employ maximally occupied (pseudo) π-systems to transfer electron density from bromine to the periphery. While the formation of the (3c-5e) π-system suffices to avoid hypervalency in 1, the formation of the (4c-7e) π-system in 2 or 3 still leaves the bromine formally hypervalent and (HO)2BrO requires delocalization of bromine density into σ*-SMOs over the trans O-Br-O moiety. Molecular orbital theory is employed to describe the mechanisms for the avoidance of hypervalency and for spin delocalization and spin polarization. The (4c-7e) π-system in 2 is truly remarkable in that it contains five π-symmetric spin molecular orbitals (SMO) with unique shapes.