Dimensional Potential Energy Surface Research Articles

Vibrations of a chelated proton in a protonated tertiary diamine

Vibrational spectra of the conjugate acid of Me(2)NCH(2)CH(2)CH(2)CH(2)NMe(2) (N,N,N',N'-tetramethylputrescine) have been examined in the gaseous and crystalline phases using Infrared Multiple Photon Dissociation (IRMPD) spectroscopy, Inelastic Neutron Scattering (INS), and high pressure Raman spectroscopy. A band observed near 530 cm(-1) is assigned to the asymmetric stretch of the bridging proton between the two nitrogens, based on deuterium substitution and pressure dependence. The NN distance measured by X-ray crystallography gives a good match to DFT calculations, and the experimental band position agrees with the value predicted from theory using a 2-dimensional potential energy surface. The reduced dimensionality potential energy surface, which treats the ion as though it possesses a linear NHN geometry, shows low barriers to proton transit from one nitrogen to the other, with zero point levels close to the barrier tops. In contrast, two other related systems containing strong hydrogen bonds do not exhibit the same spectroscopic signature of a low barrier hydrogen bond (LBHB). On the one hand, the IRMPD spectra of the conjugate acid ions of the amino acid N,N,N',N'-tetramethylornithine (in which the two nitrogens have different basicities) show fewer bands and no comparable isotopic shifts in the low frequency domain. On the other hand, the IRMPD spectrum of the shorter homologue Me(2)NCH(2)CH(2)CH(2)NMe(2) (N,N,N',N'-tetramethyl-1,3-propanediamine), for which the NHN bond angle deviates substantially from linearity, displays more than one band in the 1100-1400 cm(-1) domain, which vanish as a consequence of deuteration.

Theoretical study of the dynamics of Cl + O3 reaction I. Ab initio potential energy surface and quasiclassical trajectory results

We present a global full dimensional potential energy surface (PES) for the Cl + O(3)→ ClO + O(2) reaction, which is an elementary step in a catalytic cycle that leads to the destruction of ozone in the stratosphere. The PES is constructed by interpolation of quantum chemistry data using the method developed by Collins and co-workers. Ab initio data points (energy, gradients and Hessian matrix elements) have been calculated at the UQCISD/aug-cc-pVDZ (unrestricted quadratic configuration interaction with single and double excitations) level of theory. The ab initio calculations predict a markedly non-coplanar (dihedral angle of 80°) transition state for the reaction, located very early in the reactant valley and slightly below the energy of the reactants as long as the spin-orbit splitting is neglected. Quasiclassical trajectory (QCT) calculations have been carried out at several collision energies to investigate the reaction dynamics. The QCT excitation function shows no threshold, displays a minimum at a collision energy of 2.5 kcal mol(-1), and then increases monotonically at larger collision energies. This behaviour is consistent with a barrierless reaction dominated by an oxygen-abstraction mechanism. The calculated product vibrational distributions (strongly inverted for ClO) and rate constants are compared with experimental determinations. Differential cross sections (DCS) summed over all final states are found to be in fairly good agreement with those derived from crossed molecular beam experiments.

A new accurate and full dimensional potential energy surface of H5+ based on a triatomics-in-molecules analytic functional form

In this work a reliable full nine-dimensional potential energy surface for studying the dynamics of H(5)(+) is constructed, which is completely symmetric under any permutation of the nuclei. For this purpose, we develop a triatoms-in-molecules method as an extension of the more common diatoms-in-molecules one, which allows a very accurate description of the asymptotic regions by including correctly the charge-induced dipole and quadrupole interactions. Moreover, this treatment provides a semiquantitative description of all the topological features of the global potential compared with coupled cluster results. In particular, the hop of the proton between two H(2) fragments produces a double well in the potential. This resonant structure involving the five atoms produces a stabilization, lowering the barrier, and the triatoms-in-molecules yields to a barrier significantly higher than the ab initio results. Therefore, to improve the triatomics-in-molecules potential surface, two five-body terms are added, which are fitted to more than 110,000 coupled-cluster ab initio points. The global potential energy surface thus obtained in this work has an overall root mean square error of 0.079 kcal/mol for energies below 27 kcal/mol above the global well. The features of the potential are described and compared with previous available surfaces.

Vibrational spectra and geometry relaxation in core-electronic processes of N2O and CO2

Vibratinal spectra and geometry relaxation in the core-electron processes of N2O and CO2 have been studied by the high-resolution photoelectron spectroscopy and accurate ab initio calculations. We calculated reliable two dimensional potential energy surfaces of the core-hole states of N2O and CO2 and performed the vibrational analysis based on these surfaces. We also investigated the geometry changes in the π* and σ* core-excited states. Theory reproduced the details of the high-resolution spectra providing quantitative assignments and interpretations.

Dynamics simulation of N2 scattering onto W(100,110) surfaces: A stringent test for the recently developed flexible periodic London–Eyring–Polanyi–Sato potential energy surface

An efficient method to construct the six dimensional global potential energy surface (PES) for two atoms interacting with a periodic rigid surface, the flexible periodic London-Eyring-Polanyi-Sato model, has been proposed recently. The main advantages of this model, compared to state-of-the-art interpolated ab initio PESs developed in the past, reside in its global nature along with the small number of electronic structure calculations required for its construction. In this work, we investigate to which extent this global representation is able to reproduce the fine details of the scattering dynamics of N(2) onto W(100,110) surfaces reported in previous dynamics simulations based on locally interpolated PESs. The N(2)/W(100) and N(2)/W(110) systems are chosen as benchmarks as they exhibit very unusual and distinct dissociative adsorption dynamics although chemically similar. The reaction pathways as well as the role of dynamic trapping are scrutinized. Besides, elastic/inelastic scattering dynamics including internal state and angular distributions of reflected molecules are also investigated. The results are shown to be in fair agreement with previous theoretical predictions.

Dispersion dominated halogen–π interactions: energies and locations of minima

The interactions of halogen atoms with aromatic π-systems can be crucial for structural stability and ligand binding. However, many aspects of the nature and energetics of these interactions remain elusive. Therefore, we designed model systems mimicking the T-shaped complex of chloro- and bromobenzene with tyrosine as found, e.g., in serine protease-inhibitor complexes. Three dimensional potential energy surfaces (3D-PES) were calculated at a high level of theory, up to CCSD(T). On these 3D-PES the exact location of the minima and, even more important, the shape of favorable interaction regions were determined. We show that the height of the halogen atom above the tyrosine-ring is decisive, while the lateral position is of minor influence. Finally, a truncated harmonic potential is developed to modify the Amber/GAFF force field to shift the locations of the minima to the correct regions.

Newab initioPotential Energy Surfaces for Cl(2P3/2, 2P1/2)+H2 Reaction

New global three dimensional potential energy surfaces for the Cl+H2 reactive system have been constructed using accurate multireference configuration interaction calculations with a large basis set. The three lowest adiabatic potential energy surfaces correlating asymptotically with Cl(2P)+H2 have been transformed to a diabatic representation, which leads to a fourth coupling potential for non-linear geometries. In addition, the spin-orbit coupling surfaces have also been computed using the BreitPauli Hamiltonian. Properties of the new potential are described. Reaction dynamics based on the new potential agrees with the recent experimental results quite well.

Time-dependent quantum wave-packet description of H and D atom tunneling in N–H and N–D photodissociation of methylamine and methylamine-d2

The degree to which tunneling through a barrier in the N-H and N-D photodissociation channels of methylamine (CH3NH2) and its deuterated variant (CH3ND2), respectively, plays a role was investigated by time-dependent quantum wave-packet dynamics calculations. Two dimensional potential energy surfaces (PESs) of methylamine, presenting the N-H stretch and the HNC bend, were constructed employing multireference ab initio electronic-structure methods, allowing full description of the H motion on the HC-NH2 plane. The time-dependent Schrödinger equation was solved employing the Fourier method for calculating the Hamiltonian operation together with the Chebychev polynomial expansion of the evolution operator. The results show that tunneling and decay to vibrational resonant states on the first excited electronic PES are faster for the H atom than for the D. The decay into two of the resonant states found on the first PES strongly depends on the initially excited vibrational state on the ground electronic PES.

A Direct Molecular Orbital−Molecular Dynamics Study on the Diffusion of the Li Ion on a Fluorinated Graphene Surface

A direct molecular orbital−molecular dynamics (MO-MD) method has been applied to diffusion processes of the Li+ ion on a fluorinated graphene surface. A graphene sheet composed of C96F24 (denoted by F-graphene) was used as a model of the fluorinated graphene surface. The total energy and energy gradient on the full dimensional potential energy surface of the Li+C96F24 system were calculated at each time step in the trajectory calculation. The calculations were carried out at the AM1 level. Simulation temperatures were chosen in the range 200−1000 K. At low temperatures, below 200 K, the diffusion of lithium ion did not occur, and the ion vibrates around an equilibrium point. At around room temperature (∼300 K), the lithium ion diffused freely on the surface, but the ion did not approach to the edge region of the surface. This is due to the repulsive interaction with positively charged carbon atom connecting to the fluorine atom where the C−F bond is polarized as Cδ+−Fδ-. The repulsive interaction strongly...

Vibrational energy levels of methyl chloride calculated from full dimensional ab initio potential energy surface

The vibrational levels for two isotopic species of methyl chloride have been calculated in the region 0–3500 cm −1 from the ab initio potential energy surface (PES). The isotopic shift of vibrational levels of molecules 12CH 3 35Cl and 12CH 3 37Cl has been calculated. The correlation consistent basis sets cc-pVTZ and cc-pVQZ are employed to calculate energy values for 7957 and 3758 points correspondingly from a large domain of the nine-dimensional internal coordinate space. The analytic global PES is fitted with the standard deviation of 4.5 cm −1.

The theoretical prediction of infrared spectra of trans- and cis-hydroxycarbene calculated using full dimensional ab initio potential energy and dipole moment surfaces

Accurate infrared spectra of the two hydroxycarbene isomers are computed by diagonalizing the Watson Hamiltonian including up to four mode couplings using full dimensional potential energy and dipole moment surfaces calculated at the CCSD(T)/cc-pVTZ (frozen core) and CCSD6-311G(**) (all electrons correlated) levels, respectively. Anharmonic corrections are found to be very important for these elusive higher-energy isomers of formaldehyde. Both the energy levels and intensities of stretching fundamentals and all overtone transitions are strongly affected by anharmonic couplings between the modes. The results for trans-HCOHHCOD are in excellent agreement with the recently reported IR spectra, which validates our predictions for the cis-isomers.

Quantum scattering of SiS with H2: Potential energy surface and rate coefficients at low temperature

Rotational excitation of the interstellar species SiS with H2 is investigated. We present a new four dimensional potential energy surface for the SiS-H2 system. Both molecules were treated as rigid rotors. Potential was obtained from the electronic structure calculations using a single- and double-excitation coupled cluster method with perturbative contributions from connected triple excitations [CCSD(T)]. The four atoms were described using the aug-cc-pVTZ basis sets. Bond functions were placed at mid-distance between the SiS center of mass and the center of mass of H2 for a better description of the van der Waals interaction. Additionally, at seven characteristic geometries, we calculated perturbational components of the interaction energy using symmetry-adapted perturbation theory approach to explain the anisotropy of the potential energy surface. Coupled-state calculations of the inelastic integral cross sections of SiS in collisions with para-H2 and ortho-H2 were calculated at low energies. After Boltzmann thermal averaging, rate coefficients were obtained for temperatures ranging from 5 to 50 K. Significant differences exist between para- and ortho-H2 results. The strongest collision-induced rotational SiS transitions are the transitions with Deltaj=2 for collisions with para-H2 and the transitions with Deltaj=1 for collisions with ortho-H2.

Classical dynamics study of atomic oxygen sticking on the β-cristobalite (1 0 0) surface

The sticking of oxygen atoms with collision energies in the range 0.1–1 eV on a clean (1 0 0) β-cristobalite surface with surface temperatures between 300–1100 K has been investigated using classical trajectories for normal and off-normal incidence. A full dimensional adiabatic potential energy surface (PES) based on a dense grid of density functional theory (DFT) points was constructed by means of the corrugation reducing procedure. Sticking probabilities are very high (>0.9) for all conditions, increasing with collision energy and decreasing with surface temperature. This behavior can be interpreted by decomposing the sticking between adsorption and absorption, which show different trends. The large attractive character of the PES favors the absorption or penetration into the big unit cell of the β-cristobalite, with a predominant direct mechanism instead of a dynamic trapping, accordingly also with the quick dissipation of the oxygen energy into the slab. Calculated thermal initial sticking coefficients seem to depend on the type of silica structure. Moreover, these initial thermal coefficients are much higher than values derived using expressions obtained from standard transition state theory even when using as parameters values extracted from DFT calculations. Therefore, the use of these expressions in kinetic models for O or N recombination over silica should be reconsidered.

Rotational excitation of SiS molecules by collisions with He atoms

Context. Observations of molecular emission at millimeter and infrared wavelengths, supplemented by careful and detailed modeling, are powerful tools for investigating the physical and chemical conditions of astrophysical objects. Modeling of molecular emission requires excitation calculations using radiative as well as collisional rates with the most abundant species. Aims. The present paper focuses on the calculation of rate coefficients among the first rotational levels of the SiS molecule in its ground vibrational state in collision with He. Methods. A new accurate two dimensional potential energy surface system was calculated for the SiS-He system using highly correlated ab initio calculations. Dynamical calculations of pure rotational (de)excitation of SiS by collisions with He were performed for the first rotational levels using the near exact close-coupling approach. Results. Collisional cross sections among the 26 first rotational levels of SiS were calculated for kinetic energies up to 1500 cm −1 . Rate coefficients between the rotational levels were calculated for temperatures ranging from 5 to 200 K. A propensity rule that favors even ∆j transitions is found and is explained by the small asymmetry of the SiS-He potential energy surface.

The Conformers of Hydroxyacetaldehyde

Both two and eighteen dimensional quantum diffusion Monte Carlo (DMC) calculations were used to study the isomers of hydroxyacetaldehyde. A total of four unique minima, and the transition states connecting them, were located. Both two and eighteen dimensional potential energy surfaces were generated and used in the DMC runs. The rotational constants for the global minimum were predicted for all experimentally identified isotopomers and an approximate equilibrium structure obtained by combining our theoretical results with the experimentally observed rotational constants. The results obtained for the remaining isomers indicate that not all of them can be isolated in the gas phase.

Quasiclassical Trajectory Study of the CH3+ + HD → CH2D+ + H2 Reaction

A full dimensional ab initio potential energy surface for the CH5+ system based on coupled cluster electronic structure calculations and capable of describing the dissociation of methonium ion into methyl cation and molecular hydrogen (J. Phys. Chem. A 2006, 110, 1569) is used in quasiclassical trajectory calculations of the reaction CH3++HD-->CH2D++H2 for low collision energies of relevance to astrochemistry. Cross sections for the exchange are obtained at several relative translational energies and a fit to the energy dependence of the cross sections is used to obtain the rate constant at temperatures between 10 and 50 K. The calculated rate constant at 10 K agrees well with the previously reported experimental value. Internal energy distributions of the products are presented and discussed in the context of zero-point energy "noncompliance".

The ground state tunneling splitting and the zero point energy of malonaldehyde: A quantum Monte Carlo determination

Quantum dynamics calculations of the ground state tunneling splitting and of the zero point energy of malonaldehyde on the full dimensional potential energy surface proposed by Yagi et al. [J. Chem. Phys. 1154, 10647 (2001)] are reported. The exact diffusion Monte Carlo and the projection operator imaginary time spectral evolution methods are used to compute accurate benchmark results for this 21-dimensional ab initio potential energy surface. A tunneling splitting of 25.7+/-0.3 cm-1 is obtained, and the vibrational ground state energy is found to be 15 122+/-4 cm-1. Isotopic substitution of the tunneling hydrogen modifies the tunneling splitting down to 3.21+/-0.09 cm-1 and the vibrational ground state energy to 14 385+/-2 cm-1. The computed tunneling splittings are slightly higher than the experimental values as expected from the potential energy surface which slightly underestimates the barrier height, and they are slightly lower than the results from the instanton theory obtained using the same potential energy surface.

Ro-vibrational excitation of CS by He

Context. Over the next few years, ALMA and Herschel missions will perform high spatial and spectral resolution studies at infrared and sub-millimeter wavelengths. Modeling of the spectra will require accurate radiative and collisional rates for species of astrophysical interest. Aims. The present paper focuses on the calculation of ro-vibrational excitation rate coefficients of CS by He, useful for studies of high-temperature interstellar environments. Methods. A new accurate three dimensional (3D) potential energy surface for the CS-He system, which explicitly takes into account the r-dependence of the CS vibration, was calculated. The dynamic calculations were performed using the VCC-IOS approximation. Results. Cross sections among the 38 first rotational levels of o = 0, v = 1 and v = 2 are calculated for total energies up to 10 000 cm -1 . Rate coefficients between the ro-vibrational levels are calculated for temperatures from 300 K to 1500 K.

Diffusion Dynamics of the Li Atom on Amorphous Carbon: A Direct Molecular Orbital−Molecular Dynamics Study

Direct molecular orbital-molecular dynamics (MO-MD) calculation was applied to diffusion processes of the Li atom on a model surface of amorphous carbon and compared with the diffusion mechanism of Li+ ion. A carbon sheet composed of C96H24 was used as the model surface. The total energy and energy gradient on the full dimensional potential energy surface of the LiC96H24 system were calculated at each time step in the trajectory calculation. The optimized structure, where the Li atom is located at the center of mass of the model surface, was used as the initial structure at time zero. Simulation temperatures were chosen in the range of 200-1250 K. The dynamics calculations showed that the Li atom vibrates around the initial position below 250 K, and it moves above 300 K. At middle temperature, the Li atom translates freely on the surface. At higher temperature (1000 K), the Li atom moves from the center to edge region of the model surface and is trapped in the edge. The activation energy calculated for the Li atom is larger than that for the Li+ ion. This difference is due to the fact that the Li atom diffuses together with an unpaired electron on the carbon surface. The diffusion mechanism of the Li atom was discussed on the basis of the theoretical results.

A Globalab initioPotential Energy Surface for F+H2HF+H

A global three dimensional potential energy surface for the F+H2HF+H reaction has been developed by spline interpolation of about 15,000 symmetry-unique ab initio points, obtained from the multi-reference configuration interaction level with Davidson correction using the aug-cc-pV5Z basis set. In the entrance channel the spin-orbit coupling energy is also included.