An ab initio based global potential energy surface describing CH5+ --> CH3+ + H2

A full-dimensional, ab initio based potential energy surface (PES) for CH(5)(+), which can describe dissociation is reported. The PES is a precise fit to 36173 coupled-cluster [CCSD(T)] calculations of electronic energies done using an aug-cc-pVTZ basis. The fit uses a polynomial basis that is invariant with respect to permutation of the five H atoms, and thus describes all 120 equivalent minima. The rms fitting error is 78.1 cm(-1) for the entire data set of energies up to 30,000 cm(-1) and a normal-mode analysis of CH(5)(+) also verifies the accuracy of the fit. Two saddle points have been located on the surface as well and compared with previous theoretical work. The PES dissociates correctly to the fragments CH(3)(+) + H(2) and the equilibrium geometry and normal-mode analyses of these fragments are also presented. Diffusion Monte Carlo calculations are done for the zero-point energies of CH(5)(+) (and some isotopologs) as well as for the separated fragments of CH(5)(+), CH(3)(+) + H(2) and those of CH(4)D(+), CH(3)(+) + HD and CH(2)D(+) + H(2). Values of D(0) are reported for these dissociations. A molecular dynamics calculation of CH(4)D(+) dissociation at one total energy is also performed to both validate the applicability of the PES for dynamics studies as well as to test a simple classical statistical prediction of the branching ratio of the dissociation products.     

Calculations on the rate of the ion-molecule reaction between NH3+ and H2

The rate coefficient for the ion-molecule reaction NH3(+) + H2 --> NH4(+) + H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50-100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH3(+) + H2 + He --> NH4(+) + H + He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.     

Calculations on the competition between association and reaction for C3H(+) + H2

A potential energy surface has been calculated for the competing associative and reactive ion-molecule processes involving the reactants C3H(+) + H2. Our ab initio results show that the linear ion C3H+ and H2 can directly access the deep potential well of the propargyl ion H2CCCH+, which is calculated to lie 390 kJ mol-1 below the zero-point energy of the reactants. Isomerization between the propargyl ion and the lower energy, cyclic C3H3+ ion, calculated to lie 501 kJ mol-1 below the zero-point energy of reactants, can subsequently occur via two pathways. One of these pathways involves a transition state lying 22 kJ mol-1 below the energy of the reactants while the other, which occurs at much lower energies, involves two transition states and an intermediate. The dissociation of c-C3H3+ into c-C3H2(+) + H is calculated to occur directly, without any intermediate potential energy maximum, but the energy of the products lies 7.3 kJ mol-1 above the energy of the reactants. Using the minimum energy potential pathway and properties of the stationary point structures determined via ab initio methods, we have calculated both the association rate coefficient to produce C3H3+ as a function of density and the branching ratio between the propargyl and cyclic structures of the ion. Our results are in good agreement with some experimental results and in conflict with others. Specifically, we agree with the 1:1 branching ratio measured for the propargyl and cyclic isomers of C3H3+ at 80 and 300 K and we agree with the rate coefficient for radiative association measured at 80 K. We cannot reproduce reported measurements that the reactive channel (C3H2(+) + H) is the dominant channel at 80 K and at low gas densities, or that the association channel at high densities saturates at an effective rate coefficient well below the Langevin value -2x10(-11) cm3 s-1 at 300K and 1x10(-10) cm3 s-1 at 80K.     

Probing the structure of CH5+ by dissociative charge exchange

Dissociative charge exchange of CH5+ with Cs, coupled with quasiclassical trajectory calculations on an ab initio PES for CH5, has been used to probe the structure of the CH5+ cation. Product kinetic energy release distributions and branching ratios for CH5 --> CH4 + H and CH5 --> CH3 + H2 have been compared. The agreement of the product branching ratios provides evidence for the fluxional nature of CH5+.     

Experimental and theoretical differential cross sections for the N(2D) + H2 reaction

In this paper, we report a combined experimental and theoretical study on the dynamics of the N(2D) + H2 insertion reaction at a collision energy of 15.9 kJ mol(-1). Product angular and velocity distributions have been obtained in crossed beam experiments and simulated by using the results of quantum mechanical (QM) scattering calculations on the accurate ab initio potential energy surface (PES) of Pederson et al. (J. Chem. Phys. 1999, 110, 9091). Since the QM calculations indicate that there is a significant coupling between the product angular and translational energy distributions, such a coupling has been explicitly included in the simulation of the experimental results. The very good agreement between experiment and QM calculations sustains the accuracy of the NH2 ab initio ground state PES. We also take the opportunity to compare the accurate QM differential cross sections with those obtained by two approximate methods, namely, the widely used quasiclassical trajectory calculations and a rigorous statistical method based on the coupled-channel theory.     

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".     

Calibration-quality adiabatic potential energy surfaces for H3(+) and its isotopologues

Calibration-quality ab initio adiabatic potential energy surfaces (PES) have been determined for all isotopologues of the molecular ion H(3)(+). The underlying Born-Oppenheimer electronic structure computations used optimized explicitly correlated shifted Gaussian functions. The surfaces include diagonal Born-Oppenheimer corrections computed from the accurate electronic wave functions. A fit to the 41,655 ab initio points is presented which gives a standard deviation better than 0.1 cm(-1) when restricted to the points up to 6000 cm(-1) above the first dissociation asymptote. Nuclear motion calculations utilizing this PES, called GLH3P, and an exact kinetic energy operator given in orthogonal internal coordinates are presented. The ro-vibrational transition frequencies for H(3)(+), H(2)D(+), and HD(2)(+) are compared with high resolution measurements. The most sophisticated and complete procedure employed to compute ro-vibrational energy levels, which makes explicit allowance for the inclusion of non-adiabatic effects, reproduces all the known ro-vibrational levels of the H(3)(+) isotopologues considered to better than 0.2 cm(-1). This represents a significant (order-of-magnitude) improvement compared to previous studies of transitions in the visible. Careful treatment of linear geometries is important for high frequency transitions and leads to new assignments for some of the previously observed lines. Prospects for further investigations of non-adiabatic effects in the H(3)(+) isotopologues are discussed. In short, the paper presents (a) an extremely accurate global potential energy surface of H(3)(+) resulting from high accuracy ab initio computations and global fit, (b) very accurate nuclear motion calculations of all available experimental line data up to 16,000 cm(-1), and (c) results suggest that we can predict accurately the lines of H(3)(+) towards dissociation and thus facilitate their experimental observation.     

Quantum reactive scattering of H + hydrocarbon reactions

A practical quantum-dynamical method is described for predicting accurate rate constants for general chemical reactions. The ab initio potential energy surfaces for these reactions can be built from a minimal number of grid points (average of 50 points) and expressed in terms of analytical functionals. All the degrees of freedom except the breaking and forming bonds are optimised using the MP2 method with a cc-pVTZ basis set. Single point energies are calculated on the optimised geometries at the CCSD(T) level of theory with the same basis set. The dynamics of these reactions occur on effective reduced dimensionality hyper-surfaces accounting for the zero-point energy of the optimised degrees of freedom. Bonds being broken and formed are treated with explicit hyperspherical time independent quantum dynamics. Application of the method to the H + CH(4)--> H(2)+ CH(3), H + C(2)H(6)--> H(2)+ C(2)H(5), H + C(3)H(8)--> H(2)+n-C(3)H(7)/H(2)+i-C(3)H(7) and H + CH(3)OH --> H(2)+ CH(3)O/H(2)+ CH(2)OH reactions illustrate the potential of the approach in predicting rate constants, kinetic isotope effects and branching ratios. All studied reactions exhibit large quantum tunneling in the rate constants at lower temperatures. These quantum calculations compare well with the experimental results.     

Direct versus resonances mediated F+OH collisions on a new 3A" potential energy surface

A theoretical study of the F(2P) + OH(2Pi) --> HF(1Sigma+) + O(3P) reactive collisions is carried out on a new global potential energy surface (PES) of the ground 3A" adiabatic electronic state. The ab initio calculations are based on multireference configuration interaction calculations, using the aug-cc-pVTZ extended basis sets of Dunning et al. A functional representation of the PES shows no nominal barrier to reaction, contrary to previous results by others. Wave packet and quasiclassical trajectory calculations have been performed for this PES to study the F + OH(v = 0,j) reactive collision. The comparison was performed at fixed and constant values of the total angular momentum from 0 to 110 and relative translational energy up to 0.8 eV. The reaction presents a dynamical barrier, essentially due to the zero-point energy for the bending vibration near the saddle point. This determines two different reaction mechanisms. At energies higher than approximately 0.125 eV the reaction is direct, while below that value it is indirect and mediated by heavy-light-heavy resonances. Such resonances, also found in the simulations of the photodetachment spectrum of the triatomic anion, manifest themselves in the quasiclassical simulations, too, where they are associated to periodic orbits.     

Vibrational and rotational cooling of NO+ in collisions with He

A quantum mechanical investigation of the vibrational and rotational deactivation of NO(+) in collisions with He atoms in the cold and ultracold regime is presented. Ab initio potential energy calculations are carried out at BCCD(T) level and a new global 3D potential energy surface (PES) is obtained by fitting ab initio points within the reproducing kernel Hilbert space method. As a first test of this PES the bound state energies of the (3)He-NO(+) and (4)He-NO(+) complexes are calculated and compared to previous rigid rotor calculations. The efficiency of the vibrational and the rotational cooling of this molecular ion using a buffer gas of helium is then investigated by performing close coupling scattering calculations for collision energy ranging from 10(-6) to 2000 cm(-1). The calculations are performed for the two isotopes (3)He and (4)He and the results are compared to the available experimental data.     

Thermochemistry and accurate quantum reaction rate calculations for H2/HD/D2 + CH3

Accurate quantum-mechanical results for thermodynamic data, cumulative reaction probabilities (for J = 0), thermal rate constants, and kinetic isotope effects for the three isotopic reactions H2 + CH3 --> CH4 + H, HD + CH3 --> CH4 + D, and D2 + CH3 --> CH(3)D + D are presented. The calculations are performed using flux correlation functions and the multiconfigurational time-dependent Hartree (MCTDH) method to propagate wave packets employing a Shephard interpolated potential energy surface based on high-level ab initio calculations. The calculated exothermicity for the H2 + CH3 --> CH4 + H reaction agrees to within 0.2 kcal/mol with experimentally deduced values. For the H2 + CH3 --> CH4 + H and D2 + CH3 --> CH(3)D + D reactions, experimental rate constants from several groups are available. In comparing to these, we typically find agreement to within a factor of 2 or better. The kinetic isotope effect for the rate of the H2 + CH3 --> CH4 + H reaction compared to those for the HD + CH3 --> CH4 + D and D2 + CH3 --> CH(3)D + D reactions agree with experimental results to within 25% for all data points. Transition state theory is found to predict the kinetic isotope effect accurately when the mass of the transferred atom is unchanged. On the other hand, if the mass of the transferred atom differs between the isotopic reactions, transition state theory fails in the low-temperature regime (T < 400 K), due to the neglect of the tunneling effect.     

Ab initio potential energy and dipole moment surfaces for H5O2 +

Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H(5)O(2) (+). Tens of thousands of coupled-cluster [CCSD(T)] and second-order Moller-Plesset (MP2) calculations of electronic energies, using aug-cc-pVTZ basis, were done. The energies were fit very precisely in terms of all the internuclear distances, using standard least-square procedures, however, with a fitting basis that satisfies permutational symmetry with respect to like atoms. The H(5)O(2) (+) PES is a fit to 48 189 CCSD(T) energies, containing 7962 polynomial coefficients. The PES has a rms fitting error of 34.9 cm(-1) for the entire data set up to 110 000 cm(-1). This surface can describe various internal floppy motions, including the H atom exchanges, monomer inversions, and monomer torsions. First- and higher-order saddle points have been located on the surface and compared with available previous theoretical work. In addition, the PES dissociates correctly (and symmetrically) to H(2)O+H(3)O(+), with D(e)=11 923.8 cm(-1). Geometrical and vibrational properties of the monomer fragments are presented. The corresponding global DMS fit (MP2 based) involves 3844 polynomial coefficients and also dissociates correctly.     

Ab initio molecular dynamics of hydrogen dissociation on metal surfaces using neural networks and novelty sampling

We outline a hybrid multiscale approach for the construction of ab initio potential energy surfaces (PESs) useful for performing six-dimensional (6D) classical or quantum mechanical molecular dynamics (MD) simulations of diatomic molecules reacting at single crystal surfaces. The algorithm implements concepts from the corrugation reduction procedure, which reduces energetic variation in the PES, and uses neural networks for interpolation of smoothed ab initio data. A novelty sampling scheme is implemented and used to identify configurations that are most likely to be predicted inaccurately by the neural network. This hybrid multiscale approach, which couples PES construction at the electronic structure level to MD simulations at the atomistic scale, reduces the number of density functional theory (DFT) calculations needed to specify an accurate PES. Due to the iterative nature of the novelty sampling algorithm, it is possible to obtain a quantitative measure of the convergence of the PES with respect to the number of ab initio calculations used to train the neural network. We demonstrate the algorithm by first applying it to two analytic potentials, which model the H2/Pt(111) and H2/Cu(111) systems. These potentials are of the corrugated London-Eyring-Polanyi-Sato form, which are based on DFT calculations, but are not globally accurate. After demonstrating the convergence of the PES using these simple potentials, we use DFT calculations directly and obtain converged semiclassical trajectories for the H2/Pt(111) system at the PW91/generalized gradient approximation level. We obtain a converged PES for a 6D hydrogen-surface dissociation reaction using novelty sampling coupled directly to DFT. These results, in excellent agreement with experiments and previous theoretical work, are compared to previous simulations in order to explore the sensitivity of the PES (and therefore MD) to the choice of exchange and correlation functional. Despite having a lower energetic corrugation in our PES, we obtain a broader reaction probability curve than previous simulations, which is attributed to increased geometric corrugation in the PES and the effect of nonparallel dissociation pathways.     

Full-dimensional quantum calculations of ground-state tunneling splitting of malonaldehyde using an accurate ab initio potential energy surface

Quantum calculations of the ground vibrational state tunneling splitting of H-atom and D-atom transfer in malonaldehyde are performed on a full-dimensional ab initio potential energy surface (PES). The PES is a fit to 11 147 near basis-set-limit frozen-core CCSD(T) electronic energies. This surface properly describes the invariance of the potential with respect to all permutations of identical atoms. The saddle-point barrier for the H-atom transfer on the PES is 4.1 kcalmol, in excellent agreement with the reported ab initio value. Model one-dimensional and "exact" full-dimensional calculations of the splitting for H- and D-atom transfer are done using this PES. The tunneling splittings in full dimensionality are calculated using the unbiased "fixed-node" diffusion Monte Carlo (DMC) method in Cartesian and saddle-point normal coordinates. The ground-state tunneling splitting is found to be 21.6 cm(-1) in Cartesian coordinates and 22.6 cm(-1) in normal coordinates, with an uncertainty of 2-3 cm(-1). This splitting is also calculated based on a model which makes use of the exact single-well zero-point energy (ZPE) obtained with the MULTIMODE code and DMC ZPE and this calculation gives a tunneling splitting of 21-22 cm(-1). The corresponding computed splittings for the D-atom transfer are 3.0, 3.1, and 2-3 cm(-1). These calculated tunneling splittings agree with each other to within less than the standard uncertainties obtained with the DMC method used, which are between 2 and 3 cm(-1), and agree well with the experimental values of 21.6 and 2.9 cm(-1) for the H and D transfer, respectively.     

Theoretical study of the dynamics of the H+CH4 and H+C2H6 reactions using a specific-reaction-parameter semiempirical Hamiltonian

We present a theoretical study of the reactions of hydrogen atoms with methane and ethane molecules and isotopomers. High-accuracy electronic-structure calculations have been carried out to characterize representative regions of the potential-energy surface (PES) of various reaction pathways, including H abstraction and H exchange. These ab initio calculations have been subsequently employed to derive an improved set of parameters for the modified symmetrically-orthogonalized intermediate neglect of differential overlap (MSINDO) semiempirical Hamiltonian, which are specific to the H+alkane family of reactions. The specific-reaction-parameter (SRP) Hamiltonian has then been used to perform a quasiclassical-trajectory study of both the H+CH4 and H+C2H6 reactions. The calculated values of dynamics properties of the H+CH4-->H2+CH3 reaction and isotopologues, including alkyl product speed distributions, diatomic product internal-state distributions, and cross sections, are generally in good agreement with experiment and with the results provided by the ZBB3 PES [Z. Xie et al., J. Chem. Phys. 125, 133120 (2006)]. The results of trajectories propagated with the SRP Hamiltonian for the H+C2H6-->H2+C2H5 reaction also agree with experiment. The level of agreement between the results calculated with the SRP Hamiltonian and experiment in both the H+methane and H+ethane reactions indicates that semiempirical Hamiltonians can be improved for not only a specific reaction but also a family of reactions.     

An eight-degree-of-freedom, time-dependent quantum dynamics study for the H2+C2H reaction on a new modified potential energy surface

An eight-dimensional time-dependent quantum dynamics wave packet approach is performed for the study of the H2+C2H-->H+C2H2 reaction system on a new modified potential energy surface (PES) [L.-P. Ju et al., Chem. Phys. Lett. 409, 249 (2005)]. This new potential energy surface is obtained by modifying Wang and Bowman's old PES [J. Chem. Phys. 101, 8646 (1994)] based on the new ab initio calculation. This new modified PES has a much lower transition state barrier height at 2.29 kcal/mol than Wang and Bowman's old PES at 4.3 kcal/mol. This study shows that the reactivity for this diatom-triatom reaction system is enhanced by vibrational excitations of H2, whereas the vibrational excitations of C2H only have a small effect on the reactivity. Furthermore, the bending excitations of C2H, compared to the ground state reaction probability, hinder the reactivity. The comparison of the rate constant between this calculation and experimental results agrees with each other very well. This comparison indicates that the new modified PES corrects the large barrier height problem in Wang and Bowman's old PES.     

Ab initio potential energy surface, variational transition state theory, and quasiclassical trajectory studies of the F+CH4-->HF+CH3 reaction

In this work we present a study of the F+CH(4)-->HF+CH(3) reaction (DeltaHdegrees(298 K)=-32.0 kcal mol(-1)) using different methods of the chemical reaction theory. The ground potential energy surface (PES) is characterized using several ab initio methods. Full-dimensional rate constants have been calculated employing the variational transition state theory and using directly ab initio data. A triatomic analytical representation of the ground PES was derived from ab initio points calculated at the second- and fourth-order M?ller-Plesset levels with the 6-311+G(2df,2pd) basis set, assuming the CH(3) fragment to be a 15 a.m.u. pseudoatom in the fitting process. This is suggested from experiments that indicate that the methyl group is uncoupled to the reaction coordinate. A dynamics study by means of the quasiclassical trajectory (QCT) method and employing this analytical surface was also carried out. The experimental data available on the HF internal states distributions are reproduced by the QCT results. Very recent experimental information about the reaction stereodynamics is also borne out by our QCT calculations. Comparisons with the benchmark F+H(2) and analogous Cl+CH(4) reactions are established throughout.     

Ab initio rate constants from hyperspherical quantum scattering: application to H + CH4 --> H2 + CH3

A general and practical procedure is described for calculating rate constants for chemical reactions using a minimal number of ab initio calculations and quantum-dynamical computations. The method exploits a smooth interpolating functional developed in the hyperspherical representation. This functional is built from two Morse functions and depends on a relatively small number of parameters with respect to conventional functionals developed to date. Thus only a small number of ab initio points needs to be computed. The method is applied to the H + CH4 --> H2 + CH3 reaction. The quantum scattering calculations are performed treating explicitly the bonds being broken and formed. All the degrees of freedom except the breaking and forming bonds are optimized ab initio and harmonic vibrational frequencies and zero-point energies for them are calculated at the MP2(full) level with a cc-pVTZ basis set. Single point energies are calculated at a higher level of theory with the same basis set, namely CCSD(T, full). We report state-to-state cross sections and thermal rate constants for the title reaction and make comparisons with previous results. The calculated rate constants are in good agreement with experiments.     

Quasiclassical trajectory study of the Cl+CH4 reaction dynamics on a quadratic configuration interaction with single and double excitation interpolated potential energy surface

An ab initio interpolated potential energy surface (PES) for the Cl+CH(4) reactive system has been constructed using the interpolation method of Collins and co-workers [J. Chem. Phys. 102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); Theor. Chem. Acc. 108, 313 (2002)]. The ab initio calculations have been performed using quadratic configuration interaction with single and double excitation theory to build the PES. A simple scaling all correlation technique has been used to obtain a PES which yields a barrier height and reaction energy in good agreement with high level ab initio calculations and experimental measurements. Using these interpolated PESs, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions has been carried out for the Cl+CH(4) and Cl+CD(4) reactions, and the theoretical results have been compared with the available experimental data. It has been shown that the calculated total reaction cross sections versus collision energy for the Cl+CH(4) and Cl+CD(4) reactions is very sensitive to the barrier height. Besides, due to the zero-point energy (ZPE) leakage of the CH(4) molecule to the reaction coordinate in the quasiclassical trajectory (QCT) calculations, the reaction threshold falls below the barrier height of the PES. The ZPE leakage leads to CH(3) and HCl coproducts with internal energy below its corresponding ZPEs. We have shown that a Gaussian binning (GB) analysis of the trajectories yields excitation functions in somehow better agreement with the experimental determinations. The HCl(v'=0) and DCl(v'=0) rotational distributions are as well very sensitive to the ZPE problem. The GB correction narrows and shifts the rotational distributions to lower values of the rotational quantum numbers. However, the present QCT rotational distributions are still hotter than the experimental distributions. In both reactions the angular distributions shift from backward peaked to sideways peaked as collision energy increases, as seen in the experiments and other theoretical calculations.     

Quantum and classical studies of vibrational motion of CH5+ on a global potential energy surface obtained from a novel ab initio direct dynamics approach

We report a full dimensional, ab initio based potential energy surface for CH(5) (+). The ab initio electronic energies and gradients are obtained in direct-dynamics calculations using second-order M?ller-Plesset perturbation theory with the correlation consistent polarized valence triple zeta basis. The potential energy and the dipole moment surfaces are fit using novel procedures that ensure the full permutational symmetry of the system. The fitted potential energy surface is tested by comparing it against additional electronic energy calculations and by comparing normal mode frequencies at the three lowest-lying stationary points obtained from the fit against ab initio ones. Well-converged diffusion Monte Carlo zero-point energies, rotational constants, and projections along the CH and HH bond lengths and the tunneling coordinates are presented and compared with the corresponding harmonic oscillator and standard classical molecular dynamics ones. The delocalization of the wave function is analyzed through comparison of the CH(5) (+) distributions with those obtained when all of the hydrogen atoms are replaced by (2)H and (3)H. The classical dipole correlation function is examined as a function of the total energy. This provides a further probe of the delocalization of CH(5) (+).