Structures of water octamers (H2O)8: exploration on ab initio potential energy surfaces by the scaled hypersphere search method

The potential energy surface (PES) of water octamers has been explored by the scaled hypersphere search method. Among 164 minima on the PES (based on MP2/6-311++G(3df,2p)//B3LYP/6-311+G(d,p) calculations), the cubic structure with D2d symmetry has been confirmed to be the global minimum. In a thermodynamic simulation using these 164 structures, the cubic structure with S4 symmetry has the highest population at low temperature, though double rings can become dominant as temperature going up, in good accord with a recent Monte Carlo simulation using an empirical potential. A transition temperature from cubic to noncubic has significantly been underestimated when potential energy data of B3LYP/6-311+G(d,p) calculations are employed in the simulation. This serious discrepancy between the MP2 and the B3LYP results suggests an importance of dispersion interactions for discussions on thermodynamics of water octamers.     

Quantum chemistry study of H+(H2O)8: a global search for its isomers by the scaled hypersphere search method, and its thermal behavior

The structures of the protonated water cluster H+(H2O)8 have been globally explored by the scaled hypersphere search method. On the Hartree-Fock potential energy surface 174 isomers were found, among which 168 were computed to be minima at the B3LYP/6-31+G** level, and their energies were further refined at the level of MP2/6-311++G(3df,2p). The global minimum on the potential energy surface computed at the B3LYP/6-31+G** level shows a cagelike structure with the "Eigen" motif, while the lowest-free-energy isomer has a five-membered-ring structure at 170 K and a chain form at 273 K. The present results are well in line with previous experimental findings. In addition, the ADMP (atom-centered density matrix propagation) simulation indicates that the extra proton in the lowest-free-energy isomer (170 K), which has a five-membered ring and the "Zundel" feature, is often in an asymmetrical hydrogen bond.     

Gas-phase reaction of the isobutenyl anion with N2O from ab initio calculations

Calculations using conventional ab initio theory are performed to investigate the reaction mechanism associated with the gas-phase ion/molecule reaction of isobutenyl anion with N₂O. As a result, our theoretical findings strongly suggest that the main pathway is the reaction pattern of end-N attack and that the corresponding reaction mechanism basically relates to hydrogen migration, which may yield products cis-CH₂(CH₃)CCN ₂ ^? , trans-CH₂(CH₃)CCN ₂ ^? , and H₂O. Those are in good agreement with the experimental observations. Moreover, based on the NBO, Activation Strain model and methyl group effect analysis, we also explored the characters of rate-determining step of the main pathway.     

Linewidths of C2H2 perturbed by H2: experiments and calculations from an ab initio potential

In this work we present a theoretical and experimental study of the acetylene-hydrogen system. A potential surface considering rigid monomers has been obtained by ab initio quantum chemistry methods. This 4-dimensional potential is further employed to compute, using the close-coupling approach and the coupled-states approximation, pressure broadening coefficients of C(2)H(2) isotropic Raman Q lines over a temperature range of 77 to 2000 K. Experimental data for the acetylene nu(2) Raman lines broadened by molecular hydrogen are obtained using stimulated Raman spectroscopy. The comparison of theoretical values with experimental data at 143 K is promising. Approximations to increase the computational efficiency are proposed.     

Microsolvation of hydrogen sulfide: exploration of H2S.(H2O)n and SH-.H3O+.(H2O)n-1 (n = 5-7) cluster structures on ab initio potential energy surfaces by the scaled hypersphere search method

The scaled hypersphere search method was applied to ab initio potential energy surfaces of the H2S.(H2O)n/SH-.H3O+.(H2O)n-1 system with n = 5-7. Local minima databases including 121, 326, and 553 structures for n = 5-7, respectively, were obtained based on calculations at the MP2/6-311++G(3df,2p)//B3LYP/6-31+G** level. In these small cluster sizes, the SH-.H3O+.(H2O)n-1 type is still unstable relative to the H2S.(H2O)n type, and the global minima for H2S.(H2O)n are very similar to those of pure water clusters of (H2O)n+1. Thermodynamic simulations based on the present databases showed a structure transition from the well-mixed (H2O)n+1-like global minimum at low temperatures to unmixed complexes between H2S and (H2O)n at high temperatures.     

Atomic layer deposition of TiO2 from TiI4 and H2O onto SiO2 surfaces: ab initio calculations of the initial reaction mechanisms

The initial surface reactions involved in the atomic layer deposition (ALD) of TiO2 from TiI4 and H2O onto a SiO2 substrate have been investigated using electronic structure calculations based on cluster models. The detailed atomic growth mechanisms on different types of functionalities on the SiO2 substrate have been proposed. The effects of quantum tunneling and hindered rotations of adsorbates on the rate of surface reactions have been investigated. The effects of tunneling were found to be negligible for all reactions, because typical ALD temperatures range from 150 to 450 degrees C. However, the rotational contributions to the rate constants must be taken into account in certain cases. All of the three surface functionalities investigated exhibit high chemical reactivity toward TiI4 precursors at typical ALD temperatures. The rate constants of the second half-reactions between Ti intermediates and H2O are 5-8 orders of magnitude smaller than the first half-reactions between TiI4 and the surface functionalities. Although the iodine release reaction has been used to explain previous experimental measurements, it is predicted to be unfavorable (kinetically and thermodynamically) and is unlikely to occur at typical ALD temperatures. Substitution of TiI4 with TiCl4 as the metal precursor can increase the binding energies of the absorbates onto the surface due to the high electronegativity of the Cl ligands. However, the activation barriers are not significantly different between these two metal precursors. More importantly, our calculations predict that TiI4 precursors tend to produce TiO2 films with fewer impurities than the TiCl4 precursors.     

Kinetic isotope effects for the H2 + C2H ↔ C2H2 + H reaction based on the ab initio calculations and a global potential energy surface

In the present paper, kinetic isotope effects of the title reaction are studied with canonical variational transition state theory on the modified Wang Bowman (MWB) potential energy surface (PES) (Chem Phys Lett 2005, 409, 249) and the ab initio calculations at the quadratic configuration interaction (QCISD (T, full))/aug‐cc‐pVTZ//QCISD (full)/cc‐pVTZ level. The calculated rate constants for the isotopic variants of this title reaction on the MWB PES have good agreement with those of the present ab initio calculations over the temperature range of 20–5000 K for the forward reactions and 800–5000 K for the reverse reactions, respectively. In particular, the forward rate constants for the title reaction and its isotopically substituted reactions have negative temperature dependences at about 40 K. Rate expressions are presented for all the studied reactions. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 289–298, 2010     

Flexible H2O2 in water: electronic structure from photoelectron spectroscopy and ab initio calculations

The effect of hydration on the electronic structure of H(2)O(2) is investigated by liquid-jet photoelectron spectroscopy measurements and ab initio calculations. Experimental valence electron binding energies of the H(2)O(2) orbitals in water are, on average, 1.9 eV red-shifted with respect to the gas-phase molecule. A smaller width of the first peak was observed in the photoelectron spectrum from the solution. Our experiment is complemented by simulated photoelectron spectra, calculated at the ab initio level of theory (with EOM-IP-CCSD and DFT methods), and using path-integral sampling of the ground-state density. The observed shift in ionization energy upon solvation is attributed to a combination of nonspecific electrostatic effects (long-range polarization) and of the specific interactions between H(2)O(2) and H(2)O molecules in the first solvation shell. Changes in peak widths are found to result from merging of the two lowest ionized states of H(2)O(2) in water due to conformational changes upon solvation. Hydration effects on H(2)O(2) are stronger than on the H(2)O molecule. In addition to valence spectra, we report oxygen 1s core-level photoelectron spectra from H(2)O(2)(aq), and observed energies and spectral intensities are discussed qualitatively.     

Global mapping of equilibrium and transition structures on potential energy surfaces by the scaled hypersphere search method: applications to ab initio surfaces of formaldehyde and propyne molecules

Technical details of a new global mapping technique for finding equilibrium (EQ) and transition structures (TS) on potential energy surfaces (PES), the scaled hypersphere search (SHS) method (Ohno, K.; Maeda, S. Chem. Phys. Lett. 2004, 384, 277), are presented. On the basis of a simple principle that reaction pathways are found as anharmonic downward distortions of PES around an EQ point, the reaction pathways can be obtained as energy minima on the scaled hypersphere surface, which would have a constant energy when the potentials are harmonic. Connections of SHS paths between each EQ are very similar to corresponding intrinsic reaction coordinate (IRC) connections. The energy maximum along the SHS path reaches a region in close proximity to the TS of the reaction pathway, and the subsequent geometry optimization from the SHS maximum structure easily converges to the TS. The SHS method, using the one-after-another algorithm connecting EQ and TS, considerably reduces the multidimensional space to be searched to certain limited regions around the pathways connecting each EQ with the neighboring TS. Applications of the SHS method have been made to ab initio surfaces of formaldehyde and propyne molecules to obtain systematically five EQ and nine TS for formaldehyde and seven EQ and 32 TS for propyne.     

An intermolecular potential function for the Fe(II)–H2O system from ab initio calculations

An intermolecular potential for the Fe(II)–H₂O system has been determined from ab initio calculations which have been obtained with Huzinaga's MINI -2 basis set. Interaction energies for more than 100 points of the potential energy surface were fitted to an analytical function that contains 11 adjustable parameters. The goodness of the fitting and its applicability to the study of Fe(H₂O) clusters and to Monte Carlo simulations are discussed.     

Insights into the photochemical processes of ClC(O)SCl from ab initio calculations

All possible unimolecular processes upon photolysis of ClC(O)SCl in the UV-visible region have been characterized in the present paper through the optimized stationary structures and computed potential-energy profiles of the S0, S1, T2, and S2 states with the MP2, B3LYP, CASSCF, and MR-CI methods in conjugation with the cc-pVDZ basis set. Upon photoexcitation in the range of 300-400 nm, the ClC(O)SCl molecules are excited to the S1 state. From this state, the dissociation into ClC(O)S + Cl takes place immediately and subsequently Cl2 and SCO are formed. The C-Cl and C-S bond fissions that start from the S2 state are the dominant channels upon photodecomposition of ClC(O)SCl in the gas and condensed phases in the wavelength range of 200-248 nm. The formed Cl, C(O)SCl, ClCO, and SCl radicals are very reactive, and the Cl2, SCO, CO, and SCl2 molecules are subsequently produced as stable products in the condensed phase.     

Direct ab initio dynamics calculations on the rate constants for the hydrogen-abstraction reaction of C2H5F with O (3P)

The hydrogen-abstraction reaction C₂H₅F+O → C₂H₄F+OH has been studied by a dual-level direct dynamics method. For the reaction, three reaction channels, one for α-abstraction and two for β-abstraction, have been identified. The potential-energy surface information is obtained at the MP2(full)/6-311G(d,p) and PMP2(full)/6-311G(3df,3pd) (single-point) levels. By canonical variational transition-state theory, rate constants for each reaction channel are calculated with a small-curvature tunneling correction. The total rate constant is calculated from the sum of the individual rate constants and the temperature dependence of the branching ratios is obtained over a wide range of temperatures from 300 to 5,000 K. The agreement of the rate constants with experiment is good in the experimental temperature range from 1,000 to 1,250 K. The calculated results indicate that at low temperatures α-abstraction is most likely to be the major reaction channel, while β-abstraction channels will significantly contribute to the whole reaction rate as the temperature increases. Received: 23 January 2002 / Accepted: 23 June 2002 / Published online: 20 September 2002     

Abstraction kinetics of H-atom by OH radical from pinonaldehyde (C10H16O2): ab initio and transition-state theory calculations

The kinetics and abstraction rate coefficients of hydroxyl radical (OH) reaction with pinonaldehyde were computed using G3(MP2) theory and transition-state theory (TST) between 200 and 400 K. Structures of the reactants, reaction complexes (RCs), product complexes (PCs), transition states (TSs), and products were optimized at the MP2(FULL)/6-31G* level of theory. Fifteen transition states were identified for the title reaction and confirmed by intrinsic reaction coordinate (IRC) calculations. The contributions of all the individual hydrogens in the substrate molecule to the total reaction are computed. The quantum mechanical tunneling effect was computed using Wigner's and Eckart's methods (both symmetrical and unsymmetrical methods). The reaction exhibits a negative temperature dependent rate coefficient, k(T) = (1.97 ± 0.34) × 10(-13) exp[(1587 ± 48)/T] cm(3) molecule(-1) s(-1), k(T) = (3.02 ± 0.56) × 10(-13) exp[(1534 ± 52/T] cm(3) molecule(-1) s(-1), and k(T) = (4.71 ± 1.85) × 10(-14) exp[(2042 ± 110)/T] cm(3) molecule(-1) s(-1) with Wigner's, Eckart's symmetrical, and Eckart's unsymmetrical tunneling corrections, respectively. Theoretically calculated rate coefficients are found to be in good agreement with the experimentally measured ones and other theoretical results. It is shown that hydrogen abstraction from -CHO position is the major channel, whereas H-abstraction from -COCH(3) is negligible. The atmospheric lifetime of pinonaldehyde is computed to be few hours and found to be in excellent agreement with the experimentally estimated ones.     

Insights into dynamics of the S2 state of thiophosgene from ab initio calculations

The S2 potential energy surface for Cl2CS dissociation has been characterized with a combined complete active space self-consistent field and multireference configuration interaction method. The S3/S2 minimum-energy intersection has been determined with the state-averaged complete active space self-consistent field method. The S2 direct dissociation was found to have a barrier of 6.0 kcal/mol, leading to formation of Cl(X2P)+ClCS(A2A") in the excited electronic state. Dynamics of the S2 state of Cl2CS can be summarized as follows: (1) The S2-S0 fluorescence occurs with high quantum yield at low excess energies; (2) Both the S(2) dissociation and the S2-->S3 internal conversion cause the loss of the S2-S0 fluorescence upon photoexcitation at 235-253 nm; (3) The S2-->S3 internal conversion (IC) followed by the direct IC to the ground electronic state results in the fragments produced in the ground state, while the S2 dissociation leads to formation of the fragments in excited electronic states.     

DFT and ab initio study on the reaction mechanism of CH2SH + O2

The mechanism for the CH₂SH + O₂ reaction was investigated by DFT and ab initio chemistry methods. The geometries of all possible stationary points were optimized at the B3LYP/6-311+G(d,p) level, and the single point energy was calculated at the CCSD(T)/cc-pVXZ(X = D and T), G3MP2 and BMC-CCSD levels. The results indicate that the oxidation of CH₂SH by O₂ to form HSCH₂OO is a barrierless process. The most favorable channel is the rearrangement of the initial adduct HSCH₂OO (IM1) to form another intermediate H₂C(S)OOH (IM3) via a five-center transition state, and then the C–O bond fission in IM3 leads to a complex CH₂S. . .HO₂ (MC1), which finally gives out to the major product CH₂S + HO₂. Due to high barriers, other products including cis- and trans-HC(O)SH + HO could be negligible. The direct abstraction channel was also determined to yield CH₂S + HO₂, with the barrier height of 22.3, 18.1 and 15.0 kcal/mol at G3MP2, CCSD(T)/cc-pVTZ and BMC-CCSD levels, respectively, it is not competitive with the addition channel, in which all stationary points are lower than reactant energetically. The other channels to produce cis- and trans-CHSH + HO₂ are also of no importance.     

Systematic ab initio calculations on the energetics and stability of covalent O4

Ab initio calculations with highly correlated methods together with extensive basis sets have been used to obtain the most accurate heat of formation and stability with respect to dissociation (into molecular oxygen) for the chemically bound tetraoxygen molecule. Our calculations show that the heat of formation is significantly smaller and that the barrier to dissociation is larger than previously assumed. In particular, we have shown that the previous theoretical estimate for the heat of formation of tetraoxygen was in error by a significant amount (18%-24%) owing to lack of accuracy in the theoretical method then used. Our best estimates places that value in the range 93-95 kcal/mol and this should be taken into consideration when discussing the possible relevance of tetraoxygen in a variety of experiments, as well as in the fundamental atmospheric chemical processes where oxygen species participate.     

Quantum calculations of vibrational energies of H3O2- on an ab initio potential

We report a full-dimensional potential energy surface for H3O2-, based on fitting 66,965 ab initio electronic energies. A major feature of this potential is a barrier of roughly 200 cm-1 to internal rotation of the two hydroxyl groups about a line connecting the two oxygen atoms and the bridging hydrogen atom. The potential is used in calculations of vibrational energies, performed with the "Reaction Path" version of the code "MULTIMODE". The results are compared to recent infrared messenger experiments and are used to propose interpretations of the experimental results.     

Analytical potential from ab initio calculations for the Fe+-H2O and Feo-H2O systems

Analytical intermolecular potentials for the Fe⁺?H₂O and Fe^o?H₂O systems have been determined from ab initio calculations. Interaction energies for a lot of points along the two potential energy surfaces were calculated using Huzinga's MINI ?2 basis set. The results obtained were fitted to an analytical function containing 11 adjustable parameters that we have already used with success for the Fe²⁺?H₂O system. The goodness of the generated intermolecular potentials is discussed.