Direct solution of the many-body Schrödinger equation in the hyperspherical formalism: Formulation of the CFHH–GLF method

To accelerate the convergence of the HH expansion, we modified the HH –GLF method, a new simple hyperspherical harmonic method proposed recently by us, into the CFHH –GLF method. Applications of the CFHH –GLF method to the three-body systems He and e^? e^? e⁺ exhibit very fast convergence with number of HH basis sets. With only 36 HH and five GLF , we obtain the ground-state energy of ?2.90371 au for He, compared with the exact value of ?2.90372 au, and with only 36 HH and 10 GLF , we obtained the ground-state energy of -0.26188 au for e^? e^? e⁺, compared with the exact value of ?0.26200 au. We formulate the CFHH –GLF method in this article. © 1994 John Wiley & Sons, Inc.     

Interaction dipole moment in Rg–Xe (Rg = He, Ne, Ar, and Kr) heterodiatoms from conventional ab initio and density functional theory calculations

We have obtained interaction dipole moment curves for the rare gas heterodiatoms Rg...Xe (Rg = He, Ne, Ar, and Kr) from conventional ab initio and density functional theory calculations with flexible Gaussian-type basis sets. All methods seem to reproduce fairly similar dipole moment curves for all pairs. Our best values for the interaction dipole moment (at the respective experimental equilibrium separation R _e) were obtained at the coupled-cluster theory with single, double, and perturbatively linked triple excitations level of theory: μ_int(RgXe)/eα₀ = − 0.0025(He), − 0.0047(Ne), − 0.0055(Ar), and − 0.0037(Kr). The same trend (in absolute terms) is observed at the MP2 level of theory for the derivative of the dipole moment at R _e, as (dμ_int (RgXe)/dR) _e /e = 0.0043 (He), 0.0082 (Ne), 0.0091 (Ar), and 0.0059 (Kr). Around R _e , μ_int(HeXe) ≡ μ_HeXe varies at the MP2 level of theory as [μ_HeXe(R) − μ_HeXe(R _e)]/ea₀ = 0.0043(R − R _e) − 0.0033(R − R _e)² + 0.0018(R − R _e)³ − 0.0005(R − R _e)⁴.     

CEPA calculations on open-shell molecules. III. Potential curves for the six lowest excited states of He2 in the vicinity of their equilibrium distances

CEPA-PNO and PNO-CI calculations have been performed for the potential energy curves of the He ₂ ⁺ ground state and the six lowest excited states of He₂ in the range of 1.4 a₀ ≤R ≤ 3.5 a₀. The calculated equilibrium distances as well as the spectroscopic constants are in very good agreement with molecular constants as derived experimentally from the rotation-vibration spectrum of He₂ by Ginter, except for thec ³∑ _g ⁺ state. This latter discrepancy is probably due to an “obligatory” hump in thec ³∑ _g ⁺ state occurring at 3.5 a₀ which cannot be properly treated in our calculation. The relative energetic positions of the six lowest states and their ionization energies are reproduced by our calculations with an accuracy of 0–400 cm⁻¹. Extrapolation of our results to infinite basis sets leads to estimates of the dissociation energies of He₂ excited states which cannot be measured spectroscopically because of the humps in all these states.     

Reduction mechanism in class A methionine sulfoxide reductases: a theoretical chemistry investigation

Ab initio calculations at the B3LYP/6–311 ++G(2df,2p) and B3LYP/6–31G(d) level have been carried out to investigate the reaction mechanism of methionine sulfoxide reductases of class A. These enzymes reduce oxidized methionine in vivo and therefore play an important role in repairing protein damage caused by the oxidative stress. Our calculations have been carried out for a model reaction in a model active site. Several reaction mechanisms have been explored that can roughly be described as (2H⁺ + 2e⁻) or (H⁺ + e⁻). The results suggest that the actual reaction mechanism is of the (2H⁺ + 2e⁻) type corresponding to a more or less asynchronous-concerted double-proton transfer reaction leading to the formation of methionine (dimethylthioether in our model) and a sulfenic acid Cys-SOH. The Michaelis complex would involve one deprotonated Cys and one protonated Glu residues in the active site, this protonation state being mandatory to stabilize the sulfoxide substrate. Then, proton transfer from Glu to the substrate takes place, followed by proton transfer from one Tyr residue and fast reorganization of the system. The overall activation energy barrier is estimated to fall in the range 7–9 kcal/mol, much lower than the predicted barrier in DMSO solution (29.6 kcal/mol) reported before.     

Atom-atom potential functions for simulation of DNA—counterion interaction in aqueous solution

A system of atom-atom potential functions for computer simulation of aqueous solutions of DNA fragments and counterions was developed. Hydration of Na⁺, K⁺, and dimethyl phosphate (DMP⁻) ions was simulated by the Monte Carlo method. The obtained energy and structural characteristics of the solutions reproduce well the experimental data and are in good agreement with the results ofab initio calculations carried out by other authors. Translated fromIzvestiya Akademii Nauk Seriya Khimicheskaya, No. 11, pp. 2166–2173, November, 1998.     

The unimolecular chemistry of protonated glycine and the proton affinity of glycine: a computational model

The potential energy hypersurface of protonated glycine, GH⁺, has been investigated. The calculated G2(MP2) value for the proton affinity (PA) of glycine, PA _calc=895kJ mol⁻¹, is in good agreement with the experimental value which has been estimated to lie in the range 864kJ mol⁻¹ < PA _exp <891kJ mol⁻¹. Ab initio quantum chemical calculations of relevant parts of the potential energy surface of GH⁺ give a reaction model which is consistent with the observed mass spectrometric fragmentation pattern. The lowest energy unimolecular reactions of GH⁺ are two distinct processes: (1) loss of CO, which has a substantial barrier for the reverse reaction, and (2) loss of CO plus H₂O, which has no barrier for the reverse reaction. Received: 15 November 1996 / Accepted: 6 May 1997     

Modelling spur chemistry for alkaline and acidic water at high temperatures

The effect of pH and associated ionic strength on the primary yields in the radiolysis of pressurised water has been assessed by diffusion-kinetic calculations for temperatures in the range 100–300°C. Account has been taken for ionic strength I up to 0.1 mol kg⁻¹, assuming that the counter ions of H⁺ in acid solutions and of OH⁻ in base solutions have unit charge. In acid solutions, the H⁺ ions react with e⁻ _aq. The decrease in G(e⁻ _aq) and the increase in G(H) with decreasing pH becomes substantial for [H⁺] ≥ 1 × 10⁻⁴ m, but the primary yields of oxidising species are almost constant. In alkaline solutions, the OH⁻ anions affect the spur chemistry of radiation-generated protons and hydroxyl radicals for [OH⁻] ≥ 1 × 10⁻⁴ m. The scavenging of H atoms and hydrogen peroxide becomes significant for [OH⁻] ≥ 1 × 10⁻² m. The total yields G(OH) + G(O⁻) and G(H₂O₂) + G(HO₂ ⁻) are independent of base concentration below 0.01 m. In more alkaline solutions, G(OH) + G(O⁻) increases, whereas G(H₂O₂) + G(HO₂ ⁻) decreases with increasing [OH⁻]. Calculations showed the substantial yield of the reaction O⁻ + e⁻ _aq in 0.1 m base solution. Spur chemistry in alkaline hydrogenated water is not affected by the presence of H₂ if less than 0.001 m of hydrogen is added.     

Pseudopotential study of lanthanum and lutetium dimers

Relativistic energy-consistent small-core lanthanide pseudopotentials of the Stuttgart–Bonn variety and extended valence basis sets have been used for the investigation of the dimers La₂ and Lu₂. It was found that the ground states for La₂ and Lu₂ are most likely ¹∑ _g ⁺ (σ _g ²π _u ⁴) and ³∑ _g ⁻ (4f ¹⁴4f ¹⁴σ _g ²σ _u ²π_u ²), respectively. The molecular constants including error bars were derived from multireference configuration interaction as well as coupled-cluster calculations, taking into account corrections for atomic spin–orbit splitting as well as possible basis set superposition errors. The theoretical values for La₂ (R _e=2.70±0.03 ?, D _e=2.31±0.13 eV, ω_e=186±13 cm⁻¹) show good agreement with the experimental binding energy (D _e=2.52±0.22 eV), but the experimental vibrational constant in an Ar matrix (ω_e=236±0.8 cm⁻¹) is significantly higher. For Lu₂ the theoretical values (R _e=3.07±0.03 ?, D _e=1.40±0.12 eV, ω_e=123±1 cm⁻¹) are in overall excellent agreement with experimental data (D _e=1.43±0.34 eV, ω_e=122± 1 cm⁻¹). The electronic structures of La₂ and Lu₂ are compared to those other lanthanide dimers and trends in the series are discussed. Received: 25 March 2002 / Accepted: 2 June 2002 / Published online: 21 August 2002     

A systematic theoretical investigation of the valence excited states of the diatomic molecules B2, C2, N2 and O2

A quantitative survey on the performance of multireference (MR), configuration interaction with all singles and doubles (CISD), MRCISD with the Davidson correction and MR-average quadratic coupled cluster (AQCC) methods for a wide range of excited states of the diatomic molecules B₂, C₂, N₂ and O₂ is presented. The spectroscopic constants r _e, ω_e, T _e and D _e for a total of 60 states have been evaluated and critically compared with available experimental data. Basis set extrapolations and size-extensivity corrections are essential for highly accurate results: MR-AQCC mean-errors of 0.001 ?, 10 cm⁻¹, 300 cm⁻¹ and 300 cm⁻¹ have been obtained for r _e, ω_e, T _e and D _e, respectively. Owing to the very systematic behavior of the results depending on the basis set and the choice of method, shortcomings of the calculations, such as Rydberg state coupling or insufficient configuration spaces, can be identified independently of experimental data. On the other hand, significant discrepancies with experiment for states which indicate no shortcomings whatsoever in the theoretical treatment suggest the re-evaluation of experimental results. The broad variety of states included in our survey and the uniform quality of the results indicate that the observed systematics is a general feature of the methods and, hence, is molecule-independent. Received: 12 June 2000 / Accepted: 1 September 2000 / Published online: 21 December 2000     

Accurate calculations of dissociation energies of weakly bonded He<Subscript>2</Subscript> and Be<Subscript>2</Subscript> molecules by MRCI method

The He₂ and Be₂ ground state potential curves have been calculated by extrapolating to an infinite basis BSSE corrected MRCI total energies obtained with large Gaussian basis sets, large reference configuration spaces, and pseudo-natural molecular orbitals. The calculated D _e = 11.0031 K and R _e = 5.607 a.u. of He₂ are in an excellent agreement with D _e = 11.006 ± 0.004 K and R _e = 5.608 ± 0.012 a.u. obtained recently by SAPT with SM energy correction. The obtained Be₂ non-relativistic D _e = 822 cm⁻¹ and relativistically corrected D _e = 818 cm⁻¹ are in a good agreement with experimental D _e = 790 ± 30 cm⁻¹ and the value of 829 ± 64 cm⁻¹ obtained recently by a quantum Monte Carlo method.     

Reduction of Cr(VI) Assisted by Sol-Gel Generated Electron-Hole Centers

We report an in-situ harvesting technique of electron-hole (e⁻-h⁺) carriers (e.g., the defect electrons in the O^{2 −} matrix and the self-trapped holes, Si–O⁻–Si) generated during sol-gel processing. In the absence of redox species, the e⁻-h⁺ centers created during room temperature sol-gel polycondensation steps are quickly annihilated and deactivated. However, when Cr(VI) ions are pre-dispersed in sol-gel solutions, the ejected electrons can be effectively harvested for the reduction of Cr(VI) to Cr(III) ions which are encapsulated in the silica gel matrix. The Cr(VI) ions, the possible intermediate oxidation states of chromium ions such as Cr(V) and/or Cr(IV), and the stable Cr(III)-hole complexes in the sol-gel matrix are investigated using uv-visible spectroscopy, electron paramagnetic resonance spectroscopy, and cyclic voltammetry. The chemical stability of Cr(VI) and Cr(III) in sol-gel networks is compared to that in aqueous solutions. The results indicate that the utilization of e⁻-h⁺ carriers generated in the sol-gel can be an effective and selective means for investigating the redox process of Cr(VI) and encapsulating the stable Cr(III) ions in the confined sol-gel environments.     

Toward spectroscopic accuracy of ab initio calculations of vibrational frequencies and related quantities: a case study of the HF molecule

Calculations at various coupled-cluster (CC) levels with and without the inclusion of linear r _{i j} -dependent terms are performed for the HF molecule in its ground state with a systematic variation of basis sets. The main emphasis is on spectroscopic properties such as the equilibrium distance r _e and the harmonic vibration frequency ω_e. Especially with the R12 methods (including linear r _{i j} -dependent terms), convergence to the basis set limit is reached. However, the results (at the basis set limit) are rather sensitive to the level of the treatment of electron correlation. The best results are found for the CCSDT1-R12 and CCSD[T]-R12 methods (CCSD[T] was previously called CCSD+T(CCSD)), while CCSD(T) overestimates ω_e by ≈6 cm⁻¹. The good agreement of conventional CCSD(T) with experiment for basis sets far from saturation (e.g. truncated at g-functions) is probably the result of a compensation of errors. The contribution of core-correlation is non-negligible and must be included (effect on ω_e≈5 cm⁻¹). Relativistic effects are also important (23 cm⁻¹), while adiabatic effects are much smaller (<1cm⁻¹) and non-adiabatic effects on ω_e can be simulated in replacing nuclear by atomic masses; for rotation nuclear masses appear to be the better choice, at least for hydrides. From a potential curve based on calculations with the CCSDT1-R12 method with relativistic corrections, the IR spectrum is computed quantum-mechanically. Both the band heads and the rotational structures of the observed spectra are reproduced with a relative error of ≈10⁻⁴ for the three isotopomers HF, DF, and TF. Received: 3 July 1998 / Accepted: 4 August 1998 / Published online: 28 October 1998     

Theoretical study of the C3S molecule

For the most stable linear isomer of C₃S in its X¹Σ⁺ state a six-dimensional potential energy surface (PES) has been calculated ab initio by coupled cluster – connected triples (CCSD(T)) method. The analytic form of the PES has been transformed in a quartic force field in dimensionless normal coordinates and employed in calculations of spectroscopic constants using second-order perturbation theory. The PES and the full kinetic energy operator in internal coordinates have been used to calculate variationally the anharmonic ro-vibrational energies for J=0 and J=1. The two experimental band origins of C₃S observed in the gas phase, ν₁ and ν₁+ν₅−ν₅, agree very well with the theoretical values. The anharmonic ro-vibrational levels, including the bending modes up to 2200 cm⁻¹, are reported. The singlet ground state PES has a saddle point at about 1.25 eV above the linear minimum and two other higher lying cyclic local minima. The only dipole- and spin-allowed electronic transition between 0 and 5 eV is calculated to be the ¹Π−X¹Σ⁺ transition with a vertical transition energy of 353.2 nm in good agreement with the matrix value of 378 nm. The dissociative paths C + C₂S, C₂ + CS and C₃ + S of low lying singlet and triplet states have been investigated. Electronic Supplementary Material: Supplementary material is available in the online version of this article at dx.doi.org/10.1007/s00214-005-0683-7 Dedicated to Professor H. Stoll.     

A theoretical study of the acetylide anion, HCC−

The results of various ab initio calculations are reported for the electronic ground state of the acetylide anion. An “Eyring's lake” in the T-shaped configuration is identified with six different methods (SCF, MP2, CCSD, CCSD-T, CCSD(T), and CEPA–1). The equilibrium bond lengths of HCC⁻ are estimated to be r _e (CH)=1.0689(3) ? and R _e (CC)=1.2464(2) ?, and the ground-state rotational constant is predicted to be B ₀=41636(20)MHz. The large permanent dipole moment of μ₀=−3.093D should facilitate detection of the anion by microwave spectroscopy. The band centers are predicted to be 3211.3cm⁻¹(ν₁), 511.1cm⁻¹(ν₂), and 1805.0cm⁻¹(ν₃). A large transition dipole moment of 0.477 D is calculated for the ν₂ band. Rovibrational levels of HCC⁻ up to approximately 20 000 cm⁻¹ above equilibrium are calculated with DVR-DGB and FBR methods on the basis of a previous CEPA–1 potential energy surface. Different energy patterns are found and discussed, for which anharmonic and Coriolis resonances are shown to play an important role. Received: 27 July 1998 Accepted: 12 August 1998 / Published online: 19 October 1998     

Theoretical studies on the structure of M+BF−4 ion pairs M = Li+, NH+4: the role of electrostatics and electron correlation

A systematic investigation of the M⁺BF₄ ⁻ (M = Li or NH₄) ion-pair conformers has been carried out using an electrostatic docking model based on the molecular electrostatic potential topography of the free anion. This method provides a guideline for the subsequent ab initio molecular orbital calculations at the Hartree-Fock (HF) and second-order M?ller-Plesset perturbation theory (MP2) levels. It has been demonstrated that the model presented here yields more than 75% of the HF interaction energy when Li⁺ is the cation involved and more than 90% for the case of NH₄ ⁺. Inclusion of MP2 correlation in the HF-optimized geometries leads to stationary point geometries with different numbers of imaginary frequencies and in some places where the energies of two adjacent conformers are very close, the energy rank order is altered. The HF lowest-energy minima for the Li⁺BF₄ ⁻ and NH₄ ⁺BF₄ ⁻ show a bidentate and tridentate coordinating cation, respectively, whereas at the MP2 level, this ordering is reversed. Received: 9 September 1997 / Accepted: 5 November 1997     

Theoretical and experimental study of the structure and stability of multiply Na-substituted glucose and 2,4,6-trihydroxyacetophenone derivatives

The equilibrium geometric parameters and the energetic and spectroscopic characteristics of low lying conformers for series of polyhydroxyl molecules and ions in which sodium atoms are successively substituted for the hydroxyl hydrogen atoms have been calculated by the density functional theory B3LYP method with the 6−31G* and 6−311+G** basis sets. The glucose derivatives [Glu − nH + nNa] and [Glu − nH + (n + 1)Na]⁺ (n = 1−5) and the 2,4,6-trihydroxyacetophenone derivatives [THAP − nH + nNa] and [THAP − nH + (n + 1)Na]⁺ (n = 1−4) have been considered. The affinities of the neutral [Glu − nH + nNa] and [THAP − nH + nNa] molecules for adding Na⁺ cations, as well as the energies of successive substitution of Na atoms for H atoms in the Glu and THAP molecules and the Glu⁺ and THAP⁺ ions in their reaction with sodium acetate molecules, have been estimated. Computations show that the first substitution of Na for H in ions is slightly exothermic and, presumably, can spontaneously occur under common conditions. Further substitutions are endothermic, but the required energy inputs are small. Therefore, successive substitutions for two, three, or more hydroxyl H atoms in the molecules and ions under consideration are possible at relatively low energy inputs. The computation results and conclusions are compared with the MALDI TOF mass spectral data for Na-substituted glucose and 2,4,6-trihydroxyacetophenone derivatives in the [glucose + CH₃COONa + THAP] system where, in addition to common Glu · Na⁺ and THAP · Na⁺ ion-molecular complexes, multiply substituted positive ions of the [Glu − nH + (n + 1)Na]⁺ (n = 1−4) and [THAP − nH + (n + 1)Na]⁺ (n = 1−3) type have been identified.     

Oxygen reduction in acid media by a Ru<Subscript>x</Subscript>Fe<Subscript>y</Subscript>Se<Subscript>z</Subscript>(CO)<Subscript>n</Subscript> cluster catalyst dispersed on a glassy carbon-supported Nafion film

Electrocatalytic oxygen reduction was studied on a Ru_xFe_ySe_z(CO)_n cluster catalyst with Vulcan carbon powder dispersed into a Nafion film coated on a glassy carbon electrode. The synthesis of the electrocatalyst as a mixture of crystallites and amorphous nanoparticles was carried out by refluxing the transition metal carbonyl compounds in an organic solvent. Electrocatalysis by the cluster compound is discussed, based on the results of rotating disc electrode measurements in a 0.5 M H₂SO₄. A Tafel slope of −80.00±4.72 mV dec⁻¹ and an exchange current density of 1.1±0.17×10⁻⁶ mA cm⁻² was calculated from the mass transfer-corrected curve. It was found that the electrochemical reduction reaction follows the kinetics of a multielectronic (n=4e⁻) charge transfer process producing water, i.e. O₂+4H⁺+4e⁻→2H₂O. Electronic Publication     

Isomerism in OBe3F3 + cation: anab initio study

Ab initio MP2/6-31G^*//HF/6-31G^*+ZPE(HF/6-31G^*) calculations of the potential energy surface in the vicinity of stationary points and the pathways of intramolecular rearrangements between low-lying structures of the OBe₃F₃ ⁺ cation detected in the mass spectra of μ₄-Be₄O(CF₃COO)₆ were carried out. Ten stable isomers with di- and tricoordinate oxygen atoms were localized. The relative energies of six structures lie in the range 0–8 kcal mol⁻¹ and those of the remaining four structures lie in the range 20–40 kcal mol⁻¹. Two most favorable isomers, aC _2v isomer with a dicoordinate oxygen atom, planar six-membered cycle, and one terminal fluorine atom and a pyramidalC _3v isomer with a tricoordinate oxygen atom and three bridging fluorine atoms, are almost degenerate in energy. The barriers to rearrangements with the breaking of one fluorine bridge are no higher than 4 kcal mol⁻¹, except for the pyramidalC _3v isomer (∼16 kcal mol⁻¹). On the contrary, rearrangements with the breaking of the O−Be bond occur with overcoming of a high energy barrier (∼24 kcal mol⁻¹). A planarD _3h isomer with a tricoordinate oxygen atom and linear O−Be−H fragments was found to be the most favorable for the OBe₃H₃ ⁺ cation, a hydride analog of the OBe₃F₃ ⁺ ion; the energies of the remaining five isomers are more than 25 kcal mol⁻¹ higher. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 420–430, March, 1999.     

Raman spectra and structural features of ClOF2 + cation in a solution of anhydrous HF

The Raman spectra of ClOF₂ ⁺ cation in solutions of anhydrous HF were studied. In the ClOF₂ ⁺HF₂ ⁻ and ClOF₂ ⁺BF₄ ⁻−HF systems, this cation exists as a pyramidal structure (C _s symmetry), while in the ClOF₂ ⁺AuF₆ ⁻−HF system, it exists as a planar structure (C _2v symmetry). Based on nonempirical calculations by the Hartree-Fock-Roothaan method, an explanation for the dependence of the structure of the ClOF₂ ⁺ cation on the nature of the anion was proposed. For the Cl−O bond vibrations, the correlation functions of vibrational and rotational relaxations were calculated, and the characteristic times of these processes were determined. The main contribution to the formation of the band contours corresponding to the above-mentioned modes is made by the vibrational dephasing. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 432–437, March, 1998.