SCF and localized orbitals in ethylene: MBPT/CC results and comparison with one-million configuration Cl

Many-body perturbation theory (MBPT) and coupled-cluster (CC) calculations are performed on the ethylene molecule employing canonical SCF and simple bond-orbital localized orbitals (LO). Full fourth-order MBPT [i.e. SDTQ MBPT(4)], CC doubles (CCD) and CC singles and doubles (CCSD) energies are compared with the over one-million configuration ‘bench-mark” Cl calculation of Saxe et al. Though the SCF and LO reference determinant energies differ by 0.29706 hartree, the CCSD energy difference is only 1.7 mhartrees (mh). Our most extensive SCF orbital calculation, CCSD plus fourth-order triples, is found to be lower in energy than the CI result by 5.3 mh.     

Geometries and stabilities of various configurations of benzene dimer: details of novel V-shaped structure revealed

Benzene dimer configurations namely T-shaped, parallel-displaced, sandwich, and V-shaped that were proposed by experimental studies are investigated using second- and fourth-order Møller–Plesset perturbation theory. The MP2 method with aug-cc-pVDZ and aug-cc-pVTZ basis sets unequivocally shows that the parallel-displaced configuration is considerably more stable than T-shaped structure. On the other hand, the MP4(SDTQ)/aug-cc-pVDZ level predicts that the T-shaped and parallel-displaced configurations are nearly isoenergetic, which is parallel to the previous results of estimated CCSD(T)/CBS level reported recently. The lowest energy T-shaped configuration is stabilized by 0.17 kcal/mol over the parallel-displaced configuration at the MP4(SDTQ)/aug-cc-pVDZ level. Although the structures of all the four different types of configurations are found to be stable at both MP2 and full MP4 methods, the V-shaped configuration is the least stable among them. The calculated interaction energy of ?2.3 kcal/mol for the lowest energy T-shaped structure at the MP4(SDTQ)/aug-cc-pVDZ level is in good agreement with the experimental value of ?2.4 ± 0.4 kcal/mol. We conclude that the MP4(SDTQ) with a reasonably good basis set can be used for systems involving π–π interactions to obtain qualitative and quantitative results.     

Ab initio calculations on the barrier height for the hydrogen addition to ethylene and formaldehyde. The importance of spin projection

Heats of reaction and barrier heights have been computed for H + CH₂CH₂ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH using unrestricted Hartree-Fock and Møller–Plesset perturbation theory up to fourth order (with and without spin annihilation), using single-reference configuration interaction, and using multiconfiguration self-consistent field methods with 3-21G, 6-31G(d), 6-31G(d,p), and 6-311G(d,p) basis sets. The barrier height in all three reactions appears to be relatively insensitive to the basis sets, but the heats of reaction are affected by p-type polarization functions on hydrogen. Computation of the harmonic vibrational frequencies and infrared intensities with two sets of polarization functions on heavy atoms [6-31G(2d)] improves the agreement with experiment. The experimental barrier height for H + C₂H₄ (2.04 ± 0.08 kcal/mol) is overestimated by 7?9 kcal/mol at the MP2, MP3, and MP4 levels. MCSCF and CISD calculations lower the barrier height by approximately 4 kcal/mol relative to the MP4 calculations but are still almost 4 kcal/mol too high compared to experiment. Annihilation of the largest spin contaminant lowers the MP4SDTQ computed barrier height by 8?9 kcal/mol. For the hydrogen addition to formaldehyde, the same trends are observed. The overestimation of the barrier height with Møller-Plesset perdicted barrier heights for H + C₂H₄ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH at the MP4SDTQ /6-31G(d) after spin annihilation are respectively 1.8, 4.6, and 10.5 kcal/mol.     

Thermogravimetric investigation of the dehydration thermodynamics of amorphous silica after its hydrothermal and thermovaporous treatment

After hydrothermal and thermovaporous treatment of chemically pure amorphous aqueous silicic acid in solutions of NaOH and NH₄OH and in water vapour it is possible, using complex thermal analysis, to detect the weight loss and heat effects corresponding to evaporation of various forms of combined water, and to estimate the heats of evaporation of these forms. From the obtained data, the following water forms have been identified: (1) at 200–300° capillary-condensed water formations of the cluster type evaporate;ΔH _deh is about 8 kcal/mole H₂O; (2) at 250–400°, molecules of water linked by hydrogen bonds with hydroxyl groups on the surface and in the volume of the particles;ΔH _deh. is about 5 kcal/mole H₂O; (3) at 350 600°, molecules of water coordinated to silicon atoms in the volume of the particles;ΔH _deh is approximately 1 kcal/mole H₂O. The total evaporation heat changes from 10 kcal/mole H₂O when water of form 1 predominates, to 5 kcal/mole H₂O when forms 2 and 3 predominate.     

Formic acid catalyzed gas-phase reaction of H2O with SO3 and the reverse reaction: a theoretical study

The formic acid catalyzed gas‐phase reaction between H₂O and SO₃ and its reverse reaction are respectively investigated by means of quantum chemical calculations at the CCSD(T)//B3LYP/cc‐pv(T+d)z and CCSD(T)//MP2/aug‐cc‐pv(T+d)z levels of theory. Remarkably, the activation energy relative to the reactants for the reaction of H₂O with SO₃ is lowered through formic acid catalysis from 15.97 kcal mol^?1 to ?15.12 and ?14.83 kcal mol^?1 for the formed H₂O ??? SO₃ complex plus HCOOH and the formed H₂O ??? HCOOH complex plus SO₃, respectively, at the CCSD(T)//MP2/aug‐cc‐pv(T+d)z level. For the reverse reaction, the energy barrier for decomposition of sulfuric acid is reduced to ?3.07 kcal mol^?1 from 35.82 kcal mol^?1 with the aid of formic acid. The results show that formic acid plays a strong catalytic role in facilitating the formation and decomposition of sulfuric acid. The rate constant of the SO₃+H₂O reaction with formic acid is 10⁵ times greater than that of the corresponding reaction with water dimer. The calculated rate constant for the HCOOH+H₂SO₄ reaction is about 10^?13 cm³ molecule^?1 s^?1 in the temperature range 200–280 K. The results of the present investigation show that formic acid plays a crucial role in the cycle between SO₃ and H₂SO₄ in atmospheric chemistry.     

AB initio study of thermodynamic stability and structure of cage molecules B4N4H8 and Be4O4H8

Stable molecular structures of heterocubane systems B₄N₄H₈ 2 and Be₄O₄H₈, isoelectronic to the cubane molecule, are investigated by ab initio (RHF/6-31G**, MP2(full)/6-31C**, and MP2(full)/6-311+ + G**) methods and are shown to be highly thetmodynamically stable. Decomposition of structure 2 into two 1,3,2,4-diazadiboroethidine molecules 6 or four iminobomne NBNH molecules 11 is an endothermal process taking 10.1 (RHF/6-31G**), 39.6 (MP2(full)/6-31G**) kcaUmole and 140.6 (RHF/6-31G**), 161.4 (MP2(full)/6-31G**) kcal/mole, respectively. Decomposition of structure 3 into two 1,3,2,4-dioxydi-beryllothidine molecules 12 or four molecules 13 is also an endothermal reaction taking 22.1 (RHF/6-31G**), 39.8 (MP2(full)/6-31G**) kcal/mole and 127.1 (RHF/6-31G**), 155.2 (MP2(full)/631G**) kcal/mole, respectively. The geometrical characteristics of simple molecules BeH₂ 15, Be₂ 16 and 17, Be₂H₂ 18, Be₂H₄ 19, BeO 20, and Be₂O₂ 21 are calculated. Translated from Zhumal Struktumoi Khim ii, Vol. 41, No. 1, pp. 3-13, January–February, 2000     

A simple,self-consistent electrostatic model for quantitative prediction of the activation energies of four-center reactions. II.*

A simple electrostatic model of point dipoles is used which permits direct calculation of the activation energies for the addition of the molecules H₂O, H₂S, H₃N, and H₃P to olefins. These calculated values agree with the known experimental data to within ±2 kcal/mole on the average. It was found that the best fit could be obtained with a polar transition state that corresponded to a reduction in bond order from 1 to ½ for the bond-breaking coordinates and an increase in bond order from 0 to 0.18 for the bond-forming coordinates. The replacement of a hydrogen atom of the species H₂O, H₂S, H₃N, or H₃P by a polarizable methyl group is expected to stabilize the charge on the central atoms. The following stabilization energies for the pairs H₂O? CH₃OH, H₂S? CH₃SH, H₃N? CH₃NH₂, H₃P? CH₃PH₂ were calculated: ?4.8 kcal/mole, ?0.7 kcal/mole, ?1.9 kcal/mole, ?0.8 kcal/mole, respectively.     

The gas phase hydrogen‐bonded dimers of HOCl: A high‐level quantum chemical study

The O···H? O and Cl···H? O hydrogen bonding interactions were analyzed for HOCl dimers by using B3LYP, MP2, CCSD, and MP4(SDTQ) methods in conjunction with the various basis sets. Five isomers were found for the HOCl dimer. The ZPE and BSSE corrected binding energies were computed at the different levels of theory. At the optimized geometries obtained at CCSD/AUG‐cc‐pVDZ level, energies were re‐evaluated at MP4(SDTQ)/AUG‐cc‐pVTZ and CCSD(T)/cc‐pVTZ levels of theory. We found an average of ?20.9 and ?9.6 kJ/mol for the strength of the O···H and Cl···H hydrogen bonding interactions, respectively. Excitation and vertical ionization energies as well as rotational constants were computed at different levels of theory. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis were used to elucidate the nature of the interactions of HOCl dimers. The interaction energies were decomposed by Morokuma methodology. We have computed Δ_fH°(HOCl) and Δ_fH°(HOCl⁺) using the atomization reactions. The Δ_fH°₂₉₈(HOCl) values are ?17.85 and ?18.05 kcal/mol by using CBS‐Q and CBS‐QB3 extrapolation models, respectively, in good agreement with the results given in JANAF tables. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Density functional theory and post-Hartree-Fock studies on molecular structure and harmonic vibrational spectrum of formaldehyde

Density functional theory (DFT) with the Becke's three-parameter exchange correlation functional and the functional of Lee, Yang and Parr, gradient-corrected functionals of Perdew, and Perdew and Wang [the DFT(B3LYP), DFT(B3P86) and DFT(B3PW91) methods, respectively], and several levels of conventional ab initio post-Hartree-Fock theory (second- and fourth-order perturbation theory M?ller-Plesset MP2 and MP4(SDTQ), coupled cluster with the single and double excitations (CCSD), and CCSD with perturbative triple excitation [CCSD(T)], configuration interaction with the single and double excitations [CISD], and quadratic configuration interaction method [QCISD(T)], using several basis sets [ranging from a simple 6-31G(d,p) basis set to a 6-311+ +G(3df, 2pd) one], were applied to study of the molecular structure (geometrical parameters, rotational constants, dipole moment) and harmonized infrared (IR) spectrum of formaldehyde (CH₂O). High-level ab initio methods CCSD(T) and QCISD(T) with the 6-311+ +G(3df, 2pd) predict correctly molecular parameters, vibrational harmonic wavenumbers and the shifts of the harmonic IR spectrum of ¹²CH₂ ¹⁶O upon isotopic substitution. Received: 30 January 1997 / Accepted: 7 May 1997     

Theoretical Studies on Mechanism and Rate Constant of Gas Phase Hydrolysis of Glyoxal Catalyzed by Sulfuric Acid

The gas phase hydration of glyoxal (HCOCHO) in the presence of sulfuric acid (H2SO4) were studied by the high-level quantum chemical calculations with M06-2X and CCSD(T) theoretical methods and the conventional transition state theory (CTST). The mechanism and rate constant of the ve di erent reaction paths are consid-ered corresponding to HCOCHO+H₂O, HCOCHO+H₂O H₂O, HCOCHO H₂O+H₂O, HCOCHO+H₂O H₂SO₄ and HCOCHO H₂O+H₂SO₄. Results show that H₂SO₄ has a strong catalytic ability, which can signi cantly reduce the energy barrier for the hydration reaction of glyoxal. The energy barrier of hydrolysis of glyoxal in gas phase is lowered to 7.08 kcal/mol from 37.15 kcal/mol relative to pre-reactive complexes at the CCSD(T)/6-311++G(3df, 3pd)//M06-2X/6-311++G(3df, 3pd) level of theory. The rate constant of the H₂SO₄ catalyzed hydrolysis of glyoxal is 1.34×10^-11cm³/(molecule s), about 10¹³ higher than that involving catalysis by an equal number of water molecules, and is greater than the reaction rate of glyoxal reaction with OH radicals of 1.10×10^-11cm³/(molecule s) at the room temperature, indicating that the gas phase hydrolysis of glyoxal of H₂SO₄ catalyst is feasible and could compete with the reaction glyoxal+OH under certain atmospheric condi-tions. This study may provide useful information on understanding the mechanistic features of inorganic acid-catalyzed hydration of glyoxal for the formation of oligomer     

Examination of the structures,energetics, and vibrational frequencies of small sulfur-containing prototypical dimers, (H2S)2 and H2O/H2S

The optimized geometries, vibrational frequencies, and dissociation energies from MP2 and CCSD(T) computations with large correlation consistent basis sets are reported for (H₂S)₂ and H₂O/H₂S. Anharmonic vibrational frequencies have also been computed with second-order vibrational perturbation theory (VPT2). As such, the fundamental frequencies, overtones, and combination bands reported in this study should also provide a useful road map for future spectroscopic studies of the simple but important heterogeneous H₂O/H₂S dimer in which the hydrogen bond donor and acceptor can interchange, leading to two unique minima with very similar energies. Near the CCSD(T) complete basis set limit, the HOH⋯SH₂ configuration (H₂O donor) lies only 0.2 kcal mol⁻¹ below the HSH⋯OH₂ structure (H₂S donor). When the zero-point vibrational energy is included, however, the latter configuration becomes slightly lower in energy than the former by <0.1 kcal mol⁻¹. © 2018 Wiley Periodicals, Inc.     

The structure of the H3O+ (hydronium) ion

Near Hartree-Fock level ab initio molecular orbital calculations on H₃O⁺ and a minimum energy structure with θ(HOH) = 112.5° and r(OH) = 0.963 Å and an inversion barrier of 1.9 kcal/mole. By comparing these results to calculations on NH₃ and H₂O, where precise experimental geometries are known, we estimate the “true” geometry of isolated H₃O⁺ to have a structure with θ(HOH) = 110-112°, r(OH) = 0.97–0.98 Å and an inversion barrier of 2–3 kcal/mole. Our prediction for the proton affinity of water is ≈ 170 kcal/mole, which is somewhat smaller than the currently accepted value.     

Molecular orbital theory of the hydrogen bond. VIII. Hydrogen bonding in H2OH2CO in relaxed singlet and triplet n → π* states

Ab initio SCF CI calculations with a minimal STO-3G basis set have been performed on the hydrogen bonded dimers in which H₂O is the proton donor to H₂CO in its relaxed singlet and triplet n→π^* states. Two dimers which are easily interconverted are found in the singet n→π^* state with hydrogen bond energies of 1.82 and 1.71 kcal/mole. The equilibrium dimer in the triplet state has a hydrogen bond energy of 2.97 kcal/mole. In both states, hydrogen bond formation occurs at the carbon atom. The structures of the dimers and the nature of the intermolecular surfaces in the regions of hydrogen bond formation are examined. Electron densities and distributions are also discussed.     

Synthesis,characterization, and antibacterial activity of a manganese(II) complex of triaryltriazole

A manganese(II) complex of 4-(4-methylphenyl)-3,5-bis(2-pyridyl)-4H-1,2,4-triazole (MBPT) was synthesized and characterized by X-ray crystallography. [Mn(MBPT)₂(H₂O)₂](ClO₄)₂?·?4H₂O is a divalent mononuclear manganese(II) complex with manganese coordinated to four nitrogens from two triazole ligands and two oxygens from two water molecules in a slightly distorted octahedral geometry. The complex and ligand were tested in vitro for their antibacterial activities. The title complex showed a wide range of bactericidal activities.     

Near-degeneracy effects and efficiency of MBPT calculations of molecular properties. Polarizability curve of H2

The polarizability curve of H₂ is calculated by using the finite-field perturbation method. All self-consistency effects are accounted for at the HF level and many-body perturbation theory (MBPT) is used to evaluate the correlation contributions. Using a single HF determinant as a reference in MBPT calculations makes the near-degeneracy effects of essential importance on increasing the interatomic distance. Nevertheless, applying the MBPT scheme with appropriate fourth-order terms gives nearly exact values of both components of the polarizability tensor for interatomic distances up to ≈3.6 au.     

Correlation effects on energy curves for proton transfer. The cation [H5O2]+

Potential curves for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺ were calculated by both ab initio and semi-empirical LCAO MO SCF CI methods. The energy barrier of the symmetric double minimum potential in [H₅O₂]⁺ is very sensitive to electron correlation. At an OO distance of 2.74 Å it decreases from the HF value of 9.5 kcal/mole to about 7.0 kcal/mole. The results of the semi-empirical calculations agree well with the ab initio data as long as only relative effects are regarded. The partitioning of correlation energy into contributions of individual electron pairs is very similar for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺. In this example the proton transfer appears as a superposition of two “contracted ionic dissociation” processes. An interpretation of the behaviour of correlation during these processes is presented.     

SCF LCAO MO studies on the hydration of ions: The system F− · 2H2O

The energy surface of the dihydrated fluoride anion (F·2H₂O)^–1 is studied for a number of different geometry points near the equilibrium structure within the SCF LCAO MO framework, using an extended gaussian basis set to approximate the molecular wavefunctions. For the first and second hydration step of the fluoride anion the corresponding hydration energies are calculated to beB ₁ ^scf =24.1 kcal/mole andB ₂ ^SCF =20.8 kcal/mole (experimental measurements: 23.3 kcal/mole and 16.6 kcal/mole, respectively). The hydration energies and equilibrium bond distances obtained for the dihydrated fluoride anion (F·2H₂O)^– are compared with those found for the monohydrate (FHOH)^– and with corresponding results of the dihydrated lithium cation (Li · 2H₂O)⁺. The system (F·2H₂O)^– is taken as a very simple model to discuss some basic features of the hydration process of small ions and to study the influence of a negative ion on an adjacent hydrogen bond.We would like to thank our technical staff for valuable help in carrying out these calculations.     

Potential energy surface and thermochemistry for the direct gas phase reaction of germane and water

Gas phase reaction between germane GeH₄ and water H₂O was investigated at CCSD(T)/[aug-cc-pVTZ-pp for Ge + Lanl2dz for H and O]//MP2/6-31G(d,p) level. Only the hydrogen elimination channels are monitored. Within the energy range of 100 kcal/mol, we located nine equilibrium and six transition states on the potential energy surface (PES) of the Ge–O–H systems. GeH₄ reacts with H₂O exothermically (by 2.37 kcal/mol) without a barrier to form a non-planar complex GeH₄·H₂O which isomerizes to GeH₃OH·H₂ and H₂GeOH₂·H₂ with a barrier of 44.34 kcal/mol and 53.75 kcal/mol respectively. The first step of hydrogen elimination gives two non-planar species, GeH₃OH and H₂GeOH₂ but germinol GeH₃OH is found to be more stable. Further thermal decomposition reactions of GeH₃OH involving hydrogen elimination have been studied extensively using the same method. The final hydrogen elimination step gives HGeOH which can exist in cis and trans forms. As the trans form is more stable, only the trans form is considered on the potential energy surface (PES) of the reaction. The important thermochemical parameters (∆_rE_tot + ZPE), ∆_rH and ∆_rG for the H₂ elimination pathways are predicted accurately.     

Thermochemistry of the hydrates of k2CoCl4

A study of the dissociation pressure of crystalline K₂CoCl₄·2H₂O. The reactions can be summed up as K₂CoCl₄·nH₂O(c) = K₂CoCl₄·mH₂O(c)+(nm)H₂O (v). Below 50°C, n = 2 and 1, m = 1 and 0, above 50°C, n = 2 and m = 0. Below 50°C, the dihydrate is octahedral, the monohydrate and anhydrous compounds are tetrahedral. ΔH° and ΔS° are respectively 7.0 kcal and 14.2 e.u. for the loss of the first mole of water and 12.7 kcal and 32.0 e.u. for the loss of the last mole of water. Above 50°C, ΔH° and ΔS° are respectively 29.8 kcal and 77.6 e.u. for the loss of both waters. The changes in structure are discussed using the spectral and magnetic properties as indications for structural changes.     

Thermal decomposition of (NH4)2MoS4 and (NH4)2WS4heat of formation of (NH4)2MoS4

The reactions (NH₄)₂MeS₄ = 2 NH₃ + H₂S + MeS₃ (Me = Mo, W) were investigated by measuring the decomposition vapour pressures. Thermochemical data were obtained from these measurements: ΔH = 52 kcal/mole and ΔS = 105 cal/deg.mole for the decomposition of the tetrathiomolybdate. Similarly, ΔH = 69 kcal/mole and ΔS = 106 cal/deg.mole were obtained for the decomposition of the tetrathiotungstate. The normal heat of formation of (NH₄)₂MoS₄ was found to be ΔH = ?140 kcal/mole. The kinetics of thermal decomposition of the above reactions were also measured.