Theoretical studies on the stability of the H3O radical based on ab initio UHF-CI calculations

An UHF-CI investigation of parts of the energy surface for the H₃O radical is reported. Several types of basis sets have been used and the CI expansion included all singly and doubly replaced configurations using an UHF determinant as the reference state. H₃O, constrained-to C_3v symmetry is in the best approximation found to be 20.5 kcal/mol less stable than H₂O + H. A small local barrier of 4.6 kcal/mol for dissociation is found on the UHF level of approximation. Correlation effects lower this barrier to 3.4 kcal/mol making the existence of a quasibound state with a measurable lifetime improbable. The height of the barrier was found to be very sensitive to the detailed form of the diffuse singly occupied orbital.     

Synthesis,Crystal Structure,and Magnetic Properties of {[Cu(phen)]2(C4H2O4)2} · 4.5H2O with C4H4O4 = Maleic Acid

Reaction of CuCl₂ · 2H₂O, phenanthroline, maleic acid and NaOH in CH₃OH/H₂O (1:1 v/v) at pH = 7.0 yielded blue {[Cu(phen)]₂(C₄H₂O₄)₂} · 4.5H₂O, which crystallizes in the monoclinic space group C2/c (no. 15) with cell dimensions: a = 18.127(2)Å, b = 12.482(2)Å, c = 14.602(2)Å, β = 103.43(1)°, U = 3213.5(8)ų, Z = 4. The crystal structure consists of the centrosymmetric dinuclear {[Cu(phen)]₂(C₄H₂O₄)₂} complex molecules and hydrogen bonded H₂O molecules. The Cu atoms are each square‐pyramidally coordinated by two N atoms of one phen ligand and three carboxyl O atoms of two maleato ligands with one carboxyl O atom at the apical position (d(Cu‐N) = 2.008, 2.012Å, equatorial d(Cu‐O) = 1.933, 1.969Å, axial d(Cu‐O) = 2.306Å). Two square‐pyramids are condensed via two apical carboxyl O atoms with a relatively larger Cu···Cu separation of 3.346(1)Å. The dinuclear complex molecules are assembled via the intermolecular π—π stacking interactions into 1D ribbons. Crossover of the resulting ribbons via interribbon π—π stacking interactions forms a 3D network with the tunnels occupied by H₂O molecules. The title complex behaves paramagnetically between 5—300 K, following the Curie‐Weiss law _χm(T—θ) = 0.435 cm³ · mol^—1 · K with θ = 1.59 K.     

Two new phosphinic amides: Synthesis,crystal structure,and theoretical study of hydrogen bonding

Two novel phosphinic amides, (C₆H₅)₂P(O)(NH?cyclo?C₇H₁₃) (I) and (C₆H₅)₂P(O)(NH?cyclo?C₆H₁₁) (II) were synthesized and characterized by spectroscopic methods and X-ray crystallography. Both compounds crystallize in the orthorhombic chiral space group P2₁2₁2₁ and in both structures, the N—H···O hydrogen bonds lead to one-dimensional arrangements along the a axis. The molecular geometries and vibrational frequencies of I and II were investigated with quantum chemical calculations at the B3LYP/6–311G^** level of theory. Furthermore, the hydrogen bonds were studied by means of the Bader theory of atoms in molecules (AIM) and natural bond orbital (NBO) analysis.     

The structure and stability of the acetylene dication

The dication C₂H has been investigated by ab initio molecular orbital theory. It is found to have a linear (D_∞h), structure with a triplet (³σ^?_g) ground state. Deprotonation to C₂H⁺ is exothermic by 9.8 kcal/mol, but this process is hindered by a large barrier of 65 kcal/mol.     

Vinyloxyborane and its isomers. An ab initio study of the C2H5BO potential energy surface,the barrier to 1,3-shifts in β-ketoboranes,and the mechanism of the carbonylation reaction of boranes

Vinyloxyboranes, CH₂?CH? ;O? ;BR₂, are shown by ab initio molecular orbital theory to be more stable than the isomeric β-aldoboranes, R₂B? CH₂? CH?O, by ca. 19 kcal/mol. The MP2/6-31G*/6-31G* + ZPE barrier for the [1,3] boron shift is only 10.9 kcal/mol (R ? Me) relative to the aldoborane. Other C₂H₅BO isomers (β-ketoboranes, boraepoxides and organoboron oxides), which are related to the proposed stages in the carbonylation reaction of boranes, are shown to be plausible intermediates. However, some of the computed barriers for methyl group migrations are unrealistically large, up to ca. 63 kcal/mol.     

Kinetic study of reactions of C2H5O2 with NO at 298 K and 0.55 – 2 torr

The kinetics of C₂H₅O₂ and C₂H₅O₂ radicals with NO have been studied at 298 K using the discharge flow technique coupled to laser induced fluorescence (LIF) and mass spectrometry analysis. The temporal profiles of C₂H₅O were monitored by LIF. The rate constant for C₂H₅O + NO → Products (2), measured in the presence of helium, has been found to be pressure dependent: k₂ = (1.25±0.04) × 10^?11, (1.66±0.06) × 10^?11, (1.81±0.06) × 10^?11 at P (He) = 0.55, 1 and 2 torr, respectively (units are cm³ molecule^?1 s^?1). The Lindemann-Hinshelwood analysis of these rate constant data and previous high pressure measurements indicates competition between association and disproportionation channels: C₂H₅O + NO + M → C₂H₅ONO + M (2a), C₂H₅O + NO → CH₃CHO + HNO (2b). The following calculated average values were obtained for the low and high pressure limits of k_2a and for k_2b : k = (2.6±1.0) × 10^?28 cm⁶ molecule^?2 s^?1, k = (3.1±0.8) × 10^?11 cm³ molecule^?1 s^?1 and k_2b ca. 8 × 10^?12 cm³ molecule^?1 s^?1. The present value of k, obtained with He as the third body, is significantly lower than the value (2.0±1.0) × 10^?27 cm⁶ molecule^?2 s^?1 recommended in air. The rate constant for the reaction C₂H₅O₂ + NO → C₂H₅O + NO₂ (3) has been measured at 1 torr of He from the simulation of experimental C₂H₅O profiles. The value obtained for k₃ = (8.2±1.6) × 10^?12 cm³ molecule^?1 s^?1 is in good agreement with previous studies using complementary methods. © 1995 John Wiley & Sons, Inc.     

Stationary points on the potential energy surface of O2−HF and O2−H2O

The determination of minima and saddle points on the potential energy surfaces of the hydrogen bonded species O₂^?HF and O₂^?H₂O is performed with unrestricted Hartree-Fock calculations. Geometries, electron density distributions, and relative energies for every stationary point are reported. Only one true minimum is found for O₂^?HF and for O₂^?H₂O, and this approximately corresponds to a structure where the partially positive hydrogen atom is located along one of the superoxide ion electron lone-pair directions. Calculated ΔH, ΔS, and ΔG values for the reaction between O₂^? and H₂O are in good agreement with experimental data.     

Erdalkaliquadratate. IV. SrC4O4 · 3 H2O Typ II

Alkaline Earth Squarates. IV. SrC₄O₄ · 3 H₂O Type II In SrC₄O₄ · 3 H₂O (type II) Sr²⁺ and C₄O₄^2? build up a threedimensional framework. Sr²⁺ is 9-coordinated by three water molecules and six oxygen atoms of the squarate dianions, which form a distorted mono-capped square anti prism. Strong hydrogen bonding has to be assumed between water and that O_squarate which is bound weakly to Sr²⁺. C? C and C? O bond lengths are typical of delocalization of the, π-electron system. Dehydration to SrC₄O₄ · 1 H₂O is reversible. The single crystal nature, however, is destroyed during this process.     

Internal conrotation and disrotation in H2BCH2BH2 and diborylmethane 1,3 H exchange

The paths of correlated internal disrotation (barrier less than 0.4 kcal/mol) and conrotation (barrier around 1.9 kcal/mol) of the two BH₂ groups in H₂BCH₂BH₂ have been computed employing ab initio [MP2(full)/6–31G**] and density functional theory (Becke3LYP/6–311+G**) methods. Two B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative interactions stabilize the C_s symmetric H₂BCH₂BH₂ isomer ( 1 ). The B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative stabilization, evaluated by homodesmotic reactions and using the orbital deletion procedure (which “deactivates” the “vacant” born p orbital), is less than 6 kcal/mol in diborylmethane. The B(SINGLE BOND)C(DOTTED BOND)B(p) stabilization is shown to be remarkably large in C₄B₆H₁₀ (Td). At MP2(fu)/6–31G**, disproportionation into 1 and methane is only 5.6 kcal/mol exothermic. The 1,3 H exchange in diborylmethane is an asynchronous process and proceeds via a doubly bridged cyclic intermediate with 9.3 kcal/mol barrier. Structures with “planar tetracoordinate” carbon are stabilized considerably by BH₂ substituents, but they are still high in energy. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1792–1803, 1997     

A novel mechanism for the isomerization of N2O4 and its implication for the reaction with H2O and acid rain formation

We have discovered, by high‐level quantum‐chemical calculations, a new and predominant isomerization mechanism for N₂O₄ → ONONO₂ via a roaming‐like transition state occurring unimolecularly or bimolecularly during collision with H₂O. The new mechanism allows N₂O₄ to react with H₂O with a significantly lower barrier (< 13.1 kcal/mol) than the commonly known tight transition state (∼30‐45 kcal/mol) by concurrent stretching of the N N bond and rotation of one of the NO₂ groups to form trans‐ONONO₂, which then undergoes a rapid metathetical reaction with H₂O in the gas phase and in aqueous solution. The results have a significant implication for the hydrolysis of N₂O₄ in water to produce HONO and HNO₃. Rate constants for the isomerization and hydrolysis reactions have been predicted for atmospheric modeling applications.     

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure,Redox Properties,and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru^V?O complex [Ru^V(N₄O)(O)]²⁺, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol^?1, respectively. The overall free energy activation barrier (ΔG^≠=25.5 kcal mol^?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru^III(N₄O)(HOCl)]²⁺ (ΔG=15.0 kcal mol^?1) and the substitution of HOCl by a water molecule (ΔG^≠=10.5 kcal mol^?1). For water oxidation, nucleophilic attack on Ru^V?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol^?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [Ru^V(N₄O)(O)]²⁺ leads to the [(N₄O)Ru^IV? O₂? Ru^III(N₄O)]⁴⁺ complex with a calculated free energy barrier of 12.0 kcal mol^?1.     

Chemisorption of O and O2 on Ag(110): An LCGTO-LSD cluster study

As part of a study aimed at better understanding of molecular and dissociative chemisorption of oxygen on Ag(110), linear combinations of Gaussian type orbitals-local spin density (LCGTO -LSD ) calculations have been performed for O, O^?, O₂, O^?₂, O^2?₂ and a variety of silver clusters interacting with O or O₂. For atomic O adsorption a very small cluster, Ag₄, chosen to model the long-bridge site already affords very good agreement with both recent EXAFS experiments and recent ab initio calculations. We calculate O to be 0.25 Å above the surface (exp. 0.2 Å). The Ag₄? O vibrational frequency is estimated to be 400 cm^?1, in reasonable accord with the experimental EELS value of 325 cm^?1. Determination of the geometry for O₂ (ads.) and, ultimately, of the dissociation path are far more difficult tasks. An extensive search for local minima in the vicinity of the LB site is being carried out. Results to date for small, Ag₂ and Ag₄, clusters have furnished insight into the factors influencing the structure. Overlap between the π* orbital of the O₂ moiety and Ag s functions is a key factor; that is, there is an important covalent component of the binding. For geometries with O₂ parallel to the surface, this is achieved by twisting the O₂ fragment with respect to the [11¯0] grooves (geometries either parallel or perpendicular to the grooves yield zero π‖*?s overlap by symmetry). The structure with O₂ perpendicular to the surface also achieves reasonable overlap and lies close in energy to the most stable ‘parallel’ geometry.     

Surface interaction of H2O and H2S onto Ca12O12 nanocluster: Quantum‐chemical analyses

To find the selectivity of H₂S, we explicate the adsorption properties of water (H₂O) and hydrogen sulfide (H₂S) molecules on the external surfaces of free Ca₁₂O₁₂ nanocages using the density functional theory method. More specifically, binding energies, natural bond orbital charge transfer, dipole moment, molecular electrostatic potential, frontier molecular orbitals, density of states, and global indices of activities are calculated to deeply understand the impacts of the aforementioned molecules on the electronic and chemical properties of Ca₁₂O₁₂ nanocages. Our theoretical findings indicate that although H₂O seems to be adsorbed in molecular form, the H₂S molecule is fully dissociated during the adsorption process because of the weak bond between sulfur and hydrogen atoms of the molecule. Interestingly, the highest occupied molecular orbital–lowest unoccupied molecular orbital energy gap of the nanocage is decreased by 1.87 eV upon H₂S adsorption, indicating that the electrical conductivity of the nanocage is strongly increased by the dissociation process. In addition, the values of softness and electrophilicity for the H₂S‐Ca₁₂O₁₂ complex are higher than those for the free nanocage. Our results suggest that Ca₁₂O₁₂ nanoclusters show promise in the adsorption/dissociation of H₂S molecules, which can be used further for designing its selective sensor.     

Mechanism of pyrolysis of C2Cl6

Using published data on the kinetics of pyrolysis of C₂Cl₆ and estimated rate parameters for all the involved radical reactions, a mechanism is proposed which accounts quantitatively for all the observations: The steady-state rate law valid for after about 0.1% reaction is and the reaction is verified to proceed through the two parallel stages suggested earlier whose net reaction is A reported induction period obtained from pressure measurements used to follow the rate is shown to be compatible with the endothermicity of reaction A, giving rise to a self-cooling of the gaseous mixture and thus an overall pressure decrease. From the analysis, the bond dissociation energy DH⁰(C₂Cl₅? Cl) is found to be 70.3 ± 1 kcal/mol and ΔH_f300⁰(·C₂Cl₅) = 7.7 ± 1 kcal/mol. The resulting π? bond energy in C₂Cl₄ is 52.5 ± 1 kcal/mol.     

Syntheses and crystal structures of [Na(H2O)](C17H13O6SO3)* 2H2O and [NKH2O)6]C17H13O6SO3)2*H2O

Two hydrates of sodium 5,7‐dihydroxy‐6,4′‐dimethoxyisoflavone‐3′‐sulfonate ([Na(H₂O)J(C₁₇H₁₃O₆SO₃)*2H₂O,] 1) and nickel 5,7‐dihydroxy‐6,4′‐dimethoxyisoflavone‐3′‐sulfonate ([Ni(H₂O)₆](C₁₇H₁₃O₆SO₃)₂*4H₂O, 2) were synthesized and characterized by IR, 'H NMR and X‐ray diffraction analyses. The hydrate 1 crystallizes in the mono‐clinic system, space group P2(1) with a=0.8201(9) nm, b=0.8030(8) nm, c= 1.5361(16) nm, β=102.052(12)°, V =0.9893(18) nm³, D,= 1.579 g/cm³, Z=2, μ=0.252 nm^?1, F(000)=488, R=0.0353, wR=0.0873. The hydrate 2 belongs to triclinic system, space group P‐1 with a=0.7411(3) nm, b=0.8333(3) nm, c=1.7448(7) nm, α= 86.361(6)°, β=86.389(5)°, γ= 88.999(3)°, V=1.0731(7) nm³, D,=1.587 g/cm³, Z=1, μ=0.649 m^?1, F(000)= 534. In the structure of 1, the sodium cation is coordinated by six oxygen atom and two adjacent ones are bridged by three oxygen atoms to form an octahedron chain. The C? H…?… hydrogen bonds exist between two isoflavone molecules in the structure of 2. Meanwhile, hydrogen bonds in two compounds, link themselves to assemble two three‐dimensional network structures, respectively.     

Quantum chemical studies of propene,ethylene, acetylene and dihydrogen reactivity in the insertion reaction into the titanium-alkyl bond

Using the ab initio method of SCF MO LCAO

1 SCF MO LCAO: Self-consistent field molecular orbital linear combination of atomic orbitals.

in a valency-splitted basis of the Gaussian functions we have studied the addition of various monomers (C₃H₈, C₂H₄, C₂H₂) and dihydrogen to the titanium-alkyl bond in the complex H₂TiCH₃. The structure of transition states in the insertion reaction, heats of π-complex formation and activation energies for the insertion of the coordinated monomers have been calculated. The calculation results show that the reactivity decreases in the order C₂H₂ > C₂H₄ > C₃H₈ > H₂. According to the results obtained, the energy of the π^*-antibonding orbital of monomers can serve as an index of relative reactivity in the insertion reaction into the metal-alkyl bond.     

Valence-bond calculations on N2 and isoelectronic species

Valence-bond calculations are reported for the isoelectronic series of molecules and ions: N₂, CO, BF, NO⁺ and CN^?. The most important structures are N?N, C?O, B_π? F, N⁺?O and C?N. Hybridization of the 2s and 2p orbitals is important. Only two or three structures are required to obtain an energy lower than that obtained with the molecular orbital approximation. Structures in which the electronegative element loses a σ-orbital or gains a π-orbital are favored. π-bonds tend to be favored over σ-bonds. The bond in NO⁺ resembles that in CO, whereas that in CN^? resembles the bonding in N₂.     

Role of CA2+ ion in the abortifacient action of prostaglandins. II. Molecular orbital and conformation energy calculations of PGA1

This paper reports some theoretical studies of PGA₁ using conformational and molecular orbital techniques. The conformation energy (CE ) calculations, using empirical potential-energy functions, for intrinsic torsional rotations around C₁₂? C₁₃ (θ), C₇? C₈ (Ψ), and C₁₄? C₁₅ (?) show a number of energy minima. The relative value of the CE for these minima ranges from 4.09 to 8.01 kcal/mol. An additional rotation around C₄? C₅ (χ) giving “twist” to the carboxyl chains lowers the CE value by 2–3 kcal/mol and a conformation with CE value 2.39 kcal/mol less than crystallographic one is obtained. The interchain interaction energy showed changes with conformations. No significant change in the interchain interaction energy was observed due to “twist” in the carboxyl chain. The isopotential mapping study demonstrated the probable ionic binding site near the carboxyl and the ring (O₉) oxygens. Conformational and molecular orbital results are discussed in the light of the reduced abortifacient potency of PGA₁ with respect to PGF_2α and the possible role of Ca²⁺ ions in this action.     

An Unexplored O2‐Involved Pathway for the Decarboxylation of Saturated Carboxylic Acids by TiO2 Photocatalysis: An Isotopic Probe Study

The aerobic decarboxylation of saturated carboxylic acids (from C₂ to C₅) in water by TiO₂ photocatalysis was systematically investigated in this work. It was found that the split of C¹? C² bond of the acids to release CO₂ proceeds sequentially (that is, a C₅ acid sequentially forms C₄ products, then C₃ and so forth). As a model reaction, the decarboxylation of propionic acid to produce acetic acid was tracked by using isotopic‐labeled H₂¹⁸O. As much as ≈42 % of oxygen atoms of the produced acetic acids were from dioxygen (¹⁶O₂). Through diffuse reflectance FTIR measurements (DRIFTS), we confirmed that an intermediate pyruvic acid was generated prior to the cut‐off of the initial carboxyl group; this intermediate was evidenced by the appearance of an absorption peak at 1772 cm^?1 (attributed to C?O stretch of α‐keto group of pyruvic acid) and the shift of this peak to 1726 cm^?1 when H₂¹⁶O was replaced by H₂¹⁸O. Consequently, pyruvic acid was chosen as another model molecule to observe how its decarboxylation occurs in H₂¹⁶O under an atmosphere of ¹⁸O₂. With the α‐keto oxygen of pyruvic acid preserved in the carboxyl group of acetic acid, ≈24 % new oxygen atoms of the produced acetic acid were from molecular oxygen at near 100 % conversion of pyruvic acid. The other ≈76 % oxygen atoms were provided by H₂O through hole/OH radical oxidation. In the presence of conduction band electrons, O₂ can independently accomplish such C¹? C² bond cleavage of pyruvic acid to generate acetic acid with ≈100 % selectivity, as confirmed by an electrochemical experiment carried out in the dark. More importantly, the ratio of O₂ participation in decarboxylation increased along with the increase of pyruvic acid conversion, indicating the differences between non‐substituted acids and α‐keto acids. This also suggests that the O₂‐dependent decarboxylation competes with hole/OH‐radical‐promoted decarboxylation and depends on TiO₂ surface defects at which Ti_4c sites are available for the simultaneous coordination of substrates and O₂.     

N—H...O and O—H...O hydrogen‐bonded supramolecular networks in 4‐chloroanilinium, 2‐hydroxyanilinium and 3‐hydroxyanilinium hydrogen phthalates

The title salts, 4‐chloroanilinium hydrogen phthalate (PCAHP), C₆H₇ClN⁺·C₈H₅O₄⁻, 2‐hydroxyanilinium hydrogen phthalate (2HAHP), C₆H₈NO⁺·C₈H₅O₄⁻, and 3‐hydroxyanilinium hydrogen phthalate (3HAHP), C₆H₈NO⁺·C₈H₅O₄⁻, all crystallize in the space group P2₁/c. The asymmetric unit of 2HAHP contains two independent ion pairs. The hydrogen phthalate ions of 2HAHP and 3HAHP show a short intramolecular O—H...O hydrogen bond, with O...O distances ranging from 2.3832 (15) to 2.3860 (14) Å. N—H...O and O—H...O hydrogen bonds, together with short C—H...O contacts in PCAHP and 3HAHP, generate extended hydrogen‐bond networks. PCAHP forms a two‐dimensional supramolecular sheet extending in the (100) plane, whereas 2HAHP has a supramolecular chain running parallel to the [100] direction and 3HAHP has a two‐dimensional network extending parallel to the (001) plane.