Further theoretical examination of the F + H2 entrance channel barrier

A theoretical study has been conducted to clarify the discrepancy between ab initio predictions of the entrance channel barrier in the F + H₂ reaction and the experimentally observed thresholds for product formation. New results using two different models are in better agreement with the experimental excergicity and predict a zero-point energy corrected barrier of 2.0 kcal mol⁻¹. Estimates of the resulting reaction cross section and thermal activation energy suggest that this value differs from experiment by about 0.5 kcal mol⁻¹. The theoretically predicted isotope effect on the thermal activation energy is larger than either of the recently suggested experimental values, which, however, are subject to large uncertainties.     

Direct determination of the rate constant for the reaction CO + N2O → CO2 + N2

The rate constant for the bimolecular reaction CO + N₂O → CO₂ + N₂ was determined by comparison of calculated infrared emission profiles of CO₂ with those observed in shock-tube experiments in the temperature range 1350–2100 K for CO-N₂O-He-Ar mixtures. The rate constant was found to be k₁ = 3.2 × 10¹¹exp(−85 kJ/RT) cm³ mol⁻¹ s⁻¹.     

Ab initio MO and TST calculations for the rate constant of the HNO+NO2→HONO+NO reaction

Potential-energy surfaces for various channels of the HNO+NO₂ reaction have been studied at the G2M(RCC,MP2) level. The calculations show that direct hydrogen abstraction leading to the NO+cis-HONO products should be the most significant reaction mechanism. Based on TST calculations of the rate constant, this channel is predicted to have an activation energy of 6–7 kcal/mol and an A factor of ca. 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at ambient temperature. Direct H-abstraction giving NO+trans-HONO has a high barrier on PES and the formation of trans-HONO would rather occur by the addition/1,3-H shift mechanism via the HN(O)NO₂ intermediate or by the secondary isomerization of cis-HONO. The formation of NO+HNO₂ can take place by direct hydrogen transfer with the barrier of ca. 3 kcal/mol higher than that for the NO+cis-HONO channel. The formation of HNO₂ by oxygen abstraction is predicted to be the least significant reaction channel. The rate constant calculated in the temperature range 300–5000 K for the lowest energy path producing NO+cis-HONO gave rise to © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 729–736, 1998     

An ab initio study of the structures and energetics of CH3OCl and CH3ClO

The equilibrium structures, vibrational spectra, and heats of formation for CH₃OCl and CH₃ClO have been estimated using high levels of ab initio molecular orbital theory. The lowest energy isomer is found to be CH₃OCl, and its heat of formation is estimated to be −13.5±2 kcal mol⁻¹, in good agreement with bond additivity estimates. Results for the CH₃ClO isomer are presented for the first time, and it is found to be 40.5 kcal mol⁻¹ higher in energy relative to CH₃OCl. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 29–35, 1999     

On the Stability of Interactions between Pairs of Anions – Complexes of MCl3− (M=Be,Mg, Ca,Sr, Ba) with Pyridine and CN−

The ability of the central M atom of the MCl₃⁻ anion, with M=Be, Mg, Ca, Sr, Ba, to engage in a noncovalent bond with an approaching nucleophile is gauged by ab initio methods. The N atom of pyridine forms a M⋅⋅⋅N bond with an interaction energy between 12 and 21 kcal mol⁻¹, even though the π-hole above the M atom is not necessarily positive in sign. Despite a strong Coulombic repulsion between two anions, CN⁻ is also able to approach the M atom so as to engage in a metastable complex that is higher in energy than the individual anions. The energy barrier separating this complex from its constituent anion pair is roughly 20 kcal mol⁻¹. Despite the endothermic formation reaction energy of the CN⁻⋅⋅⋅MCl₃⁻ complex, the electron topology signals a strong interaction, more so than in pyridine⋅⋅⋅MCl₃⁻ with its exothermic binding energy. The dianionic complex is held together largely on the strength of interorbital interactions, thereby overcoming a repulsive electrostatic component. The latter is partially alleviated by the pyramidalization of the MCl₃ unit which makes its π-hole more positive. The complex sinks below the separate monomers in energy when the system is immersed in an aqueous medium, with a binding energy that varies from as much as 20 kcal mol⁻¹ for Be down to 1.2 kcal mol⁻¹ for Ba.     

How effective is internal excitation in promoting the HCl + 1,3-butadiene addition reaction?

Excitation of the C-H overtones of 1,3-butadiene in the presence of HCl to an energy ≈ 16 kcal mol⁻¹ in excess of the activation energy causes the formation of no detectable addition products. This indicates that the rate constant for the formation of 1-chloro-2-butene is at least slower than 2.1× 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹.     

The thermodynamic stability of P8, A CBS‐Q study

The thermodynamic stabilities of P₂, P₄, and three P₈ cage structure were investigated through high‐precision CBS‐Q calculations. The CBS‐Q values for the bond energy of P₂ (ΔE_o: +115.7 kcal mol⁻¹) and the formation of P₄ from P₂ (Δ E_o:‐56.6 kcal mol⁻¹) were in excellent agreement with the experimental values (E_o: +117 and ‐56.4 kcal mol⁻¹ respectively). Among the P₈ cages, the cubane structure was the least stable (Δ E_o +37 kcal vs. 2×P₄). The most stable P₈ isomer adopts a cuneane structure resembling S₄N₄, and is more stable than white phosphorus at T = 0 K (Δ E_o −3.3 kcal mol⁻¹), but still unstable under standard conditions for entropic reasons (Δ G^o of +8.1 kcal mol⁻¹ vs. 2×P₄). The CBS‐Q energies represent significant revisions (6–20 kcal mol⁻¹) of previous computational predictions obtained by high‐level single method calculations. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:453–457, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20119     

Proton shuttle efficiency of bicarbonate: A theoretical study on tautomerization and CO2 hydration

The efficiency of bicarbonate molecule (HCO₃⁻) as a proton shuttle in the tautomerization and (non)enzymatic CO₂ hydration reactions has been investigated with the aid of computational chemistry methods (DFT and ab initio). The results revealed that bicarbonate can decrease the barrier height of tautomerization (keto-enol, azo-hydrazo and imine-amine) more than 70%. This value is around 45% for water molecules. Also, HCO₃⁻ can catalyze the CO₂ hydration both inside (enzymatic) and outside (nonenzymatic) the active site of human carbonic anhydrases II (HCA II). In the absence of enzyme, bicarbonate molecule can lower the CO₂ hydration from ∼50 kcal mol⁻¹ in the gas phase to ∼14 kcal mol⁻¹ in the aqueous media. This reaction maintains its barrier (∼15 kcal mol⁻¹) for bicarbonate-Zn complex in the active site of enzyme; it has been observed that amino acid residues, mainly Thr199 and Glu106, are actively involved in the proton transfer network and facilitate CO₂ hydration ability of bicarbonate.     

Energetic Ordering of Hydrogen Bond Strengths in Methanol-Water Clusters: Insights via Molecular Tailoring Approach

In this work, we examine the strength of various types of individual hydrogen bond (HB) in mixed methanol-water M_nW_m, (n+m=2 to 7) clusters, with an aim to understand the relative order of their strength, using our recently proposed molecular tailoring-based approach (MTA). Among all the types of HB, it is observed that the O_M−H…O_W HBs are the strongest (6.9 to 12.4 kcal mol⁻¹). The next ones are O_M−H…O_M HBs (6.5 to 11.6 kcal mol⁻¹). The O_W−H…O_W (0.2 to 10.9 kcal mol⁻¹) and O_W−H…O_M HBs (0.3 to 10.3 kcal mol⁻¹) are the weakest ones. This energetic ordering of HBs is seen to be different from the respective HB energies in the dimer i. e., O_M−H…O_M (5.0 to 6.0 kcal mol⁻¹)>O_W−H…O_M (1.5 to 6.0 kcal mol⁻¹)>O_M−H…O_W (3.8 to 5.6 kcal mol⁻¹)>O_W−H…O_W (1.2 to 5.0 kcal mol⁻¹). The plausible reason for the difference in the HB energy ordering may be attributed to the increase or decrease in HB strengths due to the formation of cooperative or anti-cooperative HB networks. For instance, the cooperativity contribution towards the different types of HB follows: O_M−H…O_W (2.4 to 8.6 kcal mol⁻¹)>O_M−H…O_M (1.3 to 6.3 kcal mol⁻¹)>O_W−H…O_W (−1.0 to 6.5 kcal mol⁻¹)>O_W−H…O_M (−1.2 to 5.3 kcal mol⁻¹). This ordering of cooperativity contribution is similar to the HB energy ordering obtained by the MTA-based method. It is emphasized here that, the interplay between the cooperative and anti-cooperative contributions are indispensable for the correct energetic ordering of these HBs.     

Kinetics and mechanism of the thermal decomposition of methyl isocyanate

The kinetics of gas-phase decomposition of methyl isocyanate have been investigated in the range of 427–548°C. Two decomposition routes are followed; the predominant one is a radical-chain process giving CO, H₂, and HCN as major products, which has an order of 1.5 and an Arrhenius equation given by log k(L^1/2/mol^1/2·s) = (13.12 ± 0.06) ? (56,450 ± 1670) cal/mol/2.303 RT. The minor route is the bimolecular formation of N,N′-dimethylcarbodiimide and CO₂, which from the low activation parameters E_a = 31.6 kcal, A = 10^5.30 L^1/2/mol^1/2·s, and the reaction order of 1.57 appears to be heterogeneous.     

Conversion of CO2 and C2H6 to Propanoic Acid over a Au‐Exchanged MCM‐22 Zeolite

Finding novel catalysts for the direct conversion of CO₂ to fuels and chemicals is a primary goal in energy and environmental research. In this work, density functional theory (DFT) is used to study possible reaction mechanisms for the conversion of CO₂ and C₂H₆ to propanoic acid over a gold‐exchanged MCM‐22 zeolite catalyst. The reaction begins with the activation of ethane to produce a gold ethyl hydride intermediate. Hydrogen transfers to the framework oxygen leads then to gold ethyl adsorbed on the Brønsted‐acid site. The energy barriers for these steps of ethane activation are 9.3 and 16.3 kcal mol^?1, respectively. Two mechanisms of propanoic acid formation are investigated. In the first one, the insertion of CO₂ into the Au?H bond of the first intermediate yields gold carboxyl ethyl as subsequent intermediate. This is then converted to propanoic acid by forming the relevant C?C bond. The activation energy of the rate‐determining step of this pathway is 48.2 kcal mol^?1. In the second mechanism, CO₂ interacts with gold ethyl adsorbed on the Brønsted‐acid site. Propanoic acid is formed via protonation of CO₂ by the Brønsted acid and the simultaneous formation of a bond between CO₂ and the ethyl group. The activation energy there is 44.2 kcal mol^?1, favoring this second pathway at least at low temperatures. Gold‐exchanged MCM‐22 zeolite can therefore, at least in principle, be used as the catalyst for producing propanoic acid from CO₂ and ethane.     

Formation and decomposition of Lanthanum Monoxocarbonate

Amorphous lanthanum carbonate was prepared by hydrolysis of lanthanum isopropoxide using ammonia water in the atmosphere. Lanthanum monoxocarbonate, La₂O(CO₃)₂ · H₂O, crystallizes when this amorphous material was washed with hot water. The crystallization and thermal behavior of the crystalline material are studied by X-ray diffraction, thermal analysis, and infrared spectroscopy. The decomposition of La₂O(CO₃)₂ · H₂O into type-IA (LaO)₂CO₃ is observed at 440 to 540°C. Decomposition isotherms are described by the contracting cube equation, the activation energy being 42.6 kcal mol^?1. Type-IA (LaO)₂CO₃ subsequently decomposes to A-type La₂O₃ at 750 to 870°C. The kinetics is also interpreted in terms of the contracting cube equation, the activation energy being 58.3 kcal mol^?1.     

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.     

A structural and theoretical study of intermolecular interactions in nicotinohydrazide dihydrochloride

In the title compound [systematic name: 3‐(azaniumylcarbamoyl)pyridinium dichloride], C₆H₉N₃O²⁺·2Cl⁻, the ions are connected by N—H...Cl hydrogen bonds to form layers and C—H...Cl interactions expand the layers into a three‐dimensional net. The energies of the N—H...Cl interactions range from typical for very weak interactions (0.17 kcal mol⁻¹) to those observed for relatively strong interactions (29.1 kcal mol⁻¹). C—H...Cl interactions can be classified as weak and mildly strong (energies ranging from 2.2 to 8.2 kcal mol⁻¹). Despite the short contacts existing between the parallel aromatic rings of the cations, π–π interactions do not occur.     

Adsorption and dissociation of carbon trioxide on Ag(100)

Using density functional theory methods, we have studied carbon trioxide, its adsorption and dissociation on Ag(100). In the gas phase, two isomers are found, D_3h and C_2v, with the latter of 2.0 kcal mol^?1 lower in energy at the PW91PW91/6?31G(d) level. For CO₃ on Ag(100), the calculated adsorption energy is 91.2 and 89.1 kcal mol^?1 for the bi‐coord perpendicular and tri‐coord parallel structures, respectively. Upon the adsorption, 0.50 ～ 0.56 electron is transferred from silver to CO₃, indicative of significant ionic characters of the adsorbate‐surface bonding. In addition, the geometry of CO₃ is largely changed by its strong interaction with silver. For CO_3(ad) → O_(ad) + CO_2(gas), the energy barrier is calculated to be 19.8 kcal mol^?1 through the bi‐coord path. The process is endothermic with an enthalpy change of +17.3 ～ +26.7 kcal mol^?1 and the weakly chemisorbed CO₂ is identified as an intermediate on the potential energy surface. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Thermodynamic properties of potassium metasilicate (K2SiO3)

The standard enthalpy of formation of potassium metasilicate (K₂SiO₃), determined by hydrofluoric acid solution calorimetry, was found to be ΔH^o_f,298 = −363.866±0.421 kcal mol⁻¹ (−1522.415±1.762 kj mol⁻¹). The standard enthalpy of formation from the oxides was found to beΔH^o₂₉₈ = −64.786±0.559 kcal mol⁻¹ (−271.065±2.339 kJ mol⁻¹).These experimentally determined data were combined with data from the literature to calculate the Gibbs energies of formation and equilibrium constants of formation over the temperature range of the literature data. The standard enthalpies of formation and Gibbs energies of formation are given as functions of temperature. The standard Gibbs energy of formation is ΔG_f,298.15⁰ = −341.705 kcal mol⁻¹ (−1429.694 kJ mol⁻¹).     

Hydrogen Cyanide Exchange on [Al(HCN)6]3+ – A DFT Study

The hydrogen cyanide exchange mechanism of [Al(HCN)₆]³⁺ has been investigated by DFT calculations (B3LYP/6‐311+G**). The calculations provide theoretical evidence that the hydrogen cyanide exchange proceeds via a limiting dissociative (D) mechanism involving a stable five‐coordinate intermediate [Al(HCN)₅ · (HCN)₂]³⁺. The activation energy for the D‐mechanism is 23.4 kcal · mol^–1, which is 2.8 kcal · mol^–1 lower than for the seven‐coordinate transition state [Al(HCN)₇]^3+? for the alternative associative (A) pathway. The difference in stability between the two intermediates [Al(HCN)₅ · (HCN)₂]³⁺ (12.1 kcal · mol^–1) and [Al(HCN)₇]³⁺ (25.7 kcal · mol^–1) in comparison to [Al(HCN)₆ · (HCN)]³⁺ is much more pronounced and further supports a limiting dissociative mechanism.     

Photophysical properties of benzil in solution: triplet state deactivation pathways

The fluorescence and phosphorescence from benzil in dilute benzene and cyclohexane solutions (2 × 10⁻⁴ M) were studied by both conventional luminescence and time-correlated single-photon techniques in the temperature range 8 – 69 °C. The fluorescence (λ = 502 nm) did not show a substantial temperature dependence and was free from thermal and triplet-triplet annihilation delayed contributions at the low concentration used. The phosphorescence (λ = 562 nm) was temperature dependent and its decay was controlled by an activation energy (E_a = 7.4 ± 0.5 kcal mol⁻¹) which was slightly larger than the spectroscopic single-triplet splitting (6.1 kcal mol⁻¹). The photophysical parameters derived from the lifetimes of the two emissions was not consistent with the model of thermal equilibration between S₁ and T₁.     

Determination of Astaxanthin and Canthaxanthin Triplet Properties in Different Polarities of the Solvent by Laser Flash Photolysis

Triplet‐triplet extinction coefficients for astaxanthin ( I ) and canthaxanthin ( II ) in different deaerated polarity solutions of MeCN and benzene were evaluated by laser flash photolysis at 298 K in the spectral region from 350 to 650 nm by energy transfer method, employing 2‐acetonaphthone as sensitizer. The triplet‐triplet extinction coefficients in MeCN and benzene were different in terms of the carotenoid present. The maximum triplet‐triplet extinction coefficient was 0.1–1.7×10⁵ L·mol⁻¹·cm⁻¹ in different solvents. The rate constants of triplet decay were I : 1.25×10¹⁰ L·mol⁻¹·s⁻¹, II : 1.12×10¹⁰ L·mol⁻¹·s⁻¹ in MeCN; and I : 1.75×10¹⁰ L·mol⁻¹·cm⁻¹, II : 3.27×10¹⁰ L·mol⁻¹·s⁻¹ in benzene. The bimolecular rate constants of energy transfer from triplet excited 2‐acetonaphthone to carotenoids were determined from the linear regression of the decay rate constant of 2‐acetonaphthone triplet at varying carotenoid concentrations. The triplet lifetimes of ³AST* and ³CAN* in different solvents were also determined. The results indicated that triplet energy transfer was nearly diffusion‐controlled.     

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.