Can chlorine anion catalyze the reaction of HOCl with HCl?

The reaction of HOCl + HCl → Cl₂ + H₂O in the presence of chlorine anion Cl⁻ has been studied using ab initio methods. The overall exothermicity is 15.5 kcal mol⁻¹ and this reaction has been shown to have a high activation barrier of 46.5 kcal mol⁻¹. Cl⁻ is found to catalyze the reaction via the formation of HOCl·Cl⁻, ClH·HOCl·Cl⁻ and Cl⁻·H₂) intermediate ion-molecule complexes or by interacting with a concerted four-center transition state of the reaction of HOCl + HCl.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.     

Ab initio calculations of the vibrational spectra of the ammonia and fluoramide complexes with HF

The structures, energetics, vibrational frequencies and IR intensities of the H₃N HF, H₃N F₂ and NH₂FHF (three isomers) complexes were examined using the self-consistent field method within the 6-311G^** basis set. The interaction energies were calculated using the MP2 approach. The results are compared with monomer calculations and experimental data. The complex NH₂FHF was found to exist in three forms: one with the HF molecule hydrogen bonded to the nitrogen lone pair of NH₂F (D₀ =7.403 kcal mol⁻¹), another a complex formed through the F atom lone pair (D₀=4.698 kcal mol⁻¹) and third a cyclic structure (D₀=5.644 kcal mol⁻¹).     

Density functional study of molecular properties of hydrazoic acid and methyl azide

Density functional calculations for hydrazoic acid HN₃ and methyl azide CH₃N₃ and for the respective singly ionized structures HN⁺₃ and CH₃N⁺₃ are reported. An analysis of the electrostatic solvent effects, based on the self-consistent reaction field approach, on the molecular properties and conformational equilibrium of CH₃N₃ is also reported. The results are sensitive to the basis set quality and show some dependence on the different representations for the exchange-correlation functions. For HN₃ very good agreement with experiment is observed for several properties, such as the geometry, dipole moment, vibrational frequencies and for the adiabatic first ionization energy. For CH₃N₃ the energy difference between eclipsed (ec) and staggered (st) conformers (δ_ec-st) is 2.5 kJ mol⁻¹, in good agreement with the experimental value (2.9 kJ mol⁻¹). However, for CH₃N⁺₃, δ_ec-st is −3.2 kJ mol⁻, reflecting a significant modification of the methyl group rotational potential after ionization. Solvent effects on the molecular properties of CH₃N₃ are important when it is solvated in a polar medium. The most significant modifications concern the dipole moment and the frequencies related to the CH₃ symmetric stretch and torsion vibrational modes.     

A Gaussian-2 ab initio study of [C2H5S] ions: III. H2 and CH4 eliminations from CH3CHSH and CH3SCH2

Gaussian-2 ab initio calculations were performed to examine the six modes of unimolecular dissociation of cis-CH₃CHSH⁺ (1⁺), trans-CH₃CHSH⁺ (2⁺), and CH₃SCH₂⁺ (3⁺): 1⁺→CH₃⁺+trans-HCSH (1); 1⁺→CH₃+trans-HCSH⁺ (2); 1⁺→CH₄+HCS⁺ (3); 1⁺→H₂+c-CH₂CHS⁺ (4); 2⁺→H₂+CH₃CS⁺ (5); and 3⁺→H₂+c-CH₂CHS⁺ (6). Reactions (1) and (2) have endothermicities of 584 and 496 kJ mol⁻¹, respectively. Loss of CH₄ from 1⁺ (reaction (3)) proceeds through proton transfer from the S atom to the methyl group, followed by cleavage of the C–C bond. The reaction pathway has an energy barrier of 292 kJ mol⁻¹ and a transition state with a wide spectrum of nonclassical structures. Reaction (4) has a critical energy of 296 kJ mol⁻¹ and it also proceeds through the same proton transfer step as reaction (3), followed by elimination of H_2. Formation of CH₃CS⁺ from 2⁺ (reaction (5)) by loss of H₂ proceeds through protonation of the methine (CH) group, followed by dissociation of the H₂ moiety. Its energy barrier is 276 kJ mol⁻¹. On both the MP2/6-31G* and QCISD/6-31G* potential-energy surfaces, the H₂ 1,1-elimination from 3⁺ (reaction (6)) proceeds via a nonclassical intermediate resembling c-CH₃SCH₂⁺ and has a critical energy of 269 kJ mol⁻¹.     

Ab initio MO study on the global minimum structure of C3H3 anion: propynl-1-yl versus allenyl anions

The geometrical structures of the C₃H⁻₃ anion are surveyed at the coupled-cluster doubles (CCD) level of theory with the aug-cc-pVDZ basis set. To clarify the CCD geometries, the stable two isomers -- propynl-l-yl 1 and allenyl 2 anions -- are further optimized at the coupled-cluster singles, doubles (triples) (CCSD(T)) level of theory both with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. The final energies are calculated at the CCSD(T) and the complete active space self-consistent field (CASSCF) multi-reference internally contracted CI (MRCI) levels of theory with the aug-cc-pVTZ basis set. At the MRCI level of theory including both the corrections due to the cluster energies (MRCI+Q) and the zero-point vibrational energies, the allenyl anion 2 is about 1.3 kcal mol⁻¹ lower in energy than the propynl-l-yl anion 1. These results contrast with the previous theoretical estimates, where the propynl-l-yl anion 1 is 2-3 kcal mol⁻¹ lower in energy than the allenyl anion 2. The activation energies of the intramolecular hydrogen transfer in the 1 → 2 conversion reactions are 63.5 kcal mol⁻¹ at the MRCI+Q level of theory with the aug-cc-pVTZ basis set including the zero-point energy corrections. The adiabatic electron affinity of the planer propargyl (H₂CCCH) radical, which is the global minimum of the C₃H₃ radical, is calculated to be 0.976 eV (after correction for the zero-point energy changes) at the CCSD(T) level of theory with the aug-cc-pVTZ basis set. The present electron affinity is in fairly good agreement with the experimental one (0.893 eV) observed by Oakes and Ellison.     

Pentacoordinate silicon compounds: Stereochemical non-rigidity of chelates formed by intramolecular ring-closure. Crystal structure of 8-dimethylamino-1-trifluorosilylnaphthalene

The application of dynamic NMR spectroscopy to the study of stereochemical non-rigidity in pentacoordinate chelated organosilicon compounds is described. It is shown that in the compounds Me₂ iXYZ, non-dissociative ligand permutation at silicon can be distinguished unambiguously from processes associated with rupture of the chelate ring and nitrogen inversion. The crystal and molecular structure of 8-Me₂NC₁₀H₆SiF₃ has been determined. Pentacoordination of the silicon atom is confirmed, with the donor nitrogen atom and a fluorine atom occupying axial sites in an overall trigonal bipyramidal geometry. The N → Si separation is 2.3 Å (average of two distinct but closely related molecular conformations), which is less than the C¹---C⁸ distance in the naphthalene nucleus, indicating a substantial bonding interaction. NMR studies of the dynamic behaviour of the Me₂N group, and where possible (¹⁹F, ¹H) of the monodentate ligands in 8-dimethylamino-1-silylnaphthalene compounds, together with the results for the chelated benzylaminosilicon compounds, confirm that inversion of the absolute configuration at the silicon atom is not achieved by this process. The free energies of activation for non-dissociative ligand permutation at a silicon range from less than 7 kcal mol⁻¹ [SiH₃, Si(OR)₃], which is below the limit of direct measurement, to 13 kcal mol⁻¹ for Me₂NCH(Me)C₆H₄SiF₃; difunctional silicon chelate compounds (Cl, F, OR) display values from 9–12 kcal mol⁻¹. These are comparable with those determined for fluxional processes in acyclic pentacoordinate silicon compounds.     

Kinetic study of thermal decomposition of CoOOH

The kinetics of the thermal decomposition of CoOOH powder has been studied isothermally in a temperature range of 260—310°C in air. The reaction was found to proceed by the advance of a two-dimensional reaction interface. The kinetics results indicate that there are two phases in the decomposition in this temperature range: up to 280°C with an activation energy E₁ = 34.75 kcal mol⁻¹ and above 280°C with E₂ = 18.91 kcal mol⁻¹. A reaction mechanism is proposed to account for these observations.     

Thermochemistry of organosilicon compounds VII. Permethylcyclosilazanes and 1,1,3,3-tetramethyldisilazane

The enthalpy of formation (ΔH_f⁰), enthalpy of evaporation (ΔH_v⁰) and enthalpy of atomization (ΔH_a) of permethylcyclosilazanes (Me₂SiNH)_n (n = 3, 4) and 1,1,3,3-tetramethyldisilazane (Me₂SiH)₂NH have been determined. The enthalpies of formation of these compounds were compared with those calculated by the Benson-Buss-Franklin and Tatevskii additive schemes. In higher permethylcyclosilazanes the energy of the endocyclic Si---N bond is 306 ± 2 kJ mol⁻¹ (73 kcal mol⁻¹), that is 12 ± 2 kJ mol⁻¹ (3 kcal mol⁻¹) lower than the energy of the acyclic Si---N bond. The strain energy of the cyclotrisilazane ring is estimated to be 10.5 kJ mol⁻¹ (2.5 kcal mol⁻¹), whereas the energy of the ring Si---N bond is 295 kJ mol⁻¹ (70.5 kcal mol⁻¹).

The thermochemical data for permethylcyclosilazanes were compared with the corresponding values for permethylcyclosiloxanes calculated from the results of previously reported studies.     

Ab initio and density function theory computational studies of the CH4 + H → CH3 + H2 reaction

The activation barrier for the CH₄ + H → CH₃ + H₂ reaction was evaluated with traditional ab initio and Density Functional Theory (DFT) methods. None of the applied ab initio and DFT methods was able to reproduce the experimental activation barrier of 11.0-12.0 kcal/mol. All ab initio methods (HF, MP2, MP3, MP4, QCISD, QCISD(T), G1, G2, and G2MP2) overestimated the activation energy. The best results were obtained with the G2 and G2MP2 ab initio computational approaches. The zero-point corrected energy was 14.4 kcal mol⁻¹. Some of the exchange DFT methods (HFB) computed energies which were similar to the highly accurate ab initio methods, while the B3LYP hybrid DFT methods underestimated the activation barrier by 3 kcal mol⁻¹. Gradient-corrected DFT methods underestimated the barrier even more. The gradient-corrected DFT method that incorporated the PW91 correlational functional even generated a negative reaction barrier. The suitability of some computational methods for accurately predicting the potential energy surface for this hydrogen radical abstraction reaction was discussed.     

Transients and cooperative action of β-carotene, vitamin E and C in biological systems in vitro under irradiation

Using N^•₃ species as specific electron acceptor a defined ascorbate radical: AH^•↔A^•−+H⁺ (λ_max=360 nm, =3400 dm³ mol⁻¹ cm⁻¹) is observed. The attack of DMSO^•+ on vit.E results in a vit.E^• radical (k=1×10⁹ dm³ mol⁻¹ s⁻¹; λ_max=425 nm, =2400 dm³ mol⁻¹ cm⁻¹; 2k=4.7×10⁸ dm³ mol⁻¹ s⁻¹). Vit.E-acetate leads to the formation of a radical cation (vit.E-ac^•+). β-carotene reacts also with DMSO^•+ forming a radical cation, β-car^•+ (k=1.75×10⁸ dm³ mol⁻¹ s⁻¹; λ_max=942 nm, =14 600 dm³ mol⁻¹ cm⁻¹), which probably leads to the formation of a dimer radical cation, (β-car)^•+₂ (k=2.5×10⁷ dm³ mol⁻¹ s⁻¹).

Using E.coli bacteria (AB1157) as a model system in vitro it was found that all three vitamins are rather efficient radiation protecting agents. They can also increase the activity of cytostatica, e.g., mitomycin C (MMC), by electron transfer process. The mixture of vit.E-ac and β-car acts contradictory, but adding vit.C to it a strong cooperative enhancement of the MMC activity is observed once again. A relationship between the pulse radiolysis and the radiation biological data is found and discussed. A possible explanation of the previously reported trials concerning the role of vit.E and β-car on the increased occurence of lung and other types of cancer in smokers and drinkers is presented.     

Ab initio molecular orbital study on three feasible mechanisms for substitution of the vinylic carbon in F2C=C(OMs)BMe3−

A substitution on 2,2-difluorovinylic carbon was investigated by using ab initio molecular orbital calculations. Three feasible mechanisms, which are the S_N1-like, the S_N2-type and the addition-elimination mechanisms, were ex- amined for a model borate, 2,2-difluoro-1-mesyloxyvinyl(trimethyl)borate. Four TSs were obtained depending on the position of Li⁺ around the vinylborate although activation energies in the gas phase are rather high (ca. 30–40 kcal mol⁻¹) in comparison with that expected from the experimental conditions. It was confirmed at the SCRF-IPCM calculations that the solvent effect reduces the acti- vation energy of one S_N2-type mechanism very much (4. l kcal mol⁻¹ at the B3LYP/6-31+G^*//RHF/6-31+G^/s* level of theory) while those for the other mechanisms do not change very much. Therefore, the S_N2-type mechanism is applicable to the substitution reaction observed for the vinylborate.     

Tautomeric and conformational stabilities of methylphosphonic dicyanide, methoxydicyanophosphine, and their isocyano analogs: an ab initio calculation

The relative energies and structural parameters of the equilibrium forms and the potential functions of internal rotation of methylphosphonic dicyanide, CH₃(=O)(CN)₂, methoxydicyanophosphine, CH₃OP(CN)₂, and their isocyano analogs, CH₃P(=O)(NC)₂ and CH₃OP(NC)₂, have been calculated at the RHF/6-31G* level. The total energy of the more stable oxo forms CH₃P(=O)(CN)₂ and CH₃P(=O)(NC)₂ are 10–20 kcal mol⁻¹ lower than the energies of the aci forms CH₃OP(CN)₂ and CH₃OP(NC)₂. The relative stabilities of the cyano and isocyano isomers are almost the same in the case of the oxo forms, but for the aci forms the energies of the cyano isomers are 8 kcal mol⁻¹ lower than those of the isocyano isomers. The potential curves for internal rotation in the aci forms are characterized by a deep minimum corresponding to the trans arrangement of the methyl group and the lone pair of electrons on the phosphorus atom. Two less pronounced minima are symmetrically situated with respect to relative maximum corresponding to the transition cis form. The potential curves of internal rotation in the oxo form possess three minima corresponding to staggered configurations of the methyl group and phosphorus atom bonds. The energy characteristics and geometrical parameters of the studied molecules are compared with known data for similar compounds.     

First steps in radiation-induced hydrogel synthesis: radical formation and oligomerization in dilute aqueous N-isopropylacrylamide solutions

The reactions of hydroxyl radical, hydrogen atom and hydrated electron intermediates of water radiolysis with N-isopropylacrylamide (NIPAAm) were studied by pulse radiolysis in dilute aqueous solutions. OH, H and e_aq⁻ react with NIPAAm with rate coefficient of (6.9±1.2)×10⁹, (6.6±1)×10⁹, and (1.0±0.2)×10¹⁰ mol⁻¹ dm³ s⁻¹. In OH and H radical addition to the double bond mainly -carboxyalkyl type radicals form, (OHCH₂CH^√C(N-i-C₃H₇)O and CH₃CH^√C(N-i-C₃H₇)O). In reaction of e_aq⁻ oxygen atom centered radical anion is produced (CH₂CHC^√(N-i-C₃H₇)O⁻), the anion undergoes reversible protonation with pK_a=8.7. There is also an irreversible protonation on the β-carbon atom that produces the same radical as forms in H atom reaction (CH₃CH^√C(N-i-C₃H₇)O). The -carboxyalkyl type radicals at low NIPAAm concentration (0.1–1 mmol dm⁻³) mainly disappear in self-termination reactions, 2k_t,m=8.4×10⁸ mol⁻¹ dm³ s⁻¹. At higher concentrations the decay curves reflect the competition of the self-termination and radical addition to monomer (propagation). The termination rate coefficient of oligomer radicals containing a few monomer units is 2k_t≈2×10⁸ mol⁻¹ dm³ s¹.     

Protonation of group 14 oxides: the proton affinity of SnO

Molecular orbital calculations are reported for the monoxides, XO, of group 14 elements (X = C, Si, Ge and Sn) and for both isomers, XOH⁺ and HXO⁺, of the protonated monoxides. Structure optimisation has been carried out using the Density Functional Theory employing the B3LYP procedure and at both Hartree-Fock and MP2 (full) levels, all with a variety of medium-sized Gaussian basis sets. In all XO molecules the oxygen atom is the preferred site for protonation, except when X = C where HCO⁺ is the lower energy isomer. Barriers to interconversion between the two isomers XOH⁺ and HXO⁺ are over-estimated by the Hartree-Fock calculations, but with wave functions that include electron correlations they generally fall into the range 27-44 kcal mol⁻¹. Proton affinities increase as the atomic number of X increases, and values calculated by averaging over all wave functions that include electron correlation, give the following proton affinities: for CO, 141.5; for SiO, 189.3; for GeO, 196.1; and for SnO, 215.6 (all in kcal mol⁻¹).     

Theoretical Study on Ionic Liquid Based on 1-Ethyl-3-Methyl- Imidazolium Cation and Hexafluorophosphate or Tetrafluoroborate

The Hartree-Fock and DFT/B3LYP methods have been employed to investigate the electronic structures of 1-ethy1-3-methyl-imidazolium cation(EMIM~ ),BF_4~-,PF_6~-,EMIM~ -BF_4~-,and EMIM~ -PF_6~- using the Gaussian-94 soft-package at 6-31 G(d,p)basis set level for hydrogen,carbon,nitrogen,boron, phosphorus,and fluorine atoms.Comparison of the electronic structures of the lowest energy of EMIM~ - BF_4~- and EMIM~ -PF_6~- pairs,and single EMIM~ ,BF_4~- and PF_6~- showed that the optimized EMIM~ -BF_4~- and EMIM~ -PF_6~- pair conformers were BF_4~- and PF_6~- outside the 5-ring plane between the ethyl group and the methyl group.The cohesion of C—H…F hydrogen bond between cation and anion is reinforced by charge assistance.The interaction energy between EMIM~ and PF_6~- is 328.8 kJ/mol at the B3LYP level and 326.6 kJ/mol at the Hartree-Fock level,whereas that between EMIM~ and BF_4~- is 353.5 kJ/mol at the B3LYP level and 350.5 kJ/mol at the Hartree-Fock level.The low energy interactions caused by bulky asymmetric EMIM~ ,and charge dispersion of cation and anion give rise to the low melting point of ionic liquid EMIM~ -BF_4~- and EMIM~ -PF_6~-.The two hydrogen bonding models of single hydrogen bond formation,and the hydrogen transfer between C_2 in EMIM~ and F in BF_4~- or PF_6~- were principally depicted.     

Electronic structures and electronic spectra of the linkage isomers NSO and SNO

The relative stabilities and electronic structures of the linkage isomers NSO⁻ and SNO⁻ have been determined by the MNDO and ab initio Hartree—Fock—Slater methods. Both approaches predict a higher stability for SNO⁻ by ca. 100 kcal mol⁻¹, but an overlap population analysis indicates substantially higher bond orders for NSO⁻ compared to SNO⁻. The calculations also reveal a low energy pathway with a barrier of ca. 6 kcal mol⁻¹ for the isomerization process NSO⁻ → SNO⁻. Good agreement was found between the observed UV-visible absorption bands for NSO⁻ (λ_max 379 nm) and SNO⁻ (λ_max 340 nm) and calculated values of the electronic transition energies.     

Theoretical studies of the NTO unimolecular decomposition

This work studies 39 decomposition paths among 18 intermediates and 14 transition states. Three types of intra-molecular proton migration and the direct scission of C–NO₂ were regarded as the initial steps of the unimolecular decomposition of NTO. The activation energies of the radicalization C–NO₂ homolysis step are 79.158, 79.781 and 80.652 kcal mol⁻¹. The activation energies of the ionization C–NO⁻¹₂ scission step are 262.488, 263.138 and 272.278 kcal mol⁻¹. The bottle neck activation energies of the C–NO₂H cleavage are 54.936, 63.257 and 71.247 kcal mol⁻¹. Two paths have the smallest bottle neck activation energy. Both of them have two proton migration steps and one internal rotation step prior to C–NO₂H cleavage. At lower temperatures, energy accumulated slowly. When the energy is high enough and reaction time is long enough for structure transformation, these two mechanisms should be the most probable decomposition paths. At high temperatures, the shortest (four steps) mechanism which goes through radicalization C–NO₂ scission should be the dominant path. There are five tautomers found in this study. Four of them are intra-molecular proton migration tautomers. The other one is an internal rotational tautomer. Their energy barriers for structure transfer are lower than any of the activation energies of the decomposition reactions. It may be regarded as one explanation of the insensitive property of NTO.     

A molecular mechanics study on novel Ln(0) π-arene compounds

To examine the steric effects on the stability of Ln(0) π-arene compounds, molecular mechanics (MMP2) calculations are performed on Gd(η-C₆H₆)₂ and Ln(η-Bu^t₃C₆H₃)₂ (where Ln is Gd, Yb and Y ). The small potential-well depth ( ≈ 2 kcal mol⁻¹) and the large Gd-C equilibrium distance ( > 3.3 Å) explains the instability of Gd(η-C₆H₆)₂, while the difference in the stability between Gd(η-Bu^t₃C₆H₃)₂ and Yb(η-Bu^t₃C₆H₃)₂ can be attributed to the difference in the van der Waalsradii of the two metals and the more contracted 5d orbitals on the Yb atom.     

Semiempirical and ab initio calculations on the alkaline hydrolysis of the β-lactam ring Influence of the solvent

We used semiempirical and ab initio calculations to investigate the nucleophilic attack of the hydroxyl ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions. We also used ab initio calculations and the PM3 semiempirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential energy barrier of 20.7 kcal mol⁻¹ due to the desolvation of the hydroxyl ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a water solvation sphere of 20 molecules around the solute, the potential energy barrier is lowered to 17.5 kcal mol⁻¹. Ab initio calculations using the SCRF method predict a potential energy barrier of 13.6 kcal mol⁻¹. These three values, especially the last two, are very close to the experimental value of 16.7 kcal mol⁻¹.