The fragmentations of [M-H]- anions derived from underivatised peptides. The side-chain loss of H2S from Cys. A joint experimental and theoretical study

Loss of H₂S is the characteristic Cys side‐chain fragmentation of the [M? H]^? anions of Cys‐containing peptides. A combination of experiment and theory suggests that this reaction is initiated from the Cys enolate anion as follows: RNH‐^?C(CH₂SH)CONHR′ Ø [RNHC(?CH₂)CONHR′ (HS^?)] Ø [RNHC(?CH₂)CO‐HNR′‐H]^?+H₂S. This process is facile. Calculations at the HF/6‐31G(d)//AM1 level of theory indicate that the initial anion needs only ≥20.1 kcal mol^?1 of excess energy to effect loss of H₂S. Loss of CH₂S is a minor process, RNHCH(CH₂SH)CON^?‐R′ Ø RNHCH(CH₂S^?)CONHR′ Ø RNH ^?CHCONHR+CH₂S, requiring an excess energy of ≥50.2 kcal mol^?1. When Cys occupies the C‐terminal end of a peptide, the major fragmentation from the [M–H]^? species involves loss of (H₂S+CO₂). A deuterium‐labelling study suggests that this could either be a charge‐remote reaction (a process which occurs remote from and uninfluenced by the charged centre in the molecule), or an anionic reaction initiated from the C‐terminal CO₂^? group. These processes have barriers requiring the starting material to have an excess energy of ≥79.6 (charge‐remote) or ≥67.1 (anion‐directed) kcal mol^?1, respectively, at the HF/6‐31G(d)//AM1 level of theory. The corresponding losses of CH₂O and H₂O from the [M? H]^? anions of Ser‐containing peptides require ≥35.6 and ≥44.4 kcal mol^?1 of excess energy (calculated at the AM1 level of theory), explaining why loss of CH₂O is the characteristic side‐chain loss of Ser in the negative ion mode. Copyright © 2003 John Wiley & Sons, Ltd.     

Collision-Induced dissociations of deprotonated peptides. Dipeptides containing aspartic or glutamic acids

Deprotonated dipeptides, on collisional activation, fragment by the characteristic process NH₂CH(R¹) CONHCH(R²)CO₂^? → NH₂^?C(R¹)CONHCH(R²)CO₂H → ^?NHCH(R²)CO₂H + NH₂C(R¹)?C?O, when R¹ and R² = H or alkyl. However, when one of the constituent amino acids is either aspartic acid or glutamic acid, the standard cleavage becomes minor in comparison with fragmentation through the α-side-chain of Asp or Glu. For example, [Asp-Leu - H]^? and [Leu-Asp - H]^? both fragment principally by loss of water, a fragmentation not normally noted for peptides. In addition, [Leu-Asp - H]^? loses CO₂ and also forms HO₂CCH?CHCO₂^?˙. These fragmentations establish that Asp is the C-terminal amino acid. In contrast, isomeric Glu dipeptides, e.g. [Glu-Ala - H]^? and [Ala-Glu - H]^? undergo similar fragmentation, both competitively losing H₂O and CO₂. Both spectra also contain a product ion at m/z 128, identified as the pyroglutamate anion. Product ion and deuterium-labelling studies have been used in an attempt to elucidate the complex fragmentation mechanisms in these systems.     

Diagnostic cyclisation reactions which follow phosphate transfer to carboxylate anion centres for energised [M-H]- anions of pTyr-containing peptides

The low‐energy negative ion phosphoTyr to C‐terminal ‐CO₂PO₃H₂ rearrangement occurs for energised peptide [M–H]^– anions even when there are seven amino acid residues between the pTyr and C‐terminal amino acid residues. The rearranged C‐terminal ‐CO₂PO₂H(O^–) group effects characteristic S_Ni cyclisation/cleavage reactions. The most pronounced of these involves the electrophilic central backbone carbon of the penultimate amino acid residue. This reaction is aided by the intermediacy of an H‐bonded intermediate in which the nucleophilic and electrophilic reaction centres are held in proximity in order for the S_Ni cyclisation/cleavage to proceed. The ΔG_reaction is +184 kJ mol^?1 with the barrier to the S_Ni transition state being +240 kJ mol^?1 at the HF/6‐31 + G(d)//AM1 level of theory. A similar phosphate rearrangement from pTyr to side chain CO₂^– (of Asp or Glu) may also occur for energised peptide [M–H]^– anions. The reaction is favourable: ΔG_reaction is ?44 kJ mol^?1 with a maximum barrier of +21 kJ mol^?1 (to the initial transition state) when Asp and Tyr are adjacent. The rearranged species R¹‐Tyr‐NHCH(CH₂CO₂PO₃H^–)COR² (R¹ = CHO; R² = OCH₃) may undergo an S_Ni six‐centred cyclisation/cleavage reaction to form the product anion R¹‐Tyr(NH^–). This process has a high energy requirement [ΔG_reaction = +224 kJ mol^?1, with the barrier to the S_Ni transition state being +299 kJ mol^?1]. Copyright © 2011 John Wiley & Sons, Ltd.     

The Reaction of Ozone with the Hydroxide Ion: Mechanistic Considerations Based on Thermokinetic and Quantum Chemical Calculations and the Role of HO4− in Superoxide Dismutation

The reaction of OH^? with O₃ eventually leads to the formation of ^.OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol. 1982 , 16, 676–681), it was suggested that the first step occurred by O transfer: OH^?+O₃→HO₂^?+O₂ and that ^.OH was generated in the subsequent reaction(s) of HO₂^? with O₃ (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one‐step O transfer such as that mentioned above would require the release of O₂ in its excited singlet state (¹O₂, O₂(¹Δ_g)); this state lies 95.5 kJ mol^?1 above the triplet ground state (³O₂, O₂(³Σ_g^?)). The low experimental rate constant of 70 M ^?1 s^?1 is not incompatible with such a reaction. However, according to our calculations, the reaction of OH^? with O₃ to form an adduct (OH^?+O₃→HO₄^?; ΔG=3.5 kJ mol^?1) is a much better candidate for the rate‐determining step as compared with the significantly more endergonic O transfer (ΔG=26.7 kJ mol^?1). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO₄^? have been probed: HO₄^?→HO₂^?+¹O₂ is spin allowed, but markedly endergonic (ΔG=23.2 kJ mol^?1). HO₄^?→HO₂^?+³O₂ is spin forbidden (ΔG=?73.3 kJ mol^?1). The decay into radicals, HO₄^?→HO₂^.+O₂^.?, is spin allowed and less endergonic (ΔG=14.8 kJ mol^?1) than HO₄^?→HO₂^?+¹O₂. It is thus HO₄^?→HO₂^.+O₂^.? by which HO₄^? decays. It is noted that a large contribution of the reverse of this reaction, HO₂^.+O₂^.?→HO₄^?, followed by HO₄^?→HO₂^?+³O₂, now explains why the measured rate of the bimolecular decay of HO₂^. and O₂^.? into HO₂^?+O₂ (k=1×10⁸ M ^?1 s^?1) is below diffusion controlled. Because k for the process HO₄^?→HO₂^.+O₂^.? is much larger than k for the reverse of OH^?+O₃→HO₄^?, the forward reaction OH^?+O₃→HO₄^? is practically irreversible.     

Synthesis,structure, and kinetic studies on [RuCl2(NCCH3)2(cod)]

[RuCl₂(NCCH₃)₂(cod)], an alternative starting material to [RuCl₂(cod)] _n for the preparation of ruthenium(II) complexes, has been prepared from the polymer compound and isolated in yields up to 87% using a new work-up procedure. The compound has been obtained as a yellow solid without water of crystallization. The complexes [RuCl₂(NCR)₂(cod)] spontaneously transform into dimers [Ru₂Cl(μ-Cl)₃(cod)₂(NCR)] (R?=?Me, Ph). ¹H NMR kinetic experiments for these transformations evidenced first-order behavior. [RuCl₂(NCPh)₂(cod)] dimerizes slower by a factor of ten than [RuCl₂(NCCH₃)₂(cod)]. The following activation parameters, ΔH ^#?=?114?±?3?kJ?mol^?1 and ΔS ^#?=?66?±?9?J?K^?1?mol^?1 for R?=?CH₃CN (ΔG ^#?=?94?±?5?kJ?mol^?1, 298.15?K) and ΔH ^#?=?122?±?2?kJ?mol^?1 and ΔS ^#?=?75?±?6?J?K^?1?mol^?1 for R?=?Ph (ΔG ^#?=?100?±?4?kJ?mol^?1, 298.15?K), have been calculated from the first-order rate constants in the temperature range 294–323?K. The kinetic parameters are in agreement with a two-step mechanism with dissociation of acetonitrile as the rate-determining step. The molecular structures of [Ru₂Cl(μ-Cl)₃(cod)₂(NCR)] (R?=?Me, Ph) have been determined by X-ray diffraction.     

Is the HO4− Anion a Key Species in the Aqueous‐Phase Decomposition of Ozone?

The role of the HO₄^? anion in atmospheric chemistry and biology is a matter of debate, because it can be formed from, or be in equilibrium with, key species such as O₃ + HO^? or HO₂ + O₂^?. The determination of the stability of HO₄^? in water therefore has the greatest relevance for better understanding the mechanism associated with oxidative cascades in aqueous solution. However, experiments are difficult to perform because of the short‐lived character of this species, and in this work we have employed DFT, CCSD(T) complete basis set (CBS), MRCI/aug‐cc‐pVTZ, and combined quantum mechanics/molecular mechanics (QM/MM) calculations to investigate this topic. We show that the HO₄^? anion has a planar structure in the gas phase, with a very large HOO? OO bond length (1.823 Å). In contrast, HO₄^? adopts a nonplanar configuration in aqueous solution, with huge geometrical changes (up to 0.232 Å for the HOO? OO bond length) with a very small energy cost. The formation of the HO₄^? anion is predicted to be endergonic by 5.53±1.44 and 2.14±0.37 kcal mol^?1 with respect to the O₃ + HO^? and HO₂ + O₂^? channels, respectively. Moreover, the combination of theoretical calculations with experimental free energies of solvation has allowed us to obtain accurate free energies for the main reactions involved in the aqueous decomposition of ozone. Thus, the oxygen transfer reaction (O₃ + OH^? → HO₂ + O₂^?) is endergonic by 3.39±1.80 kcal mol^?1, the electron transfer process (O₃ + O₂^? → O₃^? + O₂) is exergonic by 31.53±1.05 kcal mol^?1, supporting the chain‐carrier role of the superoxide ion, and the reaction O₃ + HO₂^? → OH + O₂^? + O₂ is exergonic by 12.78±1.15 kcal mol^?1, which is consistent with the fact that the addition of small amounts of HO₂^? (through H₂O₂) accelerates ozone decomposition in water. The combination of our results with previously reported thermokinetic data provides some insights into the potentially important role of the HO₄^? anion as a key reaction intermediate.     

Dinitrogen Activation by Fryzuk’s [Nb(P2N2)] Complex and Comparison with the Laplaza–Cummins [Mo{N(R)Ar}3] and Schrock [Mo(N3N)] Systems

The reaction profile of N₂ with Fryzuk’s [Nb(P₂N₂)] (P₂N₂=PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh) complex is explored by density functional calculations on the model [Nb(PH₃)₂(NH₂)₂] system. The effects of ligand constraints, coordination number, metal and ligand donor atom on the reaction energetics are examined and compared to the analogous reactions of N₂ with the three‐coordinate Laplaza‐Cummins [Mo{N(R)Ar}₃] and four‐coordinate Schrock [Mo(N₃N)] (N₃N=[(RNCH₂CH₂)₃N]^3?) systems. When the model system is constrained to reflect the geometry of the P₂N₂ macrocycle, the N? N bond cleavage step, via a N₂‐bridged dimer intermediate, is calculated to be endothermic by 345 kJ mol^?1. In comparison, formation of the single‐N‐bridged species is calculated to be exothermic by 119 kJ mol^?1, and consequently is the thermodynamically favoured product, in agreement with experiment. The orientation of the amide and phosphine ligands has a significant effect on the overall reaction enthalpy and also the N? N bond cleavage step. When the ligand constraints are relaxed, the overall reaction enthalpy increases by 240 kJ mol^?1, but the N₂ cleavage step remains endothermic by 35 kJ mol^?1. Changing the phosphine ligands to amine donors has a dramatic effect, increasing the overall reaction exothermicity by 190 kJ mol^?1 and that of the N? N bond cleavage step by 85 kJ mol^?1, making it a favourable process. Replacing Nb^II with Mo^III has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 160 kJ mol^?1. The reaction profile for the model [Nb(P₂N₂)] system is compared to those calculated for the model Laplaza and Cummins [Mo{N(R)Ar}₃] and Schrock [Mo(N₃N)] systems. For both [Mo(N₃N)] and [Nb(P₂N₂)], the intermediate dimer is calculated to lie lower in energy than the products, although the final N? N cleavage step is much less endothermic for [Mo(N₃N)]. In contrast, every step of the reaction is favourable and the overall exothermicity is greatest for [Mo{N(R)Ar}₃], and therefore this system is predicted to be most suitable for dinitrogen cleavage.     

The structure and dissociation pathways of protonated methanol: An ab initio molecular orbital study

Ab initio molecular orbital calculations with moderately large polarization basis sets and including valence-electron correlation have been used to examine the structure and dissociation mechanisms of protonated methanol [CH₃OH₂]⁺. Stable isomers and transition structures have been characterized using gradient techniques. Protonated methanol is found to be the only stable isomer in the [CH₅O]⁺ potential surface. There is no evidence for a tightly-bound complex, [HOCH₂]⁺…?H₂, analogous to the preferred structure [CH₃]⁺…?H₂ of [CH₅]⁺. Protonated methanol is found to possess a pyramidal arrangement of bonds at the oxygen atom with a barrier to inversion of 8kJ mol^?1. The lowest energy fragmentation pathways are dissociation into methyl cation and water (predicted to require 284 kJ mol^?1 with zero reverse activation energy) and loss of molecular hydrogen (endothermic by 138 kJ mol^?1 but with a reverse activation barrier of 149 kJ mol^?1). The results offer a possible explanation as to why production of [CH₂OH]⁺ from the reaction of methyl cation with water is not observed. Other dissociation processes examined include loss of a hydrogen atom to yield the methylenoxonium radical cation or methanol radical cation (requiring 441 and 490 kJ mol^?1, respectively) and loss of a proton to yield neutral methanol (requiring 784 kJ mol^?1).     

Dynamic proton magnetic resonance studies on complex spin systems. Non-mutual three-spin exchange in N-(trideuteriomethyl)-2-cyanoaziridine

At room temperature and below, the proton NMR spectrum of N-(trideuteriomethyl)-2-cyanoaziridine consists of two superimposed ABC patterns assignable to two N-invertomers; a single time-averaged ABC pattern is observed at 158.9°C. The static parameters extracted from the spectra in the temperature range from –40.3 to 23.2°C and from the high-temperature spectrum permit the calculation of the thermodynamic quantities ΔH⁰ = ?475±20 cal mol^?1 (?1.987 ± 0.084 kJ mol^?1) and ΔS⁰ = 0.43±0.08 cal mol^?1 K^?1 (1.80±0.33 J mol^?1 K^?1) for the cis ? trans equilibrium. Bandshape analysis of the spectra broadened by non-mutual three-spin exchange in the temperature range from 39.4–137.8°C yields the activation parameters ΔH_tc^≠ = 17.52±0.18 kcal mol^?1 (73.30±0.75 kJ mol^?1), ΔS_tc^≠ = ?2.08±0.50 cal mol^?1 K^?1 (?8.70±2.09 J mol^?1 K^?1) and ΔG_tc^≠ (300 K) = 18.14±0.03 kcal mol^?1 (75.90±0.13 kJ mol^?1) for the trans → cis isomerization. An attempt is made to rationalize the observed entropy data in terms of the principles of statistical thermodynamics.     

Negative ion chemical ionization mass spectrometry—binding of molecules to bromide and iodide anions

The analytical potential of negative ion chemical ionization (NICI) mass spectrometry utilizing dibromodifluoro-methane (CF₂Br₂) and iodomethane (CH₃I)/methane (CH₄) as reagent gases is examined. The NICI mass spectrum of CF₂Br₂ contains Br^?, [HBr₂]^? and [CF₂Br₃]^? anions. Weak acids (i.e. those acids with approximately ΔH°_(acid) values between 1674 and 1464 kJ mol^?1) react with Br^? to produce minor yields of the hydrogen?bonded bromide attachment [MH + Br]^? anion or are unreactive. Strong acids (i.e. those acids with approximately ΔH°_(acid) > 1464 kJ mol^?1) produce primarily [MH + Br]^? anions with a minor yield of proton transfer [M ? H]^? anion. The NICI spectrum of CH₃I/CH₄ is dominated by I^?. Weak acids react with I^? to yield minor amounts of [MH + 1]^? or are unreactive. Strong acids produce only [MH + l]^? anions. From a consideration of the gas-phase basicity of the halide anion and the binding energy of the hydrogen-bonded halide attachment adduct, thermochemical data are used as a potential guide to rationalize or predict the ions observed in NICI mass spectra.     

Vitamin C: an experimental and theoretical study on the gas‐phase structure and ion energetics of protonated ascorbic acid

In order to investigate the gas‐phase mechanisms of the acid catalyzed degradation of ascorbic acid (AA) to furan, we undertook a mass spectrometric (ESI/TQ/MS) and theoretical investigation at the B3LYP/6‐31 + G(d,p) level of theory. The gaseous reactant species, the protonated AA, [C₆H₈O₆]H⁺, were generated by electrospray ionization of a 10^?3 M H₂O/CH₃OH (1 : 1) AA solution. In order to structurally characterize the gaseous [C₆H₈O₆]H⁺ ionic reactants, we estimated the proton affinity and the gas‐phase basicity of AA by the extended Cooks's kinetic method and by computational methods at the B3LYP/6‐31 + G(d,p) level of theory. As expected, computational results identify the carbonyl oxygen atom (O2) of AA as the preferred protonation site. From the experimental proton affinity of 875.0 ± 12 kJ mol^?1 and protonation entropy ΔS_p 108.9 ± 2 J mol^?1 K^?1, a gas‐phase basicity value of AA of 842.5 ± 12 kJ mol^?1 at 298 K was obtained, which is in agreement with the value issuing from quantum mechanical computations. Copyright © 2016 John Wiley & Sons, Ltd.     

Proton and sodium ion affinities of glycine and its sodium salt in the gas phase. Ab initio calculations

The energies of protonation and Na⁺ cationization of glycine (GLY) and its (GLY ? H + Na) salt in the gas phase were calculated using ab inltio calculations. The proton affinity of GLY, valued at the MP2/6–31G*//3-21G level, is 937 kJ mol^?1. The amino function is confirmed to be the most favourable site of protonation: ‘proton affinities’ of the carbonyl and hydroxyl functions are calculated to be 75 and 180 kJ mol^?1, respectively, lower than that of NH₂ at the MP2/6-31G*//3–21G level. Calculations performed up to the MP2/6–31G*//3–21G level give the Na⁺ affinity of GLY as 189 kJ mol^?1 and the H⁺ and Na⁺ affinities of (GLY – H + Na) as 1079 and 298 kJ mol^?1, respectively. The geometries of all neutral and protonated species optimized with the 3–21G basis set are described. Both H* and Na⁺ cations complex preferably between the nitrogen atom and the carbonyl oxygen atom, leading to pseudo-five-membered ring structures in which Na? O and Na? N bonds lengths are greater than 2 Å.     

A dataset of highly accurate homolytic NBr bond dissociation energies obtained by Means of W2 theory

Homolytic N? Br bond dissociation constitutes the initial step of numerous reactions involving N‐brominated species. However, little is known about the strength of N? Br bonds toward homolytic cleavage. We herein report accurate bond dissociation energies (BDEs) for a set of 18 molecules using the high‐level W2 thermochemical protocol. The BDEs (at 298 K) of the species in this set range from 162.2 kJ mol^?1 (N‐bromopyrrole) to 260.6 kJ mol^?1 ((CHO)₂NBr). In order to compute BDEs of larger systems, for which W2 theory is not applicable, we have benchmarked a wide range of more economical theoretical procedures. Of these, G3‐B3 offers the best performance (root‐mean‐square deviations = 2.9 kJ mol^?1), and using this method, we have computed N? Br BDEs for four widely used N‐brominated compounds. These include (BDEs are given in parentheses): N‐bromosuccinimide (281.6), N‐bromoglutarimide (263.2), N‐bromophthalimide (274.7), and 1,3‐dibromo‐5,5‐dimethylhydantoin (218.2 and 264.8 kJ mol^?1). © 2015 Wiley Periodicals, Inc.     

Alginate polyelectrolyte ionotropic gels. XVIII. Oxidation of alginate polysaccharide by potassium permanganate in alkaline solutions: Kinetics of decomposition of intermediate complex

The kinetics of decomposition of [Alg · Mn ^VIO₄^2?] intermediate complex have been investigated spectrophotometrically at a constant ionic strength of 0.5 mol dm^?3. The decomposition reaction was found to be first-order in the intermediate concentration. The results showed that the rate of reaction was base-catalyzed. The kinetic parameters have been evaluated and found to be ΔS^? = ?103.88±6.18 J mol^?1 K^?1, ΔH^? = 51.61 ± 1.02 kJ mol^?1, and ΔG^? = 82.57 ± 2.86 kJ mol^?1, respectively. A reaction mechanism consistent with the results is discussed. © 1993 John Wiley & Sons, Inc.     

Ligand Dynamics in the Solid: Lithium‐fluorenide and Lithium‐benzo[b]fluorenide N,N,N′,N′‐Tetramethylethylenediamine (tmeda) Complexes

The dynamic behavior of the N,N,N′,N′‐tetramethylethylenediamine (tmeda) ligand has been studied in solid lithium‐fluorenide(tmeda) ( 3 ) and lithium‐benzo[b]fluorenide(tmeda) ( 4 ) using CP/MAS solid‐state ¹³C‐ and ¹⁵N‐NMR spectroscopy. It is shown that, in the ground state, the tmeda ligand is oriented parallel to the long molecular axis of the fluorenide and benzo[b]fluorenide systems. At low temperature (<250 K), the ¹³C‐NMR spectrum exhibits two MeN signals. A dynamic process, assigned to a 180° rotation of the five‐membered metallacycle (π‐flip), leads at elevated temperatures to coalescence of these signals. Line‐shape calculations yield ΔH^?=42.7 kJ mol^?1, ΔS^?=?5.3 J mol^?1 K^?1, and =44.3 kJ mol^?1 for 3 , and ΔH^?=36.8 kJ mol^?1, ΔS^?=?17.7 J mol^?1 K^?1, and =42.1 kJ mol^?1 for 4 , respectively. A second dynamic process, assigned to ring inversion of the tmeda ligand, was detected from the temperature dependence of T_1ρ, the ¹³C spin‐lattice relaxation time in the rotating frame, and led to ΔH^?=24.8 kJ mol^?1, ΔS^?=?49.2 J mol^?1 K^?1, and =39.5 kJ mol^?1 for 3 , and ΔH^?=18.2 kJ mol^?1, ΔS^?=?65.3 J mol^?1 K^?1, and =37.7 kJ mol^?1 for 4 , respectively. For (D₁₂)‐ 3 , the rotation of the CD₃ groups has also been studied, and a barrier E_a of 14.1 kJ mol^?1 was found.     

Kinetics and mechanism of the reduction of monochloramine by hydroxylamine and hydroxylammonium ion

The kinetics and mechanism by which monochloramine is reduced by hydroxylamine in aqueous solution over the pH range of 5–8 are reported. The reaction proceeds via two different mechanisms depending upon whether the hydroxylamine is protonated or unprotonated. When the hydroxylamine is protonated, the reaction stoichiometry is 1:1. The reaction stoichiometry becomes 3:1 (hydroxylamine:monochloramine) when the hydroxylamine is unprotonated. The principle products under both conditions are Cl^–, NH⁺₄, and N₂O. The rate law is given by ?[d[NH₂Cl]/dt] = k₊[NH₃OH⁺][NH₂Cl] + k₀[NH₂OH][NH₂Cl]. At an ionic strength of 1.2 M, at 25°C, and under pseudo‐first‐order conditions, k₊= (1.03 ± 0.06) ×10³ L · mol^?1 · s^?1 and k₀=91 ± 15 L · mol^?1 · s^?1. Isotopic studies demonstrate that both nitrogen atoms in the N₂O come from the NH₂OH/NH₃OH⁺. Activation parameters for the reaction determined at pH 5.1 and 8.0 at an ionic strength of 1.2 M were found to be ΔH? = 36 ± 3 kJ · mol^–1 and Δ S? = ?66 ± 9 J · K^?1 · mol^?1, and Δ H? = 12 ± 2 kJ · mol^?1 and Δ S? = ?168 ± 6 J · K^?1 · mol^?1, respectively, and confirm that the transition states are significantly different for the two reaction pathways. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 124–135, 2006     

An ab initio study of the geometry,energy, and selected force constants for the three planar conformers of carbonic acid,and the bicarbonate ion; and of the energy for the reaction H2O + CO2 → H2CO3

Ab initio calculations using the unscaled 4-31G basis set have been carried out on the cc, tc, and tt conformers of carbonic acid and the bicarbonate ion, with full geometry optimization assuming the structures to be planar. The complete harmonic force field is reported for the (most stable) tt conformer and for the bicarbonate ion, also selected quadratic force constants for the cc and tc conformers. The changes in certain bond lengths and stretching force constants in the cc → tc, tc → tt, and cc → tt conformer conversion reactions are indicative of intramolecular hydrogen bonding, C?O…H? O and H? O…H? O, which is examined in greater detail by partitioning the overall conformer conversion energy into distortion and bonding energy components. The fundamental vibration frequencies for the tt conformer and the bicarbonate ion are calculated from the force constant matrices, and hence, using a scaling factor based on a comparison of calculated and experimental values for the bicarbonate ion and trans-formic acid, a value is predicted for the zero-point energy of the tt conformer. A new estimate of ΔH? for the hydration reaction, H₂O + CO₂ → H₂CO₃, at 298 K in the gas phase; is made from thermochemical data, +20.2 ± 3.4 kJ mol^?1, which, together with estimates of (H₂₉₈? – H₀?) and the zero-point energy for H₂CO₃, gives +8.1 ± 7.0 kJ mol^?1 for ΔE_T(expt). ΔE_T calculated from the 4-31G basis set data is -29.1 kJ mol^?1. Comparison of the experimental value, the Hartree–Fock limit value, and values calculated with a variety of basis sets for the bond separation reaction, CO₂ + CH₄ → 2H₂CO, suggests that the differences, ΔE_T(expt) minus ΔE_T(SCF ), are due mainly to basis set limitations and not substantial correlation energy contributions.     

Kinetics of the thermal unimolecular decomposition of hex-1-ene-3-yne. Heat of formation and resonance stabilization energy of the 3-ethenylpropargyl radical

The thermal unimolecular decomposition of hex-1-ene-3-yne (HEY) has been investigated over the temperature range 949–1230 K using the technique of very low-pressure pyrolysis (VLPP). One reaction pathway is the expected C₅? C₆ bond fission to form the resonance-stabilized 3-ethenylpropargyl radical. There is a concurrent process producing molecular hydrogen which probably occurs via the intermediate formation of hexatrienes and cyclohexa-1,3-diene. RRKM calculations yield the extrapolated high-pressure rate parameters at 1100 K given by the expressions 10^16.0±0.3 exp(?300.4 ± 12.6 kJ mol^?1/RT) s^?1 for bond fission and 10^13.2+0.4 exp(?247.7 ± 8.4 kJ mol^?1/RT) for the overall formation of hydrogen. The A factors were assigned from the results of previous studies of related alkynes, alkenes, and alkadienes. The activation energy for the bond fission reaction leads to ΔH [H₂CCHCC?H₂] = 391.9, DH [H₂CCHCCCH₂? H] = 363.3, and a resonance stabilization energy of 56.9 ± 14.0 kJ mol^?1 for the 3-ethenylpropargyl radical, based on a value of 420.2 kJ mol^?1 for the primary C? H bond dissociation energy in alkanes. Comparison with the revised value of 46.6 kJ mol^?1 for the resonance energy of the unsubstituted propargyl radical indicates that the ethenyl substituent (CH₂?CH) on the terminal carbon atom has only a small effect on the propargyl resonance energy. © John Wiley & Sons, Inc.     

Base hydrolysis of (αβ S)-(o-methoxy benzoato) (tetraethylenepentamine) cobalt(III) ion: A comparative study of the role of ion pairs and micelles

The kinetics of base hydrolysis of (αβ S)-(o -methoxy benzoato) (tetraethylenepentamine)cobalt(III) obeyed the rate law: k_obs = k_OH[OH^?], in the range 0.05 ? [OH^?]_T, mol dm^?3 ? 1.0, I = 1.0 mol dm^?3, and 20.0–40.0°C. At 25°C, k_OH = 13.4 ± 0.4 dm³ mol^?1 s^?1, ΔH^≠ = 93 ± 2 kJ mol^?1 and ΔS^≠ = 90 ± 5 JK^?1 mol^?1. Several anions of varying charge and basicity, CH₃CO₂^?, SO₃^2?, SO₄^2?, CO₃^2?, C₂O₄^2?, CH₂(CO₂)₂^2?, PO₄^3?, and citrate^3? had no effect on the rate while phthalate^2?, NTA^3?, EDTA^4?, and DTPA^5? accelerated the process via formation of the reactive ion pairs. The anionic (SDS), cationic (CTAB), and neutral (Triton X-100) micelles, however, retarded the reaction, the effect being in the order SDS> CTAB > Triton X-100. The importance of electrostatic and hydrophobic effects of the micelles on the selective partitioning of the reactants between the micellar and bulk aqueous pseudo-phases which control the rate are discussed. © 1994 John Wiley & Sons, Inc.     

Rate constants and transition‐state geometry of reactions of alkyl,alkoxyl, and peroxyl radicals with thiols

Enthalpy, activation energy, and rate constant of 9 alkyl, 3 acyl, 3 alkoxyl, and 9 peroxyl radicals with alkanethiols, benzenethiol, and L ‐cysteine are calculated. The intersection parabolas model is used for activation energy calculations. Depending on the structure of attacking radical, the activation energy of reactions with alkylthiols varies from 3 to 43 kJ mol^?1 for alkyl radicals, from 7 to 9 kJ mol^?1 for alkoxyl, and from 18 to 35 kJ mol^?1 for peroxyl radicals. The influence of adjacent π‐bonds on activation energy is estimated. The polar effect is found in reactions of hydroxyalkyl and acyl radicals with alkylthiols. The steric effect is observed in reactions of alkyl radicals with tert‐alkylthiols. All these factors are characterized via increments of activation energy. Quantum chemical calculations of activation energy and geometry of transition state were performed for model reactions: C^?H₃ + CH₃SH, CH₃O^? + CH₃SH, and HO₂^? + CH₃SH with using density functional theory and Gaussian‐98. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 284–293, 2009