Variational transition state theory with multidimensional tunnelling and kinetic isotope effects in the reactions of C2H6, C2H5D and C2D6 with .CCl3 to produce CHCl3 and CDCl3

ABSTRACT

Rate constants for the reactions of C₂H₆, C₂H₅D and C₂D₆ with .CCl₃. for the production of CHCl₃ and CDCl₃ (k₁, k₂, k₃ and k₄) were computed using variational transition state theory coupled with hybrid-meta density functional theory (MPWB1K) over the temperature range of 200–2900 K. The ground-state vibrational adiabatic potential was plotted for all channels. Small- and large-curvature tunnelling were determined to include quantum effects in the calculation of rate constants. Harmonic vibrational frequencies along the reaction path were calculated in curvilinear coordinates with scaled frequencies. Anharmonicity was included in the lowest-frequency torsion. The position of formation and dissociation of bonds was specified using the variation in harmonic vibrational frequencies along the reaction path. Representative tunnelling energy and the thermally averaged transmission probability at 298 K (P(E)exp?( ? ΔE/RT)) were determined for the reactions in which tunnelling is important. The kinetic isotope effect was used to calculate the considerable contributions of tunnelling and vibration. The expressions for rate constants were determined using nonlinear least-square fitting over the temperature range of 200–2900 K.     

Isotope Effects by Comparing 1H‐ and 2D‐NMR Spectra of 9,9′‐Bisbicyclo[4.3.0]‐cyclonona‐2,4,7‐triene

Abstract

Inverse secondary kinetic isotope effects are determined for the dimerization of all‐cis‐cyclononatetraenyl radical, 1, to its corresponding dimer, all‐cis‐9,9′‐bicyclonona‐1,3,5,7‐tetraene, 2, (step 1, k _H/k _D=0.5), and cyclization of the latter to 9,9′‐bisbicyclo[4.3.0]cyclonona‐2,4,7‐triene, 3 (step 2, k _H/k _D=0.75). These results are obtained by comparison of ¹H‐ and ²D‐NMR spectra of 3 and employment of a simple statistical method for acquiring kinetic data. This new strategy appears superior to conventional methods in being fast, simple, and less expensive.     

Reaction of the muonium substituted cyclohexadienyl radical with 2,3-dimethyl-1,3,-butadiene

The rate constant for the title reaction is represented by log (k _M/M^–1s^–1)=6.9(4)–700(350)K/T between 279 and 343K, whereas the H analogous radical, C₆H₇, reacts at room temperature withk _H 12 M^–1s^–1 with dimethylbutadiene. The title reaction is proposed to be transfer of the light hydrogen isotope, Mu, and the large kinetic isotope effect is discussed.Support by the Swiss National Foundation for Scientific Research and by the Swiss Institute for Nuclear Research (SIN) are gratefully acknowledged.     

The concentration dependence of the proton chemical shift and the deuterium quadrupole coupling parameter for binary solutions of ethanol

Concentration dependent experimental measurements of the ethanol hydroxyl proton chemical shift σ_H for binary solutions were carried out. The solvents used were carbon tetrachloride (CCl₄), benzene, chloroform, acetonitrile, acetone and dimethylsulphoxide (DMSO). The chemical shift values range from 0.69 ppm (relative to TMS) for dilute ethanol (extrapolated to infinite dilution) in CCl₄ to 5.34 ppm for neat liquid ethanol. Ab initio calculations of the ethanol-solvent hydrogen bond energies show a correlation with the values for the chemical shift. The hydrogen bond energies for ethanol-solvent dimers range from 0.63 kcal mol^?1 for ethanol-CCl₄ to 9.34 kcal mol^?1 for ethanol-DMSO. Theoretical calculations show a linear correlation between the deuterium quadrupole coupling parameter XD ^ar d the isotropic proton chemical shift σ_H: X_D(kHz) = 291.48 ? 14.96 σ_H, where σ_H is the proton chemical shift in ppm relative to TMS (R ² = 0.99). Using the concentration dependent chemical shift data and this equation, X_D ia observed to range from 280 kHz for very dilute concentrations in CCl₄, where the primary species is ethanol monomer, to 210 kHz for the neat liquid that is comprised primarily of cyclic pentamers.     

Primary kinetic hydrogen isotope effects in deprotonations of a nitroalkane by intramolecular phenolate groups

Rate constants and kinetic isotope effects have been determined for the formation of nitronate anions from the ethers 1‐(2‐methoxyphenyl)‐2‐nitropropane, 7 (X = H, L = H and D) and 1‐(2‐methoxy‐5‐nitrophenyl)‐2‐nitropropane, 7 (X = NO₂, L = H and D), and from the corresponding phenols, 1‐(2‐hydroxyphenyl)‐2‐nitropropane, 3 (X = H, L = H and D), and 1‐(2‐hydroxy‐5‐nitrophenyl)‐2‐nitropropane, 3 (X = NO₂, L = H and D), in aqueous basic medium. For the ethers 7 , rates of deprotonation by hydroxide are comparable with those found for deprotonations of 2‐nitropropane, with k^H/k^D (25 °C) = 7.7 and 7.8, respectively. In both the cases, the isotope effects are conventionally temperature dependent. For the corresponding phenols 3 , conditions have been established under which the deprotonations of the nitroalkane are dominated by intramolecular deprotonation by the kinetically first‐formed phenolate anion, with an estimated effective molarity EM ～ 250. For 3 (X = H, L = H or D), k^H/k^D (25 °C) = 7.8, with E ? E = 6.9 kJ mol^?1 and A^H/A^D = 0.5. For 3 (X = NO₂, L = H or D), rates of intramolecular deprotonation are reduced 30‐fold, and an elevated kinetic isotope effect is found (k^H/k^D (25 °C) = 10.7). Activation parameters (E ? E = 17.8 kJ mol^?1 and A^H/A^D = 0.008) are compatible with an enhanced tunnelling contribution to reactivity in the H‐isotopomer. Copyright © 2009 John Wiley & Sons, Ltd.     

Anilinolysis of O‐butyl phenyl phosphonochloridothioate in acetonitrile: Synthesis,characterization, kinetic study,and reaction mechanism

This paper describes a simple optimized method for the synthesis of O‐butyl phenyl phosphonochloridothioate ( 4 ) under mild conditions. The target compounds were characterized by ¹H‐nuclear magnetic resonance (NMR), ¹³C‐NMR, and ³¹P‐NMR spectroscopy, as well as mass spectroscopy. The apparent structure of 4 was confirmed by optimization using the B3LYP/6‐311 + G(d,p) level in the Gaussian 09 program in acetonitrile. The nucleophilic substitution reactions of 4 with X‐anilines (XC₆H₄NH₂) and deuterated X‐anilines (XC₆H₄ND₂) were investigated kinetically in acetonitrile at 55.0°C. The free energy relationship with X in the anilines looked biphasic concave upwards with a break region between X = H and X = 3‐MeO, giving large negative ρ_X and small positive β_X values. The deuterium kinetic isotope effects were secondary inverse (k_H/k_D < 1: 0.789‐0.995) and the magnitudes, (k_H/k_D), increased when the nucleophiles were changed from weakly basic to strongly basic anilines. A concerted S_N2 mechanism is proposed on the basis of the selectivity parameters and the variation trend of the deuterium kinetic isotope effects with X.     

Isotope Effects during Metabolism of (+)- and (−)- Trans Tramadol Isotopomers by Human Liver Microsomes

Abstract

As for the unlabelled (±)-1(e)-(m-methoxyphenyl)-2(e)-(dimethylaminomethyl)cyclohexane 1(a)-ol, tramadol, (T), pronounced stereochemical effects were observed for the isotopomers with regard to the extent of formation of O- and N-demethylated metabolites. The pharmacological active O-demethylated metabolite M1 was formed from the unlabelled as well as from the labelled (+)-tramadols about 6 times less than from the corresponding (?)-T. In contrast, the N-demethylated metabolite M2 was formed from the (+)-tramadols about 1.5 times more than from the corresponding (?)-T. These steric effects were modulated by the smaller apparent isotope effects for the isotopomers. Isotope effects were expressed as k_H/k_D (calculated from concentrations) and as ^DV, ^DV/K and ^HK_m/^DK_m (from Eadie-Hofstee plots).     

Isotope effect in electron capture by protons into the 2S-state of hydrogen

We have investigated the isotope effect for electron capture into the 2S-state of hydrogen in close collisions for the processes H⁺ in H₂ and H⁺ in D₂. The differential cross sections and transition probabilities P_2S obtained for capture into H(2S) as a function of energy at a fixed angle of θ = 1° exhibit distinctive features, but no apparent isotope effects are detected.     

Vibrational spectrum of HCCl3 and the force field of chloroform

From high resolution infrared absorption studies in the gaseous phase, the fundamental vibrational frequencies and some overtone and combination bands of H¹³CCl₃ have been determined. The infrared spectrum of H¹²CCl₃ has been reinvestigated with higher resolution in order to get accurate ¹²C/¹³C isotope frequency shifts. These shifts in conjunction with ³⁵Cl/³⁷Cl frequency shift data for H¹²CCl₃, H¹³CCl₃, D¹²CCl₃, and the centrifugal distortion constants of H¹²CCl₃ are used to determine the general valence force field, which reproduces all data within the error limits.     

Substitution of Hydroxyl Groups by Deuterium in the Electrochemical Reduction of Methanesulfonates

Abstract

The electrochemical reduction of dodecyl methanesulfonate (ROMs; R = n-C₁₂H₂₅ ^?, Ms = CH₃SO₂ ^?) as model compound was studied in deuterated media. This reaction could be of potential use in syntheses of partially deuterated compounds by conversion of C?OH-groups into C?D-bonds in the presence of other reactive groups. In order to obtain detailed information about the origin of the introduced hydrogen all individual components (solvent, supporting electrolyte, substrate) of the reaction system were deuterium labelled in the model reaction. The experiments indicate, that this reduction does not proceed along a uniform pathway. Dodecyl anions play the dominating role in the reaction mechanism. Deuterium is transferred into the product dodecane-d ₁ from all components of the reaction mixture in the order of their relative proton acidities Ms-d ₃ > Bu₄N⁺-d ₃₆ > DMF-d ₇ > R-1,1-d ₂-OMs.

In the presence of D²O a hydrogen isotope exchange was found with the methyl group of MsO^?, catalyzed by electrogenerated bases.     

Thioenols and thioamides substituted by two β‐EWGs. Comparison with analogous amides and enols

Condensation of organic isothiocyanates with active methylene compounds gave nine thioamides RNHCSCHYY′ or their isomeric thioenols RNHC(SH) = CYY′ for substrates in which Y and Y′ are electron‐withdrawing groups (EWG). These included derivatives of Meldrum's acid (MA) which showed 100% thioenol in all solvents. For other compounds the percentages of thioenol in CDCl₃ when R = Ph are 100% when Y = CN and Y′ = CO₂Me or Y′ = CO₂CH₂CCl₃, 6% when Y = Y′ = CO₂CH₂CF₃, and 0% when Y = Y′ = CO₂Me. The chemical shift of SH (highest values 12.0–16.0 ppm) served as a probe for the thioenol structures and also for the extent of hydrogen bonding to the SH group. In contrast to simple ketones and thioketones in which thioenolization is favored over enolization by factors as large as 10⁶, for intramolecular competition K_Thioenol/K_Enol ratios are much lower than for systems not substituted by β‐EWGs. X‐ray crystallography of the 5‐anilido‐MA derivative shows a hydrogen‐bonded thioenol structure. δ(OH), δ(NH), K_Enol, and crystallographic data for analogous thioenol and enol systems are compared. Copyright © 2008 John Wiley & Sons, Ltd.     

Hydrogen isotope exchange in proton-conducting oxides during proton and deuteron irradiation

It has been found that during accelerator beam deuteron irradiation of a proton-conducting oxide containing protium H/D isotope exchange between beam ions and dissolved ions takes place. This isotope exchange was also observed during high-energy proton irradiation of the oxide containing dissolved deuterium atoms. These results provide evidence to a new type of hydrogen isotope exchange. Any appreciable effects of conjugate diffusion of hydrogen and oxygen ions and of the interface processes on the isotope exchange rate were eliminated. In this type of exchange the rate of replacement of H⁺ by D⁺ and of D⁺ by H⁺ was determined only by the properties of the crystal. The discovered effect was used in our study to obtain experimental data characterizing the dynamic and equilibrium behavior of hydrogen isotopes in the oxide BaZr_0.9Y_0.1O_{3 ? δ}.     

Understanding the reaction between muonium atoms and hydrogen molecules: zero point energy,tunnelling, and vibrational adiabaticity

The advent of very precise measurements of rate coefficients in reactions of muonium (Mu), the lightest hydrogen isotope, with H₂ in its ground and first vibrational state and of kinetic isotope effects with respect to heavier isotopes has triggered a renewed interests in the field of muonic chemistry. The aim of the present article is to review the most recent results about the dynamics and mechanism of the reaction Mu+H₂ to shed light on the importance of quantum effects such as tunnelling, the preservation of the zero point energy, and the vibrational adiabaticity. In addition to accurate quantum mechanical (QM) calculations, quasiclassical trajectories (QCT) have been run in order to check the reliability of this method for this isotopic variant. It has been found that the reaction with H₂(v=0) is dominated by the high zero point energy (ZPE) of the products and that tunnelling is largely irrelevant. Accordingly, both QCT calculations that preserve the products’ ZPE as well as those based on the Ring Polymer Molecular Dynamics methodology can reproduce the QM rate coefficients. However, when the hydrogen molecule is vibrationally excited, QCT calculations fail completely in the prediction of the huge vibrational enhancement of the reactivity. This failure is attributed to tunnelling, which plays a decisive role breaking the vibrational adiabaticity when v=1. By means of the analysis of the results, it can be concluded that the tunnelling takes place through the ν₁=1 collinear barrier. Somehow, the tunnelling that is missing in the Mu+H₂(v=0) reaction is found in Mu+H₂(v=1).     

FTIR and 1H NMR Spectral Study of 3-(Substituted Benzamido)-5,6,7,8-Tetrahydro-5,8-methanoiso-quinolines And Related Compounds

The title benzamide derivatives were synthesized. FTIR spectra of the benzamides having substituents p-NH₂, p-OCH₃, p-CH₃ 3, p-F, H 5,p-CI, p-CF₃, p-CN, p-NO₂ 9 and m-Cl 10 and their related compounds were measured in dilute n-heptane, CCl₄, CHCl₃ and CH₂Cl₂ solutions. ¹H NMR spectra of the benzamides were also measured in CDCl₃ solution. Substituent effects on these spectra were investigated. The results were theoretically considered together with CNDO/2 calculation ones. Conformational analysis of 5 was carried out by the AMl method, and the results were compared with the X-ray structural data of 3, 5, 9 and 10. The intermolecular hydrogen bondings in crystals of the benzamides were examined based on the frequency shifts of their C=0 stretching vibration bands.     

NMR,IR, and ESR spectroscopic investigation of reaction of α‐silylamines with carbon tetrachloride

This paper reports about high reactivity of α‐silylamines in the reaction with CCl₄. Unlike Et₃N, α‐silylamines rapidly react with CCl₄ upon irradiation with daylight to form α‐silylamine hydrochloride salts in 92–98% yields. The influence of structure of α‐silylamines and solvent on the degree of conversion was displayed. The interaction of α‐silylamines with CCl₄ was studied by NMR, ESR, and IR spectroscopy. C‐centered radicals of α‐silylamines were detected by ESR spectroscopy with spin traps (MNP, ND, and PBN) in reaction mixtures in CH₃CN and C₆H₆ and it show the radical character of this reaction. Both CH₃CN and C₆H₆ serve as solvents as well as reagents for this reaction. A mechanism of an interaction between α‐silylamines and CCl₄ is discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Direct ab initio dynamics calculations of the reaction rate for the hydrogen abstraction reaction of NCO with CH4 and C2H6

The kinetics of the hydrogen abstraction reactions NCO + CH₄ (R1) and NCO + C₂H₆ (R2) have been studied over a wide temperature range. The minimum energy paths (MEPs) were calculated at the MP2/cc-pVDZ level and single-point calculations were refined at the G3MP2 level. The rate constants for the title reactions were calculated using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) contributions. The fitted three-parameter formulae are k ₁ = 2.52 × 10^?22 T ^3.46 exp(2466/T) and k ₂ = 9.8 × 10^?22 T ^3.2 exp(411.8/T) cm³ molecule^?1 s^?1 for (R1) and (R2), respectively. The calculated rate constants were found to be in good agreement with the available experimental data. Deuterium kinetic isotope effects were also investigated. Both reactions show a significant kinetic isotope effect in the low-temperature range.     

High‐temperature study of superconducting hydrogen and deuterium sulfide

Hydrogen‐rich compounds are extensively explored as candidates for a high‐temperature superconductors. Currently, the measured critical temperature of 203 K in hydrogen sulfide (H₃S) is among the highest over all‐known superconductors. In present paper, using the strong‐coupling Eliashberg theory of superconductivity, we compared in detail the thermodynamic properties of two samples containing different hydrogen isotopes H₃S and D₃S at 150 GPa. Our research indicates that it is possible to reproduce the measured values of critical temperature 203 K and 147 K for H₃S and D₃S by using a Coulomb pseudopotential of 0.123 and 0.131, respectively. However, we also discuss a scenario in which the isotope effect is independent of pressure and the Coulomb pseudopotential for D₃S is smaller than for H₃S. For both scenarios, the energy gap, specific heat, thermodynamic critical field and related dimensionless ratios are calculated and compared with other conventional superconductors. We shown that the existence of the strong‐coupling and retardation effects in the systems analysed result in significant differences between values obtained within the framework of the Eliashberg formalism and the prediction of the Bardeen‐Cooper‐Schrieffer theory.     

Transesterification of an RNA model in buffer solutions in H2O and D2O

Transesterification of a phosphodiester bond of RNA models has been studied in various buffer solutions, under neutral and slightly alkaline conditions in H₂O and D₂O. The results show that imidazole is the only buffer system where a clear buffer catalysis on the cleavage of a phosphodiester bond is observed. The rate enhancement in sulphonic acid buffers is smaller, and a sulphonate base, particularly, is inactive as a catalyst. The rate‐enhancing effect of imidazole is, however, catalytic, and the catalytic inactivity of sulphonate buffers can be attributed to their structure and/or charge. The catalysis by imidazole is a complex system which, in addition to first‐order reactions, involves a process that shows a second‐order dependence in imidazole concentration. The latter reaction becomes significant in acidic imidazole buffers (pH < pK_a), as the buffer concentration increases. The kinetic solvent deuterium isotope effect k_H/k_D, referring to first‐order catalysis by imidazole base, is 2.3 ± 0.3. That referring to second‐order catalysis is most probably much larger, but an accurate value could not be obtained. Copyright © 2007 John Wiley & Sons, Ltd.     

Thermal decomposition of dichloroketene and its reaction with H atoms

The thermal unimolecular decomposition of dichloroketene CCl₂CO → CCl₂ + CO (reaction 1) was studied experimentally and computationally. Dichloroketene was produced by the pulsed laser photolysis of hexachloroacetone, and the kinetics of its decay due to reaction 1 was monitored using photoionization mass spectrometry. Rate constants of reaction 1 were determined in direct time-resolved experiments as a function of temperature (740–870 K) and bath gas density ([He] = (3–25) × 10¹⁶ atom cm⁻³, [N₂] = 12 × 10¹⁶ molecule cm⁻³). Reaction 1 is in the falloff region under these conditions. The potential energy surface (PES) of reaction 1 was studied using quantum chemical methods. The experimental k₁ (T, P) dependence was reproduced with an RRKM/master equation model based on quantum chemical calculations. Parameterized expressions for the rate constants of reaction 1 and the reverse reaction, that of CCl₂ with CO, were obtained over wide ranges of temperatures and pressures. The enthalpy of formation of CCl₂CO was determined in quantum chemical calculations. The kinetics of the reaction of dichloroketene with hydrogen atoms (reaction 2), an important channel of destruction of CCl₂CO in flames, was studied computationally. The PES of reaction 2 was studied using quantum chemical methods. Temperature and pressure dependences of the rate constants of the four dominant reaction channels were obtained in transition state theory and master equation calculations; the technique of isodesmic reactions for transition states was applied to the channel of Cl atom abstraction. Analysis of the experimental data and the computational models of reactions 1 and 2 demonstrates that thermal decomposition is a major pathway of destruction for dichloroketene in combustion systems.     

Implant redistribution in high-dose ion implanted and annealed silicon

Abstract

Diffusion constants of helium in gold, silver and aluminium are determined from thermal desorption experiments, giving:

Au: D ₀ = 10^?1.0 cm²/sec, ΔH = 1.70 eV

Ag: D ₀ = 10^?1.2 cm²/sec, ΔH = 1.50 eV

Al: D ₀ = 10^+0.1 cm²/sec, ΔH = 1.35 eV

The results are compared to self-diffusion and to the diffusion of other light elements in metals. Possible diffusion mechanisms are discussed.