Normal Kinetic Isotope Effect Research Articles

Low temperature water–gas shift: Applications of a modified SSITKA–DRIFTS method under conditions of H2 co-feeding over metal/ceria and related oxides

Three applications of a modified SSITKA–DRIFTS technique demonstrate the importance of designing catalysts for fuel processor applications in terms of a formate mechanism. Switching between 12CO and 13CO was carried out under steady state water–gas shift conditions with a feed containing co-fed H2. The dynamic responses of surface species and CO2 product were evaluated in terms of their times required to achieve 50% fractional isotopic exchange. In all cases studied, the formate ν(CH) band was observed to switch at a rate similar to the gas phase asymmetric ν(CO2) band. In the first application, the method was employed to monitor and corroborate the normal kinetic isotope effect that has been suggested to link the rate limiting step of water–gas shift to formate decomposition via C–H bond cleaving. Also observed was that when the ν(CO) band for adsorbed CO on Pt was completely exchanged, the ν(CO2) band had only achieved 50% exchange, casting doubt on the proposed mechanism involving the direct reaction of Pt–CO with O adatoms on ceria to produce gas phase CO2. In a second study, the impact of metal loading on the formate switching rate led to a remarkable decrease in the switching times of both formates and CO2 product, emphasizing the need to express the reaction pathway in terms of a bifunctional mechanism involving both the oxide, where formates are formed at Type II bridging OH groups, and the metal, where dehydrogenation of formate proceeds rapidly. The results suggest that when the loading of Pt was greater, a higher fraction of rapidly reacting formate close to the metal–ceria interface was present. In contrast, when the Pt loading was low, a considerable fraction of formate was located further from the metal and required time to diffuse to the metal–oxide interface prior to decomposition. Finally, a demonstration of the usefulness of the technique in determining whether an associative mechanism holds true for other partially reducible oxide components is provided. Oxides included ceria, thoria, zirconia, and a mixed ceria–zirconia oxide sample. Limitations of the modified SSITKA technique regarding experimentation and data interpretation are also considered.

Kinetics and Mechanism of the Aminolysis of Diphenyl Phosphinic Chloride with Anilines

The aminolyses of diphenyl phosphinic chloride (1) with substituted anilines in acetonitrile at 55.0 oC are investigated kinetically. Large Hammett ρ X (ρnuc = ?4.78) and Bronsted β X (βnuc = 1.69) values suggest extensive bond formation in the transition state. The primary normal kinetic isotope effects (kH/kD = 1.42-1.82) involving deuterated aniline (XC6H4ND2) nucleophiles indicate that hydrogen bonding results in partial deprotonation of the aniline nucleophile in the rate-limiting step. The faster rate of diphenyl phosphinic chloride (1) than diphenyl chlorophosphate (2) is rationalized by the large proportion of a frontside attack in the reaction of 1. These results are consistent with a concerted mechanism involving a partial frontside nucleophilic attack through a hydrogen-bonded, four-center type transition state.

Mechanism of Rhodium-Catalyzed Carbene Formation from Diazo Compounds

[reaction: see text] A large and normal nitrogen-15 kinetic isotope effect of 1.035 +/- 0.003 provides direct support for the proposed mechanism for the rhodium-catalyzed carbene formation from diazo compounds, which involves the fast formation of a metal-diazo complex followed by rate-limiting extrusion of N2. The large magnitude of the KIE indicates extensive C-N bond fission in the transition state.

Deuterium Kinetic Isotope Effects in Microsolvated Gas-Phase E2 Reactions

This work describes the first experimental studies of deuterium kinetic isotope effects (KIEs) for the gas-phase E2 reactions of microsolvated systems. The reactions of F(-)(H(2)O)(n) and OH(-)(H(2)O)(n), where n = 0, 1, with (CH(3))(3)CX (X = Cl, Br), as well as the deuterated analogs of the ionic and neutral reactants, were studied utilizing the flowing afterglow-selected ion flow tube technique. The E2 reactivity is found to decrease with solvation. Small, normal kinetic isotope effects are observed for the deuteration of the alkyl halide, while moderately inverse kinetic isotope effects are observed for the deuteration of the solvent. Minimal clustering of the product ions is observed, but there are intriguing differences in the nature and extent of the clustering process. Electronic structure calculations of the transition states provide qualitative insight into these microsolvated E2 reactions.

A combined experimental and theoretical study of the reaction between methylglyoxal and OH/OD radical: OH regeneration

Experimental studies have been conducted to determine the rate coefficient and mechanism of the reaction between methylglyoxal (CH(3)COCHO, MGLY) and the OH radical over a wide range of temperatures (233-500 K) and pressures (5-300 Torr). The rate coefficient is pressure independent with the following temperature dependence: k(3)(T) = (1.83 +/- 0.48) x 10(-12) exp((560 +/- 70)/T) cm(3) molecule(-1) s(-1) (95% uncertainties). Addition of O(2) to the system leads to recycling of OH. The mechanism was investigated by varying the experimental conditions ([O(2)], [MGLY], temperature and pressure), and by modelling based on a G3X potential energy surface, rovibrational prior distribution calculations and master equation RRKM calculations. The mechanism can be described as follows: Addition of oxygen to the system shows that process (4) is fast and that CH(3)COCO completely dissociates. The acetyl radical formed from reaction (4) reacts with oxygen to regenerate OH radicals (5a). However, a significant fraction of acetyl radical formed by reaction (R4) is sufficiently energised to dissociate further to CH(3) + CO (R4b). Little or no pressure quenching of reaction (R4b) was observed. The rate coefficient for OD + MGLY was measured as k(9)(T) = (9.4 +/- 2.4) x 10(-13) exp((780 +/- 70)/T) cm(3) molecule(-1) s(-1) over the temperature range 233-500 K. The reaction shows a noticeable inverse (k(H)/k(D) < 1) kinetic isotope effect below room temperature and a slight normal kinetic isotope effect (k(H)/k(D) > 1) at high temperature. The potential atmospheric implications of this work are discussed.

Measurements of the 12C/13C Kinetic Isotope Effects in the Gas-Phase Reactions of Light Alkanes with Chlorine Atoms

The carbon kinetic isotope effects (KIEs) of the reactions of several light non-methane hydrocarbons (NMHC) with Cl atoms were determined at room temperature and ambient pressure. All measured KIEs, defined as the ratio of the Cl reaction rate constants of the light isotopologue over that of the heavy isotopologue (Clk12/Clk13) are greater than unity or normal KIEs. For simplicity, measured KIEs are reported in per mil according to Clepsilon=(Clk12/Clk13 -1)x1000 per thousand unless noted otherwise. The following average KIEs were obtained (all in per thousand): 10.73+/-0.20 (ethane), 6.44+/-0.14 (propane), 6.18+/-0.18 (methylpropane), 3.94+/-0.01 (n-butane), 1.79+/-0.42 (methylbutane), 3.22+/-0.17 (n-pentane), 2.02+/-0.40 (n-hexane), 2.06+/-0.19 (n-heptane), 1.54+/-0.15 (n-octane), 3.04+/-0.09 (cyclopentane), 2.30+/-0.09 (cyclohexane), and 2.56+/-0.25 (methylcyclopentane). Measurements of the 12C/13C KIEs for the Cl atom reactions of the C2-C8 n-alkanes were also made at 348 K, and no significant temperature dependence was observed. To our knowledge, these 12C/13C KIE measurements for alkanes+Cl reactions are the first of their kind. Simultaneous to the KIE measurement, the rate constant for the reaction of each alkane with Cl atoms was measured using a relative rate method. Our measurements agree with published values within+/-20%. The measured rate constant for methylcyclopentane, for which no literature value is available, is (2.83+/-0.11)x10-10 cm3 molecule-1 s-1, 1sigma standard error. The Clepsilon values presented here for the C2-C8 alkanes are an order of magnitude smaller than reported methane Clepsilon values (Geophys. Res. Lett., 2000, 27, 1715), in contrast to reported OHepsilon values for methane (J. Geophys. Res. (Atmos.), 2001, 106, 23, 127) and C2-C8 alkanes (J. Phys. Chem. A, 2004, 108, 11537), which are all smaller than 10 per thousand. This has important implications for atmospheric modeling of saturated NMHC stable carbon isotope ratios. 13C-structure reactivity relationship values (13C-SRR) for alkane-Cl reactions have been determined and are similar to previously reported values for alkane-OH reactions.

Laboratory measurements of the 12C/13C kinetic isotope effects in the gas-phase reactions of unsaturated hydrocarbons with Cl atoms at 298 ± 3 K

The carbon kinetic isotope effects (KIEs) in the reactions of several unsaturated hydrocarbons with chlorine atoms were measured at room temperature and ambient pressure using gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). All measured KIEs, defined as the ratio of the rate constants for the unlabeled and labeled hydrocarbon reaction k 12/k 13, are greater than unity or normal KIEs. The KIEs, reported in per mil according to Cl ɛ = (k 12/k 13−1) × 1000‰ with the number of experimental determinations in parenthesis, are as follows: ethene, 5.65 ± 0.34 (1); propene, 5.56 ± 0.18 (2); 1-butene, 5.93 ± 1.16 (1); 1-pentene, 4.86 ± 0.63 (1); cyclopentene, 3.75 ± 0.14 (1); toluene, 2.89 ± 0.31 (2); ethylbenzene, 2.17 ± 0.17 (2); o-xylene, 1.85 ± 0.54 (2). To our knowledge, these are the first reported KIE measurements for reactions of unsaturated NMHC with Cl atoms. Relative rate constants were determined concurrently to the KIE measurements. For the reactions of cyclopentene and ethylbenzene with Cl atoms, no rate constant has been reported in refereed literature. Our measured rate constants are: cyclopentene (7.32 ± 0.88) relative to propene (2.68 ± 0.32); ethylbenzene (1.15 ± 0.04) relative to o-xylene (1.35 ± 0.21), all × 10−10 cm3 molecule−1 s−1. The KIEs in reactions of aromatic hydrocarbons with Cl atoms are similar to previously reported KIEs in Cl-reactions of alkanes with the same numbers of carbon atoms. Unlike the KIEs for previously studied gas-phase hydrocarbon reactions, the KIEs for alkene–Cl reactions do not exhibit a simple inverse dependence on carbon number. This can be explained by competing contributions of normal and inverse isotope effects of individual steps in the reaction mechanism. Implications for the symmetries of the transition state structures in these reactions and the potential relevance of Cl-atom reactions on stable carbon isotope ratios of atmospheric NMHC are discussed.

Kinetic and Mechanism of the Addition of Benzylamines to α-Phenyl-β-thiophenylacrylonitriles in Acetonitrile

Nucleophilic addition reactions of p-substitutedbenzylamines <TEX>$(XC _6H_4CH _2NH _2)$</TEX> to <TEX>$\alpha$</TEX>-phenyl-<TEX>$\beta$</TEX>-thiophenyl-acrylonitriles (<TEX>$YC _4SH _2CH=C(CN)C_6H_4$</TEX>Y') have been studied in acetonitrile at 25.0, 30.0, and 35.0 <TEX>${^{\circ}C}$</TEX>. The reactions take place in single step in which the <TEX>$C_\beta$</TEX> -N bond formation and proton transfer to <TEX>$C_\alpha$</TEX> of <TEX>$\alpha$</TEX>-phenyl-<TEX>$\beta$</TEX>-thiophenylacrylonitriles occur concurrently with four-membered cyclic transition structure. These mechanistic conclusions are drawn based on (i) the large negative <TEX>$\rho$</TEX>x and large positive <TEX>$\rho$</TEX>Y' values and also large magnitude of <TEX>$\rho$</TEX>X, (ii) the negative sign and large magnitude of the cross-interaction constants (<TEX>$\rho$</TEX>XY), (iii) the normal kinetic isotope effects (<TEX>$k_H/k_D$</TEX> > 1.0), and (iv) relatively low <TEX>$\Delta H ^\neq$</TEX> and large negative <TEX>$\Delta S ^\neq$</TEX> values.

Mechanism of Taurine: α‐Ketoglutarate Dioxygenase (TauD) from Escherichia coli

AbstractThe iron(II)‐ and α‐ketoglutarate‐dependent dioxygenases comprise enzymes that catalyze a variety of important reactions in biology, including steps in the biosynthesis of collagen and antibiotics, the degradation of xenobiotics, the repair of alkylated DNA, and the sensing of oxygen and response to hypoxia. In these reactions, the reductive activation of oxygen is coupled to hydroxylation of the substrate and decarboxylation of the co‐substrate, α‐ketoglutarate. It is believed that a single, conserved mechanistic pathway for formation of a high‐valent iron intermediate that attacks the substrate is operant in all members of this family. Application of a combination of rapid kinetic and spectroscopic techniques to the reaction of taurine/α‐ketoglutarate dioxygenase (TauD), one member of this large enzyme family, has led to the detection of two reaction intermediates. The first intermediate, which is termed J, is a high‐spin FeIV‐oxo complex. Decay of J exhibits a large, normal C1 deuterium kinetic isotope effect, demonstrating that it is the species activating the C–H bond for hydroxylation. The second intermediate is an FeII‐containing product(s) complex. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2005)

Intramolecular NH···S Hydrogen Bonding in the Zinc Thiolate Complex [TmPh]ZnSCH2C(O)NHPh: A Mechanistic Investigation of Thiolate Alkylation as Probed by Kinetics Studies and by Kinetic Isotope Effects

The zinc thiolate complex [Tm(Ph)]ZnSCH2C(O)N(H)Ph, which features a tetrahedral [ZnS4] motif analogous to that of the Ada DNA repair protein, may be obtained by the reaction of Zn(NO3)2 with [Tm(Ph)]Li and Li[SCH2C(O)N(H)Ph] ([Tm(Ph)] = tris(2-mercapto-1-phenylimidazolyl)hydroborato ligand). Structural characterization of [Tm(Ph)]ZnSCH2C(O)N(H)Ph by X-ray diffraction demonstrates that the molecule exhibits an intramolecular N-H...S hydrogen bond between the amide N-H group and thiolate sulfur atom, a structure that is reproduced by density functional theory (DFT) calculations. The thiolate ligand of [Tm(Ph)]ZnSCH2C(O)N(H)Ph is subject to alkylation, a reaction that is analogous to the function of the Ada DNA repair protein. Specifically, [Tm(Ph)]ZnSCH2C(O)N(H)Ph reacts with MeI to yield PhN(H)C(O)CH2SMe and [Tm(Ph)]ZnI, a reaction which is characterized by second-order kinetics that is consistent with either (i) an associative mechanism or (ii) a stepwise dissociative mechanism in which the alkylation step is rate determining. Although the kinetics studies are incapable of distinguishing between these possibilities, a small normal kinetic isotope effect of kH/kD = 1.16(1) at 0 degrees C for the reaction of [Tm(Ph)]ZnSCH2C(O)N(H*)Ph (H* = H, D) with MeI is suggestive of a dissociative mechanism on the basis of DFT calculations. In particular, DFT calculations demonstrate that a normal kinetic isotope effect requires thiolate dissociation because it results in the formation of [PhN(H)C(O)CH2S]- which, as an anion, exhibits a stronger N-H...S hydrogen bonding interaction than that in [Tm(Ph)]ZnSCH2C(O)N(H)Ph. Correspondingly, mechanisms that involve direct alkylation of coordinated thiolate are predicted to be characterized by kH/kD < or = 1 because the reaction involves a reduction of the negative charge on sulfur and hence a weakening of the N-H...S hydrogen bonding interaction.

Temperature dependence and deuterium kinetic isotope effects in the HCO + NO reaction

The reactions of HCO and DCO with NO have been measured by the laser photolysis/continuous-wave (CW) laser-induced fluorescence (LIF) method from 296 to 623 K, probing the ( B ˜ 2 A ′ ← X ˜ 2 A ′ ) HCO (DCO) system. The HCO + NO rate coefficient is (1.81 ± 0.10) × 10 −11 cm 3 molecule −1 s −1 and the DCO + NO rate coefficient is (1.61 ± 0.12) × 10 −11 cm 3 molecule −1 s −1 at 296 K. Both rate coefficients decrease with increasing temperature between 296 and 623 K. The kinetic isotope effect is k H/ k D = 1.12 ± 0.09 at 296 K and increases to 1.25 ± 0.15 at 623 K. The normal kinetic isotope effect supports abstraction as the principal mechanism for the reaction, in agreement with recent computational results.

Catalytic links among the water–gas shift, water-assisted formic acid decomposition, and methanol steam reforming reactions over Pt-promoted thoria

Implied in the proposed water–gas shift (WGS) mechanisms for Pt/ceria and Pt/thoria catalysts is the presumption that reduced defect centers are formed on the surface. This X-ray absorption near-edge spectroscopy study provides direct results indicating that Pt facilitates reduction in the surface shell of thoria. Mechanistic arguments from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) are provided suggesting that the active sites for WGS, water-assisted formic acid decomposition, and methanol steam reforming are associated with oxygen-deficient centers. In all cases, a high H 2O/reactant (i.e., carbon monoxide, formic acid, or methanol) ratio was used. For WGS, CO reacted with type II bridging OH groups at reduced centers to generate surface formate intermediates, the decomposition of which is suggested to be the rate-limiting step by the observation of a normal kinetic isotope effect (NKIE) associated with the formate coverage as monitored by DRIFTS under steady-state conditions using CO + H 2O and CO + D 2O feeds. The same NKIE was observed in steady-state reaction tests. Formic acid dissociated on the surface of thoria to yield the same surface formate species as observed when CO adsorbs. An identical NKIE associated with formate decomposition was observed when switching from a feed containing HCOOH + H 2O and DCOOH + H 2O, establishing two important commonalities: (1) similarity in the mechanistic pathway and (2) importance of the role of type II bridging OH groups at reduced centers in the catalysis. Methanol steam reforming likely proceeded through a mechanism involving adsorption at reduced centers to generate type II methoxy species, with subsequent conversion to formate, unidentate carbonate, and finally CO 2. The higher NKIE when switching between H-labeled and D-labeled feeds suggests that conversion of methoxy species to formate may be the rate-limiting step. The methanol steam reforming reaction was selective to CO 2 at low conversion, but CO selectivity increased at higher conversions, suggesting competition with the secondary reaction of reverse WGS at higher temperature. Pt/thoria was more selective at higher conversion for CO 2 than a similarly loaded Pt/ceria catalyst. These results suggest that from a mechanistic standpoint, the two materials are virtually analogs of one another.

Kinetics and Mechanism of the Addition of Benzylamines to α-Cyano-β-phenylacrylamides in Acetonitrile

Nucleophilic addition reactions of benzylamines (BA; XC 6 H 4 CH 2 NH 2 ) to α-cyano-β-phenylacrylamides (CPA; YC 6 H 4 CH=C(CN)CONH 2 ) have been investigated in acetonitrile at 25.0 °C. The rate is first order with respect to BA and CPA and no base catalysis is observed. The addition of BA to CPA occurs in a single step in which the addition of BA to C β of CPA and proton transfer from BA to C α of CPA take place concurrently with a four-membered cyclic transition state structure. The magnitude of the Hammett (px) and Bronsted (β x ) coefficients are rather small suggesting an early tansition state (TS). The sign and magnitude of the cross-interaction constant, ρXY(= -0.26), is comparable to those found in the normal bond formation processes in the S N 2 and addition reactions. The normal kinetic isotope effect (k H /k D > 1.0) and relatively low ΔH # and large negative ΔS ≠ values are also consistent with the mechanism proposed.

Aminolysis of Aryl N-Ethyl Thionocarbamates: Cooperative Effects of Atom Pairs O and S on the Reactivity and Mechanism

[reaction: see text] The aminolysis reactions of aryl N-ethyl thionocarbamates (ETNC/EtHN-C(=S)-OC6H4Z) with benzylamines (XC6H4CH2NH2) in acetonitrile are investigated at 30.0 degrees C. The rate of ETNC is slower by a factor of ca. 3 than the corresponding aminolysis of aryl N-ethyl thiocarbamate (AETC/EtHN-(C=O)-SC6H4Z), which has been interpreted in terms of cooperative effects of atom pairs O and S on the reactivity and mechanism. For concerted processes, these effects predict a rate sequence, -C(=S)-S- < -C(=S)-O- < -C-(=O)-S- < -C-(=O)-O-, and the present results are consistent with this order. The negative cross-interaction constant, rho(XZ) = -0.87, the magnitude of betaZ (= 0.36-0.50) and failure of the RSP are in accord with the concerted mechanism. The normal kinetic isotope effects, kH/kD = 1.52-1.78, involving deuterated benzylamines suggest a hydrogen-bonded cyclic transition state. Other factors influencing the mechanism are also discussed.

Low temperature water gas shift: the link between the catalysis of WGS and formic acid decomposition over Pt/ceria

Partial reduction of the ceria surface leads to the formation of bridging Type II OH groups, as reported previously. These were found to react with formic acid to yield bidentate formate and water, or with CO directly to form bidentate formate. In both WGS and formic acid decomposition via dehydrogenation whereby a high water/CO or water/formic acid ratio was utilized, a normal kinetic isotope effect was observed consistent with the involvement of formate C–H bond cleaving in the rate limiting step. In the current investigation, switching between a feed containing DCOOH to HCOOH led to an increase in the formic acid conversion, as well as a slight increase in the CO2 selectivity. The overall change in the CO2 yield was approximately 1.3, very close to the normal kinetic isotope effect of 1.4 reported in several studies of water gas shift on metal promoted ceria. By in situ DRIFTS, a slower D-formate decomposition rate versus H-formate in the presence of water was also observed during transient decomposition of the stabilized formate. Based on the identification of the same adsorbed reactive species and similar kinetic isotope effects, the results suggest an analogous mechanism operates for formic acid decomposition and water gas shift.

Water-gas shift: steady state isotope switching study of the water-gas shift reaction over Pt/ceria using in-situ DRIFTS

The stability of surface formates generated by reaction of bridging OH groups with CO is an important first criterion supporting the idea that the rate limiting step of WGS involves formate decomposition. The second important factor is that, in the presence of water, shown directly by the measurements obtained during this steady state isotope switching study, the forward decomposition of surface formates to CO2 and H2 is strongly auto-catalyzed by H2O, in agreement with the findings of Shido and Iwasawa. Based on a normal kinetic isotope effect previously observed with H2O:D2O switching and the response of surface formate coverages to the WGS rate under steady state conditions when a high H2O:CO ratio is employed, the conclusion is drawn that a surface formate mechanism is likely operating for the low temperature water gas shift reaction.

Water-gas shift: an examination of Pt promoted MgO and tetragonal and monoclinic ZrO2 by in situ drifts

In situ DRIFTS measurements on unpromoted and Pt promoted MgO and ZrO2 (both tetragonal and monoclinic) indicate that at high H2O/CO ratios, where the reaction rate has been reported to be zero order in H2O and first order in CO, the mechanism involved in the catalysis of water-gas shift is likely a surface formate mechanism, in agreement with Shido and Iwasawa. Pt was found to catalyze the removal of surface carbonates and to facilitate the generation of active OH groups relative to the unpromoted catalyst. Comparison with Pt/ceria revealed that the OH groups involved in the catalysis of magnesia and zirconia may be those of the bridging variety which occur at defect sites. That is, water dissociated over vacancies to produce bridging OH groups, as observed by infrared spectroscopy. The existence of such an adsorbed species is implied in the zero reaction order for water, where kinetics suggests that the surface should be saturated by an adsorbed water species. The lower extent of vacancy formation for magnesia and zirconia-based materials in comparison with ceria could explain a lower surface population of active bridging OH groups. CO was used as a probe molecule of the reduced centers, as it reacts with bridging OH groups to generate surface formates, a proposed WGS intermediate, and the decomposition of which is proposed to be the rate-limiting step. The trends in formate intensity by CO adsorption and CO conversion in WGS catalytic testing both followed the order: Pt/ceria>Pt/m-zirconia>Pt/t-zirconia>Pt/magnesia. In all cases, a normal kinetic isotope effect was observed in switching from H2O to D2O, consistent with a link between the rate-limiting step and the decomposition of surface formates, as noted previously by Shido and Iwasawa for Rh/ceria, MgO, and ZnO.

The proton-bound dimer of acetone

Kinetic isotope effects (KIE) affect rates of ion–molecule reactions and provide access to important information on binding patterns of gaseous ions and neutral molecules and their reaction mechanisms.1 Mass spectrometric techniques have been widely used to measure KIE, and Cooks’ kinetic method has been invaluable to measure the intrinsic nature and extents of KIE, most particularly the relatively small values normally associated with secondary KIE.2 Recently, using electrospray ionization mass spectrometry (ESIMS), Schroder et al.3 used Cooks’ kinetic method to measure the secondary KIE associated with the 5–20 eV collision-induced dissociation (CID) of the proton-bound dimer of acetone-d0 and acetone-d6, presumably the loosely bonded ion 1′, and found an inverse secondary KIE of 0.82–0.79. This value is in marked contrast to the normal KIE of 1.17 reported by us4 for 15 eV CID with argon of the mass-selected proton-bound dimer of acetone-d0 and acetoned6 generated in the gas-phase by chemical ionization (CI). For the metastable ion, KIE of 1.012d and 0.933 have also been measured. Hence, for ions presumed to be 1′, different instruments and methods provide KIE ranging widely from 1.17 to 0.79. These values vary far beyond the experimental uncertainties, from normal to inverse KIE!

Aqueous Organometallic Chemistry: Synthesis, Structure, and Reactivity of the Water-Soluble Metal Hydride CpRu(PTA)2H

The reaction of CpRu(PTA)2Cl (1) with KOH in methanol afforded the water-soluble hydride CpRu(PTA)2H (2) in good yield. The solid-state structures of both CpRu(PTA)2H and the previously reported CpRu(PTA)2Cl are described and exhibit classic piano stool geometries. Complex 2 is one of a very few structurally characterized CpRu(PR3)2H compounds known and the only one that is water-soluble. The ruthenium hydride is stable and soluble (S25 °C = 20 mg/mL) in deoxygenated water; however, the complex does react with chlorinated solvents to yield 1. Complex 2 undergoes H/D exchange with D2O, and the kinetics of this process were monitored by 31P NMR spectroscopy as a function of temperature. Activation parameters for the reaction of 2 with D2O were obtained: ΔH⧧ = 68 ± 2 kJ/mol; ΔS⧧ = −94 ± 7 J/mol·K. A normal kinetic isotope effect of 7.9 was calculated for the H/D exchange reaction described here. These kinetic parameters suggest an associative mechanism with little Ru−H (or Ru−D) bond cleavage at the transit...

Kinetic Isotope Effects in the Reactions of D Atoms with CH4, C2H6, and CH3OH: Quantum Dynamics Calculations

A practical procedure of calculating quantum rate constants for polyatomic reactions presented previously has been extended in this paper to investigate the kinetic isotope effects (KIEs) for the following reactions: D + CH4, D + C2H6, D + C2D6, and D + CH3OH. The method involves treating the quantum dynamics explicitly for bonds being broken and formed with a potential energy surface obtained from ab initio calculations (MP2 for geometry optimizations and vibrational frequencies, CCSD(T) for points on reduced dimensionality surface). Rate constants are evaluated, and we find the existence of inverse and normal kinetic isotope effects among the listed reactions. Comparison with the available experimental measurements for the D + CH4 and D + CH3OH reactions confirms the inverse KIE found in the present calculations.