Carbon-13 Kinetic Isotope Effects Research Articles

RuII(tpy)(bpy)Cl]+-Catalyzed reduction of carbon dioxide. Mechanistic insights by carbon-13 kinetic isotope effects.

In this work, we examine the use of competitive 13C kinetic isotope effects (13C KIEs) on CO2 reduction reactions that produce CO and formic acid as a means to formulate reaction mechanisms. The findings reported here mark a further advancement in the combined 13C KIE measurements and theoretical calculations methodology for probing CO2 conversion reactions.

Theoretical analysis on the kinetic isotope effects of bimolecular nucleophilic substitution (S(N)2) reactions and their temperature dependence.

Factors affecting the kinetic isotope effects (KIEs) of the gas-phase SN2 reactions and their temperature dependence have been analyzed using the ion-molecule collision theory and the transition state theory (TST). The quantum-mechanical tunneling effects were also considered using the canonical variational theory with small curvature tunneling (CVT/SCT). We have benchmarked a few ab initio and density functional theory (DFT) methods for their performance in predicting the deuterium KIEs against eleven experimental values. The results showed that the MP2/aug-cc-pVDZ method gave the most accurate prediction overall. The slight inverse deuterium KIEs usually observed for the gas-phase SN2 reactions at room temperature were due to the balance of the normal rotational contribution and the significant inverse vibrational contribution. Since the vibrational contribution is a sensitive function of temperature while the rotation contribution is temperature independent, the KIEs are thus also temperature dependent. For SN2 reactions with appreciable barrier heights, the tunneling effects were predicted to contribute significantly both to the rate constants and to the carbon-13, and carbon-14 KIEs, which suggested important carbon atom tunneling at and below room temperature.

Investigating Inner-Sphere Reorganization via Secondary Kinetic Isotope Effects in the C−H Cleavage Reaction Catalyzed by Soybean Lipoxygenase: Tunneling in the Substrate Backbone as Well as the Transferred Hydrogen

This work describes the application of NMR to the measurement of secondary deuterium (2° (2)H) and carbon-13 ((13)C) kinetic isotope effects (KIEs) at positions 9-13 within the substrate linoleic acid (LA) of soybean lipoxygenase-1. The KIEs have been measured using LA labeled with either protium (11,11-h2-LA) or deuterium (11,11-d2-LA) at the reactive C11 position, which has been previously shown to yield a primary deuterium isotope effect of ca. 80. The conditions of measurement yield the intrinsic 2° (2)H and (13)C KIEs on k(cat)/K(m) directly for 11,11-d2-LA, whereas the values for the 2° (2)H KIEs for 11,11-h2-LA are obtained after correction for a kinetic commitment. The pattern of the resulting 2° (2)H and (13)C isotope effects reveals values that lie far above those predicted from changes in local force constants. Additionally, many of the experimental values cannot be modeled by electronic effects, torsional strain, or the simple inclusion of a tunneling correction to the rate. Although previous studies have shown the importance of extensive tunneling for cleavage of the primary hydrogen at C11 of LA, the present findings can only be interpreted by extending the conclusion of nonclassical behavior to the secondary hydrogens and carbons that flank the position undergoing C-H bond cleavage. A quantum mechanical method introduced by Buhks et al. [J. Phys. Chem. 1981, 85, 3763] to model the inner-sphere reorganization that accompanies electron transfer has been shown to be able to reproduce the scale of the 2° (2)H KIEs.

Experimental and Theoretical Study of the Carbon-13 and Deuterium Kinetic Isotope Effects in the Cl and OH Reactions of CH3F

A laser flash photolysis-resonance fluorescence technique has been employed to determine absolute rate coefficients for the CH3F + Cl reaction in N2 bath gas in the temperature range of 200-700 K and pressure range of 33-133 hPa. The data were fitted to a modified Arrhenius expression k(T) = 1.14 x 10(-12) x (T/298)2.26 exp{-313/T}. The OH and Cl reaction rates of (13)CH3F and CD3F have been measured by long-path FTIR spectroscopy relative to CH3F at 298 +/- 2 K and 1013 +/- 10 hPa in purified air. The FTIR spectra were fitted using a nonlinear least-squares spectral fitting method including line data from the HITRAN database and measured infrared spectra as references. The relative reaction rates defined by alpha = k(light)/k(heavy) were determined to be k(OH+CH3F)/k(OH+CD3F) = 4.067 +/- 0.018, k(OH+CH3F)/k(OH+(13)CH3F) = 1.067 +/- 0.006, k(Cl+CH3F)/k(Cl+CD3F) = 5.11 +/- 0.07, and k(Cl+CH3F)/k(Cl+(13)CH3F) = 1.016 +/- 0.006. The carbon-13 and deuterium kinetic isotope effects in the OH and Cl reactions of CH3F have been further investigated by quantum chemistry methods and variational transition state theory.

Study of the Carbon-13 and Deuterium Kinetic Isotope Effects in the Cl and OH Reactions of CH4 and CH3Cl

Relative rate experiments have been carried out for three isotopologues of chloromethane and their reactions with Cl atoms and OH radicals. The OH and Cl reaction rates of CH2DCl and CHD2Cl were measured by long-path FTIR spectroscopy relative to CH3Cl at 298+/-2 K and 1013+/-10 hPa in purified air. The FTIR spectra were fitted using a nonlinear least squares spectral fitting method including measured high-resolution infrared spectra as references. The relative reaction rates defined by alpha=klight/kheavy were determined to be kOH+CH3Cl/kOH+CH2DCl=1.41+/-0.05, kOH+CH3Cl/kOH+CHD2Cl=2.03+/-0.05, kCl+CH3Cl/kCl+CH2DCl=1.42+/-0.04, and kCl+CH3Cl/kCl+CHD2Cl=2.27+/-0.04. The carbon-13 and deuterium kinetic isotope effects in the OH and Cl reactions of CH3Cl were investigated further using variational transition state theory, and the results were compared to similar calculations performed for the CH4+OH/Cl reaction systems. The calculations show that the order of magnitude difference for the carbon-13 kinetic isotope effect in the OH reaction of CH3Cl compared to CH4 reported by Gola et al. (Atmos. Chem. Phys. 2005, 5, 2395) can be explained by the lower barrier to internal rotation of the OH radical in the transition state of the CH4+OH reaction than in the CH3Cl+OH reaction. The deuterium kinetic isotope effects can be explained in terms of combined variational effects and tunneling.

Temperature Dependence of Carbon-13 Kinetic Isotope Effects of Importance to Global Climate Change

We report here a theoretical study of the 13C kinetic isotope effect (KIE) and its temperature dependence for the reaction OH + CH4 --> H2O + CH3, the major sink of atmospheric methane in the troposphere. The KIE values at various atmospherically significant temperatures were determined by direct dynamics using variational transition state theory with multidimensional tunneling contributions (VTST/MT). The potential energy surfaces (PESs) were generated by hybrid density functional theory as well as by recently developed doubly hybrid density functional theory methods. Comparisons of our calculated KIEs with experimental data and theoretical values in the literature reveal the critical contributions due to multidimensional tunneling and torsion anharmonicity as well as the critical issue of the choice of internal rotational axis.

13C kinetic isotope effects in the copper(I)-mediated living radical polymerization of methyl methacrylate.

Carbon-13 kinetic isotope effects (KIEs) have been determined for free-radical and copper-mediated living radical polymerizations of methyl methacrylate at 60 degrees C. While free-radical polymerization shows only one primary 13C KIE, on the least-substituted double bond carbon (k12/k13 = 1.045), two significant KIEs are observed, one on each double bond carbon, for copper-mediated polymerization (k12/k13(H2C=) = 1.050, k12/k13(=C <) = 1.010), showing that copper-mediated living radical polymerization does not propagate via a simple free radical process.

Carbon-14 kinetic isotope effects in the debromination of dibromocinnamic acid

The kinetic isotope effect (KIE) method was applied to study the mechanism of elimination of bromine from erythro-a,b-dibromocinamic acid. The large 14C KIE for both a- and b-position of side chain of erythro-a,b-dibromocinamic acid proves that elimination of bromine leading to formation of (E)-cinnamic acid proceeds via E2 mechanism.

Carbon-13 Kinetic Isotope Effects in the Decarbonylation Of Liquid Formic Acid in Water Solution

Carbon-13 kinetic isotope effects (13C KIEs) in the decarbonylation of formic acid diluted with water 1:1 (V:V) has been studied in the 130–181°C temperature interval in sealed vacuum all glass reactions vessels. The experimental 13C KIEs are higher than the 13C KIEs values extrapolated from low 50–100°C temperature interval 13C KIEs observed in the decarbonylation of water free liquid formic acid but less than 13C KIEs expected theoretically assuming the complete carbon—oxygen (13C‒16O) bond rupture in the transition state. For instance the (k12/k13) KIE found in this study is 1.0375 at 170.2°C while the “full” 13C KIE is expected to be 1.0429 at this temperature. The 13C KIE extrapolated to 170.2°C from the low temperature data published earlier is 1.0299. - The Arrhenius activation energy for the decarbonylation of formic acid in 1:1 water solution E = 31.3 kcal/mol is by 6 kcal/mol higher than that which was obtained with 99,9% pure formic acid (E = 25,660 kcal/mol). The entropy of activation ΔS ≠ increased from -21.4 e.u. (pure formic acid) to -15.3 e.u. in the decarbonylation of formic acid diluted with water 1:1. The increases of the enthalpy of activation, of the entropy of activation and partly of the carbon-13 kinetic isotope effect observed in the decarbonylation of formic acid in water solution have been rationalized by suggesting isotopic equilibria interfering the pure kinetic fractionation of 13C and intervention of the water molecules into the process of transfer of protons to the formic acid molecules lowering the absolute rate of their decomposition.

Carbon-13 Kinetic Isotope Effects in CO Oxidation by Ag

In the catalytic oxidation of carbon monoxide over silver wool the 13C kinetic isotope effects in the 343−453 K temperature range were experimentally determined and the following temperature dependence was found: 100 ln(k12/k13) = (3.398 − 630/T) ± 0.083. A reaction CO/O2 gas mixture of 1:2 ratio was used in a static system with initial pressures ranging from 20 to 40 kPa. Under these conditions the reaction is of order 1 with respect to CO and order 0 with respect to O2 and CO2 pressure. The apparent activation energy is 59.3 ± 1.7 kJ mol-1. In our theoretical interpretation of the experimental data various geometries of (CO2)⧧ and (CO3)⧧ transition states were applied, and only a (CO2)⧧ with an interbond angle of 110° and CO stretching force constants of 1700 and 1000−1400 N m-1, respectively, with an asymmetric reaction coordinate was found to be acceptable.

Kinetic Isotope Effects for Dialkylglycine Decarboxylase via a High-Precision Continuous-Flow Method

Kinetic Isotope Effects for Dialkylglycine Decarboxylase via a High-Precision Continuous-Flow Method

Carbon-13 Kinetic Isotope Effects in the Decarbonylation of Liquid Formic Acid Assisted with Acetic Anhydride

The kinetic isotope effect (KIE) in the decarbonylation of formic acid(FA) assisted with acetic anhydride (AA) has been studied in the temperature interval 18–100°C. The 13C KIE lying in the range from 1.0541 (at 18.4°C) to 1.0463(at 100.1°C) indicates that the rupture of the carbon—oxygen single bond is the reaction step which determines the 13C fractionation in the observed production of carbon monoxide from formic acid assisted with Ac2O similarly to what has been observed in the decarbonylation of pure liquid formic acid. The magnitude of the 13C KIE (smaller than the value expected for “full C—O bond rupture”) indicates again that there are intrinsic side processes lowering the 13C fractionation in the course of decarbonylations of formic acid. The mechanism of decarbonylation of FA in the liquid medium consisting initially of “HCOOH–Ac2O” has been discussed and the most probable main and side minor reaction channels have been selected. The mechanistic conclusions have been corroborated by presenting the supplementary 13C KIE data for CO formation in a “HCOOH–Ac2O–EDS” system, where the “overall 13C KIE” is observed (EDS = etidronate disodium). The presented isotope data and their interpretation contribute to the full understanding of the decarbonylation of formic acid.

Carbon-13 Kinetic Isotope Effects in the Catalytic Oxidation of CO over Ag

The 13C kinetic isotope effects in the catalytic oxidation of carbon monoxide over silver wool were found in the temperature range 343−453 K with the following temperature dependence: 100 ln(k12/k13) = (3.398−630/T) ± 0.083. A reaction CO/O2 gas mixture of 1:2 ratio was used in a static system with initial pressures ranging from 20 to 40 kPa. Under these conditions the reaction obeys the rate law dPCO/dt = −k1PCO and has an apparent activation energy of 59.3 ± 1.7 kJ mol-1. In our theoretical interpretation, based on Bigeleisen's formalism, various geometries of (CO2)⧧ were applied as transition states of the rate determining step, and only a (CO2)⧧ with an interbond angle of 110° and stretching force constants of 1700 and 1000−1400 N m-1 could explain the experimental results.

Temperature Dependence of the Carbon-13 Isotope Effect in the Decarbonylation of Concentrated Formic Acid Solutions of Sodium Chloride and of Metaphosphoric Acid Reagent

Abstract The temperature dependence of the carbon-13 kinetic isotope effect (13C KIE) in the decarbonylation of concentrated formic acid solutions of NaCl and of metaphosphoric acid reagent has been determined in the temperature interval 50–110°C and compared with the 13C KIE observed in the decarbonylation of pure liquid formic acid. The (k 12/k 13 ) values lay in the range 1.058–1.047, expected assuming that the C[sbnd]O vibration is lost in the course of activation of formic acid molecules leading to decarbonylation. Metaphosphoric acid reagent present in liquid formic acid at saturation concentrations eliminates all side “minor” processes of physical and chemical nature which caused the slight decrease of the experimental 13C kinetic fractionation observed in the course of decarbonylation of pure formic acid from the expected values of 13C KIE. Sodium chloride is a less effective breaker of the structure of liquid formic acid than the metaphosphoric acid reagent.

Carbon-13 kinetic isotope effects in the decarbonylation of liquid formic acid of natural isotopic composition initiated by phosphorus pentoxide

Carbon-13 kinetic isotope effects in the decarbonylation of liquid formic acid of natural isotopic composition initiated by phosphorus pentoxide

Carbon-13 kinetic isotope effects in the decarbonylation of liquid formic acid of natural isotopic composition initiated by phosphorus pentoxide

The isotopic composition of the consecutive fractions of carbon monoxide produced in the decarbonylation of liquid formic acid of natural isotopic composition initiated by addition of phosphorus pentoxide has been measured in the temperature interval 19–100°C and the observed gradual decrease of theδPDB values and the increase of thek12/k13 ratio of the isotopic specific rate constants (KIE values) for each next fraction of CO have been interpreted in terms of conclusions presented in the first paper from this series1 concerning the decarbonylation of HCOOH (F.A.) in concentrated and diluted with water phosphoric acid media. The initial fast dehydration of F.A. by phosphoric anhydride, P2O5, proceeds at room temperture with about 1% carbon-13 KIE. The (k12/k13) values increase with time, as the decarbonylation slows down due to the hydration of phosphorus pentoxide with water generated in dehydration of HCOOH and reach the “plateau” values characteristic for each reaction temperature. These increasing very slowly with reaction times at intermediate temperatures maximum values of (k12/k13) ratios are quite close to values of13C KIE observed in the decarbonylation of pure F.A. (k12/k13=1.0443 at 81°C). Addition of water to liquid F.A. at 90°C and at 100°C caused the further increase of the13C KIE. The detailed discussion of the13C KIE in the “HCOOH−P2O5” system has been given.

Carbon-13 kinetic isotope effect of the decarbonylation of oxalic acid

Carbon-13 intramolecular kinetic isotope effects in the decarbonylation of oxalic acid dihydrate of natural isotopic composition by SO3 and by fuming sulphuric acid at room temperature and decarbonylation of oxalic acid dihydrate by 100% H3PO4 in the temperature interval 80–150°C have been determined. The obtained isotopic and kinetic results have been compared with the earlier13C experimental and theoretical studies in other solvents.

Kinetic Carbon Isotope Effects in the Decarbonylation of Lactic Acid of Natural Isotopic Composition and of [1-14C] Lactic Acid

Abstract Studies of carbon-13 and carbon-14 kinetic isotope effects (K. I. E.) in the decarbonylation of lactic acid (L. A.) in sulphuric acid and in phosphoric acids media have been summarized and compared with earlier studies of 14C and 13C K. I. E. in the decarbonylation of formic and oxalic acids in concentrated sulphuric acid. Supplementary data concerning the decarbonylation of L. A. in sulphuric acid diluted with water and in pyrophosphoric acid are presented and discussed. The observed temperature dependences of 13C and 14C K. I. E. in concentrated H2SO4 and in concentrated phosphoric acids media have been rationalized by invoking a change of the mechanism of decarbonylation of L. A. in concentrated sulphuric and phosphoric acids with temperature. Preliminary calculational results concerning 13C and 1∗∗C K. I. E. in decarbonylation processes are also presented. In H2SO4 diluted with water and in H3PO4 diluted with water the temperature dependence of 13C and 14C K. I. E. is normal and well reproduc...

Carbon-13 kinetic isotope effects in the decarbonylation of formic acid of natural isotopic composition in phosphoric acid media

Carbon-13 kinetic isotope effects in the decarbonylation of formic acid of natural isotopic composition in phosphoric acid media

Carbon-13 kinetic isotope effects in the decarbonylation of lactic acid of natural isotopic composition in phosphoric acid medium

The13C kinetic isotope effect fractionation in the decarbonylation of lactic acid (LA) of natural isotopic composition by concentrated phosphpric acids (PA) and by 85% H3PO4 has been studied in the temperature interval of 60–150°C. The values of the13C(1) isotope effects in the decarbonylation of lactic acid in 100% H3PO4, in pyrophosphoric acid and in more concentrated phosphoric acids are intermediate between the values calculated assuming that the C(1)−OH bond is broken in the rate-controllin gstep of dehydration and those calculated for rupture of the carbon-carbon bond in the transition state. In the temperature interval of 90–130°C the experimental13C fractionation factors determined in concentrated PA approach quite closely the13C fractionation corresponding to C(2)−C(1) bond scission. the13C(1) kinetic isotope effects in the decarbonylation of LA in 85% orthophosphoric acid in the temperature range of 110–150°C coincide with the13C isotope effects calculated assuming that the frequency corresponding to the C(1)−OH vibration is lost in the transition state of decarbonylation. A change of the mechanism of decarbonylation of LA in going from concentrated PA medium to 85% H3PO4 has been suggested. A possible secondary18O and a primary18O kinetic isotope effect in decarbonylation of lactic acid in phosphoric acids media have been discussed, too.