The kinetic deuterium isotope effect in the thermal dehydration of boric acid

The kinetic deuterium isotope effect in the thermal dehydration process from H₃BO₃ to HBO₂(III) was determined using simultaneous TG and DSC. The rate constant ratio of H₃BO₃ to D₃BO₃ obtained by the analysis of isothermal TG and DSC curves was found to be smaller than unity. Both activation energy, E, and frequency factor, A, for the dehydration of H₃BO₃ proved to be larger than those of D₃BO₃, using non-isothermal TG and DSC. The origin of the deuterium kinetic isotope effect in the thermal dehydration of boric acid is also briefly discussed.     

Deuterium isotope effects in the thermal dehydration of K2Mg(SO4)2 · 6H2O

Deuterium isotope effects in the thermal stability, enthalpy change, and kinetic parameters were examined by means of TG-DSC recorded simultaneously for the dehydration stages of

The thermal stability and enthalpy change for the hydrate were smaller than those for its deuterium analog in stage (1). In addition, the rate constant for the hydrate was larger than that for the deuterate in this stage. As for stages (2) and (3), any isotope effect in these parameters was not recognized in practice. The correlation of these isotope effects observed in stage (1) is briefly discussed     

Secondary kinetic isotope effects: Deuterium abstraction by chlorine atoms from the CD3 group in CD3CH2Cl,CD3CHDCl,and CD3CD2Cl

The occurrence and magnitude of secondary kinetic isotope effects in the gas phase has been determined for deuterium abstraction from the CD₃ group in CD₃CH₂Cl, CD₃CHDCl, and CD₃CD₂Cl by photochemically generated ground-state chlorine atoms. Over the temperature range 10–94°C a discernible “inverse” kinetic isotope effect is observed. Both the pre-exponential factors and activation energies decrease with deuterium substitution in the vicinal chloromethyl group. The opposing trends result in a net effect close to unity.     

Deuterium isotope effects and mechanism of the gas phase reaction [C3H7]+→[C3H5]+ + H2

The deuterium kinetic isotope effect and the deuterium isotope effect upon kinetic energy release have been calculated for the loss of H₂ from the [C₃H₇]⁺ ion. The calculations are based on the transition state structure suggested recently from ab initio calculations on the reaction pathway. The results obtained are in good agreement with experimental data.     

Deuterium isotope effect on the heats of hydration of barium chloride

Heats of solution of barium chloride, its mono- and dihydrates, and the deuterated hydrates at 25°C have been measured and combined to calculate the deuterium isotope effect on the heats of hydration of barium chloride. Substitution of D₂O for H₂O in the hydrates enhances the heats of dehydration by 1–2%.     

Zur theorie der α-ketosa¨uren: Mechanismus der enolisierung einiger brenztraubensa¨ureamide

The mechanism of enolisation of pyruvamide is discussed by the influence of substituents on the kinetic CH₃-acidity, by general base-catalysis of enolisation, by the enthalpy and entropy of activation and primary kinetic and kinetic solvent deuterium isotope effects respectively. A Bro¨nsted coefficient β = 0·71 has been obtained in the general base catalysis of pyruvdiethylamide enolisation. The effect of car☐ylsubstituents on the kinetic CH₃-acidity is produced not only by an inductive mechanism. The importance of solvent structure is demonstrated by a strong negative entropy of activation for the H₂O-catalysed reaction. In the H₂O-catalysed enolisation of pyruvdiethylamide a large kinetic deuterium solvent isotope effect k_o^H₂O/k_o^D₂O = 2·39) was obtained at 25°C. In contrast, when hydroxid is the catalyst, the primary kinetic deuterium isotope effect is unusually low (k_H/k_D = 3·5). Thus, in comparison to other keto compounds, a different mechanism of enolisation for the pyruvic acid derivatives must be postulated. Some aspects of this mechanism are discussed in the paper.     

A deuterium isotope effect on the lifetime of the second triplet state of naphthalene

From a Stern-Volmer kinetic analysis of the naphthalene-T₂ sensitized reaction of endodicyclopentadiene with benzene as the intermediate excitation carrier, it was concluded that deuterium substitution increases the T₂-lifetime of naphthalene. An isotope effect of 1.4 ± 0.3 was observed for per-deuteronaphthalene.     

The mechanism for elimination of acetic acid from 1-acetoxy- and 2-acetoxytetralin

By employing deuterium substitution and metastable ion defocusing methods, it has been determined that 1-acetoxytetralin undergoes a highly regiospecific (>98%) 1,4-elimenation of acetic acid. The mechanism closely parallels that for loss of water from 1-tetralol in terms of specificity. However, unlike the water loss, which shows a significant kinetic isotope effect (K_H/K_D = 2.0) and a large release of translational energy (270 meV), the expulsion of acetic acid occurs without an isotope effect and with release of only 10 meV of kinetic energy. Competitive with acetic acid loss is the elimination of ketene which has been shown to occur by a 4-centered transition state. The 2-acetoxytetralin exhibits the more traditional 1,2-elimination of acetic acid which contrasts with a 1,3-elemination of water for the corresponding alcohol.     

Deuterium kinetic isotope effect in the oxidation of sodium (2-D2)propionate with permanganate in water solutions

Kinetic parameters characterizing the oxidation of sodium(2-¹H₂) propionate and the oxidation of sodium (2-²H₂) propionate with permanganate in water solutions have been determined and compared with kinetic parameters derived from the investigation of the deuterium isotope effect on the activation parameters in the permanganate and manganate oxidation of sodium (2-³H₂) propionate in water solutions of sodium hydroxide.     

Hydrolyse acide du p-nitrophényl-diazométhane dans les mélanges H2O?D2O: analyse des effets isotopiques

The velocity of the hydrogen ion catalysed hydrolysis of p-nitrophenyl-diazo-methane (I) has been measured in H₂O? D₂O mixtures, giving an isotopic α_i = 0.49. The product isotope effect r = 5.1, determined from product analyses, combined with the (overall) solvent isotope effect k_H/k_D = 2.81, yields the primary kinetic isotope effect (k_H/k_D)^I = 3.8, and the secondary kinetic isotope effect (k_H/k_D)^II = 0.75. The CICH₂COOH-catalysed hydrolysis of I in H₂O? D₂O mixtures gave a straight-line plot of k_n/k_H versus the atomic fraction n of deuterium. With four carboxylic acids, as catalysts, values of about 4.3 for the kinetic (overall) isotope effects were observed.     

Deuterium and Hydrogen Tunneling in the Hydrogenation of 4‐Oxocyclohexa‐2,5‐dienylidene

4‐Oxocyclohexa‐2,5‐dienylidene is a highly reactive triplet ground state carbene that is hydrogenated in solid H₂, HD, and D₂ at temperatures as low as 3 K. The mechanism of the insertion of the carbene into dihydrogen was investigated by IR and EPR spectroscopy and by kinetic studies. H or D atoms were observed as products of the reaction with H₂ and D₂, respectively, whereas HD produces exclusively D atoms. The hydrogenation shows a very large kinetic isotope effect and remarkable isotope selectivity, as was expected for a tunneling reaction. The experiments, therefore, provide clear evidence for both hydrogen tunneling and the rare deuterium tunneling in an intermolecular reaction.     

Deuterium kinetic isotope effect in the oxidation of sodium (2-D2)valerate with manganate in 3M aqueous NaOH solutions

Sodium (2-²H₂)valerate has been obtained by exchange technique and the kinetic parameters characterizing the oxidation of both sodium valerate of normal isotopic composition and sodium ,-dideuteriovalerate with manganate in 3M aqueous NaOH solutions have been determined. A discussion of the kinetic deuterium isotope effect observed in the oxidation of CH₃CH₂CH₂CD₂COONa is given.     

Deshydratation du di-t-butylmenthylcarbinol dans l'acide acetique anhydre: Probleme de l'etape determinante

The products of the acid-catalysed dehydration of di-t-butylmethylcarbinol (1) in acetic acid are 2-t-butyl-3,3-dimethylbut-1-ene (2) and 2,3,3,4,4-pentamethylpent-1-ene (3). The rate constant for the rearrangement of 2 to 3 (k_rearr) is about 10 times smaller than that of dehydration (k_I). The kinetic isotope effect on the dehydration rate of d₃-methyl-(1) is that expected for a secondary isotope effect upon carbonium ion formation; there appears to be no primary kinetic isotope effect. The rate determining step is, then, probably heterolysis of the protonated alcohol, contrary to what is observed in aqueous media where carbonium ion deprotonation is rate determining. At low acidity (H₀ > - 1) the (log k_I)/ — H₀ correlation has a slope close to unity but at higher acidity log k_I increases faster than —H₀.     

Investigation of site selectivity of the stereoselective deprotonation of cyclohexene oxide using kinetic resolution of isotopic enantiomers in natural abundance

Stereoselective deprotonation of epoxides with lithium amides can occur by abstraction of protons from more than one site. The site selectivity of the deprotonation of cyclohexene oxide by several chiral and achiral lithium amides has been investigated. ²H NMR has been used to measure the relative abundances of the isotopomers of the epoxide containing one deuterium. An isotopic stereoisomer, with deuterium in the site undergoing abstraction, reacts slower than its enantiomer and other isotopomers having protium in the same site due to a kinetic isotope effect. This results in a kinetic resolution yielding a relative excess of the less reactive isotopic stereoisomer. Thus, the relative abundance of such an enantiomer increases when compared with those having protium at the site in question as the reaction proceeds. It can be concluded that deprotonation of cyclohexene oxide using some chiral- and non-chiral lithium amides occurs by β_syn-deprotonation.     

Inverse kinetic isotope effect in Magtrieve™ mediated oxidation or deoximation of benzaldoxime: mechanistic implication

Magtrieve? (CrO₂) mediated reactions with benzaldoxime (1a) and its deuterium congener (d-1a) led to the observation of inverse deuterium kinetic isotope effect (i-DKIE) for the substrate’s oxidation (Eq. 1a) as well as deoximation (Eq. 2a) process. Disappearance of the starting material 1a and formation of the products—1,3-dipolar cycloaddition product (4a) as well as benzaldehyde (3a)—followed a typical 1st-order kinetics. The observed k_D/k_H values, in the range of 2–4, suggest for a strong secondary isotope effect which was further evidenced by the fact that d-labeling was retained in 3a. Therefore, the observed i-SDKIE supports our original hypothesis that aldoxime (with sp²-C) interacts with CrO₂ in a rate determining step to form a tetrahedral (now with sp³-C) structure, possibly like 6, which may act as a common intermediate for both the pathways.     

Overall kinetics of photosystem II: pH Dependence and deuterium isotope effect

The overall kinetics of photosystem II was scanned by means of a double flash technique. Oxygen evolution by pea thylakoids provided with an artificial electron acceptor was measured under a regime of double flashes of variant intervals. The pH optimum in H₂O lies near p¹H 7.2 (p¹H meaning the pH in H₂O), with a first-order rate constant of 800 s⁻¹ at 20°C. In deuterium oxide (D₂O), a plateau of maximum reaction rate was found between p²H 6.6 and 7.8 (p²H meaning the “pH” in D₂O), the highest rate constant being 550 s⁻¹. The apparent kinetic deuterium isotope effect is therefore 1.45. Outside this plateau region, there seems to exist another isotope effect of 1.2 to 1.3. These effects are small but may nevertheless reflect the fact that more than one step of the photosystem II reaction sequence is involved in the splitting of a bond to hydrogen. However, the effects may also be solvent effects or located on the acceptor side of the photosystem.     

The Formation and Reactions of Hydrogen Cyanide During Isobutane-SCR over Fe-MFI Catalysts

The extent to which HCN is produced as a significant product during isobutane-SCR over Fe-MFI catalysts has been investigated together with its origin and further conversion. Catalysts made by both vapor-phase sublimation and solid-state ion exchange can produce well over 100-ppm HCN, which is a major intermediate for N₂ production, but the peak concentration is less with the former material due to a higher oxidation activity. HCN production is confined to temperatures giving partial conversion of isobutane, when deposited material suppresses the very high intrinsic activity of Fe-MFI for the further conversion of HCN to N₂, largely via hydrolysis to ammonia and NH₃-SCR. Activity for the oxidation of NO to NO₂ is similarly suppressed. This oxidation step is likely to be the rate determining one in isobutane-SCR rather than the activation of the alkane since there is no deuterium kinetic isotope effect. Decomposition of isobutyronitrile, formed by dehydration of a primary nitroso species via its oxime tautomer, is a possible source of HCN. This decomposition gives propene as a coproduct, which reacts completely during isobutane-SCR and may also be a source of deposits through oligomerization. Secondary and tertiary nitroso intermediates cannot react in this way and, based on the reactions of nitroanalogues, are more likely to undergo elimination to form alkenes. The overall effect is to convert alkane-SCR to SCR with alkenes containing the same number or one less carbon atom.     

Theoretical Study on the Gas‐Phase SN2 Reaction of Microhydrated Fluoride with Methyl Fluoride

The rate constants for the gas‐phase S_N2 reaction of F^?(H₂O) with CH₃F have been calculated using the dual‐level variational transition state theory including multidimensional tunneling from 50 to 500 K. Tunneling was found to dominate the reaction below 200 K. The deuterium, ¹³C, and ¹⁴C kinetic isotope effects (KIEs) and solvent (D₂O) isotope effects (SKIEs) were also calculated in the same temperature range. The results indicated that the deuterium and heavy water substitutions resulted in inverse KIEs (0.6～0.8 ) while the ¹³C and ¹⁴C substitutions resulted in normal KIEs (1.0～1.2) at room temperature. The calculated carbon KIEs increased significantly below 80 K due to the differences in the magnitude of the tunneling effects for different isotopic substitutions.     

Kinetic isotope effect for hydrogen/deuterium abstraction by chlorine atoms from (CH3)2NNO2 and (CD3)2NNO2

The kinetic isotope effect for the abstraction of hydrogen/deuterium from dimethylnitramine and dimethylnitramine-d₆ by chlorine atoms has been studied in the temperature range 273–353 K. The rate constant ratio k_H0/k_D is given by the Arrhenius expression, k_H/k_D=(0.92 ± 0.07)exp(286 ± 250/RT), where R is expressed in cal mol^?1 K^?1. The absolute rate constant for the deuterium abstraction reaction is extrapolated as k_D=(1.50 ± 0.90) × 10^?10 exp(?1,486 ± 370/RT) cm³ molecule^?1 s^?1. The temperature dependence of the kinetic isotope effect was calculated using the conventional transition-state theory, and the obtained values for k_H/k_D and ΔE_{H, D} are in good agreement with the experimental value for a bent transition state geometry, with two new vibrational frequencies of 340 cm^?1 (272 cm^?1) corresponding to the in-plane and out-of-plane motions of hydrogen (deuterium) atoms in the Cl…H…C arrangement. © 1993 John Wiley & Sons, Inc.     

Zur theorie der α-ketosäuren: Sekundäre thermodynamische und kinetische deuterium isotopieeffekte bie nucleophilen additionsreaktionen an die α-carbonyl-gruppe von pyruvamid

Equilibrium constants for the addition of a series of O, N and S-nucleophilic reagents to the α-carbonyl group of pyruvamide are reported. The secondary deuterium isotope effects of the addition constants are nearly independent of the nature of the O- and S-nucleophilic reagent; the corresponding isotope effects of N-nucleophilic reagents are significantly larger. The values of the kinetic secondary deuterium isotope effects for the attack of H₂O to the α-carbonyl group are useful as a guide to the transition state of the uncatalysed and the OH^?-catalysed reaction.