Calculation Of Isotope Effects Research Articles

Collisional energy transfer in highly excited molecules: deuteration effects

Average energy-transfer values are reported from trajectory studies of Xe and He colliding with highly excited azulene-d{sub 0} and azulene-d{sub 8}. The calculated isotope effect is small, in agreement with experiments on related systems. This suggests that the low-frequency modes are important in determining the amount of energy transfer. Furthermore, the fact that both calculated and observed changes with deuteration are small for helium suggests that the classical description of the energy transfer is adequate. The biased random walk model for collisional energy transferred is found to reproduce the observation of a small isotope effect. The calculated fraction and magnitude of supercollisions are in accord with extant data and are predicted to be relatively unaffected by deuteration. 33 refs., 2 figs., 2 tabs.

D/H isotope effects in ?-complexes of deuterated hexamethylbenzenes with the nitrosonium cation

The isotope effects of deuterium, manifested in the13C NMR spectra of complexes of deuterated hexamethylbenzenes C6(CD3)n·(CH3)6−n with the nitrosonium cation, have been studied. The small values observed for the isotopic perturbation are evidence of π-bonding of the NO+ group the hexamethylbenzene molecule. The applicability of an additive scheme of calculation of isotope effects for the ring carbon atoms of the complexes, based on the increments of replacement of the CH3 group by CD3 in hexamethylbenzene, has been demonstrated.

Microcanonical variational theory of radical recombination by inversion of interpolated partition function. Isotope effect in CH 3 + H/D

Application of the title theory to the recombinations CH 3 + H and CH 3 + D leads to the value of ≈ 2 for the isotope effect. Analysis of this result shows that the increase over the value 2 1 2 expected from simple transition state theory comes almost entirely from the ratio of rotational constants of the doubly degenerate rotation of the symmetric-top CH 3…X (X=H/D) transition state which represents X moving away from CH 3. However the calculated isotope effect remains below the experimental value of ≈ 2.7. Possible reasons for the discrepancy are discussed.

Stereomutation of cyclopropane revisited. An ab initio investigation of the potential surface and calculation of secondary isotope effects

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStereomutation of cyclopropane revisited. An ab initio investigation of the potential surface and calculation of secondary isotope effectsStephen J. Getty, Ernest R. Davidson, and Weston Thatcher BordenCite this: J. Am. Chem. Soc. 1992, 114, 6, 2085–2093Publication Date (Print):March 1, 1992Publication History Published online1 May 2002Published inissue 1 March 1992https://doi.org/10.1021/ja00032a023RIGHTS & PERMISSIONSArticle Views146Altmetric-Citations62LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (3 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Theoretical kinetic isotope effects for the hydride transfer from formate to carbon dioxide: A comparison of theory with experiment

AbstractHeavy atom and deuterium isotope effects for the hydride transfer from formate to carbon dioxide were determined using MNDO, AM1, and PM3 semiempirical methods and the ab initio RHF/3‐21G*, 3‐21 + G, 6‐31G*, and 6‐31 + G* basis sets. Frequencies were also determined for ground state structures using 6‐31G* basis set at the MP2 level of theory. The calculated isotope effects compared favorably with the experimentally determined values of Hermes et al. (Biochemistry, 23, 5479 (1984)). The calculated ground state frequencies were high for most vibrational modes as compared with measured values, however some ab initio methods underestimate the carbon hydrogen stretching frequencies. The heavy atom isotope effects were influenced to a large extent by the excitation factor, the mass factor and the moment of inertia factor of the Bigeleisen equation.

Calculations of intrinsic isotope effects and the detection of tunneling in enzyme-catalyzed reactions

The equation of Northrop [1975, Biochemistry, 14, 2644] for calculating intrinsic isotope effects from observed deuterium and tritium isotope effects of V K , in which hydrogen is the reference isotope, has been extended to experimental designs using either deuterium or tritium as a reference. Partial derivatives of the intrinsic equations allow calculation of the relative precision of the three referenced isotope effects and these favor the order deuterium > tritium > hydrogen. In comparisons of observed and calculated isotope effects when hydrogen tunneling is present, both the precision and the magnitude of the difference was greater for intrinsic calculations than for exponentiations based upon a breakdown in the Swain-Schaad relationship.

ChemInform Abstract: Carbon Kinetic Isotope Effects on the Hydrolysis of Aryl Carbonates.

Carbon kinetic isotope effects on the hydrolysis of aryl carbonates in 1:1 (v/v) dioxane-water at 25°C at various pHs were used to calculate isotope effects on the reactions with hydroxide ion and with water. For diphenyl carbonate k OH 12 /k OH 13 =1.0482 and k w 12 /k w 13 =1.0509. For bis (p-methoxyphenyl)carbonate k OH 12 /k OH 13 =1.0493 and k w 12 /k w 13 =1.0518. For bis(p-nitrophenyl)carbonate k OH 12 /K OH 13 =1.0476. The small change in the isotope effect with substituent group fro both alkaline and neutral hydrolysis is in contrast to the large substituent effect on the rate constants for both reactions. The nearly constant isotope effect indicates that transition-state structures are similar for all substrates

Carbon kinetic isotope effects on the hydrolysis of aryl carbonates

Carbon kinetic isotope effects on the hydrolysis of aryl carbonates in 1:1 (v/v) dioxane-water at 25°C at various pHs were used to calculate isotope effects on the reactions with hydroxide ion and with water. For diphenyl carbonate k OH 12 /k OH 13 =1.0482 and k w 12 /k w 13 =1.0509. For bis (p-methoxyphenyl)carbonate k OH 12 /k OH 13 =1.0493 and k w 12 /k w 13 =1.0518. For bis(p-nitrophenyl)carbonate k OH 12 /K OH 13 =1.0476. The small change in the isotope effect with substituent group fro both alkaline and neutral hydrolysis is in contrast to the large substituent effect on the rate constants for both reactions. The nearly constant isotope effect indicates that transition-state structures are similar for all substrates

On the influence of structural instabilities on superconductivity in the framework of the electron-phonon mechanism

A model for the coupling of an anharmonic optical phonon branch, producing a structural phase transition at T = T 0, with an electron band has been treated within the Eliashberg theory. The features of the model are discussed in relation to the HTSC-system La 2− x Sr x CuO 4. The superconducting transition temperature T c has its maximum value if the lattice is most unstable ( T c = T 0). In general the calculated isotope effect differs significantly from the BCS-value due to lattice anharmonicity, even zero or negative values are possible.

Concerted synchronous two-hydron transfer within the formic acid dimer. A kinetic isotope effect study using ab initio calculations

Calculated isotope effects on the two-hydron transfer within the formic acid dimer calculated at different levels of theory including perturbational electron correlation correction (MP2) together with normal mode analysis form a basis for the interpretation of isotope effects in two-hydron transfer reactions.

Theoretical calculation of equilibrium isotope effects using ab initio force constants: application to NMR isotope perturbation studies

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTheoretical calculation of equilibrium isotope effects using ab initio force constants: application to NMR isotope perturbation studiesMartin Saunders, Keith E. Laidig, and Max WolfsbergCite this: J. Am. Chem. Soc. 1989, 111, 25, 8989–8994Publication Date (Print):December 1, 1989Publication History Published online1 May 2002Published inissue 1 December 1989https://doi.org/10.1021/ja00207a001RIGHTS & PERMISSIONSArticle Views660Altmetric-Citations124LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (735 KB) Get e-Alerts Get e-Alerts

Weak Coupling Electron-Phonon for High Tc Superconductors

Our opinion is that, in the high Tc copper oxides, the electronic correlations are not large enough to allow the localization of the electrons of the half-filled d-p sub-band. Thus, we treat them as itinerant electrons, in a bidimensional structure. And we show that, contrary to a widely held opinion, the electron-phonon interaction can induce high Tc superconductivity in these compounds, even in the weak coupling limit. This is due to the fact that, because of the bidimensionality, the electronic density of states is sharply peaked in the neighbourhood of the Fermi energy. A small coupling between nearest neighbouring CuO2 planes is sufficient to prevent a very large reduction of Tc by the critical fluctuations. The calculated isotope effect is much smaller than usually in the BCS theory. And, in our weak coupling theory, the antiferromagnetic (AF) phase is much more rapidly destabilized by dopping or internal charge transfer than the superconducting phase, which takes place when the AF phase has vanished.

Quasiclassical trajectory study of the reaction C( 3P)+HI→CH(X 2Π) + I

Three-dimensional quasiclassical trajectory calculations have been performed for the C( 3P) + HI/DI→CH (X 2Π)/CD (X 2Π) + I reactions by using LEPS potential energy surfaces. The experimental vibrational distribution of the product CH has been compared with the calculated one. It has been qualitatively suggested that the potential energy surface for the C + HI reaction is highly attractive, and that the smaller product vibrational energy disposal, 〈ƒ ν〉, relative to those of the reactions of Cl, F, and O with HI is due to secondary encounters on its surface. This conclusion has been also supported by the calculated isotope effect on the product vibrational energy disposal for the C + HI/DI reactions.

Kinetic isotope effect study of reductions of benzophenone with complex metal hydrides

Carbon-14 and deuterium kinetic isotope effects were determined for reductions of benzophenone with NaBH/sub 4/, LiBH/sub 4/, and LiAlH/sub 4/ at 25.0/sup 0/C. The observed /sup 12/k//sup 14/k at the carbonyl carbon and k/sub H4//k/sub D4/ of the reducing agents were 1.066 and 0.75, 1.043 and 1.089 and 1.024 and 1.10, respectively, for NaBH/sub 4/, LiBH/sub 4/ and LiAlH/sub 4/ reductions. Kinetic isotope effect calculations with various transition-state models suggested that the extent of hydride transfer from Al or B to the carbonyl carbon is increasing in the order LiAlH/sub 4/ (0.35) < LiBH/sub 4/ (0.55) < NaBH/sub 4/ (0.75). Nucleophilic solvent participation to the BH/sub 4/ moiety is weak if present for the SBH reduction.

Theoretical analysis of isotope effects on ozone formation in oxygen photochemistry

In situ measurements of stratospheric ozone and laboratory studies of ozone production in electric discharge through oxygen have shown previously that ozone containing heavy isotopes of oxygen (17O, 18O) may be formed preferentially. In order to assess the relevance of the latter experiment to the stratospheric measurements, detailed understanding of the effect of isotopic substitution on the O3 formation reaction O + O2 + M → O3 + M and on the O atom exchange reaction O + O2 → O2 + O is necessary. In this work, an estimate of the effect of isotopic substitution on the recombination rate is made by use of approximate dynamical theories and statistical mechanics. The results indicate the possibility of isotope effects on the O + O2 recombination rate of the order of several percent at stratospheric temperatures. In general, recombination reactions involving heavy (mass 49, 50) O3 formation are found to be slower than the reaction leading to normal (mass 48) O3 formation. The calculated isotope effects are sufficiently small that the uncertainties in the model input and the approximations in the dynamical theories will probably make the quantitative nature of these results subject to considerable uncertainty. This isotope effect should not be observable in the atmosphere given the precision of the current measurements but may be crucial in the understanding of the laboratory experiments, where observed enhancements are only of the order of several percent. Possible reasons for this discrepancy between the observed enhancement and predicted depletion are presented.

Temperature-dependent stereoselectivity and hydrogen deuterium kinetic isotope effect for .gamma.-hydrogen transfer to 2-hexyloxy radical. The transition state for the Barton reaction

The diastereomers of 5-deuterio-2-hexanol, prepared stereospecifically from acetol and lactic acid by using fermentation procedures, have been subjected to the Barton reaction with Ag/sub 2/CO/sub 3//Br/sub 2/ in pentane over the temperature range -8.65 to 30.05/sup 0/C. The derived 2,5-dimethyltetrahydrofuran deuterium incorporation yields both the C-5 diastereotopic hydrogen stereoselectivity and the isotope effect (k/sub H//k/sub D/) for the intramolecular hydrogen-transfer step. The isotope effect is classically temperature dependent, 5.82 (+- 0.07), 30.05; 6.01 (+- 0.22), 20.55; 6.81 (+- 0.17), 10.55; 6.90 (+- 0.08), 1.65; 7.47 (+- 0.49), -8.65/sup 0/C, while the stereoselectivity is small, 1.23 (1.65/sup 0/C), and temperature independent. Molecular mechanics calculations show that conformations with linear arrangements of C/sub 5/, H, and O show small steric energy differences between the diastereotropic hydrogens at C-5. Calculations of isotope effects and their temperature dependencies for model transition states match the experimental data for only C/sub 5/, H, and O angles greater than 150/sup 0/.

The energy levels of hydrogen isotopes mesic molecular complexes

A method to calculate isotope effects in diatomic molecules is developed. The energy levels of hydrogen isotopes and mesic molecular complexes which are used for calculation ofddμ anddtμ mesic molecule resonant formation rates are found. The obtained values agree with the experimental data within ∼ 5·10−4 eV accuracy.

Model calculations of isotope effects. 10. Temperature-independent isotope effects in hydrogen transfer. Do they signify a bent transition state?

Model calculations of isotope effects. 10. Temperature-independent isotope effects in hydrogen transfer. Do they signify a bent transition state?

Calculations of isotope effects in elimination reactions. New experimental criteria for tunneling in slow proton transfers

Isotope-effect calculations have been carried out on the model reaction CCH/sub 2/CH/sub 2/Cl + OH/sup -/ ..-->.. CCH=CH/sub 2/ + Cl/sup -/ + H/sub 2/O. When the bending motions of the nontransferred ..beta..-hydrogen are coupled with the stretching motion of the transferred ..beta..-hydrogen, a tunnel correction to the secondary ..beta..-hydrogen isotope effect is introduced. The kinetic secondary effect can be larger than the equilibrium secondary effect, in agreement with recent experiments. The contribution of tunneling to the secondary kinetic effect decreases with increased mass of the transferred atom. Deviations from the expected relation between deuterium and tritium isotope effects, both primary and secondary, can occur when tunneling is important, and are large enough to observe easily when k/sub H//k/sub D/ (or K/sub H//k/sub T/) calculated from k/sub D//k/sub T/ is compared with directly calculated k/sub H//k/sub D/ (or k/sub H//k/sub T/). Deuterium substitution at the ..beta..-carbon also reduces the contribution of tunneling to ..beta..-carbon isotope effects. Tunneling causes a decrease in the ratio of Arrhenius preexponential factors, A/sub light//A/sub heavy/, not only for primary hydrogen isotope effects but also for secondary hydrogen isotope effects and carbon isotope effects. The ratio increases again when the mass of the transferred atom increases.more » Deviations from the rule of the geometric mean can be expected when isotopic substitution at one position affects the contribution of tunneling to the isotope effect at another. These predictions provide new experimental tests for tunneling in slow proton transfers. 30 references, 7 tables.« less

An Examination of the Relationship Between Transition State Geometry in Hydron Transfer Reactions and the Temperature Dependence of the Primary Kinetic Isotope Effect

AbstractSeveral recent papers by Kwart feature hydron transfer reactions in which the primary kinetic isotope effect is temperature independent over wide temperature ranges. It has been asserted (without theoretical justification) that this phenomenon is associated with transition state X….H….Y bond angles which are significantly less than 180°, whereas normal temperature dependence may be associated with transition states in which the angle is close to 180°. This work employs model calculations of isotope effects, principally for [1,5] H‐shifts in 1,3‐pentadiene, to examine Kwarts's hypothesis. No model tested yielded a temperature‐independent isotope effect of substantial magnitude. The transition state (angle = 133°) was located on the MNDO potential energy surface and the isotope effect, calculated by a new and fast computational procedure, was again temperature dependent. Model calculations of secondary hydrogen isotope effects were carried out. When bending motions of the non‐reacting hydrogens were coupled to C….H stretching modes in the force constant matrix, it was found that kH/kD was greater than the equilibrium secondary isotope effect, kH/KD, and that kH/kD for hydrogen transfer exceeded kH/kD for deuteron transfer when simple tunnel corrections were incorporated. A novel interpretation to account for the observation of some temperature‐independent isotope effects is advanced.