Model Calculations Of Isotope Effects Research Articles

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?

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.

Model calculations of isotope effects. VII. Deprotonation of 2-nitropropane by oxy anion bases

Model calculations of primary and secondary deuterium isotope effects for the hydroxide-induced deprotonation of 2-nitropropane are reported. Various transition-state models have been examined in an effort to reproduce experimental results. A purely pyramidal transition state in which proton transfer has run far ahead of carbon rehybridization and charge delocalization is a successful model as far as isotope effects are concerned, but may fail on other counts. Three incipient trigonal models for the transition state have been tested, and, although none can be firmly eliminated by the resultant isotope effects, those involving the proton transfer's running ahead of electron delocalization and perhaps carbon rehybridization are favoured.

Model calculations of isotope effects. IX. Origin of large hydrogen and carbon isotope effects in the deprotonation of 2-nitropropane by pyridine bases

The abnormally large primary hydrogen and carbon kinetic isotope effects found in the deprotonation of 2-nitropropane by hindered pyridine bases are investigated by means of model calculations. Transition-state models have been varied between tight and loose extremes, and between carbanion-like and nitronate-like structures. The only models that reproduce the experimental findings are those in which the sum of the bond orders to the transferring proton is less than unity (loose transition states) and which are subject to tunnelling corrections.

Model calculations of isotope effects. VI. The structure of the transition state for hydrogen atom transfer from ethane to methyl radicals

Kinetic hydrogen isotope effects for the reaction C2H6 + CDB → C2H5 + CHD3 have been calculated for a large number of transition state models, bond orders being based on an ab initio structure for the ethyl radical. Various empirical relationships for transition state force fields in terms of partial bond orders were examined for each model structure. No transition state model reproduced the experimental intermolecular and intramolecular isotope effects over the temperature range, but when an Eckart tunnel correction was applied a single model gave satisfactory agreement.

ChemInform Abstract: MODEL CALCULATIONS OF ISOTOPE EFFECTS. PART 5. SECONDARY HYDROGEN ISOTOPE EFFECTS IN THE SOLVOLYSIS OF ISOPROPYL HALIDES

Chemischer InformationsdienstVolume 12, Issue 51 Preparative Organic Chemistry ChemInform Abstract: MODEL CALCULATIONS OF ISOTOPE EFFECTS. PART 5. SECONDARY HYDROGEN ISOTOPE EFFECTS IN THE SOLVOLYSIS OF ISOPROPYL HALIDES D. J. MCLENNAN, D. J. MCLENNANSearch for more papers by this author D. J. MCLENNAN, D. J. MCLENNANSearch for more papers by this author First published: December 22, 1981 https://doi.org/10.1002/chin.198151104AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume12, Issue51December 22, 1981 RelatedInformation

Model calculations of isotope effects. Part 5. Secondary hydrogen isotope effects in the solvolysis of isopropyl halides

Model reaction systems in terms of reactant and transition state geometries and force fields have been developed to simulate SN1 and SN2 solvolyses of isopropyl halides. Equilibrium and kinetic isotope effects for α-deuterium (α-kH/kD) and β-deuterium (β-kH/kD6) substitution have been calculated, with transition state models being assigned structures using the ab initio geometry of the isopropyl cation as a basis. Values of α-kH/kD were found to be dependent on the identity of the leaving group and were, for ion-pair-like transition states, considerably larger than the values suggested by Shiner as ‘maximum’α-deuterium isotope effects in limiting solvolyses. The mechanistic consequences are examined. Solvent assistance to ionization appears to be a necessary component of simple secondary substrate solvolysis in that it is necessary to include a nucleophilically solvating water molecule in order to match the calculated and experiment values for the hydrolysis of isopropyl bromide.

Model Calculations of Isotope Effects. IV. Transition State Structures and the Question of Tunnelling in Proton Transfer Reactions

Model calculations of primary hydrogen isotope effects in proton transfer reactions are reported. The geometries and force fields of transition state models have been systematically varied with respect to both reactant-like and product-like character and to tight against loose character. The models include both hypothetical cut-off molecules and 2-nitropropane. Values of kH/kD greater than 17 are calculated for loose, symmetrical transition states in which the sum of the bond orders pertaining to the transferring proton is set at 0.6, and higher than normal values of (ED-EH) and ADIAH are also associated with such transition states. It is suggested that transition state looseness is a consequence of repulsive donor-acceptor steric interactions, and that several sets of experimental results which have hitherto been rationalized by the invocation of proton tunnelling may equally well be explained by postulating loose transition states.

Model Calculations of Isotope Effects. III. Isotope Effects in Hydrogen Transfer Reactions, and Transition State Force Fields

Model calculations of kinetic isotope effects for the reactions: C2H6+·CH3 → ·C2H5+CH4 CH4 + ·CF3 → ·CH3+CHF3 are reported. Transition state geometries were those calculated by Dewar and coworkers using the MNDO semiempirical method. Transition state force fields were formulated from empirical expressions for stretching, bending, linear bending and interaction valence force constants by using bond orders as disposable parameters. Although it proved impossible to assign a best force field to either reaction, the calculated isotope effects generally were in satisfactory agreement with experiment, and were better than those calculated from the MNDO potential energy surface. Hydrogen tunnelling is apparently implicated in the CH4+ CF3 reaction.

Model calculations of isotope effects in proton transfer reactions

An electrostatic charge-cloud model, including an empirical repulsive potential is used to calculate the properties of initial, final and transition states for the reaction X–H++ Yδ–→X–+ Yδ–H+, where X– and Yδ– are spherical charge distributions. The real and imaginary frequencies thus derived are used to calculate hydrogen isotope effects as a function of δ and hence of the energy change in the reaction. It is concluded that the observed variation of isotope effects cannot be accounted for in terms of the real vibrations of the transition state, but are primarily determined by the tunnel correction. The model also accounts for the dependence of isotope effects upon steric hindrance.

Kinetic deuterium isotope effect in the reaction of methyl iodide with thiosulfate ion in aqueous solution

The kinetic deuterium isotope effect in the reaction of methyl iodide with thiosulfate ion in aqueous solution has been determined as follows: kH/kD (per 3D) = 0.966 (+0.02°), 0.968 (+10.00°), 0.970 (+19.98°). Kinetic experiments have been carried out in reaction vessels with no gas phase. The experimental data are compared with results of model calculations of isotope effects from force constants. Excellent agreement between experimental and calculated isotope effects may be obtained with the following values of the transition state bending force constants: fHCI = 0.295 mdyn Å, fHCS = 0.300 to 0.320 mdyn Å. These values are equal to 50–65% of the corresponding bending force constants in stable molecules.