Ion cyclotron resonance and deuterium kinetic isotope studies of the gas-phase reactions of the alkoxide negative ions with anhydrides and esters

1981 ◽  
Vol 34 (3) ◽  
pp. 507 ◽  
Author(s):  
G Klass ◽  
DJ Underwood ◽  
JH Bowie
Keyword(s):  
Transition State ◽  
Carbonyl Group ◽  
Isotope Effects ◽  
Negative Ions ◽  
Alkyl Ester ◽  
Negative Ion ◽  
Nucleophilic Attack ◽  
Ion Cyclotron Resonance ◽  
Gas Phase Reactions ◽  
Rearrangement Reaction

(i) The i.c.r. spectrum of the CD3O-/acetic anhydride system shows the occurrence of the negative ion McLafferty rearrangement [reaction (1)] and the characteristic elimination shown in reaction (2) (R = COMe) CD3O-+ (MeCO)2O → (CD3O)(Me)(HO)C-O-+ CH2CO (1) CD30-+ MeCO2R → -CH2C02CD3+ ROH (2) (ii) The i.c.r. spectra of CD3O-/alkyl ester systems show major peaks due to [ester-H+]- ions, together with small peaks corresponding to [ester+CD3O-]- adducts. Carboxylate and alkoxide anions (derived from the ester) are also observed in certain spectra; peaks due to these ions increase in intensity with elaboration of alkyl substituents. Reaction (2) is characteristic of all esters studied (illustrated above for acetates) which have at least one hydrogen substituent on the carbon atom α to the carbonyl group. The reaction must occur by nucleophilic attack of the alkoxide anion at the carbonyl group of the ester. Negative ion McLafferty rearrangements do not occur in alkyl ester systems. (iii) The H transfer reactions (1) and (2) show deuterium isotope effects kH/kD of 3.0 and 1.5 respectively (when the carbon adjacent to the carbonyl group contains one deuterium substituent). Isotope effect calculations suggest that reaction (1) proceeds through a 'near symmetrical' six membered transition state, whereas (2) goes through a 'reactant-like' but distorted four-centred transition state.

scholarly journals Fullerene Negative Ions: Formation and Catalysis

2020 ◽  
Author(s):  
Zineb Felfli ◽  
Kelvin Suggs ◽  
Nantambu Nicholas ◽  
Alfred Z. Msezane
Keyword(s):  
Transition State ◽  
Cross Sections ◽  
Water Oxidation ◽  
Regge Pole ◽  
Negative Ions ◽  
Negative Ion ◽  
Total Cross Sections ◽  
Hydrogen Bond Strength ◽  
Low Energy Electron ◽  
Fundamental Mechanism

We first explore negative-ion formation in fullerenes C44, C60, C70, C98, C112, C120, C132 and C136 through low-energy electron elastic scattering total cross sections calculations using our Regge-pole methodology. Water oxidation to peroxide and water synthesis from H2 and O2 are then investigated using the anionic catalysts C44ˉ to C136ˉ. The fundamental mechanism underlying negative-ion catalysis involves hydrogen bond strength-weakening in the transition state. DFT transition state calculations found C60ˉ numerically stable for both water and peroxide synthesis, C100ˉ increases the energy barrier the most and C136ˉ the most effective catalyst in both water synthesis and oxidation to H2O2.

Ion Cyclotron Resonance Studies of Alkylsilyl Ions. II. The Reactions of Ketones, Carboxylic Acids and Esters with the Trimethylsilyl Cation

1979 ◽  
Vol 32 (6) ◽  
pp. 1389 ◽  
Author(s):  
IA Blair ◽  
JH Bowie
Keyword(s):  
Carboxylic Acid ◽  
Transition State ◽  
Lewis Acid ◽  
Cyclotron Resonance ◽  
Nucleophilic Attack ◽  
Ion Cyclotron Resonance ◽  
Deuterium Isotope Effect ◽  
Trimethylsilyl Cation ◽  
A Minor ◽  
Membered Transition State

Nucleophilic attack of a ketone, carboxylic acid or ester at the electrophilic centre of the trimethylsilyl cation (Me,Si+) produces a 1:1 adduct. This adduct does not decompose in the case of ketones. Acid adducts decompose primarily by loss of methane, but a minor pathway exists which involves elimination of a ketene. Ester adducts fragment primarily by this latter process through a four-membered transition state. The transfer of hydrogen was shown to arise solely from the acyl group and was shown to proceed with a small deuterium isotope effect. Further decomposition of the resulting ion by loss of methane provides unequivocal proof that esters react predominantly through the alkyl oxygen with the Lewis acid Me3Si+.

scholarly journals Human DNMT1 transition state structure

2016 ◽  
Vol 113 (11) ◽  
pp. 2916-2921 ◽  
Author(s):  
Quan Du ◽  
Zhen Wang ◽  
Vern L. Schramm
Keyword(s):  
Transition State ◽  
Dna Methyltransferase ◽  
Catalytic Mechanism ◽  
Isotope Effects ◽  
Nucleophilic Attack ◽  
Kinetic Isotope ◽  
Methyl Transfer ◽  
Major Transition ◽  
Methylation Patterns ◽  
Rate Limiting

Human DNA methyltransferase 1 (DNMT1) maintains the epigenetic state of DNA by replicating CpG methylation signatures from parent to daughter strands, producing heritable methylation patterns through cell divisions. The proposed catalytic mechanism of DNMT1 involves nucleophilic attack of Cys1226 to cytosine (Cyt) C6, methyl transfer from S-adenosyl-l-methionine (SAM) to Cyt C5, and proton abstraction from C5 to form methylated CpG in DNA. Here, we report the subangstrom geometric and electrostatic structure of the major transition state (TS) of the reaction catalyzed by human DNMT1. Experimental kinetic isotope effects were used to guide quantum mechanical calculations to solve the TS structure. Methyl transfer occurs after Cys1226 attack to Cyt C6, and the methyl transfer step is chemically rate-limiting for DNMT1. Electrostatic potential maps were compared for the TS and ground states, providing the electronic basis for interactions between the protein and reactants at the TS. Understanding the TS of DNMT1 demonstrates the possibility of using similar analysis to gain subangstrom geometric insight into the complex reactions of epigenetic modifications.

scholarly journals Fullerene Negative Ions: Formation and Catalysis

2020 ◽  
Vol 21 (9) ◽  
pp. 3159
Author(s):  
Zineb Felfli ◽  
Kelvin Suggs ◽  
Nantambu Nicholas ◽  
Alfred Z. Msezane
Keyword(s):  
Transition State ◽  
Cross Sections ◽  
Water Oxidation ◽  
Density Functional ◽  
Regge Pole ◽  
Negative Ions ◽  
State Density ◽  
Negative Ion ◽  
Functional Theory ◽  
Total Cross Sections

We first explore negative-ion formation in fullerenes C44 to C136 through low-energy electron elastic scattering total cross sections calculations using our Regge-pole methodology. Then, the formed negative ions C44ˉ to C136ˉ are used to investigate the catalysis of water oxidation to peroxide and water synthesis from H2 and O2. The exploited fundamental mechanism underlying negative-ion catalysis involves hydrogen bond strength-weakening/breaking in the transition state. Density Functional Theory transition state calculations found C60ˉ optimal for both water and peroxide synthesis, C100ˉ increases the energy barrier the most, and C136ˉ the most effective catalyst in both water synthesis and oxidation to H2O2.

Elimination of Molecular Hydrogen and Methane from Collision-Activated Alkoxide Negative Ions in the gas Phase. An ab initio and Isotope Effect Study

1985 ◽  
Vol 38 (8) ◽  
pp. 1197 ◽  
Author(s):  
RN Hayes ◽  
JC Sheldon ◽  
JH Bowie ◽  
DE Lewis
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Gas Phase ◽  
Molecular Hydrogen ◽  
Isotope Effects ◽  
Negative Ions ◽  
Kinetic Isotope Effects ◽  
Effect Study ◽  
Negative Ion ◽  
Kinetic Isotope

Ab initio calculations indicate that the collisional induced losses of molecular hydrogen from the ethoxide negative ion and methane from the t- butoxide negative ion to be stepwise processes in which the key intermediates are [H-… MeCHO ] and [Me-…Me2CO] respectively. Deuterium kinetic isotope effects observed for these and other alkoxide negative ions are in accord with the operation of a stepwise reaction.

Ion-Cyclotron Resonance Studies of Alkylsilyl Ions. I. The Reactions Between Alcohols and the Trimethylsilyl Cation

1979 ◽  
Vol 32 (1) ◽  
pp. 59 ◽  
Author(s):  
IA Blair ◽  
G Phillipou ◽  
JH Bowie
Keyword(s):  
Proton Transfer ◽  
Transition State ◽  
Cyclotron Resonance ◽  
Nucleophilic Attack ◽  
Ion Cyclotron Resonance ◽  
Trimethylsilyl Cation ◽  
Ion Cyclotron ◽  
Decomposition Pathway ◽  
Membered Transition State

Nucleophilic attack of an alcohol (ROH) at the electrophilic silicon centre of the trimethylsilyl cation (Me3Si+) produces the 1 : 1 adduct Me3-O+(H)R(1). The adduct may fragment by loss of methane to yield Me2Si+-O-R; and this elimination is most pronounced when R = Me. When R ≥ C2H5, the major decomposition pathway of (1) involves elimination of the alkene [R-H] to produce Me3Si-O+H2, which may undergo further reaction with the neutral alcohol to reform (1). The proton transfer which accompanies the elimination of [R- H] from (1) originates predominantly from C2 of the alcohol; this suggests the intermediacy of a four-membered transition state in this reaction.

Electron impact studies. XCIII. The reactions of negative ions from 3- and 4-nitrophthalic anhydrides

1975 ◽  
Vol 28 (3) ◽  
pp. 563 ◽  
Author(s):  
JH Bowie ◽  
BD Williams
Keyword(s):  
Electron Impact ◽  
Phthalic Anhydride ◽  
Cyclotron Resonance ◽  
Mass Spectra ◽  
Negative Ions ◽  
Negative Ion ◽  
Ion Cyclotron Resonance ◽  
Impact Studies ◽  
Ion Molecule Reactions ◽  
Positive Ions

Possible structures for some rearrangement ions in the negative ion mass spectra of 3- and 4-nitro- phthalic anhydride are suggested from an examination of the + E spectra obtained by the conversion of the negative ions to positive ions by charge-stripping reactions. The reactivities of the non-decomposing negative ions have been investigated by observation of their ion-molecule reactions with the neutral anhydride in the cell of an ion cyclotron resonance spectrometer.

Computational simulation of mechanism and isotope effects on acetal heterolysis as a model for glycoside hydrolysis

2020 ◽  
Vol 92 (1) ◽  
pp. 75-84 ◽  
Author(s):  
John H. Glancy ◽  
Daniel M. Lee ◽  
Emily O. Read ◽  
Ian H. Williams
Keyword(s):  
Free Energy ◽  
Transition State ◽  
Isotope Effects ◽  
Computational Simulation ◽  
Nucleophilic Attack ◽  
Free Energy Surface ◽  
Mean Values ◽  
Deuterium Substitution ◽  
The Mean ◽  
Glycoside Hydrolysis

AbstractDFT calculations for the equilibrium isotope effect for deuterium substitution at the anomeric centre Cα in 2-(p-nitrophenoxy)tetrahydropyran with continuum solvation show significant variation in the range of relative permittivity 2 ≤ ε ≤ 10. One-dimensional scans of potential energy (with implicit solvation by water) or of free energy (from QM/MM potentials of mean force with explicit aqueous solvation with a hybrid AM1/OPLS method) for heterolysis of the bond between Cα and the nucleofuge do not show a transition state. A two-dimensional free-energy surface that considers also the distance between Cα and a nucleophilic water indicates a pre-association DN*ANint‡ mechanism with a transition state involving nucleophilic attack upon an ion-pair intermediate, and this is supported by good agreement between the mean values of the calculated and experimental α-D KIEs. However, the magnitudes of the standard deviations about the mean values for the making and breaking C–O bonds suggest that the transition state is rather plastic, with Cα–Onu≈2 ± 0.4 Å and Cα–Olg≈3 ± 0.5 Å. Not only is nucleophilic solvent assistance necessary, but there is also evidence for electrophilic assistance through specific hydrogen bonding to the nucleofuge.

Article

1999 ◽  
Vol 77 (4) ◽  
pp. 459-462
Author(s):  
J Andraos ◽  
Y Chiang ◽  
S J Eustace ◽  
A J Kresge ◽  
S W Paine ◽  
...  
Keyword(s):  
Aqueous Solution ◽  
Carbonyl Group ◽  
Flash Photolysis ◽  
Isotope Effects ◽  
Nucleophilic Attack ◽  
Hydroxide Ion ◽  
Solvent Isotope ◽  
Solvent Isotope Effects

Five ketenes, phenyl(ethyl)ketene, phenyl(methylthio)ketene, diphenylketene, pentafluorophenylketene, and 1-naphthylketene, were generated flash photolytically and solvent isotope effects (H2O vs. D2O) on their hydroxide-ion-catalyzed hydration in aqueous solution were determined. The values obtained are all weakly inverse and closely similar (kHO/kDO = 0.76-0.97), as expected for these fast, hydroxide-ion-consuming reactions, known to proceed by nucleophilic attack of hydroxide on the ketene carbonyl group. The characteristic magnitude of these isotope effects should prove useful in identifying new examples of this reaction.Key words: ketenes, flash photolysis, photo-Wolff reaction, solvent isotope effects on hydroxide ion consumption.

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