Fragmentation reactions of the enolate ions of 2-pentanone

The reaction of [OH]^? with 2-pentanone produces two enolate ions, [CH₃CH₂CH₂COCH₂]^? and [CH₃COCHCH₂CH₃]^?, by proton abstraction from C(1) and C(3), respectively. Using deuterium isotopic labelling the fragmentation reactions of each enolate have been delineated for collisional activation at both high (8 keV) and low (5–100 eV) collisional energies. The primary enolate ion fragments mainly by elimination of ethene. Two mechanisms operate: elimination of C(4) and C(5) with hydrogen migration from C(5), and elimination of C(3) and C(4) with migration of the C(5) methyl group. Minor fragmentation of the primary enolate also occurs by elimination of propane and elimination of C₂H₅; the latter reaction involves specifically the terminal ethyl group. The secondary enolate ion fragments mainly by loss of H₂ and by elimination of CH₄; for the latter reaction four different pathways are operative. Minor elimination of ethene also is observed involving migration of a C(5) hydrogen to C(3) and elimination of C(4) and C(5) as ethene.     

Unimolecular fragmentation reactions of protonated and methyl-cationated n-octanones and N-nonanones. Sequential fragmentation reactions

Protonated and methyl-cationated n-octanonts and n-nonanones have been prepared by chemical ionization methods and the unimolecular fragmentation reactions occurring on the metastable ion time-scale have been exam ined. The [R₁R₂COR₃]⁺ (R₃ = H or CH₃) species fragment by elimination of R₃OH and by elimination of neutral alkenes. The elimination of (R₁ — H), where R₁ is the larger alkyl group of the original ketone, is particularly important. In addition, alkenyl ions are observed corresponding, nominally, to elimination of C₃H₇OR₃ from the ionized octanones and to elimination of C₃H₇OR₃ and C₄H₉OR₃ from the nonanones. These ions are shown to arise largely, if not completely, by sequential elimination of R₃OH plus an olefin (C₃H₆ or C₄H₈) from [R₁R₂COR₃]⁺. A comparison is made of the unimolecular fragmentation reactions occurring in the second field-free region and in the radio-frequency-only quadrupole of a hybrid BEQQ mass spectrometer.     

Isomerisation of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules

The recombination of CF₂Cl with CH₂Cl and CFCl₂ with CH₂F were employed to generate CF₂ClCH₂Cl* and CFCl₂CH₂F* molecules with 381 and 368 kJ mol^?1, respectively, of vibrational energy in a room‐temperature bath gas. The unimolecular reactions of these molecules, which include HCl elimination, HF elimination, and isomerisation by interchange of chlorine and fluorine atoms, were characterized. The three rate constants for CFCl₂CH₂F were 2.9×10⁷, 0.87×10⁷ and 0.04×10⁷ s^?1 for HCl elimination, isomerisation and HF elimination, respectively. The isomerisation reaction must be included to have a complete characterization of the unimolecular kinetics of CFCl₂CH₂F. The rate constants for HCl elimination and HF elimination from CF₂ClCH₂Cl were 14×10⁷and 0.37×10⁷ s^?1, respectively. Isomerisation that has a rate constant less than 0.08×10⁷ s^?1 is not important. These experimental rate constants were matched to calculated statistical rate constants to assign threshold energies, which are 264, 268, and 297 kJ mol^?1, respectively, for isomerisation, HCl elimination, and HF elimination for CFCl₂CH₂F and 314, 251, and 289 kJ mol^?1 in the same order for CF₂ClCH₂Cl. Density functional theory was used to evaluate the models that were needed for the statistical rate constants; the computational method was B3PW91/6‐31G(d′,p′). Threshold energies for the unimolecular reactions of CF₂ClCH₂Cl and CFCl₂CH₂F are compared to those for CF₂ClCH₃ and CFCl₂CH₃ to illustrate the elevation of threshold energies by F‐ or Cl‐atom substitution at the beta carbon atom (identified by C_H). The DFT calculations systematically underestimate the threshold energy for HCl elimination.     

The insertion and H<Subscript>2</Subscript> elimination reactions of H<Subscript>2</Subscript>GeFMgF germylenoid with RH (R = Cl,SH, PH<Subscript>2</Subscript>)

In present paper, the insertion and H₂ elimination reactions of H₂GeFMgF germylenoid with RH (R = Cl, SH, PH₂) were investigated by means of B3LYP and QCISD calculation methods. One transition state and one intermediate were found along the potential energy surface in each insertion reaction, while for the H₂ elimination reactions, only one transition state was found between the reactants and products in each reaction process. Both for the insertion and H₂ elimination reactions, RH reactivity increases in the following order: H–Cl > H–SH > H–PH₂. The insertion and H₂ elimination reactions were compared, and the results demonstrated that the H₂ elimination should be more favorable than the corresponding insertion. The solvent effects on these two types of reactions were considered. The calculated results indicated that the solvents could accelerate the reactions by reducing their barrier heights.     

β'β Anionic elimination of carboxylic esters

The elimination of lithium, magnesium and aluminium enolatea of isobutyrates of medium ring cyclanols occurs in a syn fashion. A set of experimental procedures is presented. This elimination seems to be restricted to strained systems. The stereo-chemistry has been determined on stereospecifically deuterated cyclooctanol isobutyrates. The primary isotope effect k_H/k₂ was 3.0 ± 0.1 and the secondary 1.1. The name β'β elimination is proposed for this syn elimination and related elimination.     

Hydrogen migrations in mass spectrometry. VI—the chemical ionization mass spectra of substituted benzoic acids and benzyl alcohols

The H₂ and CH₄ chemical ionization mass spectra of a series of series of substituted benzoic acids and substituted benzyl alcohols have been determined. For the benzoic acids the major fragmentation reactions of the protonated molecule involve elimination of H₂O or elimination of CO₂, the latter reaction involving migration of the carboxylic hydrogen to the aromatic ring. For the benzyl alcohols the major fragmentation reactions of [MH]⁺ involve loss of H₂O or CH₂O, analogous to the CO₂ elimination reaction for the benzoic acids. It is shown that the CO₂ and CH₂O elimination reactions occur only when a conjugated aromatic ring system is present, and that for the carboxylic acid systems, methyl groups and, to a lesser extent, phenyl groups are capable of migrating. The only discernible effect of substituents on the fragmentation of [MH]⁺ is an enhancement of the H₂O loss reaction in the benzoic acid system when an amino, hydroxyl, or halogen substituent is ortho to the carboxyl function. This ‘ortho’ effect, which differs in scope from that observed in electron impact mass spectra, is attributed to an intramolecular catalysis by the ortho substituent of the 1,3 hydrogen migration in the carbonyl protonated acid followed by H₂O elimination. Apparently, this route is favoured over the direct elimination of H₂O from the carbonyl protonated acid, since the latter has a high activation energy barrier because of unfavourable orbital symmetry restrictions.     

Theoretical investigation on H2 elimination reactions of germylenoid H2GeLiF with RH (R = F, OH, and NH2)

The H₂ elimination reactions of the germylenoid H₂GeLiF with RH (R = F, OH, NH₂) have been studied by using the DFT B3LYP and QCISD methods. The calculated results indicate that all the mechanisms of the three reactions are identical to each other and under the same condition the H₂ elimination reactions should occur easily in the order of H-F > H-OH > H-NH₂. In THF solvent the H₂ elimination reactions get more difficult than in gas phase. Compared with the insertion reactions of H₂GeLiF with RH (R = F, OH, NH₂), the H₂ elimination reactions have the lower activation barriers and should be more favorable.     

Pathways for Water Loss from Doubly Protonated Peptides Containing Serine or Threonine

The doubly-protonated peptides Ala-Ala-Xaa-Ala-Ala-Ala-Arg show extensive loss of H₂O when Xaa = Ser or Thr. Using quasi-MS³ techniques the fragmentation reactions of the [M + 2H – H₂O]⁺² ions have been studied in detail. For both Ser and Thr, the [M + 2H – H₂O]⁺² ions show three primary fragmentation reactions, elimination of CH₃CH = NH, elimination of one Ala residue, and elimination of two Ala residues, in all cases forming doubly-charged products. From a study of the further fragmentation of these products, it is concluded that elimination of two Ala residues results in formation of a three-membered aziridine ring by interaction with the adjacent amide function as H₂O is lost. The elimination of one Ala residue results in formation of a five-membered oxazoline ring through interaction with the N-terminal adjacent carbonyl function as H₂O is lost. The elimination of CH₃CH = NH appears to involve formation of an eight-membered ring by interaction with the remote N-terminal carbonyl function as H₂O is lost. However, this initial structure undergoes rearrangement through interaction with the adjacent C-terminal carbonyl function prior to further fragmentation. The [MH – H₂O]⁺ ion of Ala-Ala-Ser-Ala-Ala-Ala also shows elimination of CH₃CH = NH, one Ala residue and two Ala residues.     

On the elimination of methanol from stereoisomeric arylcyclohexyl methyl ethers under electron impact

The elimination of methanol from 3- and 4-arylcyclohexyl methyl ethers under electron impact exhibits stereoselectivity, which is similar to that found for the elimination of H₂O from the corresponding arylcyclohexanols. The two eliminations also exhibit a similar substituent effect correlation. The similarity holds in the site specificity of these processes only for the trans-isomers. The cis-ethers undergo elimination of methanol by a mechanism which is different from that for the elimination of H₂O from the cis-alcohols.     

A new explanation for the carbon deposition and elimination over supported Ni, Ni-Ce and Ni-Co catalysts for CO2-reforming of methane

The influence of the addition of CeO₂ or Co₃O₄ to Ni-based catalysts on carbon deposition by CH₄ and carbon elimination by CO₂ was studied with a pulse reaction as well as BET, TGA, XPS and hydrogen chemisorption techniques. It was found that addition of the CeO₂ decreased the carbon deposition activity of CH₄ and increased the carbon elimination ability of CO₂, which could be explained by the interaction between the metal (Ni) and n-type semiconductor (CeO₂). However, the addition of the Co₃O₄, which was transformed to Co⁰ during the reaction, accelerated carbon deposition by CH₄ and resisted carbon elimination by CO₂. A novel model of carbon deposition/carbon elimination over Ni-based catalysts has been proposed.     

Understanding E2 versus SN2 Competition under Acidic and Basic Conditions

Our purpose is to understand the mechanism through which pH affects the competition between base-induced elimination and substitution. To this end, we have quantum chemically investigated the competition between elimination and substitution pathways in H₂O+C₂H₅OH₂⁺ and OH⁻+C₂H₅OH, that is, two related model systems that represent, in a generic manner, the same reaction under acidic and basic conditions, respectively. We find that substitution is favored in the acidic case while elimination prevails under basic conditions. Activation-strain analyses of the reaction profiles reveal that the switch in preferred reactivity from substitution to elimination, if one goes from acidic to basic catalysis, is related to (1) the higher basicity of the deprotonated base, and (2) the change in character of the substrates LUMO from C^β−H bonding in C₂H₅OH₂⁺ to C^β−H antibonding in C₂H₅OH.     

Electron impact studies. CXIV—The occurance of metastable ions for two- and multi- stage decomposition of negative ions derived from p-nitrophenyl carboxylic acids

Pronounced metastable peaks are observed for the two-stage decomposition and the multi - stage elimination from p-NO₂C₆H₄(CH₂)_nCO₂H (n>1). ²H and ¹⁸O labelling shows that each process involves elimination of atoms from both the nitro and carboxyl groups. A consideration of the spectrum of p-nitrophenylcyclohexane-4-carboxylic acid leads to the proposition that interaction between the two functional groups is not a necessary prerequisite for the elimination of HO?.     

Understanding the Mechanisms of Unusually Fast HH,CH,and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon–carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co‐workers have demonstrated extremely rapid C?C reductive elimination from cis‐[AuPPh₃(4‐F‐C₆H₄)₂Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy‐atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis‐[Au(PPh₃)₂(4‐F‐C₆H₄)₂]⁺ was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for H?H, C?H, and C?C bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp³ carbon atoms. Metal–carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis‐[AuPPh₃(H)CH₃]⁺ predict that at ?52 °C, about 82 % of the reaction occurs by hydrogen‐atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.     

Elimination reactions in dimethyl sulfoxide solution. The effective base with potassium tert-butoxide.

Deuterium labeling studies have established that elimination of a cyclooctylammonium iodide with dimsyl anion in DMSO proceeds through an ylide intermediate while elimination with KOC(CH₃)₃ in DMSO involves an E2 mechanism. The effective base with KOC(CH₃)₃/DMSO is shown to be the alkoxide ion.     

In-depth insight into the electronic and steric effects of phosphine ligands on the mechanism of the R-R reductive elimination from (PR3)2PdR2

The aim of this study was to investigate both the electronic and steric effects of the ancillary phosphine ligand L on the reductive elimination of Me-Me from a series of L₂PdMe₂ and LPdMe₂ complexes. Density functional theory was used to study these processes with the model ligands L = PMe₃, PH₃, PCl₃ and with the experimentally reported ligands L = PPh₃, PPh₂Me, PPhMe₂. For the model ligands we confirm that electron donation from L affects the barrier for reductive elimination from L₂PdMe₂ but not from LPdMe₂. In the former case the greater the electron donation or basicity of L, the greater the barrier and the later the transition state. This is because electron donation increases the σ^∗ antibonding between Pd and L in the transition structure. On the other hand, if L is a good π acceptor this stabilizes the occupied d_π orbital of Pd in the transition structure and lowers the barrier to reductive elimination. In the case of the reactions involving LPdMe₂ as the intermediate, it is the loss of the first L (L₂PdMe₂ → LPdMe₂ + L) which determines the differences in the barrier height. Greater electron donation leads to greater L-to-Pd σ donation and a stronger Pd-L bond, and thus a greater overall barrier. A comparison of these results with the reductive elimination of 1,3-butadiene from divinyl palladium complexes L₂PdR₂ shows that the barriers are lower in the vinyl case because of a mix of orbital factors. Our results show that there is a significant stabilizing interaction between the Pd d_π orbital and the vinyl-vinyl hybrid σ^∗/π^∗ orbitals in the reductive elimination transition structure. At the same time this Pd-R₂ orbital stabilization alleviates the potential antibonding interactions between Pd and L and makes the vinyl elimination much less susceptible to ancillary ligand effects. Energy-decomposition analyses have been used to elucidate the contributing factors to the activation energies for the reductive eliminations with the model phosphine ligands. These analyses have also been used to disentangle the electronic and steric effects involved in the larger ligand systems. The electronic effects of the experimentally reported ligands are found to be very similar to each other. On the other hand, steric effects lead to a destabilization of the reactant L₂PdMe₂ complexes but not the transition structures, which results in a decrease in the barriers to reductive elimination compared to the smaller phosphine ligands. These steric effects do not play a role in reductive elimination from LPdMe₂. These detailed analyses of the electronic and steric factors may be used to assist the design of systems which enhance or retard reductive elimination behaviour.     

The Synthesis of II-VI Binary Alloys and Organometallic Compounds via Me3SiOR Elimination Reactions

Trimethylsilyl chloride elimination reactions have been well documented. We now report the use of trimethylsilyl acetate elimination reaction in the preparation of binary alloys, HgTe and CdTe. The new compound, (MeHg)₃P, and the known compound (MeHg)₂S have been prepared by the trimethylsilyl acetate elimination reaction and characterized by NMR spectroscopy, electron ionization mass spectroscopy, and elemental analysis.     

Fragmentations of Hydroxymethyl Radical Cation: An Ab Initio Study

The probable fragmentation channels of hydroxymethyl radical cation were studied through the H‐and H₂‐abstraction and C‐O bond breaking reactions including their related isomerization reactions. The energy barriers for hydroxymethyl cation undergoing isomerization reactions are generally higher than those undergoing the concerted 1,2‐elimination reactions to generate CHO⁺ and H₂. The fragmentation reaction to form CHO⁺ and H₂ through the 1,2‐elimination pathways is the major fragmentation channel for hydroxymethyl cation, consistent with the experimental observation. H abstraction from the hydroxyl group of CH₂OH⁺ is more difficult than that from the methylene group. The feasible path to lose H is to generate CHOH₂⁺ through hydrogen transfer reaction as the first step and then to undergo H‐elimination to generate trans‐CHOH⁺. Among all the reactions found in this study, the OH‐elimination to generate CH₂⁺ has the highest energy barrier. Our calculation results indicate that the major signals contributed from the related species of hydroxymethyl cation found in the mass spectrum should be m/e 29, m/e 30.     

Energy disposal in the three-centered elimination of df from 1,1,2-trifluoroethane-1-d1

The chemical activation data for three- and four-centered hydrogen fluoride elimination from CH₂FCDF₂ have been analyzed to assign the energy released to the olefin fragment in the three-centered process and to estimate the threshold energies for elimination channels. Based upon the cis–trans isomerization rates of CHF = CHF, 78% of the total available energy was released to the olefin fragment for the αα channel. The analysis suggests the existence of an appreciable barrier (～10 kcal/mole) for the reverse reaction, addition of the CH₂FCF carbene to DF. The threshold energies for αα, αβ, and βα elimination from 1,1,2-trifluoroethane-1-d₁ were assigned as 71, 68, and 68 kcal/mole, respectively. Analysis of the chemical activation data for 1,1,2,2,-tetrafluoroethane, without distinguishing between the three- and four-centered elimination channels, suggests a threshold energy of ～75 kcal/mole.     

Competitive Reaction Pathways for the Gas‐Phase Reactivity of [Me2AlNH2]3

Reaction energy profiles for [Me₂AlNH₂]₃ have been computationally explored by using density functional theory. Both intra‐ and intermolecular methane elimination reactions, as well as Al?N bond‐breaking pathways, were considered. The results show that the energy required for Al?N bond breaking in cyclic [Me₂AlNH₂]₃ is of the same order of magnitude as the activation energies for the first (limiting) step of methane elimination (for both mono‐ and bimolecular mechanisms). Thus, dissociative and associative reaction pathways are competitive. Low‐temperature/high‐pressure conditions will favor the bimolecular pathway, whereas at high temperatures, either intramolecular methane elimination or Al?N bond‐breaking dissociative pathways will be operational.     

Interaction of dioxygen with the platinum Pt19/SnO2/H2 cluster: DFT calculation

The adsorption of the O₂ molecule onto the surface of the Pt₁₉ platinum cluster deposited onto the tin dioxide crystal surface in the presence of dissociated hydrogen molecule has been calculated by the density functional theory method within the generalized gradient approximation (GGA-PBE) with periodic boundary conditions and a projector-augmented plane-wave (PAW) basis set. It has been demonstrated that the oxygen molecule can be adsorbed without a barrier onto the free surface of the Pt₁₉/SnO₂/H₂ cluster to form a superoxy isomer with one Pt-O bond (the energy of elimination of the oxygen molecule is 0.75 eV), which converts almost without a barrier to more stable peroxide isomers with two Pt-O bonds (the energy of elimination of the O₂ molecule is 1.2?1.7 eV). The energy of elimination of the oxygen molecule from the isomers with two-coordinated oxygen positions at the cluster edges is 2.10?2.53 eV. The isomers with mono- and tricoordinated oxygen positions are less energetically favorable than the isomers with two-coordinated oxygen positions. The process of addition of the oxygen molecule to the platinum cluster and elimination of the water molecule formed in the reaction Pt₁₉/SnO₂/H₂ + O₂ → Pt₁₉/SnO₂/O + H₂O is energetically favorable by 1.6 eV.