Hydrogen Scrambling Research Articles

Generation of a cyclopentenyl cation labelled with a single deuterium at C‐4. Rate studies of the scrambling reaction

AbstractThe kinetics of the hydrogen scrambling reaction in the cyclopentenyl cation 1 were studied. The pure 4‐deutero derivative of 1 was prepared using superacid conditions and the activation energy ΔG‡ for the rearrangement of this label to the C‐1(3) position of 1 was determined using low‐temperature 2H NMR spectroscopy. In addition, the sequential rearrangement of the deuterium label from C‐4(5) → C‐1(3) → C‐2 was noted in a qualitative fashion. Based on a statistical analysis, the activation free energy ΔG‡ for a single hydrogen (deuterium) migration leading to the 4‐cyclopentenyl cation intermediate was determined to be 18.3 kcal mol−1 (−20.8 °C), in good agreement with the less direct determination provided by an earlier NMR line‐broadening and spin‐saturation transfer experiment. Copyright © 2003 John Wiley & Sons, Ltd.

The Potential Energy Surface of BH5and the Rate of the Hydrogen Scrambling

The <TEX>$BH_5$</TEX> molecule, which is suggested as an intermediate of the acidolysis of <TEX>$BH_4^-$</TEX>, contains a weak two-electron-three-center bond and it requires extremely high-level of theories to calculate the energy and structure correctly. The structures and energies of <TEX>$BH_5$</TEX> and the transition state for the hydrogen scrambling have been studied using recently developed multi-coefficient correlated quantum mechanical methods (MCCMs). The dissociation energies and the barrier heights agree very well with the previous results at the CCSD(T)/ TZ(3d1f1g, 2p1d) level. We have also calculated the potential energy curves for the dissociation of <TEX>$BH_5$</TEX> to <TEX>$BH_3$</TEX> and <TEX>$H_2$</TEX>. The lower levels of theory were unable to plot correct potential curves, whereas the MCCM methods give very good potential energy curves and requires much less computing resources than the CCSD(T)/ TZ(3d1f1g,2p1d) level. The potential energy of the <TEX>$BH_5$</TEX> scrambling has been obtained by the multiconfiguration molecular mechanics algorithm (MCMM), and the rates are calculated using the variational transition state theory including multidimensional tunneling approximation. The rate constant at 300 K is 2.1 × <TEX>$10^9s^{-1}$</TEX>, and tunneling is very important.

Acetylide Participation in Ligand Substitution and P−C Bond Cleavage in the Reaction between HRu3(CO)9(μ3,η2,η2,η1-C⋮CPh) and 4,5-Bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd). Syntheses and X-ray Structures of HRu3(CO)7[μ3,η2,η2,η1,η1,η1-Ph2PCCC(O)CH2C(O)PPh2CCPh] and Ru3(CO)7(μ,η2,η1-PhCCHPh)[μ2,η2,η1- PPhCCC(O)CH2C(O)PPh2

The triruthenium cluster HRu3(CO)9(μ3,η2,η2,η1-C⋮CPh) (1) reacts with the diphosphine ligand bpcd in the presence of Me3NO to afford the new cluster compounds HRu3(CO)7(bpcd)(μ3,η2,η2,η1-C⋮CPh) (2) and (3). Cluster 2 is the sole product from the controlled thermolysis of 1 with bpcd. Both 2 and 3 have been isolated and characterized in solution by IR and NMR (1H and 31P) spectroscopies, and by X-ray crystallography in the case of the latter cluster. The bpcd ligand in 2 chelates to the ruthenium center that is σ bound by the acetylide ligand, and in the zwitterionic compound 3 one of the PPh2 moieties of the bpcd ligand coordinates to a hydride-bridged ruthenium atom while the other PPh2 group exists as a phosphonium center due to attack on the acetylide Cα carbon. Whereas 3 slowly decomposes in solution at elevated temperatures, cluster 2 was found to undergo a clean conversion to the new cluster (4). The bridging alkenyl ligand in 4 results from the formal cleavage of one of the P−Ph bonds of the bpcd ligand, followed by the concomitant coupling of the phenyl and hydride groups with the acetylide ligand. Cluster 4 has been fully characterized in solution and the solid-state structure established by X-ray analysis. NMR data are presented from a labeling study employing HRu3(CO)7(bpcd)(μ3,η2,η2,η1-C⋮Ctol-p) that shows that the hydrogen in the μ,η2,η1-RCCHR‘ moiety of is distributed between both the phenyl and tol-p groups. The scrambling of the alkenyl hydrogen between the phenyl- and tol-p-substituted carbon atoms in the μ,η2,η1-RCCHR‘ moiety suggests that hydride transfer from the triruthenium frame occurs via a transient cluster compound that possesses a coordinated PhC⋮Ctol-p ligand.

Theoretical Study for the Potential Energy Surface of BH5 Using the Multicoefficient Correlated Quantum Mechanical Methods

The structures and energies of BH5 and the transition state for the hydrogen scrambling have been studied using recently developed multicoefficient correlated quantum mechanical methods (MCCMs). We have obtained the dissociation energies between 5.76 and 6.15 kcal/mol, and the barrier heights between 5.44 and 5.83 kcal/mol, which agree very well with the previous results at the CCSD(T)/TZ(3d1f1g,2p1d) level. We have also calculated the potential energy curves for the dissociation of BH5 to BH3 and H2. The lower levels of theory were unable to plot correct potential curves, whereas the MCCM methods give very good potential energy curves and requires much less computing resources than the CCSD(T)/TZ(3d1f1g,2p1d) level.

Factors affecting gas-phase deuterium scrambling in peptide ions and their implications for protein structure determination.

In this report, we evaluate the validity of using hydrogen/deuterium exchange in combination with collision-induced dissociation mass spectrometry (CID MS) for the detailed structural and conformational investigation of peptides and proteins. This methodology, in which partly deuterated peptide ions are subjected to collision-induced dissociation in the vacuum of a mass spectrometer, has emerged as a useful tool in structural biology. It may potentially provide quantitatively the extent of deuterium incorporation per individual amino acid in peptides and proteins, thus providing detailed structural information related to protein structure and folding. We report that this general methodology has limitations caused by the fact that the incorporated deuterium atoms migrate prior or during the CID MS analysis. Our data are focused on a variety of transmembrane peptides, incorporated in a lipid bilayer, in which the near-terminal amino acids that anchor at the lipid-water interface are systematically varied. Our findings suggest that, under the experimental conditions we use, the extent of intramolecular hydrogen scrambling is strongly influenced by experimental factors, such as the exact amino acid sequence of the peptide, the nature of the charge carrier, and therefore most likely by the gas-phase structure of the peptide ion. Moreover, the observed scrambling seems to be independent of the nature of the peptide fragment ions (i.e., protonated B and Y' ' ions, and sodiated A and Y' ions). Our results strongly suggest that scrambling may be reduced by using alkali metal cationization instead of protonation in the ionization process.

Dissociation of ethylene and several deuterated derivatives at 193 and 157 nm by direct classical trajectories

Several fragmentation reactions of ethylene were investigated by AM1 direct classical trajectories. Product yields were computed for ethylene and several deuterated derivatives at two excitation energies (193 and 157 nm). The atomic elimination channel is favoured as the energy increases, and hydrogen scrambling is found to be an important process at the energies of study, which contrasts with the mechanisms proposed up to date. The computed product yields are in better agreement with experiment than are previous RRKM calculations. The results suggest that the dissociation of ethylene occurs at, on near, the statistical limit, which disagrees with previous conclusions.

Ortho Effects in the Dissociation of Ionized N-Chlorophenyl- and N-Bromophenyl-2-Aminobenzamidines: Intramolecular Aromatic Substitution with Cyclization to Protonated 2-(2-Aminophenyl)-1H-Benzimidazoles

Electron ionization (70 eV) mass spectra, double-stage (MS2) 10 eV collision-induced dissociation (CID) product-ion mass spectra of molecular ions and triple-stage (MS3) sequential product-ion mass spectra of major fragment ions are reported for the parent N-phenyl and the isomeric ortho-, meta- and para- N-chlorophenyl- and N-bromophenyl-2-aminobenzamidines. Dissociation is greatly influenced by an ortho effect that favors the loss of ortho H atoms and particularly the loss of the ortho halogen substituents. Dissociation occurs via a two-step intramolecular aromatic substitution reaction and a distonic-ion intermediate inhibits scrambling of ring hydrogens thus favoring the loss of the ortho substituents. The ortho isomers form an abundant and diagnostic [M – X]+ fragment ion (X = Cl, Br) and are easily distinguished. Protonated 2-(1 H-benzimidazol-2-yl)-phenylammonium ions are likely formed; they dissociate mainly by NH3 loss upon CID and react readily with pyridine by proton transfer.

Hydrogen Scrambling in [(C5Me5)Os(dmpm)(CH3)H]+. A Density Functional and ONIOM Study

The hydrogen scrambling in the “model” compound [CpOs(PH2CH2PH2)(CH3)H*] and the experimentally used “real” compound [Cp*Os(PMe2CH2PMe2)(CH3)H*]+ was studied with the density functional (B3LYP) and...

Potential energy surface for activation of methane by Pt(+): a combined guided ion beam and DFT study.

A guided-ion beam tandem mass spectrometer is used to study the reactions of Pt(+) with methane, PtCH(2)(+) with H(2) and D(2), and collision-induced dissociation of PtCH(4)(+) and PtCH(2)(+) with Xe. These studies experimentally probe the potential energy surface for the activation of methane by Pt(+). For the reaction of Pt(+) with methane, dehydrogenation to form PtCH(2)(+) + H(2) is exothermic, efficient, and the only process observed at low energies. PtH(+), formed in a simple C-H bond cleavage, dominates the product spectrum at high energies. The observation of a PtH(2)(+) product provides evidence that methane activation proceeds via a (H)(2)PtCH(2)(+) intermediate. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies in eV (kJ/mol) of D(0)(Pt(+)-H) = 2.81 +/- 0.05 (271 +/- 5), D(0)(Pt(+)-2H) = 6.00 +/- 0.12 (579 +/- 12), D(0)(Pt(+)-C) = 5.43 +/- 0.05 (524 +/- 5), D(0)(Pt(+)-CH) = 5.56 +/- 0.10 (536 +/- 10), and D(0)(Pt(+)-CH(3)) = 2.67 +/- 0.08 (258 +/- 8). D(0)(Pt(+)-CH(2)) = 4.80 +/- 0.03 eV (463 +/- 3 kJ/mol) is determined by measuring the forward and reverse reaction rates for Pt(+) + CH(4) right harpoon over left harpoon PtCH(2)(+) + H(2) at thermal energy. We find extensive hydrogen scrambling in the reaction of PtCH(2)(+) with D(2). Collision-induced dissociation (CID) of PtCH(4)(+), identified as the H-Pt(+)-CH(3) intermediate, with Xe reveals a bond energy of 1.77 +/- 0.08 eV (171 +/- 8 kJ/mol) relative to Pt(+) + CH(4). The experimental thermochemistry is favorably compared with density functional theory calculations (B3LYP using several basis sets), which also establish the electronic structures of these species and provide insight into the reaction mechanism. Results for the reaction of Pt(+) with methane are compared with those for the analogous palladium system and the differences in reactivity and mechanism are discussed.

Theoretical and experimental studies of the dissociation dynamics of methaniminium cation, CH2NH2+→CHNH++H2: Reaction path bifurcation

The unimolecular dissociation of CH2NH2+ has been investigated experimentally and theoretically. Kinetic energy release distribution was obtained by analyzing the mass-analyzed ion kinetic energy profile. Critical configurations along the reaction path were investigated by electronic structure calculations at the HF, MP2, QCISD, and B3LYP levels using the 6-31G** and 6-311+G** basis sets. Reaction path bifurcation was observed at all the levels. The bifurcation point was in the entrance region (before the transition state) at the HF level. This point moved to the exit region when the electron correlation effect was included at the MP2, QCISD, and B3LYP levels. A global potential energy surface incorporating this bifurcation feature was constructed by interpolation at the MP2/6-311+G** level. Classical trajectories were calculated on this surface and product mode-specific energies were evaluated. Based on these data, various experimental observations, lack of hydrogen scrambling in particular, could be adequately explained.

Theoretical study of the mechanisms of palladation of methylenecyclopropane and [3 + 2] cycloadditions.

Palladation of methylenecyclopropane and [3 + 2] cycloaddition with alkenes have been studied theoretically. In palladium complexes of methylenecyclopropane or trimethylenemethane, the eta 2 forms have been shown to be more stable than the palladacyclobutane forms. However, the barrier heights to yield palladacyclobutanes are not very high and the methylene-bridged carbon can be converted to the metal-attached carbon on palladium via a ring-opened transition state, allowing the scrambling of hydrogens in methylenecyclopropane. With respect to the palladium-catalyzed [3 + 2] cycloaddition reaction of methylenecyclopropane with alkenes, the path via palladacyclobutanes has been demonstrated to be more favorable than the path via an eta 2 complex. These results are discussed by using the paired interaction orbital scheme.

Theoretical Studies of Carbocations in Ion Pairs. 4.1The Interconversion of the 1-Propyl Cation and Protonated Cyclopropane

The structure of the 1-propyl cation in the ion pair with the model anion trihydrofluoroborate, proven in earlier work to be appropriate for such studies, was investigated by ab initio calculations at the level previously reported to give the definitive structure of carbocations. In previous work, it was shown that the carbocation structure does not change with the nature of the anion. The cation structure is determined, however, by the distance between the cation and anion, d, and their relative orientation. At infinite interionic distance the only stable chemical species (energy minimum) is the protonated cyclopropane, 1. As the ions move toward each other, the cyclopropane bond opposite to the anion becomes progressively longer and eventually it breaks up in the contact ion pair. Three domains of cation stability are identified as a function of d: at long distances, ion 1 is the only energy minimum; at intermediate distances 1 and the 1-propyl cation 2 are both energy minima; at short distances, ion 2 is the only energy minimum. Thus, ionization of 1-propyl precursors forms the tight ion pair of 2 as the first intermediate. Isomers 1 and 2 differ in both the C1−C2−C3 angle and the conformation of the C2−C3 bond; the transition structure for their interconversion has been determined by calculations. At the MP4(FC)/6-311G**//MP2/6-311G** level, the two isomers have the same energy content for d = 2.40 Å, but correction for the zero-point energies obtained from the vibrational frequencies calculated at the MP2/6-311G** level reduces the energy of 2 relative to 1, thus requiring a slight upward correction in the value of d for equal stability of isomers. The interconversion of 1 and 2 is observed for a position of the anion essentially in the same plane as the three carbon atoms. Movement of the anion above the same plane results in hydrogen shift with the formation of the 2-propyl cation, 3. Some literature results in which primary carbocations could intervene as intermediates are discussed. In particular, the data on carbon and hydrogen scrambling in 3 in superacid solution are better accounted for by the results of calculations for ion pairs, with both 1 and 2 as intermediates, than by the results of calculations for isolated ions.

A Theoretical Study of the H2 Elimination from C6H7+: An Example of the Role of Back-Donation in Carbocation Chemistry

C6H7+ has been investigated by using the ab initio method at MP2-FC/6-31G* level and single point calculations on the MP2 geometries at the MP4(SDTQ)-FC/6-31G* level. The most stable structure located corresponds to the benzenium ion C6H7+ in which a proton is bonded to one of the carbon atoms of the benzene ring and an extension of the delocalization has taken place to include the attached proton, the π-electron density distribution being basically allylic in agreement with experimental findings. Two transition structures (TSs) for hydrogen scrambling were found on the C6H7+ PES. The first one corresponds to the peripheral migration of a proton in the benzenium ion and determines an energy barrier of 7.8 kcal/mol at MP2 level (10.8 at MP4//MP2 level) in good agreement with experimental data. The second scrambling TS corresponds to the interchange of the two hydrogen atoms bonded to the same carbon atom in the ring of the benzenium ion and presents an energy barrier of 59.2 kcal/mol at MP2 level (62.6 at MP4//MP2 level). The H2-elimination is an endoergic process that proceeds through a very late TS and an intermediate very close in energy to it that finally renders the products, C6H5+ + H2, without any energy barrier. The activation energy for this process (70.3 and 70.1 kcal/mol at MP2 and MP4//MP2 levels, respectively) is practically its endoergicity (71.1 kcal/mol at MP2 level and 70.8 kcal/mol at MP4//MP2 level) in accordance with the experimental finding of null energy release as relative motion between the fragments. A configuration analysis of the wave function of C6H7+ along the reaction coordinate for H2-elimination clearly shows that the back-donation from the NHOMO of C6H5+ to the LUMO of H2 plays a fundamental role in the bonding structure of the benzenium ion. The H2 elimination takes place through deactivation of the back-bonding at an early stage in the process.

Comparison of structures and energies of CH5(2+*) with CH4(+*) and their possible role in superacidic methane activation.

Contrary to previous theoretical studies at the UHF/6-31G* level, the methonium radical dication CH5(2+) is not a Cs symmetrical structure with a 2e-3c bond but a C2v symmetrical structure 1 with two 2e-3c bonds (at the UHF/6-31G**, UMP2/6-31G**, and UQCISD(T)/6-311G** levels). The Cs symmetrical structure is not even a minimum at the higher level of calculations. The four hydrogen atoms in 1 are bonded to the carbon atom by two 2e-3c bonds and the fifth hydrogen atom by a 2e-2c bond. The unpaired electron of 1 is located in a formal p-orbital (of the sp2-hybridized carbon atom) perpendicular to the plane of the molecule. Hydrogen scrambling in 1 is however extremely facile, as is in other C1 cations. It is found that the protonation of methane to CH5(+) decreases the energy for subsequent homolytic cleavage resulting in the exothermic (24.1 kcal/mol) formation of CH4(+*). Subsequent reaction with neutral methane while reforming CH5(+) gives the methyl radical enabling reaction with excess methane to ethane and H2. The overall reaction is endothermic by 11.4 kcal/mol, but offers under conditions of oxidative removal of H2 an alternative to the more energetic carbocationic conversion of methane.

ChemInform Abstract: Competitive Charge‐Remote and Anion‐Induced Fragmentations of the Non‐ 8‐enoate Anion. A Charge‐Remote Reaction Which Co‐Occurs with Hydrogen Scrambling.

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Competitive charge-remote and anion-induced fragmentations of the non-8-enoate anion. A charge-remote reaction which co-occurs with hydrogen scrambling

The non-8-enoate anion undergoes losses of the elements of C3H6, C4H8 and C6H12 on collisional activation. The mechanisms of these processes have been elucidated by a combination of product ion and labelling (2H and 13C) studies, together with a neutralisation reionisation mass spectrometric study. These studies allow the following conclusions to be made. (i) The loss of C3H6 involves cyclisation of the enolate anion of non-8-enoic acid to yield the cyclopentyl carboxylate anion and propene. (ii) The loss of ‘C4H8’ is a charge-remote process (one which proceeds remote from the charged centre) which yields the pent-4-enoate anion, butadiene and dihydrogen. This process co-occurs and competes with complex H scrambling. (iii) The major loss of ‘C6H12’ occurs primarily by a charge-remote process yielding the acrylate anion, hexa-1,5-diene and dihydrogen, but in this case no H scrambling accompanies the process. (iv) It is argued that the major reason why the two charge-remote processes occur in preference to anion-induced losses of but-1-ene and hex-1-ene from the respective 4- and 2-anions is that although these anions are formed, they have alternative and lower energy fragmentation pathways than those involving the losses of but-1-ene and hex-1-ene; viz. the transient 4-anion undergoes facile proton transfer to yield a more stable anion, whereas the 2-(enolate) anion undergoes preferential cyclisation followed by elimination of propene [see (i) above].

A quantum electronic theory of chemical processes?The inverted energy profile case: CH3+ + H2 reaction

A quantum approach to chemical processes is developed. The chemical interconversion is described as an electronic process. The reaction corresponds to histories involving quantum states belonging to different stationary molecular Hamiltonians. The system may be embedded in a weak (thermal) and/or external electromagnetic field. The electromagnetic transverse fields lead to transition moments yielding finite probability amplitudes for the system to change from one quantum state to another. Bottleneck subspaces (transition states) are defined; they mediate the interconversions in generic unimolecular and bimolecular processes. Active precursor and successor complexes are introduced to help bridge reactant and product electronic states. The stationary states are modeled with Born-Oppenheimer Hamiltonians. At a qualitative level, the theory is general. The rate, measured as a time derivative of product concentration, is expressed in terms of concentrations of active precursor and successor complexes. The kinetic coefficients are given in terms of quantum processes involving electronic bottleneck states. Stationary structures and vibrational zero-point energies characterizing the reactive CH3++H2 system are determined at a Hartree-Fock level of theory with 6-31++G** basis set. The vibrational levels are corrected with anharmonicity effects. The saddle point of index one for hydrogen scrambling reactions has been obtained and shown to be related to the CH5+ molecular complex together with the precursor and successor complexes geometries. The unusual properties of the system with respect to standard transition-state theory are fairly well described within this approach, in particular, isotope scrambling as well as photon emission during formation of the carbocation. The theory suggests that these types of reactions, which are found in outer space, may contribute to the scattering of the cosmic microwave background. © 1997 John Wiley & Sons, Inc.

A quantum electronic theory of chemical processes—The inverted energy profile case: CH3 + H2 reaction

A quantum approach to chemical processes is developed. The chemical interconversion is described as an electronic process. The reaction corresponds to histories involving quantum states belonging to different stationary molecular Hamiltonians. The system may be embedded in a weak (thermal) and/or external electromagnetic field. The electromagnetic transverse fields lead to transition moments yielding finite probability amplitudes for the system to change from one quantum state to another. Bottleneck subspaces (transition states) are defined; they mediate the interconversions in generic unimolecular and bimolecular processes. Active precursor and successor complexes are introduced to help bridge reactant and product electronic states. The stationary states are modeled with Born-Oppenheimer Hamiltonians. At a qualitative level, the theory is general. The rate, measured as a time derivative of product concentration, is expressed in terms of concentrations of active precursor and successor complexes. The kinetic coefficients are given in terms of quantum processes involving electronic bottleneck states. Stationary structures and vibrational zero-point energies characterizing the reactive CH3++H2 system are determined at a Hartree-Fock level of theory with 6-31++G** basis set. The vibrational levels are corrected with anharmonicity effects. The saddle point of index one for hydrogen scrambling reactions has been obtained and shown to be related to the CH5+ molecular complex together with the precursor and successor complexes geometries. The unusual properties of the system with respect to standard transition-state theory are fairly well described within this approach, in particular, isotope scrambling as well as photon emission during formation of the carbocation. The theory suggests that these types of reactions, which are found in outer space, may contribute to the scattering of the cosmic microwave background. © 1997 John Wiley & Sons, Inc.

Characterization of thiophene and of its alpha-coupled oligomers with 2 &lt; = n &lt; = 8 using multiphoton ionization mass spectrometry

Thiophene (T1) and the all α-coupled unsubstituted oligomers T2–T8 were analyzed in a time-of-flight mass spectrometer using multiphoton ionization (MPI) in the wavelength range λ = 230–270 nm. The MPI spectra obtained showed no resonance enhancement in the investigated wavelength range. The molecular ion was detected as the base peak in all cases. A general fragmentation pathway is described to interpret the dominant features of the MPI-mass spectra for all oligomers. 2,5-d2-thiophene (d2-T1) was investigated in order to characterize the degree of hydrogen scrambling under MPI conditions.

State-Selected Photodissociation of Acetaldehyde Molecular Ion: Hydrogen Scrambling and the Product Branching Ratio

Acetaldehyde molecular ions were produced in an ICR cell using (2 + 1) resonance-enhanced multiphoton ionization of a pulsed molecular beam of acetaldehyde via a two-photon resonant 3s ← n Rydberg transition in the wavelength range 364−354 nm. Mass-selected REMPI spectra of CH3CHO are presented. The power dependencies of ion intensities were examined to help establish the mechanisms of REMPI dissociation processes. The product branching ratios from REMPI dissociations of CH3CHO and CH3CDO were determined. A multireference configuration interaction method was employed to obtain structural and energetic information about the X 2A‘, A 2A‘‘, and B 2A‘ states of acetaldehyde molecular ion. Comparisons of theory with the experiments lead us to conclude that acetaldehyde molecular ions are excited to the B 2A‘ state and undergo both α and β cleavage processes. The α C−C cleavage yields the formyl ion without hydrogen scrambling. The product branching ratio of [HCO+]/[CH3CO+] remains constant at 2.1 ± 0.2 over...