An investigation of the decomposition of the intermediate ions produced by electron-impact. II—[C7H7]+ ions from several halogenotoluenes

The structure and decomposition of the [C₇H₇]⁺ ions produced by electron-impact from o-, m- and p-chlorotoluene, o-, m- and p-bromotoluence, and p-iodotoluence, have been investigated. By determining the relative abundance of normal and metastable ions, these [C₇H₇]⁺ ions at electron energy of 20 eV are shown to be so-called ‘tropylium ions’. The amount of the internal energy of the [C₇H₇]⁺ ion estimated by the relative ion abundance ratios, ? [C₅H₅]⁺/[C₇H₇]⁺ and m*/[C₇H₇]⁺ for the decomposition \documentclass{article}\pagestyle{empty}\begin{document}$ [{\rm C}_{\rm 7} {\rm H}_{\rm 7}]^ + \mathop \to \limits^{m^* } [{\rm C}_{\rm 5} {\rm H}_{\rm 5}]^ + + {\rm C}_{\rm 2} {\rm H}_{\rm 2} $\end{document}, is in the order iodotoluene > bromotoluene > chlorotoluene. The heats of formation of the activated complexes for the reaction \documentclass{article}\pagestyle{empty}\begin{document}$ [{\rm C}_{\rm 7} {\rm H}_{\rm 7}]^ + \mathop \to \limits^{m^* } [{\rm C}_{\rm 5} {\rm H}_{\rm 5}]^ + + {\rm C}_{\rm 2} {\rm H}_{\rm 2} $\end{document} were estimated. The values suggest that the decomposing [C₇H₇]⁺ ions from various halogenotoluenes are identical in structure.     

Isomerization of hydrocarbon ions. VIII—The electron impact induced decomposition of n-dodecane

The literature on the mass spectrometry of ²H and ¹³C labelled higher alkanes is reviewed and the decomposition behaviour of both the molecular and the fragment ions of n-dodecane, n-dodecane-1, 12-[¹³C₂] and n-dodecane-1,1,1,12,12,12-[²H₆] studied with special emphasis on metastable decompositions. It is shown that the elimination of alkane molecules and alkyl radicals from the n-dodecane molecular ion occurs primarily by simple splitting of the C? C bond. In addition, both small alkane molecule and alkyl radicals are eliminated with low probability from centreal parts of the molecular ion. The alkane elimination is less specific than the alkyl elimination. The methyl elimination shows an exceptionally high non-specificity, but is of negligible abundance in the 70 e V electron impact spectrum. The metastable ion spectra suggest, but do not prove unambiguously, that those small alkyl ions (with up to four carbon atoms) originating directly from the molecular ion, may be formed both by direct cleavage of the terminal groups and from central parts of the molecular ion. However, the majority of the small alkyl fragment ions in the 70 eV spectrum are formed by secondary decomposition explaining their apparent non-specific formation. The strikingly different fragmentation behaviour of even electron, [C_nH_2n+1]⁺, and odd electron fragment ions, results from differences in the product stabilities. Using collisional activation and metastable ion spectra it is shown that the odd electron fragments have the structure of the linear alkene (most probably the 1-alkene) molecular ion. In contrast to the molecular ions, alkyl fragment ions decompose with complicated skeletal rearrangements, which lead to substantial, but not complete, carbon randomization. The terminal hydrogen atoms, however, show little scrambling.     

Formation and stability of gaseous tolyl ions

Collisional activation mass spectra confirm that tolyl ions can be produced from a variety of CH₃C₆H₄Y compounds. High purity o-, m- and p-tolyl ions are prepared by chemical ionization of the corresponding fluorides (Y=F) as proposed by Harrison. In electron ionization of CH₃C₆H₄Y formation of the more stable tropylium and benzyl ionic isomers usually accompanies that of the o-, m- and p-tolyl ions. Isomerization of low energy [CH₃C₆H₄Y]⁺? to [Y–methylenecyclohexadiene]⁺? is proposed to account for most [benzyl]⁺ formation, while the tropylium ion appears to arise from the isomerization of tolyl ions formed with higher internal energies, [o-, m-, p-tolyl]⁺→ [benzyl]⁺→ [tropylium]⁺, consistent with Dewar's predictions from MINDO/3 calculations.     

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.     

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.     

Structure/formula informatics of isomeric sets of fluoranthenoid/fluorenoid and indacenoid hydrocarbons

The systematics of fluoranthenoid/fluorenoid and indacenoid hydrocarbons is studied. Successive circumscribing a set of isomeric structures that result in a constant number of isomers at each circumscribing step gives what is called a constant-isomer series. Constant-isomer series have a repetitive isomer number pattern in which those series with the same isomer number have a one-to-one matching in topology among their membership. The general formulas of the matching constant-isomer sets of indacenoids are reproduced by C_4p²+1H_4p+1{{\rm C}_{{{4p}^{2}}{+1}}{\rm H}_{4p+1}} and C_4p²+4p+3H_4p+3{{\rm C}_{{{4p}^{2}}{+4p+3}}{\rm H}_{4p+3}}, respectively, by successively inputting p  =  1, 2, 3, . . ., and the general formula for the unique indacenoid constant-isomers is reproduced by C_p²+3p+4H_2p+4{{\rm C}_{{p^{2}}{+3p+4}}{\rm H}_{2p+4}}. Similar general formulas for the fluoranthenoid/fluorenoid hydrocarbon constant-isomer series are also presented.     

Kinetic studies of reactions of 4‐nitrobenzofuroxan with carbanions derived from benzyltriflones in methanol

This paper reports the rate measurements for the reactions of carbanions derived from benzyltriflones, 2 , with 4‐nitrobenzofuroxan, 4 , in methanol, to give anionic σ‐adducts. ¹H NMR studies in DMSO‐d₆ indicate that the products formed by the reaction of 2 and 4 in the presence of triethylamine are consistent with the products formed by the elimination of trifluoromethylsulfinic acid from σ‐adducts, initially formed by a carbanion attack at the 5 position of 4 . The low value of β, which is the slope of the linear plot of log k₅ versus pK_a, provides evidence for the high steric requirements of the benzyltriflone anions. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 546–554, 2012     

Charge localization and aromatic stabilization in large ring ions: Mass spectra of ms-porphyrins

The major mass spectrometric fragments of ms-tetraphenylporphin and ms-tetra(p-chloro)phenylporphin are [M ? H]⁺˙ and [M ? Cl]⁺˙, respectively. Metal derivatives of these compounds give a modified characteristic fragmentation pattern with peak groups ending in the ions [M ? 4H]⁺˙, [M ? ? ? 5H]⁺˙ and [M ? 2? ? 2H]⁺˙ for the metallo ms-tetraphenylporphins, and [M ? ?Cl ? 2Cl ? 3H]⁺˙ and [M ? 2?Cl ? Cl ? H]⁺˙ for Mgms-tetra(p-chloro)phenylporphin. Deuterated metal derivatives indicate random hydrogen loss from both phenyl and pyrrole carbons. However, metal substituents do not significantly modify the fragmentation pattern in the case of ms-tetra(p-methoxy)phenylporphin. These patterns can be explained in terms of aromatic stabilization of the fragmentation products, coupled with charge localization on the π system in the free base, on the metal atom in the metallo derivatives and on the methoxy function in the p-methoxyphenyl derivative.     

Electron impact studies. CXII—The properties of positive ions produced by charge stripping from negative ions. [C6H5O]+, [C6H5S]+ and [C7H7S]+

The dissociative spectrum of the [C₆H₅S]⁺ ion derived by charge inversion from [C₆H₅S]^?, shows a variety of fragmentations including the competitive losses of H?, C₃H₄ and the formation of [CHS]⁺. The spectrum of a deuteriated derivative shows that these three processes are preceded or accompanied by H/D scrambling. The corresponding [C₆H₅O]⁺ species also undergoes hydrogen scrambling prior to fragmentation. In marked contrast, the ion [p-MeC₆H₄S]⁺ does not undergo hydrogen randomization between the methyl and aryl groups, and positional integrity is retained during fragmentation. These results are compared with the properties of the same ions produced by conventional ionization.     

Average inner product sums of electron linear momenta for atoms and ions

From the second moments of the electron-pair densities in momentum space, accurate Hartree–Fock values of the average inner product sum 〈∑ _i<j p _i ·p _j 〉 of electron linear momenta are evaluated for the 102 neutral atoms from He to Lr, the 53 singly charged cations from Li⁺ to Cs⁺, and the 43 stable anions from H⁻ to I⁻ in their experimental ground states. The present results are new for 38 species and improve the literature values for 68 species. Received: 18 July 2002 / Accepted: 4 September 2002 / Published online: 8 November 2002 Acknowledgement. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan. Correspondence to: H. Matsuyama e-mail: hisashi@mmm.muroran-it.ac.jp     

Alkene loss from metastable methyleneimmonium ions: Unusual inverse secondary isotope effect in ion-neutral complex intermediate fragmentations

The mechanism of propene elimination from metastable methyleneimmonium ions is discussed. The first field-free region fragmentations of complete sets of isotopically labelled methyleneimmonium ions (H₂C = $ \mathop {\rm N}\limits^{\rm +} $⁺R¹R²: R¹ = R² = n-C₃H₇; R¹ = R² = i-C₃H₇; R¹ = n -C₃H₇; R² = C₂H₅; R¹ = n-C₃H₇; R² = CH₃; R¹ = n-C₃H₇; R² = H) were used to support the mechanism presented. The relative amounts of H/D transferred are quantitatively correlated to two distinct mathematical concepts which allow information to be deduced about influences on reaction pathways that cannot be measured directly. Propene loss from the ions examined proceeds via ion-neutral complex intermediates. For the di-n-propyl species rate-determining and H/D distribution-determining steps are clearly distinct Whereas the former corresponds to a 1,2-hydride shift in a 1-propyl cation coordinated to an imine moiety, the latter is equivalent to a proton transfer to the imine occurring from the 2-propyl cation generated by the previous step. For the diisopropyl-substituted ions which directly form the 2-propyl cation-containing complex, the rate-determining hydride shift vanishes. The 2-propyl cation-containing complex can decompose directly or via an intermediate proton-bridged complex. Competition of these routes is not excluded by the experimental results. Assuming a 2:1:3 distribution, a preference for the α- and β-methylene of the initial n-propyl chain as the source of the hydrogen transferred is detected for n-propylimmonium ions containing a second alkyl chain R². This preference shows a clear dependence on the steric influence of R². During the transfer step isotopic substitution is found to affect the H/D distribution strongly. For the alternative route of McLafferty rearrangement leading to C₂H₄ loss, specific γ-H transfer is observed.     

Mechanistic Studies on the Gas‐Phase Dehydrogenation of Alkanes at Cyclometalated Platinum Complexes

In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L? H)]⁺ (L=2,2′‐bipyridine (bipy), 2‐phenylpyridine (phpy), and 7,8‐benzoquinoline (bq)) with linear and branched alkanes C_nH_2n+2 (n=2–4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L? H)]⁺/C₂H₆, loss of C₂H₄ dominates clearly over H₂ elimination; however, the mechanisms significantly differs for the reactions of the “rollover”‐cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen‐atom transfer from C₂H₆ to [Pt(bipy? H)]⁺, followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)]⁺, for the phpy and bq complexes [Pt(L? H)]⁺, the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L? H)(H₂)]⁺ as the ionic product accounts for C₂H₄ liberation. In the latter process, [Pt(L? H)(H₂)(C₂H₄)]⁺ (that carries H₂ trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C₂H₄ rather than H₂ is ejected. For both product‐ion types, [Pt(H)(bipy)]⁺ and [Pt(L? H)(H₂)]⁺ (L=phpy, bq), H₂ loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy? H)]⁺ with the higher alkanes C_nH_2n+2 (n=3, 4), H₂ elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C₃H₅)(bipy)]⁺. In the reactions of [Pt(L? H)]⁺ (L=phpy, bq) with propane and n‐butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of “rollover”‐cyclometalated [Pt(bipy? H)]⁺ with C_nH_2n+2 (n=2–4) less than 15 % of the generated product ions are formed by C? C bond‐cleavage processes, this value is about 60 % for the reaction with neo‐pentane. The result that C? C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2‐elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1‐ and 1,3‐elimination pathways are only of minor importance.     

Five-coordinate low-spin cobalt(II) complexes with benzeneseleninato ions and ethylenediamine as ligands

Summary Five-coordinate bis(benzeneseleninato)tris(ethylenediamine) cobalt (II)complexes are obtained by reaction of Co(H₂O)₂ (XC₆ H4 SeO₂)2 complexes (X = H, p-Cl, m-CI, p-Br, ni-Br, p-Me,p-NO₂) with ethylenediamine. The diaquo complexes (one mole)react with ethylenediamine (three moles)to form O-seleninato derivatives. Spectral and magnetic properties show that the complexes are low-spin (s = 1/2) and,on the basis of the electronic spectra a distorted trigonal geometry,D _3h , is suggested. Assignments for the electronic spectra are proposed. Conductivity data indicate that these derivatives are nonelectrolytes. Both ethylenediamine and [RSeO₂ ]^– behave as monodentate ligands.     

Optical oscillator strengths for isoelectronic ions of nitrogen

An analytic atomic active electron model potential adjusted to experimental single-particle energy levels is used to generate wave functions for the valence and excited states of O¹⁺, F²⁺, Ne³⁺, Na⁴⁺, and Mg⁵⁺. Using these wave functions in conjunction with the Born approximation and the LS-coupling scheme, we calculate optical oscillator strengths for excitations from the 1s²2s²2p³(⁴S_3/2) ground state. The results are compared to experimental data and other calculations. Systematic trends along the isoelectronic sequence are discussed.     

Collisional activation spectra of the adduct ions of 1,2,3-triarylpropenones under ammonia chemical ionization conditions

Under ammonia chemical ionization (CI) conditions triarylpropenones undergo hydrogen radical-induced olefinic bond reduction on metal surfaces, resulting in [M + 2H + NH₄]⁺ ions corresponding to the ammonium adduct of the saturated ketone. The decomposition of the adduct ions, [MNH₄]⁺ and [M + 2H + NH₄]⁺, was studied by collision-induced dissociation mass-analysed ion kinetic energy (CID-MIKE) spectroscopy in a reverse geometry instrument. From the CID-MIKE spectra of the [MNH₄]⁺, [M + 2H + NH₄]⁺, [MND₄]⁺ and [M + 2D + ND₄]⁺ ions it is clear that the fragmentation of the adduct ions involves loss of NH₃ followed by various cyclization reactions resulting in stable condensed ring systems. Elimination of ArH and ArCHO subsequent to the loss of NH₃ and formation of aroyl ion are characteristic decomposition pathways of the [MNH₄]⁺ ions, whereas elimination of ArCH₃ and formation of [ArCH₂]⁺ are characteristic of the [M + 2H + NH₄]⁺ ions of these propenones.     

Some gas phase reactions of silicenium ions derived from sym-tetramethyldisiloxane

Ion cyclotron resonance spectroscopy has been used to study reactions of the silicenium ions, Me₂HSiO iMeR (R = H and Me), generated from sym-tetramethyldisiloxane in the gas phase. These silicenium ions react with neutral sym-tetramethyldisiloxane by an addition—elimination process, to form the homologous silicenium ions, H(Me₂SiO _nSiMeR (n = 2 and 3). Analogous addition—elimination products, MeO(Me₂SiO)_nSiMeR, are derived from their reaction with methanol. Bimolecular adducts are observed in the presence of benzene and anisole, while oxygen abstraction occurs in the presence of anisole and acetone.     

Highly Stereoselective Synthesis of Phenylseleno- and p-Tolylsulfonyl Substituted 1,3-Dienes from Functionalized Allyl Alcohols

Introduction Heteroatom-substituted 1,3-dienes have been exten-sively studied because of their marked abilities to con-struct highly functionalized ring systems in cycloaddi-tions.1 For example, Danishefsky2 developed 1-methoxy-3-trimethyl-siloxy-buta-1,3-diene which has led to many creative applications in complex organic synthesis. Padwa et al.3 demonstrated that 1,3- and 2, 3-bis(phenyl-sulfonyl)-1,3-butadienes are versatile building blocks in organic synthesis via reactions such as [4+2]-c…     

Ligand influence on the electrochemical behavior of some copper(II) thiosemicarbazone complexes

The redox properties of the title mono- and binuclear copper(II) chelates have been investigated by cyclic voltammetry in DMF at a working platinum electrode. The cathodic reduction and anodic oxidation of the investigated chelates produced the corresponding electrochemical Cu^I and Cu^IIIspecies stable only in the voltammetric time scale, The effects of substituents on E_1/2, redox properties and stability towards oxidation of the complexes were related to the electron-withdrawing or releasing ability of the substituents on the C=N¹[H, CH₃ or C₆H₅] and/or N⁴H [H, C₂H₅, C₆H₅ or pClC₆H₄] groups, The electron attracting substituents stabilize the Cu(II) complexes while electron-donating groups favor oxidation to Cu(III). Changes in the E_1/2 for the complexes due to remote substituent effects could be related to changes in basicity of N⁴H.Thus, variation in N⁴1-J has more influence on E_1/2 than changes in C=N¹. The correlation between E_1/2 of the complexes and pK_a of the ligands has been attributed to the spherical potential generated by the electron density of the donor atoms at the antibonding d orbitals.     

Loss of methyl from ionized 2-hydroxy-1,3-butadiene

The loss of methyl from unstable, metastable and collisionally activated [CH₂?CH? C(OH)?CH₂]⁺˙ ions (1⁺˙) was examined by means of deuterium and ¹³C labelling, appearance energy measurements and product identification. High-energy, short-lived 1⁺˙ lose methyl groups incorporating the original enolic methene (C(1)) and the hydroxyl hydrogen atom (H(0)). The eliminations of C(1)H(1)H(1)H(4) and C(4)H(4)H(4)H(0) are less frequent in high-energy ions. Metastable 1⁺˙ eliminate mainly C(1)H(1)H(1)H(4), the elimination being accompanied by incomplete randomization of the five carbon-bound hydrogen atoms. The resulting [C₃H₃O]⁺ ions have been identified as the most stable CH₂?CH? CO⁺ species. The appearance energy for the loss of methyl from 1 was measured as AE[C₃H₃O]⁺ = 10.47 ± 0.05 eV. The critical energy for 1⁺˙ → [C₃H₃O]⁺ + CH₃˙ is assessed as E_c ? 173 kJ mol^?1. Reaction mechanisms are proposed and discussed.     

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.