3,514 research outputs found
Ab initio quantum mechanical investigation of the reaction mechanisms for the formation of C7H(7)(+) from o-, m-, and p-chlorotoluene radical cations
No full textThe reaction pathways to C7H(7)(+)(tolyl, benzyl, and tropylium) from o-, m-, and p-chlorotoluene radical cations have been studied at the SCF level of theory using the DZ and DZP basis sets. The equilibrium structures and relative energies for the local minima and transition states have been fully optimized and the harmonic vibrational frequencies have been evaluated to differentiate the local minimum and the transition state. The lowest barrier rearrangement pathway is the formation of the benzyl cation for all three isomers, corroborating the recent experimental findings of Kim and Shin. This benzyl channel involves [1,3] alpha-hydrogen migration at the entrance and the activation energy is calculated to be 40.3, 39.7, and 42.2 kcal mol(-1) for the o-, m-, and p-isomer, respectively. The other competing rearrangement pathway involves [1,2] alpha-hydrogen migration at the entrance, which leads to the tropylium ion, and the activation barrier is predicted to be 55.5, 54.2, and 56.62 kcal mol(-1) for the o-, m-, and p-isomer, respectively. The highest barrier process involves direct C-Cl bond cleavage that yields the tolyl cation with a 0 K dissociation energy of 61.6, 64.6, and 68.92 kcal mol(-1) for the o-, m-, and p-isomer, respectively.X1199sciescopu
RELATIVE STABILITIES OF ORTHO-, META- AND PARA-TOLYL CATIONS
No full textRelative energies of the tropylium ion and tolyl cations with respect to the benzyl cation were estimated using ab initio calculations. The relative energies of tolyl cations are in ascending order ortho < meta < para. After correcting for zero-point energies, correlation effects, and basis-set deficiencies, the best estimates for the relative energies were converted to 0 K heats of formation by taking Delta H(f0)degrees = 919 kJ mol(-1) for the benzyl cation as an anchor. The best Delta H(f0)degrees estimates are 896, 1087, 1093, and 1101 kJ mol(-1) for the tropylium, ortho-, meta-, and para-tolyl cations, respectively. (C) 1997 Elsevier Science B.V.X1122sciescopu
On the Sherlocks, Jane Coleman and County Kildare in the Eighteen Forties
No full textIn the late 1980s and early 1990s the author acquired about 30,000 letters written mainly in the 1840s. These pertained to estates throughout Ireland managed by the firm of James Robert Stewart and Joseph Kincaid, hereafter denoted SK. Until the letters – called the SK correspondence in what follows – became the author’s property, they had not seen light of day since the 1840s. Addressed mainly to the firm’s office in Dublin, they were written by landlords, tenants, the partners in SK, local agents, etc. After about 200 years in operation as a land agency, the firm in which members of the Stewart family were the principal partners – Messrs J. R. Stewart & Son(s) from the mid- 1880s onwards – ceased operations in the mid-1980s. Since 1994 the author has been researching the SK correspondence of the 1840s. It gives many new insights into economic and social conditions in Ireland during the decade of the great famine, and into the operation of Ireland’s most important land agency during those years. It is intended ultimately to publish details on several of the estates managed by SK in a study more comprehensive than the present article, in book form. The proposed title is Landlords, tenants, famine: business of an Irish land agency in the 1840s, a draft of which has now been completed. A majority of the letters in that study are on themes some of which one might expect - rents, distraint (seizure of assets in lieu of rent); ‘voluntary’ surrender of land in return for ‘compensation’ upon quitting quietly; formal ejectment (a matter of last resort on estates managed by SK); landlordassisted emigration (on a scale much more extensive than most historians of Ireland in the 1840s appear to believe); petitions from tenants; complaints by tenants, both about other tenants and about local agents; landlord-financed and other relief of distress both before and during the great famine; major works of improvement (on almost all of the estates managed by SK which have been investigated in detail in the draft book); applications by SK, on behalf of landlords, for government loans to finance improvements; recommendations of agricultural advisers hired by SK, etc. Thus, most of the SK correspondence is about aspects of estate management. But the firm of SK was not only a manager of land. The correspondence reveals only two estates in Kildare, each of them relatively small, managed by SK in the 1840s. These were the lands of the Sherlocks near Naas and of Jane Coleman in the Kilcullen district. The correspondence on these properties differs substantively from most of those discussed in detail in the draft of Landlords, tenants, famine: first, it is relatively small in quantity, and secondly, it contains relatively little on the core aspects of estate management indicated above. Much of that on the Sherlocks focuses on misfortunes among family members, while the correspondence on Jane Coleman highlights the benevolence of that proprietor.
SOLVATION OF THE LI+-BR--LI+ TRIPLE ION IN THE GAS PHASE
No full textThe solvation of the Li+-Br--Li+ triple ion with oxygen-donor Lewis bases was studied in the gas phase with use of Fourier-transform ion cyclotron resonance spectrometry. The Li+-Br--Li+ triple ions were prepared in an ICR cell by matrix-assisted laser desorption ionization of a lithium bromide/dibenzo-18-crown-6-ether matrix pasted on a Teflon substrate. The stepwise solvations of LiBrLi+ were examined with oxygen-containing Lewis bases: 15-crown-5-ether (15C5), 12-crown-4-ether (12C4), 1,4-dioxane, 1,3-dioxane, and tetrahydrofuran (THF). Crown ethers solvate Lif with loss of LiBr. 15-Crown-5-ether yields a 1:1 (15C5) Lif complex as the end product, whereas 12-crown-4-ether forms a 2:1 (12C4)(2) . Li+ complex. On the other hand, low dielectric constant solvents, such as dioxanes and THF, solvate the Li+-Br--Li+ triple ion rather than abstracting Lif with loss of the neutral ion pair. The maximum coordination numbers are 3 for dioxanes and 4 for THF at 293 K. The rate constants for the stepwise solvations were measured, and the binding energies were determined from the temperature dependence of the equilibrium constants. The mechanisms of stepwise solvations are discussed in light of the structures and energetics of the 1:1 complexes of Li+ and LiBrLi+ with dioxanes and THF calculated at the Hartree-Fock level with a standard 6-311G(d,p) basis set.X119sciescopu
TIME- AND PRODUCT-RESOLVED PHOTODISSOCIATIONS OF BROMOTOLUENE RADICAL CATIONS
No full textPhotodissociations of o-, m-, and p-bromotoluene radical cations have been studied in the wavelength range 575-475 nm using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. The parent ions were prepared by charge-transfer reactions of bromotoluenes with toluene-d(8) radical cations produced by two-photon ionization of toluene-d(8) at 266 nm. Bromotoluene radical cations dissociate to C7H7+ by loss of Br. The dissociation rates were measured by time-resolved photodissociation spectroscopy. Structures of C7H7+ from one-photon dissociation were identified by their bimolecular reactivities with toluene-d(8). The C7H7+ products from all three isomers were identified as the benzyl cation. No unreactive tropylium ions were detected within experimental limits. The rate constants measured in this work were combined with the previous photoelectron-photoion-coincidence results to refine activation parameters for the Rice-Ramsperger-Kassel-Marcus rate-energy curves, k(E), for the low barrier rearrangement process. The activation barriers are estimated to be 1.66, 1.80, and 1.78 eV for the o-, m-, and p-bromotoluene radical cations, respectively, whereas the entropy changes for the activation, Delta S-double dagger(1000 K), are -9.6, -7.2, and -5.6 eu., respectively. The mechanism of the rearrangement process is presented to account for the predominant formation of the benzyl cation. (C) 1997 American Institute of Physics.open1115sciescopu
EXPERIMENTAL AND THEORETICAL STUDIES OF SILACYCLOHEPTATRIENYL CATION FORMATION FROM PHENYLSILANE
No full textThe formation of C6SiH7+ from phenylsilane was studied using Fourier transform ion cyclotron resonance spectrometry and by ab initio calculations. The (parent - H or D)(+) ions were produced by electron impact (EI) dissociation of C6H5SiH3 and C6H5SiD3, and their bimolecular reactivities toward phenylsilane, cycloheptatriene, benzyl chloride, benzene-d(6), and toluene-d(8) were examined. The reactive component that abstracts hydride from phenylsilane and cycloheptatriene or chloride from benzyl chloride is mostly the phenylsilyl cation C6H5SiH2+. The identity of the unreactive component was characterized by collision-induced dissociation and bimolecular chemical reactivity. The low-energy collision-induced dissociation of the unreactive C6SiH7+ ion with argon yielded SiH+ with a loss of C6H6 and C6SiH5+ with a loss of H-2 as the primary fragments. Vibrationally hot C6SiH7+ ions from EI of C6H5SiH3 reacted with benzene-d(6) to form C6D6 . SiH+ adducts, which after few seconds of cooling delay remained unreactive toward cycloheptatriene. Ab initio calculations predict the hydride affinity of C6H6 . SiH+ that forms C6H6 + SiH2 to be comparable to or lower than that of tropylium ion. The chloride affinity of C6H6 . SiH+ that forms C6H6 + SiHCl is estimated to be similar to 4 kcal mol(-1) lower than that of the benzyl cation. Both experiment and theory suggest the C6H6 . SiH+ adduct as the unreactive C6SiH7+ component and not the silacycloheptatrienyl cation. The mechanism of the formation of C6H6 . SiH+ is presented based on the theoretical energetics of radical cations and the transition state for the [1,2] sigmatropic migration of an alpha-H.X1121sciescopu
Time-resolved photodissociations of iodotoluene radical cations
No full textUnimolecular dissociations of o-, m-, and p-iodotoluene radical cations that yield C7H7+ by loss of an iodine atom have been studied by time-resolved photodissociation (TRPD) spectroscopy using Fourier transform-ion cyclotron resonance mass spectrometry. Iodotoluene radical cations were prepared by the phoionization charge-transfer (PICT) method. The TRPD spectra were obtained in the wavelength range 480-575 run. The rate-energy data from the present work are combined with the previous rate data obtained in a higher internal energy range to extract activation parameters for unimolecular dissociations. For the direct C-I cleavage channel that leads to tolyl cations, the entropy changes for activation are assumed to be identical to that of the vinylic C-I cleavage of the iodobenzene radical cation, and activation barriers are estimated from thermochemical thresholds. For the lowest barrier rearrangement channel that leads to the benzyl cation, the entropy changes are assumed to be identical to those of the corresponding bromotoluene radical cations. Activation barriers for the rearrangement channel are determined to be 1.77, 1.88, and 1.90 eV for o, m, and p isomers, respectively. Both the product structures and activation parameters for the lowest barrier rearrangement process lend support to the McLafferty mechanism that invokes the H-atom migration from the alpha to the ipso- or ortho-carbon site and the subsequent H-atom and/or CH2-group migration on the ring to form benzyl precursors. The effects of halogen substitution (X = Cl, Br, and I) on the rearrangement process are discussed.X111213sciescopu
NONLINEAR OSCILLATIONS IN ION CYCLOTRON RESONANCE
No full textNonlinear equations of motion of an ion in a quartic electrostatic potential are solved using a perturbation method. A valid region of the quartic potential in an cubic ion trap is determined from the numerical comparison with the exact potentials. The quartic potential introduces the coupled cubic nonlinear forces to the equations of motion. Cubic nonlinearities that couple the axial and radial motions yield amplitude-dependent frequency shifts. Nonlinear solutions are derived to first order. The ion trajectory within the valid region of the quartic potential reproduces the essential features of the exact numerical results. The amplitudes of nonlinear oscillations are derived and their mass dependences are compared. The Fourier transform ion cyclotron resonance spectra are compared for the linear and nonlinear oscillations. The collisional damping is included as a perturbation in the nonlinear equations of motion. Collisions damp the cyclotron and axial oscillations but amplify the magnetron motion. The amplitude-dependent frequencies of nonlinear oscillations become time dependent due to the collisional damping of oscillation amplitudes. The collisional line broadenings of the nonlinear oscillation trajectories are compared with those of the linear oscillation trajectories.X119sciescopu
SPACE-CHARGE EFFECTS ON FOURIER TRANSFORM ION CYCLOTRON RESONANCE SIGNALS: EXPERIMENTAL OBSERVATIONS AND THREE-DIMENSIONAL TRAJECTORY SIMULATIONS
No full textSpace-charge effects were studied by monitoring Fourier transform ion cyclotron resonance spectra while scanning the laser wavelength near the origin of a two-photon resonant 3s <-- n Rydberg transition of acetaldehyde. The rotational contour of the origin band permits the experimental control of space-charge density. Both the frequency shift and the inhomogeneous line broadening were observed as a function of space-charge density. Three-dimensional ion trajectories in the presence of Coulomb interactions between ions were simulated under the quadratic and exact trapping potentials. The simulated Fourier transform ion cyclotron resonance spectra were obtained from the image-charge signals induced by a uniform field of chirp or impulse excitation. Comparisons of experiments with three-dimensional simulations reveal that the inhomogeneous line broadening observed in experiments is most likely due to both large-amplitude oscillations of ions and Coulomb interactions between different m/q ions. (C) 1997 American Society for Mass Spectrometry.X1112sciescopu
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