Statistical Theory Of Mass Spectra Research Articles

Fragmentation of Energy-Selected Fluorobenzene Ion

Abstract Dissociation of the fluorobenzene ion has been studied using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown curves were measured over internal energies from 0 to 9 eV. Asymmetric flight time distributions due to metastable dissociation were observed for the formations of C6H5+ and C4H3F+, and their absolute decay rate constants were derived from their flight time distributions. The information-theoretical approach has been employed for the analysis of kinetic energy release distribution (KERD). The results of surprisal analysis demonstrated that experimental KERDs are in good agreement with the predictions based on the statistical theory of mass spectra (QET). The heat of formation of C4H3F·+ has been estimated to be 222±2 kcal mol−1.

Collision energy effects in tanden mass spectrometry as revealed by a proton‐bound dimer ion

AbstractMass spectra resulting from collision‐induced decomposition of the proton‐bound dimer of iso‐propylamine and sec‐butylamine have been obtained as a function of laboratory collision energy over the range 10‐6000 eV. The ratio of the two principal fragment ions from the dimer ion measured as a function of collision energy is compared with the ratio expected as a function of internal energy as calcualted based on the statistical theory of mass spectra. This comparison indicates that the average energy deposited into the dimer ion upon collision reaches a maximum at a collision energy of ∼70 eV. The average internal energy of the ions at this collision energy is ∼4.3 eV. Other fragment ions which arise from higher energy decompositions are also observed in the spectra at much lower intensities. The relative intensities of these fragments indicate that the probability for large energy transfers are highest at ke V collision energies. These observations are interpreted on the basis of differences in the postcollision internal energy distributions resulting from keV and eV collisions.

Comparative Test of Approximations for Calculation of Energy-Level Densities

Two molecular models, one representing a “small” molecule and the other a “large” molecule, both with several free rotors, are used to test, by comparison with exact count, seven approximation formulas for the calculation of density of harmonic vibration–rotation states, at energies between 1000 and 20 000 cm−1. The techniques of the classical approach, the Laplace transform, steepest descents, and symmetric functions are represented among the seven formulas. Also shown is the effect of the various approximations on the energy dependence of the computed unimolecular rate constant, and on ion breakdown curves calculated by the statistical theory of mass spectra. It is concluded that the techniques of the Laplace transform [J. Chem. Phys. 46, 3736 (1967)] and steepest descents, as formulated in an Appendix, yield the best all-around approximation, while a semiclassical-type formula [J. Chem. Phys. 41, 1883 (1964)] requires least computational effort and still gives excellent results in most cases. In an Appendix, the method of steepest descents is generalized and its connection with the Laplace transform noted.

Photoionization of Ethane, Propane, and n-Butane with Mass Analysis

Relative photoionization cross-section curves have been determined for parent and fragment ions of ethane, propane, and n-butane over the photon-energy range from the ionization potential to 14.0 eV. The data have been treated to give experimental breakdown curves which are in good qualitative and fair quantitative agreement with the predictions of the statistical theory of mass spectra. The results are also compared with data obtained by charge exchange, electron impact, and photoelectron energy analysis by other workers. Heats of formation of some ions have been obtained more accurately and the ionization potential of the C2H5 radical is confirmed to be 8.4 eV. Ion-pair formation of H− is shown to occur at least for ethane and n-butane and very probably for propane.

Application of the statistical theory of mass spectra to the decomposition of C2H6+ and C2D6+

Application of the statistical theory of mass spectra to the decomposition of C2H6+ and C2D6+

Theoretical Calculations of Mass Spectra: Fragmentation of C6–C8 Alkane Ions

An improved statistical theory of mass spectra is here applied to the fragmentation of five alkanes: n-hexane, 2-methylpentane, 3-methylpentane, n-heptane, and n-octane. The calculated breakdown curves are compared with the experimental curves obtained from photoionization studies by Steiner, Geise, and Inghram. The major fragmentations, the metastable ions, and the ``missing metastable'' ions are computed for gas temperatures of 0° and 300°K. The improved quasiequilibrium theory yields satisfactory breakdown curves for the above molecules using physically reasonable activated complexes and activation energies, and without requiring introduction of artificial parameters such as an ``effective number of oscillators.''

Unimolecular Decomposition of Negative Ions Formed from the Transition-Metal Carbonyls of Ni, Fe, Cr, Mo, and W

Negative-ion mass spectra and dissociative electron-capture energetics for the principal metal-containing ions have been determined for nickel tetracarbonyl, iron pentacarbonyl, chromium hexacarbonyl, molybdenum hexacarbonyl, and tungsten hexacarbonyl. These data are utilized in the first application of the statistical theory of mass spectra to describe the unimolecular decompositions of negative ions. The mechanism proposed for the formation of the various M(CO)x− ions is an ion-pair production process followed by a series of consecutive ionic-unimolecular decomposition reactions involving the elimination of CO groups.

Appearance Potentials and Mass Spectra of Fluorinated Ethylenes. III. Calculations Based on the Statistical Theory of Mass Spectra1

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAppearance Potentials and Mass Spectra of Fluorinated Ethylenes. III. Calculations Based on the Statistical Theory of Mass Spectra1Chava Lifshitz and F. A. LongCite this: J. Phys. Chem. 1965, 69, 11, 3737–3740Publication Date (Print):November 1, 1965Publication History Published online1 May 2002Published inissue 1 November 1965https://doi.org/10.1021/j100895a014RIGHTS & PERMISSIONSArticle Views29Altmetric-Citations13LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (421 KB) Get e-Alerts

Kinetic energy of fragment ions in a radiofrequency mass spectrometer

A radiofrequency mass spectrometer has been used, with slight modification, to measure the kinetic energy of fragment ions from polyatomic molecules. Selected ions from CO 2 , CH 3 OH, C 2 H 5 OH, C 3 H 7 OH, i-C 3 H 7 OH, CH 3 I and C 2 H 5 I were examined for excess kinetic energy by means of retarding potential analysis before mass analysis. The excess kinetic energy for any given ion was always greater for 70 V electron impacts than for 50 V. Such ions as m/g 31 and 29 from ethanol have an excess of about 0·45 V kinetic energy for 50 and 70 V electrons; m/q 29 and 27 from isopropanol also have about 0·45 excess energy for 70 V electrons; CH + 3 ions from the alcohols and from the iodides showed excess kinetic energy while m/q 27 from ethyl iodide had 0·9 V excess energy for 70 V electrons. The results are interpreted as evidence of a significant num ber of dissociations from electronic states which are not in effective competition with one another. The existence of non ­ competing dissociation paths for a given molecular ion complex would preclude the application of the statistical theory of mass spectra to predict the breakdown scheme of that complex.

Die kinetische Energie ionisierter Molekülfragmente<br />III. Quasithermische Ionen von einfachen Paraffinen

From the monotonic increase of the average initial energy of paraffin fragment ions formed by electron impact the existence of unimolecular dissociation sequences is concluded as it is assumed by the statistical theory of mass spectra. For primary decomposition steps the total kinetic energies set free during the respective dissociation processes (transition energies) have been deduced from measured initial energies. The transition energies obtained in this way may be compared with theoretical data for the translational energies in the transition states (statistical energies) as calculated by means of the statistical theory of mass spectra. In case of—C bond cleavage theoretical data are close to the experimental values (≈ 0.1 ev). In case of a C—H bond cleavage, however, theoretical values are always lower than the experimental ones. In rearrangement reactions an activation energy for the reverse process may exist, which should show up, at least partly, in the transition energy. For a primary H2-abstraction process the activation energy for the reverse process aE can be calculated from thermochemical data. The observed transition energies are always lower than the calculated values. A C—C skeleton rearrangement also shows some influence of aΕ on the transition energy. The absolute effect, however, is small—less than 0.1 ev. For hydrogen shift reactions no indication of an aE-contribution was found.

Energy Distribution and Fragmentation Processes Resulting from Electron Impact on Propane and n-Butane

The internal energy distribution has been determined for propane and n-butane ions produced by electron impact. The method involves the use of differential ionization efficiency curves and depends on the validity of the linear threshold law for simple ionization. The experimental distributions are in fair agreement with theoretical calculations of Lennard-Jones and Hall and indicate that the major fraction of the parent ions are formed by simple removal of a valence electron. Only a minor fraction of the parent ions are apparently formed by removal of one valence electron together with excitation of one or more remaining electrons. Breakdown curves for propane and n-butane have been determined experimentally and, except for the energy scales, are in fair qualitative agreement with the predictions of the statistical theory of mass spectra. The discrepancy between theoretical and experimental energy scales is ascribed at least in part to inadequacy of the theoretical rate equations used in the statistical theory. There is no evidence for gross failure of the assumptions of the theory regarding energy randomization although the number of crossings of potential surfaces of the molecular ion is shown to be very much smaller than originally assumed by Rosenstock et al.

On the Classical Approximation in the Statistical Theory of Mass Spectra

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Interpretation of the Appearance Potentials of Secondary Ions

The statistical theory of mass spectra assumes that the excitation energy in a molecule-ion is essentially equipartitioned prior to unimolecular decomposition. Some of the consequences of this assumption have been deduced and the results have been used in interpreting the observed appearance potentials of secondary and tertiary ions of neopentane, n-butane, and 1-butene. Although the results are not quantitative, it is evident that considerable vibrational energy is removed with the neutral fragment. The problem of the detailed interpretation of appearance potentials is complicated by the fluctuation of the vibrational energy distribution in the products of a decomposition from the most probable value, i.e., equipartition. Some general considerations with regard to determination of minimum-energy decomposition paths and the significance of this study in relation to the statistical theory of mass spectra are also presented.

Statistical Theory of Mass Spectra: Difficulties in Interpretation of Spectra of the Lower Alkanes

Mass spectra of a group of normal alkanes, C4 through C8, have been studied as a function of ionizing voltage. Appearance potentials have also been measured for most of the ions. At all voltages the yields of the ion fragments produced by loss of hydrogen atoms and by loss of methyl radicals are surprisingly low relative to their appearance potentials. Since for the lower ionizing voltages it can safely be assumed that there are no significant contributions to the patterns from secondary decompositions, these low yields must indicate a low intrinsic probability for the primary processes by which these ions are formed. For the typical case of n-pentane, the experimental data have been analyzed from the standpoint of the statistical theory of mass spectra and it is found that frequency factors in the ratio of 1:104:108 must be proposed to account for the yields from the hydrogen atom:methyl radical:ethyl radical extraction processes at low ionizing voltages. The implications of this wide range of frequency factors are discussed briefly.

Study of the Mass Spectra of the Lower Aliphatic Alcohols

Mass spectra and appearance potentials have been determined for methanol, ethanol, and propanol. These data are used in semiempirical application of the statistical theory of mass spectra. The theory facilitates assignment of plausible mechanisms for the production of the various ion fragments. It is possible to obtain computed spectra in agreement with experimental data using an internally consistent set of structural parameters.

Ionization Efficiency Curves and the Statistical Theory of Mass Spectra

Ionization Efficiency Curves and the Statistical Theory of Mass Spectra