Structure Of Molecular Ions Research Articles

The “Coulomb-explosion” technique for determining geometrical structures of molecular ions

Traditional experimental techniques (e.g. studies on photon absorption or emission) for determining the stereochemical structures of neutral molecules are extremely difficult to apply to molecular ions because of problems in obtaining a sufficient spatial density of the ions to be studied. Recent high-resolution measurements on the energy and angle distributions of the fragments produced when fast (MeV) molecular-ion beams from an electrostatic accelerator dissociate (“Coulomb explode”) in thin foils and in gases, offer promising possibilities for deducing the stereochemical structures of the molecular ions constituting the incident beams. Bond lengths have been determined in this way for several diatomic projectiles (H 2 +, HeH +, CH +, NH +, OH +, N 2 +,O 2 +, etc.) with an accuracy of ∼0.01 Å. H 3 + has been demonstrated (for the first time) to be equilateral triangular and the interproton distance measured. Measurements on single fragments from CO 2 +, N 2O +, C 3H 3 +, and CH n + have revealed the gross structures of the projectiles. An apparatus has recently been constructed at Argonne to permit precise measurements on fragments in coincidence. The apparatus has been tested on a known structure (OH 2 +). The OH bond length was found to be 1.0 ± 0.04 A ̊ and the HOH bond angle was measured as 110 ± 2°. These values are in excellent agreement with those found in optical experiments (0.999 Å and 110.5°). This “Coulomb explosion” technique can be expected to be refined in accuracy and to be extended to a wide range of molecular ions whose structures are inaccessible by other means.

Physics with Fast Molecular-Ion Beams

Fast (MeV) molecular-ion beams provide a unique source of energetic projectile nuclei which are correlated in space and time. The recognition of this property has prompted several recent investigations of various aspects of the interactions of these ions with matter. High-resolution measurements on the fragments resulting from these interactions have already yielded a wealth of new information on such diverse topics as plasma oscillations in solids and stereochemical structures of molecular ions as well as a variety of atomic collision phenomena. The general features of several such experiments will be discussed and recent results will be presented.

Determining the stereochemical structures of molecular ions by "Coulomb-explosion" techniques with fast (MeV) molecular ion beams

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDetermining the stereochemical structures of molecular ions by "Coulomb-explosion" techniques with fast (MeV) molecular ion beamsDonald S. GemmellCite this: Chem. Rev. 1980, 80, 4, 301–311Publication Date (Print):August 1, 1980Publication History Published online1 May 2002Published inissue 1 August 1980https://doi.org/10.1021/cr60326a002RIGHTS & PERMISSIONSArticle Views330Altmetric-Citations91LEARN 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 (7 MB) Get e-Alerts Get e-Alerts

Determination of molecular-ion structures through studies of the collisionally induced dissociation of fast (MeV) molecular ions

High-resolution studies of the angle and energy distributions of fragments detected in coincidence from the foil- and gas- induced dissociation of fast (MeV) polyatomic molecular ions are described. The aim of the measurements is to explore the possibilities that these new techniques offer for determining the stereochemistry of molecular ions.

On the formation of cyclopentenium cations from all C6H10+• molecular ion structures

The structures of [C5H7]+ ions produced by methyl loss from thirty isomeric C6H10 molecular ions have been assessed from measurements of ionic heats of formation and the kinetic energy release associated with the fragmentation. It is proposed that the thermodynamically most stable [C5H7]+ ions [cyclopentenium]+ are formed at the dissociation threshold irrespective of the initial molecular ion structure. Mechanistic aspects of the fragmentations were studied by mean of deuterium labelling.Observations of metastable peak shapes for labelled compounds permitted the identification of reaction pathways having different energy requirements. The lowest energy routes involved little or no positional mixing of H and D prior to fragmentation. It is proposed that the molecular ions with sufficient energy to fragment on the μs time scale undergo successive (and reversible) isomerization by allylic 1,3-hydrogen shifts to their isomeric isolated diene molecular ions which, by an allylic 1,2-hydrogen shift, may isomerize to a methylcyclopentene molecular ion prior to loss of methyl. The sequence of isomerization of the molecular ions is alkyne → allene → conjugated diene → isolated diene. The de-conjugation step has an activation energy of ca. 10 kcal mol−1. We suggest, however, that allene molecular ions rearrange directly to isolated diene molecular ions.

The structure of molecular ions studied with fast (MeV) heavy projectiles

Fast molecular ions excited in collisions with solid or gaseous targets loose most of their binding electrons and the remaining molecular fragments are then strongly repelled by their mutual Coulomb force. High resolution measurements on the fragments arising from this “Coulomb explosion” show promise as an unique technique to determine the geometrical structure of molecular ions. Similar information may be obtained studying the influence of the “Coulomb explosion” on the Auger decay of electronic states in the fragments of the molecular ion projectiles.

Program realizing the INDO method in a basis of up to 232 orbitals with the inclusion of d orbitals

Semiempirical methods of quantum chemistry provide a powerful instrument for studying the structure and properties of complex molecules and molecular ions. Of these methods, the most widely used is the CNDO method, one of the simplest and most reliable [i]. At the same time, the use, in this method, of the averaged values of the one-center Coulomb integrals, together with the underestimation of the role of exchange correlation of the electrons, leads to the energetic equivalence of atomic states of the same configuration with different multiplicities. The underestimation of the exchange correlation is revealed in the incorrect description, in the CNDO method, of the spin density distribution when the unrestricted Hartree--Fock method is used. The INDO method, for which the complexity of the calculations is comparable with that of the CNDO method, does not have these unsatisfactory features [2]. A number of workers in the USSR [3, 4] have now compiled programs which use the CNDO method for elements of the second period in a comparatively small basis.

INTERACTIONS OF SWIFT H, H+2 AND H+3 WITH THIN FOILS

INTERACTIONS OF SWIFT H, H⁺₂ AND H⁺₃ WITH THIN FOILS

A photoionization study of the [C7H8]\documentclass{article}\pagestyle{empty}\begin{document}$\mathop +\limits_.$\end{document} ion formed from some monosubstituted alkyl benzenes

AbstractAppearance potentials for the [C7H8]\documentclass{article}\pagestyle{empty}\begin{document}$\mathop +\limits_.$\end{document}ion produced in the fragmentation of n‐butyl benzene, iso‐butyl benzene and n‐pentyl benzene have been measured by photon impact. The results indicate that, at threshold, the fragment ion has the toluene molecular ion structure.

Spectra of Molecular Ions

AbstractHistorically the study of the spectra of diatomic molecular ions has been an important contributor to the development of molecular orbital theory. In view of the importance of molecular ions for radiation chemistry on the one hand and astrophysics (especially the study of the interstellar medium) on the other, we have started a program aimed at the discovery of new molecular ion spectra and at a better understanding of the structure of molecular ions. The various experimental techniques that have been used will be described. Particular attention has been focussed on the study of absorption spectra of molecular ions, since these absorption spectra would allow the study of those excited states which are subject to predissociation and therefore not easily observed in emission work. However, up to now only very few new spectra have been observed in absorption. On the other hand a number of new emission spectra of both diatomic and polyatomic molecular ions have been observed and the results about the structures of these ions will be discussed.

Electronic Structures of Some Molecular Ions by the Unrestricted CNDO/2 and INDO Methods

Electronic structures of simple molecular ions are calculated by the unrestricted CNDO/2 and INDO methods to show that organic chemical expressions for these ions are well reproduced. The conformational dependence of the strength of the β-bond on the heteroatom with the unpaired electron is discussed.

Mass spectrometry of tautomeric compounds. Part IV. Structure of the molecular ions of 4-hydroxycoumarins

The structure of the mass spectral molecular ions of 4-hydroxycoumarins is discussed. From the fact that variation of inlet temperatures did not affect the spectral patterns, the molecular ion is considered not to be an equilibrium mixture of keto and enol forms. In contrast to earlier proposals, the enolic nature of the molecular ion is demonstrated by comparison of the spectra with those of acetylated derivatives and by deuterium-labelling experiments.

Faraday Lecture. Spectra and structures of molecular ions

The first page of this article is displayed as the abstract.

The structure of some trapped hole centres in CaF2. II

Electron spin resonance studies have been made of trapped hole centres with an F2- molecular ion structure in CaF2. x-irradiation of CaF2 at 77 °K produces self-trapped hole centres. These become mobile at higher temperatures and may be trapped at special sites in the lattice to form other, relatively stable, F2- molecular ions with axes near , and directions. The molecular ion which is stable at room temperature has a symmetry which is lower than that of the self-trapped hole and this is shown to be consistent with the hole being trapped adjacent to the site of a substitutional Na+ ion. The case is treated in detail and is found to give an axially symmetric F2- molecular ion in which the probabilities of locating the hole near each of the two nuclei are unequal. This is shown to be consistent with the hole being trapped at the site of an interstitial anion. Activation energies for thermal annealing of these centres and for the self-trapped holes have been found by pulse annealing to be, respectively, 055 ± 010 and 013 ± 002 ev.

Electron impact-induced rearrangements of substituted 1-phenylpropynes

The primary ion mass spectra of the C9H8 isomers indene and 1-phenylpropyne were remarkably similar. Examination of ion and metastable peak intensities and their energy spectrum indicate that their common M-1 (C9H7+) fragment ion has the same structure. The mass spectra of several substituted phenylpropynes were also similar to that of the parent hydrocarbon below mass m/e 115 but showed no substituent or kinetic effect. The mass spectra of 1-phenylpropyne-d3 clearly shows H/D randomization in the molecular ion and linear ion structures are proposed.

The mass spectra of carboxylic acids—III: The structures of molecular and fragment ions in benzoic acid and related molecules

The mass spectra of benzoic acid and its carboxyl-deuterated derivative have been studied in detail. Molecular and fragment ion structures are proposed and the role of ortho hydrogen atoms in rearrangements is discussed. The behaviour under electron-impact of phthalaldehydic acid and its carboxyl deuterated derivative provides evidence in support of ion structures in the benzoic acid fragmentation. From these results the fragmentation mechanism for phthalic acid is further elucidated. The mass spectrum and fragmentation mechanism of thiobenzoic acid is described and compared with that of benzoic acid.

The structure of some trapped hole centres in CaF2. - I

Electron spin resonance studies have been made of trapped hole centres with an F2- molecular ion structure in CaF2. x-irradiation of CaF2 at 77 °K produces self-trapped hole centres. These become mobile at higher temperatures and may be trapped at special sites in the lattice to form other, relatively stable, F2- molecular ions with axes near , and directions. The molecular ion which is stable at room temperature has a symmetry which is lower than that of the self-trapped hole and this is shown to be consistent with the hole being trapped adjacent to the site of a substitutional Na+ ion. The case is treated in detail and is found to give an axially symmetric F2- molecular ion in which the probabilities of locating the hole near each of the two nuclei are unequal. This is shown to be consistent with the hole being trapped at the site of an interstitial anion. Activation energies for thermal annealing of these centres and for the self-trapped holes have been found by pulse annealing to be, respectively, 055 ± 010 and 013 ± 002 ev.

Laboratory studies of electron attachment and detachment processes of aeronomic interest

Techniques for the study of electron attachment and detachment are reviewed. The rate coefficients for the various processes of aeronomic interest are then discussed. The rates of three-body and dissociative attachment by thermal electrons have been successfully determined by swarm techniques and by high frequency studies of electrons produced by high energy particles and by photoionization. Collisional and associative detachment rates for thermal energy negative ions have been measured using the swarm and flowing afterglow techniques. Radiative attachment rates for some atmospheric negative ions have been calculated from measurements of photodetachment cross sections using crossed photon and ion beam techniques. Electron beam studies and measurements of ion kinetic energy have provided much useful information regarding the dissociative attachment process and the structure of molecular negative ions. Rate coefficients for low energy processes such as the three-body attachment to O2, the radiative attachment to O, and the associative detachment of O− in collisions with various atmospheric gases are reasonably well known. Other possibly important low energy processes, such as dissociative attachment to O3, radiative attachment to O2, and the associative detachment of O2− are less well known.

The structure of the self-trapped hole in KMgF 3

Optical and electron spin resonance studies have been made of the self-trapped holes produced by irradiation of KMgF3 at 77°K. These are found to have the F2- molecular ion structure. Because of the low symmetry of anion pairs in the perovskite lattice, none of the principal axes of the tensors describing the hyperfine interaction with each of the two nuclei lie along the molecular axis. Thus the molecular ion has a `bent bond' configuration similar to that found for VF centres in LiF. The inclination of the axes of the ground state orbitals to the molecular axis is found to be 7°.