W.J. van der Hart

Excitation of the z-motion of ions in a cubic icr cell

Abstract In addition to previous discussions of the ejection of ions in the direction of the trapping plates by excitation of the cyclotron motion, it is shown that ion ejection also occurs by excitation at 2ω T , twice the trapping frequency, and at 2ω T plus the cyclotron frequency. In all cases, the observed effects can be explained by a simplified expression for the equation of motion in the z -direction.

Excitation of the z motion of ions in cubic and elongated ion cyclotron resonance cells

Abstract From recent studies, it is well known that excitation of ions by a radiofrequency voltage on the transmitter plates of a cubic ion cyclotron resonance (ICR) cell can lead to unwanted ion losses by excitation of the z motion parallel to the magnetic field. In this work it is shown that, in principle, these problems can be solved using an elongated cell. However, at high magnetic field strengths, very long cells will be required. In addition, even an elongation of the cell by only 50% results in an anharmonic trapping well where the trapping frequencies are highly dependent on the z amplitude. Possible solutions using a screened ICR cell are discussed.

Isomerization of molecular ions of alkenes and cycloalkanes studied by photodissociation in an ICR spectrometer

Photodissociation spectra and ion—molecule reaction kinetics of the molecular ions of all C4-, C5- and some of the C6-alkenes and cyclopropane, cyclobutane and cyclohexane are discussed. From the spectra it is deduced that cyclobutane molecular ions ring open directly upon ionization, whereas for the molecular ions of cyclopropane and cyclohexene a mixture of cyclic and acyclic ions is formed. The fraction of C3 H6-+ ions from cyclopropane that isomerize to the propene structure appears to be strongly dependent on the energy of the ionizing electrons. The three linear butene ions cannot be distinguished by the photodissociation spectra alone; isobutene molecular ions retain their original structure upon ionization. For some of the C5- and C6-alkenes studied it follows from both the photodissociation spectra and the kinetics of ion—molecule reactions, that upon ionization two isomeric molecular ions with strongly different reactivity and photodissociation spectra are formed. In all cases, the reactivity ions have a larger photodissociation rate in the visible region than the less reactive ions. Some possibilities in relation to the nature of these two ions are discussed. No straightforward explanation can be given for the observed phenomena. An explanation in terms of differences both in the position of the double bond and the sterical structure (non-planarity of the double bond as well as rotation of single bonds in the terminal ethyl-group) must presumably be taken into consideration.

The structures of C2H4O+ ions: Evidence suggesting the occurrence of radiative relaxation of vibrationally excited ethylene oxide molecular ions

Abstract C 2 H 4 O + ions from various sources have been studied by photodissociation in an ICR spectrometer. Molecular ions of ethylene oxide ring open directly after ionization. The radiative relaxation lifetime of the resulting vibrationally excited ions is ≈ 350 ms. Similar effects are observed for the C 2 H 4 O + ions from 1,3-dioxolan.

Ab initio molecular orbital calculations on 1,2-hydrogen shifts in the ethene, allene and propyne radical cations

Abstract MRCI//ROHF/6-31G ∗∗ ab initio calculations on the 1,2-hydrogen shift in the ethylene radical cation show that the CH 3 CH + product ion is not stable. CH 2 CHCH + obtained by a 1,2-hydrogen shift from the propyne or the allene radical cation is only stable because the ion can relax to a more stable structure, which can be described as an allyl cation with one of the terminal hydrogen atoms removed.

Photodissociation and ion/molecule reactions of C6H6+. ions studied by rapid scan and Fourier transform ion cyclotron resonance

Abstract C 6 H 6 +. ions from different neutral precursors were studied by photodissociation and by reactions with 2-propyl iodide and hexadeuterobenzene in rapid scan and Fourier transform ion cyclotron resonance spectrometers. Ejection of all ions from the ICR cell, except ions of one particular mass, proved to be essential for the ion/molecule reaction studies. From those studies it was concluded that fulvene ions are stable towards isomerization, whereas C 6 H 6 +. ions from 1,5-hexadiyne have the benzene structure. In all other cases, a mixture of at least two or three (for dimethylene cyclobutene) different ion structures was found. One of these ion structures corresponds to the structure of the parent neutral molecule. Also, in all cases, part of the ions isomerize to a non-photodissociating structure. The experimental results indicate that these ions have the benzene structure.

An ICR study of sequential two-photon dissociation of benzene, cyanobenzene and bromobenzene molecular ions

Abstract Two-photon photodissociation kinetics for the molecular ions of benzene, cyanobenzene and bromobenzene is discussed and compared with previous results. In all three cases the lifetime of the intermediate excited state is much shorter than reported before. For bromobenzene it appeared that photo-dissociation does not arise from two-photon processes only. Instead, a gradual increase of the fraction of single-photon photodissociation with decreasing wavelength through the visible band is observed.

Theoretical investigations of the nature of intramolecular interactions

The description of intramolecular interactions by a single-determinant wave function of non-orthogonal, strictly local, molecular orbitals (NOLMOs) is improved by the admission of electron charge transfer into anti-bonding orbitals. An energy decomposition scheme in terms of quasiclassical (overlap independent), interference (overlap dependent) and charge transfer components is outlined. The formalism is applied to the study of the barrier to internal rotation in ethane. It is found that vicinal CH-CH′ interactions and non-additivity effects on geminal CH-CH interactions constitute the two major contributions to the barrier.

The isomerization of C6H6 radical cations. A comparison of photodissociation results with semi-empirical molecular orbital calculations

Abstract Semi-empirical calculations on the isomerization of C6H +6 radical cations give a clear picture of the relevant processes, which is in good agreement with previous experiments. For the 1,3-hexadien-5-yne radical cation the barrier for isomerization to the benzene structure is well below the dissociation limit. For all other linear benzene isomers the barrier is much closer to the dissociation threshold but sufficiently low for an isomerization of part of the ions to stable benzene ions. The very low barrier for isomerization of the 1,5-hexadiyne radical cation to the benzene structure is ascribed to the very weak-nature of the central bond in this ion which leads to a low-energy isomerization to the 1,2,4,5-hexatetraene structure and from there to the benzene structure. The calculations furthermore show that many non-classical ion structures have heats of formation comparable to or lower than that of classical ion structures. The calculated barrier for an isomerization to the fulvene structure shows that the barrier for carbon scrambling in the benzene ion is well below the dissociation limit.

Ab initio calculations on the isomerization of C4H4 radical cations

Abstract Ab initio calculations at the MRCI//ROHF/6-31G∗∗ level on the possible isomerization pathways of C4H4 radical cations give a clear explanation of experimental results obtained from neutralization-reionization mass spectrometry and from photodissociation experiments. The observed photon-induced isomerization of vinyl acetylene radical cations to the methylene cyclopropene structure is in excellent agreement with the calculated results. It is suggested that the methylene cyclopropene radical cation is not a primary product of fragmentation of C6H6 precursor ions but is formed via a low-energy isomerization of ions having a non-classical structure. This isomerization presumably takes place within the ion/molecule complex before the fragments go apart.