Hydration of highly charged ions

Based on a series of ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulations, the broad spectrum of structural and dynamical properties of hydrates of trivalent and tetravalent ions is presented, ranging from extreme inertness to immediate hydrolysis. Main group and transition metal ions representative for different parts of the periodic system are treated, as are 2 threefold negatively charged anions. The results show that simple predictions of the properties of the hydrates appear impossible and that an accurate quantum mechanical simulation in cooperation with sophisticated experimental investigations seems the only way to obtain conclusive results.     

Mass spectra of doubly charged ions

The mass spectra of biological molecules, whose molecular mass exceeds 10 kDa, invariably contain multiply charged ions. For example, a survey scan of a small protein will produce singly, doubly and triply protonated molecules, the intensity of the doubly charged species often being greater than that of the singly charged entity. Although the spectra resulting from doubly charged peptides have not previously been studied, collisional activation of such doubly charged species may result in significant additional information pertaining to molecular structure. The techniques employed to study ions originating from multiply charged species were linked scanning of constant B/E and tandem mass spectrometry, namely low collision energy spectra acquired on a BEQQ hybrid instrument. The methodology was applied to model compounds whose tandem mass spectrometry characteristics are well known, e.g. gramicidin S and angiotensin I. The results for the product ions of the [M + 2H]²⁺ species of the models were obtained which highlight the methodology required for high-mass materials.     

Multiply charged ions in the mass spectra of aromatics

The prominence of multiply charged molecular and fragment ions upon electron-impact in the mass spectrometer is proposed as an experimental, empirical indication of aromatic character. The effects of electron withdrawing and donating substituents on the production of multiply charged ions are considered and appearance potentials are noted for several species.     

Disentangling proton connectivity networks in highly overlapped 1H-NMR spectra of polynuclear aromatics using 1D-HOHAHA

One-dimensional ( 1D ) variants of two-dimensional ( 2D ) nmr techniques can frequently provide the required information in a much shorter period of time than the 2D experiment from which they were derived. Application of the 1D homonuclear Hartmann-Hahn (1D-HOHAHA) experiment to establish proton-proton connectivity networks in highly overlapped four-spin systems in the proton nmr spectra of polynuclear aromatics is described. Selective subspectra are contrasted to the data obtained in a COSY experiment on the same molecule, benzo[f][l]benzothieno [2,3 -c]quinoline. Results from the 1D-HOHAHA technique are especially useful when component resonances from several spin systems are heavily overlapped. Relayed 1D-HOHAHA provides the means of exploiting small, long range coupling pathways of polynuclear aromatics.     

Apparent proton affinities of highly charged peptide ions

The apparent proton affinities (PA) of various charge states of three highly basic peptides [(KAP)₁₀, (KAP)₈, (KAA)₈] were measured by the “bracketing” method. For all three peptides the apparent PA decreases as the charge state increases and the magnitude of the decrease is consistent with an increase in coulombic repulsion in the highly protonated species. Based on a simple electrostatic model, theoretical PAs were predicted for each charge state and the values for (KAP)₁₀ and (KAP)₈ were within 10 kcal/mol of the experimental values. The maximum charge state of these peptides was observed in all cases even when the most volatile solvent was sufficiently basic to deprotonate that charge state in the gas phase. In solution (KAP)₈ exhibits a random coil secondary structure while (KAA)₈ exhibits an α-helix structure. Comparison of measured and calculated apparent PAs suggests that (KAP)₈ retains its solution random coil structure in the gas phase and (KAA)₈ retains the solution compact α-helix structure in the lower charge states but opens up to a β structure in the gas phase to minimize electrostatic repulsions in higher charge states.     

Crystal optics for hard-X-ray spectroscopy of highly charged ions

A twin crystal-spectrometer assembly, operated in the focusing compensated asymmetric Laue geometry has been developed for accurate spectroscopy of fast highly charged heavy ions in the hard-X-ray region. Coupled to the focusing crystal optics is a specially developed two-dimensional position-sensitive X-ray detector which is necessary for retaining spectral resolution also for fast moving sources. We summarize the crystal optics and demonstrate the usefulness of the instrument for spectroscopy of both stationary and fast moving X-ray sources. Results are reported from several tests employing a ¹⁶⁹Yb gamma-ray source and the Lyman radiation of one-electron Pb⁸¹⁺ ions traveling at a velocity corresponding to β ≡ v/c ≈ 0.59. The features of the instrument presented may be useful in many applications where it appears difficult to make the leap from conventional X-ray energy measurements to wavelength-dispersive spectroscopy based on crystal optics.     

1H-NMR spectra of some ditriazolyls and ditriazolyalkanes

Summary The proton magnetic resonance spectra of 12 azoles were measured in neutral and acidic solvents. The protonation shifts observed by comparison of the spectra inDMSO-d ₆ andTFA were attributed to an amidinium type resonance of the resulting cations. The synthesis and characterization of the azoles are also discussed.

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¹H-NMR-Spektren einiger Ditriazolyle und Ditriazolylalkane

Zusammenfassung Es wurden die¹H-NMR-Spektren von 12 Azol-Verbindungen in neutralem (DMSO-d ₆) und azidischem (TFA) Lösungsmittel gemessen und die chemischen Verschiebungswerte verglichen. Der Unterschied beider Werte beruht höchstwahrscheinlich auf einer Amidinium-Typ Resonanz der im azidischen Bereich entstandenen Kationen. Im Rahmen dieser Arbeit wurden 12 Azol-Verbindungen synthetisiert und beschrieben.

     

Doubly charged fragment ions in the field ionization mass spectra of alkylbenzenes

It is shown that the radical [C₆H₅C_mH_2m]²⁺ fragment ions found in the field ionization mass spectra of alkylbenzenes are formed via a different adsorption state of the singly charged species than in the case of the formation of [M]²⁺ molecular ions. It is further demonstrated that the primary fragmentation of molecules by the cleavage of C? C bonds results not only from decompositions of molecular ions in the gas phase but also from surface reactions.     

Analysis of 1H-NMR spectra of various end-functionalized polyisobutylenes

¹H-NMR spectra of various telechelic (i.e., ～ CH₂C(CH₃)₂Cl, ～ CH₂C(CH₃)?CH₂, ～ CH?C(CH₃)₂, and ～ CH₂CH(CH₃)CH₂OH capped) polyisobutylenes (PIB) have been analyzed. The products were prepared by living carbocationic polymerization followed by end-group functionalization. Shielding and deshielding effects strongly influence the ¹H-NMR spectra of these products. Inductive effects (chlorine-ended PIBs), magnetically anisotropic end-groups (olefin groups and phenyl rings), allylic coupling (olefin end-groups), chirality (hydroxyl end-groups), and the interaction of these effects on the ¹H-NMR spectra are discussed. Numerous heretofore unidentified resonances have been assigned and better insight into the detailed structure of end-functionalized PIBs has been obtained. © 1994 John Wiley & Sons, Inc.     

Angular dependence of 1H-NMR and 2H-NMR spectra of water absorbed in cellulose film

¹H-NMR and ²H-NMR spectra of water (H₂O and D₂O) absorbed in Visking cellulose tubing have been observed as a function of the angle Θ between the film surface and the magnetic field. ¹H-NMR spectra show broad lines, and the chemical shift and the linewidth depend on Θ. From the angular dependence of the chemical shift, the anisotropic volume diamagnetic susceptibilities of the film are determined, i.e., χ_∥ in the direction longitudinal to the tubing (stretched direction) is 0.43 ppm, and χ_⊥ in the transverse direction is 0.57 ppm. The different values of χ_∥ and χ_⊥ afford an evidence of the anisotropy of the film. ²H-NMR spectra of D₂O absorbed in the film show quadrupole splitting which also depends on Θ. The angular dependences of the linewidth (¹H-NMR) and the quadrupole splitting (²H-NMR) indicate that the H? H axes of the water molecules have a tendency to orient in the direction longitudinal to the film surface.     

Excitation and fragmentation of highly charged metal clusters in collisions with ions

We discuss the Coulomb fragmentation of highly charged metal clusters. The analogy with a classical conducting liquid drop is assessed from molecular dynamics calculations. Experimentally, the highly charged metal clusters are formed in collisions with highly charged ions (Xe²⁰⁺, Ar¹¹⁺, Ar⁸⁺, Ar³⁺, and O⁵⁺) at low velocity. We show new experimental data on the rate of emitted light charged particles that indicate an as yet unobserved fragmentation regime. Collisions of ions with metal clusters also offer a unique method to strongly excite the conducting electron gas within a short time of a few fs opening the possibility to study large amplitude electron dynamics and relaxation in microscopic systems.     

On the formation of doubly charged ions in field ionization

The observation of doubly charged ions in field ionization with blade or wire emitters does not in itself indicate that high excitation energies (≥ 17 eV) are accessible through gas phase ionization with such emitters.     

13C- and 1H-NMR. Spectra of ortho-benzoquinones. On the assignment problem in 13C spectra

¹³C- and ¹H-NMR. spectra of ortho-benzoquinone 1 and its methyl derivatives have been analysed. By means of heteronuclear double resonance experiments it is shown that assignments given in the literature for the olefinic carbon resonances of 1 and of a series of substituted ortho-benzoquinones have to be inverted. ¹³C-carbonyl frequencies of various six-membered cyclic ketones are presented.     

A reassessment of the 1H-NMR spectra of styrene/methyl methacrylate copolymers

The methoxy ¹H-NMR assignments in the 2.1–3.7δ region for styrene/methyl methacrylate copolymers have been assessed in terms of pentad sensitivity. Several of the methoxy resonances suggested by this study differ from literature assignments. The evidence in support of these reasignments is based on a comparison of calculated and observed triad distributions and on the good agreement of the value of the reactivity ratio r_M calculated from individual triads with that obtained by classical methods. A new procedure for the determination of the tacticity coefficient σ is applied.     

Formation of doubly charged ions in field ionization mass spectra of para substituted acetophenones

The doubly charged [M]²⁺, [M+1]²⁺ and [M-O]²⁺ ions are observed in the field ionization mass spectra of para substituted acetophenones. The effect of the type of the substituent on the formation of the doubly charged ions is described.     

300-MHz 1H-NMR spectra of p-cresol-formaldehyde condensates having regular repeating structures

Model p-cresol-formaldehyde condensates having regular sequences of methylene ether and methylene linkages were prepared by the self-condensation of dimethylol derivatives of p-cresol-formaldehyde condensates (2-hydroxy-5-methyl-1,3-benzenedimethanol, 3,3′-methylene-bis[2-hydroxy-5-methylbenzenemethanol] and 3,3′-[(2-hydroxy-5-methyl-m-phenylene)dimethylene]-bis[2-hydroxy-5-methylbenzenemethanol]). 300-MHz ¹H-NMR spectra of these polymers and of their acylated derivatives were recorded and used to develop resonance assignments for the various types of protons present in these polymers. The spectra were found to be sensitive to end-group and sequence distribution effects.