CH5+: the infrared spectrum observed

Dissociative recombination of CH4(+)

CH4(+) is an important molecular ion in the astrochemistry of diffuse clouds, dense clouds, cometary comae, and planetary ionospheres. However, the rate of one of the common destruction mechanisms for molecular ions in these regions, dissociative recombination (DR), is somewhat uncertain. Here, we present absolute measurements for the DR of CH4(+) made using the heavy ion storage ring CRYRING in Stockholm, Sweden. From our collision-energy dependent cross-sections, we infer a thermal rate constant of k(Te) = 1.71(±0.02) × 10(–6)(Te/300)(−0.66(±0.02)) cm3 s(–1) over the region of electron temperatures 10 ≤ Te ≤ 1000 K. At low collision energies, we have measured the branching fractions of the DR products to be CH4 (0.00 ± 0.00); CH3 + H (0.18 ± 0.03); CH2 + 2H (0.51 ± 0.03); CH2 + H2 (0.06 ± 0.01); CH + H2 + H (0.23 ± 0.01); and CH + 2H2 (0.02 ± 0.01), indicating that two or more C–H bonds are broken in 80% of all collisions.

Microsolvation-induced quantum localization in protonated methane

Nuclear quantum effects are responsible for vivid large-amplitude motion in protonated methane CH(5)(+), which enables so-called "hydrogen scrambling" that leads to the dynamical equivalence of all five protons even at low temperatures. But what is the impact of external perturbations on hydrogen scrambling of CH(5)(+) in this quantum fluxional ground state? We report ab initio path integral simulations of CH(5)(+)(H(2))(n), n = 1, 2, 3 that demonstrate cessation of hydrogen scrambling at low temperatures (20 K), but only slowdown at moderate temperatures (110 K). Importantly, different and unexpected mechanisms that are responsible for freezing the scrambling dynamics are revealed and traced back to distinct microsolvation patterns.

Hyperconjugation in Carbocations, a BLW Study with DFT approximation

The geometry of ethyl cation is discussed, and the hyperconjugation effect in carbocations is evaluated at the B3LYP/6-311G(d) level. The Block Localized Wavefunction (BLW) method is used for all evaluations of the hyperconjugation, considered as the energy gained by the delocalization onto the C⁺ atom. This energy is defined as the energy difference between the delocalized (standard) calculation, where the electrons are freely delocalized, and a localized form where the positive charge sits on the carbon center. It is evaluated for 18 carbocations, including conjugated systems. In these cases we were particularly interested in the additional stabilization brought by hyperconjugative effects. Among other effects, the β-silicon effect is computed. Hyperconjugation amounts in several cases to an energy similar to conjugation effects.

Frequency comb assisted mid-infrared spectroscopy of cold molecular ions

A mid-infrared spectrometer consisting of a high power optical parametric oscillator, a frequency comb, and a cold ion trap is described and characterized. The idler frequency at 3 μm is measured accurately by analyzing the pump and signal beat frequencies with the comb. This is done via two spectrum analyzers, allowing for a wide and continuous scanning ideal for spectroscopy of cold molecules with unknown spectra. The potential of this approach is demonstrated by measuring a ro-vibrational line of CH(5)(+) in a 22-pole ion trap via action spectroscopy of only several thousand cold ions. The current setup limits the precision of the line center frequency determination to some 100 kHz with ample room for future improvements. Following this approach, ground state combination differences of molecular ions can be predicted in order to identify them in astronomical objects.

Frontiers in spectroscopy

We review the frontiers of spectroscopy from a historical perspective, starting with the development of atomic spectroscopy about 150 years ago, followed by some comments on selected previous Faraday Discussions. As the spectrum of frontiers at the Faraday Discussion 150 is very broad, we give only a brief survey providing a map of the various frontiers approached today. This is followed by an exemplary discussion of one particular frontier towards the spectroscopic detection of symmetry violations in fundamental physics. In particular the understanding of parity violation in chiral molecules has recently made great progress. We briefly describe the advances made in recent decades as well as the current status of theory and experiments in this exciting field of research. We conclude with an outlook on open questions and frontiers of the future in spectroscopy.

ISOLOBAL BORON CARBONYL CARBOCATION ANALOGS

The isolobal [Formula: see text] relationship has been applied computationally to create a new family of positively charged boron carbonyls. These are generated by replacing one or more of the CH's in carbocations by BCO groups. The boron carbonyl cations can be hypercoordinated (e.g. H4BCO +, isolobal with [Formula: see text]), hydrogen bridged (e.g. ( BCO )2 H +, isolobal with the bridged vinyl cation [Formula: see text]), carbon bridged (e.g. [Formula: see text], isolobal with the 2-norbornyl cation), and polyhedral (e.g. [Formula: see text], isolobal with C 4 v [Formula: see text]). Furthermore, the BCO analogs have geometries and electronic structures similar to those of their carbocation counterparts, for example, characterized by 3c-2e multicenter and aromatic six interstitial electron bonding. The nucleus-independent chemical shifts (NICS) at the centers of three-membered rings or the pyramidal clusters are highly diatropic, ranging from -28 to -45 ppm. The replacement of CH by BCO favors the hypercoordinate bonding characteristic of boron compounds due to the more diffuse orbitals and lower electronegativity of boron.

CHEMICAL BONDS AND ELECTRONIC STRUCTURES OF THE METHONIUM CATIONS ${\rm CH}_{6}^{2+}$ AND ${\rm CH}_{7}^{3+}$

Six isomers with C2v, D3h, C5v, D2d, Oh, and C4v symmetries of [Formula: see text] and two isomers with C3v and C2v symmetries of [Formula: see text] are investigated at the high ab initio level combined with the natural bond orbital and the atoms-in-molecules theorems. The hyperconjugative interaction and the electron topological analyses indicate that the multiple three-center two-electron (3c-2e) hyperbond is the common chemical-bonding basis for [Formula: see text] and [Formula: see text] species. In contrast to the planar 3c-2e (triangle structure) and planar four-center four-electron (4c-4e) hyperbonds in [Formula: see text] isomeric species, the 3c-2e hyperbond in [Formula: see text] (C4v) is linear while the 4c-4e hyperbonds in [Formula: see text] (C5v, D2d, Oh) are unplanar. [Formula: see text] (C2v) and [Formula: see text] (C3v) as the global minima have many resonance structures predicted by the natural bond resonance theory, indicating the high possibility of the hydrogen scrambling which is similar to the scenario of [Formula: see text].

Jahn-Teller effects in molecular cations studied by photoelectron spectroscopy and group theory

The traditional "ball-and-stick" concept of molecular structure fails when the motion of the electrons is coupled to that of the nuclei. Such a situation arises in the Jahn-Teller (JT) effect which is very common in open-shell molecular systems, such as radicals or ions. The JT effect is well known to chemists as a mechanism that causes the distortion of an otherwise symmetric system. Its implications on the dynamics of molecules still represent unsolved problems in many cases. Herein we review recent progress in understanding the dynamic structure of molecular cations that have a high permutational symmetry by using rotationally resolved photoelectron spectroscopy and group theory. Specifically, we show how the pseudo-Jahn-Teller effect in the cyclopentadienyl cation causes electronic localization and nuclear delocalization. The fundamental physical mechanisms underlying the vaguely defined concept of "antiaromaticity" are thereby elucidated. Our investigation of the methane cation represents the first experimental characterization of the JT effect in a threefold degenerate electronic state. A special kind of isomerism resulting from the JT effect has been discovered and is predicted to exist in all JT systems in which the minima on the potential-energy surface are separated by substantial barriers.

Steric effect: A quantitative description from density functional theory

The concepts of steric energy, steric potential, and steric charge are introduced within the density functional theory framework. The steric energy, representing a hypothetical state with all electrons packed into the lowest orbital and other effects entirely excluded, is a measure of the intrinsic space occupied by an electronic system. It is exclusive, repulsive, and extensive, and it vanishes for homogeneous electron gas. When Bader's zero-flux boundary condition is adopted, atoms in molecules are found to achieve balanced steric repulsion among one another with vanished steric energy density interfaces. A few molecular systems involving conformation changes and chemical reactions have been investigated to examine the relative contribution of the steric and other effects, providing insights for a few controversial topics from a different perspective.

Quasiclassical Trajectory Study of the CH3+ + HD → CH2D+ + H2 Reaction

A full dimensional ab initio potential energy surface for the CH5+ system based on coupled cluster electronic structure calculations and capable of describing the dissociation of methonium ion into methyl cation and molecular hydrogen (J. Phys. Chem. A 2006, 110, 1569) is used in quasiclassical trajectory calculations of the reaction CH3++HD-->CH2D++H2 for low collision energies of relevance to astrochemistry. Cross sections for the exchange are obtained at several relative translational energies and a fit to the energy dependence of the cross sections is used to obtain the rate constant at temperatures between 10 and 50 K. The calculated rate constant at 10 K agrees well with the previously reported experimental value. Internal energy distributions of the products are presented and discussed in the context of zero-point energy "noncompliance".

Driving Energies of Hydrogen Scrambling Motions in CH5+

A comprehensive quantum chemistry study with natural bond orbital analysis is performed to reveal the intrinsic properties of the floppy potential energy surface of the smallest methonium ion CH5+. In contrast to the low-energy barriers of Cs(I)-->Cs(II)-->Cs(I) and Cs(I)-->C2v-->Cs(I) that correspond to the H2 rotation and H-flip motions, the remarkably larger intramolecular interaction energies are the driving power to mimic the hydrogen scrambling in CH5+. As for the H2 rotation and H-flip motions, the hyperconjugative and electrostatic interactions compete strongly. They together with other interactions compensate for each other and lead to the floppy potential energy surface.

Four decades of joy in mass spectrometry

Tremendous developments in mass spectrometry have taken place in the last 40 years. This holds for both the science and the instrumental revolutions in this field. In chemistry the research was heavily focused on organic molecules that upon electron ionization fragmented via complex mechanistic pathways as shown by isotopic labeling experiments. These studies, including ion structure determinations, were performed with use of double focusing mass spectrometers of both conventional and reversed geometry, and equipped with various types of metastable ion scanning and collision-induced dissociation techniques developed by physical and analytical chemists. Time-resolved mass spectrometry by use of the field ionization kinetics method, developed by physical chemists, was another powerful way to unravel details of unimolecular gas phase ion dissociations. Then the development of new ionization methods, such as desorption chemical ionization, field desorption, and fast atom bombardment permitted not only to analyze unvolatile, thermally labile and higher molecular weight compounds, but also to study their chemical behavior in the gas phase, initially with use of double focusing instruments and later on with multisector and hybrid mass spectrometers. These ionization methods also enabled to study organometallic compounds and increasingly the field of medium-sized to large biomolecules, the latter being exploded in the last decade by the development of electrospray- and matrix-assisted laser desorption ionization/time-of-flight mass spectrometry. Another area of research concerned the bimolecular chemistry of organic ions with organic molecules in the gas phase. Initially this was performed with use of among others drift-cell ion cyclotron resonance spectroscopy, that later on was replaced by the developed method of ion trapping and Fourier transform ion cyclotron resonance. Combination of the latter with the afore-mentioned ionization methods has shifted also in this case the research on organic molecules to organometallic/inorganic systems, and predominantly to biomolecules in the last decade. This invited review will describe the research efforts made by the author's group over the last 40 years together with some personal experiences during his career.

Nature of the Chemical Bond in Protonated Methane

Protonated methane, CH(5)(+), is a key reactive intermediate in hydrocarbon chemistry and a borderline case for chemical structure theory, being the simplest example of hypercoordinated carbon. Early quantum mechanical calculations predicted that the properties of this species could not be associated with only one structure, because it presents serious limitations of the Born-Oppenheimer approximation. However, ab initio molecular dynamics and diffusion Monte Carlo calculations showed that the most populated structure could be pictured as a CH(3) tripod linked to a H(2) moiety. Despite this controversy, a model for the chemical bonds involved in this ion still lacks. Here we present a modern valence bond model for the electronic structure of CH(5)(+). The chemical bond scheme derived directly from our calculations pictures this ion as H(3)C...H(2)(+). The fluxionality can be seen as the result of a proton transfer between C-H bonds. A new insight on the vibrational bands at approximately 2400 and approximately 2700 cm(-1) is suggested. Our results show that the chemical bond model can be profitably applied to such intriguing systems.

Infrared studies of ionic clusters: The influence of Yuan T. Lee

Beginning in the mid-1980s, a number of innovative experimental studies on ionic clusters emerged from the laboratory of Yuan T. Lee combining infrared laser spectroscopy and tandem mass spectrometry. Coupled with modern electronic structure calculations, this research explored many facets of ionic clusters including solvation, structure, and dynamics. These efforts spawned a resurgence in gas-phase cluster spectroscopy. This paper will focus on the major areas of research initiated by the Lee group and how these studies stimulated and influenced others in what is currently a vibrant and growing field.

π-Complex Structure of Gaseous Benzene−NO Cations Assayed by IR Multiple Photon Dissociation Spectroscopy

[C(6)H(6)NO](+) ions, in two isomeric forms involved as key intermediates in the aromatic nitrosation reaction, have been produced in the gas phase and analyzed by IR multiple photon dissociation (IRMPD) spectroscopy in the 800-2200 cm(-)(1) fingerprint wavenumber range, exploiting the high fluence and wide tunability of a free electron laser (FEL) source. The IRMPD spectra were compared with the IR absorption spectra calculated for the optimized structures of potential isomers, thus allowing structural information on the absorbing species. [C(6)H(6)NO](+) ions were obtained by two routes, taking advantage of the FEL coupling to two different ion traps. In the first one, an FT-ICR mass spectrometer, a sequence of ion-molecule reactions was allowed to occur, ultimately leading to an NO(+) transfer process to benzene. The so-formed ions displayed IRMPD features characteristic of a [benzene,NO](+) pi-complex structure, including a prominent band at 1963 cm(-)(1), within the range for the N-O bond stretching vibration of NO (1876 cm(-)(1)) and NO(+) (2344 cm(-)(1)). A quite distinct species is formed by electrospray ionization (ESI) of a methanol solution of nitrosobenzene. The ions transferred and stored in a Paul ion trap showed the IRMPD features of substituent protonated nitrosobenzene, the most stable among conceivable [C(6)H(6)NO](+) isomers according to computations. It is noteworthy that IRMPD is successful in allowing a discrimination between isomeric [C(6)H(6)NO](+) species, whereas high-energy collision-induced dissociation fails in this task. The [benzene,NO](+) pi-complex is characterized by IRMPD spectroscopy as an exemplary noncovalent ionic adduct between two important biomolecular moieties.