Production and Characterization of Molecular Dications: Experimental and Theoretical Efforts

On the thermodynamic stability of polycations

We present a simple approximation to estimate the largest charge that a given molecule can hold until fragmentation into smaller charged species becomes more energetically favorable. This approximation solely relies on the ionization potentials, electron affinities of the parent and fragment species, and also on the neutral parent's dissociation energy. By parameterizing these quantities, it is possible to obtain analytical phase diagrams of polycationic stability. We demonstrate the applicability of this approach by discussing the maximal charge dependence on the size of the molecular system. A numerical demonstration for linear polyenes, monocyclic annulenes, and helium clusters is provided.

High-Resolution Photoelectron Spectroscopy of the Ground and First Excited Electronic States of MgXe. J Phys Chem A 2024;128:3149-3157. [PMID: 38619915 PMCID: PMC11056989 DOI: 10.1021/acs.jpca.4c00940] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We report on the characterization of the X+ 2Σ+ ground and the A+ 2ΠΩ (Ω = 1/2, 3/2) and B+ 2Σ+ electronically excited states of MgXe+. Rotationally cold MgXe in the a 3Π0(v″ = 0) metastable electronic state was generated in a laser-ablation supersonic-beam source. Following single-photon excitation from the metastable state, the vibrational structure of the X+ state of MgXe+ was measured by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy, and the adiabatic ionization energy of the X+ ← a ionizing transition was determined to be EI/(hc) = 37,468.3(6) cm-1. Spectra of the A+ ← X+ and B+ ← X+ transitions were recorded by using the method of isolated-core Rydberg-dissociation spectroscopy. The observation of the Mg+(3p) 2P1/2 + Xe 1S0 dissociation limit enabled the determination of the dissociation energies of the X+ [D0(X+) = 2970(7) cm-1] and A+ states [D0(A1/2+) = 9781(7) cm-1 and D0(A3/2+) = 9603(7) cm-1]. We compare these results with those of earlier experimental studies and ab initio quantum-chemical calculations.

Chemical Aristocracy: He3 Dication and Analogous Noble-Gas-Exclusive Covalent Compounds

Herein, we predict the first set of covalently bonded triatomic molecular compounds composed exclusively of noble gases. Using a combination of double-hybrid DFT, CCSD(T), and MRCI+Q calculations and a range of bonding analyses, we explored a set of 270 doubly charged triatomics, which included various combinations of noble gases and main group elements. This extensive exploration uncovered nine noble-gas-exclusive covalent compounds incorporating helium, neon, argon, or combinations thereof, exemplified by cases such as He32+ and related systems. This work brings to light a previously uncharted domain of noble gas chemistry, demonstrating the potential of noble gases in forming covalent molecular clusters.

Double Photoionization of Nitrosyl Chloride by Synchrotron Radiation in the 24-70 eV Photon Energy Range

The behavior of nitrosyl chloride (ClNO) exposed to ionizing radiation was studied by direct probing valence-shell electrons in temporal coincidence with ions originating from the fragmentation process of the transient ClNO²⁺. Such a molecular dication was produced by double photoionization with synchrotron radiation in the 24-70 eV photon energy range. The experiment has been conducted at the Elettra Synchrotron Facility of Basovizza (Trieste, Italy) using a light beam linearly polarized with the direction of the polarization vector parallel to the ClNO molecular beam axis. ClNO molecules crossing the photon beam at right angles in the scattering region are generated by effusive expansion and randomly oriented. The threshold energy for the double ionization of ClNO (30.1 ± 0.1 eV) and six dissociation channels producing NO⁺/Cl⁺, N⁺/Cl⁺, N⁺/O⁺, O⁺/Cl⁺, ClN⁺/O⁺, NO⁺/Cl²⁺ ion pairs, with their relative abundance and threshold energies, have been measured.

State selective fragmentation of doubly ionized sulphur dioxide

Using multi-electron–ion coincidence measurements combined with high level calculations, we show that double ionisation of SO₂ at 40.81 eV can be state selective. It leads to high energy products, in good yield, via a newly identified mechanism, which is likely to apply widely to multiple ionisation by almost all impact processes.

Spectroscopic characterization of a thermodynamically stable doubly charged diatomic molecule: MgAr2

Although numerous doubly positively charged diatomic molecules (diatomic dications) are known from investigations using mass spectrometry and ab initio quantum chemistry, only three of them, NO²⁺, N₂²⁺ and DCl²⁺, have been studied using rotationally resolved optical spectroscopy and only about a dozen by vibrationally resolved double-ionization methods. So far, no thermodynamically stable diatomic dication has been characterized spectroscopically, primarily because of experimental difficulties associated with their synthesis in sufficient densities in the gas phase. Indeed, such molecules typically involve, as constituents, rare-gas, halogen, chalcogen, and metal atoms. We report here on a new approach to characterize molecular dications based on high-resolution photoelectron spectroscopy of the singly charged parent molecular cation and present the first spectroscopic characterization of a thermodynamically stable diatomic dication, MgAr²⁺. From the fully resolved vibrational and partially resolved rotational structures of the photoelectron spectra of ²⁴MgAr⁺ and ²⁶MgAr⁺, we determined the potential-energy function of the electronic ground state of MgAr²⁺, its dissociation (binding) energy (D₀ = 10 690(3) cm⁻¹), and its harmonic (ω_e(²⁴MgAr²⁺) = 327.02(11) cm⁻¹) and anharmonic (ω_ex_e(²⁴MgAr²⁺) = 2.477(15) cm⁻¹) vibrational constants. The analysis enables us to explain quantitatively how the strong bond arises in this dication despite the fact that Ar and Mg²⁺ both have a full-shell rare-gas electronic configuration.

We present a new method to study doubly charged molecules relying on high-resolution spectroscopy of the singly charged parent cation, and report on the first spectroscopic characterization of a thermodynamically stable diatomic dication, MgAr²⁺.

Exploring the electronic states of the hydroxyl dication OH2+: thermodynamic (meta)stability, bound-free emission spectra, and charge transfer processes

Accurate potential energy curves were constructed for a manifold of electronic states of the hydroxyl dication using a highly correlated electronic structure approach (SA-CASSCF/MRCI+Q/aug-cc-pV5Z). The existence of a bound (meta)stable ground state and bound low-lying states for OH2+ are ruled out, but do not exclude the possibility of its transient formation and dissociation along the repulsive ground state potential energy curve. Our results do not support the conclusion reported for the observation of OH2+ by electron ionization from ground state OH+. Despite the repulsive character of the low-lying states, thermodynamic stability was indeed verified for the states 2 4Π and 3 4Σ- along with a series of metastable high-lying doublet states. For the (quasi)bound states, we obtained vibrational levels, spectroscopic parameters, and dipole moment functions. Using accurate transition dipole moment functions, we also evaluated bound-free emission transition probabilities and radiative lifetimes. For transitions from v'= 0, our estimates of 92.8 ns (4Π) and 9.3 ns (4Σ-) indicate that the ones obtained by a multichannel theory of predissociating states are too short (2-60 ps). Landau-Zener cross sections averaged over the Maxwellian distribution of relative velocities, and rate coefficients for the reaction O2+ + H → O+ + H+ were obtained using the potential energy curves of the states 4Π and 4Σ- associated with the channel O2+ + H and the repulsive ones dissociating into O+ + H+ leading to good results for the rate constant thus supporting its importance to explain the distribution of O+ in astrophysical plasmas.