The Evolution of Bonding and Thermodynamic Properties of Boron‐Doped Small Carbon Clusters: An Ab Initio Study

Theoretical studies on BC_n (n=1–6) clusters are carried out using density functional theory, Møller–Plesset second‐order perturbation theory (MP2), coupled‐cluster calculations including up to triple excitations (CCSD(T)), and higher‐level approaches. All possible isomers depending on the positions of the boron atom are generated and the lowest‐energy isomers are determined for doublet and quartet electronic states. The three potential evolution paths of the clusters are determined as a function of their size. The energetic and electronic consequences for the increased size of structures differ significantly, which leads to representatives of the ground electronic state from different structural groups. The ab initio calculated thermal functions allow enhancements to the available atomization energies and improve the agreement between the calculated and experimental heat content.     

Energetics and Structures of Charged Helium Clusters: Comparing Stabilities of Dimer and Trimer Cationic Cores

We present accurate ab initio calculations of the most stable structures of He_n⁺ clusters in order to determine the more likely ionic core arrangements existing after reaching structural equilibrium of the clusters. Two potential energy surfaces are presented: one for the He₂⁺ and the other with the He₃⁺ linear ion, both interacting with one He atom. The two computed potentials are in turn employed within a classical structure optimization where the overall interaction forces are obtained within the sum‐of‐ potentials approximation described in the main text. Because of the presence of many‐body effects within the ionic core, we find that the arrangements with He₃⁺ as a core turn out to be energetically preferred, leading to the formation of He₃⁺(He)_n?3 stable aggregates. Nanoscopic considerations about the relative stability of clusters with the two different cores are shown to give us new information on the dynamical processes observed in the impact ionization experiments of pure helium clusters and the importance of pre‐equilibrium evaporation of the ionic dimers in the ionized clusters.     

Intrinsic Neutral and Anionic Structures of Glutathione

Gas‐phase intrinsic structures of intact neutral and anionic glutathione (GSH) have been determined by means of a combination of negative ion photo‐electron spectroscopy and quantum chemistry calculations. The inferred structures of the neutral parents of those peptide anions are canonical (non‐zwitterionic). These intrinsic structures are compared to those already known in aqueous solution or determined by crystallography in binding sites of enzymes.     

Adsorption of Propene on Neutral Gold Clusters in the Gas Phase

The adsorption of propene on neutral gold clusters is investigated in a collision cell under a few collision conditions. The adsorption reaction is studied by pressure‐dependent kinetic measurements and delayed unimolecular dissociation of the excited Au_n?propene complexes. The cluster size (n=9–25) and temperature (T=90–300 K) dependence of the propene adsorption is analyzed. Strong size dependences of the absorption reaction are observed; a larger propene adsorption probability was found for gold clusters composed of an even number of atoms. Propene binding energies are estimated by comparison of the temperature‐dependent unimolecular dissociation rates with rates obtained by using statistical RRKM modeling. The Au_n–propene binding energies decrease non‐monotonously with cluster size and are in the range of 1.2–0.85 eV for n=9–25. Finally, the bonding of C₃H₆ on Au_n is qualitatively described and similarities with the absorption of CO molecules on gold clusters are discussed.     

Elucidating the Thermal Decomposition of Dimethyl Methylphosphonate by Vacuum Ultraviolet (VUV) Photoionization: Pathways to the PO Radical,a Key Species in Flame‐Retardant Mechanisms

The production of phosphoryl species (PO, PO₂, HOPO) is believed to be of great importance for efficient flame‐retardant action in the gas phase. We present a detailed investigation of the thermal decomposition of dimethyl methylphosphonate (DMMP) probed by vacuum ultraviolet (VUV) synchrotron radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. This technique provides a snapshot of the thermolysis process and direct evidence of how the reactive phosphoryl species are generated during heat exposure. One of the key findings of this work is that only PO is formed in high concentration upon DMMP decomposition, whereas PO₂ is absent. It can be concluded that the formation of PO₂ needs an oxidative environment, which is typically the case in a real flame. Based on the identification of products such as methanol, formaldehyde, and PO, as well as the intermediates O?P?CH₃, H₂C?P?OH, and H₂C?P(?O)H, supported by quantum chemical calculations, we were able to describe the predominant pathways that lead to active phosphoryl species during the thermal decomposition of DMMP.     

Chemical Solutions in a Quantum Solvent: Anionic Electrolytes in 4He Nanodroplets

Variational and diffusion Monte Carlo (VMC and DMC) calculations are presented for anionic electrolytes solvated in ⁴He. The electrolytes have the general structure X^?(He)_N, with X=F, Cl, Br and I, and N varying up to 40 (41 for I^?). The overall interaction potential is obtained from accurate ab initio data for the two‐body components and then using the sum‐of‐potentials approximation. Our computational scheme is a robust procedure, giving us accurate trial wavefunctions that can be used to perform high‐quality DMC calculations. The results indicate very marked delocalization and permanence of the liquid‐like quantum features of the solvent adatoms surrounding the anionic impurities. This finding stands in contrast to the more structured, solid‐like behavior of the quantum solutions with alkali metal cations embedded in He nanodroplets. While other negatively charged species such as H^? have shown an overall repulsive interaction with He, the present calculations clearly indicate that the halogen anions remain solvated within liquid‐like solvent “bubbles” of species‐dependent size.     

Far‐Infrared Spectra of Yttrium‐Doped Gold Clusters AunY (n=1–9)

The geometric, spectroscopic, and electronic properties of neutral yttrium‐doped gold clusters Au_nY (n=1–9) are studied by far‐infrared multiple photon dissociation (FIR‐MPD) spectroscopy and quantum chemical calculations. Comparison of the observed and calculated vibrational spectra allows the structures of the isomers present in the molecular beam to be determined. Most of the isomers for which the IR spectra agree best with experiment are calculated to be the energetically most stable ones. Attachment of xenon to the Au_nY cluster can cause changes in the IR spectra, which involve band shifts and band splittings. In some cases symmetry changes, as a result of the attachment of xenon atoms, were also observed. All the Au_nY clusters considered prefer a low spin state. In contrast to pure gold clusters, which exhibit exclusively planar lowest‐energy structures for small sizes, several of the studied species are three‐dimensional. This is particularly the case for Au₄Y and Au₉Y, while for some other sizes (n=5, 8) the 3D structures have an energy similar to that of their 2D counterparts. Several of the lowest‐energy structures are quasi‐2D, that is, slightly distorted from planar shapes. For all the studied species the Y atom prefers high coordination, which is different from other metal dopants in gold clusters.     

Analyzing the Vibrational Signatures of Thiophenol Adsorbed on Small Gold Clusters by DFT Calculations

Using density functional theory, we calculate the IR and Raman signatures of the thiophenol (TP) molecule adsorbed on gold clusters by mimicking the different types of adsorption sites, and we analyze these signatures by using advanced tools implemented into the pyvib2 program. First, we follow the evolution of the vibrational normal modes from the isolated TP molecule to those of TP adsorbed on different clusters to highlight the influence of the site of adsorption on the vibrational motions. The use of the overlap matrix between the modes enables mode permutations, mode mixings, and mode splittings to be highlighted, all of which depend not only on the adsorption but also on the type of cluster and its symmetry. Second, the IR and Raman signatures were analyzed by using group coupling matrices and atomic contribution patterns based on the Hug decomposition scheme. Key results include 1) the fact that Raman spectroscopy is more sensitive than IR spectroscopy with respect to the nature of the coordination site, 2) an IR criterion that distinguishes between on‐top coordination (onefold coordinated) with respect to the bridge (twofold coordinated) and hexagonal close‐packed hollow site coordination (threefold coordinated), and 3) the best agreement to the experimental Raman spectrum with regard to signatures in the 500 to 1200 cm^?1 region is obtained for bridged, twofold coordination.     

Experimental and Theoretical Study of the Broadening and Shifting of N2H+ Rotational Lines by Helium

Pressure broadening and pressure shift of N₂H⁺ rotational lines perturbed by collisions with He are studied for the first time using experiment and theory. Results are reported from measurements at 88 K for the rotational transitions ${j = 3 \leftarrow 2}$ , ${4 \leftarrow 3}$ , ${5 \leftarrow 4}$ and ${6 \leftarrow 5}$ with frequencies ranging from 0.28 to 0.56 THz. The agreement between experiment and theoretical data derived from close coupling calculations confirms the reliability of a theoretical framework used for state‐to‐state transition rates of interest in the interpretation of spectroscopic data from interstellar molecular clouds. The influence of hyperfine effects on shifts and widths of the rotational lines is discussed in detail. Although in principle possible, experiment and theoretical considerations lead to the conclusion that hyperfine effects only play a minor role.     

Small Potassium Clusters

The molecules K ₂ , K ₃ , and K ₄ (structure shown on the right) have been isolated in krypton matrices at 15 K and characterized by Raman spectroscopy. Comparison of the experimental data with density functional calculations supports the prediction that potassium clusters are not only bonded by the valence electrons, but that there is also a contribution from the core electrons.     

Structure and Stability of (NG)nCN3Be3+ Clusters and Comparison with (NG)BeY0/+

The noble gas binding ability of CN₃Be₃⁺ clusters was assessed both by ab intio and density functional studies. The global minimum structure of the CN₃Be₃⁺ cluster binds with four noble‐gas (NG) atoms, in which the Be atoms are acting as active centers. The electron transfer from the noble gas to the Be atom plays a key role in binding. The dissociation energy of the Be? NG bond gradually increases from He to Rn, maintaining the periodic trend. The HOMO–LUMO gap, an indicator for stability, gives additional insight into these NG‐bound clusters. The temperature at which the NG‐binding process is thermodynamically feasible was identified. In addition, we investigated the stability of two new neutral NG compounds, (NG)BeSe and (NG)BeTe, and found them to be suitable candidates to be detected experimentally such as (NG)BeO and (NG)BeS. The dissociation energies of the Be? NG bond in monocationic analogues of (NG)BeY (Y=O, S, Se, Te) were found to be larger than in the corresponding neutral counter‐parts. Finally, the higher the positive charge on the Be atoms, the higher the dissociation energy for the Be? NG bond becomes.     

Magic Clusters PtMg2,3H5− Facilitated by Local σ-Aromaticity

The concept of local aromaticity has been successfully utilized in understanding the stability of certain atomic clusters. However, all the skeleton atoms in these clusters are covered by at least one local aromatic feature, collectively making the multiple local aromaticities coexist globally. Herein we show the robustness of local aromaticity as a tool for the discovery of novel magic clusters: not all of the skeleton atoms need to be covered by an aromatic feature to make the cluster magic. In this study, the PtMg_2,3H₅⁻ cluster anions are generated by a unique high-current pulsed discharge ion source and found to be magic numbers using mass spectrometry. Photoelectron spectroscopy and calculations confirm that only the PtH₄²⁻ kernels in these clusters are locally aromatic. Based on these results, we propose that local aromaticity can be gainfully utilized as a new potential magic rule in the search for magic numbers.