π- and σ-Coordinated Al in AlC2- and AlCSi-. A Combined Photoelectron Spectroscopy and ab Initio Study

Reconsidering the Structures of C2 Al4 - and C2 Al5. Chemistry 2023;29:e202301338. [PMID: 37498677 DOI: 10.1002/chem.202301338]

The study of C2 Al4 -/0 and C2 Al5 -/0 was conducted using anion photoelectron spectroscopy and quantum chemical computations. The present findings reveal that C2 Al4 - has a boat-like structure, with a single C2 unit surrounded by four aluminum atoms. In contrast, the neutral C2 Al4 species adopts a D2h planar structure with two planar tetracoordinate carbon (ptC) units, consistent with previous reports. Furthermore, the global minimum isomer of C2 Al5 - adopts a D3h symmetry, where the C2 unit interacts with five aluminum atoms. It was also found that a lower symmetry structure of C2 Al5 - , where all five aluminum atoms are located on the same side of the C2 unit, albeit slightly higher in energy compared to the D3h structure. These computations show that the D3h structure of C2 Al5 - is highly stable, exhibiting a large HOMO-LUMO gap.

Structural Evolution of Carbon-Doped Aluminum Clusters AlnC- (n = 6-15): Anion Photoelectron Spectroscopy and Theoretical Calculations

Carbon-doped aluminum cluster anions, Al_nC^- (n = 6-15), were generated by laser vaporization and investigated by mass-selected anion photoelectron spectroscopy. The geometric structures of Al_nC^- (n = 6-15) anions were determined by the comparison of theoretical calculations with the experimental results. It is found that the most stable structure of Al₆C^- is a carbon endohedral triangular prism. The Al₇C^- anion is a magic cluster with high stability. The structures of Al_7-9C^- can be viewed as the additional aluminum atoms attached around the triangular prism Al₆C^-. Two isomers of Al₁₀C^- have been detected in the experiments. The most stable one has a planar tetracoordinate carbon structure. The second one derives from Al₉C^- with the carbon atom located in a pentagonal bipyramid. The Al₁₁C^- anion has a bilayer structure composed of one planar tetracoordinate carbon and one aluminum-centered hexagon, in which the major interactions between two layers are multicenter bonds. The structures of Al_12-14C^- can be viewed as evolving from Al₁₁C^- by adding aluminum atoms to interact with the carbon atom. In Al₁₅C^-, the carbon atom stays at the surface with a tetracoordinate structure, and an icosahedral Al₁₃ unit can be identified as a part of the geometric structure of Al₁₅C^-.

Photoelectron spectroscopy and theoretical study of AlnC5-/0 (n = 1-5) clusters: structural evolution, relative stability of star-like clusters, and planar tetracoordinate carbon structures

The AlnC5- (n = 1-5) clusters were detected in the gas-phase and were investigated via mass-selected anion photoelectron spectroscopy. The structures of AlnC5-/0 (n = 1-5) were explored by theoretical calculations. It is found that the structures of AlC5-/0 and Al2C5-/0 are linear while those of Al3C5-/0, Al4C5-/0, and Al5C5-/0 are two-dimensional. The most stable structures of AlC5-/0 and Al2C5-/0 are linear with the Al atoms attached to the ends of C5 chain. The most stable structures of Al3C5-/0 can be viewed as three Al atoms interacting with a nonlinear C5 chain. The most stable structure of Al4C5- anion is a planar structure composed of a C2 unit, a C3 unit, and two Al2 units, while that of the neutral Al4C5 cluster has four Al atoms connected to different positions of a distorted C5 chain. The global minimum structures of Al5C5-/0 are planar structures composed of an Al4C quadrilateral, two C2 groups, and an Al atom connected to two C2 groups. Planar tetracoordinate carbon (ptC) has been identified in the structures of both anionic and neutral Al5C5. It is worth mentioning that the star-like structure of Al5C5- is slightly higher in energy than the ground state structure. The comparison of theoretical calculations with the experimental spectra indicates the star-like structure of Al5C5- may also appear in our experiments.

Anion Photoelectron Spectroscopy and Theoretical Studies of Al4C6-/0: Global Minimum Triangle-Shaped Structures and Hexacoordinated Aluminum

Anion photoelectron spectroscopy and theoretical calculations were used to investigate the structural and bonding properties of Al₄C₆^-/0 clusters. The vertical detachment energy of Al₄C₆^- was measured to be 3.36 ± 0.08 eV. The structure of the Al₄C₆^- anion is confirmed to be a bowl-shaped distorted triangle with an Al atom at the center and three Al atoms at the vertices. The global minimum isomer of neutral Al₄C₆ has a planar triangle-shaped structure with D_3h symmetry. Both anionic and neutral Al₄C₆ have a hexacoordinated Al atom surrounded by three C≡C groups. Compared with the structure of neutral Al₄C₆, the structure of Al₄C₆^- is distorted due to the addition of the excess electron. The molecular orbital analysis shows that the singly occupied molecular orbital of Al₄C₆^- mainly locates on one side of the triangle plane and the neutral Al₄C₆ has a large highest occupied molecular orbital and lowest unoccupied molecular orbital gap. Theoretical calculations indicate that neutral Al₄C₆ has some aromaticity.

The pure rotational spectrum of the T-shaped AlC2 radical (X[combining tilde]2A1)

The pure rotational spectrum of the AlC2 radical (X[combining tilde]2A1) has been measured using Fourier transform microwave/millimeter-wave (FTMmmW) techniques in the frequency range 21-65 GHz. This study is the first high-resolution spectroscopic investigation of this molecule. AlC2 was created in a supersonic jet from the reaction of aluminum, generated by laser ablation, with a mixture of CH4 or HCCH, diluted in argon, in the presence of a DC discharge. Three transitions (NKa,Kc = 101 → 000, 202 → 101, and 303 → 202) were measured, each consisting of multiple fine/hyperfine components, resulting from the unpaired electron in the species and the aluminum-27 nuclear spin (I = 5/2). The data were analyzed using an asymmetric top Hamiltonian and rotational, fine structure, and hyperfine constants determined. These parameters agree well with those derived from previous theoretical calculations and optical spectra. An r0 structure of AlC2 was determined with r(Al-C) = 1.924 Å, r(C-C) = 1.260 Å, and θ(C-Al-C) = 38.2°. The Al-C bond was found to be significantly shorter than in other small, Al-bearing species. The Fermi contact term established in this work indicates that the unpaired electron in the valence orbital has considerable 3pza1 character, suggesting polarization towards the C2 moiety. A high degree of ionic character in the molecule is also evident from the quadrupole coupling constant. These results are consistent with a T-shaped geometry and an Al+C2- bonding scheme. AlC2 is a possible interstellar molecule that may be present in the circumstellar envelopes of carbon-rich AGB stars.

Probing the electronic structure and Au-C chemical bonding in AuC2(-) and AuC2 using high-resolution photoelectron spectroscopy

We report photoelectron spectroscopy (PES) and high-resolution PE imaging of AuC2(-) at a wide range of photon energies. The ground state of AuC2(-) is found to be linear (C∞v, (1)Σ(+)) with a …8π(4)4δ(4)17σ(2)9π(4)18σ(2) valence configuration. Detachments from all the five valence orbitals of the ground state of AuC2(-) are observed at 193 nm. High-resolution PE images are obtained in the energy range from 830 to 330 nm, revealing complicated vibronic structures from electron detachment of the 18σ, 9π, and 17σ orbitals. Detachment from the 18σ orbital results in the (2)Σ(+) ground state of neutral AuC2, which, however, is bent due to strong vibronic coupling with the nearby (2)Π state from detachment of a 9π electron. The (2)Σ(+)-(2)Π vibronic and spin-orbit coupling results in complicated vibronic structures for the (2)Σ(+) and (2)Π3/2 states with extensive bending excitations. The electron affinity of AuC2 is measured accurately to be 3.2192(7) eV with a ground state bending frequency of 195(6) cm(-1). The first excited state ((2)A') of AuC2, corresponding to the (2)Π3/2 state at the linear geometry, is only 0.0021 eV above the ground state ((2)A') and has a bending frequency of 207(6) cm(-1). The (2)Π1/2 state, 0.2291 eV above the ground state, is linear with little geometry change relative to the anion ground state. The detachment of the 17σ orbital also results in complicated vibronic structures, suggesting again a bent state due to possible vibronic coupling with the lower (2)Π state. The spectrum at 193 nm shows the presence of a minor species with less than 2% intensity relative to the ground state of AuC2(-). High-resolution data of the minor species reveal several vibrational progressions in the Au-C stretching mode, which are assigned to be from the metastable (3)Π2,1,0 spin-orbit excited states of AuC2(-) to the (2)Π3/2,1/2 spin-orbit states of neutral AuC2. The spin-orbit splittings of the (3)Π and (2)Π states are accurately measured at the linear geometry. The current study provides a wealth of electronic structure information about AuC2(-) and AuC2, which are ideal systems to investigate the strong Σ-Π and spin-orbit vibronic couplings.

Gas phase electronic spectrum of T-shaped AlC2 radical

Gas phase electronic transitions for the C 2B2<--X 2A1 and D 2B1<--X 2A1 band systems of T-shaped AlC2 (C2v) radical have been measured in the 345-475 nm range. Vibrational analyses of both band systems are reported. Simulation of several rotationally resolved bands confirms previously obtained rotational parameters for the C 2B2 state. The radical is produced by ablating an aluminum rod in the presence of acetylene gas. The resulting supersonic molecular beam is probed using both mass-selective resonant two-color two-photon ionization and laser induced fluorescence. Ab initio calculations and vertical electronic excitation energies help the assignment. Vibrational frequencies for the X 2A1, C 2B2, and D 2B1 states have been determined. Rotational analysis of a number of bands yields spectroscopic constants for one vibronic state in the C 2B2 manifold and the origin band of the D 2B1<--X 2A1 system.

Fourier transform infrared observation and density functional theory study of the AlC3 and AlC3Al linear chains trapped in solid Ar

The vibrational spectra of linear AlC(3) and AlC(3)Al, formed by trapping the products of the dual laser evaporation of aluminum and carbon rods in solid Ar at approximately 10 K, were observed. Fourier transform infrared (FTIR) measurements of (13)C isotopic shifts are in good agreement with the predictions of density functional theory (DFT) B3LYP6-311+G(3df) calculations, enabling the first assignments of the nu(3)(sigma(u)) and nu(4)(sigma(u)) fundamentals of ((3)Sigma(g) (+)) linear AlC(3)Al at 1624.0 and 528.3 cm(-1), respectively, and the nu(2)(sigma) vibrational fundamental of ((2)Pi) linear AlC(3) at 1210.9 cm(-1).

Observation of Triatomic Species with Conflicting Aromaticity: AlSi2- and AlGe2-

We created mixed triatomic clusters, AlCGe(-), AlSi(2)(-), and AlGe(2)(-), and studied their electronic structure and chemical bonding using photoelectron spectroscopy and ab initio calculations. Excellent agreement between theoretical and experimental photoelectron spectra confirmed the predicted global minimum structures for these species. Chemical bonding analysis revealed that the AlSi(2)(-) and AlGe(2)(-) anions can be described as species with conflicting (sigma-antiaromatic and pi-aromatic) aromaticity. The AlCGe(-) anion represents an interesting example of chemical species which is between classical and aromatic.

On the Competition between Linear and Cyclic Isomers in Second-Row Dicarbides

Second-row dicarbides C(2)X (X = Na-Cl) are investigated with quantum mechanical techniques. The cyclic-linear competition in these systems is studied, and the bonding scheme for these compounds is discussed in terms of the topological analysis of the electronic density. C(2)Na, C(2)Mg, C(2)Al, and C(2)Si are found to prefer a C(2)(v)-symmetric arrangement corresponding to a T-shape structure. On the other hand, for C(2)P, C(2)S, and C(2)Cl the linear isomer is predicted to be the ground state. A detailed analysis of the variation of the electronic energy and orbital energies with the geometry has been carried out. A simple theoretical model, taking into account the main interactions between the valence orbitals of both fragments, the X atom and the C(2) molecule, allows an interpretation of the main features of these compounds.

A theoretical study of the structures, energetics, stabilities, reactivities, and out-of-plane distortive tendencies of skeletally substituted benzenes (CH)(5)XH and (CH)(4)(XH)(2) (X = B(-), N(+), Al(-), Si, P(+), Ga(-), Ge, and As(+))

Ab initio molecular orbital theory at Hartree-Fock (HF), post-Hartree-Fock (MP2 and CCSD(T)), and the hybrid density functional theory (B3LYP) calculations were done on the mono-(CH)(5)XH and diskeletally substituted (CH)(4)(XH)(2) benzenes (X = B(-), N(+), Al(-), Si, P(+), Ga(-), Ge, and As(+)). The computed relative energies of the disubstituted isomers show interesting trends. While the ortho-isomer is the most stable for X = Ga(-), Ge, and As(+), meta was found to be the most stable for X = B(-), N(+), Al(-) and Si, and para was found to be the most stable for X = P(+). Various intricate factors that govern the relative stabilities, such as the sum of bond strengths in the twin Kekule forms, rule of topological charge stabilization (TCS), and electrostatic repulsion were critically examined. The sum of bond strengths in the twin Kekule forms was proved to be quite a successful measure in predicting the relative stability orders between ortho- and meta-/para-isomers. The rule of TCS breaks down especially in the presence of overwhelming factors such as the differences in the cumulative bond strengths of the two positional isomers; however, the stability ordering between the para- and meta-isomers is successfully predicted in most cases. The tendency for ring puckering increases a great deal especially when the substituents are from 3rd or 4th row. Extension of the popular inverse relationship between the thermodynamic stability and reactivity was found to be inapplicable for this class of compounds. The computed singlet-triplet energy differences and the chemical hardness (eta) values indicate that the skeletal substitution weakens the pi-strength of the benzenoid system and increases their reactivity.