Bond orders of the diatomic molecules

Purely single-bonded spiral nitrogen chains stabilized by trivalent lanthanum ions

Inspired by the single-bonded nitrogen chains stabilized by tetravalent cerium, pentavalent tantalum, and hexavalent tungsten atoms, we explored the possibility of single-bonded nitrogen polymorphs stabilized by trivalent lanthanum ions. To achieve this, we utilized the crystal structure search method on the phase diagram of binary La-N compounds. We identified three novel thermodynamically stable phases, the C2/c LaN3, P-1 LaN4, and P-1 LaN8. Among them, the C2/c phase with infinite helical poly-N6 chains becomes thermodynamically stable above 50 GPa. Each nitrogen atom in the poly-N6 chain acquires one extra electron, and the spiral chain is purely single-bonded. The C2/c phase has an indirect band gap of ∼1.6 eV at 60 GPa. Notably, the band gap exhibits non-monotonic behavior, decreases first and then increases with increasing pressure. This abnormal behavior is attributed to the significant bonding of two La-N bonds at around 35 GPa. Phonon spectrum calculations and AIMD simulations have confirmed that the C2/c phase can be quenched to ambient conditions with slight distortion, and it exhibits excellent detonation properties. Additionally, we also discovered armchair-like nitrogen chains in LaN4 and the armchair and zigzag-like mixed nitrogen chains in LaN8. These results provide valuable insights into the electronic and bonding properties of nitrides under high pressure and may have important implications for the design and development of novel functional materials.

Eliminating all bonds from the ground state gives rise to ionic bonding in high-spin states of heterodiatomics

Understanding chemical bonding in second-row diatomics has been central to elucidating the basics of bonding itself. Bond strength and the number of bonds are the two factors that decide the reactivity of molecules. While bond strengths have been theoretically computed accurately and experimentally determined, the number of bonds is a more contentious issue, especially for complicated multi-reference systems like C2. We have developed an experimentally verifiable approach to determine bond numbers from excited spin state potential energy surfaces. On applying this to a series of second-row heterodiatomics, we obtain the surprising phenomenon of an inverted charge transfer ionic state after all the ground state bonds are broken via higher spin states. These ionic states are ubiquitous in all heterodiatomics and unexpected in non-metallic systems.

Analysis of chemical bonding of the ground and low-lying states of Mo2 and of Mo2Clx complexes, x = 2 - 10

In the present study, we perform accurate calculations via multireference configuration interaction and coupled cluster methodologies on the dimolybdenum molecule in conjunction with complete series of correlation and weighted core correlation consistent basis sets up to quintuple size. The bonding, dissociation energies, and spectroscopic parameters of the seven states that correlate to the ground state products are calculated. The ground state has a sextuple chemical bond and each of the calculated excited state has one less bond than the previous one. The calculated values for the ground(X1Σg+ ) state of Mo2 have been extrapolated to the complete basis set limits. Our final values, re=1.9324 Å and De(D0)=4.502{plus minus}0.007(4.471{plus minus}0.009) eV, are in excellent agreement with the experimental values of re=1.929, 1.938(9) Å and D0=4.476(10) eV. The Mo2 in 13Σg+ state is a weakly bound dimer, forming 5s...5pz bonds, with De=0.120 eV at re=3.53 Å. All calculated excited states (except 13Σg+) have a highly multireference character (C0=0.25-0.55). The ordering of the molecular bonding orbitals changes as the spin is increased from quintet to septet state. The quite low bond dissociation energy of the ground state is due to the splitting of the molecular bonding orbitals in two groups differing in energy by ~3 eV. Finally, the bond breaking of Mo2, as the multiplicity of spin is increased, is analyzed in parallel with the Mo-Mo bond breaking in a series of Mo2Clx complexes when x is increased. Physical insight into the nature of the sextuple bond and its low dissociation energy is provided.

Phase diagram exploration of Tc-Al-B: from bulk Tc2AlB2 to two-dimensional Tc2B2

In this study, the ternary phase diagram of the Tc-Al-B system is constructed by a combination of an evolutionary algorithm and density functional theory calculations. Four novel phases are predicted, including three binary compounds (P1̄ Al₇B₁₅, Cmcm TcAl₂, and C2 TcAl₃) and one ternary compound (Cmmm Tc₂AlB₂). All predicted structures are mechanically, dynamically, and thermodynamically stable. Among the predicted phases, P1̄ Al₇B₁₅ resembles the experimental structure of Al_0.93B₂ and Cmmm Tc₂AlB₂ corresponds to the 212-type MAB phase. Due to the in- and out-of-plane anisotropic chemical bonding in Cmmm Tc₂AlB₂, a tetragonal two-dimensional (2D) Tc₂B₂ structure could be possibly exfoliated by chemical removal of Al atoms. The electronic structure calculations indicate that the 2D Tc₂B₂ structure and its potential layered precursors are all metallic. Furthermore, the chemical reactivity of H, F, O and, OH ligands with the 2D Tc₂B₂ surface is studied and the associated 2D surface-functionalized Tc₂B₂ derivatives are found to be metallic. It is revealed that the F and O functional groups strengthen the surface atomic layer of 2D Tc₂B₂ and enhance the Young's moduli.

The binary aluminum scandium clusters AlxScy with x + y = 13: when is the icosahedron retained?

Geometrical and electronic structures of the 13-atom clusters Al_xSc_y with x + y = 13, as well as their thermodynamic stabilities were investigated using DFT calculations. Both anionic and neutral isomers of Al_xSc_y were found to retain an icosahedral shape of both Al₁₃ and Sc₁₃ systems in which an Al atom occupies the endohedral central position of the icosahedral cage, irrespective of the number of Al atoms present. Such a phenomenon occurs to maximize the number of stronger Al–Al and Sc–Al bonds instead of the weaker Sc–Sc bonds. NBO analyses were applied to examine their electron configurations and rationalize the large number of open shells and thereby high multiplicities of the mixed clusters having more than three Sc atoms. The SOMOs are the molecular orbitals belonged to the irreducible representations of the symmetry point group of the clusters studied, rather than to the cluster electron shells. Evaluation of the average binding energies showed that the thermodynamic stability of Al_xSc_y clusters is insignificantly altered as the number y goes from 0 to 7 and then steadily decreases when y attains the 7–13 range. Increase of the Sc atom number also reduces the electron affinities of the binary Al_xSc_y clusters, and thus they gradually lose the superhalogen characteristics with respect to the pure Al₁₃.

The icosahedral structure of the Al_xSc_y clusters with x + y = 13.

Seven confluence principles: a case study of standardized statistical analysis for 26 methods that assign net atomic charges in molecules

This article studies two kinds of information extracted from statistical correlations between methods for assigning net atomic charges (NACs) in molecules. First, relative charge transfer magnitudes are quantified by performing instant least squares fitting (ILSF) on the NACs reported by Cho et al. (ChemPhysChem, 2020, 21, 688–696) across 26 methods applied to ∼2000 molecules. The Hirshfeld and Voronoi deformation density (VDD) methods had the smallest charge transfer magnitudes, while the quantum theory of atoms in molecules (QTAIM) method had the largest charge transfer magnitude. Methods optimized to reproduce the molecular dipole moment (e.g., ACP, ADCH, CM5) have smaller charge transfer magnitudes than methods optimized to reproduce the molecular electrostatic potential (e.g., CHELPG, HLY, MK, RESP). Several methods had charge transfer magnitudes even larger than the electrostatic potential fitting group. Second, confluence between different charge assignment methods is quantified to identify which charge assignment method produces the best NAC values for predicting via linear correlations the results of 20 charge assignment methods having a complete basis set limit across the dataset of ∼2000 molecules. The DDEC6 NACs were the best such predictor of the entire dataset. Seven confluence principles are introduced explaining why confluent quantitative descriptors offer predictive advantages for modeling a broad range of physical properties and target applications. These confluence principles can be applied in various fields of scientific inquiry. A theory is derived showing confluence is better revealed by standardized statistical analysis (e.g., principal components analysis of the correlation matrix and standardized reversible linear regression) than by unstandardized statistical analysis. These confluence principles were used together with other key principles and the scientific method to make assigning atom-in-material properties non-arbitrary. The N@C₆₀ system provides an unambiguous and non-arbitrary falsifiable test of atomic population analysis methods. The HLY, ISA, MK, and RESP methods failed for this material.

Standardized statistical analysis of many methods to assign net atomic charges revealed relative charge transfer magnitudes and confluent correlations.