First Principle Studies on Ground and Excited Electronic States: Chemical Bonding in Main-Group Molecules, Molecular Systems with Diffuse Electrons, and Water Activation using Transition Metal Monoxides

Abstract

Potential energy profiles can be used to investigate the origins of chemical bonding of molecules. Such profiles were adopted to show the propagation of lone pair electrons of N2(X1Σg+) to the vacant orbital of O(1D) or NH(a1Δ) to produce N2O(X1Σ+) and N3H(X1Aˊ) molecules, respectively. Similarly, the combination of N2(X1Σg+) + 2O(1D) or N2O(X1Σ+) + O(1D) generates N2O2 molecule. Using multi-reference potential energy profiles chemical bonding patterns of ground and excited states of series of metal-ligand molecules were rationalized. Ground states of Li(CO)1-3, LiNO, Be(CO)1−3, and BeNO originate from some excited state of the metal. In contrast, similar metal-ammonia molecules emerge from ground state fragments. The Be…NH3 interaction guided us to proposed the :Be←:Be + Be:→Be: resonance chemical bonding pattern for Be2 dimer. The Be-Be bond can be significantly strengthened by ammonia ligation (e.g. (NH3)Be-Be(NH3) and (NH3)2Be-Be(NH3)2). Even though [Be-Be]2+ dimer is metastable, [(NH3)Be-Be(NH3)]2+ and [(NH3)2Be-Be(NH3)2]2+ create stable minima.

Ammonia and water solvate loosely bound valence electrons of metals. In such neutral or partially oxidized complexes solvated electrons orbit in a diffuse atomic s-type orbital. Fascinatingly, diffuse electrons of these complexes tend to populate quasi p-, d-, f-orbitals in low-lying electronic states. Such complexes were dubbed “solvated electron precursors” (SEPs) where an SEP is a “complex that displaces one or more electrons from its coordinated metal atom to the periphery of its ligands”. The SEPs of the simplest M(NH3)4 (M = Li, Na) bear one peripheral electron that orbit around M(NH3)4+ core. Based on the shapes of the orbitals and the excitation energies of M(NH3)4, the 1s, 1p, 1d, 2s, 2p Aufbau principle of SEPs were introduced. Be(NH3)4 and Be(H2O)4 bear two outer electrons, hence their excited state spectra show substantial multi-reference characters. Similarly, several Mg and Ca-ammine (or aqua) complexes behave as SEPs. The M (M ≡ metal): cc-pVTZ, N/O: cc-pVTZ, and H: d-aug-cc-pVTZ basis set was proven to represent excited states of these species accurately and efficiently. Ammonia or water does not solvate inner 3d electrons of titanium but solvates the valence 4s electrons which promote to higher-angular momentum p- and d-type orbitals. Two SEPs bind together to form a stable dimer. By linking two SEPs with an adjustable carbon chain, its singlet-triplet gap can be tuned into a desirable value. Twelve or twenty-four ammonia molecules can occupy the second solvation shell of a tetrahedral metal-ammine SEP (e.g. M(NH3)4@nNH3; n = 12, 24) and follow a similar Aufbau order (1s, 1p, 1d, 1f, 2s, 2p, 1g, 2d). Geometries of such clusters (species with solvated electrons and H-bonds) can be represented accurately using density functional theory with CAM-B3LYP functional.

Implementation of the correct active space in multi-reference calculations is critical for accurate description of ground and excited electronic states of transition metal monoxides. Low-lying electronic states of ZrO+ can be represented by probing a larger number of electronic states into 2p/O, 5s/Zr, 4d/Zr CASSCF active space. Interestingly, its state specific calculations tend to use 3px/O and 3py/O orbitals instead 4dxz/Zr and 4dyz/Zr in the active space and provide rather inaccurate description of electronic states. This error can be overcome by applying a larger active space that contains twelve orbitals (2p/O, 5s/Zr, 4d/Zr, and 3p/O) at CASSCF level. A similar dilemma occurs in MoO− and RuO−. Even though nine active orbitals (2p/O, 5s/Zr, 4d/Zr) can treat low-lying electronic states of NbO+, MoO+, MoO2+, RuO, RuO+, and RuO2+ accurately, the use of three more CASSCF orbitals (2p/O, 5s/Mo,Ru, 4d/Mo,Ru, 5p/Mo,Ru) is critically important to obtain the correct order of electronic states and energetics of MoO− and RuO−. Using suitable active spaces, high-level multi-reference configuration interaction calculations were performed to unravel chemical bonding patterns and spectroscopic constants of their low-lying electronic states. Water activation potential of these second-row transition metal monoxides were tested, and it was observed that in general anions are more suitable for thermodynamically favorable water activation process compared to their neutral or cationic counterparts. Furthermore, a better understanding of the electronic structures of transition metal monoxides allows us to design more practical molecular complexes for efficient water activation.