Computational Study of [Al, N, C, S] Isomers: Electronic Structure and Bonding Analysis.

[Al, N, C, S] isomers have been investigated computationally in their ground electronic states using DFT B3LYP, DH-DFT B2PLYP, MP2, and CCSD-(T) methods with the aug-cc-pVTZ basis set, and the CCSD-(T)-F12 method using the cc-pVTZ-F12 basis set. Seven isomers, AlNCS, SAlNC, SAlCN, AlSNC, AlCNS, AlNSC, and AlCSN are found to correspond to minima on the global potential energy surface, with AlNCS the lowest in energy according to all methods. Optimized equilibrium geometries are linear for all isomers, except for AlSNC and AlCSN. Natural bond orbital (NBO) and atoms in molecules (AIM) analyses yield a primarily ionic description of the aluminum-ligand bond in the metal̵ligand isomers. A greater covalent character is found in the aluminum bonds in the metal insertion products, SAlNC and SAlCN. Adiabatic dissociation energies indicate these [Al, N, C, S] species are thermodynamically stable. Transition states for the AlNCS ⇌ SAlNC and SAlCN ⇌ AlCNS isomerizations have been characterized. In contrast to HSCN, MgOCN, and AlOCN, the AlSCN isomer has not been found to correspond to a minimum on the potential energy surface. Low values of harmonic vibrational frequencies for aluminum-containing bending vibrations suggest floppy molecular structures. Rotational and hyperfine constants have been determined to assist with future microwave/millimeter wave spectroscopic studies.