The K-edge of X-ray absorption near-edge structure (XANES) spectrum can directly probe the in-situ geometric and electronic structures of an active transition-metal center in homogeneous catalysis. Theoretical calculation is indispensable in the quantitative analysis of XANES to translate complicated spectral features of K-edge absorptions into orbital and structural configurations. The present work focused on K-edge absorptions of four vanadium complex catalysts with methyl or/and chloride ligands. Linear-response time-dependent density functional theory (LR-TDDFT) and real-time time-dependent density functional theory (RT-TDDFT) methods were used to calculate the K-edge XANES spectra (Figure 1). Using LR-TDDFT, the pre-edge regions were well reproduced, and their peaks were assigned to dipole-allowed transitions of 1s electrons to the 3d4p hybridized orbitals. However, the shoulder peak regions were not fully explained due to the complicated multiple excitations and extensive density rearrangement that invalidate the linear-response approximation. RT-TDDFT, which directly solves the time-dependent Kohn—Sham equations and surpasses the linear-response region, served as a supplement to LR-TDDFT. Using broadband δ-function kick fields, the broadband spectral features, including those belonging to the shoulder peaks, were reproduced by RT-TDDFT. Using monochromic continuous wave fields, important peaks assigned by LR-TDDFT were confirmed by RT-TDDFT using the resonant excitations from 1s orbitals to linear combinations of 3d and 4p.

TDDFT predictions of broadband K-edge XANES for vanadium complexes

ACS Spring 2022

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