Understanding the working mechanism of a homogeneous transition-metal-centered catalyst requires a prediction of the structure of the intermediate coordination complex. The K-edge of X-ray absorption near-edge structure (XANES) spectrum can directly probe the geometric and electronic structures of an active transition-metal center in-situ. A theoretical calculation is indispensable in the quantitative XANES analysis because there is no other way to translate complicated spectral features of K-edge absorptions into orbital and structural configurations. In the present work, we predicted and interpreted the K-edge XANES spectra of vanadium-centered complexes with the aid of a combination of linear-response (LR) and real-time (RT) time-dependent density functional theory (TDDFT). Using LR-TDDFT, we assigned pre-edge peaks to dipole-allowed transitions of 1s electron to the 3d4p hybridized orbitals and explained the increased peak intensities via a large p-hybridization ratio (Figure 1). However, the LR approximation is insufficient for us to commit the shoulder peaks on the K-edge due to multiple excitations and large density rearrangements. We attacked the later problem by applying RT-TDDFT where the time-dependent electronic densities are generated by solving time-dependent Kohn–Sham equations in the real-time framework. We obtained the time-dependent signals for transition dipole moments using RT-TDDFT and converted them into frequency-dependent spectral intensities using a discrete Fourier transform. A semi-empirical estimation of full width at half maximum (FWHM) is no longer necessary when a natural broadening is already provided. Based on our RT-TDDFT analysis, we successfully reproduced and assigned the shoulder peaks on the raising K-edge to the core electron excitations of vanadium-centered complex compounds.

TDDFT approaches reproduce and assign feature peaks in the XANES spectra of vanadium complexes

ACS Fall 2021

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