Solvatochromism Observed in the X-ray Absorption Spectrum of Indole Dissolved in Water

Current developments in X-ray absorption spectroscopy (XAS) for liquid samples in the water window demand a rigorous understanding of the interactions between molecules or solute-solvent interactions observed in the spectra. Meanwhile, a theoretical description of such effects, in addition to inner-shell excitations, remains controversial. The controversy is mainly over whether the orbitals should be optimized in the final states or whether the orbital optimizations can be expressed by dynamic electron correlation. In the present work, we measured the XAS spectra of indole in aqueous solution at the carbon and nitrogen K-edges to compare them with those measured in the gas phase. Obvious solvatochromism was observed only in the XAS spectrum measured at the nitrogen K-edge. We then interpreted the observed solvatochromism by simulating spectra with both ΔSCF, where the orbitals were optimized in the final states, and the algebraic-diagrammatic construction through second order [ADC(2)], where the molecular orbitals optimized in the ground state were used throughout. The present results indicate that covalent interactions, such as hydrogen bonds, are the dominant causes of the solvation effects observed in XAS spectra. The present simulations with ΔSCF and ADC(2), in addition to some other reports, highlight the importance of optimizing the orbitals in the final inner-shell excited states for general inner-shell calculations with predictive accuracy.