Probing electronic structure of chiral and achiral unsaturated hydrocarbons at the carbon edge

Core-level spectroscopies provide an atom-specific probe of electronic structure, including the effects of molecular geometry and chirality. This contribution investigates the carbon K-edge electronic structure in a series of conjugated hydrocarbons and their chiral derivatives using frozen-core core-valence-separated equation-of-motion coupled-cluster methods with single and double excitations (fc-CVS-EOM-CCSD). Core ionization energies and X-ray absorption spectra (XAS) are analyzed for ethylene, benzene, toluene, butadiene, and stilbene as well as their chiralized derivatives. The computed XAS reproduce the main features of available experimental spectra and reveal how molecular structure and π-conjugation modify core-level transitions. Chiral substitution lowers molecular symmetry, activates transitions that are dipole-forbidden in the parent molecules, redistributes oscillator strength among framework-centered states, and introduces additional substituent-centered core excitations. In addition, it activates X-ray circular dichroism (XCD). Comparison of XAS and XCD demonstrates their complementary character. Whereas closely lying transitions often merge into a single XAS band, their signed rotatory strengths can produce distinct, sign-alternating features in XCD. Consequently, XCD reveals electronic structure that is often hidden in congested XAS spectra. These results demonstrate how high-level simulations of carbon K-edge XAS and XCD can connect local core-level electronic structure with molecular structure, conjugation, and chirality.