Observation of Electronic Structure Change at the First-Order Phase Transition in Quasi-Two-Dimensional Squaric Acid (H2C4O4) byC13NMR

Abstract

A direct relation between Feder's normal mode coordinate for solid squaric acid (${\mathrm{H}}_2$ ${\mathrm{C}}_4$ ${\mathrm{O}}_4$) and the line separation $\Delta \sigma$ in high-resolution ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ NMR is established. The temperature variation of $\Delta \sigma$ fits to a first-order phase transition. At the phase transition the ${\mathrm{C}}_4$ ${\mathrm{O}}_4$ unit is shown to undergo a change in its electronic structure.