Role of hydrogen bonding in charge-ordered organic conductor $\alpha$-(BEDT-TTF)${}_2$I${}_3$ probed by ${}^{127}$I nuclear quadrupole resonance

We present ${}^{127}$I nuclear quadrupole resonance spectra and nuclear relaxation of $\alpha$-(BEDT-TTF)${}_2$I${}_3$ that undergoes a charge-ordering transition. Only one of the two I${}_3$ anion sites shows a significant differentiation in the electric field gradients across the first-order transition. The charge modulation only in the BEDT-TTF layers can not reproduce; instead, an anion-donor interaction accompanied by hydrogen bonding is necessary. The dominating source for the nuclear relaxation is the local libration of the I${}_3$ anions, but an anomalous peak is detected just below the transition, as observed by ${}^{13}$C NMR.