We investigated the local magnetism of the organic conductor $\kappa \text{−}{\left(\mathrm{ET}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Cl}$ with a quasitriangular lattice of weakly dimerized molecules through ${}^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) spectroscopy. The NMR spectra and nuclear relaxation show that charge disproportionation occurs, associated with the metal-insulator transition at 31 K. The relaxation rate indicates that the paramagnetic spins in the insulating phase undergo a transition into a spin-singlet ground state with the emergence of orphan spins, a possible valence-bond-glass state. The present results are in high contrast to the spin-cluster paramagnetism of the electric dipole-liquid candidate $\kappa \text{−}{\left(\mathrm{ET}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Br}$, having nearly identical material parameters. This fact indicates that these two systems are on the verge between distinct phases in both charge and spin degrees of freedom; a spin-singlet charge-ordered state versus a spin-active Mott insulating state, competing with each other on a triangular lattice of dimerized sites.

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• Charge order
• Magnetism
• Strongly correlated systems
• Nuclear magnetic resonance