  The competition between antiferromagnetic and spin-singlet ground states
within quantum spin models and the 1/2-filled band Hubbard model has received
intense scrutiny. Here we demonstrate a frustration-induced transition from
N\'{e}el antiferromagnetism to spin-singlet in the interacting 1/4-filled band
on an anisotropic triangular lattice. While the antiferromagnetic state has
equal charge densities 0.5 on all sites, the spin-singlet state is a
paired-electron crystal, with pairs of charge-rich sites separated by pairs of
charge-poor sites. The paired-electron crystal provides a natural description
of the spin-gapped state proximate to superconductivity in many organic
charge-transfer solids. Pressure-induced superconductivity in these
correlated-electron systems is likely a transition from the 1/4-filled band
valence bond solid to a valence bond liquid.
