Cocrystal Design Gone Awry? A New Dimorphic Hydrate of Oxalic Acid

The crystal chemistry of oxalic acid has been studied since 1880, including 37
published structure determinations of the two anhydrous forms (α, β) and the dihydrate, the
latest in 2000. In attempts to cocrystallize asparagine or glutamine with oxalic acid, utilizing the
particular hydrogen bond synthon

(8), we unexpectedly obtained two previously unreported
sesquihydrates of oxalic acid. Form 1 is orthorhombic, Pnma, and the unit cell is a = 11.231(2)
Å, b = 12.330(2) Å, c = 6.908(1) Å. Form 2 crystallizes in the triclinic crystal system in space
group P1̄, and the unit cell is a = 6.337(2) Å; b = 7.247(3) Å, c = 10.571(4)Å, α = 94.34(1)°,
β = 100.244(9)°, γ = 97.67(1)°. Both of the new hydrate forms of oxalic acid utilized the

(8)
hydrogen bonded synthon.
Keywords: Cocrystals; polymorphs; hydrates