In the title compound, C₁₄H₈ClNO₃, the central ester CO₂ group is twisted away from the quinoline and furoyl rings by 57.46 (5) and 2.0 (1)°, respectively. In the crystal, mol­ecules are linked by weak C—HO inter­actions, forming chains in [001].

In the title compound, C₁₄H₈BrNO₅, the benzene rings form a dihedral angle of 62.90 (7)°. The central ester group is twisted away from the nitro-substituted and bromo-substituted rings by 71.67 (7) and 8.78 (15)°, respectively. The nitro group forms a dihedral angle of 7.77 (16)° with the benzene ring to which it is attached. In the crystal, mol­ecules are linked by weak C—HO inter­actions, forming C(12) chains which run along [001]. Halogen–halogen inter­actions [BrBr = 3.523 (3) Å] within the chains stabilized by C—HO inter­actions are observed.

In the title nitroaryl benzoate derivative, C₁₄H₉NO₅, the aromatic rings form a dihedral angle of 46.37 (8)°. The central ester moiety, —C—(C=O)—O—, is essentially planar (r.m.s. deviation for all non-H atoms = 0.0283 Å) and forms a dihedral angle of 54.06 (9)° with the 4-formyl-2-nitro­phenyl ring and 7.99 (19)° with the benzoate ring. In the crystal, mol­ecules are inter­twined by weak C—HO inter­actions, forming helical chains along [100].

In the title compound, C₁₄H₈ClNO₅, the benzene rings form a dihedral angle of 19.55 (9)°. The mean plane of the central ester group [r.m.s. deviation = 0.024 Å] forms dihedral angles of 53.28 (13) and 36.93 (16)°, respectively, with the nitro- and chloro-substituted rings. The nitro group forms a dihedral angle of 19.24 (19)° with the benzene ring to which it is attached. In the crystal, mol­ecules are linked by weak C—HO hydrogen bonds, forming C(7) chains, which run along [100].

In the crystal structure of 6-meth­oxy­quinoline N-oxide di­hydrate, C₁₀H₉NO₂·2H₂O, (I), the presence of two-dimensional water networks is analysed. The water mol­ecules form unusual water channels, as well as two inter­secting mutually perpendicular columns. In one of these channels, the O atom of the N-oxide group acts as a bridge between the water mol­ecules. The other channel is formed exclusively by water mol­ecules. Confirmation of the mol­ecular packing was performed through the analysis of Hirshfeld surfaces, and (I) is compared with other similar iso­quinoline systems. Calculations of bond lengths and angles by the Hartree-Fock method or by density functional theory B3LYP, both with 6-311++G(d,p) basis sets, are reported, together with the results of additional IR, UV-Vis and theoretical studies.