Similarities and differences in the structures of 5-bromo-6-hy­droxy-7,8-dimethyl­chroman-2-one and 6-hy­droxy-7,8-dimethyl-5-nitro­chroman-2-one

The title compounds, C₁₁H₁₁BrO₃, (I), and C₁₁H₁₁NO₅, (II), respectively, are derivatives of 6-hy­droxy-5,7,8-trimethyl­chroman-2-one substituted at the 5-position by a Br atom in (I) and by a nitro group in (II). The pyran­one rings in both mol­ecules adopt half-chair conformations, and intra­molecular O—H...Br [in (I)] and O—H...O_nitro [in (II)] hydrogen bonds affect the dispositions of the hy­droxy groups. Classical inter­molecular O—H...O hydrogen bonds are found in both mol­ecules but play quite dissimilar roles in the crystal structures. In (I), O—H...O hydrogen bonds form zigzag C(9) chains of molecules along the a axis. Because of the tetragonal symmetry, similar chains also form along b. In (II), however, similar contacts involving an O atom of the nitro group form inversion dimers and generate R₂²(12) rings. These also result in a close inter­molecular O...O contact of 2.686 (4) Å. For (I), four additional C—H...O hydrogen bonds combine with π–π stacking inter­actions between the benzene rings to build an extensive three-dimensional network with mol­ecules stacked along the c axis. The packing in (II) is much simpler and centres on the inversion dimers formed through O—H...O contacts. These dimers are stacked through additional C—H...O hydrogen bonds, and further weak C—H...O inter­actions generate a three-dimensional network of dimer stacks.