A second monoclinic polymorph of methyl 4-hydroxybenzoate

A second monoclinic polymorph of methyl 4-hydroxybenzoate, C8H8O3, is reported. The unit-cell dimensions are different from those of the previously reported monoclinic form [Vujovic & Nassimbeni (2006 ▶). Cryst. Growth Des. 6, 1595–1597]. The asymmetric unit contains three crystallographically independent molecules, as observed in the previous form. The crystal structure is stabilized by intermolecular O—H⋯O and C—H⋯O hydrogen bonds and C—H⋯π interactions, which link the molecules into a three-dimensional network.

A second monoclinic polymorph of methyl 4-hydroxybenzoate, C 8 H 8 O 3 , is reported. The unit-cell dimensions are different from those of the previously reported monoclinic form [Vujovic & Nassimbeni (2006). Cryst. Growth Des. 6, 1595-1597]. The asymmetric unit contains three crystallographically independent molecules, as observed in the previous form. The crystal structure is stabilized by intermolecular O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions, which link the molecules into a threedimensional network.

Comment
The crystal structure of the title compound at room temperature and at 113 K has been reported previously (Lin, 1983;Vujovic & Nassimbeni, 2006). We report here the structure of a second monoclinic polymorph of the title compound which was elucidated at 100.0 (1) K.
The compound crystallizes in the space group Cc with three independent molecules in the asymmetric unit, similar to the first monoclinic polymorph (Vujovic & Nassimbeni, 2006). However, the cell parameters of the present monoclinic polymorph differ significantly from the previous polymorph [a = 13.006 (3) Å, b = 17.261 (4) Å, c = 12.209 (2) Å and β = 129.12 (3)°]. The corresponding bond lengths and angles of the three independent molecules agree with each other and also with those in the other monoclinic polymorph (Vujovic & Nassimbeni, 2006). Each of the independent molecules are planar.
In the asymmetric unit, the independent molecules are linked via O-H···O and C-H···O hydrogen bonds. The crystal packing is stabilized by intermolecular O-H···O and C-H···O hydrogen bonds and C-H···π interactions which link the molecules into a three-dimensional network (Fig.2).

Experimental
Methyl 4-hydroxybenzoate was purchased from Aldrich. Single crystals were obtained by slow evaporation of an ethanol solution.

Refinement
H atoms were positioned geometrically [C-H = 0.93-0.96 Å and O-H = 0.82 Å] and refined using a riding model, with U iso (H) = 1.2U eq (C). In the absence of significant anomalous scattering, 3197 Friedel pairs were merged prior to the final refinement.

Special details
Experimental. The data was collected with the Oxford Cyrosystem Cobra low-temperature attachment. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. 0.0207 (9) 0.0252 (9) 0.0182 (10) 0.0078 (7) 0.0058 (8) 0.0009 (7) O2A 0.0188 (9) 0.0202 (8) 0.0173 (9) 0.0021 (7) 0.0039 (7) 0.0009 (7)