A triclinic polymorph of hexanedioic acid

Hexanedioic acid (or adipic acid), C6H10O4, crystallizes with two crystallographically independent half-molecules in the asymmetric unit of the triclinic unit cell, space group P , as each molecule lies across a crystallographic inversion centre. A monoclinic polymorph has been reported previously, most recently by Ranganathan, Kulkarni & Rao [J. Phys. Chem. A, (2003), 107, 6073–6081]. The molecules adopt the expected zigzag structure and are linked via centrosymmetric pairs of O—H⋯O hydrogen bonds, forming infinite one-dimensional chains along [011]. These chains are stacked along the a axis. The crystal is further stabilized by weak C—H⋯O interactions.

Hexanedioic acid (or adipic acid), C 6 H 10 O 4 , crystallizes with two crystallographically independent half-molecules in the asymmetric unit of the triclinic unit cell, space group P1, as each molecule lies across a crystallographic inversion centre. A monoclinic polymorph has been reported previously, most recently by Ranganathan, Kulkarni & Rao [J. Phys. Chem. A, (2003), 107, 6073-6081]. The molecules adopt the expected zigzag structure and are linked via centrosymmetric pairs of O-HÁ Á ÁO hydrogen bonds, forming infinite one-dimensional chains along [011]. These chains are stacked along the a axis. The crystal is further stabilized by weak C-HÁ Á ÁO interactions.

Comment
Structural investigation of crystalline solids undergoing phase transition has been an interesting area of research. Molecular solids are more interesting in that they often crystallize in different structural forms and exhibit polymorphic transformations (Dunitz, 1991). We have previously reported reversible phase transitions due to hydrogen bonds in some organic compounds Fun et al., 2003;; How et al., 2005). Aliphatic dicarboxylic acids form an interesting class of organic compounds for hydrogen bonding and phase transition studies. In the course of our research on the influence of hydrogen bonding on phase transitions, we have found that adipic acid exists in both monoclinic and triclinic polymorphs.
The triclinic form does not undergo a phase transition in sharp contrast to the behaviour of the monoclinic form (Srinivasa Gopalan et al., (1999). We report herein the crystal structure of the triclinic polymorph of adipic acid (I).
The crystal structure of the hexanedioic acid or adipic acid was previously reported by Ranganathan et al., (2003) and Srinivasa Gopalan et al., (1999Gopalan et al., ( , 2000 in the monoclinic space group P21/c. It was found that adipic acid exhibits a phase transition at around 136 K (Srinivasa Gopalan et al., 1999) and does not exhibit polymorphism (Srinivasa Gopalan et al., 2000). However, in the present work, we have found that adipic acid actually does exhibit polymorphisim in which the compound crystallized out in the triclinic space group P -1.
In the structure of (I), Fig. 1, each of the two unique adipic acid molecules, C 6 H 10 O 4 , lies across a different crystallographic inversion centre. There are two crystallographically independent half molecules in the asymmetric unit, A and B, with slightly different bond lengths and bond angles. The molecules exist in an zigzag form. Atoms O1A, O2A, C1A, C2A and C3 lie on the same plane in one molecule with a maximum deviation of 0.006 (1) Å for C1A while atoms O1B, O2B, C1B and C2B in the other molecule are also coplanar with a maximum deviation -0.005 (1) Å for atom C1B. The interplanar angle between these two planes is 61.14 (7)°. Bond lengths and angles in the title compound are within normal ranges (Allen et al., 1987) and comparable to those in related structures (Ranganathan et al., 2003;Srinivasa Gopalan et al., 1999;2000).
In the crystal packing (Fig. 2), the molecules are linked by centrosymmetric pairs of O-H···O hydrogen bonds forming infinite one-dimensional chains along the [0 1 1] directions and these molecular chains are stacked along the a axis. The crystal is stablized by O-H···O hydrogen bonds and weak C-H···O interactions (Table 1). It is interesting to note that this triclinic polymorph has fewer O-H···O hydrogen bonds and weak C-H···O interactions in comparison to the monoclinic form (Srinivasa Gopalan et al., 1999;2000).

Experimental
Adipic acid was obtained commercially (Fluka, Germany). Single crystals of adipic acid were grown by slow evaporation of ethyl acetate solution at room temperature.

Refinement
All the H atoms were positioned geometrically and refined using a riding model with C-H = 0.93-0.96 Å and O-H = 0.82 Å. The U iso values were constrained to be -1.2U eq of the carrier atom for all hydrogen atoms. The highest residual electron density peak is located at 0.72 Å from C1A and the deepest hole is located at 0.70 Å from C3A.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K. 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. 108.6 C1B-C2B-H2BA 108.5 C3A-C2A-H2AA 108.6 C3B-C2B-H2BA 108.5 C1A-C2A-H2AB 108.6 C1B-C2B-H2BB 108.5 C3A-C2A-H2AB 108.6 C3B-C2B-H2BB 108.5