2-(2-Methyl-benzo-yl)benzoic acid: catemeric hydrogen bonding in a γ-keto acid.

The crystal structure of the title compound, C15H12O3, displays catemeric aggregation involving O-H⋯O hydrogen bonds progressing from the carboxyl group of one mol-ecule to the ketone O atom of another glide-related neighbor. The mol-ecule is twisted, with the toluene 80.61 (3)° out of plane with respect to the phenyl group of the benzoic acid. The acid group makes a dihedral angle of 13.79 (14)° with the attached phenyl ring. The mol-ecules are achiral, but the space group glide planes create alternating conformational chirality in the chain units. The four hydrogen-bonding chains progress along [001] in an A-A-B-B pattern (right-to-left versus left-to-right), and are related to each other by the center of symmetry at (0.5, 0.5, 0.5) in the chosen cell. There is one close contact (2.54 Å) between a phenyl H atom and the acid carbonyl from a symmetry-related mol-ecule.

The hydrated and anhydrous gold(III) tetrachloride salts of L-ecgonine, an important forensic toxicology marker for cocaine.

The structure of the hydrated gold(III) tetrachloride salt of L-ecgonine {hydronium tetrakis[(1R,2R,3S,5S,8S)-3-hydroxy-8-methyl-8-azoniabicyclo[3.2.1]octane-2-carboxylate pentakis[tetrachloridoaurate(III)] hexahydrate}, (C(9)H(16)NO(3))(4)(H(3)O)[AuCl(4)](5).6H(2)O, demonstrates an unprecedented stoichiometric relationship between the cations and anions in the unit cell. The previous tropane alkaloid structures, including the related hydrochloride salts, all have a cation-anion ratio of 1:1, as does the anhydrous salt described here, namely (1R,2R,3S,5S,8S)-3-hydroxy-8-methyl-8-azoniabicyclo[3.2.1]octane-2-carboxylate tetrachloridoaurate(III), (C(9)H(16)NO(3))[AuCl(4)]. The hydrated salt, however, consists of four monopositive N-protonated units of the alkaloid and five [AuCl(4)](-) counter-ions, plus seven solvent water molecules. The H atom required for change balance has been assigned to a water molecule. In addition, the hydrate has a novel arrangement, with all seven of the water molecules and all of the O atoms in the cations participating in an alternating arrangement of interleaved sheets of the anionic species. Both the hydrate and the anhydrous salt of the same toxicologically important marker for cocaine show that the cation and anion are in close proximity to each other, as was found in the gold(III) tetrachloride salt of L-cocaine.

4-Oxocyclo-hexa-neacetic acid: catemeric hydrogen bonding and spontaneous resolution of a single conformational enanti-omer in an achiral ∊-keto acid.

The asymmetric unit of the title compound, C(8)H(12)O(3), consists of a single conformational enanti-omer, which aggregates in the catemeric acid-to-ketone hydrogen-bonding mode [O⋯O = 2.682 (4) Šand O-H⋯O = 172 (6)°]. Four hydrogen-bonding chains of translationally related mol-ecules pass through the cell orthogonal to the 4(3) screw axis along c, alternating in the 110 and the 10 direction, with alignment with respect to this axis of + + - -. Successive chains are rotated by 90° around the c axis. One C-H⋯O=C close contact, involving the carboxyl group, exists.

3-Acetyl-benzoic acid.

In the crystal structure of the title compound, C(9)H(8)O(3), essentially planar mol-ecules [the carboxyl group makes a dihedral angle of 4.53 (7)° with the plane of the ring, while the acid group forms a dihedral angle of 3.45 (8)° to the ring] aggregate by centrosymmetric hydrogen-bond pairing of ordered carboxyl groups. This yields dimers which have two orientations in a unit cell, creating a herringbone pattern. In addition, two close C-H⋯O inter-molecular contacts exist: one is between a methyl H atom and the ketone of a symmetry-related mol-ecule and the other involves a benzene H atom and the carboxyl group O atom of another mol-ecule. The crystal studied was a non-merohedral twin with twin law [100, 00, 0] and a domain ratio of 0.8104(14): 0.1896(14).

3-Oxocyclo-butane-carboxylic acid: hydrogen bonding in a small-ring γ-keto acid.

The title ketocarboxylic acid, C(5)H(6)O(3), is the smallest carboxy-cyclanone to have its crystal structure determined. It adopts a chiral conformation, by rotation of its carboxyl O atoms away from the plane of skeletal symmetry that passes through the carboxyl carbon and both atoms of the ketone carbonyl. The four-membered ring is non-planar, with a shallow fold of 14.3 (1)° along a line connecting the two α-carbons of the ketone group. In the crystal, the molecules are linked by centrosymmetric hydrogen-bond pairing of ordered carboxylic acid groups [O⋯O = 2.6392 (12) Šand O-H⋯O = 175.74 (15)°], yielding two different sets of dimers, related by by a 2(1) screw axis in c, in the cell. A C-H⋯O interaction is also present.

(±)-2-Oxocyclo-penta-neacetic acid: catemeric hydrogen bonding in a γ-keto acid.

The title racemate, C(7)H(10)O(3), aggregates in the solid as acid-to-ketone hydrogen-bonding catemers [O⋯O = 2.7050 (13) Šand O-H⋯O = 166.1 (17)°] having glide-related components. Four such heterochiral chains, paired centrosymmetrically about (, , ) in the cell, proceed through the cell in the 010 direction, with alignment with respect to the c axis of ++--.

The hydro-chloride salt of l-ecgonine, a congener of cocaine.

The title compound, (1R,2R,3S,5S,8S)-3-hydr-oxy-8-methyl-8-azoniabicyclo-[3.2.1]octane-2-carboxylic acid chloride, C(9)H(16)NO(3) (+)·Cl(-), is both a metabolite and a precursor of the tropane alkaloid l-cocaine. The carboxyl group is not involved in dimerization, but instead donates a hydrogen bond to the chloride counter-ion, which participates in two additional hydrogen bonds. The chloride ion is thus trigonally hydrogen bonded to three l-ecgonine cations. The quarternary N proton is intra-molecularly hydrogen bonded to the carboxyl C=O group, an arrangement identical to that reported for both (-)-nor-cocaine and the tetrachloroaurate(III) salt of l-cocaine. One close inter-molecular C-H⋯O contact exists.

(±)-trans-3-Benzoyl-bicyclo-[2.2.2]octane-2-carboxylic acid.

The title keto acid, C(16)H(18)O(3), displays significant twisting of all three ethyl-ene bridges in its bicyclo-[2.2.2]octane structure owing to steric inter-actions; the bridgehead-to-bridgehead torsion angles are 13.14 (12), 13.14 (13) and 9.37 (13)°. The compound crystallizes as centrosymmetric carboxyl dimers [O⋯O = 2.6513 (12) Šand O-H⋯O = 178°], which have two orientations within the cell and contain no significant carboxyl disorder.

(±)-trans-3-Oxo-1,2,3,4,4a,9,10,10a-octa-hydro-phenanthrene-10a-carboxylic acid: catemeric hydrogen bonding in a δ-keto acid.

The title compound, C(15)H(16)O(3), aggregates as hydrogen-bonded catemers progressing from each carboxyl to the ketone of a screw-related neighbor [O⋯O = 2.6675 (14) Šand O-H⋯O = 170°]. Two parallel centrosymmetrically related single-strand hydrogen-bonding helices proceed through the cell in the b-axis direction. The packing includes three inter-molecular C-H⋯O=C close contacts, involving both the ketone and the carboxyl group. The structure is isomorphous with that of the previously described Δ(4) α,ß-unsaturated ketone.

4-(2-Benzoyl-ethyl)benzoic acid.

The title compound, C(16)H(14)O(3), adopts a conformation in which each functional group is almost coplanar with its adjacent ring, while the two aromatic rings are twisted with respect to one another with a dihedral angle of 78.51 (3)°. The compound dimerizes by standard centrosymmetric hydrogen-bonded carboxyl pairing [O⋯O = 2.6218 (11) Šand O-H⋯O = 176 (2)°]. The packing includes two inter-molecular C-H⋯O close contacts with the ketone group.

(2RS,8aRS)-6-Oxo-1,2,3,4,6,7,8,8a-octa-hydro-naphthalene-2-carboxylic acid.

The title racemate, C(11)H(14)O(3), aggregates in the crystal structure as acid-to-ketone O-H⋯O hydrogen-bonding catemers whose components are glide-related. The relative stereochemistry at the carboxyl group arises spontaneously during the synthesis. Two inter-molecular C-H⋯O=C close contacts were found, both involving the acid group.

(2SR,4aSR,8aSR)-6-Oxoperhydro-naphthalene-2-carboxylic acid.

In the title racemic compound, C(11)H(16)O(3), the mol-ecule adopts a conformation that places its carboxyl group in an equatorial position. Mol-ecules aggregate by hydrogen-bond pairing of carboxyl groups, yielding centrosymmetric dimers that are arranged into layers in the (020) planes.

3-Oxo-5beta-24-norcholanic acid: acid-to-acid hydrogen-bonding catemers employing the rare anti carboxyl conformation in the aggregation of a steroid keto acid.

The title ketocarboxylic acid [systematic name: (5R,8R,9S,10S,13R,14S,17R,20R)-3-oxo-24-norcholanic acid], C(23)H(36)O(3), forms acid-to-acid hydrogen-bonding chains [O...O = 2.620 (2) A and O-H...O = 163 (3) degrees ] in which all carboxyl groups adopt the rare anti conformation, while the ketone group does not participate in the hydrogen bonding. The occurrence and energetics of this conformation are discussed. One intermolecular C-H...O close contact exists, which plays a role in stabilizing the hydrogen-bonding arrangement.

(+/-)-3-Oxocyclopentanecarboxylic acid: The smallest keto acid known to aggregate by catemeric hydrogen bonding.

The title compound, C6H8O3, is the smallest keto acid yet found to aggregate in the solid as acid-to-ketone hydrogen-bonded catemers. Four translational chains pass through the cell in the a direction [O...O = 2.6915 (14) A and O-H...O = 166 degrees]. Two intermolecular C-H...O close contacts exist, involving both carbonyl functions.

(-)-3,7-Dioxo-5beta-cholanic acid: dual hydrogen-bonding modes in a diketonic bile-acid derivative.

The asymmetric unit of the title compound, C(24)H(36)O(4), contains three molecules, all differing in their side-chain conformations and all linked by hydrogen bonding confined entirely within a three-molecule block. One connection is of the acid-to-ketone type [O...O = 2.7055 (19) Angstrom and O-H...O = 180 degrees ] and the other involves carboxyl pairing [O...O = 2.6485 (18) and 2.6598 (18) Angstrom, and O-H...O = 168 and 174 degrees ]. Numerous intermolecular C-H...O close contacts connect neighbouring molecules.

(+)-3,11-Dioxo-5alpha-androstanecarboxylic acid: catemeric hydrogen bonding in a steroidal diketo acid.

The title compound, C(20)H(28)O(4), aggregates catemerically, with hydrogen-bonding links from each carboxylic acid group to the 3-oxo group in the A ring of a molecule translationally related in both the a and the b directions [O...O = 2.7537 (18) Angstrom and O-H...O = 162 degrees ]. The 11-oxo group in the C ring is not involved in the hydrogen bonding. A single intermolecular C-H...O close contact connects the carboxyl C=O group to a methyl group on an adjacent molecule.

Two (+)-alpha,4-dimethyl-2-oxocyclohexaneacetic acids: hydrogen bonding in a terpenoid gamma-keto acid and in a diastereomeric lactol.

The (+)-(alphaS,1S,4R)-diastereomer of the title structure, C10H16O3, aggregates in the solid as non-symmetric dimers with disorder in both carboxyl groups [O...O = 2.710 (5) and 2.638 (5) A]. The two molecules constituting the asymmetric unit pair around a pseudo-twofold rotational axis and differ only slightly in their distances and angles, but one methyl group displays rotational disorder absent in the other molecule. Five intermolecular C-H...O close contacts exist, involving both ketone groups. The (+)-(alphaR,1R,4R)-diastereomer exists in the crystal in its closed-ring lactol form, (3R,3aR,6R,7aR)-2,3,3a,4,5,6,7,7a-octahydro-7a-hydroxy-3,6-dimethylbenzo[b]furan-2-one, C(10)H(16)O(3), and aggregates as hydrogen-bonded catemers that extend from the hydroxyl group of one molecule to the carbonyl group of a neighbor screw-related along b [O...O = 2.830 (3) A and O-H...O = 169 degrees]. One close intermolecular C-H...O contact exists involving the carbonyl group.

2-Hydroxyisophthalic acid: hydrogen-bonding patterns in the monohydrate and the tetraphenylphosphonium salt. An instance of dramatic acidity enhancement by symmetric, internally hydrogen-bonded anion stabilization.

The monohydrate of the title phenolic diacid (C8H6O5.H2O, 2-hydroxybenzene-1,3-dioic acid or 3-carboxysalicylic acid) adopts a planar conformation, with the phenol hydrogen internally hydrogen-bonded to the carbonyl of one highly ordered carboxyl, which, in turn, donates a hydrogen bond to the oxygen of water. The second carboxyl is disordered and hydrogen-bonded both to water and to the disordered carboxyl of a centrosymmetrically related neighbor in a static disorder arrangement extending over two full asymmetric units. The water accepts either one or two hydrogen bonds and donates a long bifurcated hydrogen bond shared equally by O atoms of the phenol and the disordered carboxyl. The hydrogen bonding includes no standard carboxyl pairing and is entirely two-dimensional. The resulting planar ribbons stack translationally at a distance of 3.413 (8) A, in an offset arrangement having non-translational interplanar distances of 0.821 (5) and 2.592 (6) A. This structure is compared with two previously reported for this compound. The title compound forms a monoanion, whose tetraphenylphosphonium salt is described (C32H25O5P, tetraphenylphosphonium 2,6-dicarboxyphenolate, tetraphenylphosphonium 2-oxidoisophthalic acid or tetraphenylphosphonium 3-carboxysalicylate). The phenol oxygen is the site of formal negative charge on the anion, which is stabilized in a planar arrangement by symmetrical hydrogen bonds from both ortho-carboxyl groups. The energetics of this arrangement, the phenol and carboxyl acidities, and factors affecting those acidities and providing anion stabilization are discussed.

(-)-Parasantonic acid and its enol lactone, (+)-parasantonide: observation of the rare acid-to-acid catemeric hydrogen-bonding mode in a gamma,epsilon-diketocarboxylic acid.

The title diketo acid, (-)-alpha,3a,7-trimethyl-5,8-dioxo-1,4-ethanoperhydropentalene-1-acetic acid, C(15)H(20)O(4), is shown to aggregate in the solid state as acid-to-acid hydrogen-bonded catemers, whose chains follow 2(1) screw axes from each carboxyl H atom to the C=O group of a neighboring carboxyl group [O.O = 2.672 (4) A and O.H-O = 173 degrees ]. Two parallel counterdirectional screw-related single-strand hydrogen-bonded chains pass through the cell in the a direction. Two intermolecular C=O.H-C close contacts are present in this compound. Both this diketo acid and its enol lactone, (+)-parasantonide [systematic name: (-)-alpha,3a,7-trimethyl-5-oxo-1,4-ethenoperhydropentalene-1,8-carbolactone], C(15)H(18)O(3), have an R configuration at the methylated chiral center adjacent to the carboxyl group, unlike the precursor from which they are derived, viz. (-)-santonic acid.