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.

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).

Improvements in complex mixture analysis by NMR: DQF-COSY iDOSY.

Diffusion-ordered NMR spectroscopy (DOSY NMR) is a highly useful tool for the study of complex mixtures via NMR. Often, spectral overlap limits the ability of obtaining cleanly separated subspectra of the components due to inherently instable multiexponential fits or data inversion procedures. Three-dimensional DOSY variants offer the advantage of separating individual peaks in an additional dimension, such that robust monoexponential fits to cross-peaks may be used to determine the diffusion coefficients with higher accuracy. For sensitivity reasons, methods based on proton nuclei are preferable. We show that a double-quantum-filtered COSY-DOSY experiment provides advantages over COSY-DOSY, while high signal-to-noise ratios are maintained. We demonstrate the viability of the technique by applying it to a solution of single-stranded DNA oligomers and to a mixture of unprocessed beeswax and decanol.

trans-4-Acetylcyclohexanecarboxylic acid and (+/-)-trans-2-acetylcyclohexanecarboxylic acid: hydrogen-bonding patterns in two isomeric keto acids.

Both title compounds, C(9)H(14)O(3), display carboxyl-dimer hydrogen-bonding patterns. The 4-acetyl isomer adopts a chiral conformation with negligible disordering of the methyl and carboxyl groups and forms centrosymmetric dimers across the b and c edges of the chosen cell [O.O = 2.667 (3) A and O-H.O = 175 degrees ]. Intermolecular C-H.O close contacts were found for both carbonyl groups. In the 2-acetyl isomer, there is no intramolecular interaction between the carboxyl and acetyl groups and the hydrogen bonding involves centrosymmetric carboxyl dimerization across the ab and ac faces of the chosen cell [O.O = 2.668 (2) A and O-H.O = 173 degrees ]. The carboxyl group is negligibly disordered, but significant rotational disordering was found for the acetyl methyl group. An intermolecular C-H.O close contact was found involving the ketone group.

(+)-3,11-Dioxoandrost-4-ene-17 beta-carboxylic acid: catemeric hydrogen bonding in a steroidal keto acid.

The title keto acid, C(20)H(26)O(4), forms carboxyl-to-ketone hydrogen-bonding catemers [O...O = 2.653 (5) A and O[bond]H...O = 172 (5) degrees ], linking translationally related molecules via the A-ring ketone. The two molecules in the cell form two parallel counter-directional chains, screw-related in b. A total of four intermolecular C[bond]H...O[double bond]C close contacts was found, involving both ketone functions.

1'-Acetylferrocene-1-carboxylic acid: an instance of hydrogen bonding in the rare dimeric acid-to-ketone mode.

The title compound, [Fe(C(6)H(5)O(2))(C(7)H(7)O)], adopts a conformation involving partial staggering of its rings and aggregates in the solid as acid-to-ketone hydrogen-bonding dimers [O.O = 2.720(4)A and O-H...O = 164 degree] having centrosymmetrically related components. Close intermolecular C-H...O contacts were found to both carboxyl O atoms.