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The various conformers of the dicarboxylic acids HO2C--(CH2)n--CO2H, n = 1-4, were obtained using density functional methods (DFT), both in the gas phase and in the aqueous phase using a polarized continuum model (PCM). Several new conformers were identified, particularly for the two larger molecules glutaric (n = 3) and adipic acid (n =4). The PCM results show that the stability of most conformers were affected, many becoming unstable in the aqueous phase; and the energy ordering of conformers is also different. The results suggest that conformational preferences could be important in determining the design and stability of appropriate synthetic receptors for glutaric and adipic acid. Geometry changes between gas and aqueous phases were most marked in those conformers containing an intramolecular hydrogen bond. Additional calculations have probed the strength of intramolecular hydrogen bonds in these dicarboxylic acids. In the cases of glutaric and adipic acid, the strength of the intramolecular hydrogen bond were estimated to be around 28-29 kJ/mol, without any vibrational energy correction. The intramolecular hydrogen bond energies in malonic and succinic acid were also estimated from the calculated H-bond distances using an empirical relationship. Intramolecular H-bond redshifts of 170-250 cm(-1) have been estimated from the results of the harmonic frequency analyses.

The experimental electron density distribution (EDD) in 8-hydroxyquinoline cocrystallized with salicylic acid, 1, has been determined from a multipole refinement of high-resolution X-ray diffraction data collected at 100 K. The experimental EDD is compared with theoretical densities resulting from high-level ab initio and BHandH calculations using Atoms in Molecules theory. 1 crystallizes in the triclinic crystal system, and the asymmetric unit consists of a neutral salicylic acid molecule, a salicylate anion, and an 8-hydroxyquinolinium cation exhibiting a number of inter- and intramolecular hydrogen bonds and π-π interactions. Topological analysis reveals that π-π interactions are of the "closed-shell" type, characterized by rather low and flat charge density. In general, the agreement of the topological values (ρ(bcp) and ∇(2)ρ(bcp)) between experiment and theory is good, with mean differences of 0.010 e Å(-3) and 0.036 e Å(-5), respectively. The energetics of the π-π interactions have been estimated, and excellent agreement is observed between the relative energy and the strength of π-stacking derived from the Espinosa approach, with an average difference of only 4.4 kJ mol(-1).

The experimental charge density distribution of dimethyl-trans-2-oxohexahydro-pyrimidine-4,6-dicarboxylate 1 has been determined using single-crystal X-ray diffraction data measured at 100 K, in terms of the rigid-pseudoatom formalism. Multipole refinement converged at R(F) = 0.034 for 7283 reflections with I > 3 sigma (I) and sin theta/lambda < or = 1.13 A(-1). Covalent and hydrogen bonding interactions are analyzed using a topological analysis of the Laplacian of the charge density. The experimentally derived electrostatic potential mapped onto the reactive surface of the molecule reveals the potential binding sites of 1.

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