The importance of cation->aromatic polarization effects on cation-interactions has been explored. Theoretical calculations demonstrate that polarization is a large contribution to cation-aromatic interactions, and particularly to cation-interactions. For a series of compounds with a similar aromatic core, polarization is constant and makes small inf luence in the relative cation-binding energies. However, when the aromatic core changes polarization contributions might be very different. We found that the generalized molecular interaction potential with polarization is a very fast and powerful tool for the prediction of cation binding of aromatic compounds.Cation-is a strong, and quite specific, interaction, which plays a key role in molecular recognition (for an excellent review see ref. 1). Thus, cation-interactions are relevant in host-guest complexes (1-5). In addition, they are common in protein structure (6)(7)(8), where these interactions seem to be related with general mechanisms of substrate-enzyme binding (8)(9)(10)(11)(12) and probably with some catalytic processes (13-16). Finally, Dougherty and coworkers (17, 18) recently have suggested that these interactions are essential for the recognition and action of ion channels.Cation-complexes involve aromatic molecules having large and well-defined -electron distributions, and the cations lie perpendicular to the aromatic plane. Dougherty and coworkers (1,7,18,19) have shown that, in the case of nonpolarizable cations such as Na ϩ , the preferential binding to different aromatic compounds can be explained from electrostatic considerations. Particularly, for a series of 11 derivatives of benzene, they found an excellent correlation (r ϭ 0.991, slope ϭ 1.01, intercept ϭ 11.6 kcal͞mol) between the self-consistent field (SCF) binding energies and the molecular electrostatic potential (MEP; Eq. 1) at the SCFoptimized position of the Na ϩ in the complex (19). More interesting, when the MEP was computed at a common position for all of the molecules (2.47 Å above the aromatic ring), the correlation with the SCF binding energy was also excellent (r ϭ 0.992, slope ϭ 1.04, intercept ϭ 12.3 kcal͞ mol). This finding suggests that a simple MEP calculation can provide very accurate estimates of the relative values of cation binding energy for a given series of aromatic compounds (18,19).where R B and R A stands for the positions of the cation and nuclei, and c stands for the coefficients of atomic orbitals in the molecular orbital-linear combination atomic orbitals (MO-LCAO) approximation.The pure electrostatic model seems very useful for a qualitative description of cation-interactions. However, the electrostatic component of the cation-interaction energy is always larger (in absolute terms) than the MEP value. Therefore, other interactions are involved in the formation of cation-complexes. For the particular case of Na ϩ , the charge transfer is expected to be small. Indeed, the polarization of the -electron system by the cation will have a significant contribu...

The theory of atoms in molecules is used to examine the nature of anti-hydrogen bond (anti-H bond) interaction. Contrary to what is found in normal hydrogen bond (H bond) complexes, which are characterized by lengthening of the X−H bond and a red shift of its stretching frequency, the anti-H bond leads to a shortening of the X−H bond length and a blue shift of its vibrational frequency. The topological properties of the electron density have been determined for a series of C−H···π complexes, which exhibit either anti-H bond or normal H bond character, as well as for the complexes C6H5F···HCCl3 and C6H6···HF, which are representative cases of anti- and normal H bonds. Inspection of the set of topological criteria utilized to characterize conventional H bonds shows no relevant difference in the two classes of H···π complexes. Analysis of the results suggests that the specific features of the anti-H bond originates from the redistribution of electron density in the C−H bond induced upon complexation, which in turn evidences the different response − dispersion versus electrostatic− of the interacting monomer for stabilizing the complex.

Structures and stabilities of H-bonded adenine···uracil Watson−Crick (AU WC) and two uracil···uracil nucleic acid base pairs possessing C−H···O contacts (UU7, UU−C) were determined using gradient optimization with inclusion of electron correlation via the second-order Møller−Plesset (MP2) perturbational method with a 6-31G** basis set of atomic orbitals. In the AU WC pair, closest contacts occur between the N6(A) and O4(U), N1(A) and N3(U), and C2(A) and O2(U) atoms:  2.969, 2.836, and 3.568 Å, respectively. For the UU7 pair the closest contact corresponds to O4···N1 and C5···O2 pairs with distances of 2.860 and 3.257 Å, respectively, while in UU−C pair the closest contact was found for O4···N3 and C5···O4 heteroatoms with distances of 2.913 and 3.236 Å, respectively. The nature of all intermolecular contacts in the sense of a conventional H-bonding or improper, blue-shifting H-bonding was determined on the basis of harmonic vibrational analysis, atom-in-molecules (AIM) Bader analysis of electron density, and natural bond orbital analysis (NBO) performed at the MP2/6-31G** level of theory. N−H stretch frequencies of A and U exhibited a red shift and intensity decrease upon formation of AU WC base pair. It unambiguously proves the existence of the N−H···O and O···H−N H-bonds. The frequency shift of the C−H stretching frequency of A upon complex formation is, however, marginal (∼2 cm-1). Bader AIM analysis and NBO analysis confirmed the existence of N−H···O and O···H−N H-bonds in the AU WC pair but was inconclusive in the case of C−H···O contact. The results thus clearly show that the C2−H2···O2 contact in the AU or AT WC base pair corresponds neither to standard H-bond nor to improper, blue-shifting H-bond. The opposite result, however, has been found for the UU pairs; here the vibrational analysis shows a red shift and intensity increase of both N−H and C−H stretching vibrational frequencies upon the formation of the pair. This is a clear manifestation of the presence of two H-bonds of the N−H···O and O···H−C types. Bader AIM analysis as well as NBO analysis confirmed the existence of both H-bonds.