Noncovalent interactions: a challenge for experiment and theory

The nature and characteristics of the CH/π interaction are discussed by comparison with other weak molecular forces such as the CH/O and OH/π interaction. The CH/π interaction is a kind of hydrogen bond operating between a soft acid CH and a soft base π-system (double and triple bonds, C6 and C5 aromatic rings, heteroaromatics, convex surfaces of fullerenes and nanotubes). The consequences of CH/π hydrogen bonds in supramolecular chemistry are reviewed on grounds of recent crystallographic findings and database analyses. The topics include intramolecular interactions, crystal packing (organic and organometallic compounds), host/guest complexes (cavity-type inclusion compounds of cyclodextrins and synthetic macrocyclic hosts such as calixarenes, catenanes, rotaxanes and pseudorotaxanes), lattice-inclusion type clathrates (including liquid crystals, porphyrin derivatives, cyclopentadienyl compounds and C60 fullerenes), enantioselective clathrate formation, catalytic enantioface discriminating reactions and solid-state photoreaction. The implications of the CH/π concept for crystal engineering and drug design are evident.

CH/π hydrogen bonds in crystals

Kraitchman has shown that a single isotopic substitution on an atom is sufficient to determine directly the coordinates of that atom with respect to the principal axes of the original molecule. Kraitchman's formulas represent exact solutions of the equations for the equilibrium moments of inertia. However, the effects of the zero‐point vibrations are such that the coordinates obtained by substitution from the ground state moments of inertia I0 are systematically less than r0. These coordinates have here been called r (substitution) or rs, and it is found that rs≃(r0+re)/2, and Is= ∑ imirsi2≃(I0+Ie)/2.In the usual method of solution, the coordinate of one atom is determined from the equation for I0, and therefore the difference I0—Is must be made up by this one coordinate. This introduces a large error in the structures normally determined from ground state constants, and results in variations of 0.01 A in structures determined from different sets of isotopic species. If instead, we obtain the structure on...

A new criterion for the determination of molecular structures from ground state rotational constants

The CH/π hydrogen bond is an attractive molecular force occurring between a soft acid and a soft base. Contribution from the dispersion energy is important in typical cases where aliphatic or aromatic CH groups are involved. Coulombic energy is of minor importance as compared to the other weak hydrogen bonds. The hydrogen bond nature of this force, however, has been confirmed by AIM analyses. The dual characteristic of the CH/π hydrogen bond is the basis for ubiquitous existence of this force in various fields of chemistry. A salient feature is that the CH/π hydrogen bond works cooperatively. Another significant point is that it works in nonpolar as well as polar, protic solvents such as water. The interaction energy depends on the nature of the molecular fragments, CH as well as π-groups: the stronger the proton donating ability of the CH group, the larger the stabilizing effect. This Perspective focuses on the consequence of this molecular force in the conformation of organic compounds and supramolecular chemistry. Implication of the CH/π hydrogen bond extends to the specificity of molecular recognition or selectivity in organic reactions, polymer science, surface phenomena and interactions involving proteins. Many problems, unsettled to date, will become clearer in the light of the CH/π paradigm.

The CH/π hydrogen bond in chemistry. Conformation, supramolecules, optical resolution and interactions involving carbohydrates

Density functional theory (DFT) calculations were performed to evaluate the antioxidant activity of molecules commonly present in many Mediterranean foods. A series of interesting systems, including tyrosol, hydroxytyrosol, gallic, and caffeic acids, belonging to the family of phenols, resveratrol of the stilbenes family, epicatechin, kaempferol, and cianidin as examples of flavonoids and, at last, a simplified model of α-tocopherol (6-hydroxy-2,2,5,7,8-pentamethylchroman (HPMC)) were studied. Conformational behavior was examined at the B3LYP/6-311++G(3df,2p) level of theory, in the gas phase and in two solvents with different polarity (water and benzene), with the aim to compute the bond dissociation enthalpy (BDE) for the O−H bonds and the adiabatic ionization potentials (IP). BDE and IP for these systems do not follow the same trends in gas and solution phases:  the major differences with respect to vacuum are found as when water computations are performed. On the basis of the computed BDE and IP value...

Antioxidant Properties of Phenolic Compounds: H-Atom versus Electron Transfer Mechanism

The last few years have seen a very rapid growth in understanding the influence of non-bonded, particularly hydrogen-bonded
 interactions, on the shapes and conformations of flexible molecules, including those of pharmacological
 or biological importance, and of the supramolecular structures of their hydrated clusters. This has
 come about through the combination of a wide range of newly developed spectroscopic strategies, many of which are laser-based, coupled with powerful and widely available ab initio codes for structural computation. The
 consequent rapid growth of a new link between the worlds of chemistry and biophysics is surveyed in a review
 which introduces the range of present strategies, their origins, and their application to studies of neutrotransmitters,
 amides and peptides, amino acids and nucleic acid bases. It concludes with a prospectus for the
 future.

Getting into shape: Conformational and supramolecular landscapes in small biomolecules and their hydrated clusters

Conformational landscapes in amino acids: infrared and ultraviolet ion-dip spectroscopy of phenylalanine in the gas phase

High-resolution Fourier-transform infrared spectroscopy of the v6 and Coriolis perturbation allowed v10 modes of ketenimine.

The conformational landscapes of 2-amino-1-phenylethanol and its 1:1 water complexes have been investigated by UV band contour, UV−UV hole-burning, and IR−UV ion dip spectroscopy, coupled with ab initio computation. The two molecular conformers observed are both stabilized by an intramolecular hydrogen bond, located in the folded (gauche) OCCN side chain, which links the proton donor OH group to the terminal amino group and leads to a significant constriction of the side chain. In the dominant 1:1 water complex, the intramolecular hydrogen bond is disrupted by the first water molecule, which inserts between the OH group and the nitrogen atom, to form a cyclic H-bonded structure. The side chain expands significantly in order to accommodate the water molecule within the neighborhood of both the hydroxyl and amino groups.

Infrared Ion Dip Spectroscopy of a Noradrenaline Analogue: Hydrogen Bonding in 2-Amino-1-phenylethanol and Its Singly Hydrated Complex

Surface-enhanced Raman scattering (SERS) spectra of fonofos, an organophosphorous pesticide (OPP), were recorded using citrate-reducedsilver(Ag)andgold(Au)colloidalnanoparticlesintheformofdriedfilms.Inthisstudy,significantenhancements wereachievedwithfonofosconcentrationsdownto ∼10 ppmwiththeAgcolloids,demonstratingapotentialforthetechnique in the analysis of fonofos residues. Successful formation of the SERS-active metal aggregates was indicated by the presence of a plasmon band at longer wavelengths in the UV‐visible spectrum. Transmission electron microscopy (TEM) images revealed distinctively different morphologies of the aggregates formed by the two metals, while the observed SERS spectral features of fonofos were also found to be different for molecules adsorbed on Ag and Au colloidal surfaces. This evidence suggests metal-induced changes in adsorption behavior of the fonofos analyte, leading to the different binding structures onto the different metal surfaces. Comparisons between the most prominent SERS-enhanced bands and the precise mode descriptions predictedthroughdensityfunctionaltheory(DFT)simulationsattheB3LYP/6-311+G(d,p)levelallowedanin-depthorientation analysisoftheadsorbedspeciesonmetalsurfaces.Nevertheless,theringvibrationswerefoundtopossessamajorcontribution in the observed SERS enhancementsfor both metal nanoparticles. Copyright c � 2010 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article.

Evan G. Robertson

Papers

Getting into shape: Conformational and supramolecular landscapes in small biomolecules and their hydrated clusters

Conformational landscapes in amino acids: infrared and ultraviolet ion-dip spectroscopy of phenylalanine in the gas phase

High-resolution Fourier-transform infrared spectroscopy of the v6 and Coriolis perturbation allowed v10 modes of ketenimine.

Infrared Ion Dip Spectroscopy of a Noradrenaline Analogue: Hydrogen Bonding in 2-Amino-1-phenylethanol and Its Singly Hydrated Complex

Surface-enhanced Raman spectroscopic analysis of fonofos pesticide adsorbed on silver and gold nanoparticles