A Highly Efficient Neutral Anion Receptor in Polar Environments by Synergy of Anion-π Interactions and Hydrogen Bonding.

Herein, it is shown how anion recognition in highly polar solvents by neutral metal-free receptors is feasible when multiple hydrogen bonding and anion-π interactions are suitably combined. A neutral aromatic molecular tweezer functionalized with azo groups is shown to merge these two kinds of interactions in a unique system and its efficiency as an anion catcher in water is evaluated using first-principles quantum methods. Theoretical calculations unequivocally prove the high thermodynamic stability in water of a model anion, bromide, captured within the tweezer's cavity. Thus, static calculations indicate anion-tweezer interaction energies within the range of covalent or ionic bonds and stability constants in water of more than 10 orders of magnitude. First-principles molecular dynamics calculations also corroborate the stability through the time of the anion-tweezer complex in water. It shows that the anion is always found within the tweezer's cavity due to the combination of the tweezer-anion interactions plus a hydrogen bond between the anion and a water molecule that is inside the tweezer's cavity.

Theoretical vibrational Raman and surface-enhanced Raman scattering spectra of water interacting with silver clusters.

In this study, we analyzed the Raman spectrum of a water molecule adsorbed on a cluster of 20 silver atoms, and the plasmonic electromagnetic effect of the silver surface was also considered to give a theoretical prediction of the surface-enhanced Raman scattering spectrum. The calculations were performed at the density functional theory (DFT) level by using both frozen and unfrozen silver clusters. Two different models were used to consider the plasmonic enhancement; one of them was a modified classical (dipole) model and the other was the coupled perturbed Hartree-Fock method with excitation frequencies obtained from time-dependent DFT calculations and with proper detuning of these frequencies. The importance of small geometrical distortions of the silver surface in the orientation of the adsorbed water was shown. Moreover, it was shown how the symmetry of the transition dipole moment and the symmetry of the vibrational modes influence the Raman intensities of the SERS spectrum.

Theoretical design of molecular grippers for anion recognition based on subporphyrazines and subphthalocyanines.

Subphthalocyanines (SubPcs) are 14 π-electron aromatic macrocycles containing three 1,3-diiminoisoindol units N-fused around a central boron atom. Their particular concave π-extended structure makes them potentially useful in fields related to optoelectronics and molecular electronics. Also, their structure makes them potential receptors of complementary convex-shaped molecules. Inspired by the properties of subphthalocyanines and by previous studies about the possibility of obtaining neutral receptors composed of aromatic rings that can bind anions, we performed a theoretical study to analyze the ability of subphthalocyanines and some SubPc analogues to capture anions by non-covalent interactions. This may be relevant, since they can produce a variety of chromogenic reagents for anion detection that are not destroyed in the detection process, since the non-covalent interactions are weak enough to reverse the procedure. We characterized complexes formed between several SubPc derivatives and different anions, and also studied the influence of this process on the optical properties of these molecules. The stabilization trend reflected by the energetic results and the changes shown in the absorption wavelengths of the host molecule under complexation confirm our hypothesis about the possibility of these kinds of molecules being used as chromogenic reagents for anion capture, even in solvents with a large dielectric constant like water.

A computational study of the protonation of simple amines in water clusters.

The microsolvation study of a group of amines with a variable number of water molecules was performed by conducting a theoretical analysis of the properties of the clusters formed by the amines with up to seven molecules of water. We describe the microsolvation of several amines focusing on the dissociation of a water molecule that transfers a proton to the amine and forms a hydroxide ion. Ab initio calculations were performed on these clusters employing the DFT/B3LYP and MP2 methods with the 6-311++G(2d,p) basis set. Several stationary points for each cluster were thus located and characterized as minima from frequency calculations. Intermolecular BSSE corrected interaction energies were obtained. The protonation mechanism of the amines was examined in terms of some parameters that include the lengths of the bonds involved in the process of proton transfer and the frequencies associated with certain O-H and N-H stretching modes. On the basis of the calculations, all studied amines present similar behavior but trimethylamine, whose limitations to be integrated in the water hydrogen bond network cause the instability of some of their complexes. The cyclic configurations are the most stable structures up to five water molecules due to the presence of cooperative effects associated with the hydrogen bonds of water molecules. However, when the number of water molecules increases the spatial forms become the most stable configurations. The dissociated forms were not found to have the most stable configuration in any of the studied systems but energetic differences between the dissociated and non-dissociated forms decrease with the number of water molecules.

Anion-π aromatic neutral tweezers complexes: are they stable in polar solvents?

The impact of the solvent environment on the stabilization of the complexes formed by fluorine (T-F) and cyanide (T-CN) substituted tweezers with halide anions has been investigated theoretically. The study was carried out using computational methodologies based on density functional theory (DFT) and symmetry adapted perturbation theory (SAPT). Interaction energies were obtained at the M05-2X/6-31+G* level. The obtained results show a large stability of the complexes in solvents with large dielectric constant and prove the suitability of these molecular tweezers as potential hosts for anion recognition in solution. A detailed analysis of the effects of the solvent on the electron withdrawing ability of the substituents and its influence on the complex stability has been performed. In particular, the interaction energy in solution was split up into intermonomer and solvent-complex terms. In turn, the intermonomer interaction energy was partitioned into electrostatic, exchange, and polarization terms. Polar resonance structures in T-CN complexes are favored by polar solvents, giving rise to a stabilization of the intermonomer interaction, the opposite is found for T-F complexes. The solvent-complex energy increases with the polarity of the solvent in T-CN complexes, nonetheless the energy reaches a maximum and then decreases slowly in T-F complexes. An electron density analysis was also performed before and after complexation, providing an explanation to the trends followed by the interaction energies and their different components in solution.

Ultrafast ring-opening/closing and deactivation channels for a model spiropyran-merocyanine system.

The photochemistry of a model merocyanine-spiropyran system was analyzed theoretically at the MS-CASPT2//SA-CASSCF(14,12) level. Several excited singlet states were studied in both the closed spiropyran and open merocyanine forms, and the paths to the different S(1)/S(0) conical intersections found were analyzed. After absorption of UV light from the spiropyran form, there are two possible ultrafast routes to efficient conversion to the ground state; one involves the rupture of the C(spiro)-O bond leading to the open form and the other involves the lengthening of the C(spiro)-N bond with no photoreaction. From the merocyanine side the excited state can reach a very broad S(1)/S(0) conical intersection region that leads the system to the closed form after rotation of the central methine bond. Alternatively, rotation of the other methine bonds connects the system through different S(1)/S(0) conical intersections to several merocyanine isomers. The present work provides a theoretical framework for the recent experimental results (Buback , J. J. Am. Chem. Soc. 2010, 132, 1610-1619) and sheds light on the complex photochemistry of these kinds of compounds.

Enhancing the interactions between neutral molecular tweezers and anions.

Several structural modifications to the original molecular tweezers of the Klärner's group were made with the aim of improving its binding capacity towards anions. The proposed modifications raise the molecular electrostatic potential inside the cavity and provide more conformational flexibility. The complexes of these new molecules with the halide anions Cl(-), Br(-), I(-) were optimized at the MPW1B95/6-31+G* level of theory. The molecular interactions were analyzed by single point density fitted local second-order Møller-Plesset perturbation theory (DF-LMP2) and DF-LMP2 spin-component-scaled MP2 (SCS-MP2), calculations were performed with the cc-pVTZ basis set. In view of the large magnitude of the interaction energies computed and the stability of the complexes in different solvents, this kind of molecule is a good candidate as molecular host for anion recognition.