Double Hook Perylene Diimide as a New Receptor for PAHs: An Experimental and Theoretical Study.

In a one-step reaction, we prepared a dibenzylamine perylene diimide derivative (PDI). Its double hook structure allows for self-association with a constant of Kd ∼108  M-1 determined by fluorescence. We confirmed its ability to bind PAHs using UV/Vis, fluorescence, and 1 H NMR titrations in CHCl3 . The complex formation signature in UV/vis is a new band at 567 nm. The calculated binding constants (Ka ∼104  M-1 ) follow the trend pyrene>perylene>phenanthrene>naphthalene>anthracene. Theoretical modeling of these systems using DFT ωB97X-D/6-311G(d,p) proved helpful in rationalizing the complex formation and the observed association trend. The distinctive signal in UV/vis is due to a charge transfer in the complex from orbitals in the guest to the host. SAPT(DFT) confirmed that the driving forces in the complex formation are exchange and dispersion (π-π interactions). Still, the recognition ability depends on the electrostatic component of the interaction, a minor fraction.

Interaction energy of Cl2 and Br2 with H2 O: Exchange, dispersion and density the crucial ingredients.

Modern Density Functional Theory models are now suitable for many molecular and condensed phase studies. The study of noncovalent interactions, a well-known drawback, is no longer an insurmountable obstacle through design and empirical corrections. However, using empirical corrections as in the DFT-D methods might not be an all-in-one solution. This work uses a simple system, X2 -H2 O with X = Cl or Br, with two different interactions, halogen-bonded (XB) and hydrogen-halogen (HX), to investigate the capability of current density functional approximations (DFA) in predicting interaction energies with eight different exchange-correlation functionals. SAPT(DFT) provides, for all the studied cases, better predictions than the widely used supermolecular approach. In addition, the components of the interaction energy suggest where some of the shortcomings originate in each DFA. The analysis of the functionals used confirms that PBE0 and ω-B97X-D have a physically correct behavior. Using SAPT(DFT) and PBE0, and ω-B97X-D, we obtained the interaction energy of Cl2 and Br2 inside different clathrate cages and satisfactorily compared with wavefunction results; hence, the lower and upper limits of this value are defined: Cl2 @512 , -5.3 ± 0.3 kcal/mol; Cl2 @512 62 , -5.5 ± 0.1 kcal/mol; Br2 @512 62 , -7.6 ± 1.0 kcal/mol; Br2 @512 63 , -10.6 ± 1.0 kcal/mol; Br2 @512 64 , -10.9 ± 0.8 kcal/mol.

Density Functional Study on the Fundamental and Valence Excited States of Dibromine in T, P, and H Clathrate Cages.

This work evaluates the performance of different DFT models in the accurate prediction of the guest-host intermolecular potentials for the ground and excited states of Br2 in the tetrakaidecahedral (T), pentakaidecahedral (P), and hexakaidecahedral (H) clathrate cages. Of a set of density functionals, we found that PBE0-D3 and wb97XD provide a physically sound and quantitatively correct description of the interaction and transition energies of low-lying valence excited states of Br2 inside these clathrate cages. The importance of correctly modeling dispersive interactions is also analyzed. This study provides the first detailed potential energy surface of the ground and excited states of Br2 in the largest H cage. Comparisons with the LCC2 method and experimental electronic shifts probe the reliability of PBE0-D3 and wb97XD to describe weak intermolecular forces in the ground and excited states.

Density-Difference-Driven Optimized Embedding Potential Method To Study the Spectroscopy of Br2 in Water Clusters.

A variant of the density difference driven optimized embedding potential (DDD-OEP) method, proposed by Roncero et al. (J. Chem. Phys. 2009, 131, 234110), has been applied to the calculation of excited states of Br2 within small water clusters. It is found that the strong interaction of Br2 with the lone electronic pair of the water molecules makes necessary to optimize specific embedding potentials for ground and excited electronic states, separately and using the corresponding densities. Diagnosis and convergence studies are presented with the aim of providing methods to be applied for the study of chromophores in solution. Also, some preliminary results obtained for the study of electronic states of Br2 in clathrate cages are presented.

Performance of local correlation methods for halogen bonding: The case of Br2-(H2O)n,n = 4,5 clusters and Br2@5(12)6(2) clathrate cage.

The performance of local correlation methods is examined for the interactions present in clusters of bromine with water where the combined effect of hydrogen bonding (HB), halogen bonding (XB), and hydrogen-halogen (HX) interactions lead to many interesting properties. Local methods reproduce all the subtleties involved such as many-body effects and dispersion contributions provided that specific methodological steps are followed. Additionally, they predict optimized geometries that are nearly free of basis set superposition error that lead to improved estimates of spectroscopic properties. Taking advantage of the local correlation energy partitioning scheme, we compare the different interaction environments present in small clusters and those inside the 5(12)6(2) clathrate cage. This analysis allows a clear identification of the reasons supporting the use of local methods for large systems where non-covalent interactions play a key role.

Ground state analytical ab initio intermolecular potential for the Cl(2)-water system.

The chlorine/water interface is of crucial importance in the context of atmospheric chemistry. Modeling the structure and dynamics at this interface requires an accurate description of the interaction potential energy surfaces. We propose here an analytical intermolecular potential that reproduces the interaction between the Cl2 molecule and a water molecule. Our functional form is fitted to a set of high level ab initio data using the coupled-cluster single double (triple)/aug-cc-p-VTZ level of electronic structure theory for the Cl2 - H2O complex. The potential fitted to reproduce the three minima structures of 1:1 complex is validated by the comparison of ab initio results of Cl2 interacting with an increasing number of water molecules. Finally, the model potential is used to study the physisorption of Cl2 on a perfectly ordered hexagonal ice slab. The calculated adsorption energy, in the range 0.27 eV, shows a good agreement with previous experimental results.

Communication: Ab initio study of O4H+: a tracer molecule in the interstellar medium?

The structure and energetics of the protonated molecular oxygen dimer calculated via ab initio methods is reported. We find structures that share analogies with the eigen and zundel forms for the protonated water dimer although the symmetrical sharing of the proton is more prevalent. Analysis of different fragmentation channels show charge transfer processes which indicate the presence of conical intersections for various states including the ground state. An accurate estimate for the proton affinity of O4 leads to a significantly larger value (5.6 eV) than for O2 (4.4 eV), implying that the reaction H3(+) + O4 → O4H(+) + H2 is exothermic by 28 Kcal/mol as opposed to the case of O2 which is nearly thermoneutral. This opens up the possibility of using O4H(+) as a tracer molecule for oxygen in the interstellar medium.

Large shift and small broadening of Br2 valence band upon dimer formation with H2O: an ab initio study.

Valence electronic excitation spectra are calculated for the H(2)O···Br(2) complex using highly correlated ab initio potentials for both the ground and the valence electronic excited states and a 2-D approximation for vibrational motion. Due to the strong interaction between the O-Br and the Br-Br stretching motions, inclusion of these vibrations is the minimum necessary for the spectrum calculation. A basis set calculation is performed to determine the vibrational wave functions for the ground electronic state and a wave packet simulation is conducted for the nuclear dynamics on the excited state surfaces. The effects of both the spin-orbit interaction and temperature on the spectra are explored. The interaction of Br(2) with a single water molecule induces nearly as large a shift in the spectrum as is observed for an aqueous solution. In contrast, complex formation has a remarkably small effect on the T = 0 K width of the valence bands due to the fast dissociation of the dihalogen bond upon excitation. We therefore conclude that the widths of the spectra in aqueous solution are mostly due to inhomogeneous broadening.

Charge separation process in water clusters containing HCl. Molecular dynamics study using semiempirical hamiltonians.

The acid dissociation of HCl in water clusters was studied using the PM3-MAIS model in a direct molecular dynamic simulations framework. Several trajectories for each cluster size were computed to improve the sampling on the potential energy surface and to increase the statistical significance of our study. We have analyzed the emergence of well-defined hydration structures around the ions and the effect they have on the charge separation process. Surface and bulk solvation situations are examined for both ions in terms of the competing circumstances for water molecules in the cluster. Our results show that the prevailing situation in all these clusters is a water mediated interaction between the ions. This situation is related to the significant interplay of several factors such as charge transfer occurring between the ions and their first hydration shells. Our results show that no significant charge redistribution occurs during the Eigen-Zundel transformation.

An ab initio calculation of the valence excitation spectrum of H2O...Cl2: comparison to condensed phase spectra.

Valence electronic excitation spectra are calculated for the H(2)O...Cl(2) dimer using state-of-the art ab initio potentials for both the ground and the valence excited states, a basis set calculation of the ground state nuclear wave function, and a wave packet analysis to simulate the dynamics on the excited state surface. The peak of the H(2)O...Cl(2) dimer spectrum is blue-shifted by 1250 cm(-1) from that of the free Cl(2) molecule. This is less than the value previously estimated from vertical excitation energies but still significantly more than the blue shift in aqueous solution and clathrate-hydrate solid. Seventy percent of the blue shift is attributed to ground state stabilization, the rest to excited state repulsion. Spin-orbit effects are found to be small for this dimer. Homogeneous broadening is found to be slightly smaller for the dimer than for the free Cl(2). The reflection principle and spectator model approximations were tested and found to be quite satisfactory. This is promising for an eventual simulation of the condensed phase spectra.

On the unusual properties of halogen bonds: a detailed ab initio study of X2-(H2O)(1-5) clusters (X = Cl and Br).

Halogen bonds have received a great deal of attention in recent years. Their properties, sometimes paralleled with those of hydrogen bonds, have not yet been fully understood. In this work, we investigate the nature of the intermolecular interactions between Cl(2) and Br(2) with water. Our analysis of several features of MP2/aug-cc-pVDZ-optimized stable clusters with different number and arrangement of water molecules shows that two different kinds of halogen-water coordination patterns are involved in the stability and properties found for these systems: halogen bonds (X-X...O) and halogen-hydrogen interactions, (X-X...H-O-H). Both types of interactions result in a large polarization of the halogen molecule, which leads to important cooperative effects on these structures. Although the general structural aspects of these clusters can be understood in terms of dipole-quadrupole forces at long range, where it is the dominant term, the SAPT analysis shows that factors such as polarization of pi densities and dispersion become increasingly important close to equilibrium. In particular, we show that the halogen-hydrogen interactions are weaker than halogen-oxygen interactions mainly due to the electrostatic and dispersion forces. We also calculate vibrational and electronic shifts that should be helpful for the interpretation of experimental results and for investigating the microsolvation phenomena for halogens in an aqueous environment.

Theoretical study of the experimental coordination behavior of N-(2-aminophenyl)-D-glycero-D-gulo-heptonamide to Hg(II) ion.

The reactivity of N-(2-aminophenyl)-d-glycero-d-gulo-heptonamide (adgha), with the group 12 cations, Zn(II), Cd(II), and Hg(II), was studied in DMSO-d(6) solution. The studied system showed a selective coordination to Hg(II), and the products formed were characterized by (1)H and (13)C NMR in DMSO-d(6) solution and fast atom bombardment (FAB(+)) mass spectra. The expected coordination compounds, [Hg(adgha)](NO(3))(2) and [Hg(adgha)(2)](NO(3))(2), were observed as unstable intermediates that decompose to bis-[2-(d-glycero-d-gulo-hexahydroxyhexyl)-benzimidazole-κN]mercury(II) dinitrate, [Hg(ghbz)(2)](NO(3))(2). The chemical transformation of the complexes was followed by NMR experiments, and the nature of the species formed is sustained by a theoretical study done using DFT methodology. From this study, we propose the structure of the complexes formed in solution, the relative stability of the species formed, and the possible role of the solvent in the observed transformations.