DFT Study of the Adsorption and SERS of Pyridine on M10N10 (M, N = Cu, Ag) Tetrahedral Clusters.

This work presents a theoretical detailed analysis of the surface-enhanced Raman spectroscopy (SERS) of the pyridine-M10N10 (M, N = Ag, Cu) tetrahedral (Td) clusters considering two binding positions: vertex (V) and surface (S). In addition to the well-known monometallic Td structure, we added two different bimetallic Ag-Cu compositions, named Td1 and Td2 geometries. Density functional methodology with the use of BP86 and CAM-B3LYP exchange-correlation functionals (XCs) and LANL2DZ pseudopotential has been employed for analyzing the electronic structure and geometries, the chemical static (CHEM), and resonant Raman mechanisms (RR): charge transfer RR-CT and intracluster excitation RR-CR. The static CHEM mechanism shows an increase in the enhancement factors (EFs) of Py-V concerning Py-S positions, which can also be distinguished by the averaged adsorption energies and bond polarizabilities. The static SERS response for Cu-Py-V junction is from 5 to 10 times greater than Ag-Py-V EFs and up to 28 times greater than Py-S complexes. For the static Raman, we found that the analyses of ν8a and ν1 normal modes are related to the EF changes and allow us to distinguish V from S complexes. The TDDFT calculations show striking differences between BP86 and CAM-B3LYP XCs analyzed spectra, and CAM-B3LYP granted a clear distinction between V and S for the location of CT-type transitions. In addition, important differences were obtained from the analysis of the charge transfer excitations between both XCs. Resonant Raman calculations evidenced significant enhancements for RR-CT and RR-CR as compared to the static enhancements, and RR-CT can be distinguished from the RR-CR mechanism, while specific normal modes help to differentiate the vertex from the surface Py-junction. Bimetallic Ag-Cu nanostructures represent promising choices for SERS substrates, showing EFs higher than those of monometallic Ag.

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.

A DFT study of the adsorption and surface enhanced Raman spectroscopy of pyridine on Au20, Ag20, and bimetallic Ag8Au12 clusters.

This work presents a theoretical detailed analysis of the surface-enhanced Raman spectroscopy (SERS) of the pyridine - Au20, pyridine - Ag20, and pyridine - Ag8Au12 model systems considering different symmetries of the clusters. In addition to the well-known Td geometry of this twenty atoms metal cluster, low energy structures have been analyzed (Cs and Cb). Density functional methodology with the use of PBE, PBE0, and scalar - relativistic pseudopotentials have been employed for the electronic structure calculations of these molecule-metal complexes. The projected state density analysis has shown a different behavior that distinguishes vertex (V) pyridine position from surface (S) adsorption site on both Td and Cs geometries. Adsorption of pyridine on the V position is always energetically favored as compared to S substitution. The chemical mechanism of enhancement has been analyzed through charge-transfer in the adsorption of pyridine to the metal cluster and charge-transfer excitations between both moieties. There is a close relationship between the amount of charge transfer from the pyridine molecule to the cluster and the SERS enhancements. The highest enhancement factors were obtained precisely for the Au20 cubic structure, where 102 order enhancements were calculated. A back - donation of charge triggers the highest enhancements obtained for this case. The bimetallic case also shows an improvement in the enhancements as compared to Td and Cs geometries. In addition, a direct relationship between resonant charge - transfer excitations in the 500-530 nm range and SERS enhancement have been obtained. Cs and cubic clusters show higher chemical enhancements than Td structures, thus representing a promise as SERS substrates.

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.

Theoretical Study on the Mechanism of the Thermal Retro-Cycloaddition of Isoxazolinofullerenes.

The retro-cycloaddition thermal reaction of isoxazolino[4,5:1,2][60]fullerenes to pristine fullerene seems to be guided by the electronic nature of the substituted nitrile oxide 1,3-dipole in the isoxazoline ring. Trapping experiments proved that the reaction mechanism occurs by thermal removal of the nitrile oxide 1,3-dipole in a process that is favored in the presence of a big excess of a highly efficient dipolarophile such as maleic anhydride. Theoretical gas phase calculations carried out at the B3LYP/6-31G(d) and M06-2X/6-31G(d) levels of theory underpin the experimental findings and predict that compound 1c, bearing the p-(CH3)2N-Ph substituent on the isoxazoline ring and with a remarkable experimental conversion efficiency in just 12 h, showed the lowest activation energy. Solvent calculations have predicted the same behavior in gas phase. Different approaches such as electrostatic natural population analysis and Houk's distortion/interaction model have been applied to understand how the electronic nature of these substituents affects the retro-cycloaddition reaction process. Analysis of the values of the condensed Fukui functions and dual descriptor shed light on the mechanism of the retro-cycloaddition reaction.

Pyrrolyl-Silicon Compounds as Precursors for Donor-Acceptor Systems Stabilized by Noncovalent Interactions.

Pyrrolyl-silicon compounds were investigated by different theoretical approaches. Model monomers consisted of a pyrrole ring N-substituted with silylmethoxy and silylhydroxy end groups through a propyl chain spacer, designated as PySi and PySiOH. Geometrical, vibrational, and electronic properties, as well as chemical reactivity, are discussed and compared with pyrrole (Py) and N-propylpyrrole (N-PrPy) that were studied in parallel for reference purposes and methods validation. The electronic distribution between PySi and PySiOH differs importantly, the former being an electron donor, as Py and N-PrPy. Conversely, PySiOH presents donor-acceptor character with the LUMO energy level localized on the silanol end group. Global and local reactivity descriptors predict PySiOH more reactive than PySi with two preferential reactive sites: electron-rich Py ring and electron-deficient silanol group. On the basis of experimental studies, oligomers of PySiOH linked α-α' via Py rings (α-α'PynSiOH, n = 2, 3) were considered as model molecules of hydrolyzed PySi. The most stable structures were derived from randomly generated α-α'PynSiOH that were optimized at semiempirical AM1 and refined with M05-2X/6-31G(d,p). Conformational analysis of dimer and trimer structures points to stability enhanced by molecular packing. Nonetheless, NBO and RDG results indicate that oligomer stability is dictated by the cooperative contribution of hydrogen bonding between silanol end groups and dispersive vdW interactions between silanol and the π system of the Py ring. The latter interaction resulting from electron delocalization induced by an electron-deficient silanol group seems to determine the smaller gap energy of T-shaped OH-π arrangements. The theoretical findings support the peculiar chemical behavior revealed by experiment.

Electron density analysis of 1-butyl-3-methylimidazolium chloride ionic liquid.

An analysis of the electron density of different conformers of the 1-butyl-3-methylimidazolium chloride (bmimCl) ionic liquid by using DFT through the BVP86 density functional has been obtained within the framework of Bader's atom in molecules (AIM), localized orbital locator (LOL), natural bond orbital (NBO), and deformed atoms in molecules (DAM). We also present an analysis of the reduced density gradients that deliver the non-covalent interaction regions and allow to understand the nature of intermolecular interactions. The most polar conformer can be characterized as ionic by AIM, LOL, and DAM methods while the most stable and the least polar shows shared-type interactions. The NBO method allows to comprehend what causes the stabilization of the most stable conformer based on analysis of the second-order perturbative energy and the charge transferred among the natural orbitals involved in the interaction.

Dumbbell-type fullerene-steroid hybrids: a join experimental and theoretical investigation for conformational, configurational, and circular dichroism assignments.

New [60]fullerene-steroid conjugates (4-6) have been synthesized by 1,3-dipolar cycloaddition and Bingel-Hirsch cyclopropanation reactions from suitably functionalized epiandrosterone and [60]fullerene. Since a new stereocenter is created in the formation of the Prato monoaduct, two different diastereomers were isolated by HPLC (4, 5) whose absolute configurations were assigned according to the highly reliable "sector rule" on fullerenes. A further reaction of the malonate-containing diastereomer 5 with a second C60 molecule has afforded dumbbell fullerene 6 in which the two fullerene units are covalently connected through an epiandrosterone moiety. The new compounds have been spectroscopically characterized and their redox potentials, determined by cyclic voltametry, reveal three reversible reduction waves for hybrids 4 and 5, whereas these signals are split in dumbbell 6. Theoretical calculations at semiempirical (AM1) and single point B3LYP/6-31G(d) levels have predicted the most stable conformations for the hybrid compounds (4-6), showing the importance of the chlorine atom on the D ring of the steroid. Furthermore, TDDFT calculations have allowed assignments of the experimentally determined circular dichroism (CD) of the [60]fullerene-steroid hybrids based on the sign and position of the Cotton effects, despite the exceptionally large systems under study.

Interaction of brassinolide with essential amino acid residues: a theoretical approach.

The interaction of the most active natural brassinosteroid, brassinolide, with the twenty natural amino acids is studied applying the multiple minima hypersurface method to model the molecular interactions explicitly. The resulting thermodynamic data gives useful information about the amino acids with the greatest association for brassinolide and the stabilities of such complexes. Density functional theory (DFT) optimizations were further carried out to test the performance of semiempirical calculations. Additional calculations with a more accurate DFT method were performed to explore the formation of this type of molecular complexes. The semiempirical geometries and stability order of these complexes are in good agreement with the DFT calculations. Each group of amino acids possesses a preferential zone of interaction with brassinolide, forming the polar-charged amino acids the most stable complexes. This study could contribute to future investigations of the interaction of brassinosteroids with the receptor protein in plants.

A theoretical approach to the solvation of brassinosteroids.

The interaction of three different brassinosteroids with water was studied by the Multiple Minima Hypersurface (MMH) procedure to model molecular interactions explicitly. The resulting thermodynamic data give useful information on properties of molecular association with water. This application can serve as a tool for future investigations and modelling concerning interactions of brassinosteroids with receptor proteins in plants. DFT/B3LYP calculations were also made in order to correlate and test the performance of the current AM1 Hamiltonian calculations of these complexes, which are inherent to MMH routine. Diol functionalities located in ring A and lateral chain appears as the sites that show the highest affinity to water. The oxalactone group does not appear to be a key structural requirement in the association with water. Parallel calculations with a "polarizable continuum method" (PCM) agreed with the reported experimental order of biological activities, where Brassinolide exhibited the best solubility features.