Within the field of nanotechnology, nanoparticles are one of the most prominent and promising candidates for technological applications. Self-assembly of nanoparticles has been identified as an important process where the building blocks spontaneously organize into ordered structures by thermodynamic and other constraints. However, in order to successfully exploit nanoparticle self-assembly in technological applications and to ensure efficient scale-up, a high level of direction and control is required. The present review critically investigates to what extent self-assembly can be directed, enhanced, or controlled by either changing the energy or entropy landscapes, using templates or applying external fields.

Directed Self-Assembly of Nanoparticles

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Metals and metal oxides are widely used as catalysts for materials production, clean energy generation and storage, and many other important industrial processes. However, metal-based catalysts suffer from high cost, low selectivity, poor durability, susceptibility to gas poisoning and have a detrimental environmental impact. In 2009, a new class of catalyst based on earth-abundant carbon materials was discovered as an efficient, low-cost, metal-free alternative to platinum for oxygen reduction in fuel cells. Since then, tremendous progress has been made, and carbon-based metal-free catalysts have been demonstrated to be effective for an increasing number of catalytic processes. This Review provides a critical overview of this rapidly developing field, including the molecular design of efficient carbon-based metal-free catalysts, with special emphasis on heteroatom-doped carbon nanotubes and graphene. We also discuss recent advances in the development of carbon-based metal-free catalysts for clean energy conversion and storage, environmental protection and important industrial production, and outline the key challenges and future opportunities in this exciting field. Reducing or even eliminating the need for precious-metal catalysts is crucial for the commercialization of clean energy technologies and various important industrial processes. Carbon materials have recently been shown to be cost-effective and efficient metal-free catalysts in clean energy generation and storage, environmental protection and chemical production.

Carbon-based metal-free catalysts

Gold nanorods have received much attention due to their unique optical and electronic properties which are dependent on their shape, size, and aspect ratio. This article covers in detail the synthesis, functionalization, self-assembly, and sensing applications of gold nanorods. The synthesis of three major types of rods is discussed: single-crystalline and pentahedrally-twinned rods, which are synthesized by wet chemistry methods, and polycrystalline rods, which are synthesized by templated deposition. Functionalization of these rods is usually necessary for their applications, but can often be problematic due to their surfactant coating. Thus, general strategies are provided for the covalent and noncovalent functionalization of gold nanorods. The review will then examine the significant progress that has been made in controllable assembly of nanorods into various arrangements. This assembly can have a large effect on measurable properties of rods, making it particularly applicable towards sensing of a variety of analytes. Other types of sensing not dependent on nanorod assembly, such as refractive-index based sensing, are also discussed.

Functional Gold Nanorods: Synthesis, Self-Assembly, and Sensing Applications

Benzene is the emblematic example of an aromatic molecule, and the problem of its structure has given rise to a chemical serial story running over several decades. The epistemological digest of this story written by Stephen G. Brush1,2 shows how this problem has been at the root of important concepts such as those of mesomery and resonance. Before the advent of quantum mechanics, chemists had thought of the benzene structures in terms of two center bonds attempting to preserve the valence of the carbon atom and to explain its chemical properties. Kekulé’s theory of the structure of the benzene molecule3 invokes the oscillatory hypothesis in which “the fourth valence of each carbon oscillates between its neighbors, synchronously with all the other fourth valences, so that the structure switches rapidly between the two structures”,1 whereas Claus proposed a diagonal hypothesis4 in which the fourth valence of each carbon is directed toward the carbon in the para position. The latter hypothesis has been rejected because it enables only two derivatives, and it has been revised to remove this inconsistency: instead of forming a bond, the fourth valence stops near the center of the ring in the Armstrong-Baeyer formula,5 or there is only one bridging bond as in the Dewar’s bridged benzene formula.6 In Thiele’s partial valence model,7 the adjacent carbon-carbon bonds are considered as intermediate between single and double bonds. These formulas were later considered by K. C. Ingold to set up his intra-annular tautomerism,8 which appears to be the generalization of Kekulé’s oscillatory hypothesis. Ingold’s tautomerism hypothesis was later called mesomerism.9 The mesomery is an important concept in chemistry, which implicitly introduces the electron delocalization in the context of the prequantum electronic theory. The first applications of quantum chemistry to the benzene problem led on the molecular orbital (MO) side Erich Hückel to propose his famous 4n + 2 rule10 and on the valence bond (VB) side Pauling and Wheland to identify resonance with Ingold’s mesomerism.11,12 An enlighting contribution of modern VB theory on the benzene structure has been brought by Shaik et al.,13 who have shown that the hexagonal symmetry of benzene is due to the σ-system because the π component is distortive along a Kekulean distortion. * Authors to whom correspondence should be addressed (telephone +34-972-418912; fax +34-972-418356; e-mail miquel.sola@ udg.es or silvi@lct.jussieu.fr). ‡ Universitat de Girona. § Université Pierre et Marie Curie. 3911 Chem. Rev. 2005, 105, 3911−3947

Theoretical evaluation of electron delocalization in aromatic molecules by means of atoms in molecules (AIM) and electron localization function (ELF) topological approaches.

The implementation of the n-center electron delocalization indices, n-DIs, and n-order electron localization indices, n-LIs, within the framework of the quantum theory of atoms in molecules, QTAIM, is performed. n-DIs are shown to be very useful to study the local aromaticity in monocyclic and polycyclic compounds. Total and π n-DIs from n = 4 to 7 were computed for a series of typical 4, 5, 6, and 7-center aromatic and antiaromatic rings. For n ≥ 5 the π n-DI accounts for the 95% of the total n-DI and can be employed alone to measure the aromaticity. A scaling factor on the n-DIs is required in order to compare the aromaticity of [5c-6e] and [6c-6e] rings, the same correction allows to estimate the relative aromatic stabilization of polycyclic compounds using the sum of its values for individual rings. This is called Effective Scaled Electron Delocalization, ESED. The comparison with other aromaticity indices reflects a good correlation between ESED and both resonance energies, and HOMA indices. The most important differences between scaled π n-DIs and NICS(0) indices are found for compounds that contain rings with different number of centers or π electrons. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2007

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jcc.20468

QTAIM N-center delocalization indices as descriptors of aromaticity in mono and poly heterocycles.

The conformational preferences of two model compounds for the OCH2O anomeric unit: methanediol and dimethoxymethane analyzed within the framework of the QTAIM theory provide a new interpretation of the anomeric effect. The characteristic stabilization of the gauche conformers of these compounds is accompanied by a progressive reduction of the electron population of the hydrogens of the central methylene as the number of their gauche interactions to lone pairs rises. The electron population removed from these atoms during the conformational change is gained in the gauche conformers by atoms of larger atomic number, which results in a more negative molecular energy. Also, the variations displayed by atomic populations and the QTAIM delocalization indexes are not keeping in line with the hyperconjugative model of the anomeric effect. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007

Atoms in molecules interpretation of the anomeric effect in the O ? C ? O unit

We report experimental and theoretical demonstration regarding the hydrogen-bonding mechanism behind the end-to-end assembly of gold nanorods modified with bifunctional linking molecules. Time-dependent assembly studies were carried out for different linking molecules at both higher and lower pH values with respect to their respective pKa values and were all found to be in agreement with a hydrogen-bonding theory. The results indicate that hydrogen bonding between protonated and unprotonated linking molecules is responsible for nanorod assembly in aqueous solution. Complementary information regarding the stability of the hydrogen-bonded configurations was obtained by density functional calculations for different protonation states.

Evidence for Hydrogen-Bonding-Directed Assembly of Gold Nanorods in Aqueous Solution

A complete conformational analysis on the isolated and polarizable continuum model (PCM) modeled aqueous solution cation, quinonoidal, and anion forms of pelargonidin, comprising the diverse tautomers of the latter forms, was carried out at the B3LYP/6-31++G(d,p) level. The results indicate that the most stable conformer of cationic and quinonoidal forms of pelargonidin are completely planar in the gas phase, whereas that of the anionic form is not planar. In contrast, PCM calculations show that the plane of the B ring is slightly rotated with regard to the AC bicycle in the most stable conformer of the cation and quinonoidal form. The most stable conformers of the cation, both in gas phase and aqueous solution, display anti and syn orientations for, respectively, C2−C3−O−H and C6−C5−O−H dihedral angles, whereas syn and anti orientation of hydroxyls at 7 and 4‘ positions are nearly isoenergetic. The most stable tautomer of quinonoidal pelargonidin is obtained by deprotonating hydroxyl at C5 in gas phase b...

A density functional theory study on pelargonidin.

A study of the intramolecular hydrogen bond (IHB) in 1,2-ethanediol and 1,2-dihydroxybenzene (catechol) was carried out using the QTAIM theory. Atomic and bond properties defined within this theory were calculated for different donor–acceptor distances, Hd⋯Oa, in both molecules, and different H–Oa–C–C dihedral angles for 1,2-dihydroxybenzene, optimising the remaining geometry. Though no conformer of both compounds present IHB, it appears when the Hd⋯Oa distance is reduced in both molecules or when the H–Oa–C–C angle is rotated in 1,2-dihydroxybenzene. The evolution of integrated and local properties follows the criteria of Koch and Popelier for hydrogen bond formation when the interatomic distance is modified, but display the opposite trends when the IHB is formed by rotating the dihedral angle.

Ricardo A. Mosquera

Papers

QTAIM N-center delocalization indices as descriptors of aromaticity in mono and poly heterocycles.

Atoms in molecules interpretation of the anomeric effect in the O ? C ? O unit

Evidence for Hydrogen-Bonding-Directed Assembly of Gold Nanorods in Aqueous Solution

A density functional theory study on pelargonidin.

Do 1,2-ethanediol and 1,2-dihydroxybenzene present intramolecular hydrogen bond?