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The demarcation of the chemical elements into metals and non-metals dates back to the dawn of Dmitri Mendeleev's construction of the periodic table; it still represents the cornerstone of our view of modern chemistry. In this contribution, a particular emphasis will be attached to the question 'Why do the chemical elements of the periodic table exist either as metals or non-metals under ambient conditions?' This is perhaps most apparent in the p-block of the periodic table where one sees an almost-diagonal line separating metals and non-metals. The first searching, quantum-mechanical considerations of this question were put forward by Hund in 1934. Interestingly, the very first discussion of the problem-in fact, a pre-quantum-mechanical approach-was made earlier, by Goldhammer in 1913 and Herzfeld in 1927. Their simple rationalization, in terms of atomic properties which confer metallic or non-metallic status to elements across the periodic table, leads to what is commonly called the Goldhammer-Herzfeld criterion for metallization. For a variety of undoubtedly complex reasons, the Goldhammer-Herzfeld theory lay dormant for close to half a century. However, since that time the criterion has been repeatedly applied, with great success, to many systems and materials exhibiting non-metal to metal transitions in order to predict, and understand, the precise conditions for metallization. Here, we review the application of Goldhammer-Herzfeld theory to the question of the metallic versus non-metallic status of chemical elements within the periodic system. A link between that theory and the work of Sir Nevill Mott on the metal-non-metal transition is also highlighted. The application of the 'simple', but highly effective Goldhammer-Herzfeld and Mott criteria, reveal when a chemical element of the periodic table will behave as a metal, and when it will behave as a non-metal. The success of these different, but converging approaches, lends weight to the idea of a simple, universal criterion for rationalizing the instantly-recognizable structure of the periodic table where … the metals are here, the non-metals are there … The challenge of the metallic and non-metallic states of oxides is also briefly introduced. This article is part of the theme issue 'Mendeleev and the periodic table'.

A new strategy is reported here to synthesize both nitrogen deficient and protonated graphitic carbon nitride (g-C 3 N 4 ) nanosheets by the conjoint use of NH 4 Cl as a dynamic gas template together with hypophosphorous acid (H 3 PO 2 ) as a doping agent. The NH 4 Cl treatment allows for the scalable production of protonated g-C 3 N 4 nanosheets. With the corresponding co-addition of H 3 PO 2 , nitrogen vacancies, accompanied by both additional protons and interstitially-doped phosphorus, are introduced into the g-C 3 N 4 framework, and the electronic bandgap of g-C 3 N 4 nanosheets as well as their optical properties and hydrogen-production performance can be precisely tuned by careful adjustment of the H 3 PO 2 treatment. This conjoint approach thereby results in improved visible-light absorption, enhanced charge-carrier separation and a high H 2 evolution rate of 881.7 μmol h -1 achieved over the H 3 PO 2 doped g-C 3 N 4 nanosheets with a corresponding apparent quantum yield (AQY) of 40.4% (at 420 nm). We illustrate that the synergistic H 3 PO 2 doping modifies the layered g-C 3 N 4 materials by introducing nitrogen vacancies as well as protonating them, leading to significant photocatalytic H 2 evolution enhancements, while the g-C 3 N 4 materials doped with phosphoric acid (H 3 PO 4 ) are simply protonated further, revealing the varied doping effects of phosphorus having different (but accessible) valence states., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Alkalides, the alkali metals in their ?1 oxidation state, represent some of the largest and most polarizable atomic species in condensed phases. This study determines the solvation environment around the sodide anion, Na ? , in a system of co-solvated Li + . We present isotopically varied total neutron scattering experiments alongside empirical potential structure refinement and ab initio molecular dynamics simulations for the alkali?alkalide system, LiNa?10MeNH 2 . Both local coordination modes and the intermediate range liquid structure are determined, which demonstrate that distinct structural correlations between cation and anion in the liquid phase extend beyond 8.6 ?. Indeed, the local solvation around Na ? is surprisingly well defined with strong solvent orientational order, in contrast to the classical description of alkalide anions not interacting with their environment. The ion-paired Li(MeNH 2 ) 4 + ?Na ? species appears to be the dominant alkali?alkalide environment in these liquids, whereby Li + and Na ? share a MeNH 2 molecule through the amine group in their primary solvation spheres.

A rapid and non-invasive method to determine the dispersity of emulsions is developed based on the interrelationship between the droplet size distribution and the dielectric properties of emulsions. A range of water-in-oil emulsions with different water contents and droplet size distributions were analysed using a microwave cavity perturbation technique together with dynamic light scattering. The results demonstrate that the dielectric properties, as measured by non-invasive microwave cavity analysis, can be used to characterise the dispersity of emulsions, and is also capable of characterizing heavy oil emulsions. This technique has great potential for industrial applications to examine the sedimentation, creaming and hence the stability of emulsions.

The flexibility window in zeolites was originally identified using geometric simulation as a hypothetical property of SiO 2 systems. The existence of the flexibility window in hypothetical structures may help us to identify those we might be able to synthesize in the future. We have previously found that the flexibility window in silicates is connected to phase transitions under pressure, structure amorphization and other physical behaviours and phenomena. We here extend the concept to ordered aluminosilicate systems using softer 'bar' constraints that permit additional flexibility around aluminium centres. Our experimental investigation of pressure-induced amorphization in sodalites is consistent with the results of our modelling. The softer constraints allow us to identify a flexibility window in the anomalous case of goosecreekite., Competing Interests: The authors declare no competing interests.

Metal-amine solutions provide a unique arena in which to study electrons in solution, and to tune the electron density from the extremes of electrolytic through to true metallic behavior. The existence and structure of a new class of concentrated metal-amine liquid, Li-NH 3 -MeNH 2 , is presented in which the mixed solvent produces a novel type of electron solvation and delocalization that is fundamentally different from either of the constituent systems. NMR, ESR, and neutron diffraction allow the environment of the solvated electron and liquid structure to be precisely interrogated. Unexpectedly it was found that the solution is truly homogeneous and metallic. Equally surprising was the observation of strong longer-range order in this mixed solvent system. This is despite the heterogeneity of the cation solvation, and it is concluded that the solvated electron itself acts as a structural template. This is a quite remarkable observation, given that the liquid is metallic., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We present the results of a muon spin relaxation study of the solid phases of the expanded metal, Li(NH3)4. No discernible change in muon depolarization dynamics is witnessed in the lowest temperature phase (≤25 K) of Li(NH3)4, thus suggesting that the prevailing view of antiferromagnetic ordering is incorrect. This is consistent with the most recent neutron diffraction data. Discernible differences in muon behavior are reported for the highest temperature phase of Li(NH3)4 (82-89 K), attributed to the onset of structural dynamics prior to melting.

A series of MoO 3 /H-ZSM-5 (Si/Al = 25) catalysts were prepared via calcination at a lower-than-usual temperature (400 °C) and subsequently evaluated in the methanol-to-hydrocarbon reaction at that same temperature. The catalytic properties of those catalysts were compared with the sample prepared at the more conventional, higher temperature of 500 °C. For the lower temperature preparations, molybdenum oxide was preferentially dispersed over the zeolite external surface, while only the higher loading level of MoO 3 (7.5 wt% or higher) led to observable inner migration of the Mo species into the zeolite channels, with concomitant partial loss of the zeolite Brønsted acidity. On the MoO 3 modified samples, the early-period gas yield, especially for valuable propylene and C 4 products, was noticeably accelerated, and is gradually converted into an enhanced liquid aromatic formation. The 7.5 wt% MoO 3 /H-ZSM-5 sample prepared at 400 °C thereby achieved a balance between the zeolite surface dispersion of Mo species, their inner channel migration and the corresponding effect on the intrinsic Brønsted acidity of the acidic zeolite. That loading level also possessed the highest product selectivity (after 5 h reaction) to benzene, toluene and xylenes, as well as higher early-time valuable gas product yields in time-on-stream experiments. However, MoO 3 loading levels of 7.5 wt% and above also resulted in earlier catalyst deactivation by enhanced coke accumulation at, or near, the zeolite channel openings. Our research illustrates that the careful adoption of moderate/lower temperature dispersion processes for zeolite catalyst modification gives considerable potential for tailoring and optimizing the system's catalytic performance.

A qualitative account of the occurrence and magnitude of superconductivity in the transition metals is presented, with a primary emphasis on elements of the first row. Correlations of the important parameters of the Bardeen-Cooper-Schrieffer theory of superconductivity are highlighted with respect to the number of d-shell electrons per atom of the transition elements. The relation between the systematics of superconductivity in the transition metals and the periodic table high-lights the importance of short-range or chemical bonding on the remarkable natural phenomenon of superconductivity in the chemical elements. A relationship between superconductivity and lattice instability appears naturally as a balance and competition between localized covalent bonding and so-called broken covalency, which favours d-electron delocalization and superconductivity. In this manner, the systematics of superconductivity and various other physical properties of the transition elements are related and unified., (© 2015 The Author(s) Published by the Royal Society. All rights reserved.)

The classification of a chemical element as either 'metal' or 'non-metal' continues to form the basis of an instantly recognizable, universal representation of the periodic table (Mendeleeff D. 1905 The principles of chemistry, vol. II, p. 23; Poliakoff M. & Tang S. 2015 Phil. Trans. R. Soc. A 373: , 20140211). Here, we review major, pre-quantum-mechanical innovations (Goldhammer DA. 1913 Dispersion und Absorption des Lichtes; Herzfeld KF. 1927 Phys. Rev. 29: , 701-705) that allow an understanding of the metallic or non-metallic status of the chemical elements under both ambient and extreme conditions. A special emphasis will be placed on recent experimental advances that investigate how the electronic properties of chemical elements vary with temperature and density, and how this invariably relates to a changing status of the chemical elements. Thus, the prototypical non-metals, hydrogen and helium, becomes metallic at high densities; and the acknowledged metals, mercury, rubidium and caesium, transform into their non-metallic forms at low elemental densities. This reflects the fundamental fact that, at temperatures above the absolute zero of temperature, there is therefore no clear dividing line between metals and non-metals. Our conventional demarcation of chemical elements as metals or non-metals within the periodic table is of course governed by our experience of the nature of the elements under ambient conditions. Examination of these other situations helps us to examine the exact divisions of the chemical elements into metals and non-metals (Mendeleeff D. 1905 The principles of chemistry, vol. II, p. 23)., (© 2015 The Author(s) Published by the Royal Society. All rights reserved.)

We report the experimental investigation of hydrogen storage and release in the lithium amide-lithium hydride composite (Li-N-H) system. Investigation of hydrogenation and dehydrogenation reactions of the system through in situ synchrotron X-ray powder diffraction experiments allowed for the observation of the formation and evolution of non-stoichiometric intermediate species of the form Li1+xNH2-x. This result is consistent with the proposed Frenkel-defect mechanism for these reactions. We observed capacity loss with decreasing temperature through decreased levels of lithium-rich (0.7 ≤ x ≤ 1.0) non-stoichiometric phases in the dehydrogenated material, but only minor changes due to multiple cycles at the same temperature. Annealing of dehydrogenated samples reveals the reduced stability of intermediate stoichiometry values (0.4 ≤ x ≤ 0.7) compared with the end member species: lithium amide (LiNH2) and lithium imide (Li2NH). Our results highlight the central role of ionic mobility in understanding temperature limitations, capacity loss and facile reversibility of the Li-N-H system.

(1)H, (7)Li, (14)N, and (23)Na high resolution nuclear magnetic resonance (NMR) measurements are reported for fluid solutions of lithium and sodium in anhydrous liquid ammonia across the metal-to-nonmetal transition (MNM transition), paying particular attention to the phenomenon of liquid-liquid phase separation which occurs in the composition/temperature region close to the MNM transition. Our results are discussed in terms of the electronic structure of fluid metal-ammonia solutions at low temperatures (ca. 240 K). We find that the electronic phase transition to the metallic state in these solutions, especially at temperatures close to the liquid-liquid critical consolute temperature, occurs from a nonmetallic, electrolytic solution containing a predominance of electron spin-paired, (diamagnetic) charged bosonic states. The possible implications of these observations to the nature of the liquid-liquid phase separation are discussed, both from the views of N. F. Mott, regarding the MNM transition in sodium-ammonia solutions, and those of R. A. Ogg, regarding the possibility of high-temperature superconductivity in these solutions. Similarities between the electronic structure of metal-ammonia solutions and the high-temperature cuprate superconductors are also briefly emphasized.

A facile solvent evaporation induced self-assembly (SEISA) strategy was developed to synthesize mesoporous N-doped anatase TiO2 (SE-meso-TON) using a single organic complex precursor derived in situ from titanium butoxide and ethylenediamine in ethanol solution. After the evaporation of ethanol in a fume hood and subsequent calcinations at 450 °C, the obtained N-doped TiO2 (meso-TON) anatase was of finite crystallite size, developed porosity, large surface area (101 m(2) g(-1)) and extended light absorption in the visible region. This SE-meso-TON also showed superior photocatalytic activity to the SG-meso-TON anatase prepared via sol-gel synthesis. On the basis of characterization results from XRD, XPS, N2 adsorption-desorption and ESR, the enhanced visible-light-responsive photocatalytic activity of SE-meso-TON was assigned to its developed mesoporosity and reduced oxygen vacancies.

In microwave chemistry there is a common misconception that small, highly conducting particles heat profusely when placed in a large microwave electric field. However, this is not the case; with the simple physical explanation that the electric field (which drives the heating) within a highly conducting particle is highly screened. Instead, it is the magnetic absorption associated with induction that accounts for the large experimental heating rates observed for small metal particles. We present simple principles for the effective heating of particles in microwave fields from calculations of electric and magnetic dipole absorptions for a range of practical values of particle size and conductivity. For highly conducting particles, magnetic absorption dominates electric absorption over a wide range of particle radii, with an optimum absorption set by the ratio of mean particle radius a to the skin depth δ (specifically, by the condition a = 2.41δ). This means that for particles of any conductivity, optimized magnetic absorption (and hence microwave heating by magnetic induction) can be achieved by simple selection of the mean particle size. For weakly conducting samples, electric dipole absorption dominates, and is maximized when the conductivity is approximately σ ≈ 3ωε(0) ≈ 0.4 S m(-1), independent of particle radius. Therefore, although electric dipole heating can be as effective as magnetic dipole heating for a powder sample of the same volume, it is harder to obtain optimized conditions at a fixed frequency of microwave field. The absorption of sub-micron particles is ineffective in both magnetic and electric fields. However, if the particles are magnetic, with a lossy part to their complex permeability, then magnetic dipole losses are dramatically enhanced compared to their values for non-magnetic particles. An interesting application of this is the use of very small magnetic particles for the selective microwave heating of biological samples.

An effective, template-free synthesis methodology has been developed for preparing mesoporous nitrogen-doped SrTiO(3) (meso-STON) using glycine as both a nitrogen source and a mesopore creator. The N-doping, large surface area and developed porosity endow meso-STON with excellent activity in visible-light-responsive photodegradation of organic dyes.

Lead dioxide has been used for over a century in the lead-acid battery. Many fundamental questions concerning PbO2 remain unanswered, principally: (i) is the bulk material a metal or a semiconductor, and (ii) what is the source of the high levels of conductivity? We calculate the electronic structure and defect physics of PbO2, using a hybrid density functional, and show that it is an n-type semiconductor with a small indirect band gap of ∼0.2  eV. The origin of electron carriers in the undoped material is found to be oxygen vacancies, which forms a donor state resonant in the conduction band. A dipole-forbidden band gap combined with a large carrier induced Moss-Burstein shift results in a large effective optical band gap. The model is supported by neutron diffraction, which reveals that the oxygen sublattice is only 98.4% occupied, thus confirming oxygen substoichiometry as the electron source.

A combination of inelastic neutron scattering (INS) spectroscopy and Raman spectroscopy with periodic density functional theory calculations is used to provide a complete assignment of the vibrational spectra of α-lithium amidoborane (α-LiNH(2)BH(3)). The Born charge density and the atomic motion up to the decomposition temperature have been modelled. These models not only explain the nature of bonding in α-LiNH(2)BH(3) but also provide an insight into the atomic mechanisms of its decomposition. The (INS) measurements were performed in the range of 0-4000 cm(-1) on the high-resolution time-of-flight TOSCA INS spectrometer at the ISIS Spallation Neutron Source at the Rutherford Appleton Laboratory.

An ultrasound-assisted, precipitation-deposition method has been developed to synthesise visible-light-responsive BiOBr-ZnFe(2)O(4) heterojunction photocatalysts. The heterojunctions with suitable BiOBr/ZnFe(2)O(4) ratios have a fascinating micro-spherical morphology and exhibit exceptional photocatalytic activity in visible-light degradation of Rhodamine B., (© The Royal Society of Chemistry 2011)

A stepwise phase transition in the formation of lithium amidoborane via the solid-state reaction of lithium hydride and ammonia borane has been identified and investigated. Structural analyses reveal that a lithium amidoborane-ammonia borane complex (LiNH(2)BH(3).NH(3)BH(3)) and two allotropes of lithium amidoborane (denoted as alpha- and beta-LiNH(2)BH(3), both of which adopt orthorhombic symmetry) were formed in the process of synthesis. LiNH(2)BH(3).NH(3)BH(3) is the intermediate of the synthesis and adopts a monoclinic structure that features layered LiNH(2)BH(3) and NH(3)BH(3) molecules and contains both ionic and dihydrogen bonds. Unlike alpha-LiNH(2)BH(3), the units of the beta phase have two distinct Li(+) and [NH(2)BH(3)](-) environments. beta-LiNH(2)BH(3) can only be observed in energetic ball milling and transforms to alpha-LiNH(2)BH(3) upon extended milling. Both allotropes of LiNH(2)BH(3) exhibit similar thermal decomposition behavior, with 10.8 wt % H(2) released when heated to 180 degrees C; in contrast, LiNH(2)BH(3).NH(3)BH(3) releases approximately 14.3 wt % H(2) under the same conditions.

Lithium borohydride absorbs anhydrous ammonia to form four stable ammoniates; Li(NH(3))(n)BH(4), mono-, di-, tri-, and tertraammoniate. This paper focuses on the monoammoniate, Li(NH(3))BH(4), which is readily formed on exposure of LiBH(4) to ammonia at room temperature and pressure. Ammonia loss from Li(NH(3))BH(4) commences around 40 degrees C and the compound transforms directly to LiBH(4). The crystal structure of Li(NH(3))BH(4) is reported here for the first time. Its close structural relationship with LiBH(4) provides a clear insight into the facile nature and mechanism of ammonia uptake and loss. These materials not only represent an excellent high weight-percent ammonia system but are also potentially important hydrogen stores.

A detailed molecular orbital (MO) analysis of the structure and electronic properties of the great variety of species in lithium-ammonia solutions is provided. In the odd-electron, doublet states we have considered: e-@(NH3)n (the solvated electron, likely to be a dynamic ensemble of molecules), the Li(NH3)4 monomer, and the [Li(NH3)4+.e-@(NH3)n] ion-pairs, the Li 2s electron enters a diffuse orbital built up largely from the lowest unoccupied MOs of the ammonia molecules. The singly occupied MOs are bonding between the hydrogen atoms; we call this stabilizing interaction H-->H bonding. In e-@(NH3)n the odd electron is not located in the center of the cavities formed by the ammonia molecules. Possible species with two or more weakly interacting electrons also exhibit H-->H bonding. For these, we find that the singlet (S=0) states are slightly lower in energy than those with unpaired (S=1, 2...) spins. TD-DFT calculations on various ion-pairs show that the three most intense electronic excitations arise from the transition between the SOMO (of s pseudosymmetry) into the lowest lying p-like levels. The optical absorption spectra are relatively metal-independent, and account for the absorption tail which extends into the visible. This is the source of Sir Humphry Davy's "fine blue colour" first observed just over 200 years ago.

Metal lite: High-resolution neutron powder diffraction data reveals that the body-centered cubic crystal structure of lithium(0)tetraamine transforms to a simple cubic structure below 22 K. The detailed structure determinations will allow new insights into the coupled structural and electronic properties of the lightest metal.

Visible-light-responsive tubular N-doped TiO2 nanocrystallites were synthesized via an environment-conscious solvothermal treatment of protonated titanate nanotubes in an NH4Cl/ethanol/water solution.

The years 2007 and 2008 mark the bi-centenary of two brilliant discoveries by Sir Humphry Davy: the isolation of sodium and potassium (1807) and the subsequent first observation (1808) of the beautiful blue and bronze colours now known to be characteristic of the solvated electron(1) in potassium-ammonia systems. In celebration of these dazzling discoveries, we reflect on Davy's many extraordinary contributions to science, technology and poetry. Humphry Davy, a truly great man, of Cornish spirit, brought immeasurable benefits to humankind.

All superconductors are solids in their superconducting state, this canonical electronic state of matter presently having only been observed well below the melting temperature of the solid. The discovery of high-temperature superconductivity in cuprates has widened significantly our horizons of the theoretical understanding of the physical phenomenon. A number of observations point to the possibility that superconductors with a high superconducting transition temperature may not be conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, but rather derive from the Bose-Einstein condensation of real-space pairs. While BCS superconductors exist in the solid state (probably with the exception of metallic liquid hydrogen at ultrahigh pressures), we argue here that a superconducting charged Bose liquid may be found in a true liquid state of condensed matter at ambient pressure. An experimental scenario is outlined in fluid metal-ammonia solutions for stabilizing and observing a high-temperature superconducting liquid (ca. 230 K) or at least a vitreous superconductor in the corresponding quenched solutions (ca. 160 K).

Large crystallites of high purity zeolite rho were synthesized by controlled monitoring of the aging and heating period of the mother gel. The microwave conductivity of Rb-rho doped with up to 20 Rb atoms per unit cell was measured over the temperature range 15-300 K, and the structures of three of the samples were examined through Rietveld analysis of powder neutron diffraction data. At low concentrations of rubidium dopant the observed microwave responses were dominated by polarization effects. In the sample Rb(17)/Rb-rho a strongly temperature-dependent electronic contribution to the conductivity was observed above approximately =150 K. In Rb(20)/Rb-rho, conductivities in the range 1.5-2.3 Sm(-1) were observed between 15 and 300 K. This residual conductivity at 15 K, unprecedented in a zeolite, indicates that the sample is indeed metallic; however, the values of conductivity measured are low in comparison to conventional metals and comparable to those of doped semiconductors. The evolution of the conducting behaviour is discussed in relation both to observed structural and to possible electronic changes occurring within the samples on metal doping.

The nature and surroundings of lithium cations in lithium-exchanged X and A zeolites following loading with the alkali metals Na, K, Rb, and Cs have been studied through (7)Li solid-state NMR spectroscopy. It is demonstrated that the lithium in these zeolites is stable with respect to reduction by the other alkali metals. Even though the lithium cations are not directly involved in chemical interactions with the excess electrons introduced in the doping process, the corresponding (7)Li NMR spectra are extremely sensitive to paramagnetic species that are located inside the zeolite cavities. This sensitivity makes (7)Li NMR a useful probe to study the formation, distribution, and transformation of such species.

We utilize the experience gained in our previous studies on the "chemistry of vibronic coupling" in simple homonuclear and heteronuclear molecules to begin assembling theoretical guidelines for the construction of potentially superconducting solids exhibiting large electron-phonon coupling. For this purpose we analyze similarities between vibronic coupling in isolated molecules and in extended solids. In particular, we study vibronic coupling along the antisymmetric stretch coordinate (Q(as)) in linear symmetric AAA molecules, and along the optical phonon "pairing" mode coordinate (Q(opt)) in corresponding one-dimensional [A]( infinity ) chains built of equidistant A atoms. This is done for a broad range of chemical elements (A). The following similarities between vibronic coupling in molecules and phonon coupling in solids emerge from our calculations: 1) The HOMO/LUMO electronic energy gap in an AAA molecule increases along Q(as), and the highest occupied crystal orbital/lowest unoccupied crystal orbital gap in [A]( infinity ) chain increases along Q(opt). 2) The maximum vibronic instability is invariably obtained for a half-filled, singly occupied molecular orbital in AAA molecules, and for a corresponding half-filled band in [A]( infinity ) chains. 3) The vibronic stability of an AAA molecule increases with a decrease of the AA bond length, as does the vibronic stability of [A]( infinity ) chains (external pressure may lead to a reversal of a Peierls distortion). 4) The high degree of s-p mixing and ionic/covalent forbidden curve crossing dramatically enhance the vibronic instability of both AAA molecules and [A]( infinity ) chains. We also introduce one quantitative relationship: The parameter log(R) (where R is molar refractivity, a parameter used by Herzfeld to prescribe the conditions for the metallization of the elements) correlates with a parameter f(AA) (defined as twice the electronegativity of A, divided by the equilibrium AA bond length), used by two of us previously to describe vibronic coupling in AAA molecules for a broad range of elements (A=halogen, H, or an alkali metal). We hope to illustrate that key chemical aspects of vibronic coupling in simple molecules may thus be profitably transferred to corresponding materials in the solid state.

Properties of materials determined by their size are indeed fascinating and form the basis of the emerging area of nanoscience. In this article, we examine the size dependent electronic structure and properties of nanocrystals of semiconductors and metals to illustrate this aspect. We then discuss the chemical reactivity of metal nanocrystals which is strongly dependent on the size not only because of the large surface area but also a result of the significantly different electronic structure of the small nanocrystals. Nanoscale catalysis of gold exemplifies this feature. Size also plays a role in the assembly of nanocrystals into crystalline arrays. While we owe the beginnings of size-dependent chemistry to the early studies of colloids, recent findings have added a new dimension to the subject.