3c/4e σ̂-Type Long-Bonding: A Novel Transitional Motif toward the Metallic Delocalization Limit

Electronic properties and collision cross sections of AgOkHm± (k, m = 1-4) aerosol ionic clusters

Experimental evidence shows that hydroxylated metal ions are often produced during cluster synthesis by atmospheric pressure spark ablation. In this work, we predict the ground state equilibrium structures of AgOkHm± clusters (k and m = 1-4), which are readily produced when spark ablating Ag, using the coupled cluster with singles and doubles (CCSD) method. The stabilization energy of these clusters is calculated with respect to the dissociation channel having the lowest energy, by accounting perturbative triples corrections to the CCSD method. The interatomic interactions in each of the systems have been investigated using the frontier molecular orbital (FMO), natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) methods. Many of the ground states of these ionic clusters are found to be stable, corroborating experimental observations. We find that clusters having singlet spin states are more stable in terms of dissociation than the clusters that have doublet or triplet spin states. Our calculations also indicate a strong affinity of the ionic and neutral Ag atom towards water and hydroxyl radicals or ions. Many 3-center, 4-electron (3c/4e) hyperbonds giving rise to more than one resonance structure are identified primarily for the anionic clusters. The QTAIM analysis shows that the O-H and O-Ag bonds in the clusters of both polarities are respectively covalent and ionic. The FMO analysis indicates that the anionic clusters are more reactive than the cationic ones. Using the cluster structures predicted by the CCSD method, we calculate the collision cross sections of the AgOkHm± family, with k and m ranging from 1 to 4, by the trajectory method. In turn, we predict the electrical mobilities of these clusters when suspended in helium at atmospheric pressure and compare them with experimental measurements.

High-Density “Windowpane” Coordination Patterns of Water Clusters and Their NBO/NRT Characterization

Cluster mixture models for liquid water at higher pressures suggest the need for water clusters of higher coordination and density than those commonly based on tetrahedral H-bonding motifs. We show here how proton-ordered water clusters of increased coordination and density can assemble from a starting cyclic tetramer or twisted bicyclic (Möbius-like) heptamer to form extended Aufbau sequences of stable two-, three-, and four-coordinate “windowpane” motifs. Such windowpane clusters exhibit sharply reduced (~90°) bond angles that differ appreciably from the tetrahedral angles of idealized crystalline ice I_h. Computed free energy and natural resonance theory (NRT) bond orders provide quantitative descriptors for the relative stabilities of clusters and strengths of individual coordinative linkages. The unity and consistency of NRT description is demonstrated to extend from familiar supra-integer bonds of the molecular regime to the near-zero bond orders of the weakest linkages in the present H-bond clusters. Our results serve to confirm that H-bonding exemplifies resonance–covalent (fractional) bonding in the sub-integer range and to further discount the dichotomous conceptions of “electrostatics” for intermolecular bonding vs. “covalency” for intramolecular bonding that still pervade much of freshman-level pedagogy and force-field methodology.

The Orbital Origins of Chemical Bonding in Ge-Sb-Te Phase-Change Materials

Layered phase‐change materials in the Ge−Sb−Te system are widely used in data storage and are the subject of intense research to understand the quantum‐chemical origin of their unique properties. To uncover the nature of the underlying periodic wavefunction, we have studied the interacting atomic orbitals including their phases by means of crystal orbital bond index and fragment crystal orbital analysis. In full accord with findings based on projected force constants, we demonstrate the role of multicenter bonding along straight atomic connectivities. While the resulting multicenter bonding resembles three‐center‐four‐electron bonding in molecules, its solid‐state manifestation leads to distinct long‐range consequences, thus serving to contextualize the material properties usually termed “metavalent”. Eventually we suggest multicenter bonding to be the origin of their astonishing bond‐breaking and phase‐change behavior, as well as the too small “van‐der‐Waals” gaps between individual layers.

Long-bonding and bonding nature in noble gas insertion compounds MNgBY of transition metal-boron bond

The nature of inert gas bonding has always been an important topic. The bonds of noble gases cover the entire range of chemical bonds, from the weakest van der Waals forces, to non-covalent interactions, and to covalent bonds. Two types of methods were used to investigate the properties of chemical bonds in the inert gas inserted compound MNgBY with the transition metal M = Cu/Ag/Au and substituents Y = O/S/NH, one based on orbital analysis and the other based on electron density analysis. The NBO/NRT analysis shows that in these compounds there exists long-bonding striding the noble gas between the transitional metal and boron, similar to the noble gas insertion compounds HNgX of hydrohalide, and so a three-center four-electron bond exists among the M-Ng-B part. The electron density analyses show that the M-Ng bond between the metal Cu/Ag/Au and noble gas and the Ng-B bond in the Cu/Ag compounds are partial covalent but the Ng-B bond in Au compounds is a typical covalent bond. The large relativistic effects of Au cause the bonds in Au compounds shorter and stronger than the bonds in Ag/Cu compounds. The properties of the M-Ng and Ng-B bonds are not affected by substituents Y, but the bond lengths are sensitive to substituents.

Synthesis, Structure, and Reactivity of a Terminal Cadmium Hydride Compound, [κ3-TismPriBenz]CdH

The terminal cadmium hydride compound, [κ³-Tism^PriBenz]CdH, which features the tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, may be obtained via the reactions of either [κ³-Tism^PriBenz]CdN(SiMe₃)₂ or [Tism^PriBenz]CdOSiPh₃ with PhSiH₃. The Cd-H bond of [κ³-Tism^PriBenz]CdH undergoes (a) metathesis reactions with MeI, Me₃SiX (X = Cl, Br, I, NCO), and Me₃SnX (X = Cl, Br, I) to afford the corresponding [Tism^PriBenz]CdX derivative, (b) insertion with CO₂ and CS₂ to afford respectively [Tism^PriBenz]Cd(κ¹-O₂CH) and [Tism^PriBenz]Cd(κ¹-S₂CH), and (c) hydride abstraction with B(C₆F₅)₃ to afford {[Tism^PriBenz]Cd}[HB(C₆F₅)₃] that possesses a rare trigonal monopyramidal geometry for cadmium.

Isolation and reactivity of an elusive diazoalkene

Most functional groups, especially those consisting of the abundant elements of organic matter-carbon, nitrogen and oxygen-have been extensively studied and only very few remain speculative due to their high intrinsic reactivity. In contrast to the well-explored chemistry of diazoalkanes (R₂C=N₂), diazoalkenes (R₂C=C=N₂) have been postulated in several organic transformations, but remain elusive long-sought intermediates. Here, we present a room-temperature stable diazoalkene, utilizing a dinitrogen transfer from nitrous oxide. This functional group shows dual-site nucleophilicity (C and N atoms) and features a bent C-C-N entity (124°) and a long N-N bond together with a remarkable low infrared absorption (1,944 cm^-1). Substitution of N₂ by an isocyanide leads to a vinylidene ketenimine. Furthermore, photochemically triggered loss of dinitrogen might proceed through a transient triplet vinylidene. We anticipate the existence of a stable diazoalkene functional group to pave an exciting avenue into the chemistry of low-valent carbon and unsaturated carbenes.

Sulfur Tetrahydride and Allied Superhydride Clusters: When Resonance Takes Precedence

Sulfur offers a variety of bonding surprises compared to the parent oxygen atom of the chalcogen family. In the present work, we employ standard quantum chemistry methods to characterize formation of previously unrecognized sulfur tetrahydride (C_4v -symmetric SH₄ ) from hydrogen sulfide (H₂ S) and molecular hydrogen (H₂ ) on the ground state potential energy surface. The unusual intramolecular interactions of SH₄ defy Lewis-like bonding conceptions, exhibiting the dominance of resonance-type donor-acceptor delocalizations well beyond those of SF₄ (C_2v sawhorse geometry) and other known tetrahalides. The distressed character of SH₄ bonding also leads to exotic intermolecular structural motifs in clusters of pure (SH₄ )_n and mixed (SH₄ ⋅⋅⋅H₂ S)_n composition. We evaluate structural, spectroscopic, and electronic properties for various 2D/3D coordination patterns and discuss how (SH₄ ⋅⋅⋅H₂ S)_n -type building blocks may relate to recent experimental studies of superconductivity in high-pressure materials of "SH₃ " stoichiometry.

Structure-reactivity studies on hypervalent square-pyramidal dithieno[3,2-b:2',3'-d]phospholes

A series of neutral pentacoordinate dithieno[3,2-b:2',3'-d]phosphole compounds were synthesized by [4 + 1] cycloaddition with o-quinones. Counter to the expected trigonal bipyramidal geometry, the luminescent hypervalent dithienophospholes exhibit square pyramidal geometry with inherently Lewis acidic phosphorus center that is stabilized via supramolecular π-stacking interactions in the solid state and in solution. Due to their Lewis-acid character, the compounds react with nucleophiles, suggesting their potential as mediator in organic transformations. The new species thus present an intriguing structural plaform for the design of neutral P(v) Lewis acids with useful reactivities.

NBO/NRT Two-State Theory of Bond-Shift Spectral Excitation

We show that natural bond orbital (NBO) and natural resonance theory (NRT) analysis methods provide both optimized Lewis-structural bonding descriptors for ground-state electronic properties as well as suitable building blocks for idealized "diabatic" two-state models of the associated spectroscopic excitations. Specifically, in the framework of single-determinant Hartree-Fock or density functional methods for a resonance-stabilized molecule or supramolecular complex, we employ NBO/NRT descriptors of the ground-state determinant to develop a qualitative picture of the associated charge-transfer excitation that dominates the valence region of the electronic spectrum. We illustrate the procedure for the elementary bond shifts of SN2-type halide exchange reaction as well as the more complex bond shifts in a series of conjugated cyanine dyes. In each case, we show how NBO-based descriptors of resonance-type 3-center, 4-electron (3c/4e) interactions provide simple estimates of spectroscopic excitation energy, bond orders, and other vibronic details of the excited-state PES that anticipate important features of the full multi-configuration description. The deep 3c/4e connections to measurable spectral properties also provide evidence for NBO-based estimates of ground-state donor-acceptor stabilization energies (sometimes criticized as "too large" compared to alternative analysis methods) that are also found to be of proper magnitude to provide useful estimates of excitation energies and structure-dependent spectral shifts.

Symmetry of three-center, four-electron bonds

Three-center, four-electron bonds provide unusually strong interactions; however, their nature remains ununderstood. Investigations of the strength, symmetry and the covalent versus electrostatic character of three-center hydrogen bonds have vastly contributed to the understanding of chemical bonding, whereas the assessments of the analogous three-center halogen, chalcogen, tetrel and metallic Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> -type long bonding are still lagging behind. Herein, we disclose the X-ray crystallographic, NMR spectroscopic and computational investigation of three-center, four-electron [D–X–D]⁺ bonding for a variety of cations (X⁺ = H⁺, Li⁺, Na⁺, F⁺, Cl⁺, Br⁺, I⁺, Ag⁺ and Au⁺) using a benchmark bidentate model system. Formation of a three-center bond, [D–X–D]⁺ is accompanied by an at least 30% shortening of the D–X bonds. We introduce a numerical index that correlates symmetry to the ionic size and the electron affinity of the central cation, X⁺. Providing an improved understanding of the fundamental factors determining bond symmetry on a comprehensive level is expected to facilitate future developments and applications of secondary bonding and hypervalent chemistry.

The factors determining the symmetry and the fundamental nature of the three-center, four-electron bonds are assessed.

Charge-Shift Bonding in Xenon Hydrides: An NBO/NRT Investigation on HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CCH, CN) via H-Xe Blue-Shift Phenomena

Noble-gas bonding represents curiosity. Some xenon hydrides, such as HXeY (Y = Cl, Br, I) and their hydrogen-bonded complexes HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CN, CCH), have been identified in matrixes by observing H-Xe frequencies or its monomer-to-complex blue shifts. However, the H-Xe bonding in HXeY is not yet completely understood. Previous theoretical studies provide two answers. The first one holds that it is a classical covalent bond, based on a single ionic structure H-Xe+ Y-. The second one holds that it is resonance bonding between H-Xe+ Y- and H- Xe+-Y. This study investigates the H-Xe bonding, via unusual blue-shifted phenomena, combined with some NBO/NRT calculations for chosen hydrogen-bonded complexes HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CN, CCH). This study provides new insights into the H-Xe bonding in HXeY. The H-Xe bond in HXeY is not a classical covalent bond. It is a charge-shift (CS) bond, a new class of electron-pair bonds, which is proposed by Shaik and Hiberty et al. The unusual blue shift in studied hydrogen-bonded complexes is its H-Xe CS bonding character in IR spectroscopy. It is expected that these studies on the H-Xe bonding and its IR spectroscopic property might assist the chemical community in accepting this new-class electron-pair bond concept.

What Is the Nature of Supramolecular Bonding? Comprehensive NBO/NRT Picture of Halogen and Pnicogen Bonding in RPH2···IF/FI Complexes (R = CH3, OH, CF3, CN, NO2)

We employ a variety of natural bond orbital (NBO) and natural resonance theory (NRT) tools to comprehensively investigate the nature of halogen and pnicogen bonding interactions in RPH₂···IF/FI binary complexes (R = CH₃, OH, CF₃, CN, and NO₂) and the tuning effects of R-substituents. Though such interactions are commonly attributed to “sigma-hole”-type electrostatic effects, we show that they exhibit profound similarities and analogies to the resonance-type 3-center, 4-electron (3c/4e) donor-acceptor interactions of hydrogen bonding, where classical-type “electrostatics” are known to play only a secondary modulating role. The general 3c/4e resonance perspective corresponds to a continuous range of interatomic A···B bond orders (b_AB), spanning both the stronger “covalent” interactions of the molecular domain (say, b_AB ≥ ½) and the weaker interactions (b_AB ˂ ½, often misleadingly termed “noncovalent”) that underlie supramolecular complexation phenomena. We show how a unified NBO/NRT-based description of hydrogen, halogen, pnicogen, and related bonding yields an improved predictive utility and intuitive understanding of empirical trends in binding energies, structural geometry, and other measurable properties that are expected to be manifested in all such supramolecular interaction phenomena.

Resonance Theory Reboot

What is now called "resonance theory" has a long and conflicted history. We first sketch the early roots of resonance theory, its heritage of diverse physics and chemistry conceptions, and its subsequent rise to reigning chemical bonding paradigm of the mid-20th century. We then outline the alternative "natural" pathway to localized Lewis- and resonance-structural conceptions that was initiated in the 1950s, given semi-empirical formulation in the 1970s, recast in ab initio form in the 1980s, and successfully generalized to multi-structural "natural resonance theory" (NRT) form in the 1990s. Although earlier numerical applications were often frustrated by the ineptness of then-available numerical solvers, the NRT variational problem was recently shown to be amenable to highly efficient convex programming methods that yield provably optimal resonance weightings at a small fraction of previous computational costs. Such convexity-based algorithms now allow a full "reboot" of NRT methodology for tackling a broad range of chemical applications, including the many familiar resonance phenomena of organic and biochemistry as well as the still broader range of resonance attraction effects in the inorganic domain. We illustrate these advances for prototype chemical applications, including (i) stable near-equilibrium species, where resonance mixing typically provides only small corrections to a dominant Lewis-structural picture, (ii) reactive transition-state species, where strong resonance mixing of reactant and product bonding patterns is inherent, (iii) coordinative and related supramolecular interactions of the inorganic domain, where sub-integer resonance bond orders are the essential origin of intermolecular attraction, and (iv) exotic long-bonding and metallic delocalization phenomena, where no single "parent" Lewis-structural pattern gains pre-eminent weighting in the overall resonance hybrid.

A theoretical investigation on Cu/Ag/Au bonding in XH2P⋯MY(X = H, CH3, F, CN, NO2; M = Cu, Ag, Au; Y = F, Cl, Br, I) complexes

Intermolecular interaction of XH₂P···MY (X = H, CH₃, F, CN, NO₂; M = Cu, Ag, Au; Y = F, Cl, Br, I) complexes was investigated by means of an ab initio method. The molecular interaction energies are in the order Ag < Cu < Au and increased with the decrease of R_P···M. Interaction energies are strengthened when electron-donating substituents X connected to XH₂P, while electron-withdrawing substituents produce the opposite effect. The strongest P···M bond was found in CH₃H₂P···AuF with -70.95 kcal/mol, while the weakest one was found in NO₂H₂P···AgI with -20.45 kcal/mol. The three-center/four-electron (3c/4e) resonance-type of P:-M-:Y hyperbond was recognized by the natural resonance theory and the natural bond orbital analysis. The competition of P:M-Y ↔ P-M:Y resonance structures mainly arises from hyperconjugation interactions; the bond order of b_P-M and b_M-Y is in line with the conservation of the idealized relationship b_P-M + b_M-Y ≈ 1. In all MF-containing complexes, P-M:F resonance accounted for a larger proportion which leads to the covalent characters for partial ionicity of MF. The interaction energies of these Cu/Ag/Au complexes are basically above the characteristic values of the halogen-bond complexes and close to the observed strong hydrogen bonds in ionic hydrogen-bonded species.

Resonance bonding in XNgY (X = F, Cl, Br, I; Ng = Kr or Xe; Y = CN or NC) molecules: an NBO/NRT investigation

Several noble-gas-containing molecules XNgY were observed experimentally. However, the bonding in such systems is still not understood. Using natural bond orbital and natural resonance theory (NBO/NRT) methods, the present work investigated bonding of the title molecules. The results show that each of the studied XNgY molecules should be better described as a resonance hybrid of ω-bonding and [Formula: see text]-type long-bonding structures: X:^- Ng⁺ - Y, X - Ng⁺: Y^-, and X^{^}Y. The ω-bonding and long-bonding make competing contributions to the composite resonance hybrid due to the accurately preserved bond order conservation principle. We find that the resonance bonding is highly tunable for these noble-gas-containing molecules due to its dependence on the nature of the halogen X or the central noble-gas atoms Ng. When the molecule XNgY consists of a relatively lighter Ng atom, a relatively low-electronegative X atom, and the CN fragment rather than NC, the long-bonding structure X^{^}Y tends to be highlighted. In contrast, the heavy Ng atom and high-electronegative X atom will enhance the ω-bonding structure. Overall, the present work provides electronic principles and chemical insights that help understand the bonding in these XNgY species.

Understanding and modulating the high-energy properties of noble-gas hydrides from their long-bonding: an NBO/NRT investigation on HNgCO+/CS+/OSi+ and HNgCN/NC (Ng = He, Ar, Kr, Xe, Rn) molecules

The noble-gas hydrides, HNgX (X is an electronegative atom or fragment), represent potential high-energy materials because their two-body decomposition process, HNgX → Ng + HX, is strongly exoergic. Our previous studies have shown that each member of the HNgX (X = halogen atom or CN/NC fragment) molecules is composed of three leading resonance structures: two ω-bonding structures (H-Ng+ :X- and H:- Ng+-X) and one long-bonding structure (H∧X). The last one paints a novel [small sigma, Greek, circumflex]-type long-bonding picture. The present study focuses on the relationship between this novel bonding motif and the unusual energetic properties. We chose HNgCO+/CS+/OSi+/CN/NC, with the formula HNgAB (Ng = He, Ar, Kr, Xe, Rn; AB = CO+/CS+/OSi+/CN/NC) as the research system. We first investigated the bonding of HNgCO+ and its analogous HNgCS+/OSi+ species using NBO/NRT methods, and quantitatively compared the bonding with that in HNgCN/NC molecules. NBO/NRT results showed that each of the HNgCO+/CS+/OSi+ molecules could be better represented as a resonance hybrid of ω-bonding and long-bonding structures, but the long-bonding is much weaker than that in HNgCN/NC molecules. Furthermore, we introduced the long-bonding concept into the rationalization of the high-energy properties, and found a good correlation between the highly exothermic two-body dissociation channel and the long-bond order, bH-A. We also found that the long-bond order is highly tunable for these noble-gas hydrides due to its dependence on the nature of the electronegative AB fragments or the central noble-gas atoms, Ng. On the basis of these results, we could optimize the energetic properties by changing the long-bonding motif of our studied molecules. Overall, this study shows that the long-bonding model provides an easy way to rationalize and modulate the unusual energy properties of noble-gas hydrides, and that it is helpful to predict some noble-gas hydrides as potential energetic materials.

Theoretical Prediction of Robust Second-Row Oxyanion Clusters in the Metastable Domain of Antielectrostatic Hydrogen Bonding

We provide ab initio and density functional theory evidence for a family of surprisingly robust like-charged clusters of common HSO₄^- and H₂PO₄^- oxyanions, ranging up to tetramers of net charge 4-. Our results support other recent theoretical and experimental evidence for "antielectrostatic" hydrogen-bonded (AEHB) species that challenge conventional electrostatic conceptions and force-field modeling of closed-shell ion interactions. We provide structural and energetic descriptors of the predicted kinetic well-depths (in the range 3-10 kcal/mol) and barrier widths (in the range 2-4 Å) for simple AEHB dimers, including evidence of extremely strong hydrogen bonding in the fluoride-bisulfate dianion. For more complex polyanionic species, we employ natural-bond-orbital-based descriptors to characterize the electronic features of the cooperative hydrogen-bonding network that are able to successfully defy Coulomb explosion. The computational results suggest a variety of kinetically stable AEHB species that may be suitable for experimental detection as long-lived gas-phase species or structural units of condensed phases, despite the imposing electrostatic barriers that oppose their formation under ambient conditions.

The bismuth tetramer Bi4: the ν3 key to experimental observation

The spectroscopic identification of Bi₄ has been very elusive. Two constitutional Bi₄ isomers of T_d and C_2v symmetry are investigated and each is found to be a local energetic minimum.

Long-Bonding in HNgCN/NC (Ng = Noble Gas) Molecules: An NBO/NRT Investigation

Recently, Weinhold et al. put forward one new concept of σ̂-type long-bonding and applied it to the comprehension of halogen noble-gas hydrides HNgY (Ng = noble gas; Y = halogen) bonding. The present study extends this new concept into HNgX (X = CN, NC) pseudohalogen molecules. At the B3LYP and CCSD(T) levels of theory, we perform natural bond orbital (NBO) and natural resonance theory (NRT) studies on the HNgX molecules and compare them with the previous results for HNgY molecules. The NBO/NRT results clearly reveal that each of the HNgX, but not the HHeCN, HNeCN, and HNeNC molecules, is composed of three leading resonance structures: two ω-bonding structures H-Ng⁺:X^-, H:^-Ng⁺-X and one long-bonded structure H^∧X. This result indicates that the conventional long-bonding exists in these pseudohalogen molecules, like the long-bonding in HNgY molecules. Unexpectedly, we identify a new type of longer long-bonded structure H^∧C in HNeNC molecule at the B3LYP level, which disappears at the CCSD level. This misleading prediction at the B3LYP level can be traced back to the singlet diradical character, which induces a low-quality geometry. Therefore, the geometry reoptimization of the noble-gas hydrides is indispensable using CASSCF-based methods, if the noble-gas hydrides fail the "Stable" test because of diradical-type instability.

The classification and representation of main group element compounds that feature three-center four-electron interactions

This article provides a means to classify and represent compounds that feature 3-center 4-electron (3c-4e) interactions in terms of the number of electrons that each atom contributes to the interaction. Specifically, Class I 3c-4e interactions are classified as those in which two atoms provide one electron each and the third atom provides a pair of electrons (i.e. LX₂), while Class II 3c-4e interactions are classified as those in which two atoms each provide a pair of electrons and the third atom contributes none (i.e. L₂Z). These classes can be subcategorized according to the nature of the central atom. Thus, Class I interactions can be categorized according to whether the central atom provides one (i.e.μ-X) or two (i.e.μ-L) electrons, while Class II interactions can be categorized according to whether the central atom provides none (i.e.μ-Z) or two (i.e.μ-L) electrons. The use of appropriate structure-bonding representations for these various interactions provides a means to determine the covalent bond classification of the element of interest.

3c/4e -type long-bonding competes with ω-bonding in noble-gas hydrides HNgY (Ng = He, Ne, Ar, Kr, Xe, Rn; Y = F, Cl, Br, I): a NBO/NRT perspective

Novel resonance bonding for the HNgY molecule is demonstrated based on natural resonance theory analyses. Ng/Y affects the ω-bonding vs. long-bonding propensity in each of the HNgY molecules.

From ordinary to blue emission in peralkylated n-oligosilanes: the calculated structure of delocalized and localized singlet excitons

Excited singlet state structures believed to be responsible for the Franck-Condon-allowed and the strongly Stokes-shifted (blue) emissions in linear permethylated oligosilanes (Si(n)Me(2n+2)) have been found and characterized with time-dependent density functional (TD-DFT) methods for chain lengths 4 ≤ n ≤ 16. For chain lengths with n > 7, the S1 relaxed structures closely resemble the S0 equilibrium structures where all valence angles are tetrahedral and all backbone dihedral angles are transoid. At chain lengths with n < 8 more strongly distorted structures with one long Si-Si bond built from silicon 3p orbitals are encountered. The large Stokes shift is due more to a large destabilization of the ground state than the relaxation in the S1 excited state. For n = 7, both types of minima were located, exactly reproducing the borderline between the large-radius and the small-radius self-trapped excitons known from experiments.

Anti-electrostatic hydrogen bonds

Ab initio and hybrid density functional techniques were employed to characterize a surprising new class of H-bonded complexes between ions of like charge. Representative H-bonded complexes of both anion-anion and cation-cation type exhibit appreciable kinetic stability and the characteristic theoretical, structural, and spectroscopic signatures of hydrogen bonding, despite the powerful opposition of Coulomb electrostatic forces. All such "anti-electrostatic" H-bond (AEHB) species confirm the dominance of resonance-type covalency ("charge transfer") interactions over the inessential (secondary or opposing) "ionic" or "dipole-dipole" forces that are often presumed to be essential for numerical modeling or conceptual explanation of the H-bonding phenomenon.