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The η2-formyl borane system 3 is readily available by carbonylation of the vicinal P/BH frustrated Lewis pair (FLP) 1. It serves as a frustrated C/B Lewis pair toward carbon dioxide or phenylisocyanate. In the presence of B(C6F5)3, it forms the coupling product between two CO-derived units. The resulting compound 13 rearranged to a doubly O-borylated endiolate, with both of the central carbon atoms originating from carbon monoxide. The subsequent treatment with a silane gave a rare macrocyclic silicon endiolate.

Black orthorhombic (B-γ) CsSnI3 with reduced biotoxicity and environmental impact and excellent optoelectronic properties is being considered as a promising eco-friendly candidate for high-performing perovskite solar cells (PSCs). A major challenge in a large-scale implementation of CsSnI3 PSCs includes the rapid transformation of Sn2+ to Sn4+ (within a few minutes) under an ambient-air condition. Here, we demonstrate that ambient-air stable B-γ CsSnI3 PSCs can be fabricated by incorporating N,N'-methylenebis(acrylamide) (MBAA) into the perovskite layer and by using poly(3-hexylthiophene) as the hole transporting material. The lone electron pairs of -NH and -CO units of MBAA are designed to form coordination bonding with Sn2+ in the B-γ CsSnI3, resulting in a reduced defect (Sn4+) density and better stability under multiple conditions for the perovskite light absorber. After a modification, the highest power conversion efficiency (PCE) of 7.50% is documented under an ambient-air condition for the unencapsulated CsSnI3-MBAA PSC. Furthermore, the MBAA-modified devices sustain 60.2%, 76.5%, and 58.4% of their initial PCEs after 1440 h of storage in an inert condition, after 120 h of storage in an ambient-air condition, and after 120 h of 1 Sun continuous illumination, respectively.

An intramolecular ethylene-bridged PH/BH system, isolated as a dimer, reduces carbon monoxide to the -CH2-O- state. In the presence of B(C6F5)3 the frustrated PH/BH Lewis pair reacts with carbon mo...

The ability to synthesize well-defined block copolymers (BCPs) from one-pot comonomer mixtures has powerful chemical and practical implications. However, controlling sequences between highly reactive, homologous comonomers such as acrylates during polymerization is challenging. Here we present a Lewis pair polymerization strategy that uniquely utilizes preferential Lewis acid coordination to differentiate between comonomers, distinctive kinetics, and compounded thermodynamic and kinetic differentiation to precisely control sequences and suppress tapering and misincorporation errors, thus achieving well-defined and resolved di- or tri-BCPs of acrylates.

Integration of distinct substrate activation modes in a catalytic circle is critical for the development of new, powerful synthetic methodologies toward complex and value-added chemicals from simple and readily available feedstocks. Here, we describe a highly selective difunctionalization of imines through incorporation of activation of CO2 by intramolecular N/B Lewis pairs into a copper catalytic cycle. Experimental and computational studies on the mechanistic aspect revealed an α-borylalkylamido intermediate, a metal amide-based Lewis pair formed by borylation of a C–N double bond, and enabled an unprecedented CO2 fixation pattern that is in sharp contrast to the traditional CO2 insertion into transition-metal-element bonds. The unique lithium cyclic boracarbamate products could be easily transformed into multifunctional N-carboxylated α-amino boronates. The highly diastereoselective reactions of chiral N-tert-butanesulfinyl aldimines were also achieved. We hope that our findings may inspire further dev...

The judicial placement of main group elements in conjugated structures is emerging as a key route to novel functional hybrid materials. We demonstrate here that the formation of B–N Lewis pairs at the periphery of anthracene leads to buckling of the backbone while also dramatically lowering the LUMO energy. The resulting BN-substituted contorted polycyclic aromatic hydrocarbons show large bathochromic shifts in the absorption and emission relative to all-carbon analogs. In the presence of light, they rapidly react with O2 without the need for an external photosensitizer, resulting in selective and reversible formation of the corresponding endoperoxides.

The reactivity of the homo- and heteronuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) toward methane was studied using Fourier transform ion cyclotron resonance mass spectrometry, in conjunction with high-level quantum mechanical calculations. The most reactive cluster by both experiment and theory is [Al2O2](•+). In its favorable pathway, this cluster abstracts a hydrogen atom by means of proton-coupled electron transfer (PCET) instead of following the conventional hydrogen-atom transfer (HAT) route. This mechanistic choice originates in the strong Lewis acidity of the aluminum site of [Al2O2](•+), which cleaves the C-H bond heterolytically to form an Al-CH3 entity, while the proton is transferred to the bridging oxygen atom of the cluster ion. In addition, a comparison of the reactivity of heteronuclear and homonuclear oxide clusters [XYO2](+) (X, Y = Al, Si, Mg) reveals a striking doping effect by aluminum. Thus, the vacant s-p hybrid orbital on Al acts as an acceptor of the electron pair from methyl anion (CH3(-)) and is therefore eminently important for bringing about thermal methane activation by PCET. For the Al-doped cluster ions, the spin density at an oxygen atom, which is crucial for the HAT mechanism, acts here as a spectator during the course of the PCET mediated C-H bond cleavage. A diagnostic plot of the deformation energy vis-à-vis the barrier shows the different HAT/PCET reactivity map for the entire series. This is a strong connection to the recently discussed mechanism of oxidative coupling of methane on magnesium oxide surfaces proceeding through Grignard-type intermediates.

The so-called "Lewis pair" is a ubiquitous phenomenon in chemistry and is often used as an intuitive construct to predict and rationalize chemical structure and behavior. Concepts from the very general Valence Shell Electron Pair Repulsion (VSEPR) model to the most esoteric reaction mechanism routinely rely on the notion that electrons tend to exist in pairs and that these pairs can be thought of as being localized to a particular region of space. It is precisely this localization that allows one to intuit how these pairs might behave, generally speaking, so that reasonable predictions may be made regarding molecular structure, intermolecular interactions, property trends, and reaction mechanisms, etc. Of course, it is rather unfortunate that the Lewis model is entirely qualitative and yields no information regarding how any specific electron pair is distributed. Here we demonstrate a novel electronic structure analysis technique that predicts and analyzes precise quantitative details about the relative and absolute distribution of individual electron pairs. This Single Electron Pair Distribution Analysis (SEPDA) reveals quantitative details about the distribution of the well-known Lewis pairs, such as how they are distributed in space and how their relative velocities change in various chemical contexts. We show that these distributions allow one to image the explicitly pairwise electronic behavior of bonds and lone pairs. We further demonstrate how this electronic behavior changes with several conditions to explore the nature of the covalent chemical bond, non-covalent interactions, bond formation, and exotic 3-center-2-electron species. It is shown that indications of the strength of bonded and non-bonded interactions may also be gleaned from such distributions and SEPDA can be used as a tool to differentiate between interaction types. We anticipate that SEPDA will be of broad utility in a wide variety of chemical contexts because it affords a very detailed, visual and intuitive analysis technique that is generally applicable.

The full mechanism of the hydroboration of CO2 by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh2–C6H4 (Bcat = catecholboryl) was determined using computational and experimental methods. The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step.

The first highly active phosphine (P)/borane (B) Lewis pair polymerization is promoted unexpectedly by P-B adducts. The P and B site cooperativity is essential for achieving effective polymerization, as shown by this study examining the reactivity of a library of P/B Lewis pairs toward polymerization of a renewable acrylic monomer.

Herein we report on the catalytic polymerization of diverse Michael-type monomers with high precision by using simple but highly active combinations of phosphorus-containing Lewis bases and organoaluminum compounds. The interacting Lewis pair catalysts enable the control of molecular weight and microstructure of the produced polymers. The reactions show a linear Mn vs consumption plot thus proving a living type polymerization. The initiation has been investigated by end-group analysis with ESI mass spectrometric analysis. With these main-group element Lewis acid base pairs, it is not only possible to polymerize sterically demanding, functionalized as well as heteroatom containing monomers but also, for the first time, to catalytically polymerize extended Michael systems, like 4-vinylpyridine.

Dialkylphosphinyl radical 1 was synthesized as thermally stable yellow crystals and found to be monomeric both in solution and in the solid state. EPR spectrum showed that the spin density of 1 is mainly localized on the 3p orbital of the dicoordinated phosphorus atom. A distinct absorption band due to the electronic transition from nonbonding electron pair orbital to singly occupied 3p orbital on the phosphorus atom of 1 was observed at 445 nm in solution. Phosphinyl radical 1 underwent facile reaction with carbon tetrachloride, hydrogen abstraction, and a unique reaction with a persistent radical, galvinoxyl, giving a cyclic phosphaalkene and a silylether.

The hydrated electron is one of the most fundamental nucleophiles in aqueous solution, yet it is a transient species in liquid water, making it challenging to study. The solvation thermodynamics of the electron are important for determining the band structure and properties of water and aqueous solutions. However, a wide range of values for the electron solvation enthalpy (-1.0 to -1.8 eV) has been obtained from previous methods, primarily because of the large uncertainty as to the value for the absolute proton solvation enthalpy. In the gas phase, electron interactions with water can be investigated in stable water clusters that contain an excess electron, or an electron and a solvent-separated monovalent or divalent metal ion. Here, we report the generation of stable water clusters that contain an excess electron and a solvent-separated trivalent metal ion that are formed upon electron capture by hydrated trivalent lanthanide clusters. From the number of water molecules lost upon electron capture, adiabatic recombination energies are obtained for La(H(2)O)(n)(3+) (n = 42-160). The trend in recombination energies as a function of hydration extent is consistent with a structural transition from a surface-located excess electron at smaller sizes (nor= approximately 56) to a more fully solvated electron at larger sizes (nor= approximately 60). The recombination enthalpies for n60 are extrapolated as a function of the geometrical dependence on cluster size to infinite size to obtain the bulk hydration enthalpy of the electron (-1.3 eV). This extrapolation method has the advantages that it does not require estimates of the absolute proton or hydrogen hydration enthalpies.

Bithiophene-based flexible Lewis pairs with P(O)R2 (R = phenyl, isopropyl) and BMes2 (Mes = 2,4,6-trimethylphenyl) functionalities are able to toggle between closed, Lewis adduct and open, unbound Lewis pair structures. The open structure is favored in strong hydrogen bond donating solvents or at higher temperatures giving rise to an intense charge-transfer (CT) luminescence, while the closed structure without this emission dominates in non-hydrogen bond donating solvents or at lower temperatures. Intermediate solvents result in an equilibrium mixture of both structures, which shows unusual mixed emission that is dependent on excitation wavelength.

Recent crystallographic data unambiguously demonstrate that neither Ar'GeGeAr' nor Ar'CrCrAr' molecules adopt the expected linear (VSEPR-like) geometries. Does the adoption of trans-bent geometries indicate that Ar'MMAr' molecules are not "maximally bonded" (i.e., bond order of three for M = Ge and five for M = Cr)? We employ theoretical hybrid density functional (B3LYP/6-311++G) computations and natural bond orbital-based analysis to quantify molecular bond orders and to elucidate the electronic origin of such unintuitive structures. Resonance structures based on quintuple M-M bonding dominate for the transition metal compounds, especially for molybdenum and tungsten. For the main group, M-M bonding consists of three shared electron pairs, except for M = Pb. For both d- and p-block compounds, the M-M bond orders are reflected in torsional barriers, bond-antibond splittings, and heats of hydrogenation in a qualitatively intuitive way. Trans-bent structures arise primarily from hybridization tendencies that yield the strongest sigma-bonds. For transition metals, the strong tendency toward sd-hybridization in making covalent bonds naturally results in bent ligand arrangements about the metal. In the p-block, hybridization tendencies favor high p-character, with increasing avidity as one moves down the Group 14 column, and nonlinear structures result. In both the p-block and the d-block, bonding schemes have easily identifiable Lewis-like character but adopt somewhat unconventional orbital interactions. For more common metal-metal multiply bonded compounds such as [Re2Cl8]2-, the core Lewis-like fragment [Re2Cl4]2+ is modified by four hypervalent three-center/four-electron additions.

Carbene-stabilized [L(3)P](+3) cations have been synthesized for the first time by a reaction between 1-chloro-2,3-bis(dialkylamino)cyclopropenium salts and P(SiMe(3))(3). In addition, the first structural characterization of such an entity is reported. Consistent with the X-ray data, density functional calculations indicate that these P-centered cations, despite their high positive charge, still feature a nonbonding electron pair on the P-atom (HOMO) and a very low-lying LUMO depicting them as poor σ-donors and excellent π-acceptors.

The synthesis and systematic study of the reaction chemistry of a bifunctional phosphinidene cluster, Fe3(CO)9(μ3-PH)2 (3) is reported. Reaction chemistry at one P−H functional site is effectively communicated through the Fe3(CO)9 core, and impacts on the reactivity of the second P−H site. Deprotonation of the acidic protons in 3 allows access to a lone electron pair on phosphorus. The first proton is readily removed with NEt3/[PPN]Cl ([PPN]+ = (Ph3P)2N+) to produce the [PPN]+ salt of [Fe3(CO)9(μ3-PH)(μ3-P)]- ([4]-). Removal of both phosphorus-bound hydrogen atoms in 3 requires the use of 2 equiv of n-BuLi, to produce Li2[Fe3(CO)9(μ3-P)2] (Li2[5]). Quenching of Li2[5] with MeOH-d4 generates [Fe3(CO)9(μ3-PD)(μ3-P)]-. The lone electron pairs on the cluster-bound phosphorus atoms undergo analogous reaction chemistry to organophosphines. Alkylation at the phosphorus site is carried out by reaction of [Et3NH][4] with MeOSO2CF3 generating the monosubstituted cluster, Fe3(CO)9(μ3-PMe)(μ3-PH) (6) as the primary p...

Hydroboration of dimesitylnorbornenylphosphane with Piers' borane [HB(C6F5)2] gave the frustrated Lewis pair (FLP) 4 in good yield. It has the -PMes2 Lewis base attached at the 2-endo position and the -B(C6F5)2 group 3-exo oriented at the norbornane framework. The vicinal FLP 4 was shown by X-ray diffraction and by spectroscopy to be a rare example of an intramolecular noninteracting pair of a Lewis acid and Lewis base functionality. The FLP 4 rapidly splits dihydrogen heterolytically at ambient temperature to yield the phosphonium/hydrido borate zwitterion 5. It adds to the carbonyl group of benzaldehyde and to carbon dioxide to yield the adducts 6 and 7, respectively. Compounds 5-7 were characterized by X-ray diffraction. Compound 4 adds to the S═O function of sulfur dioxide to give a pair of diastereomeric heterocyclic six-membered ring products due to the newly formed sulfur chirality center, annulated with the norbornane skeleton, which were investigated by (31)P/(11)B single and double resonance solid state NMR experiments. Compound 8 was also characterized by X-ray diffraction. The FLP 4 undergoes a clean N,N-addition to nitric oxide (NO) to give a norbornane annulated five-membered heterocyclic persistent FLP-NO aminoxyl radical 12 (characterized, e.g., by X-ray diffraction and EPR spectroscopy). Additionally, the FLP radical was characterized by (1)H solid state NMR spectroscopy. The radical 12 undergoes a H-atom abstraction reaction with 1,4-cyclohexadiene to yield the respective diamagnetic FLP-NOH product 13, which was also characterized by X-ray diffraction and solid state NMR spectroscopy.

Polar neutrals featuring an electronegative atom (O or N) are observed to form adducts with C 60 2+ in the gas phase in selected-ion flow tube experiments at 294±3 K and 0.35±0.01 Torr. Addition is ascribed to the donation of an electron pair from the electronegative atom to the site of charge concentration on the fullerene ion's surface, forming a dative bond between the donor atom and a carbon atom of the fullerene shell and resulting in charge localization upon the donor atom

During a search for conditions appropriate for Pt-catalyzed CO reduction using dihydrogen directly, metal-free conditions were discovered instead. A bulky, strong phosphazene base forms a “frustrated” Lewis pair (FLP) with a trialkylborane in the secondary coordination sphere of a rhenium carbonyl. Treatment of the FLP with dihydrogen cleanly affords multiple hydride transfers and C−C bond formation.

The results from the crystal structure determination and a normal coordinate analysis show that the XeF 5 plane of XeF 5 − is considerably more rigid than that in the fluxional IF 7 molecule due to the increased repulsion from the xenon free valence electron pairs. Local density functional calculations were carried out for XeF 5 − and XeF 4 with a double-numerical basis set augmented by polarization functions and confirm the experimentally observed geometries and vibrational spectra. It is shown that the bonding in XeF 5 − closely resembles that in XeF 4 . In a valence bond description, it can be visualized as the two axial positions being occupied by two sp-hybridized free valence electron pairs and the equatorial fluorines being bound by two Xe 5p electron pairs through semiionic multicenter four-electron bonds

Activation of the CH bond in CH{sub 4} and t-BuO(ads) by H{sup {sm bullet}} transfer to O adsorbed on the Ag(110) surface has been studied by the atom-superposition and electron-delocalization molecular orbital theory. Both abstractions are found to proceed with a low-energy barrier because of the presence of low-lying empty Ag surface orbitals at the Fermi level. Transfer of an electron to the Fermi level alleviates the initial closed-shell repulsion between the CH {sigma}-bond electron pair and the surface O (which is formally O{sup 2{minus}}) lone-pair orbital that ultimately forms the OH {sigma} bond. It is concluded that surface O behaves like a radical anion O{sup {sm bullet}{minus}} or R{sub 3}O{sup {sm bullet}} in its ability to form a single bond. These calculations also suggest the possibility that surface oxygen atoms might insert into CH bonds to form alcohols, which is a result of their ability to exhibit O atom or carbene (R{sub 2}C:) bonding properties.

In solution, the self-exchange reactions for oxygen-centered pi-radicals, e.g., PhO. + PhOH PhOH + PhO., are known to occur with low activation enthalpies (E(a) approximately equal to 2 kcal/mol). For the PhO./PhOH couple and, we conclude, for other O-centered pi-radicals, exchange occurs by proton-coupled electron transfer (PCET) with the proton transferred between oxygen electron pairs while the electron migrates between oxygen orbitals orthogonal to the -O- - -H- - -O- transition state plane (Mayer et al. J. Am. Chem. Soc. 2002, 123, 11142). Iminoxyls, R(2)C=NO., are sigma-radicals with substantial spin density on the nitrogen. The R(2)C=NO./R(2)C=NOH self-exchange has a significant E(a) (Mendenhall et al. J. Am. Chem. Soc. 1973, 95, 627). For this exchange, DFT calculations have revealed a counterintuitive cisoid transition state in which the seven atoms, >C=NO- - -H- - -ON=C

As a simple yet strongly binding electron donor-acceptor (EDA) complex, BH(3)NH(3) serves as a good example to study the electron pair donor-acceptor complexes. We employed both the ab initio valence bond (VB) and block-localized wave function (BLW) methods to explore the electron transfer from NH(3) to BH(3). Conventionally, EDA complexes have been described by two diabatic states: one neutral state and one ionic charge-transferred state. Ab initio VB self-consistent field (VBSCF) computations generate the energy profiles of the two diabatic states together with the adiabatic (ground) state. Our calculations evidently demonstrated that the electron transfer between NH(3) and BH(3) falls in the abnormal regime where the reorganization energy is less than the exoergicity of the reaction. The nature of the NH(3)-BH(3) interaction is probed by an energy decomposition scheme based on the BLW method. We found that the variation of the charge-transfer energy with the donor-acceptor distance is insensitive to the computation levels and basis sets, but the estimation of the amount of electron transferred heavily depends on the population analysis procedures. The recent resurgence of interest in the nature of the rotation barrier in ethane prompted us to analyze the conformational change of BH(3)NH(3), which is an isoelectronic system with ethane. We found that the preference of the staggered structure over the eclipsed structure of BH(3)NH(3) is dominated by the Pauli exchange repulsion.

The reaction of (CpReH(2))(2)B(4)H(4) with monoborane leads to the sequential formation of (CpRe)(2)B(n)()H(n)() (n = 7-10, 1-4). These species adopt closed deltahedra with the same total connectivities as the closo-borane anions [B(n)()H(n)()](2)(-), n = 9-12, but with flattened geometries rather than spherical shapes. These rhenaborane clusters are characterized by high metal coordination numbers, Re-Re cross-cluster distances within the Re-Re single bond range, and formal cluster electron counts three skeletal electron pairs short of that required for a canonical closo-structure of the same nuclearity. An open cluster, (CpReH)(2)B(7)H(9) (5), is isolated that bears the same structural relationship to arachno-B(9)H(15) as 1-4 bear to the closo-borane anions. Chloroborane permits the isolation of (CpReH)(2)B(5)Cl(5) (6), an isoelectronic chloro-analogue of known open (CpWH(2))(2)B(5)H(5) and (CpRe)(2)B(6)H(4)Cl(2) (7), a triple-decker complex containing a planar, six-membered 1,2-B(6)H(4)Cl(2) ring. Both are putative five- and six-boron intermediates in the formation of 1. Electronic structure calculations (extended Hückel and density functional theory) yield geometries in agreement with the structure determinations, large HOMO-LUMO gaps in accord with the high stabilities, and (11)B chemical shifts accurately reflecting the observed shifts. Analyses of the bonding in 1-4 reveal that the CpRe.CpRe interaction generates fragment orbitals that are able to contribute the "missing" three skeletal electron pairs required for skeletal bonding. The necessity of a Re.Re interaction for strong cluster bonding requires a borane fragment shape change to accommodate it, thereby explaining the noncanonical geometries. Application of the debor principle of borane chemistry to the shapes of 1-4 readily rationalizes the observed geometries of 5 and 6. This evidence of the scope of transition metal fragment control of borane geometry suggests the existence of a large class of metallaboranes with structures not found in known borane or metal clusters.

A new family of related borane and carborane cages has been designed computationally. These compounds obey a new electron counting rule (6m + 2n rule) rather than Wade's rule. The structures of these cages can be conceived by combining m aromatic pyramidal and n aromatic triangular units. The interstitial electrons from the m pyramids (six electrons for each unit) and the n triangles (two electrons for each unit) constitute the total 6m + 2n skeletal electrons. The greater number of skeletal electron pairs in large closo-borane cages (e.g., B32H328- or C8B24H32) achieves stabilization through the optimal occupancy of all bonding orbitals. The favorable electronic structure, the large HOMO−LUMO gaps, the large lowest positive frequencies, and the local aromaticity of the pyramidal and triangular units (as demonstrated by the large negative NICS values) of the new large closo-cages auger well for their eventual experimental realization.

The denticity (O-, S-, and C-nucleophilic reactivity) of the dimethylsulfinyl carbanion (“dimsyl”) toward 1,3,5-trinitrobenzene (TNB) has been studied by NMR spectroscopy, and structures of adducts have been assigned. Three dimsyl adducts are observed for the first time and have been ascribed to the O-adduct 15, the S-adduct 16, and the C-adduct 17. The kinetic (15 > 16 > 17) and thermodynamic preference (17 > 16 > 15) for the reactivity of dimsyl toward TNB is compared to the known O- and C-reactivity of enolate anions and the O- and S-reactivity of dimethyl sulfoxide. Thus, dimsyl apparently represents a unique system in which three adjacent atoms having unshared electron pairs can utilize these in covalent bond formation.

A generally applicable electron-counting rule-the mno rule-that integrates macropolyhedral boranes, metallaboranes, and metallocenes and any combination thereof is presented. According to this rule, m + n + o number of electron pairs are necessary for a macropolyhedral system to be stable. Here, m is the number of polyhedra, n is the number of vertices, and o is the number of single-vertex-sharing condensations. For nido and arachno arrangements, one and two additional pairs of electrons are required. Wade's n + 1 rule is a special case of the mno rule, where m = 1 and o = 0. B20H16, for example has m = 2 and n = 20, leading to 22 electron pairs. Ferrocene, with two nido polyhedral fragments, has m = 2, n = 11, and o = 1, making the total 2 + 11 + 1 + 2 = 16. The generality of the mno rule is demonstrated by applying it to a variety of known macropolyhedral boranes and heteroboranes. We also enumerate the various pathways for condensation by taking icosahedral B12 as the model. The origin of the mno rule is explored by using fragment molecular orbitals. This clearly shows that the number of skeletal bonding molecular orbitals of two polyhedral fragments remains unaltered during exohedral interactions. This is true even when a single vertex is shared, provided the common vertex is large enough to avoid nonbonding interactions of adjacent vertices on either side. But the presence of more than one common vertex results in the sharing of surface orbitals thereby, reducing the electronic requirements.

In the infrared spectra of solutions in liquid argon of dimethyl ether ((CH(3))(2)O) and fluoroform (HCF(3)), bands due to a 1:1 complex between these monomers have been observed. The C-H stretch of the HCF(3) moiety in the complex appears 17.7 cm(-1) above that in the monomer, and its intensity decreases by a factor of 11(2). These characteristics situate the interaction between the monomers in the realm of improper, blue-shifting hydrogen bonding. The complexation shifts the C-F stretches downward by some 9 cm(-1), while the C-H stretches in (CH(3))(2)O are shifted upward by 9-15 cm(-1), and the C-O stretches are shifted downward by 5 cm(-1). These shifts are in very good agreement with those calculated by means of correlated ab initio methods, and this validates a two-step mechanism for improper, blue-shifting hydrogen bonding. In the first step, the electron density is transferred from the oxygen lone electron pairs of the proton acceptor ((CH(3))(2)O) to fluorine lone electron pairs of the proton donor (CHF(3)) which yields elongation of all CF bonds. Elongation of CF bonds is followed (in the second step) by structural reorganization of the CHF(3) moiety, which leads to the contraction of the CH bond. It is thus clearly demonstrated that not only the spectral manifestation of H-bonding and improper H-bonding but also their nature differ.

The bis(arylimino-Cp)ZrCl2 complex (I) forms an "antagonistic" Lewis pair with B(C6F5)3. Treatment with dihydrogen yields the aminomethyl-Cp hydrogenation product (II) by means of a quasi-autocatalytic process. The salt III is an efficient catalyst for the hydrogenation of bulky imines and of silyl enolethers.

A novel size dependence in the adsorption reaction of multiple O2 molecules onto anionic silver clusters Agn- (n = 1-5) is revealed by gas-phase reaction studies in an rf-ion trap. Ab initio theoretical modeling based on DFT method provides insight into the reaction mechanism and finds cooperative electronic and structural effects to be responsible for the size selective reactivity of Agn- clusters toward one or more O2. In particular, Agn- clusters with odd n have paired electrons and therefore bind one O2 only weakly, but they are simultaneously activated to adsorb a strongly bound second oxygen molecule. For the clusters Ag3O4- and Ag5O4-, this cooperative effect results in a superoxo-like, doubly bound O2 subunit with potentially high activity in catalytic silver cluster oxidation processes.

Numerous well-behaved Karplus curves correlating dihedral angle (phi) and NMR coupling constants (3JXY) exist for X-C-Z-Y fragments with X or Y = H, C, F, and P. Absent is the enigmatic F-C-C-F moiety. By calculating the four Ramsey contributions to 3JFF for F-CH2-CH2-F, a pleasing phi/3JFF curve with both positive and negative regions arises. Unlike the H-C-C-H curve, F-C-C-F is PSO vs FC dominated. The latter and the F lone electron pairs cause both the negative J values and the substituent-induced J-leveling effect.

The environmental modifications of the polarizability, the diamagnetic contribution to the susceptibility, and the nuclear shielding of the sodium anion (Na-) in the cryptand salt Na-·Na-C222 (C222 = cryptand 222) are deduced from ab-initio quantum chemistry computations. The point charge electrostatic contribution to the anion environment in the cryptand reduces to 643 au, the free anion polarizability of 1090 au predicted taking account of electron correlation using the coupled electron pair approximation. This polarizability is found to be further diminished to around 400 au on introducing a realistic model of the full environment in the cryptand. The electron correlation contribution to the polarizability is largely suppressed on entering the condensed phase. The diamagnetic contribution to the susceptibility is found to be reduced in magnitude by 32% on introducing a model for the full environment in the cryptand. This result, combined with an average energy approximation analysis of the ab-initio polarizabilities, shows that environmental modifications of the ground-state wave functions are responsible for about one-third of the polarizability reductions, with two-thirds arising from environmental modifications of the excited states and their excitation energies. The contribution of the two 3s electrons to the diamagnetic part of the nuclear shielding is found to be enhanced by 9% when the free anion experiences the present model for the environment in the cryptand, the core contribution remaining essentially unchanged. The nucleus in the cryptated anion is predicted to be shielded by 3.58 ppm relative to that in an isolated gaseous sodium atom, compared with the 2.88 ppm nuclear shielding in the free anion relative to that in the free atom. This comparative insensitivity of the shielding is consistent with the experimental observation that the Na- shielding (uncorrected for bulk susceptibility) almost always lies within ±1 ppm of a 1.5 ppm shielding relative to the free sodium atom. This explains how the nuclear shielding can be so comparatively environment insensitive in a species interacting sufficiently strongly with its surroundings as to experience a reduction in polarizability by a factor of 2.7.

The electron localization function1-5 (ELF) has been used to study the disposition of electron pairs in H3PO. Three attractors corresponding to lone pairs on the oxygen are found behind the oxygen (away from phosphorus) staggered with respect to the PH bonds. The nature of the isosurfaces for the lone pairs and the PO bond attractor is indicative of an ionic type of environment around oxygen and supportive of the oxygen lone pair polarization and back-bonding stabilization of the PO bond. The nature of the bonding in the PO bond of phosphine oxides (R3PO) has been of great interest for many years. It has been reviewed extensively by Gilheany6,7 who points out that both experiment and ab initio calculations generally agree that the PO bond is strong, polar, and as short as conventional PO double bonds. The role of d functions as polarization functions rather than primary valence orbitals is well established.8,9 Where differences arise is in the interpretation of the bond based on different approaches. Reed and Schleyer10 view the bonding as a donor-acceptor interaction with superimposed oxygen π orbital back-bonding with the degenerate H3P moiety antibonding orbitals (negative hyperconjugation11). Gordon and co-workers12-14 use energy-localized orbitals15-17 to picture the bond as one strong PO σ bond and three equivalent oxygen orbitals characterized primarily as lone pairs polarized toward phosphorus and staggered with respect to the PR bonds. At the same time, they12 and others18,19 derive a picture based on the Boys localization scheme20,21 that involves a single lone pair orbital on oxygen pointing away from the H3P group in H3PO and three bent or banana bonds strongly polarized toward oxygen. Reed and Schleyer10 employed natural localized molecular orbitals22,23 derived from a natural bond order analysis24 to conclude that the bonding was dominated by ionic interactions and negative hyperconjugation. The fact that three quite different orbital schemes arise from basically the same Hartree-Fock (HF) density illustrates the arbitrary nature of this subdivision of charge.

Electron-counting rules for clusters allow predictions to be made about structural changes that result from the addition of electron pairs to cluster molecules. The anticipated effect is that addition of electron pairs to a cluster results in the opening up of the structure in the sequence closo, nido, arachno, hypho. There are some simple examples in the literature of systems in which addition of an electron pair to a small nido-borane, in the form of an equivalent of Lewis base, forms a conventional arachno-cluster. This results when CN{sup -} is added to B{sub 5}H{sub 9}. In most of the known cases, however, two electron pairs are added to the nido-borne to afford a hypho-cluster or the added base cleaves the borane. The structurally characterized adduct species include B{sub 5}H{sub 9}.2PMe{sub 3}, B{sub 4}H{sub 8}.(Me{sub 2}NCH{sub 2}){sub 2}, B{sub 5}H{sub 9}.(Ph{sub 2}P){sub 2}CH{sub 2}, B{sub 5}H{sub 9}.(Ph{sub 2}PCH{sub 2}){sub 2}, B{sub 5}H{sub 9}.(Me{sub 2}NCH{sub 2}){sub 2}, and B{sub 6}H{sub 10}.2PMe{sub 3}. In addition, B{sub 6}H{sub 10}.PMe{sub 3} has also been indentified from spectroscopic data. This paper describes the formation of a unique exo-arachno cluster, from the addition of phosphine bases to [(PPh{sub 3}){sub 2}(CO)OsB{sub 5}H{sub 9}] (1), in a process whichmore » is reversible. 20 refs., 1 fig.« less

It is shown that the two effects of antisymmetry (Pauli exclusion principle) within the correct SCF wavefunctions cannot bend linear H/sub 2/O or planar NH/sub 3/ without classical electronic coulombic repulsion (CER) between LMO charge clouds in the SCF energy expression. These two effects are the orthogonality of the molecular orbitals (MOs) and the electron-exchange interactions between the LMOs. This result is in direct contradiction to the presently accepted valence shell electron pair repulsion (VSEPR) model, which attributes the geometries of molecules to Pauli forces between localized electron pairs. It is argued that the Walsh MO energy correlation diagrams, based on the integral Hellmann-Feynman theorem, are a more correct simple model for molecular geometries.

The many Rayleigh-Schroedinger perturbation theory (MB-RSPT) up to the fourth order was applied to the calculations of the correlation energies in the series of molecules (including C/sub 1/ to C/sub 4/ hydrocarbons and oxygen-containing molecules) using Gaussian DZ + P basis set. The correlation effects which arise from individual types of excitation (single, double, triple, and quadruple) were analyzed as a function of the number of electrons and the bonding situation of the molecule. The lowest value of the correlation energy per electron pair was found from a series of C/sub 1/ to C/sub 3/ alkanes. The correlation energy increases in molecules with a double C=C bond, oxygen-containing single bond, adjacent double C=C bonds (CH/sub 2/CCCH/sub 2/) and triple carbon bonds and is highest in molecules with multiple carbon-oxygen bonds adjacent to another multiple bond (CH/sub 2/CCO). The bond correlation energies are also presented, and the additivity of the bond-correlation contributions is examined.

k2Cs2 = (~~~IU,/KSH+)~~C S~CH+C~,~~,+~,) Abstract: Spherands 1-5 and the related macrocycle 6 which are described in this paper are composed mainly of four to six aryloxy units attached to one another in such a way that the orbitals of the unshared electron pairs of their oxygens line an enforced cavity. These ligand systems strongly complex only Li+ and Na+ ions and reject K', Ca2+, and Mg2+ ions. Their syntheses, representative crystal structures, and a qualitative survey of their binding properties are reported. The critical ring closures of these 18-membered-ring systems involved oxidations of appropriate bisaryllithiums with Fe(acac)3 in refluxing benzene or THF to aryl radicals which coupled with one another-a reaction invented for construction of these hosts. The crystal structure of 1 clearly shows its unoccupied cavity lined with electron pairs. The crystal structures of l-LiC1 and l.NaCH3S04 indicate that no conformational reorganization of the host was involved in complexation. The crystal structures of the complexes of the polycyclic hosts 4.LiFeCI4 and 5.LiCI show the ligating systems to be highly strained and to contain severe oxygen-xygen compression.

A low-temperature ESR study of electron-capture phosphoranyl radicals in X-irradiated single crystals of tri- alkylphosphine sulfides and selenides (R3PX: X = S, Se; R = CH3, C2HS, C&l,) is presented. The principal values and direction cosines of the g tensors and ,'P and "Se hyperfine coupling tensors are determined and correlated with the X-ray structure analysis of the parent compounds. All studied compounds reveal the formation of a R3PX- radical anion, with a three-electron PLX bond in which the unpaired electron is nearly equally distributed over phosphorus and the substituent X. It is shown that the site symmetry of the radicals corresponds to the crystallographic point symmetry of the precursor molecules, resulting in pure trigonal (C3) radicals for (C2HS),PX- and C, symmetry for (CH,),PX- and (C6H11)3PX-. Ab initio quantum chemical calculations (4-31G* SCF and CASSCF) on the model H$S- predict that the three-electron bond is unstable and dissociates into PH, and SO-. The calculations do not reproduce the experimental couplings and the observed spin density distribution, but tend to localize the unpaired electron entirely on sulfur. The origin of this deficiency appears to be predominantly due to a poor description of the negative charge of the nonbonding electron pairs of the sulfur atom. Via the introduction of a positive charge in the vicinity of the sulfur atom, the theoretical spin density distribution can be brought in accordance with the experimental data. The use of quantum chemical calculations for the simulation of trapped radicals in the solid state is discussed.

Etude de la representation par repulsion de points sur une sphere de la theorie VSEPR, en relation avec la forme de la surface d'energie potentielle pour les deformations par flexion des molecules binaires tetraedriques et octaedriques en utilisant la methode de Huckel generalisee ou une methode ab initio. Bon accord du modele mecanique avec les resultats quantiques pour des constantes de force quadratiques, cubiques et d'ordre superieur lorsque les fonctions potentiel de paire additives diminuent en r ij −n (avec n, parametre caracterisant l'intensite des repulsions)

The internal rotation potential of allene has been studied with ab initio self-consistent field (SCF) methods using a double-zeta basis set of 38 contracted Gaussian functions and an extended set of 65 functions including polarization functions, and with the method self-consistent electron pairs (SCEP) using the double-zeta set. The ground state of allene is a closed shell /sup 1/A/sub 1/ state in D/sub 2d/ symmetry. In the D/sub 2/ symmetry of the twisted form, this closed shell state mixes with a 2b/sub 3/ ..-->.. 3b/sub 3/ open shell /sup 1/A/sub 1/ state which correlates with a /sup 1/A/sub u/ state in the D/sub 2h/ symmetry of the planar form of the molecule. The planar closed shell state, /sup 1/A/sub g/, is higher in energy than the /sup 1/A/sub u/ state. Examination of pair correlation energies indicates that correlation effects will not reverse this order. The internal rotation barrier is predicted to be about 49 kcal, after geometry optimization, and there is little effect on relative energies from including polarization functions.