The important role of the phosphorus lone pair in phosphole aromaticity

Calculations on phosphole systems using the G3MP2B3 model chemistry show that the phosphorus lone pair is critical to the system's aromaticity. Protonation of the lone pair results in antiaromatic molecules as measured by homomolecular homodesmotic reactions. Attempts to separate out effects of hyperconjugation on the butadiene portion of the system are unsuccessful with current practices. Because these hyperconjugation effects will tend to cancel each other in the phosphole systems, analyses using the unmodified homomolecular homodesmotic reactions are considered reasonable measures of their aromaticity. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:754–758, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20364     

Ab initio study of substituent effect on the addition of hydrogen fluoride to fluoroethylenes

An ab initio 3-21G study of the direct addition of HF to C₂H_nF_(4–n), with n = 0 to 4, has been performed to investigate the effect of the substituent on the reaction. Geometry optimization of all charge-transfer complexes and transition states has been done. Standard analysis of activation energies of addition reactions, vibrational and thermodynamical analysis, as well as Morokuma energy decomposition, BSSE correction, PMO analysis, and Pauling bond orders were used to explain the results. A subset of the reactions, including that of C₂H₄ as reference one and the two most favorable cases, was also studied at the MP2/6–31G(d,p)//HF/6–31G(d,p) level. The barriers so obtained are in agreement with the indirectly found from experimental data. It was found that the effect of the substituent is not monotonic for the additions. Decomposition of the interaction energy is shown to be adequate to explain this nonmonotonic behavior. The implications for laser chemistry of the addition of hydrogen halides to fluorosubstituted olefins is briefly discussed.     

Matrix IR spectra and quantum-chemical calculations of the products of small nickel cluster interactions with water molecules

Reactions of Ni _n clusters with water molecules were studied by IR spectroscopy in inert matrices and quantum chemistry methods. The geometric configurations, total energies, and vibrational frequencies of all the possible Ni₂(H₂O) and Ni₃(H₂O) isomers were calculated. For both systems, the main minima and transition states were found. Water was shown to dissociate to hydrogen and hydroxyl in the reactions, and, in all the complexes formed, hydrogen is situated in the bridge position on the Ni-Ni bond.     

HMO-elektronen-Struktur von Annulenen mit Bindungsalternanz

Zusammenfassung Für 4n- und (4n + 2)-Annulene mit Bindungsalternanz werden analytische Ausdrücke für die LCAO-Koeffizienten und die Elemente der Ladungs-/ Bindungsordnungsmatrix hergeleitet. Die auf die Umordnung des Orbital-schemas zurückgehende sprunghafte Änderung der Bindungsordnungen in 4n-Annulenen beim SymmetrieübergangD _(2n)h D _(4n)h wird diskutiert.

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HMO-Electron structure of annulenes with bond alternation

Analytical expressions for the LCAO-coefficients and the elements of the charge-bond order-matrix are derived for 4n and for (4n +2) annulenes with alternant bond lengths. Due to the reorganization of the orbital scheme there is a discontinuity in the bond orders of 4n -annulenes at the symmetry change fromD _(2n)h toD _(4n)h .

     

Quantum chemical study of the structure of bicyclo[2.2.0]hex-1(4)-ene and its dimers

The RHF, B3LYP, and PBE0/6-311G** quantum chemical methods are used to determine the point symmetry group and the equilibrium structure of bicyclo[2.2.0]hex-1(4)-ene (I, D _2h ), its two stable dimers (tricyclo[4.2.2.2^2,5]dodeca-1.5-diene (II, D _2h ) and 2,5-dimethylenetricyclo[4.2.2.0^1,6]decane (III, C ₂)), and pentacyclo [4.2.2.2^2,5]dodecane (IV, D ₂) that is a hypothetical intermediate in the dimerization reaction of the molecules of I. The relation of total energies is obtained with regard to zero-point vibrations: E(III) < E(II) ≪ E(IV) ≪ 2E(I).     

A comparative quantum mechanical study of bond separation energies as a measure of cyclic conjugation

Restricted Hartree-Fock (RHF), second-order Møller-Plesset (MP2), and density functional calculations [using the Becke/Lee-Yang-Parr (B-LYP) exchange/correlation gradient-corrected functionals] employing the 6-311G(d, p) and 6-311 + + G(d, p) basis sets have been carried out to calculate isodesmic bond separation energies for reactions involving a number of representative five- and six-membered ring organic compounds. The MP2 and density functional approaches yield reasonably good energies; the density functional method agrees particularly well with experiment, exhibiting a root-mean-square error of only 2.5 kcal/mol. Ring geometries are calculated satisfactorily in all approaches but are given particularly accurately by the MP2 approach. A comparison of the B-LYP bond separation energies with several other definitions of resonance energy shows that these different approaches correlate with each other in a reasonable fashion. © 1995 John Wiley & Sons, Inc.     

Theoretical study of the energies and activation barriers of elementary reactions of hydrogenation of the Ti-doped closo-aluminide cluster Al@TiAl11 and its anion Al@TiAl 11?

The potential energy surfaces, energies E, and activation barriers h of elementary reactions of addition of an H₂ molecule to the Ti-doped closo-aluminide cluster Al@TiAl₁₁ and its anion Al@Ti₁₁⁻ with an icosahedral and marquee structure in the states with different multiplicity were calculated within the B3LYP approximation of the density functional theory using the 6–31G* and 6–311+G* basis sets. The results were compared with the data calculated at the same level of theory for the related reactions of hydrogenation of bare closo-aluminides Al₁₃ and Al₁₃⁻ and their B-, C-, Si-, and Ge-doped derivatives. The computations demonstrated that, depending on the structure, charge, and multiplicity of the Al@TiAl₁₁ cluster, the hydrogenation energy varies in the range 15–23 kcal/mol. At the first stage of addition (chemisorption) of H₂, a μ-H₂ complex at the Ti atom (intermediate) forms with the distance R(Ti-H₂) ∼ 1.9–2.0 ?, which is accompanied by an energy decrease of ∼4–10 kcal/mol. The H-H bond in the μ-H₂ complex is ∼0.1 ? longer and the stretching vibration frequency V_val(HH) is ∼700–1500 cm⁻¹ (or more) lower than the corresponding characteristics of the isolated H₂ molecule. In the transition state with an imaginary frequency of ∼600i–1100i, the H₂ molecule is coordinated to the attacked edge Ti-Al_r, and its length increases to ∼0.9–1.1 ?. The activation barrier height h varies from a few kcal/mol to ∼8–10 kcal/mol when measured from the μ-H₂ complex and is within 18–22 kcal/mol when measured from the product (dihydride Al@TiAl₁₁H₂). The latter barrier (to the reverse reaction of dehydrogenation) is considerably higher than the barriers to migration of hydrogen atoms around the metal cage in the Al@TiAl₁₁H₂ dehydrides. There is a correlation between the energies E and barriers h of hydrogenation reactions and the structure, external charge, and multiplicity of the Al@TiAl₁₁ cluster. In all cases, the hydrogenation should occur significantly more readily than dehydrogenation. It was shown that these reactions can be both irreversible (for example, for an icosahedron in the singlet state) and reversible (for a marquee in the triplet state and others). The conclusion was drawn that the elementary reactions of hydrogenation and dehydrogenation for Ti-doped aluminides should occur considerably faster and under milder conditions than for bare aluminides or their analogues doped with main-group atoms. Original Russian Text ? V.K. Charkin, O.P. Kochnev, N.M. Klimenko, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 8, pp. 1345–1354.     

Ruthenium(II) carbonyl complexes of P,P and P,O donor diphosphine ligands,Ph2P(CH2)nPPh2 and Ph2P(CH2)nP(O)Ph2, n = 2, 3 and their activities in catalytic transfer hydrogenation reactions

The polymeric precursor [RuCl₂(CO)₂]_n reacts with the ligands, P∩P (a, b) and P∩O (c, d), in 1:1 M ratio to generate six-coordinate complexes [RuCl₂(CO)₂(?²-P∩P)] (1a, 1b) and [RuCl₂(CO)₂(?²-P∩O)] (1c, 1d), where P∩P: Ph₂P(CH₂)_nPPh₂, n = 2(a), 3(b); P∩O: Ph₂P(CH₂)_nP(O)Ph₂, n = 2(c), 3(d). The complexes are characterized by elemental analyses, mass spectrometry, thermal studies, IR, and NMR spectroscopy. 1a–1d are active in catalyzed transfer hydrogenation of acetophenone and its derivatives to corresponding alcohols with turnover frequency (TOF) of 75–290 h^?1. The complexes exhibit higher yield of hydrogenation products than catalyzed by RuCl₃ itself. Among 1a–1d, the Ru(II) complexes of bidentate phosphine (1a, 1b) show higher efficiency than their monoxide analogs (1c, 1d). However, the recycling experiments with the catalysts for hydrogenation of 4-nitroacetophenone exhibit a different trend in which the catalytic activities of 1a, 1b, and 1d decrease considerably, while 1c shows similar activity during the second run.     

High levelab initio stabilization energies of benzene

Summary G2 theory is shown to be reliable for calculating isodesmic and homodesmotic stabilization energies (ISE and HSE, respectively) of benzene. G2 calculations give HSE and ISE values of 92.5 and 269.1 kJ mol^–1 (298 K), respectively. These agree well with the experimental HSE and ISE values of 90.5±7.2 and 268.7±6.3 kJ mol^–1, respectively. We conclude that basis set superposition error corrections to the enthalpies of the homodesmotic or isodesmic reactions are not necessary in calculations of the stabilization energies of benzene using G2 theory. The calculated values of the enthalpies of formation of such molecules containing multiple bonds such as benzene ands-trans 1,3-butadiene, which are found from the enthalpies of isodesmic and homodesmotic reactions rather than of atomization reactions, demonstrate good performance of G2 theory. Estimates of theH _f ^o value for benzene from the G2 calculated enthalpies of homodesmotic reaction (2) and isodesmic reaction (3) are 80.9 and 82.5 kJ mol^–1 (298 K), respectively. These are very close to the experimentalH _f ^o value of 82.9±0.3 kJ mol^–1. TheH _f ^o value ofs-trans 1,3-butadiene calculated using the G2 enthalpy of isodesmic reaction (4) is 110.5 kJ mol^–1 and is in excellent agreement with the experimentalH _f ^o value of 110.0±1.1 kJ mol^–1.     

Energetics of Formation of Cyclacenes from 2,3-Didehydroacenes and Implications for Astrochemistry

The carriers of the diffuse interstellar bands (DIBs) are still largely unknown although polycyclic aromatic hydrocarbons, carbon chains, and fullerenes are likely candidates. A recent analysis of the properties of n-acenes of general formula C_4n+2H_2n+4 suggested that these could be potential carriers of some DIBs. Dehydrogenation reactions of n-acenes after absorption of an interstellar UV photon may result in dehydroacenes. Here the reaction energies and barriers for formation of n-cyclacenes from 2,3-didehydroacenes (n-DDA) by intramolecular Diels–Alder reaction to dihydro-etheno-cyclacenes (n-DEC) followed by ejection of ethyne by retro-Diels–Alder reactions are analyzed using thermally assisted occupation density functional theory (TAO-DFT) for n=10–20. It is found that the barriers for each of the steps depend on the ring strain of the underlying n-cyclacene, and that the ring strain of n-DEC is about 75 % of that of the corresponding n-cyclacene. In each case, ethyne extrusion is the step with the highest energy barrier, but these barriers are smaller than CH bond dissociation energies, suggesting that formation of cyclacenes is an energetically conceivable fate of n-acenes after multiple absorption of UV photons.     

Vibrational spectra of icosahedral (Ih and I) fullerenes

On the bases of the topological structures of the three big classes of icosahedral fullerenes: (1) C_n(I_h, n=60h²; h=1, 2,…), (2) C_n(I_h, n=20h²; h=1, 2,…), and (3) C_n(I, n=20(h²+hk+k²), h>k; h, k=1, 2,…), we derived formulas for the decomposition of their nuclear motions into irreducible representations. Hence, we obtained the infrared and Raman active modes for all of the icosahedral (I_h and I) fullerenes theoretically. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 113–117, 1998     

Activation of C2H6, C3H8, HC(CH3)3, and c-C3H6 by gas-phase Ru+ and the thermochemistry of Ru-ligand complexes

The reactions of Ru⁺ with C₂H₆, C₃H₈, HC(CH₃)₃, and c-C₃H₆ at hyperthermal energies have been studied using guided ion beam mass spectrometry. It is found that dehydrogenation is efficient and the dominant process at low energies in all four reaction systems. At high energies, C-H cleavage processes dominate the product spectrum for the reactions of Ru⁺ with ethane, propane, and isobutane. C-C bond cleavage is a dominant process in the cyclopropane system. The reactions of Ru⁺ are compared with those of the first-row transition metal congener Fe⁺ and the differences in behavior and mechanism are discussed in some detail. Modeling of the endothermic reaction cross sections yields the 0-K bond dissociation energies (in eV) of D ₀(Ru-H)=2.27±0.15, D ₀(Ru⁺-C)=4.70±0.11, D ₀(Ru⁺-CH)=5.20±0.12, D ₀(Ru⁺-CH₂)=3.57±0.05, D ₀(Ru⁺-CH₃)=1.66±0.06, D ₀(Ru-CH₃)=1.68±0.12, D ₀(Ru⁺-C₂H₂)=1.98±0.18, D ₀(Ru⁺-C₂H₃)=3.03±0.07, and D ₀(Ru⁺-C₃H₄)=2.24±0.12. Speculative bond energies for Ru⁺=CCH₂ of 3.39±0.19 eV and Ru⁺=CHCH₃ of 3.19±0.15 eV are also obtained. The observation of exothermic processes sets lower limits for the bond energies of Ru⁺ to ethene, propene, and isobutene of 1.34, 1.22, and 1.14 eV, respectively.     

Electron-pair densities of singly charged atoms

For the singly charged 53 cations from Li⁺ to Cs⁺ and 43 anions from H⁻ to I⁻ in their ground states, spherically averaged electron-pair intracule (relative-motion) density h(u), extracule (center-of-mass-motion) density d(R), and their moments uⁿ and Rⁿ are examined, where u and R are the interelectronic distance and the center-of-mass radius of a pair of electrons, respectively. The intracule and extracule densities of all the 96 ions are found to be monotonically decreasing functions, as for neutral atoms. Approximate relations d(R)8h(2R) and uⁿ/Rⁿ2ⁿ are confirmed to be valid for the charged atoms as well.     

Homoaromaticity and homoconjugation in the quinacenes: Biquinacene,triquinacene, and hexaquinacene

A thermodynamic criterion for aromatic and conjugative interactions is proposed. Enthalpies of stepwise hydrogenation of, for example, three double bonds are compensated for strain energy changes during hydrogenation. Strain energies are calculated by molecular mechanics. If the compensated values show a monotonic increase from bond 3 to bond 1, the molecule is conjugatively stabilized. If the initial rise is sharp followed by a constant H _h for bonds 2 and 1 and the molecule is cyclic, stabilization is aromatic. If the compensated H _h decreases, the interaction is destabilizing. By this set of criteria, biquinacene is unstabilized, triquinacene is homoaromatically stabilized, hexaquinacene is homoconjugatively stabilized, and cis,cis,cis-1,4,7-cyclononatriene is homoaromatically destabilized. New experimental data are presented for the biquinacenes (bicycloocta-1,7-diene and its hydrogenation products) and the hexaquinacenes.     

Reaction Mechanism of the Hydrogermylation/Hydrostannylation of Unactivated Alkenes with Two‐Coordinate EII Hydrides (E=Ge,Sn): A Theoretical Study

Quantum chemical calculations using density functional theory with the TPSS+D3(BJ) and M06‐2X+D3(ABC) functionals have been carried out to understand the mechanisms of catalyst‐free hydrogermylation/hydrostannylation reactions between the two‐coordinate hydrido‐tetrylenes :E(H)(L⁺) (E=Ge or Sn, L⁺=N(Ar⁺)(SiiPr₃); Ar⁺=C₆H₂{C(H)Ph₂}₂iPr‐2,6,4) and a range of unactivated terminal (C₂H₃R, R=H, Ph, or tBu) and cyclic [(CH)₂(CH₂)₂(CH₂)_n, n=1, 2, or 4] alkenes. The calculations suggest that the addition reactions of the germylenes and stannylenes to the cyclic and acyclic alkenes occur as one‐step processes through formal [2+2] addition of the E?H fragment across the C?C π bond. The reactions have moderate barriers and are weakly exergonic. The steric bulk of the tetrylene amido groups has little influence on the activation barriers and on the reaction energies of the anti‐Markovnikov pathway, but the Markovnikov addition is clearly disfavored by the size of the substituents. The addition of the tetrylenes to the cyclic alkenes is less exergonic than the addition to the terminal alkenes, which agrees with the experimentally observed reversibility of the former reactions. The hydrogermylation reactions have lower activation energies and are more exergonic than the stannylene addition. An energy decomposition analysis of the transition state for the hydrogermylation of cyclohexene shows that the reaction takes place with simultaneous formation of the Ge?C and (Ge)H?C′ bonds. The dominant orbitals of the germylene are the σ‐type lone pair MO of Ge, which serves as a donor orbital, and the vacant p(π) MO of Ge, which acts as acceptor orbital for the π* and π MOs of the olefin. Inspection of the transition states of some selected reactions suggests that the differences between the activation energies come from a delicate balance between the deformation energies of the interacting species and their interaction energies.     

Making Use of the Direct Process Residue: Synthesis of Bifunctional Monosilanes

The industrial production of monosilanes Me_nSiCl_4−n (n=1–3) through the Müller–Rochow Direct Process generates disilanes Me_nSi₂Cl_6−n (n=2–6) as unwanted byproducts (“Direct Process Residue”, DPR) by the thousands of tons annually, large quantities of which are usually disposed of by incineration. Herein we report a surprisingly facile and highly effective protocol for conversion of the DPR: hydrogenation with complex metal hydrides followed by Si−Si bond cleavage with HCl/ether solutions gives (mostly bifunctional) monosilanes in excellent yields. Competing side reactions are efficiently suppressed by the appropriate choice of reaction conditions.     

DFT study on the intermolecular interactions between Aun (n = 2–4) and thymine

Density functional B3LYP method with 6-31++G** basis set is applied to optimize the geometries of the luteolin, water and luteolin–(H₂O)_n complexes. The vibrational frequencies are also studied at the same level to analyze these complexes. We obtained four steady luteolin–H₂O, nine steady luteolin–(H₂O)₂ and ten steady luteolin–(H₂O)₃, respectively. Theories of atoms in molecules (AIM) and natural bond orbital (NBO) are used to investigate the hydrogen bonds involved in all the systems. The interaction energies of all the complexes corrected by basis set superposition error, are within −13.7 to −82.5 kJ/mol. The strong hydrogen bonding mainly contribute to the interaction energies, Natural bond orbital analysis is performed to reveal the origin of the interaction. All calculations also indicate that there are strong hydrogen bonding interactions in luteolin–(H₂O)_n complexes. The OH stretching modes of complexes are red-shifted relative to those of the monomer.     

Three-dimensional configurations of organic/inorganic hybrid nanostructural blocks (I). A quantum mechanical investigation for ladder-like structure of vinylsilsesquioxane

Silsesquioxanes (SSO) or polyhedral oligomericsilsesquioxanes (POSS) are generally prepared frommolecular precursors using the hydrolytic condensa-tion of trialkoxysilane, RSi(OR')3. They are organic/inorganic hybrid nanostructural blocks with theircomplete general formula Tn(T = RSiO1.5, n = 1,2, …), and the incomplete generic formula is Tn-(OH)x(OR')y[x, y = 0, 1, 2, …, T= RSiO1.5 ?(x+y)/2n][1,2].Each VSSO, possessing a certain structural formulaand molecular weight, may h…     

Syntheses and X-ray structures of heteroleptic octahedral Mn(II)-xanthato complexes involving N-donor ligands

A series of octahedral manganese(II) complexes involving xanthates and N-donor ligands, [Mn(S₂COiBu)₂(phen)] (1), [Mn(S₂COiBu)₂(2,2′-bpy)] (2), [Mn(S₂COnPr)₂(phen)] (3), [Mn(S₂COnPr)₂(2,2′-bpy)] (4), [Mn(S₂COMe)₂(2,2′-bpy)] (5), [Mn(S₂COnPr)₂(4,4′-bpy)]_n, and [Mn₂(S₂COnPr)₄(4,4′-bpy)₃] (6) (phen = 1,10-phenanthroline, bpy = bipyridine) was prepared. Complexes were characterized by elemental analysis, FTIR spectroscopy, TG/DSC analysis, and single-crystal X-ray diffraction. The structures are built of monomeric molecules of the complexes, except for 6 with the 4,4′-bipyridine ligand, which contains a binuclear complex and 1D polymeric zigzag chain in one crystal.     

Theoretical Study of the Catalytic Cycle for Ethylene Hydrogenation on a Dipalladium Cluster

A theoretical study of the potential energy surface is carried out for the catalytic cycle of ethylene hydrogenation on a Pd₂cluster using the reaction-path Hamiltonian. The catalytic cycle consists of five related reactions involving ten stationary points. The isomerization of the bridged Pd₂H₂complex into the transcomplex with a maximal barrier of 21.5 kcal/mol rather than the activation of the H–H bond is the most important reaction step. A conclusion is drawn that catalysts based on dipalladium complexes in which the dihydride product readily forms a transform can be active in ethylene hydrogenation.