Structure and properties of single crystalline CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2

Single crystals of CaMg(2)Bi(2), EuMg(2)Bi(2), and YbMg(2)Bi(2) were obtained from a Mg-Bi flux cooled to 650 °C. These materials crystallize in the CaAl(2)Si(2) structure-type (P ?3m1, No. 164), and crystal structures are reported from refinements of single crystal and powder X-ray diffraction data. EuMg(2)Bi(2) displays an antiferromagnetic transition near 7 K, which is observed via electrical resistivity, magnetization, and specific heat capacity measurements. Magnetization measurements on YbMg(2)Bi(2) reveal a weak diamagnetic moment consistent with divalent Yb. Despite charge-balanced empirical formulas, all three compounds are p-type conductors with Hall carrier concentrations of 2.0(3) × 10(19) cm(-3) for CaMg(2)Bi(2), 1.7(1) × 10(19) cm(-3) for EuMg(2)Bi(2), and 4.6(7) × 10(19) cm(-3) for YbMg(2)Bi(2), which are independent of temperature to 5 K. The electrical resistivity decreases with decreasing temperature and the resistivity ratios ρ(300 K)/ρ(10 K) ≤ 1.6 in all cases, indicating significant defect scattering.     

A systematic computational study of the reactions of HO2 with RO2: the HO2 + CH2ClO2, CHCl2O2, and CCl3O2 reactions

Potential energy surfaces for the reactions of HO(2) with CH(2)ClO(2), CHCl(2)O(2), and CCl(3)O(2) have been calculated using coupled cluster theory and density functional theory (B3LYP). It is revealed that all the reactions take place on both singlet and triplet surfaces. Potential wells exist in the entrance channels for both surfaces. The reaction mechanism on the triplet surface is simple, including hydrogen abstraction and S(N)2-type displacement. The reaction mechanism on the singlet surface is more complicated. Interestingly, the corresponding transition states prefer to be 4-, 5-, or 7-member-ring structures. For the HO(2) + CH(2)ClO(2) reaction, there are two major product channels, viz., the formation of CH(2)ClOOH + O(2) via hydrogen abstraction on the triplet surface and the formation of CHClO + OH + HO(2) via a 5-member-ring transition state. Meanwhile, two O(3)-forming channels, namely, CH(2)O + HCl + O(3) and CH(2)ClOH + O(3) might be competitive at elevated temperatures. The HO(2) + CHCl(2)O(2) reaction has a mechanism similar to that of the HO(2) + CH(2)ClO(2) reaction. For the HO(2) + CCl(3)O(2) reaction, the formation of CCl(3)O(2)H + O(2) is the dominant channel. The Cl-substitution effect on the geometries, barriers, and heats of reaction is discussed. In addition, the unimolecular decomposition of the excited ROOH (e.g., CH(2)ClOOH, CHCl(2)OOH, and CCl(3)OOH) molecules has been investigated. The implication of the present mechanisms in atmospheric chemistry is discussed in comparison with the experimental measurements.     

Unimolecular reactions of CF2ClCFClCH2F and CF2ClCF2CH2Cl: observation of ClF interchange

The unimolecular reactions of CF(2)ClCFClCH(2)F and CF(2)ClCF(2)CH(2)Cl molecules formed with 87 and 91 kcal mol(-1), respectively, of vibrational energy from the recombination of CF(2)ClCFCl with CH(2)F and CF(2)ClCF(2) with CH(2)Cl at room temperature have been studied by the chemical activation technique. The 2,3- and 1,2-ClF interchange reactions compete with 2,3-ClH and 2,3-FH elimination reactions. The total unimolecular rate constant for CF(2)ClCF(2)CH(2)Cl is 0.54 +/- 0.15 x 10(4) s(-1) with branching fractions for 1,2-ClF interchange of 0.03 and 0.97 for 2,3-FH elimination. The total rate constant for CF(2)ClCFClCH(2)F is 1.35 +/- 0.39 x 10(4) s(-1) with branching fractions of 0.20 for 2,3-ClF interchange, 0.71 for 2,3-ClH elimination and 0.09 for 2,3-FH elimination; the products from 1,2-ClF interchange could be observed, but the rate constant was too small to be measured. The D(CH(2)F-CFClCF(2)Cl) and D(CH(2)Cl-CF(2)CF(2)Cl) were evaluated by calculations for some isodesmic reactions and isomerization energies of CF(3)CFClCH(2)Cl as 84 and 88 kcal mol(-1), respectively; these values give the average energies of formed molecules at 298 K as noted above. Density functional theory was used to assign vibrational frequencies and moments of inertia for the molecules and their transition states. These results were combined with statistical unimolecular reaction theory to assign threshold energies from the experimental rate constants for ClF interchange, ClH elimination and FH elimination. These assignments are compared with results from previous chemical activation experiments with CF(3)CFClCH(2)Cl, CF(3)CF(2)CH(3,) CF(3)CFClCH(3) and CF(2)ClCF(2)CH(3).     

Crystal Structures of [Bu2Sn(O2PPh2)2], [Ph2Sn(O2PPh2)2], and [PhClSn(O2PPh2)OMe]2. Raman Spectra of [Ph2Sn(O2PPh2)2] and [PhClSn(O2PPh2)OMe]2

[(n‐Bu)₂Sn(O₂PPh₂)₂] ( 1 ), and [Ph₂Sn(O₂PPh₂)₂] ( 2 ) have been synthesized by the reactions of R₂SnCl₂ (R=n‐Bu, Ph) with HO₂PPh₂ in Methanol. From the reaction of Ph₂SnCl₂ with diphenylphosphinic acid a third product [PhClSn(O₂PPh₂)OMe]₂ ( 3 ) could be isolated. X‐ray diffraction studies show 1 to crystallize in the monoclinic space group P2₁/c with a = 1303.7(1) pm, b = 2286.9(2) pm, c = 1063.1(1) pm, β = 94.383(6)°, and Z = 4. 2 crystallizes triclinic in the space group , the cell parameters being a = 1293.2(2) pm, b = 1478.5(4) pm, c = 1507.2(3) pm, α = 98.86(3)°, β = 109.63(2)°, γ = 114.88(2)°, and Z = 2. Both compounds form arrays of eight‐membered rings (SnOPO)₂ linked at the tin atoms to form chains of infinite length. The dimer 3 consists of a like ring, in which the tin atoms are bridged by methoxo groups. It crystallizes triclinic in space group with a = 946.4(1) pm, b = 963.7(1) pm, c = 1174.2(1) pm, α = 82.495(6)°, β = 66.451(6)°, γ = 74.922(6)°, and Z = 1 for the dimer. The Raman spectra of 2 and 3 are given and discussed.     

Synthesis and catalytic epoxidation activity with TBHP and H2O2 of dioxo-, oxoperoxo-, and oxodiperoxo molybdenum(VI) and tungsten(VI) compounds containing monodentate or bidentate phosphine oxide ligands: crystal structures of WCl2(O)2(OPMePh2)2, WCl2(O)(O2)(OPMePh2)2, MoCl2(O)2dppmO2.C4H10O, WCl2(O)2dppmO2, Mo(O)(O2)2dppmO2, and W(O)(O2)2dppmO2

The dioxo tungsten(VI) and molybdenum(VI) complexes WCl2(O)2(OPMePh2)2, WCl2(O)2dppmO2, and MoCl2(O)2dppmO2, the oxoperoxo compounds WCl2(O)(O2)(OPMePh2)2, WCl2(O)(O2)dppmO2, and MoCl2(O)(O2)dppmO2, and the oxodiperoxo complexes, W(O)(O2)2dppmO2 and Mo(O)(O2)2dppmO2 have been prepared and characterized by IR spectroscopy, 31P NMR spectroscopy, elemental analysis, and X-ray crystallography. The structural and X-ray crystallographic data of compounds WCl2(O)2(OPMePh2)2, WCl2(O)(O2)(OPMePh2)2, MoCl2(O)2dppmO2.4H10O, WCl2(O)2dppmO2, Mo(O)(O2)2dppmO2, and W(O)(O2)2dppmO2 are also detailed. All complexes were studied as catalysts for cis-cyclooctene epoxidation in the presence of tert-butyl hydroperoxide (TBHP) or H2O2 as an oxidant. The Mo-based catalysts showed a superior reactivity over W-based catalysts in the TBHP system. On the other hand, in the H2O2 system, the W-based catalysts (accomplishing nearly 100% epoxidation of cyclooctene in 6 h) are more reactive than the Mo catalysts (<45% under some conditions). Various solvent systems have been investigated, and ethanol is the most suitable solvent for the H2O2 system.     

Photocycloaddition of 2-Cyanocycloalk-2-Enones to 2, 3-Dimethylbut-2-Ene

Irradiation of 5,5-dimethyl-6-oxocyclohex- l-ene- l-carbonitrile ( 1 ) in the presence of 2,3-dimethylbut-2-ene afforded 3,3,4,4,7,7-hexamethyl-3,4,4a,5,6,7-hexahydroindeno[1,7-c,d]-],2-oxazole (3) in nearly quantitative yield. In contrast, 4,4-dimethyl-5-oxo-cyclopent-l-ene-l-carbonitrile ( 2 ) under the same conditions reacted not to a tricyclic isoxazole but to a 2:1 mixture of 3,3,6,6,7,7-hexamethyl-2-oxo-bicyclo[3.2.0]heptane-l-carbonitrile ( 4 ) and trans-3,3-dimethyl-2-oxo-5-(2,3-dimethylbut-3-en-2-yl)cyclopentane-l-carbonitrile ( 5 ), respectively.     

Synthesis of 2-hetaryl-2-(tetrahydrofuran-2-ylidene)acetonitriles

The reaction of azolylacetonitriles with γ-chlorobutyryl chlorides gave the corresponding 2-(1-R-1,3-di- hydro-2H-benzimidazol-2-ylidene)-, 2-(1,3-benzothiazol-2-ylidene)-, and 6-chloro-2-(4-methylthiazol-2-ylidene)-3-oxohexanenitriles. A study of the intramolecular cyclization of 2-(quinazolin-2-ylidene)-3-oxo-6-hexanenitriles and -heptanenitriles has led to the development of an efficient method for the preparation of 2-(quinazolin-2-yl)-2-(tetrahydrofuran-2-ylidene)acetonitriles. The (Z,E)-isomerism of the synthesized 2-hetaryl-2-(tetrahydrofuran-2-ylidene)acetonitriles was studied.     

MRCI investigation of different isomers of Ni2O2H2(+)

The geometrical and electronic structures of different isomers of Ni(2)O(2)H(2)(+) are investigated by multireference configuration interaction (MRCI) calculations using natural atomic orbital basis sets. The lowest-lying isomer, Ni(2)(OH)(2)(+), has a rhombic shape with two OH groups bridging the Ni atoms. The next isomer in energetic order with a relative energy of 0.29 eV consists of a linear NiONi(OH(2))(+) chain. Other structures with a rhombic shape, (NiH)(2)O(2)(+), with H bound to the Ni atoms have considerably higher energies, above 4 eV. Especially the low-lying isomers are characterised by a large number of low-lying electronic terms. The product Ni(2)O(2)H(2)(+) of the reaction of Ni(2)O(2)(+) with small alkanes is likely to have the rhombic Ni(2)(OH)(2)(+) structure. The reaction energy of the reaction Ni(2)O(2)(+) + H(2)→ Ni(2)(OH)(2)(+) is estimated to be about -3.5 eV.     

Kinetic study of the reactions of Cl atoms with CF3CH2CH2OH, CF3CF2CH2OH, CHF2CF2CH2OH, and CF3CHFCF2CH2OH

The reaction kinetics of chlorine atoms with a series of partially fluorinated straight-chain alcohols, CF(3)CH(2)CH(2)OH (1), CF(3)CF(2)CH(2)OH (2), CHF(2)CF(2)CH(2)OH (3), and CF(3)CHFCF(2)CH(2)OH (4), were studied in the gas phase over the temperature range of 273-363 K by using very low-pressure reactor mass spectrometry. The absolute rate coefficients were given by the expressions (in cm(3) molecule(-1) s(-1)): k(1) = (4.42 +/- 0.48) x 10(-11) exp(-255 +/- 20/T); k(1)(303) = (1.90 +/- 0.17) x 10(-11), k(2) = (2.23 +/- 0.31) x 10(-11) exp(-1065 +/- 106/ T); k(2)(303) = (6.78 +/- 0.63) x 10(-13), k(3) = (8.51 +/- 0.62) x 10(-12) exp(-681 +/- 72/T); k(3)(303) = (9.00 +/- 0.82) x 10(-13) and k(4) = (6.18 +/- 0.84) x 10(-12) exp(-736 +/- 42/T); k(4)(303) = (5.36 +/- 0.51) x 10(-13). The quoted 2sigma uncertainties include the systematic errors. All title reactions proceed via a hydrogen atom metathesis mechanism leading to HCl. Moreover, the oxidation of the primarily produced radicals was investigated, and the end products were the corresponding aldehydes (R(F)-CHO; R(F) = -CH(2)CF(3), -CF(2)CF(3), -CF(2)CHF(2), and -CF(2)CHFCF(3)), providing a strong experimental indication that the primary reactions proceed mainly via the abstraction of a methylenic hydrogen adjacent to a hydroxyl group. Finally, the bond strengths and ionization potentials for the title compounds were determined by density functional theory calculations, which also suggest that the alpha-methylenic hydrogen is mainly under abstraction by Cl atoms. The correlation of room-temperature rate coefficients with ionization potentials for a set of 27 molecules, comprising fluorinated C2-C5 ethers and C2-C4 alcohols, is good with an average deviation of a factor of 2, and is given by the expression log(k) (in cm(3) molecule(-1) s(-1)) = (5.8 +/- 1.4) - (1.56 +/- 0.13) x (ionization potential (in eV)).     

Synthesis and Crystal Structure of a Dinuclear Compound Cu2[(O=PPh2)2N]2(μ-O2PPh2)2

A neutral dinuclear compound Cu2[(O=PPh2)2N]2(μ-O2PPh2)2 has been unexpec- tedly prepared from the reaction of [Cu2(dppa)2(MeCN)4](ClO4)2 (dppa = bis(diphenylphosphino)- amine) and Sn(edt)2 (edt = ethane-1,2-dithiolate) in dichloromethane in air. ESR spectrum shows that the original Cu+ ion has been oxidized. The crystal structure determined by an X-ray single diffrac- tometer displays that the concurrence of the redox and hydrolysis of the origin ligand dppa is taken with the formation of two new ligands. The complex crystallizes in triclinic, space group P with a = 9.4269(13), b = 10.7522(13), c = 16.787(2)(A), ( = 92.042(5), β = 104.591(7), γ = 96.167(6)°, V = 1633.6(4)(A)3, C72H60 Cu2N2O8P6, Mr = 1394.12, Z = 1, Dc = 1.419 g/cm3, F(000) = 718, μ = 0.856 mm-1, the final R = 0.0530 and wR = 0.1096 for 4210 observed reflections with I > 2σ(I).     

Infrared spectra of CO2-H2 complexes

Infrared spectra of weakly bound CO(2)-H(2) complexes have been studied in the region of the CO(2) v(3) asymmetric stretch, using a tunable diode laser probe and a pulsed supersonic jet expansion. For CO(2)-paraH(2), results were obtained for three isotopic species, (12)C(16)O(2), (13)C(16)O(2), and (12)C(18)O(2). These spectra were analyzed using an asymmetric rotor Hamiltonian, with results that resembled those obtained previously for OCS- and N(2)O-paraH(2), except that half the rotational levels were missing due to the symmetry of CO(2) and the spin statistics of the (16)O or (18)O nuclei. However, for CO(2)-orthoH(2), more complicated spectra were observed which could not be assigned, in contrast with OCS- and N(2)O-H(2) where the paraH(2) and orthoH(2) spectra were similar, though distinct. The CO(2)-paraH(2) complex has a T-shaped structure with and intermolecular distance of about 3.5 Angstroms, and the CO(2) v(3) vibration exhibits a small redshift (-0.20 cm(-1)) in the complex.     

Properties of fluorosulfate-based ionic liquids and geometries of (FO2SOH)OSO2F- and (FO2SOH)2O2SOF-

A room temperature ionic liquid (IL) based on the fluorosulfate anion (SO(3)F(-)) has been synthesized by the reaction of 1-ethyl-3-methylimidazolium (EMIm(+)) chloride and fluorosulfuric acid (HOSO(2)F). The viscosity, ionic conductivity, and electrochemical window of EMImSO(3)F at 25 °C are 46.6 mPa s, 10.8 mS cm(-1), and 4.3 V, respectively. According to a solvatochromic measurement using ILs, there is a trend in the donor ability of fluoro- and oxofluoroanions, PF(6)(-) < BF(4)(-) < N(SO(2)CF(3))(2)(-) < SO(3)CF(3)(-) < SO(3)F(-) < PO(2)F(2)(-), which is explained by the atomic charges obtained from quantum mechanical calculations. The 1 : 2 and 1 : 3 stoichiometric reactions of EMImCl and HOSO(2)F give EMIm(FO(2)SOH)OSO(2)F and EMIm(FO(2)SOH)(2)O(2)SOF, respectively. Both the salts are liquid at room temperature without a HOSO(2)F dissociation pressure (< 1 Pa) and have low viscosity and high ionic conductivity (9.2 mPa s and 30.8 mS cm(-1) for EMIm(FO(2)SOH)OSO(2)F and 5.1 mPa s and 43.2 mS cm(-1) for EMIm(FO(2)SOH)(2)O(2)SOF). The vibrational modes and bonding properties of these anionic species are discussed with the aid of quantum mechanical calculations. The (FO(2)SOH)OSO(2)F(-) anion in EMIm(FO(2)SOH)OSO(2)F does not have an inversion centre, which stands in contrast to the one with an inversion centre (e.g. observed in solid Cs(FO(2)SOH)OSO(2)F). The (FO(2)SOH)(2)O(2)SOF(-) anion in EMIm(FO(2)SOH)(2)O(2)SOF is characterized by vibrational spectroscopy under C(s) symmetry.     

High-spin cobalt(II) ions in square planar coordination: structures and magnetism of the oxysulfides Sr2CoO2Cu2S2 and Ba2CoO2Cu2S2 and their solid solution

The antiferromagnetic structures of the layered oxychalcogenides (Sr(1-x)Ba(x))(2)CoO(2)Cu(2)S(2) (0 ≤ x ≤ 1) have been determined by powder neutron diffraction. In these compounds Co(2+) is coordinated by four oxide ions in a square plane and two sulfide ions at the apexes of an extremely tetragonally elongated octahedron; the polyhedra share oxide vertexes. The magnetic reflections present in the diffraction patterns can in all cases be indexed using a √2a × √2a × c expansion of the nuclear cell, and nearest-neighbor Co(2+) moments couple antiferromagnetically within the CoO(2) planes. The ordered magnetic moment of Co(2+) in Sr(2)CoO(2)Cu(2)S(2) (x = 0) is 3.8(1) μ(B) at 5 K, consistent with high-spin Co(2+) ions carrying three unpaired electrons and with an additional significant unquenched orbital component. Exposure of this compound to moist air is shown to result in copper deficiency and a decrease in the size of the ordered moment to about 2.5 μ(B); there is a strong correlation between the size of the long-range ordered moment and the occupancy of the Cu site. Both the tetragonal elongation of the CoO(4)S(2) polyhedron and the ordered moment in (Sr(1-x)Ba(x))(2)CoO(2)Cu(2)S(2) increase with increasing Ba content, and in Ba(2)CoO(2)Cu(2)S(2), which has Co(2+) in an environment that is close to purely square planar, the ordered moment of 4.5(1) μ(B) at 5 K is over 0.7 μ(B) larger than that in Sr(2)CoO(2)Cu(2)S(2), so the unquenched orbital component in this case is even larger than that observed in octahedral Co(2+) systems such as CoO. The experimental observations of antiferromagnetic ground states and the changes in properties resulting from replacement of Sr by Ba are supported by ab initio calculations on Sr(2)CoO(2)Cu(2)S(2) and Ba(2)CoO(2)Cu(2)S(2). The large orbital moments in these systems apparently result from spin-orbit mixing of the unequally populated d(xz), d(yz), and d(z(2)) orbitals, which are reckoned to be almost degenerate when the CoO(4)S(2) polyhedron reaches its maximum elongation. The magnitudes of the ordered moments in high-spin Co(2+) oxide, oxychalcogenide, and oxyhalide systems are shown to correlate well with the tetragonal elongation of the coordination environment. The large orbital moments lead to an apparently magnetostrictive distortion of the crystal structures below the Nee?l temperature, with the symmetry lowered from tetragonal I4/mmm to orthorhombic Immm and the size of the distortion correlating well with the size of the long-range ordered moment for all compositions and for temperature-dependent data gathered on Ba(2)CoO(2)Cu(2)S(2).     

Formation of a stable dicarbenoid and an unsaturated C2P2S2 ring from two-electron oxidation of the [C(PPh2S)2]2- dianion

Two-electron oxidation of the [C(PPh(2)S)(2)](2-) dianion with iodine afforded an unexpected mixture of a dimeric Li-I carbenoid [(Et(2)O)(mu-Li)][(mu(4)-Li){IC(PPh(2)S)(2)}(2)] and a novel, unsaturated six-membered C(2)P(2)S(2) ring in [(SPh(2)P)(2)C(2)(PPh(2))(2)S(2)].     

Reactivity of Lewis pairs (R2PCH2AlMe2)2 with carbon dioxide

Species R(2)PCH(2)AlMe(2) (R = Me, Ph) are stable Lewis adducts but still react with CO(2) both in solution and in the solid state. The CO(2) adducts undergo a rearrangement unprecedented for ambiphilic molecules to form aluminium carboxylates. A new spirocyclic compound was also obtained by double Lewis pair activation of CO(2).     

Host-guest interactions in a series of self-assembled As2L2Cl2 macrocycles

The reaction of AsCl3 with H2L (where L = a rigid dithiolate) results in the self-assembly of As2L2Cl2 supramolecular macrocycles. For ligands 4,4'-bis(mercaptomethyl)biphenyl (H2), 4,4'-bis(mercaptomethyl)-trans-stilbene (H2), and 1,4-dimethoxy-2,5-bis(mercaptomethyl)benzene (H2), the macrocyclic cavities of the resulting assemblies are large enough to host aromatic solvent molecules, as revealed by single crystal X-ray structures of the inclusion complexes. As2L2Cl(2) macrocycles form in solution as a mixture of diastereomers, but the diastereomers can be selectively crystallized and separated. Crystallization of syn- or anti-As(2)3(2)Cl2 can be controlled using host-guest interactions by the prudent choice of crystallization solvents. anti-As(2)3(2)Cl2 crystallizes exclusively from chloroform and benzene, while a [(syn-As(2)(2)Cl(2))(2).p-xylene] dimer crystallizes from p-xylene and a mixture of [(syn-As(2)3(2)Cl(2))(anti-As(2)3(2)Cl2) x toluene] and [(syn-As(2)3(2)Cl2)2 x toluene] dimers crystallize from toluene.     

N,N''''-(2-苯并咪唑基甲基)亚氨基甲基膦酸的双核镍化合物Ni2(bbimp)2(4,4''''-bipy)(H2O)2·2H2O和Ni2(bbimp)2(H2O)2][Ni(bbimp)(H2O)2]2·4H2O的晶体结构和磁学性质

该文报道了N,N′-(2-苯并咪唑基甲基)亚氨基甲基膦酸{bbimpH_2,[(C7H5N2)CH2]2NCH2PO3H2}的2个镍化合物Ni2(bbimp)2(4,4′-bipy)(H2O)2·2H2O(1)和[Ni2(bbimp)2(H2O)2][Ni(bbimp)(H2O)2]2·4H2O(2)。化合物1是4,4′-联吡啶作为桥连配体的中性双核结构。化合物2含有1个中性的[Ni2(bbimp)2(H2O)2]双核分子与2个中性的[Ni(bbimp)(H2O)2]单核分子。双核分子单元中的2个Ni!离子被2个膦酸氧桥连。在化合物2中,膦酸氧桥连的2个Ni!离子之间存在铁磁性相互作用。     

Coupled-cluster study of isomers of H2SO2

A theoretical study has been made on six isomers of H2SO2 using coupled-cluster singles and doubles with noniterative triple excitations (CCSD(T)). The isomers studied are sulfoxylic acid (S(OH)2; C2 and Cs conformers), sulfinic acid (HS(=O)OH; 2 C1 conformers), dihydrogen sulfone (H2SO2; C2v), sulfhydryl hydroperoxide (HSOOH; C1), thiadioxirane (Cs), and dihydrogen persulfoxide (H2SOO; Cs). Molecular geometries, harmonic vibrational frequencies, and infrared intensities of all species were obtained using the CCSD(T) method and the 6-311++G(2d,2p) basis set. All aforementioned species were found to be local minima, with the exception of thiadioxirane, which has one imaginary frequency. A prior possible infrared observation of sulfinic acid was reassessed on the basis of the present data. In agreement with previous MP2 results, the present CCSD(T) data provide support for at most 4 of the 8 observed frequencies. The CCSD(T) frequencies and intensities should be of assistance in future identification of H2SO2 isomers by vibrational spectroscopy. Relative energies were calculated using the CCSD(T) method and several larger basis sets. As found previously, the lowest energy species is C2 S(OH)2, followed by Cs S(OH)2, HS(=O)OH, H2SO2, HSOOH, thiadioxirane, and H2SOO. Expanding the basis set significantly reduces the relative energies of HS(=O)OH and H2SO2. The CCSD(T) method was used with extended basis sets (up to aug-cc-pV(Q+d)Z) and basis set extrapolation in two reaction schemes to calculate the DeltaH degrees t (25 degrees C) of C2 S(OH)2. The two reaction schemes gave -285.8 and -282.7 kJ mol-1, which are quite close to a prior theoretical estimate (-290 kJ mol-1).     

Crotonaldehyde formation from decomposition of ICH2CH2OH on powdered TiO2

Adsorption and reactions of 2-iodoethanol on TiO(2) have been studied by Fourier transform infrared spectroscopy. ICH(2)CH(2)OH possesses two reactive centers of C-I and C-OH. It is found that its decomposition leads to the formation of crotonaldehyde on TiO(2). A reaction sequence of ICH(2)CH(2)OH --> ICH(2)CH(2)O- --> CH(3)CHO --> CH(3)CH=CH-CHO is proposed. Although the decomposition routes of C(2)H(5)OH and C(2)H(5)I, both forming C(2)H(5)O- on TiO(2), suggest that -OCH(2)CH(2)O- may play a role in the crotonaldehyde formation, reaction of HOCH(2)CH(2)OH on TiO(2) shows that this is not the case. Adsorbed H(2)O is formed in the ICH(2)CH(2)OH decomposition on TiO(2); however, it is found that ICH=CH(2), possibly generated by ICH(2)CH(2)OH dehydration, is not important in the crotonaldehyde formation.     

Ni(Et2PCH2NMeCH2PEt2)2]2+ as a functional model for hydrogenases

The reaction of Et(2)PCH(2)N(Me)CH(2)PEt(2) (PNP) with [Ni(CH(3)CN)(6)](BF(4))(2) results in the formation of [Ni(PNP)(2)](BF(4))(2), which possesses both hydride- and proton-acceptor sites. This complex is an electrocatalyst for the oxidation of hydrogen to protons, and stoichiometric reaction with hydrogen forms [HNi(PNP)(PNHP)](BF(4))(2), in which a hydride ligand is bound to Ni and a proton is bound to a pendant N atom of one PNP ligand. The free energy associated with this reaction has been calculated to be -5 kcal/mol using a thermodynamic cycle. The hydride ligand and the NH proton undergo rapid intramolecular exchange with each other and intermolecular exchange with protons in solution. [HNi(PNP)(PNHP)](BF(4))(2) undergoes reversible deprotonation to form [HNi(PNP)(2)](BF(4)) in acetonitrile solutions (pK(a) = 10.6). A convenient synthetic route to the PF(6)(-) salt of this hydride involves the reaction of PNP with Ni(COD)(2) to form Ni(PNP)(2), followed by protonation with NH(4)PF(6). A pK(a) of value of 22.2 was measured for this hydride. This value, together with the half-wave potentials of [Ni(PNP)(2)](BF(4))(2), was used to calculate homolytic and heterolytic Ni-H bond dissociation free energies of 55 and 66 kcal/mol, respectively, for [HNi(PNP)(2)](PF(6)). Oxidation of [HNi(PNP)(2)](PF(6)) has been studied by cyclic voltammetry, and the results are consistent with a rapid migration of the proton from the Ni atom of the resulting [HNi(PNP)(2)](2+) cation to the N atom to form [Ni(PNP)(PNHP)](2+). Estimates of the pK(a) values of the NiH and NH protons of these two isomers indicate that proton migration from Ni to N should be favorable by 1-2 pK(a) units. Cyclic voltammetry and proton exchange studies of [HNi(depp)(2)](PF(6)) (where depp is Et(2)PCH(2)CH(2)CH(2)PEt(2)) are also presented as control experiments that support the important role of the bridging N atom of the PNP ligand in the proton exchange reactions observed for the various Ni complexes containing the PNP ligand. Similarly, structural studies of [Ni(PNBuP)(2)](BF(4))(2) and [Ni(PNP)(dmpm)](BF(4))(2) (where PNBuP is Et(2)PCH(2)N(Bu)CH(2)PEt(2) and dmpm is Me(2)PCH(2)PMe(2)) illustrate the importance of tetrahedral distortions about Ni in determining the hydride acceptor ability of Ni(II) complexes.