Ab initio study of complexes with two cations as N-H donors to F-: structures and spin-spin coupling constants across N-H-F hydrogen bonds

A systematic ab initio study has been carried out to determine the MP2/6-31+G(d,p) structures and EOM-CCSD coupling constants across N-H-F-H-N hydrogen bonds for a series of complexes F(H(3)NH)(2)(+), F(HNNH(2))(2)(+), F(H(2)CNH(2))(2)(+), F(HCNH)(2)(+), and F(FCNH)(2)(+). These complexes have hydrogen bonds with two equivalent N-H donors to F(-). As the basicity of the nitrogen donor decreases, the N-H distance increases and the N-H-F-H-N arrangement changes from linear to bent. As these changes occur and the hydrogen bonds between the ion pairs acquire increased proton-shared character, (2h)J(F)(-)(N) increases in absolute value and (1h)J(H)(-)(F) changes sign. F(H(3)NH)(2)(+) complexes were also optimized as a function of the N-H distance. As this distance increases and the N-H...F hydrogen bonds change from ion-pair to proton-shared to traditional F-H...N hydrogen bonds, (2h)J(F)(-)(N) initially increases and then decreases in absolute value, (1)J(N)(-)(H) decreases in absolute value, and (1h)J(H)(-)(F) changes sign. The signs and magnitudes of these coupling constants computed for F(H(3)NH)(2)(+) at short N-H distances are in agreement with the experimental signs and magnitudes determined for the F(collidineH)(2)(+) complex in solution. However, even when the N-H and F-H distances are taken from the optimized structure of F(collidineH)(2)(+), (2h)J(F)(-)(N) and (1h)J(H)(-)(F) are still too large relative to experiment. When the distances extracted from the experimental NMR data are used, there is excellent agreement between computed and experimental coupling constants. This suggests that the N-H-F hydrogen bonds in the isolated gas-phase F(collidineH)(2)(+) complex have too much proton-shared character relative to those that exist in solution.     

Chiral recognition of zinc(II) ion complexes composed of bicyclo[3.3.0] octane-2,6-diol and <Emphasis Type="Italic">s</Emphasis>-Naproxen probed by collisional-induced dissociation

Chiral recognition of racemic bicyclo[3.3.0] octane-2,6-diol(B) was achieved in the gas phase using s-Naproxen(A) as reference, using the kinetics of competitive unimolecule dissociation of tetrameric zinc(II)-bound complexes by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer(ESI-FTMS). As undergoing a mild competitive collision-induced dissociation(CID) experiment with a constant pressure argon gas introduced by leak valve, the tetrameric cluster ion [A(2)B(2)Z(n)(II)-H](+) forms only two trimeric ions and R(chiral) is subsequently obtained in the kinetic method. Further studies obtained the difference of Gibbs free energy of [ABZ(n)(II)-H](+)(Delta Delta G(ABZn(II)-H](+))) by dissociating [A(2)BZ(n)(II)-H](+), resulting two fragment ions [ABZ(n)(II)-H](+) and [A(2)Z(n)(II)-H](+), which can be established to a linear relationship between Delta Delta G([ABZn(II)-H](+)) and R(chiral)' basing on the kinetic method. The value of R(chiral)' suggested that Delta Delta G([ABZn(II)-H](+)) could be regarded as zero. Meanwhile, dissociation of [AB(2)Z(n)(II)-H](+) generated only one daughter ion [ABZ(n)(II)-H](+) in a stable pressure. Thus, a linear relationship was established between the difference of Gibbs free energy of [AB(2)Z(n)(II)-H](+)(Delta Delta G([AB(2)Zn(II)-H](+))) and R(chiral)" if the Delta Delta G([ABZn(II)-H](+)) can be negligible. Because there is also a linear relationship of R(chiral) in the tetrameric ion [A(2)B(2)Z(n)(II)-H](+) and the Gibbs energy difference of trimeric cluster ion [A(2)BZ(n)(+)(II)-H](Delta Delta G([A(2)BZn(II)-H](+))) plus that of [AB(2)Z(n)(II)-H](+), Delta Delta G([A(2)BZ(n)(II)-H]+]) is easy to be calculated in the dissociation process of tetrameric ion. Stable of R(chiral), R(chiral)' and R(chiral)" under different pressures show T(eff) does not affect the chiral recognition of cluster ions in the condition selected. If an only-one-daughter-ion fragment process of [A(2)BZ(n)(II)-H](+) was existed, R(chiral)' relating to this dissociation would be calculated just like R(chiral)" of [AB(2)Z(n)(II)-H](+) does. Conclusion was obtained that [A(2)BZ(n)(II)-H](+) makes more contribution to chiral recognition of tetrameric ion measured by kinetic method than [AB(2)Z(n)(II)-H](+) does as R(chiral)' and R(chiral)" were applied as index to evaluate the Gibbs free energy difference of these two trimeric cluster ions. Further discussion shows that steric interactions and pi-pi stacking interactions are the major factors responsible for the observed efficient chiral recognition in this system.     

Structures of tetrafluorocyclopropene, hexafluorocyclobutene, octafluorocyclopentene and related perfluoroalkene radical anions revealed by electron spin resonance spectroscopic and computational studies

Isotropic and anisotropic ESR spectra were observed for the radical anions of hexafluorocyclobutene (c-C(4)F(6)(-)), octafluorocyclopentene (c-C(5)F(8)(-)) and perfluoro-2-butene (CF(3)CF=CFCF(3)(-)) in gamma-irradiated plastically crystalline neopentane, tetramethylsilane (TMS) and TMS-d(12) matrices, or the rigid 2-methyltetrahydrofuran (MTHF) matrix. The isotropic spectra of c-C(4)F(6)(-) and c-C(5)F(8)(-) are characterized by three different sets of pairs of (19)F nuclei with the isotropic hyperfine (hf) splittings of 15.2 (2F), 6.5 (2F), 1.1 (2F) mT for c-C(4)F(6)(-) and 14.7 (2F), 7.4 (2F), 1.0 (2F) mT for c-C(5)F(8)(-). By comparison with the results of ab initio quantum chemical computations, the large triplet (19)F hf splittings of ca. 15 mT are assigned to the two fluorines attached to the C=C bond. The UHF, B3LYP and MP2 computations predict that the geometrical structures of the perfluoroalkenes are strongly distorted by one-electron reduction to form their radical anions; c-C(3)F(4)(-): C(2) symmetry ((2)A state) <-- C(2)(v) ((1)A(1)), c-C(4)F(6)(-): C(1) ((2)A) <-- C(2)(v) ((1)A(1)) and c-C(5)F(8)(-): C(1) ((2)A) <-- C(s) ((1)A'). The structural distortion arises from a mixing of the pi* and higher-lying sigma* orbitals at the C=C carbons similar to that previously found for CF(2)=CF(2)(-) with a C(2)(h) distortion. The isotropic (19)F hf splittings computed with the B3LYP method with 6-311+G(2df,p) basis set for the geometry optimized by the UHF and/or MP2 methods are within 6% error of the experimental values. The experimental anisotropic spectra of c-C(4)F(6)(-), c-C(5)F(8)(-) and CF(2)=CF(2)(-) were satisfactorily reproduced by the ESR spectral simulation method using the computed hf principal values and orientation of (19)F nuclei. In addition, the electronic excitation energies and oscillator strengths for the CF(2)=CF(2)(-), c-C(3)F(4)(-), c-C(4)F(6)(-) and c-C(5)F(8)(-) radical anions were computed for the first time by TD-DFT methods.     

Application of the valence bond mixing configuration diagrams to hypervalency in trihalide anions: a challenge to the Rundle-Pimentel model

The X(3)(-) hypercoordinated anions (H, F, Cl, Br, I) are studied by means of the breathing-orbital valence bond ab initio method. The valence bond wave functions describe the different X(3)(-) complexes in terms of only six valence bond structures and yield energies relative to the two exit channels, X(2) + X(-) and X(2)(-) + X(*), in very good agreement with reference CCSD(T) calculations. Although H(3)(-) is unstable and dissociates to H(2) + H(-), all the trihalogen anions are stable intermediates, Br(3)(-) and I(3)(-) being more stable than F(3)(-) and Cl(3)(-). As a challenge to the traditional Rundle-Pimentel model, the different energies of the hypercoordinated species relative to the normal-valent dissociation products X(2) + X(-) are interpreted in terms of valence bond configuration mixing diagrams and found to correlate with a single parameter of the X(2) molecule, its singlet-triplet energy gap. Examination of the six-structure wave functions show that H(3)(-), Cl(3)(-), Br(3)(-), and I(3)(-) share the same bonding picture and can be mainly described in terms of the interplay of two Lewis structures. On the other hand, F(3)(-) is bonded in a different way and possesses a significant three-electron bonding character that is responsible for the dissociation of this complex to F(2)(-) + F(*), instead of the more stable products F(2) + F(-). This counterintuitive preference for the thermodynamically disfavored exit channel is found to be an experimental manifestation of the large charge-shift resonance energy that generally characterizes fluorine-containing bonds.     

Validation of the M-C/H-C bond enthalpy relationship through application of density functional theory

Density functional theory has been used to calculate H-C and M-C bond dissociation enthalpies in order to evaluate the feasibility of correlating relative M-C bond enthalpies Delta H(M-C)rel with H-C bond enthalpies Delta H(H-C) via computational methods. This approach has been tested against two experimental correlations: a study of (a) Rh(H)(R)(Tp')(CNCH2CMe3) [R = hydrocarbyl, Tp' = HB(3,5-dimethylpyrazolyl)3] (Wick, D. D.; Jones, W. D. Organometallics 1999, 18, 495) and (b) Ti(R)(silox)2(NHSit-Bu3) (silox = OSit-Bu3) (Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1997, 119, 10696). We show that the observation that M-C bond enthalpies increase more rapidly with different substituents than H-C bond enthalpies is reproduced by theory. Quantitative slopes of the correlation lines are reproduced within 4% of the experimental values with a B3PW91 functional and with very similar correlation coefficients. Absolute bond enthalpies are reproduced within 6% for H-C bonds, and relative bond enthalpies for M-C bonds are reproduced within 30 kJ mol(-1) for Rh-C bonds and within 19 kJ mol(-1) for Ti-C bonds. Values are also calculated with the BP86 functional.     

Increasing stability of the fullerenes with the number of carbon atoms: the experimental evidence

The values of the molar standard enthalpies of formation, Delta(f)H(o)(m)(C(76), cr) = (2705.6 +/- 37.7) kJ x mol(-1), Delta(f)H(o)(m)(C(78), cr) = (2766.5 +/- 36.7) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), cr) = (2826.6 +/- 42.6) kJ x mol(-1), were determined from the energies of combustion, measured by microcombustion calorimetry on a high-purity sample of the D(2) isomer of fullerene C(76), as well as on a mixture of the two most abundant constitutional isomers of C(78) (C(2nu)-C(78) and D(3)-C(78)) and C(84) (D(2)-C(84), and D(2d)-C(84). These values, combined with the published data on the enthalpies of sublimation of each cluster, lead to the gas-phase enthalpies of formation, Delta(f)H(o)(m)(C(76), g) = (2911.6 +/- 37.9) kJ x mol(-1); Delta(f)H(o)(m)(C(78), g) = (2979.3 +/- 37.2) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), (g)) = (3051.6 +/- 43.0) kJ x mol(-1), results that were found to compare well with those reported from density functional theory calculations. Values of enthalpies of atomization, strain energies, and the average C-C bond energy were also derived for each fullerene. A decreasing trend in the gas-phase enthalpy of formation and strain energy per carbon atom as the size of the cluster increases is found. This is the first experimental evidence that these fullerenes become more stable as they become larger. The derived experimental average C-C bond energy E(C-C) = 461.04 kJ x mol(-1) for fullerenes is close to the average bond energy E(C-C) = 462.8 kJ x mol(-1) for polycyclic aromatic hydrocarbons (PAHs).     

Infrared spectra of SF6-.(H2O)n (n=1-3): incipient reaction and delayed onset of water network formation

We present data on the microsolvation of an extended charge distribution with SF(6)(-) as a model system. Infrared spectroscopy, aided by ab initio calculations, shows that the first two water molecules attach to the ion by a combination of single ionic H bonds, sharing one of the F atoms, and weak electrostatic interactions with other F atoms in the ion. No water-water bonds are formed at the dihydrate level, which is an unusual observation, given the strong propensity of water to form H-bonded networks. The onset of water networks occurs with the addition of the third water molecule. Moreover, the attachment of the first two water molecules considerably weakens the SF bond of the F atom involved in bonding to both ligands, indicating a possible mechanism for water-induced reactions.     

Influence of ligand polarizability on the reversible binding of O2 by trans-[Rh(X)(XNC)(PPh3)2] (X = Cl, Br, SC6F5, C2Ph; XNC = xylyl isocyanide). Structures and a kinetic study

The complexes trans-[Rh(X)(XNC)(PPh 3) 2] (X = Cl, 1; Br, 2; SC 6F 5, 3; C 2Ph, 4; XNC = xylyl isocyanide) combine reversibly with molecular oxygen to give [Rh(X)(O 2)(XNC)(PPh 3) 2] of which [Rh(SC 6F 5)(O 2)(XNC)(PPh 3) 2] ( 7) and [Rh(C 2Ph)(O 2)(XNC)(PPh 3) 2] ( 8) are sufficiently stable to be isolated in crystalline form. Complexes 2, 3, 4, and 7 have been structurally characterized. Kinetic data for the dissociation of O 2 from the dioxygen adducts of 1- 4 were obtained using (31)P NMR to monitor changes in the concentration of [Rh(X)(O 2)(XNC)(PPh 3) 2] (X = Cl, Br, SC 6F 5, C 2Ph) resulting from the bubbling of argon through the respective warmed solutions (solvent chlorobenzene). From data recorded at temperatures in the range 30-70 degrees C, activation parameters were obtained as follows: Delta H (++) (kJ mol (-1)): 31.7 +/- 1.6 (X = Cl), 52.1 +/- 4.3 (X = Br), 66.0 +/- 5.8 (X = SC 6F 5), 101.3 +/- 1.8 (X = C 2Ph); Delta S (++) (J K (-1) mol (-1)): -170.3 +/- 5.0 (X = Cl), -120 +/- 13.6 (X = Br), -89 +/- 18.2 (X = SC 6F 5), -6.4 +/- 5.4 (X = C 2Ph). The values of Delta H (++) and Delta S (++) are closely correlated (R (2) = 0.9997), consistent with a common dissociation pathway along which the rate-determining step occurs at a different position for each X. Relative magnitudes of Delta H (++) are interpreted in terms of differing polarizabilities of ligands X.     

Very strong hydrogen bonds in a bent chain structure of fluorohydrogenate anions in liquid Cs(FH)2.3F

The structure of liquid Cs(FH)(2.3)F was revealed using a combination of high-energy x-ray and neutron diffraction measurements. We found that the strongest intermolecular H-F hydrogen bonds at an average distance of 1.36 A are accompanied by the formation of a high degree of bending of the oligomer chain in the melt, with [angle]FHF=150 degrees . A reverse Monte Carlo simulation showed that the average number of atoms per chain is 4.4. A detailed chain analysis of the atomic configuration revealed that (FH)(2)F(-) oligomer chains are the major entities in the liquid, and asymmetrical FHF(-) are formed owing to the strong H-F hydrogen bonds. The results suggest that an average of one or two HF molecules bond to each of the 11 fluorine atoms surrounding a cesium ion.     

Marked counteranion effects on single-site olefin polymerization processes. Correlations of ion pair structure and dynamics with polymerization activity, chain transfer, and syndioselectivity

Counteranion effects on the rate and stereochemistry of syndiotactic propylene enchainment by the archetypal C(s)-symmetric precatalyst [Me(2)C(Cp)(Flu)]ZrMe(2) (1; Cp = C(5)H(4); Flu = C(13)H(8), fluorenyl) are probed using the cocatalysts MAO (2), B(C(6)F(5))(3) (3)(,) B(2-C(6)F(5)C(6)F(4))(3) (4)(,) Ph(3)C(+)B(C(6)F(5))(4)(-) (5), and Ph(3)C(+)FAl(2-C(6)F(5)C(6)F(4))(3)(-) (6), offering greatly different structural and ion pairing characteristics. Reaction of 1 with 3 affords [Me(2)C(Cp)(Flu)]ZrMe(+) MeB(C(6)F(5))(3)(-) (7). In the case of 4, this reaction leads to formation the micro-methyl dinuclear diastereomers [([Me(2)C(Cp)(Flu)]ZrMe)(2)(micro-Me)](+) MeB(2-C(6)F(5)C(6)F(4))(3)(-) (8). A similar reaction with 6 results in diastereomeric [Me(2)C(Cp)(Flu)]ZrMe(+) FAl(2-C(6)F(5)C(6)F(4))(3)(-) (10) ion pairs. The molecular structures of 7 and 10 have been determined by single-crystal X-ray diffraction. Reorganization pathways available to these species have been examined using EXSY and dynamic NMR, revealing that the cation-MeB(C(6)F(5))(3)(-) interaction is considerably weaker/more mobile than in the FAl(2-C(6)F(5)C(6)F(4))(3)(-)-derived analogue. Polymerizations mediated by 1 in toluene over the temperature range of -10 degrees to +60 degrees C and at 1.0-5.0 atm propylene pressure (at 60 degrees C) reveal that activity, product syndiotacticity, m and mm stereodefect generation, and chain transfer processes are highly sensitive to the nature of the ion pairing. Thus, the complexes activated with 4 and 5, having the weakest ion pairing, yield the highest estimated propagation rates, while with 6, having the strongest pairing, yields the lowest. The strongly coordinating, immobile FAl(2-C(6)F(5)C(6)F(4))(3)(-) anion produces the highest/least temperature-dependent product syndiotacticity, lowest/least temperature-dependent m stereodefect abundance, and highest product molecular weight. These polypropylene microstructural parameters, and also M(w), are least sensitive to increased propylene pressure for FAl(2-C(6)F(5)C(6)F(4))(3)(-), but highest with MeB(C(6)F(5))(3)(-). In general, mm stereodefect production is only modestly anion-sensitive; [propylene] dependence studies reveal enantiofacial propylene misinsertion to be the prevailing mm-generating process in all systems at 60 degrees C, being most dominant with 6, where mm stereodefect abundance is lowest. For 1,3-dichlorobenzene as the polymerization solvent, product syndiotacticity, as well as m and mm stereodefects, become indistinguishable for all cocatalysts. These observations are consistent with a scenario in which ion pairing modulates the rates of stereodefect generating processes relative to monomer enchainment, hence net enchainment syndioselectivity, and also dictates the rate of termination relative to propagation and the preferred termination pathway. In comparison to 3-6, propylene polymerization mediated by MAO (2) + 1 in toluene reveals an estimated ordering in site epimerization rates as 5 > 4 > 2 > 3 > 6, while product syndiotacticities rank as 6 > 2 > 5 approximately 4 > 3.     

Vacuum-UV negative photoion spectroscopy of SF5CF3

Ion pair formation, generically described as AB-->A(+)+B(-), from vacuum-UV photoexcitation of trifluoromethyl sulfur pentafluoride, SF(5)CF(3), has been studied by anion mass spectrometry using synchrotron radiation in the photon energy range of 10-35 eV. The anions F(-), F(2)(-), and SF(x)(-) (x=1-5) are observed. With the exception of SF(5)(-), the anions observed show a linear dependence of signal with pressure, showing that they arise from ion pair formation. SF(5)(-) arises from dissociative electron attachment, following photoionization of SF(5)CF(3) as the source of low-energy electrons. Cross sections for anion production are put on to an absolute scale by calibration of the signal strengths with those of F(-) from both SF(6) and CF(4). Quantum yields for anion production from SF(5)CF(3), spanning the range of 10(-7)-10(-4), are obtained using vacuum-UV absorption cross sections. Unlike SF(6) and CF(4), the quantum yield for F(-) production from SF(5)CF(3) increases above the onset of photoionization.     

X-ray structures and anionotropic rearrangements of Di-tert-butyl-substituted thiiranium and thiirenium ions. A structure-reactivity relationship

The X-ray structures of c-2,t-3-di-tert-butyl-r-1-methylthiiranium 8 BF(4)(-), t-2,t-3-di-tert-butyl-r-1-methylthiiranium ion 10 BF(4)(-), and 2,3-di-tert-butyl-1-methylthiirenium 11 BF(4)(-) have been determined. The DeltaG()(298) values for the rearrangements from the cis and the trans tert-butyl groups of 8 SbCl(6)(-) to thietanium ion (two intramolecular S(N)2 displacements) and for the rearrangement of 11 SbCl(6)(-) to thietium ion (an intramolecular S(N)2-Vin displacement) are linearly correlated with the strengths of the C-S breaking bonds, suggesting that the two mechanisms are, in the absence of steric hindrance, uniquely governed by the nucleofugality of the sulfonium leaving group.     

Toward the comparison of rare earth element and actinide behavior in materials: a computational study of Ce- and U-bearing britholites

We present a computational investigation into the nature of bonds formed by f-elements in materials. The paper presents an example of the incorporation of rare earth elements (REE) and actinides in minerals derived from fluorapatite: Ca(10)(PO(4))(6)F(2). These minerals, called britholites, allow many substitutions on all three Ca, P, and F sites and are considered as potential host phases for radioactive elements separated from nuclear waste. REE and actinides have very similar physical and chemical properties, but REE are not radioactive and much more easily handled. REE are, therefore, very often used as a surrogate for actinides in experimental studies. The representative elements of rare earths and actinides chosen for this first investigation are cerium and uranium, respectively. We have studied all the various configurations of Ca(9)X(PO(4))(6)(-)(y)()(SiO(4))(y)()F(2), where X stands for Ce(3+), Ce(4+), U(3+), and U(4+), and y is equal to 1 and 2 for three-time and four-time charged cations, respectively. Calculations have been performed within the density functional theory (DFT) framework according to the computation scheme determined in a previous study. The analysis of the energies of the various configurations shows that the incorporation of all the cations considered stabilizes the apatitic structure. This stabilization, however, is greater for four-time charged cations than for three-time charged ones, which shows that Ce and U are both preferentially substituted in the +IV oxidation state. In addition, the substitution in one of the two cationic sites of the apatitic structure is always more favorable. Then, the geometry analysis shows a larger decrease in size of this cationic site for U than for Ce, as well as different volume variations for Ce and U substitutions in the two cationic sites. This cannot be explained by steric effects alone. Finally, the electronic density analysis yields three essential results: U and Ce form significantly covalent bonds, U forms bonds more covalent than Ce, and finally four-time charged cations form more covalent bonds than three-time charged ones. The comparison of these results with the formation enthalpies of the various phases shows a positive correlation between the covalence degree of the bonds formed by the f-element and the stability of the structure. In addition, our results prove that Ce- and U-bearing britholites exhibit very similar energetic, structural, and electronic properties. Ce, therefore, appears to be a good simulant for U.     

Transition-state energy and position along the reaction coordinate in an extended activation strain model.

We investigate palladium-induced activation of the C-H, C-C, C-F, and C-Cl bonds in methane, ethane, cyclopropane, fluoromethane, and chloromethane, using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Our purpose is to arrive at a qualitative understanding, based on accurate calculations, of the trends in activation barriers and transition state (TS) geometries (e.g. early or late along the reaction coordinate) in terms of the reactants' properties. To this end, we extend the activation strain model (in which the activation energy Delta E(not equal) is decomposed into the activation strain Delta E(not equal)(strain) of the reactants and the stabilizing TS interaction Delta E(not equal)(int) between the reactants) from a single-point analysis of the TS to an analysis along the reaction coordinate zeta, that is, Delta E(zeta)=Delta E(strain)(zeta)+Delta E(int)(zeta). This extension enables us to understand qualitatively, trends in the position of the TS along zeta and, therefore, the values of the activation strain Delta E(not equal)(strain)=Delta E(strain)(zeta(TS)) and TS interaction Delta E(not equal)(int)=Delta E(int)(zeta(TS)) and trends therein. An interesting insight that emerges is that the much higher barrier of metal-mediated C-C versus C-H activation originates from steric shielding of the C-C bond in ethane by C-H bonds. Thus, before a favorable stabilizing interaction with the C-C bond can occur, the C-H bonds must be bent away, which causes the metal-substrate interaction Delta E(int)(zeta) in C-C activation to lag behind. Such steric shielding is not present in the metal-mediated activation of the C-H bond, which is always accessible from the hydrogen side. Other phenomena that are addressed are anion assistance, competition between direct oxidative insertion (OxIn) versus the alternative S(N)2 pathway, and the effect of ring strain.     

Structure and Thermodynamics of NaF-AlF(3) Melts with Addition of CaF(2) and MgF(2)

The NaF-AlF(3) system with additions of CaF(2) and MgF(2) has been studied with Raman and vapor pressure measurements for 3 >/= CR (NaF/AlF(3) molar ratio) >/= 1 and up to 50 mol % additive. The results show that the binary melt can be described using the two equilibria AlF(6)(3)(-) = AlF(6)(2)(-) + F(-) and AlF(5)(2)(-) = AlF(4)(-) + F(-) with equilibrium constants 0.25 and 0.05, respectively, at 1293 K. Both reactions have positive reaction enthalpies. The first equilibrium is strongly shifted to the right resulting in a melt mixture with very low AlF(6)(3)(-) concentrations even at the Na(3)AlF(6) composition. Evidence for nonideal mixing of anions was found. For the ternaries, models based on Raman data are presented and compared with vapor pressure measurements. Good agreement is observed when association between the additives, CaF(2) or MgF(2), with the AlF(5)(2)(-) ions in the melt was considered. This association could be experimentally observed through a band broadening and a slight shift in the AlF(5)(2)(-) band frequency. Our vapor pressures and Raman data both indicate that MgF(2) clearly acts as an acid when added to NaF-AlF(3) melts of any composition. When CaF(2) is added, a slight decrease of vapor pressure occurs. Raman data indicate a decrease of AlF(4)(-) concentration, corresponding to a dissociation of CaF(2) with liberation of F(-) ions. All these results are, however, very much dependent on the initial melt composition. These data are explained in terms of acid-base, dilution, and association reactions of the solute with the solvent.     

Relationship between symmetry of porphyrinic pi-conjugated systems and singlet oxygen (1Delta g) yields: low-symmetry tetraazaporphyrin derivatives

We have investigated the excited-state properties and singlet oxygen ((1)Delta(g)) generation mechanism in phthalocyanines (4M; M = H(2), Mg, or Zn) and in low-symmetry metal-free, magnesium, and zinc tetraazaporphyrins (TAPs), that is, monobenzo-substituted (1M), adjacently dibenzo-substituted (2AdM), oppositely dibenzo-substituted (2OpM), and tribenzo-substituted (3M) TAP derivatives, whose pi conjugated systems were altered by fusing benzo rings. The S(1)(x) and S(1)(y) states (these lowest excited singlet states are degenerate in D(4)(h) symmetry) split in the low-symmetry TAP derivatives. The excited-state energies were quantitatively determined from the electronic absorption spectra. The lowest excited triplet (T(1)(x)) energies were also determined from phosphorescence spectra, while the second lowest excited triplet (T(1)(y)) states were evaluated by using the energy splitting between the T(1)(x) and T(1)(y) states previously reported (Miwa, H.; Ishii, K.; Kobayashi, N. Chem. Eur. J. 2004, 10, 4422-4435). The singlet oxygen quantum yields (Phi(Delta)) are strongly dependent on the pi conjugated system. In particular, while the Phi(Delta) value of 2AdH(2) is smallest in our system, that of 2OpH(2), an isomer of 2AdH(2), is larger than that of 4Zn, in contrast to the heavy atom effect. The relationship between the molecular structure and Phi(Delta) values can be transformed into a relationship between the S(1)(x) --> T(1)(y) intersystem crossing rate constant (k(ISC)) and the energy difference between the S(1)(x) and T(1)(y) states (DeltaE(S)(x)(T)(y)). In each of the Zn, Mg, and metal-free compounds, the Phi(Delta)/tau(F) values (tau(F): fluorescence lifetime), which are related to the k(ISC) values, are proportional to exp(-DeltaE(S)(x)(T)(y)), indicating that singlet oxygen ((1)Delta(g)) is produced via the T(1)(y) state and that the S(1)(x) --> T(1)(y) ISC process follows the energy-gap law. From the viewpoint of photodynamic therapy, our methodology, where the Phi(Delta) value can be controlled by changing the symmetry of pi conjugated systems without heavy elements, appears useful for preparing novel photosensitizers.     

Quantitative measure for the "nakedness" of fluoride ion sources

A quantitative measure for the donor strength or "nakedness" of fluoride ion donors is presented. It is based on the free energy change associated with the transfer of a fluoride ion from the donor to a given acceptor molecule. Born-Haber cycle calculations were used to calculate both the free energy and the enthalpy change for this process. The enthalpy change is given by the sum of the fluoride ion affinity of the acceptor (as defined in strict thermodynamic convention) and the lattice energy difference (DeltaU(POT)) between the fluoride ion donor and the salt formed with the acceptor. Because, for a given acceptor, the fluoride affinity has a constant value, the relative enthalpy (and also the corresponding free energy) changes are governed exclusively by the lattice energy differences. In this study, BF(3), PF(5), AsF(5), and SbF(5) were used as the acceptors, and the following seven fluoride ion donors were evaluated: CsF, N(CH(3))(4)F (TMAF), N-methylurotropinium fluoride (MUF), hexamethylguanidinium fluoride (HMGF), hexamethylpiperidinium fluoride (HMPF), N,N,N-trimethyl-1-adamantylammonium fluoride (TMAAF), and hexakis(dimethylamino)phosphazenium fluoride (HDMAPF). Smooth relationships between the enthalpy changes and the molar volumes of the donor cations were found which asymptotically approach constant values for infinitely large cations. Whereas CsF is a relatively poor F(-) donor [(U(POT)(CsF) - U(POT)(CsSbF(6))) = 213 kJ mol(-)(1)], when compared to N(CH(3))(4)F [(U(POT)(TMAF) - U(POT)(TMASbF(6))) = 69 kJ mol(-)(1)], a 4 times larger cation (phosphazenium salt) and an infinitely large cation are required to decrease DeltaU(POT) to 17 and 0 kJ mol(-)(1), respectively. These results clearly demonstrate that very little is gained by increasing the cation size past a certain level and that secondary factors, such as chemical and physical properties, become overriding considerations.     

A Novel Inorganic-organic Hybrid Borate with Unique One-dimensional Infinite Aluminium-oxygen Chains

A novel inorganic-organic hybrid borate,[Al2(fum)(H3BO3)(OH) 4]n·n(H3BO3) (1,H2fum = fumaric acid) ,has been synthesized and characterized by single-crystal X-ray diffraction,FTIR and elemental analysis. Crystal data for compound 1: orthorhombic,space group Pnma,a = 14.108(3) ,b = 6.9412(14) ,c = 14.995(3) ,V = 1468.3(5) 3,Z = 4,Mr = 359.72,Dc = 1.627 g/cm3,μ = 0.254 mm-1,F(000) = 736,the final R = 0.0492 and wR = 0.1650 with I > 2σ(I) . In compound 1,each AlⅢ ion is coordinated by six oxygen atoms to adopt a distorted octahedral geometry. Both fumarate anion and the coordinated boric acid act as bidentate bridging ligands to link two neighboring AlⅢ centers simultaneously. Each AlⅢ ion is bridged by two μ2-hydroxyl ligands to construct an infinite wave-like [Al2(fum)(H3BO3)(OH) 4]n chain. These one-dimensional chains form hydrogen bonds with free boric acid molecules giving rise to a three-dimensional supramolecular network.     

Dichlororuthenium(IV) complex of meso-tetrakis(2,6-dichlorophenyl)porphyrin: active and robust catalyst for highly selective oxidation of arenes, unsaturated steroids, and electron-deficient alkenes by using 2,6-dichloropyridine N-oxide

[Ru(IV)(2,6-Cl2tpp)Cl2], prepared in 90 % yield from the reaction of [Ru(VI)(2,6-Cl2tpp)O2] with Me3SiCl and structurally characterized by X-ray crystallography, is markedly superior to [Ru(IV)(tmp)Cl2], [Ru(IV)(ttp)Cl2], and [Ru(II)(por)(CO)] (por=2,6-Cl2tpp, F20-tpp, F28-tpp) as a catalyst for alkene epoxidation with 2,6-Cl2pyNO (2,6-Cl2tpp=meso-tetrakis(2,6-dichlorophenyl)porphyrinato dianion; tmp=meso-tetramesitylporphyrinato dianion; ttp=meso-tetrakis(p-tolyl)porphyrinato dianion; F20-tpp=meso-tetrakis(pentafluorophenyl)porphyrinato dianion; F28-tpp=2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato dianion). The "[Ru(IV)(2,6-Cl2tpp)Cl2]+2,6-Cl2pyNO" protocol oxidized, under acid-free conditions, a wide variety of hydrocarbons including 1) cycloalkenes, conjugated enynes, electron-deficient alkenes (to afford epoxides), 2) arenes (to afford quinones), and 3) Delta5-unsaturated steroids, Delta4-3-ketosteroids, and estratetraene derivatives (to afford epoxide/ketone derivatives of steroids) in up to 99 % product yield within several hours with up to 100 % substrate conversion and excellent regio- or diastereoselectivity. Catalyst [Ru(IV)(2,6-Cl2tpp)Cl2] is remarkably active and robust toward the above oxidation reactions, and turnover numbers of up to 6.4x10(3), 2.0x10(4), and 1.6x10(4) were obtained for the oxidation of alpha,beta-unsaturated ketones, arenes, and Delta5-unsaturated steroids, respectively.     

Thermochemistry of the fluoroformyloxyl radical: a computational study based on coupled cluster theory

The standard enthalpy of formation of FCO(2) (X (2)B(2)) was determined by a computational approach based on coupled cluster theory [CCSD(T)] with energies extrapolated to the basis-set limit, with additional corrections accounting for core-valence correlation, scalar relativity, spin-orbit coupling, and zero-point vibrational motions. Utilizing a variety of independent reaction schemes, our best estimate is Delta(f)H(o)(0)(FCO(2)) = -86.0 +/- 0.6 kcal mol(-1) [Delta(f)H(o)(298) )(FCO(2)) = -86.7 +/- 0.6 kcal mol(-1)], which is shown to be more accurate than previous theoretical and experimental values. The chosen computational procedure was also applied to HCO (X (2)A'), where we find excellent agreement with experiment, and to FCO (X (2)A'), where we recommend an improved value of Delta(f)H(o)(0)(FCO) = -42.1 +/- 0.5 kcal mol(-1) [ Delta(f)H(o)(298)(FCO) = -42.0 +/- 0.5 kcal mol(-1)]. Further theoretical results concern the C-F bond dissociation energy, electron affinity, ionization energy, first and second excitation energies in FCO(2), fluoride ion affinity of CO(2), and equilibrium geometries of the molecules treated presently. For FCO (X (2)A') we propose an improved equilibrium structure: r(e)(CF) = 132.5(2) pm, r(e)(CO) = 116.7(2) pm, and theta(e)(FCO) = 127.8(2)(o).