Underpotential deposition of hydrogen on benzene-modified Pt(111) in aqueous H2SO4

The Pt(111) electrode is modified by an overlayer of C6H6 (ads) upon its cycling in the 0.05-0.80 V range in aq H2SO4 + 1 mM C6H6. The C6H6 (ads) overlayer significantly changes the underpotential-deposited H (H(UPD)) and anion adsorption, and cyclic-voltammetry (CV) profiles show a sharp cathodic peak and an asymmetric anodic one in the 0.05-0.80 V potential range. The C6H6 (ads) layer blocks the (bi)sulfate adsorption but facilitates the adsorption of one monolayer of H(UPD). Cycling of the benzene-modified Pt(111) in benzene-free aq 0.05 H2SO4 from 0.05 to 0.80 V results in a partial desorption of C6H6 (ads) and in a partial recovery of the CV profile characteristic of an unmodified Pt(111). The peak potential of the cathodic and anodic feature is independent of the scan rate, s (10 < or = s < or = 100 mV s(-1)), and the peak current density increases linearly with an increase of the scan rate. Temperature variation modifies the peak potential and current density but does not affect the charge density of the cathodic or anodic feature. Temperature-dependent studies allow us to determine the thermodynamic state function for the H(UPD) adsorption and desorption. Delta G degrees(ads)(H(UPD))assumes values from -4 to -12 kJ mol(-1), while has values from 9 to 14 kJ mol(-1). The values of delta Delta G degrees (delta Delta G degrees = delat Delta G degrees(ads) + delta Delta D degrees(des)) decrease almost linearly from 6 kJ mol(-1) at theta(H(UPD) --> 0 to 0 kJ mol(-1) at theta(H(UPD) --> 1. The nonzero values of delta Delta G degrees testify that the adsorbing and desorbing H(UPD) adatoms interact with an energetically different substrate. The lateral interactions changed from repulsive (omega = 29 kJ mol(-1) at theta(H(UPD) --> 0) to attractive (omega = -28 kJ mol(-1) at theta(H(UPD) --> 1) as the H(UPD) coverage increases. The values of delta S degrees(ads)(H(UPD)) increase from 19 to 56 J K(-1) mol(-1), while those of delta S degrees(des)(H(UPD)) decrease from 45 to -30 J K(-1) mol(-1) with an increase of H(UPD) coverage. The values of delta H degrees(des)(H(UPD)) and delta H degrees(des)(H(UPD)) vary from 0 to 27 kJ mol(-1). The Pt(111)-H(UPD) surface bond energy at the benzene-modified Pt(111) electrode falls in the 191-218 kJ mol(-1) range and is weaker than in the case of the unmodified Pt(111) electrode in the same electrolyte.     

Calorimetric and computational study of thiacyclohexane 1-oxide and thiacyclohexane 1,1-dioxide (thiane sulfoxide and thiane sulfone). Enthalpies of formation and the energy of the S=O bond

A rotating-bomb combustion calorimeter specifically designed for the study of sulfur-containing compounds [J. Chem. Thermodyn. 1999, 31, 635] has been used for the determination of the enthalpy of formation of thiane sulfone, 4, Delta(f)H(o) m(g) = -394.8 +/- 1.5 kJ x mol(-1). This value stands in stark contrast with the enthalpy of formation reported for thiane itself, Delta(f)H(o) m(g) = -63.5 +/- 1.0 kJ x mol(-1), and gives evidence of the increased electronegativity of the sulfur atom in the sulfonyl group, which leads to significantly stronger C-SO2 bonds. Given the known enthalpy of formation of atomic oxygen in the gas phase, Delta(f)H(o) m(O,g) = +249.18 kJ x mol(-1), and the reported bond dissociation energy for the S=O bond in alkyl sulfones, BDE(S=O) = +470.0 kJ x mol(-1), it was possible to estimate the enthalpy of formation of thiane sulfoxide, 5, a hygroscopic compound not easy to use in experimental calorimetric measurements, Delta(f)H(o) m(5) = -174.0 kJ x mol(-1). The experimental enthalpy of formation of both 4 and 5 were closely reproduced by theoretical calculations at the G2(MP2)+ level, Delta(f)H(o) m(4) = -395.0 kJ x mol(-1) and Delta(f)H(o) m(5) = -178.0 kJ x mol(-1). Finally, calculated G2(MP2)+ values for the bond dissociation energy of the S=O bond in cyclic sulfoxide 5 and sulfone 4 are +363.7 and +466.2 kJ x mol(-1), respectively.     

Energetics of cresols and of methylphenoxyl radicals

Combustion calorimetry studies were used to determine the standard molar enthalpies of formation of o-, m-, and p-cresols, at 298.15 K, in the condensed state as Delta(f)H(m) degrees (o-CH(3)C(6)H(4)OH,cr) = -204.2 +/- 2.7 kJ.mol(-1), Delta(f)H(m) degrees (m-CH(3)C(6)H(4)OH,l) = -196.6 +/- 2.1 kJ.mol(-1), and Delta(f)H(m) degrees (p-CH(3)C(6)H(4)OH,cr) = -202.2 +/- 3.0 kJ.mol(-1). Calvet drop calorimetric measurements led to the following enthalpy of sublimation and vaporization values at 298.15 K: Delta(sub)H(m) degrees (o-CH(3)C(6)H(4)OH) = 73.74 +/- 0.46 kJ.mol(-1), Delta(vap)H(m) degrees (m-CH(3)C(6)H(4)OH) = 64.96 +/- 0.69 kJ.mol(-1), and Delta(sub)H(m) degrees (p-CH(3)C(6)H(4)OH) = 73.13 +/- 0.56 kJ.mol(-1). From the obtained Delta(f)H(m) degrees (l/cr) and Delta(vap)H(m) degrees /Delta(sub)H(m) degrees values, it was possible to derive Delta(f)H(m) degrees (o-CH(3)C(6)H(4)OH,g) = -130.5 +/- 2.7 kJ.mol(-1), Delta(f)H(m) degrees (m-CH(3)C(6)H(4)OH,g) = -131.6 +/- 2.2 kJ.mol(-1), and Delta(f)H(m) degrees (p-CH(3)C(6)H(4)OH,g) = -129.1 +/- 3.1 kJ.mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by the B3LYP/cc-pVDZ, B3LYP/cc-pVTZ, B3P86/cc-pVDZ, B3P86/cc-pVTZ, MPW1PW91/cc-pVTZ, CBS-QB3, and CCSD/cc-pVDZ//B3LYP/cc-pVTZ methods, were used to obtain the differences between the enthalpy of formation of the phenoxyl radical and the enthalpies of formation of the three methylphenoxyl radicals: Delta(f)H(m) degrees (C(6)H(5)O*,g) - Delta(f)H(m) degrees (o-CH(3)C(6)H(4)O*,g) = 42.2 +/- 2.8 kJ.mol(-1), Delta(f)H(m) degrees (C(6)H(5)O*,g) - Delta(f)H(m) degrees (m-CH(3)C(6)H(4)O*,g) = 36.1 +/- 2.4 kJ.mol(-1), and Delta(f)H(m) degrees (C(6)H(5)O*,g) - Delta(f)H(m) degrees (p-CH(3)C(6)H(4)O*,g) = 38.6 +/- 3.2 kJ.mol(-1). The corresponding differences in O-H bond dissociation enthalpies were also derived as DH degrees (C(6)H(5)O-H) - DH degrees (o-CH(3)C(6)H(4)O-H) = 8.1 +/- 4.0 kJ.mol(-1), DH degrees (C(6)H(5)O-H) - DH degrees (m-CH(3)C(6)H(4)O-H) = 0.9 +/- 3.4 kJ.mol(-1), and DH degrees (C(6)H(5)O-H) - DH degrees (p-CH(3)C(6)H(4)O-H) = 5.9 +/- 4.5 kJ.mol(-1). Based on the differences in Gibbs energies of formation obtained from the enthalpic data mentioned above and from published or calculated entropy values, it is concluded that the relative stability of the cresols varies according to p-cresol < m-cresol < o-cresol, and that of the radicals follows the trend m-methylphenoxyl < p-methylphenoxyl < o-methylphenoxyl. It is also found that these tendencies are enthalpically controlled.     

Energetics of hydroxybenzoic acids and of the corresponding carboxyphenoxyl radicals. Intramolecular hydrogen bonding in 2-hydroxybenzoic acid

The energetics of the phenolic O-H bond in the three hydroxybenzoic acid isomers and of the intramolecular hydrogen O-H- - -O-C bond in 2-hydroxybenzoic acid, 2-OHBA, were investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of monoclinic 3- and 4-hydroxybenzoic acids, at 298.15 K, were determined as Delta(f)(3-OHBA, cr) = -593.9 +/- 2.0 kJ x mol(-1) and Delta(f)(4-OHBA, cr) = -597.2 +/- 1.4 kJ x mol(-1), by combustion calorimetry. Calvet drop-sublimation calorimetric measurements on monoclinic samples of 2-, 3-, and 4-OHBA, led to the following enthalpy of sublimation values at 298.15 K: Delta(sub)(2-OHBA) = 94.4 +/- 0.4 kJ x mol(-1), Delta(sub)(3-OHBA) = 118.3 +/- 1.1 kJ x mol(-1), and Delta(sub)(4-OHBA) = 117.0 +/- 0.5 kJ x mol(-1). From the obtained Delta(f)(cr) and Delta(sub) values and the previously reported enthalpy of formation of monoclinic 2-OHBA (-591.7 +/- 1.3 kJ x mol(-1)), it was possible to derive Delta(f)(2-OHBA, g) = -497.3 +/- 1.4 kJ x mol(-1), Delta(f)(3-OHBA, g) = -475.6 +/- 2.3 kJ x mol(-1), and Delta(f)(4-OHBA, cr) = -480.2 +/- 1.5 kJ x mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3PW91/aug-cc-pVDZ, MPW1PW91/aug-cc-pVDZ, and MPW1PW91/aug-cc-pVTZ) and the CBS-QMPW1 methods, were used to derive the enthalpies of formation of the gaseous 2-, 3-, and 4-carboxyphenoxyl radicals as (2-HOOCC(6)H(4)O(*), g) = -322.5 +/- 3.0 kJ.mol(-1) Delta(f)(3-HOOCC(6)H(4)O(*), g) = -310.0 +/- 3.0 kJ x mol(-1), and Delta(f)(4-HOOCC(6)H(4)O(*), g) = -318.2 +/- 3.0 kJ x mol(-1). The O-H bond dissociation enthalpies in 2-OHBA, 3-OHBA, and 4-OHBA were 392.8 +/- 3.3, 383.6 +/- 3.8, and 380.0 +/- 3.4 kJ x mol(-1), respectively. Finally, by using the ortho-para method, it was found that the H- - -O intramolecular hydrogen bond in the 2-carboxyphenoxyl radical is 25.7 kJ x mol(-1), which is ca. 6-9 kJ x mol(-1) above the one estimated in its parent (2-OHBA), viz. 20.2 kJ x mol(-1) (theoretical) or 17.1 +/- 2.1 kJ x mol(-1) (experimental).     

Gas-phase acidities and O-H bond dissociation enthalpies of phenol, 3-methylphenol, 2,4,6-trimethylphenol, and ethanoic acid

Energy-resolved, competitive threshold collision-induced dissociation (TCID) methods are used to measure the gas-phase acidities of phenol, 3-methylphenol, 2,4,6-trimethylphenol, and ethanoic acid relative to hydrogen cyanide, hydrogen sulfide, and the hydroperoxyl radical using guided ion beam tandem mass spectrometry. The gas-phase acidities of Delta(acid)H298(C6H5OH) = 1456 +/- 4 kJ/mol, Delta(acid)H298(3-CH3C6H4OH) = 1457 +/- 5 kJ/mol, Delta(acid)H298(2,4,6-(CH3)3C6H2OH) = 1456 +/- 4 kJ/mol, and Delta(acid)H298(CH3COOH) = 1457 +/- 6 kJ/mol are determined. The O-H bond dissociation enthalpy of D298(C6H5O-H) = 361 +/- 4 kJ/mol is derived using the previously published experimental electron affinity for C6H5O, and thermochemical values for the other species are reported. A comparison of the new TCID values with both experimental and theoretical values from the literature is presented.     

Dissociation dynamics of sequential ionic reactions: heats of formation of tri-, di-, and monoethylphosphine

The sequential ethene (C2H4) loss channels of energy-selected ethylphosphine ions have been studied using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy in which ion time-of-flight (TOF) distributions are recorded as a function of the photon energy. The ion TOF distributions and breakdown diagrams have been modeled in terms of the statistical RRKM theory for unimolecular reactions, providing 0 K dissociation onsets, E0, for the ethene loss channels. Three RRKM curves were used to model the five measurements, since two of the reactions differ only by the internal energy of the parent ion. This series of dissociations provides a detailed check of the calculation of the product energy distribution for sequential reactions. From the determined E0's, the heats of formation of several ethylphosphine neutrals and ions have been determined: Delta(f)H degrees 298K[P(C(2)H(5))3] = -152.7 +/- 2.8 kJ/mol, Delta(f)H degrees 298K[P(C(2)H(5))3+] = 571.6 +/- 4.0 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2] = -89.6 +/- 2.1 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2+] = 669.9 +/- 2.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)] = -36.5 +/- 1.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)+] = 784.0 +/- 1.9 kJ/mol. These values have been supported by G2 and G3 calculations using isodesmic reactions. Coupled cluster calculations have been used to show that the C2H4 loss channel, which involves a hydrogen transfer step, proceeds without a reverse energy barrier.     

Thermodynamics of

Formation of cobalt(II)-thiocyanato complexes in nonionic surfactant solutions of poly(ethylene oxide) type with varying poly(ethylene oxide) chain lengths of 7.5 (Triton X-114), 30 (Triton X-305), and 40 (Triton X-405) has been studied by titration spectrophotometry and calorimetry at 298 K. Data were analyzed by assuming formation of a series of ternary complexes Co(NCS)(n)Y(m)((2-n)+) (Y=surfactant) with an overall formation constant beta(nm). In all the surfactant systems examined, data obtained can be explained well in terms of formation of Co(NCS)(+) and Co(NCS)(2) in an aqueous phase (aq), and Co(NCS)(4)Y(2-) in micelles, and their formation constants, enthalpies, and entropies have been determined. The beta(41)/beta(20) ratio increases and the corresponding enthalpy becomes significantly less negative with an increasing number of ethylene oxide groups. This suggests that micelles of these nonionic surfactants have a heterogeneous inner structure consisting of ethylene oxide and octylphenyl moieties. Indeed, on the basis of molar volumes of ethylene oxide and octylphenyl groups, intrinsic thermodynamic parameters have been extracted for the reaction Co(NCS)(2)(aq)+2NCS(-)(aq)=Co(NCS)(4)Y(2-) (Delta(r)G degrees, Delta(r)H degrees, and Delta(r)S degrees ) at each moiety. The Delta(r)G degrees, Delta(r)H degrees, and Delta(r)S degrees values are -16 kJ mol(-1), -15 kJ mol(-1), and 3 J K(-1) mol(-1), respectively, for the ethylene oxide moiety, and -15 kJ mol(-1), -70 kJ mol(-1), and -183 J K(-1) mol(-1) for octylphenyl. Significantly less negative Delta(r)H degrees and Delta(r)S degrees values for ethylene oxide imply that the hydrogen-bonded network structure of water is extensively formed at the ethylene oxide moiety, and the structure is thus broken around the Co(NCS)(4)(2-) complex with weak hydrogen-bonding ability. Copyright 2001 Academic Press.     

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.     

Activation energy for the disproportionation of HBrO2 and estimated heats of formation of HBrO2 and BrO2

The kinetics of the reaction HBrO(2) + HBrO(2) --> HOBr + BrO(3)(-) + H(+) is investigated in aqueous HClO(4) (0.04-0.9 M) and H(2)SO(4) (0.3-0.9 M) media and at temperatures in the range 15-38 degrees C. The reaction is found to be cleanly second order in [HBrO(2)], with the experimental rate constant having the form k(exp) = k + k'[H(+)]. The half-life of the reaction is on the order of a few tenths of a second in the range 0.01 M < [HBrO(2)](0) < 0.02 M. The detailed mechanism of this reaction is discussed. The activation parameters for kare found to be E(double dagger) = 19.0 +/- 0.9 kJ/mol and DeltaS(double dagger) = -132 +/- 3 J/(K mol) in HClO(4), and E(double dagger) = 23.0 +/- 0.5 kJ/mol and DeltaS(double dagger) = -119 +/- 1 J/(K mol) in H(2)SO(4). The activation parameters for k' are found to be E(double dagger) = 25.8 +/- 0.5 kJ/mol and DeltaS(double dagger) = -106 +/- 1 J/(K mol) in HClO(4), and E(double dagger) = 18 +/- 3 kJ/mol and DeltaS(double dagger) = -130 +/- 11 J/(K mol) in H(2)SO(4). The values Delta(f)H(29)(8)(0)[BrO(2)(aq)] = 157 kJ/mol and Delta(f)H(29)(8)(0)[HBrO(2)(aq)] = -33 kJ/mol are estimated using a trend analysis (bond strengths) based on the assumption Delta(f)H(29)(8)(0)[HBrO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOBr(aq)] and Delta(f)H(29)(8)(0)[HBrO(3)(aq)] as Delta(f)H(29)(8)(0)[HClO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOCl(aq)] and Delta(f)H(29)(8)(0)[HClO(3)(aq)]. The estimated value of Delta(f)H(29)(8)(0)[BrO(2)(aq)] agrees well with calculated gas-phase values, but the estimated value of Delta(f)H(29)(8)(0)[HBrO(2)(aq)], as well as the tabulated value of Delta(f)H(29)(8)(0)[HClO(2)(aq)], is in substantial disagreement with calculated gas-phase values. Values of Delta(r)H(0) are estimated for various reactions involving BrO(2) or HBrO(2).     

Synthesis, crystal structure, and H/D exchange of the inside protonated form of the cage imine 4,8,12-triaza-1-azoniatricyclo[6.6.3.2(4,12)]nonadec-1(15)-ene. A model for proton transfer through an aliphatic membrane

The reaction of the inside protonated form of the tricyclic amine 1,4,8,12-tetraazatricyclo[6.6.3.2(4,12)]nonadecane (1) with iron(III) affords the inside monoprotonated form of the corresponding imine 4,8,12-triaza-1-azoniatricyclo[6.6.3.2(4,12)]nonadec-1(15)-ene (2), which was isolated as the tetrabromozincate salt (2a) in a yield of 78%. The crystal structure of 2a has been solved by X-ray diffraction at T = 120 K. In the imine cation the acidic hydrogen atom and the lone pairs of the nitrogen atoms are oriented toward the inside of the cavity. The acidic hydrogen atom is bound to a nitrogen atom belonging to the triazacyclononane entity. The imine double bond is situated between the N-atom of the triazacyclononane entity and the C-atom belonging to one of the three trimethylene bridges. The imine 2 is stable in acidic solution and the inside coordinated proton is very robust in acidic solution. In basic solution the imine reacts fast to give a quantitative formation of the inside protonated form of the hemiaminal 1,4,8,12-tetraazatricyclo[6.6.3.2(4,12)]nonadecan-5-ol (3). The equilibrium constant K(im) = [3][H(+)]/[2] was determined at three different temperatures from potentiometric measurements, which gave K(im) = 1.57(1) x 10(-5) M at 25 degrees C, Delta S degrees = -83(1) J mol(-1) K(-)(1),and Delta H degrees = 2.6(3) kJ mol(-1) at I = 1.0 M (NaCl). The inside coordinated proton in 3 is labile in basic solution and the rate for NH/ND exchange was determined by (1)H NMR at three different temperatures. The reaction followed the expression k(obs) = k(ex)[OD(-)] with k(ex) = 0.0978(30) dm(3) mol(-1) s(-1) at 25 degrees C, Delta S(++) = 87(4) J mol(-1) K(-1), and Delta H(++) = 104.9(11) kJ mol(-1) at I = 1.0 M (NaCl). The exchange rate is more than 5 x 10(6) times faster than that of the parent saturated cage 1. This extreme enhancement of reactivity is explained by an intramolecular proton transfer reaction mediated by hydroxy and oxy groups flipping in and out of the cavity, which mechanistically has resemblance to the transport of ions in a biological system.     

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).     

Octahedral-tetrahedral equilibrium and solvent exchange of cobalt(II) ions in primary alkylamines

The enthalpy differences (Delta H degrees ) of the equilibrium between the octahedral and tetrahedral solvated cobalt(II) complexes were obtained in some primary alkylamines such as propylamine (pa, 36.1 +/- 2.3 kJ mol(-1)), n-hexylamine (ha, 34.9 +/- 1.0 kJ mol(-1)), 2-methoxyethylamine (meea, 44.8 +/- 3.1 kJ mol(-1)), and benzylamine (ba, 50.1 +/- 3.6 kJ mol(-1)) by the spectrophotometric method. The differences in the energy levels between the two geometries of the cobalt(II) complexes in the spherically symmetric field (Delta E(spher)) were estimated from the values of Delta H degrees by offsetting the ligand field stabilization energies. It was indicated that the value of Delta E(spher) is the decisive factor in determining the value of Delta H degrees and is largely dependent on the electronic repulsion between the d-electrons and the donor atoms and the interelectronic repulsion in the d orbitals. The comparison between activation enthalpies (Delta H(++)) for the solvent exchange reactions of octahedral cobalt(II) ions in pa and meea revealed that the unexpectedly large rate constant and small Delta H(++) in pa are attributed to the strong electronic repulsion in the ground state and removal of the electronic repulsion in the dissociative transition state, which can give the small Delta E(spher) between the ground and transition states. Differences in the solvent exchange rates and the DeltaH(++) values of the octahedral metal(II) ions in some other solvents are discussed in connection with the electronic repulsive factors.     

TPEPICO spectroscopy of vinyl chloride and vinyl iodide: neutral and ionic heats of formation and bond energies

The 0 K dissociative ionization onsets of C2H3X --> C2H3(+) + X (X = Cl, I) are measured by threshold photoelectron-photoion coincidence spectroscopy. The heats of formation of C2H3Cl (Delta H(f,0K)(0) = 30.2 +/- 3.2 kJ mol(-1) and Delta(H f,298K)(0) = 22.6 +/- 3.2 kJ mol(-1)) and C2H3I (Delta(H f,0K)(0) = 140.2 +/- 3.2 kJ mol(-1) and Delta(H f,298K)(0) = 131.2 +/- 3.2 kJ mol(-1)) and C- X bond dissociation enthalpies as well as those of their ions are determined. The data help resolve a longstanding discrepancy among experimental values of the vinyl chloride heat of formation, which now agrees with the latest theoretical determination. The reported vinyl iodide heat of formation is the first reliable experimental determination. Additionally, the adiabatic ionization energy of C2H3I (9.32 +/- 0.01 eV) is measured by threshold photoelectron spectroscopy.     

Gaseous rust: thermochemistry of neutral and ionic iron oxides and hydroxides in the gas phase

The experimental and theoretical thermochemistry of the gaseous neutral and ionic iron oxides and hydroxides FeO, FeOH, FeO(2), OFeOH, and Fe(OH)(2) and of the related cationic water complexes Fe(H(2)O)(+), (H(2)O)FeOH(+), and Fe(H(2)O)(2)(+) is analyzed comprehensively. A combination of data for the neutral species with those of the gaseous ions in conjunction with some additional measurements provides a refined and internally consistent compilation of thermochemical data for the neutral and ionic species. In terms of heats of formation at 0 K, the best estimates for the gaseous, mononuclear FeO(m)H(n)(-/0/+/2+) species with m = 1, 2 and n = 0-4 are Delta(f)H(FeO(-)) = (108 +/- 6) kJ/mol, Delta(f)H(FeO) = (252 +/- 6) kJ/mol, Delta(f)H(FeO(+)) = (1088 +/- 6) kJ/mol, Delta(f)H(FeOH) = (129 +/- 15) kJ/mol, Delta(f)H(FeOH(+)) = (870 +/- 15) kJ/mol, Delta(f)H(FeO(2)(-)) = (-161 +/- 13) kJ/mol, Delta(f)H(FeO(2)) = (67 +/- 12) kJ/mol, Delta(f)H(FeO(2)(+)) = (1062 +/- 25) kJ/mol, Delta(f)H(OFeOH) = (-84 +/- 17) kJ/mol, Delta(f)H(OFeOH(+)) = (852 +/- 23) kJ/mol, Delta(f)H(Fe(OH)(2)(-)) = -431 kJ/mol, Delta(f)H(Fe(OH)(2)) = (-322 +/- 2) kJ/mol, and Delta(f)H(Fe(OH)(2)(+)) = (561 +/- 10) kJ/mol for the iron oxides and hydroxides as well as Delta(f)H(Fe(H(2)O)(+)) = (809 +/- 5) kJ/mol, Delta(f)H((H(2)O)FeOH(+)) = 405 kJ/mol, and Delta(f)H(Fe(H(2)O)(2)(+)) = (406 +/- 6) kJ/mol for the cationic water complexes. In addition, charge-stripping data for several of several-iron-containing cations are re-evaluated due to changes in the calibration scheme which lead to Delta(f)H(FeO(2+)) = (2795 +/- 28) kJ/mol, Delta(f)H(FeOH(2+)) = (2447 +/- 30) kJ/mol, Delta(f)H(Fe(H(2)O)(2+)) = (2129 +/- 29) kJ/mol, Delta(f)H((H(2)O)FeOH(2+)) = 1864 kJ/mol, and Delta(f)H(Fe(H(2)O)(2)(2+)) = (1570 +/- 29) kJ/mol, respectively. The present compilation thus provides an almost complete picture of the redox chemistry of mononuclear iron oxides and hydroxides in the gas phase, which serves as a foundation for further experimental studies and may be used as a benchmark database for theoretical studies.     

On the parallel mechanism of the dissociation of energy-selected P(CH3)3+ ions

Energy selected trimethyl phosphine ions were prepared by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. This ion dissociates via H, CH(3), and CH(4) loss, the latter two involving hydrogen transfer steps. The ion time-of-flight distribution and the breakdown diagram are analyzed in terms of the statistical RRKM theory, which includes tunneling. Ab initio and DFT calculations provide the vibrational frequencies required for the RRKM modeling. CH(3) loss could produce both the P(CH(3))(2)(+) by a simple bond dissociation step, and the more stable HP(CH(2))CH(3)(+) ion by a hydrogen transfer step. Quantum chemical calculations are extensively used to uncover the reaction scheme, and they strongly suggest that the latter product is exclusively formed via an isomerization step in the energy range of the experiment. The data analysis, which includes modeling with the trimethyl phosphine thermal energy distribution, provides accurate onset energies for both H (E(0K) = 1024.1 +/- 3.5 kJ/mol) and CH(3) (E(0K) = 1024.8 +/- 3.5 kJ/mol) loss reactions. From this analysis, we conclude that the Delta(f)H(298K) degrees [HP(CH(2))(CH(3))(+)] = 783 +/- 8 kJ/mol and Delta(f)H(298K) degrees [P(CH(2))(CH(3))(2)(+)] = 711 +/- 8 kJ/mol.     

The dynamical properties of the aromatic hydrogen bond in NH4(C6H5)4B from quasielastic neutron scattering

NH(4)(C(6)H(5))(4)B represents a prototypical system for understanding aromatic H bonds. In NH(4)(C(6)H(5))(4)B an ammonium cation is trapped in an aromatic cage of four phenyl rings and each phenyl ring serves as a hydrogen bond acceptor for the ammonium ion as donor. Here the dynamical properties of the aromatic hydrogen bond in NH(4)(C(6)H(5))(4)B were studied by quasielastic incoherent neutron scattering in a broad temperature range (20< or =T< or =350 K). We show that in the temperature range from 67 to 350 K the ammonium ions perform rotational jumps around C(3) axes. The correlation time for this motion is the lifetime of the "transient" H bonds. It varies from 1.5 ps at T=350 K to 150 ps at T=67 K. The activation energy was found to be 3.14 kJ mol, which means only 1.05 kJ mol per single H bond for reorientations around the C(3) symmetry axis of the ammonium group. This result shows that the ammonium ions have to overcome an exceptionally low barrier to rotate and thereby break their H bonds. In addition, at temperatures above 200 K local diffusive reorientational motions of the phenyl rings, probably caused by interaction with ammonium-group reorientations, were found within the experimental observation time window. At room temperature a reorientation angle of 8.4 degrees +/-2 degrees and a correlation time of 22+/-8 ps were determined for the latter. The aromatic H bonds are extremely short lived due to the low potential barriers allowing for molecular motions with a reorientational character of the donors. The alternating rupture and formation of H bonds causes very strong damping of the librational motion of the acceptors, making the transient H bond appear rather flexible.     

The thermodynamics of the transformation of graphite to multiwalled carbon nanotubes

The thermodynamic quantities associated to the transformation from graphite to multiwalled carbon nanotubes (MWCNTs) were determined by electromotive force (emf) and differential scanning calorimetry (DSC) measurements. From the emf versus T data of galvanic cell Mo|Cr(3)C(2), CrF2, MWCNTs|CaF2 s.c.|Cr(3)C(2), CrF2, graphite|Mo with CaF2 as solid electrolyte, Delta(r)H(T) degrees= 8.25 +/- 0.09 kJ mol(-1) and Delta(r)S(T) degrees= 11.72 +/- 0.09 JK(-1) mol(-1) were found at average temperature T = 874 K. The transformation enthalpy was also measured by DSC of the Mn(7)C(3) formation starting from graphite or MWCNTs. Thermodynamic values at 298 K were calculated to be: Delta(r)H(298) degrees = 9.0 +/- 0.8 kJ mol(-1) as averaged value from both techniques and Delta(r)S(298) degrees approximately Delta(r)S(T) degrees. At absolute zero, the residual entropy of MWCNTs was estimated 11.63 +/- 0.09 JK(-1) mol(-1), and transformation enthalpy Delta(r)H(0) degrees approximately Delta(r)H(298) degrees. The latter agrees satisfactorily with the theoretical calculations for the graphite-MWCNTs transformation. On thermodynamic basis, the transformation becomes spontaneous above 704 +/- 13 K.     

Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)

The formation constants of UO2SO4 (aq), UO2(SO4)2(2-), and UO2(SO4)3(4-) were measured in aqueous solutions from 10 to 75 degrees C by time-resolved laser-induced fluorescence spectroscopy (TRLFS). A constant enthalpy of reaction approach was satisfactorily used to fit the thermodynamic parameters of stepwise complex formation reactions in a 0.1 M Na(+) ionic medium: log 10 K 1(25 degrees C) = 2.45 +/- 0.05, Delta r H1 = 29.1 +/- 4.0 kJ x mol(-1), log10 K2(25 degrees C) = 1.03 +/- 0.04, and Delta r H2 = 16.6 +/- 4.5 kJ x mol(-1). While the enthalpy of the UO2(SO4)2(2-) formation reaction is in good agreement with calorimetric data, that for UO2SO4 (aq) is higher than other values by a few kilojoules per mole. Incomplete knowledge of the speciation may have led to an underestimation of Delta r H1 in previous calorimetric studies. In fact, one of the published calorimetric determinations of Delta r H1 is here supported by the TRLFS results only when reinterpreted with a more correct equilibrium constant value, which shifts the fitted Delta r H1 value up by 9 kJ x mol(-1). UO2(SO 4) 3 (4-) was evidenced in a 3 M Na (+) ionic medium: log10 K3(25 degrees C) = 0.76 +/- 0.20 and Delta r H3 = 11 +/- 8 kJ x mol(-1) were obtained. The fluorescence features of the sulfate complexes were observed to depend on the ionic conditions. Changes in the coordination mode (mono- and bidentate) of the sulfate ligands may explain these observations, in line with recent structural data.     

Computational evidence for methyl-donated hydrogen bonds and hydrogen-bond networking in 1,2-ethanediol-dimethyl sulfoxide

The 1:1 complex of 1,2-ethanediol with dimethyl sulfoxide was studied using density functional theory. A network of three hydrogen bonds holds the complex together, including two in which each methyl group donates to the same hydroxyl oxygen. Four lines of evidence support the existence of methyl-donated hydrogen bonds. The interaction energy is 36 +/- 5 kJ/mol using Becke's three parameter hybrid theory with the 1991 nonlocal correlation functional of Perdew and Wang, and a moderately large basis set (B3PW91/6-311++G**//B3PW91/6-31+G**). To determine the energy of each hydrogen bond, a relaxed potential energy scan was performed in a smaller basis set to break the weaker hydrogen bonds by forced systematic rotation of the methyl groups. Two cross-checking analyses show cooperative effects that cause individual hydrogen bond energies in the network to be nonadditive. When one methyl hydrogen bond is broken, the remaining interactions stabilize the complex by storing an additional 2-3 kJ/mol. With all hydrogen bonds intact, the O[bond]H...O[bond]S hydrogen bond contributes 26 +/- 2 kJ/mol stability, and each weak methyl bond stores 5 +/- 2 kJ/mol.     

Theoretical Study on Thermodynamic Properties and Stabilities of n-Silanes

Abstract

First-principle computations were performed on the n-silane series (Si_nH_2n+2, n = 1–10). The heat of formation (Δ_fH), Gibbs free energy of formation (Δ_fG), bond length, and bond dissociation energy (BDE) for both the Si?Si and Si?H bonds were predicted. The values of Δ_fH and Δ_fG from the accurate high level G3 method for lower silanes (n ≤ 5) were compared with experimental values and used as benchmarks. Thermodynamic properties derived from the G3 method in combination with the permutation reaction are in better agreement with experiments than those with the atomization reaction. The increments of the Δ_fH and Δ_fG values with an increasing SiH₂-unit for n-silanes are 40.39 and 58.93 kJ/mol on average, respectively. The length of Si?Si bond increases slightly on average as the series number increases and then tends to be a constant for higher silanes. The BDEs for both the Si?Si and Si?H bonds initially decrease for lower silanes, and then approach a constant for higher silanes. The BDEs of the Si?Si bonds are smaller than those of Si?H bonds. The higher silanes are more unstable than the lower silanes. The average BDE of Si?Si bond at the MP2 level is ca. 302 kJ/mol, which is only half the experimental BDE value of the C?C bond (618 kJ/mol).

GRAPHICAL ABSTRACT