Kinetics and thermodynamics of copper(II) binding to apoazurin

The binding of copper(II) to apoazurin has been probed by isothermal titration calorimetry in cholamine buffer at pH 7.0. The standard enthalpy change was determined to be -10.0 +/- 1.4 kcal/mol. Each calorimetric trace reveals an initial exothermic phase followed by an endothermic phase. The calorimetric data could be fit to a kinetic model involving a bimolecular combination of copper(II) and apoazurin in an exothermic process (k = 2 +/-1 x 103 M-1 s-1, DeltaH degrees = -19 +/- 3 kcal/mol) to form an intermediate that spontaneously converts to Cu(II)-azurin in an endothermic process (k = 0.024 +/- 0.01 s-1, DeltaH degrees = +9 +/- 3 kcal/mol). These data suggest that copper(II) first combines with apoazurin in an irreversible process to form an intermediate that converts to copper(II)-azurin in a process driven by the release of water. The overall standard free energy of copper(II) binding to apoazurin is estimated to be -18.8 kcal/mol.     

Equilibrium thermodynamic studies in water: reactions of dihydrogen with rhodium(III) porphyrins relevant to Rh-Rh, Rh-H, and Rh-OH bond energetics

Aqueous solutions of rhodium(III) tetra p-sulfonatophenyl porphyrin ((TSPP)Rh(III)) complexes react with dihydrogen to produce equilibrium distributions between six rhodium species including rhodium hydride, rhodium(I), and rhodium(II) dimer complexes. Equilibrium thermodynamic studies (298 K) for this system establish the quantitative relationships that define the distribution of species in aqueous solution as a function of the dihydrogen and hydrogen ion concentrations through direct measurement of five equilibrium constants along with dissociation energies of D(2)O and dihydrogen in water. The hydride complex ([(TSPP)Rh-D(D(2)O)](-4)) is a weak acid (K(a)(298 K) = (8.0 +/- 0.5) x 10(-8)). Equilibrium constants and free energy changes for a series of reactions that could not be directly determined including homolysis reactions of the Rh(II)-Rh(II) dimer with water (D(2)O) and dihydrogen (D(2)) are derived from the directly measured equilibria. The rhodium hydride (Rh-D)(aq) and rhodium hydroxide (Rh-OD)(aq) bond dissociation free energies for [(TSPP)Rh-D(D(2)O)](-4) and [(TSPP)Rh-OD(D(2)O)](-4) in water are nearly equal (Rh-D = 60 +/- 3 kcal mol(-1), Rh-OD = 62 +/- 3 kcal mol(-1)). Free energy changes in aqueous media are reported for reactions that substitute hydroxide (OD(-)) (-11.9 +/- 0.1 kcal mol(-1)), hydride (D(-)) (-54.9 kcal mol(-1)), and (TSPP)Rh(I): (-7.3 +/- 0.1 kcal mol(-1)) for a water in [(TSPP)Rh(III)(D(2)O)(2)](-3) and for the rhodium hydride [(TSPP)Rh-D(D(2)O)](-4) to dissociate to produce a proton (9.7 +/- 0.1 kcal mol(-1)), a hydrogen atom (approximately 60 +/- 3 kcal mol(-1)), and a hydride (D(-)) (54.9 kcal mol(-1)) in water.     

Quantum Monte Carlo calculations of the dissociation energy of the water dimer

We report diffusion quantum Monte Carlo (DMC) calculations of the equilibrium dissociation energy D(e) of the water dimer. The dissociation energy measured experimentally, D(0), can be estimated from D(e) by adding a correction for vibrational effects. Using the measured dissociation energy and the modern value of the vibrational energy Mas et al., [J. Chem. Phys. 113, 6687 (2000)] leads to D(e)=5.00+/-0.7 kcal mol(-1), although the result Curtiss et al., [J. Chem. Phys. 71, 2703 (1979)] D(e)=5.44+/-0.7 kcal mol(-1), which uses an earlier estimate of the vibrational energy, has been widely quoted. High-level coupled cluster calculations Klopper et al., [Phys. Chem. Chem. Phys. 2, 2227 (2000)] have yielded D(e)=5.02+/-0.05 kcal mol(-1). In an attempt to shed new light on this old problem, we have performed all-electron DMC calculations on the water monomer and dimer using Slater-Jastrow wave functions with both Hartree-Fock approximation (HF) and B3LYP density functional theory single-particle orbitals. We obtain equilibrium dissociation energies for the dimer of 5.02+/-0.18 kcal mol(-1) (HF orbitals) and 5.21+/-0.18 kcal mol(-1) (B3LYP orbitals), in good agreement with the coupled cluster results.     

Measurement of electrostatic interactions in protein folding with the use of protein charge ladders

This paper describes a new method for the measurement of the role of interactions between charged groups on the energetics of protein folding. This method uses capillary electrophoresis (CE) and protein charge ladders (mixtures of protein derivatives that differ incrementally in number of charged groups) to measure, in a single set of electrophoresis experiments, the free energy of unfolding (DeltaG(D-N)) of alpha-lactalbumin (alpha-LA) as a function of net charge. These same data also yield the hydrodynamic radius, R(H), and net charge measured by CE, Z(CE), of the folded and denatured proteins. Alpha-LA unfolds to a compact denatured state under mildly alkaline conditions; a small increase in R(H) (11%, 2 A) coincides with a large increase in Z(CE) (71%, -4 charge units), relative to the folded state. The increase in Z(CE), in turn, predicts a large pH dependence of free energy of unfolding (-22 kJ/mol per unit increase in pH), due to differences in proton binding in the folded and denatured states. The free energy of unfolding correlates with the square of net charge of the members of the charge ladder. The differential dependence of DeltaG(D-N) on net charge for holo-alpha-LA, (partial differential) DeltaG(D-N)/(partial differential)Z = -0.14Z kJ/mol per unit of charge. This dependence of DeltaG(D-N) on net charge is a result of a net electrostatic repulsion among charge groups on the protein. These results, together with data from pH titrations, show that both the effects of electrostatic repulsion and differences in proton binding in the folded and denatured states can play an important role in the pH dependence of this protein; the relative magnitude of these effects varies with pH. The combination of charge ladders and CE is a rapid and efficient tool that measures the contributions of electrostatics to the energetics of protein folding, and the size and charge of proteins as they unfold. All this information is obtained from a single set of electrophoresis experiments.     

Experimental determination of the alpha and beta C--H bond dissociation energies in naphthalene

The acidities of the two different sites in naphthalene (1alpha and 1beta) and the electron affinities of the alpha- and beta-naphthyl radicals were measured using a Fourier transform mass spectrometer. Both carbon-hydrogen bond dissociation energies for naphthalene also were obtained, in this case via the application of a thermodynamic cycle. The final results are DeltaH(o)acid (1alpha) = 394.2+/-1.2 kcal mol(-1), DeltaH(o)acid (1beta) = 395.5+/-1.3 kcal mol(-1), EA(alpha) = 31.6+/-0.5 kcal mol(-1), EA(beta) = 31.6+/-0.5 kcal mol(-1), BDE(1alpha) = 112.2+/-1.3 kcal mol(-1) and BDE(1alpha) = 111.9+/-1.4 kcal mol(-1), and they are compared to benzene and phenyl radical as well as ab initio and density functional theory (B3LYP) calculations.     

Experimental determination of the heat of hydrogenation of phenylcyclobutadiene

The heat of hydrogenation of phenylcyclobutadiene (DeltaH degrees (hyd) = 57.4 +/- 4.9 kcal mol(-1)) was determined via a thermodynamic cycle by carrying out gas-phase measurements on 1-phenylcyclobuten-3-yl cation. This leads to an antiaromatic destabilization energy of 27 +/- 5 kcal mol(-1), a difference of 9.6 +/- 4.9 kcal mol(-1) for the first and second C-H bond dissociation energies of 1-phenylcyclobutene, and an estimate of 96 +/- 5 kcal mol(-1) for the heat of formation of cyclobutadiene. These results are compared to G3, G3(MP2), and B3LYP computations and represent the first experimental measurements of the energy of a monocyclic cyclobutadiene.     

N-NO bond dissociation energies of N-nitroso diphenylamine derivatives (or analogues) and their radical anions: implications for the effect of reductive electron transfer on N-NO bond activation and for the mechanisms of NO transfer to nitranions

The heterolytic and homolytic N-NO bond dissociation energies [i.e., deltaHhet(N-NO) and deltaHhomo(N-NO)] of 12 N-nitroso-diphenylamine derivatives (1-12) and two N-nitrosoindoles (13 and 14) in acetonitrile were determined by titration calorimetry and from a thermodynamic cycle, respectively. Comparison of these two sets of data indicates that homolysis of the N-NO bonds to generate NO* and nitrogen radical is energetically much more favorable (by 23.3-44.8 kcal/mol) than the corresponding heterolysis to generate a pair of ions, giving hints for the driving force and possible mechanism of NO-initiated chemical and biological transformations. The first (N-NO)-* bond dissociation energies [i.e., deltaH(N-NO)-* and deltaH'(N-NO)-*] of radical anions 1-*-14-* were also derived on the basis of appropriate cycles utilizing the experimentally measured deltaHhet(N-NO) and electrochemical data. Comparisons of these two quantities with those of the neutral N-NO bonds indicate a remarkable bond activation upon a possible one-electron transfer to the N-NO bonds, with an average bond-weakening effect of 48.8 +/- 0.3 kcal/mol for heterolysis and 22.3 +/- 0.3 kcal/mol for homolysis, respectively. The good to excellent linear correlations among the energetics of the related heterolytic processes [deltaHhet(N-NO), deltaH(N-NO)-*, and pKa(N-H)] and the related homolytic processes [deltaHhomo(N-NO), deltaH'(N-NO)-*, and BDE(N-H)] imply that the governing structural factors for these bond scissions are similar. Examples illustrating the use of such bond energetic data jointly with relevant redox potentials for analyzing various mechanistic possibilities for nitrosation of nitranions are presented.     

Kinetic and thermodynamic characterization of Co(II)-substrate radical pair formation in coenzyme B12-dependent ethanolamine ammonia-lyase in a cryosolvent system by using time-resolved, full-spectrum continuous-wave electron paramagnetic resonance spectroscopy

The formation of the Co(II)-substrate radical pair catalytic intermediate in coenzyme B12 (adenosylcobalamin)-dependent ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium has been studied by using time-resolved continuous-wave electron paramagnetic resonance (EPR) spectroscopy in a cryosolvent system. The 41% v/v DMSO/water cryosolvent allows mixing of holoenzyme and substrate, (S)-2-aminopropanol, at 230 K under conditions of kinetic arrest. Temperature step from 230 to 234-248 K initiates the cleavage of the cobalt-carbon bond and the monoexponential rise (rate constant, k(obs) = tau(obs)(-1)) of the EPR-detected Co(II)-substrate radical pair state. The detection deadtime: tau(obs) ratio is reduced by >10(2), relative to millisecond rapid mixing experiments at ambient temperatures. The EPR spectrum acquisition time is 5tau(obs), the approximately 10(2)-fold slower rate of the substrate radical rearrangement reaction relative to k(obs), and the reversible temperature dependence of the amplitude indicate that the Co(II)-substrate radical pair and ternary complex are essentially at equilibrium. The reaction is thus treated as a relaxation to equilibrium by using a linear two-step, three-state mechanism. The intermediate state in this mechanism, the Co(II)-5'-deoxyadenosyl radical pair, is not detected by EPR at signal-to-noise ratios of 10(3), which indicates that the free energy of the Co(II)-5'-deoxyadenosyl radical pair state is >3.3 kcal/mol, relative to the Co(II)-substrate radical pair. Van't Hoff analysis yields DeltaH13 = 10.8 +/- 0.8 kcal/mol and DeltaS13 = 45 +/- 3 cal/mol/K for the transition from the ternary complex to the Co(II)-substrate radical pair state. The free energy difference, DeltaG13, is zero to within one standard deviation over the temperature range 234-248 K. The extrapolated value of DeltaG13 at 298 K is -2.6 +/- 1.2 kcal/mol. The estimated EAL protein-associated contribution to the free energy difference is DeltaG(EAL) = -24 kcal/mol at 240 K, and DeltaH(EAL) = -13 kcal/mol and DeltaS(EAL) = 38 cal/mol/K. The results show that the EAL protein makes both strong enthalpic and entropic contributions to overcome the large, unfavorable cobalt-carbon bond dissociation energy, which biases the reaction in the forward direction of Co-C bond cleavage and Co(II)-substrate radical pair formation.     

Determination of the electron affinities of alpha- and beta-naphthyl radicals using the kinetic method with full entropy analysis. The C-H bond dissociation energies of naphthalene.

The C - H bond dissociation energies for naphthalene were determined using a negative ion thermochemical cycle involving the gas-phase acidity (Delta H (acid)) and electron affinity (EA) for both the alpha- and beta-positions. The gas-phase acidity of the naphthalene alpha- and beta-positions and the EAs of the alpha- and beta-naphthyl radicals were measured in the gas phase in a flowing after glow-triple quadrupole apparatus. A variation of the Cooks kinetic method was used to measure the EAs of the naphthyl radicals by collision-induced dissociation of the corresponding alpha- and beta-naphthylsulfinate adducts formed by reactions in the flow tube portion of the instrument. Calibration references included both pi and sigma radicals, and full entropy analysis was performed over a series of calibration curves measured at collision energies ranging from 3.5 to 8 eV (center-of-mass). The measured EAs are 33.0 +/- 1.4 and 31.4 +/- 1.0 kcal mol(-1) (1 kcal = 4.184 kJ) for the alpha- and beta-naphthyl radicals, respectively. The gas-phase acidities for naphthalene were measured by the DePuy silane cleavage method, which utilizes the relative abundances of aryldimethylsiloxides and trimethylsiloxide that result from competitive cleavages from a proposed penta coordinate hydroxysiliconate intermediate. The measured acidities are 394.0 +/- 5.0 and 397.6 +/- 4.8 kcal mol(-1) for the alpha- and beta- positions, respectively. The C - H bond dissociation energies calculated from the thermochemical cycle are 113.4 +/- 5.2 and 115.4 +/- 4.9 kcal mol(-1) for the alpha- and beta-positions, respectively. These energies are, to within experimental error, indistinguishable and are approximately the same as the first bond dissociation energy for benzene.     

Benzocyclobutene: The Impact of Fusing a Strained Ring onto Benzene

The gas-phase acidities of the two aromatic sites in benzocyclobutene were measured in a Fourier transform mass spectrometer using a kinetic technique (i.e., the DePuy method). Fusion of a cyclobutane ring onto benzene is found to have a slight acidifying effect at the alpha-position (3.2 +/- 1.7 kcal mol(-)(1)) and little, if any, influence on the beta-site (0.8 +/- 1.9 kcal mol(-)(1)). Energetic data (DeltaH degrees (acid) = 386.2 +/- 3.0 kcal mol(-)(1), EA = 0.84 +/- 0.11 eV, and C-H BDE = 92 +/- 4 kcal mol(-)(1)) for the benzylic position were obtained via the bracketing technique and application of a thermodynamic cycle. Differences in the reactivities of the three conjugate bases also were explored. Ab initio and density functional theory calculations were carried out to provide geometries, energies, and insights into the carbanions' electronic structures.     

Theoretical prediction of the heats of formation of C2H5O* radicals derived from ethanol and of the kinetics of beta-C-C scission in the ethoxy radical

Thermochemical parameters of three C(2)H(5)O* radicals derived from ethanol were reevaluated using coupled-cluster theory CCSD(T) calculations, with the aug-cc-pVnZ (n = D, T, Q) basis sets, that allow the CC energies to be extrapolated at the CBS limit. Theoretical results obtained for methanol and two CH(3)O* radicals were found to agree within +/-0.5 kcal/mol with the experiment values. A set of consistent values was determined for ethanol and its radicals: (a) heats of formation (298 K) DeltaHf(C(2)H(5)OH) = -56.4 +/- 0.8 kcal/mol (exptl: -56.21 +/- 0.12 kcal/mol), DeltaHf(CH(3)C*HOH) = -13.1 +/- 0.8 kcal/mol, DeltaHf(C*H(2)CH(2)OH) = -6.2 +/- 0.8 kcal/mol, and DeltaHf(CH(3)CH(2)O*) = -2.7 +/- 0.8 kcal/mol; (b) bond dissociation energies (BDEs) of ethanol (0 K) BDE(CH(3)CHOH-H) = 93.9 +/- 0.8 kcal/mol, BDE(CH(2)CH(2)OH-H) = 100.6 +/- 0.8 kcal/mol, and BDE(CH(3)CH(2)O-H) = 104.5 +/- 0.8 kcal/mol. The present results support the experimental ionization energies and electron affinities of the radicals, and appearance energy of (CH(3)CHOH+) cation. Beta-C-C bond scission in the ethoxy radical, CH(3)CH2O*, leading to the formation of C*H3 and CH(2)=O, is characterized by a C-C bond energy of 9.6 kcal/mol at 0 K, a zero-point-corrected energy barrier of E0++ = 17.2 kcal/mol, an activation energy of Ea = 18.0 kcal/mol and a high-pressure thermal rate coefficient of k(infinity)(298 K) = 3.9 s(-1), including a tunneling correction. The latter value is in excellent agreement with the value of 5.2 s(-1) from the most recent experimental kinetic data. Using RRKM theory, we obtain a general rate expression of k(T,p) = 1.26 x 10(9)p(0.793) exp(-15.5/RT) s(-1) in the temperature range (T) from 198 to 1998 K and pressure range (p) from 0.1 to 8360.1 Torr with N2 as the collision partners, where k(298 K, 760 Torr) = 2.7 s(-1), without tunneling and k = 3.2 s(-1) with the tunneling correction. Evidence is provided that heavy atom tunneling can play a role in the rate constant for beta-C-C bond scission in alkoxy radicals.     

Why does cyclopropene have the acidity of an acetylene but the bond energy of methane?

The gas-phase acidity of 3,3-dimethylcyclopropene (1) has been measured by bracketing and equilibrium techniques. Consistent with simple hybridization arguments, our value (deltaH degrees (acid) = 382.7 +/- 1.3 kcal mol(-)(1)) is indistinguishable from that for methylacetylene (i.e., deltadeltaH degrees (acid)(1 - CH(3)Ctbd1;CH) = 1.6 +/- 2.5 kcal mol(-)(1)). The electron affinity of 3,3-dimethylcyclopropenyl radical (1r) was also determined (EA = 37.6 +/- 3.5 kcal mol(-)(1)), and these quantities were combined in a thermodynamic cycle to afford the homolytic C-H bond dissociation energy. To our surprise, the latter quantity (107 +/- 4 kcal mol(-)(1)) is the same as that for methane, which cannot be explained in terms of the s-character in the C-H bonds. An orbital explanation (delocalization) is proposed to account for the extra stability of 1r. All of the results are supplemented with G3 and B3LYP computations, and both approaches are in good accord with the experimental values. We also note that for simple hydrocarbons which give localized carbanions upon deprotonation there is an apparent linear correlation between any two of the following three quantities: deltaH degrees (acid), BDE, and EA. This observation could be of considerable value in many diverse areas of chemistry.     

The electron as a protecting group. 3. Generation of acenaphthyne radical anion and the determination of the heat of formation of a strained cycloalkyne.

Acenaphthyne dicarboxylate (12) was transferred into the gas phase from solution via electrospray ionization and subsequently was sequentially fragmented in a Fourier transform mass spectrometer to afford acenaphthyne radical anion (9). Structural confirmation of 9 was achieved by converting it to acenaphthenone enolate (13) and demonstrating that this species is identical to the ion produced upon deprotonation of acenaphthenone (5). The reactivity of 9 was explored, and since an electron can serve as a protecting group, we were able to measure the heat of hydrogenation (98 +/- 4 kcal mol(-1)) and the heat of formation (160 +/- 4 kcal mol(-1)) of acenaphthyne (1) via the application of a thermodynamic cycle. Its strain energy (68 kcal mol(-1)) and acenaphthylene's (10H) first and second C-H bond dissociation energies (117 +/- 4 and 84 +/- 2 kcal mol(-1)) also were obtained. Ab initio and density functional theory calculations were carried out on the species of interest to explore their geometries and energetics. Our results were interpreted in comparison to cyclopentyne, and its predicted heat of formation (98 kcal mol(-1)) and strain energy (59 kcal mol(-1)) are reported.     

Composition and thermochemistry of silver bromide vapor

The mole fractions of AgBr and Ag3Br3 in the saturated vapor at 840 K have been evaluated from the vapor mass spectrum, by comparison with the corresponding spectrum of AgCl vapor, where the monomer/trimer ratio is known accurately from vapor molecular weight measurements. Combination of these results with new measurements of the vapor pressure of molten AgBr by the torsion-effusion method in the range 805-936 K yielded the third law enthalpies of vaporization and the standard enthalpies of formation DeltafH degrees 298(AgBr, g) = 27.8 +/- 0.3 kcal mol(-1) and DeltafH degrees 298(Ag3Br3, g) = -19.0 +/- 1 kcal mol(-1). The dissociation energy, D degrees 0(AgBr), is found to be 66.4 +/- 0.3 kcal mol(-1), or 2.88 +/- 0.01 eV, some 3.5-5 kcal mol(-1) lower than previous literature values. Approximate thermochemical stabilities of the dimer species Ag2Cl2 and Ag2Br2 have also been evaluated.     

Thermal decomposition of the perfluorinated peroxides CF3OC(O)OOC(O)F and CF3OC(O)OOCF3

Gas phase thermal decomposition of CF(3)OC(O)OOC(O)F and CF(3)OC(O)OOCF(3) was studied at temperatures between 64 and 98 degrees C (CF(3)OC(O)OOC(O)F) and 130-165 degrees C (CF(3)OC(O)OOCF(3)) using FTIR spectroscopy to follow the course of the reaction. For both substances, the decompositions were studied with N(2) and CO as bath gases. The rate constants for the decomposition of CF(3)OC(O)OOC(O)F in nitrogen and carbon monoxide fit the Arrhenius equations k(N)2 = (3.1 +/- 0.1) x 10(15) exp[-(29.0 +/- 0.5 kcal mol(-1)/RT)] and k(CO) = (5.8 +/- 1.3) x 10(15) exp[-(29.4 +/- 0.5 kcal mol(-1)/RT)], and that for CF(3)OC(O)OOCF(3) fits the equation k = (9.0 +/- 0.9) x 10(13) exp[-(34.0 +/- 0.7 kcal mol(-1)/RT)] (all in units of inverted seconds). Rupture of the O-O bond was shown to be the rate-determining step for both peroxides, and bond energies of 29 +/- 1 and 34.0 +/- 0.7 kcal mol(-1) were obtained for CF(3)OC(O)OOC(O)F and CF(3)OC(O)OOCF(3). The heat of formation of the CF(3)OCO(2)(*) radical, which is a common product formed in both decompositions, was calculated by ab initio methods as -229 +/- 4 kcal mol(-1). With this value, the heat of formation of the title species and of CF(3)OC(O)OOC(O)OCF(3) could in turn be obtained as Delta(f) degrees (CF(3)OC(O)OOC(O)F) = -286 +/- 6 kcal mol(-1), Delta(f) degrees (CF(3)OC(O)OOCF(3)) = -341 +/- 6 kcal mol(-1), and Delta(f) degrees (CF(3)OC(O)OOC(O)OCF(3)) = -430 +/- 6 kcal mol(-1).     

A kinetic study of the reactions of vanadium, iron, and cobalt with sulfur dioxide

A kinetic study of the reactions of ground state V, Fe, and Co with SO2 is reported. V, Fe, and Co were produced by the 248 nm photodissociation of VCl4, ferrocene, and Co(C5H5)(CO)2, respectively, and were detected by laser-induced fluorescence. V + SO2 proceeds by an abstraction reaction with rate constants given by k=(2.33 +/- 0.57)x 10(-10) exp[-(1.14 +/- 0.19) kcal mol(-1)/RT] cm3 molecule(-1) s(-1) over the temperature range 296-571 K. Fe + SO2 was studied in the N2 buffer range of 10-185 Torr between 294 and 498 K. The limiting, low-pressure third-order rate constants are given by k(0)=(3.45 +/- 1.19)x 10(-30) exp[-(2.81 +/- 0.24) kcal mol(-1)/RT] cm6 molecule(-2) s(-1). Co + SO2 was studied in the CO2 buffer range of 5-40 Torr between 294 and 498 K. This reaction is independent of temperature over the indicated range and has a third-order rate constant of k0=(5.23 +/- 0.28)x 10(-31) cm6 molecule(-2) s(-1). Results of this work are compared to previous work on the Sc, Ti, Cr, Mn, and Ni + SO2 systems. The reaction efficiencies for the abstraction reactions depend on the ionization energies of the transition metal atoms and on the reaction exothermicities, and the reaction efficiencies of the association reactions are strongly dependent on the energies needed to promote an electron from a 4s2 configuration to a 4s1 configuration.     

Diffusion of buffer layer assisted grown gold nanoclusters on Ru(100) and p(1 x 2)-O/Ru(100) surfaces

Patterning of metallic clusters on surfaces is demonstrated by utilizing a buffer layer assisted laser patterning technique (BLALP). This method has been employed in order to measure the diffusion of AFM and STM characterized size selected gold nanoclusters (5-10 nm diameter), over Ru(100) and p(1 x 2)-O/Ru(100) surfaces. Optical linear diffraction from gold cluster coverage gratings was utilized for the macroscopic diffusion measurements. The clusters were found to diffuse on the surface intact without significant coalescence or sintering. The barrier for metastable gold nanocluster diffusion on the surface is thought to be lower than the energy required for surface wetting. The apparent activation energy for diffusion was found to depend on the cluster size, increasing from 6.2 +/- 0.4 kcal/mol for 5 nm clusters to 10.6 +/- 0.5 kcal/mol for 9 nm clusters. The macroscopic diffusion of gold nanoclusters has been studied on the p(1 x 2)-O/Ru(100) surface as well, where surface diffusion was found to be rather insensitive to the clusters size with activation energy of 5.5 +/- 1 kcal/mol. The difference between the two surfaces is discussed in terms of a better commensurability (higher level of friction) of the gold facets at the contact area with the clean Ru(100) than in the case of the oxidized surface.     

Gas-phase properties and fragmentation behavior of cationic, dinuclear iron chloride clusters Fe(2)Cl(n)()(+) (n = 1-6)

Sector-field mass spectrometry is used to probe the fragmentation patterns of cationic dinuclear iron chloride clusters Fe(2)Cl(n)()(+) (n = 1-6). For the chlorine-rich, high-valent Fe(2)Cl(n)()(+) ions (n = 4-6), losses of atomic and molecular chlorine prevail in the unimolecular and collision-induced dissociation patterns. Instead, the chlorine deficient, formally low-valent Fe(2)Cl(n)()(+) clusters (n = 1-3) preferentially undergo unimolecular degradation to mononuclear FeCl(m)()(+) ions. In addition, photoionization is used to determine IE(Fe(2)Cl(6)) = 10.85 +/- 0.05 eV along with appearance energy measurements for the production of Fe(2)Cl(5)(+) and Fe(2)Cl(4)(+) cations from iron(III) chloride vapor. The combination of the experimental results allows an evaluation of some of the thermochemical properties of the dinuclear Fe(2)Cl(n)()(+) cations: e.g., Delta(f)H(Fe(2)Cl(+)) = 232 +/- 15 kcal/mol, Delta(f)H(Fe(2)Cl(2)(+)) = 167 +/- 4 kcal/mol, Delta(f)H(Fe(2)Cl(3)(+)) = 139 +/- 4 kcal/mol, Delta(f)H(Fe(2)Cl(4)(+)) = 113 +/- 4 kcal/mol, Delta(f)H(Fe(2)Cl(5)(+)) = 79 +/- 5 kcal/mol, and Delta(f)H(Fe(2)Cl(6)(+)) = 93 +/- 2 kcal/mol. The analysis of the data suggests that structural effects are more important than the formal valency of iron as far as the Fe-Cl bond strengths in the Fe(2)Cl(n)()(+) ions are concerned.     

The enthalpies of formation of o-, m-, and p-benzoquinone: gas-phase ion energetics, combustion calorimetry, and quantum chemical computations combined

Radical anions of o-, m-, and p-benzoquinone were produced in a Fourier transform mass spectrometer by low energy electron attachment or collision-induced dissociation and were differentiated. Classical derivatization experiments also were carried out to authenticate the ortho and meta anions. Gas-phase techniques were used to measure the proton affinities of all three radical anions and the electron affinities of o- and m-benzoquinone. By combining these results in thermodynamic cycles, we derived heats of hydrogenation of o-, m-, and p-benzoquinone (Delta(hyd)H degrees (1o, 1m, and 1p) = 42.8 +/- 4.1, 74.8 +/- 4.1, and 38.5 +/- 3.0 kcal mol(-)(1), respectively) and their heats of formation (Delta(f)H degrees (1o, 1m, and 1p) = -23.1 +/- 4.1, 6.8 +/- 4.1, and -27.7 +/- 3.0 kcal mol(-)(1), respectively). Good accord with the literature value for the para derivative was obtained. Combustion calorimetry and heats of sublimation also were measured for benzil and 3,5-di-tert-butyl-o-benzoquinone. The former heat of formation agreed with previous determinations, while the latter result (Delta(f)H degrees (g) = -73.09 +/- 0.87 kcal mol(-)(1)) was transformed to Delta(f)H degrees (1o) = -18.9 +/- 2.2 kcal mol(-)(1) by removing the effect of the tert-butyl groups via isodesmic reactions. This led to a final value of Delta(f)H degrees (1o) = -21.0 +/- 3.1 kcal mol(-)(1). Additivity was found to work well for m-benzoquinone, but BDE1 and BDE2 for 1,2- and 1,4-dihydroxybenzene differed by a remarkably small 14.1 +/- 4.2 and 23.5 +/- 3.7 kcal mol(-)(1), respectively, indicating that o- and p-benzoquinone should be excellent radical traps.     

Thermochemical studies of N-methylpyrazole and N-methylimidazole

The 351.1 nm photoelectron spectra of the N-methyl-5-pyrazolide anion and the N-methyl-5-imidazolide anion are reported. The photoelectron spectra of both isomers display extended vibrational progressions in the X2A' ground states of the corresponding radicals that are well reproduced by Franck-Condon simulations, based on the results of B3LYP/6-311++G(d,p) calculations. The electron affinities of the N-methyl-5-pyrazolyl radical and the N-methyl-5-imidazolyl radical are 2.054 +/- 0.006 eV and 1.987 +/- 0.008 eV, respectively. Broad vibronic features of the A(2)A' ' states are also observed in the spectra. The gas-phase acidities of N-methylpyrazole and N-methylimidazole are determined from measurements of proton-transfer rate constants using a flowing afterglow-selected ion flow tube instrument. The acidity of N-methylpyrazole is measured to be Delta(acid)G(298) = 376.9 +/- 0.7 kcal mol(-1) and Delta(acid)H(298) = 384.0 +/- 0.7 kcal mol(-1), whereas the acidity of N-methylimidazole is determined to be Delta(acid)G(298) = 380.2 +/- 1.0 kcal mol(-1) and Delta(acid)H(298)= 388.1 +/- 1.0 kcal mol(-1). The gas-phase acidities are combined with the electron affinities in a negative ion thermochemical cycle to determine the C5-H bond dissociation energies, D(0)(C5-H, N-methylpyrazole) = 116.4 +/- 0.7 kcal mol(-1) and D(0)(C5-H, N-methylimidazole) = 119.0 +/- 1.0 kcal mol(-1). The bond strengths reported here are consistent with previously reported bond strengths of pyrazole and imidazole; however, the error bars are significantly reduced.