Literature Thermochemistry Research Articles

Studies on the Kinetics of the CH + H2 Reaction and Implications for the Reverse Reaction, 3CH2 + H.

The reaction of CH radicals with H2 has been studied by the use of laser flash photolysis, probing CH decays under pseudo-first-order conditions using laser-induced fluorescence (LIF) over the temperature range 298-748 K at pressures of ∼5-100 Torr. Careful data analysis was required to separate the CH LIF signal at ∼428 nm from broad background fluorescence, and this interference increased with temperature. We believe that this interference may have been the source of anomalous pressure behavior reported previously in the literature (Brownsword, R. A.; J. Chem. Phys. 1997, 106, 7662-7677). The rate coefficient k1 shows complex behavior: at low pressures, the main route for the CH3* formed from the insertion of CH into H2 is the formation of 3CH2 + H, and as the pressure is increased, CH3* is increasingly stabilized to CH3. The kinetic data on CH + H2 have been combined with experimental shock tube data on methyl decomposition and literature thermochemistry within a master equation program to precisely determine the rate coefficient of the reverse reaction, 3CH2 + H → CH + H2. The resulting parametrization is kCH2+H(T) = (1.69 ± 0.11) × 10-10 × (T/298 K)(0.05±0.010) cm3 molecule-1 s-1, where the errors are 1σ.

Reactions of atomic thorium and uranium cations with CF4 studied by guided ion beam tandem mass spectrometry

The fundamental chemistry of the actinides continues to be an area of interest because of their use in nuclear fuel cycles. Here, the kinetic energy dependences of the reactions of thorium and uranium cations with CF4 were studied using guided ion beam tandem mass spectrometry. The products observed include AnFx+ (An = Th or U, x = 1–3) and CFy+ (y = 1–3). Although most reactions are exothermic according to literature thermochemistry, all cross sections display strong endothermic features indicating a restriction in the reactivity at low energies. For both actinides, only the AnF+ + CF3 product channel is observed at low energies; however, its formation proceeds very inefficiently. These results indicate that the reactions of Th+ and U+ with CF4 are not thermodynamically driven and suggest that the low-energy reactivity results from precise alignment of the reactants.

Non-adiabatic behavior in the homolytic and heterolytic bond dissociation of protonated hydrazine: A guided ion beam and theoretical investigation.

Threshold collision-induced dissociation using a guided ion beam tandem mass spectrometer was performed on protonated hydrazine and its perdeuterated variant. The dominant dissociation pathways observed were endothermic homolytic and heterolytic cleavages of the N-N bond. The data were analyzed using a statistical model after accounting for internal and kinetic energy distributions, multiple collisions, and kinetic shifts to obtain 0 K bond dissociation energies. Comparison with literature thermochemistry demonstrates that both channels behave non-adiabatically. Heterolytic bond cleavage yields NH2+ + NH3 products, but the NH2+ fragment is in the spin-restricted excited 1A1 state and not in the spin-forbidden ground 3B1 state, whereas homolytic bond cleavage leads to dissociation to the NH3+ + NH2 product asymptote with NH2 in its excited 2A1 state rather than the energetically favored 2B1 state. The rationale for the non-adiabatic behavior observed in the homolytic bond cleavage is revealed by detailed theoretical calculations of the relevant potential energy surfaces and the relevant occupied valence molecular orbitals. These calculations suggest that the non-adiabatic behavior results from conservation of the σ and π character of the binding and lone pair electrons on the nitrogen atoms.

Do protons recombine with O2[rad]− and CO2[rad]− in water clusters?

Gas phase reactions of O2−(H2O)m and CO2−(H2O)m with HCl as well as HNO3 are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. HCl and HNO3 are efficiently taken up by the clusters. In the reactions of O2−(H2O)m, a characteristic mass change is observed which is consistent with the evaporation of a hydroperoxyl radical HO2, most likely formed via proton transfer to O2−. Using CO2−(H2O)m as the ionic reactant, products indicating the formation of HOCO are hardly observed in the HCl reaction. With HNO3, however, evidence is found for elimination of HOCO, resulting from proton transfer to CO2−, when the clusters have lost almost all water molecules. Thermochemical cycles using literature thermochemistry show that the reactions are exothermic.

ChemInform Abstract: Cationic Noncovalent Interactions: Energetics and Periodic Trends

AbstractReview: 404 refs.

Cationic Noncovalent Interactions: Energetics and Periodic Trends.

In this review, noncovalent interactions of ions with neutral molecules are discussed. After defining the scope of the article, which excludes anionic and most protonated systems, methods associated with measuring thermodynamic information for such systems are briefly recounted. An extensive set of tables detailing available thermodynamic information for the noncovalent interactions of metal cations with a host of ligands is provided. Ligands include small molecules (H2, NH3, CO, CS, H2O, CH3CN, and others), organic ligands (O- and N-donors, crown ethers and related molecules, MALDI matrix molecules), π-ligands (alkenes, alkynes, benzene, and substituted benzenes), miscellaneous inorganic ligands, and biological systems (amino acids, peptides, sugars, nucleobases, nucleosides, and nucleotides). Hydration of metalated biological systems is also included along with selected proton-based systems: 18-crown-6 polyether with protonated peptides and base-pairing energies of nucleobases. In all cases, the literature thermochemistry is evaluated and, in many cases, reanchored or adjusted to 0 K bond dissociation energies. Trends in these values are discussed and related to a variety of simple molecular concepts.

Thermochemistry of the Reaction of SF6 with Gas-Phase Hydrated Electrons: A Benchmark for Nanocalorimetry.

The reaction of sulfur hexafluoride with gas-phase hydrated electrons (H2O)n(-), n ≈ 60-130, is investigated at temperatures T = 140-300 K by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. SF6 reacts with a temperature-independent rate of 3.0 ± 1.0 × 10(-10) cm(3) s(-1) via exclusive formation of the hydrated F(-) anion and the SF5(•) radical, which evaporates from the cluster. Nanocalorimetry yields a reaction enthalpy of ΔHR,298K = 234 ± 24 kJ mol(-1). Combined with literature thermochemical data from bulk aqueous solution, these result in an F5S-F bond dissociation enthalpy of ΔH298K = 455 ± 24 kJ mol(-1), in excellent agreement with all high-level quantum chemical calculations in the literature. A combination with gas-phase literature thermochemistry also yields an experimental value for the electron affinity of SF5(•), EA(SF5(•)) = 4.27 ± 0.25 eV.

Theoretical study of the anthropogenic greenhouse gas (SF5CF3) and analysis of the reaction of SF6 with CF

AbstractSF5CF3 was recently detected in the earth's atmosphere. This has lead to various experimental and theoretical studies aimed at establishing the properties of this molecule, from which to understand how it is formed, how it may impact on the atmosphere (it is a powerful greenhouse gas), and how it may be destroyed in the atmosphere. SF5CF3 is chemically very inert, and is not destroyed in the troposhere. Theoretical calculations using effective core potential (ECP) based on high‐level methods — QCISD(T), were performed for molecular systems related to SF5CF3. Existing values for the S–C bond dissociation energy in SF5CF3 span a wide range, from estimates of about 200 to 300 kJ mol−1, obtained from theoretical calculations on SF5CF3 using the DFT, G2, and G3 protocols, to 390 ± 45 kJ mol−1 derived from experimental studies of the photoionisation of SF5CF3. At the QCISD(T)/ECP + GCM (generator coordinate method) level, the dissociation energy ‐ Do (SF5 ‐CF3, 0 K), is 274.7 kJ mol−1. In addition to calculations on individual molecules, the reactions of SF6 with CF were studied. The first objective was to locate the reactant ion‐molecule complex, the transition state, and the product ion‐molecule complex for each of these systems. On the basis of literature thermochemistry, the reaction of SF6 with CF, in which the CF reactant anion abstracts F+ from SF6 to leave SF, is highly exothermic (−343 kJ mol−1). At the QCISD(T)/ECP + GCM level gives a value of −377.6 kJ mol−1. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Trends in ground-state entropies for transition metal based hydrogen atom transfer reactions.

Reported herein are thermochemical studies of hydrogen atom transfer (HAT) reactions involving transition metal H-atom donors M(II)LH and oxyl radicals. [Fe(II)(H(2)bip)(3)](2+), [Fe(II)(H(2)bim)(3)](2+), [Co(II)(H(2)bim)(3)](2+), and Ru(II)(acac)(2)(py-imH) [H(2)bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, H(2)bim = 2,2'-bi-imidazoline, acac = 2,4-pentandionato, py-imH = 2-(2'-pyridyl)imidazole)] each react with TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) or (t)Bu(3)PhO(*) (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex M(III)L and TEMPOH or (t)Bu(3)PhOH. Solution equilibrium measurements for the reaction of [Co(II)(H(2)bim)(3)](2+) with TEMPO show a large, negative ground-state entropy for hydrogen atom transfer, -41 +/- 2 cal mol(-1) K(-1). This is even more negative than the DeltaS(o)(HAT) = -30 +/- 2 cal mol(-1) K(-1) for the two iron complexes and the DeltaS(o)(HAT) for Ru(II)(acac)(2)(py-imH) + TEMPO, 4.9 +/- 1.1 cal mol(-1) K(-1), as reported earlier. Calorimetric measurements quantitatively confirm the enthalpy of reaction for [Fe(II)(H(2)bip)(3)](2+) + TEMPO, thus also confirming DeltaS(o)(HAT). Calorimetry on TEMPOH + (t)Bu(3)PhO(*) gives DeltaH(o)(HAT) = -11.2 +/- 0.5 kcal mol(-1) which matches the enthalpy predicted from the difference in literature solution BDEs. A brief evaluation of the literature thermochemistry of TEMPOH and (t)Bu(3)PhOH supports the common assumption that DeltaS(o)(HAT) approximately 0 for HAT reactions of organic and small gas-phase molecules. However, this assumption does not hold for transition metal based HAT reactions. The trend in magnitude of |DeltaS(o)(HAT)| for reactions with TEMPO, Ru(II)(acac)(2)(py-imH) << [Fe(II)(H(2)bip)(3)](2+) = [Fe(II)(H(2)bim)(3)](2+) < [Co(II)(H(2)bim)(3)](2+), is surprisingly well predicted by the trends for electron transfer half-reaction entropies, DeltaS(o)(ET), in aprotic solvents. This is because both DeltaS(o)(ET) and DeltaS(o)(HAT) have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between DeltaS(o)(HAT) and DeltaS(o)(ET) provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.

Reactions of Hf +, Ta +, and W + with O 2 and CO: Metal carbide and metal oxide cation bond energies

The reactions of Hf +, Ta +, and W + with O 2 and CO are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. All three reactions with O 2 form diatomic metal oxide cations in exothermic reactions that occur at the collision rate. In the CO systems, formation of both diatomic metal oxide and metal carbide cations is observed to be endothermic. The energy-dependent cross sections in the latter systems are interpreted to give 0 K bond energies (in eV) of D 0(HfC +) = 3.19 ± 0.03, D 0(TaC +) = 3.79 ± 0.04, D 0(WC +) = 4.76 ± 0.09, D 0(HfO +) = 6.91 ± 0.11, D 0(TaO +) = 7.10 ± 0.12, and D 0(WO +) = 6.77 ± 0.07. The present experimental values for TaO + and WC + agree well with literature thermochemistry, those for HfO + and WO + refine the available literature bond energies, and those for HfC + and TaC + are the first measurements available. The nature of the bonding in MO + and MC + is discussed and compared for these three metal ions and analyzed using theoretical calculations at a B3LYP/HW+/6-311+G(3df) level of theory. Bond energies for all MO + and MC + species are calculated using geometries calculated at this level and single point energies determined at B3LYP, CCSD, CCSD(T), QCISD, and QCISD(T) levels of theory with the same basis set. Reasonable agreement between the theoretical and experimental bond energies for the three metal oxide and three metal carbide cations is found. Potential energy surfaces for reaction of the metal cations with CO are also calculated at the B3LYP level of theory and reveal additional information about the reaction mechanisms.

Methane to Methanethiol Conversion by FeS+. A Combined Experimental and Theoretical Study†

The reaction of FeS+ with methane is examined by guided ion beam mass spectrometry and density functional theory employing the B3LYP/6-311+G* level of theory. For the FeS+/CD4 system examined in the experiments, two major product ions, Fe+ and FeSD+, are observed along with minor channels leading to FeCD3+, FeSCD3+, and FeSCD+. All products are formed in endothermic processes. The measured thresholds for the formations of Fe+ and FeSD+ are compared with computational data as well as literature thermochemistry. In the theoretical approach, two competing reaction mechanisms for the formation of Fe+, concomitant with neutral methanethiol, are investigated and used to interpret the experimental data. The lowest-energy path involves a formal 1,2-addition of H3C−H across the Fe+−S bond to generate a CH3FeSH+ insertion intermediate. This bond activation step involves spin inversion from the sextet to the quartet surface en route to the products. The occurrence of the second conceivable pathway resulting in formation of HFeSCH3+ as an intermediate can be ruled out because of the high-energy demand associated with overcoming the insertion barrier along this pathway.

Fragmentation behavior of Si2Cln+ cations (n = 1—6)

Sector-field mass spectrometry was used to probe the fragmentation patterns of the cationic silicon chlorides Si2Cln+ (n = 1—6). For almost all Si2Cln+ ions, Si—Si fragmentation predominates the Si—Cl bond cleavage both in the metastable ion and collisional activation mass spectra. Analysis of the fragmentation patterns indicates that the long-lived radical cation Si2Cl6·+ corresponds to a complex [SiCl2·SiCl4]·+ rather than the intact molecular ion of hexachlorodisilane. The behavior of Si2Cl5+ is consistent with the formation of the (trichlorosilyl)dichlorosilyl cation Cl3SiSICl2+. Structural aspects are also discussed for the other Si2Cln+ species. A semi-quantitative analysis of the fragmentation patterns in conjunction with the literature thermochemistry data was used to estimate some thermochemical properties of the Si2Cln+ cations.

Redox properties of charged and neutral iron chlorides FeCl mn (m = 1–3; n = −1, 0, +1, and +2)

Electron transfer in high-energy collision experiments is used to probe the redox chemistry of the iron chlorides FeCl m n (m = 1–3; n = −1, 0, +1, and +2). These experiments comprise charge inversion of FeCl m − anions ( m = 2–4) to cations, charge inversion of FeCl m + cations ( m = 1–3) to anions, charge stripping of FeCl m + monocations ( m = 1–3) to dications, and charge exchange of FeCl m 2+ dications ( m = 1, 2) to monocations. Ab initio calculations at the B3LYP/6-311+G∗ level of theory are used to evaluate the differences between adiabatic and vertical electron transfers; the accuracy of the calculated absolute energies for the associated electron-transfer processes predicted at this level of theory is doubted, however. The experimentally determined redox properties of the iron chlorides are in fair agreement with literature thermochemistry; new data derived in this work are: IE(FeCl 3) = 10.9 eV, IE(FeCl +) = 15.9 ± 0.4 eV, IE(FeCl 2 +) = 17.6 ± 0.7 eV, and IE(FeCl 3 +) = 16.0 ± 0.4 eV. In addition, evidence for the existence of the chlorine complexes Fe(Cl 2) + and Fe(Cl 2) 2+ is presented. According to the experimental data, diatomic FeCl 2+ is a thermochemically stable dication, whereas FeCl 2 2+ and FeCl 3 2+ are metastable with respect to the dissociations into FeCl ( m−1) + + Cl + and FeCl ( m−2) + + Cl 2 + ( m = 2, 3). Except for the dications, the dissociation behavior of the FeCl m n species ( m = 1–3; n = −1, 0, +1) is dominated by sequential losses of chlorine atoms rather than expulsion of molecular chlorine.

Gas-Phase Thermochemistry of Polyhalide Anions

A flowing afterglow−tandem mass spectrometer has been used to determine 0 K bond strengths in three hypervalent polyhalide ions: D(Cl2−Cl-) = 99 ± 5 kJ/mol, D(Br2−Br-) = 127 ± 7 kJ/mol, and D(Br2−Br3-) = 40 ± 7 kJ/mol. These bond strengths are close to the previously measured values D(I2−I-) = 126 ± 6 kJ/mol and D(I2−I3-) = 49 ± 6 kJ/mol. In contrast, Cl5- and F3- are not formed at room temperature in the flowing afterglow. Solvation energies for the trihalides derived from this work and literature thermochemistry are inversely correlated with the size of the ion, in reasonable agreement with the predictions of the Born model. The electron affinities EA(Cl3) = 4.60 ± 0.09 eV and EA(Br3) = 4.55 ± 0.10 eV can also be derived. The gas-phase bond strengths are more consistent with the three center-four electron bond model than with the expanded octet model.

Covalent Assistance in Metal-Mediated [4 + 2] Cycloadditions of Butadiene and Acetylene in the Gas Phase

The reactions of butadiene and acetylene, both complexed to atomic metal cations M+ (M = Cr, Mn, Fe, Co, Cu), have been investigated using ion-beam four-sector and Fourier transform ion cyclotron resonance mass spectrometry. For all metal ions studied, with the exception of Cu+, the reaction proceeds as a formal [4 + 2] cycloaddition involving 1,4-cyclohexadiene/M+ as an intermediate. This subsequently eliminates molecular hydrogen to generate the corresponding benzene/M+ complexes or bare M+ and C6H6. The Fe+-mediated reaction has been analyzed in detail, and isotopic labeling reveals that the cyclization step is rate-determining and that dehydrogenation of the six-membered ring occurs specifically from the C(1)/C(4)-positions of the butadiene building block. In conjunction with literature thermochemistry, qualitative potential-energy surfaces for the [4 + 2] cycloaddition are derived for M = Cr, Mn, Fe, and Co. The reactions are very efficient for Fe+ and Co+, while Cr+ and Mn+ are less capable of inducing C−C bond formation. Finally, Cu+ with its closed-shell s0d10 electronic ground state does not mediate the [4 + 2] cycloaddition at all. These differences are explained in terms of a model which invokes the active participation of the transition metal's d orbitals (covalent assistance), rather than mere Lewis-acid catalysis, which is known to catalyze many Diels−Alder reactions in the condensed phase.

Determination of the H 2ONO +2 and CH 3O(H)NO +2 bond strenghts and the proton affinities of nitric acid and methyl nitrate

The binding energies of water and methanol to NO + 2 have been measured to be 14.8 ± 2.3 and 19.2 ± 2.3 kcal/mol, respectively, using energy-resolved collision-induced dissociation of H 2ONO + 2 and CH 3O(H)NO + 2 in a flowing afterglow triple quadrupole apparatus. These values are used with literature thermochemistry to derive proton affinities for nitric acid and methyl nitrate; PA(HONO 2) = 177.7 ± 2.3 kcal/mol and PA(CH 3ONO 2) = 175.0 ± 2.5 kcal/mol. These results are in good agreement with recent calculations by Lee and Rice, but only the methyl nitrate result is in agreement with experimental results of Cacace and co-workers.

Kinetic-energy dependence of the reactions of C+(2P)+N2 from threshold to 28 eV CM

Guided-ion-beam mass spectrometry is used to examine the kinetic-energy dependence of the reaction of ground-state atomic carbon ion with molecular nitrogen. Integral reaction cross sections are measured for the three possible product channels: N2+ + C, CN+ + N, and N+ + CN, all strongly endothermic. Thresholds for all three channels are found to be consistent with literature thermochemistry, although formation of N2+ is inefficient near threshold. The observed behavior suggests that the Π3 state of CN+ is either the ground state or within 0.14 eV of the ground state, and that N2+ is formed primarily in its A 2Πu first excited state rather than the X 2Σg+ ground state. The thermochemistry measured here finds ΔfH00(CN)=102±3 kcal/mol, D00(CN)=7.82±0.14 eV, and IE(CN)=14.02±0.14 eV. The reaction mechanism for this system is explored by examination of the electronic state correlations and by phase-space calculations.

Reactions of Ar+, Ne+, and He+ with SiF4 from thermal energy to 50 eV c.m.

Guided ion-beam techniques are used to measure the cross sections for reaction of SiF4 with Ar+, Ne+, and He+ from thermal to 50 eV. Charge transfer followed by loss of F atoms are the sole processes observed. All SiF+x (x=0–4) products are observed, except for SiF+4 from reaction with Ne+ and He+, and Si+ from reaction with Ar+. At high energies, the dominant products are SiF+3 in the Ar system, and SiF+ in both the Ne and He systems. There is some evidence in the Ne system for an excited state of SiF+3 at 5.7 eV. In the Ar+ and Ne+ reactions, the observed energetics are consistent with literature thermochemistry, but with He+, reaction barriers are observed. A value of ΔH0f,298 (SiF+3)=−30.1±0.9 kcal/mol is derived, which is in agreement with previous values but is much more precise. The observed product distributions and energetics are explained by consideration of the potential energy surfaces and the difference in ionization potentials of the rare gases. Finally, the relationships of these reactions to plasma deposition and etching are discussed.

Kinetic energy dependence of Al++O2→AlO++O

The endothermic reaction of Al+ with O2 is studied using a guided ion-beam apparatus. The reaction cross section is measured as a function of kinetic energy from 0 to 20 eV. The threshold energy for the reaction, E0, is determined from an empirical model to be 3.64±0.04 eV. Phase space calculations of the cross section performed with E0 as the only adjustable parameter yield a similar result, E0=3.60±0.02 eV, and predict the absolute magnitude of the reaction cross section within experimental error. We conservatively quote the threshold energy as 3.62±0.12 eV which is somewhat higher than the thermodynamic threshold calculated using literature thermochemistry, 3.40±0.16 eV. This may indicate that a slight barrier to the reaction exists, or that AlO+ is produced in an excited state, or that the literature thermochemistry needs revision. Our results suggest D00 (AlO+)≥1.50±0.12 eV and I.P.(AlO)≤9.75±0.13 eV.

Reactions of U + and UO + with O 2, CO, CO 2, COS, CS 2 and D 2O

An ion-beam apparatus has been employed to investigate the reactions of uranium ions and uranium monoxide ions with O 2, CO, CO 2, COS, CS 2 and D 2O. Reaction cross sections as a function of ion kinetic energy are determined and compared to simple models for exothermic and endothermic reactions. With two exceptions, all exothermic reactions exhibit large cross sections which decrease with increasing kinetic energy. Although expected to be exothermic, the reactions of UO + with CO 2 and COS to form UO + 2 exhibit substantial energy barriers. Uranium ions react with CO to yield both UO + and UC + in endothermic processes. The thresholds for these reactions agree well with literature thermochemistry.