ΔHf Values Research Articles

Accurate Heats of Formation of the “Arduengo-Type” Carbene and Various Adducts Including H2 from ab Initio Molecular Orbital Theory

The heats of formation of saturated and unsaturated diaminocarbenes (imadazol(in)-2-ylidenes) have been calculated by using high levels of ab initio electronic structure theory. The calculations were done at the coupled cluster level through noniterative triple excitations with augmented correlation consistent basis sets up through quadruple. In addition, four other corrections were applied to the frozen core atomization energies: (1) a zero point vibrational correction; (2) a core/valence correlation correction; (3) a scalar relativistic correction; (4) a first-order atomic spin-orbit correction. The value of DeltaHf( 298) for the unsaturated carbene 1 is calculated to be 56.4 kcal/mol. The value of DeltaHf( 298) for the unsaturated triplet carbene (3)1 is calculated to be 142.8 kcal/mol, giving a singlet-triplet splitting of 86.4 kcal/mol. Addition of a proton to 1 forms 3 with DeltaHf( 298)(3) = 171.6 kcal/mol with a proton affinity for 1 of 250.5 kcal/mol at 298 K. Addition of a hydrogen atom to 1 forms 4 with DeltaHf( 298)(4) = 72.7 kcal/mol and a C-H bond energy of 35.8 kcal/mol at 298 K. Addition of H- to 1 gives 5 with DeltaHf( 298)(5) = 81.2 kcal/mol and 5 is not stable with respect to loss of an electron to form 4. Addition of H2 to the carbene center forms 6 with DeltaHf( 298)(6) = 41.5 kcal/mol and a heat of hydrogenation at 298 K of -14.9 kcal/mol. The value of DeltaHf( 298) for the saturated carbene 7 (obtained by adding H2 to the C=C bond of 1) is 47.4 kcal/mol. Hydrogenation of 7 to form the fully saturated imidazolidine, 8, gives DeltaHf( 298)(8) = 14.8 kcal/mol and a heat of hydrogenation at 298 K of -32.6 kcal/mol. The estimated error bars for the calculated heats of formation are +/-1.0 kcal/mol.

A statistical model for the optimization of dsc performance in the evaluation of drugs for preformulation studies

Differential scanning calorimetry (DSC) is a thermal analytical tool for preformulation studies. Extrapolated melting temperature (TP) and heat of fusion (ΔHf) can be used as parameters for optimizing the DSC performance. Two model pharmaceuticals acetaminophen and nicotinamide are used in this study. Using a factorial design for the experimental model and matrix analysis the results, the effect of sample mass, heating rate and the nitrogen flow rate were evaluated on the ΔHf values and TP values. Two levels for each of the procedural variables were used as a balanced experimental design with two sample sizes, two heating rates and two nitrogen flow rates. It was found that the change in the heating rate caused significant changes in the ΔHf values but not the Tp values for acetaminophen. However, no significant effect was found for the Tp value but ΔHf value was affected to a certain extent for nicotinamide.

Theoretical Study on Thermodynamic Properties of C1−C16 Alkanes: A 3-Parameter Least-Squared Calibration

The hybrid density-functional theory (DFT) calculated energies of C1−C16 straight-chain alkanes were obtained based on B3LYP/6-311++G(3df,2pd) single-point energies and the related thermal corrections of B3LYP/6-31G(d,p) optimized geometries. A 3-parameter modification equation and the least-squares approach are adopted to calibrate all the molecular calculated energies to produce accurate enthalpies of formation (ΔHf) and Gibbs free energies of formation (ΔGf), respectively. This study found that all compounds had 0.04% average relative error (A.R.E.) for the atomization energies, with a mean value of absolute error (M.A.E.) of just 1.1 kJ/mol for the ΔHf and 1.9 kJ/mol for the ΔGf of formation. The ΔHf values obtained in this work are superior to the improved G3(MP2)//B3LYP calculations for the same 16 alkanes in pertinent literature.

Relationship between Optical Basicity and Thermochemistry of Silicates

The optical basicity treatment of oxidic network systems, for example silicates, has been successful in correlating a range of properties; since the method has the advantage of avoiding such problems as those associated with “single ion” activities, it allows comparisons between many different systems. However, several of the properties dealt with are traditionally viewed in terms of thermodynamics. It is therefore of significance that a link has now been discovered between optical basicity and thermochemistry, and is established through the quantitative relationship between optical basicity and electronegativity. This link shows that thermodynamic stability increases with increasing negative charge borne by the oxygen atoms and, in principle, should allow heats of formation, ΔHf°, of silicate compounds to be predicted from their optical basicity values which, in turn, are calculated from chemical composition. This possibility is examined by making comparisons with experimentally obtained values of ΔHf°, ...

Thermodynamic properties of analcime solid solutions

Analcime (NaxAlxSi3-xO6 · [(3-x)/2]H2O, where x varies from ∼0.78 to ∼1.06) is one of the most common rock-forming zeolites. It forms in a wide range of geologic environments that span a range of temperature and pressure from ambient to magmatic conditions. Cluster variation method analysis of 29Si magic angle spinning nuclear magnetic resonance spectra indicates 1) the presence of at least two distinct states of short range Si/Al disorder [low (less disordered) and high (more disordered) analcime], and 2) that configurational entropy associated short-range Si-Al disorder within each of these states increases regularly with increasing Si content. Hydrofluoric acid (HF) solution calorimetry at 50°C was used to determine the enthalpy of formation of five pure analcime samples of varying composition (range of x approximately 0.95 to 1.05) and Si-Al disorder. Enthalpies of formation from the elements at 25°C (ΔHf) for these samples fall on a linear trend, except for one sample of high analcime for which ΔHf was about 6.1 ± 3.0 kJ/mol less stable than a low analcime of the same composition. Comparison with the results of previous calorimetric studies indicates negligible excess enthalpies of mixing in both low and high analcime solid solutions (that is, the solid solutions are athermal). The configurational entropies derived from cluster variation analysis were in turn used to derive activity-composition relationships for low analcime solid solutions whose compositions are bounded by an aluminous endmember (Na1.05Al1.05Si1.95O6 · 0.975H2O) and a siliceous endmember (Na0.75Al0.75Si2.25O6 · 1.125H2O). These relationships were used to retrieve thermodynamic properties for the endmembers from experimental observations of equilibria between analcime, albite, and aqueous solutions. Retrieved values of ΔHf are in excellent agreement with the calorimetric results of this study. Comparative analysis of equilibrium observations in the literature indicate that one sample of analcime from the Mont St. Hilaire alkaline intrusive complex used for analcime solubility measurements is high analcime. The Gibbs energy of disordering at 298.15 K, 1 bar consistent with the retrieval calculations is ∼ 6 kJ/mol. The thermodynamic properties of disordering for analcime indicate that hydrated low analcime is stable with respect to hydrated high analcime everywhere in Earth’s crust. Phase relations between low analcime, quartz, albite, and aqueous solutions calculated from the retrieved thermodynamic data indicate that at quartz equilibrium, low analcime should become more Si-rich with increasing temperature and pressure and that the composition of analcime is a sensitive function of the chemical potential of SiO2. Stable equilibrium between analcime, albite, quartz and H2O occurs at much lower temperatures than suggested by earlier phase equilibrium experiments. The breakdown of analcime plus quartz to form albite in geologic systems probably reflects metastable equilibrium in which the composition of analcime did not equilibrate with quartz.

Structural and Energetics Studies of Tri- and Tetra-tert-butylmethane

Applying the G3 and G3(MP2) models and using both the isodesmic and atomization schemes, the heats of formation (ΔHf) at 0 and 298 K are calculated for mono-tert-butylmethane (2,2-dimethylpropane or neopentane, abbreviated as mono-TBM), di-tert-butylmethane (di-TBM), tri-tert-butylmethane (tri-TBM), and tetra-tert-butylmethane (tetra-TBM). Upon examining the results, it is found that all of the calculated ΔHf298 values are well within ±10 kJ mol-1 of the available experimental data for the first three compounds. Hence, for tetra-TBM, a compound that has not yet been synthesized, the G3(MP2) results reported in this work should be reliable estimates. Moreover, we have found that the atomization scheme is slightly more suitable for the study of the smaller molecules, while the isodesmic scheme is more suitable for the larger molecules. Structurally, it is found that the equilibrium structures of mono-TBM, di-TBM, tri-TBM, and tetra-TBM have Td, C2, C1, and T symmetry, respectively. The energy-minimized structure of each TBM molecule is determined and all structural parameters are generally in good agreement with the available experimental data. Furthermore, it is found that the innermost C−C bond lengths increase along the series mono-TBM < di-TBM < tri-TBM < tetra-TBM, a trend that is expected from steric consideration.

DFT Calculation on the Thermodynamic Properties of Polychlorinated Dibenzo-p-dioxins: Intramolecular Cl−Cl Repulsion Effects and Their Thermochemical Implications

Polychlorinated dibenzo-p-dioxins (PCDDs) are one of the most intensively studied chemical pollutants. However, the absence of reliable thermodynamic data on PCDDs seriously limits quantitative understanding of their formation and distribution at combustion sources. By carrying out a DFT calculation (B3LYP/6-31G**) for thermodynamic properties of 75 PCDD congeners, we find that previous thermodynamic estimations on the gas-phase enthalpies of formation (ΔHf) and Gibbs free energy of PCDDs were seriously misestimated, particularly for highly chlorinated congeners, largely as a result of underestimating the intramolecular Cl−Cl repulsion energy. The disagreement between the DFT calculation and other methods becomes progressively larger with increasing chlorine substitution. Octachlorodibenzo-p-dioxin (OCDD) that has been considered as the most thermodynamically stable congener in previous calculations turns out to be much less stable. The differences in calculated ΔHf values between OCDD and the most toxic congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin, or other laterally chlorinated (2,3,7,8-substituted) toxic congeners are within 1 kcal/mol. Most ΔHf values for congeners with five to eight chlorines differ by only 1−2 kcal/mol, since the decreasing electronic energies with increasing the number of chlorines in PCDDs are counterbalanced by increasing Cl−Cl repulsion energy. The intramolecular chlorine repulsion effects in PCDDs are systematically analyzed by using isodesmic reactions.

Thermochemistry of Chlorine Fluorides ClFn,n= 1−7, and Their Singly Charged Cations and Anions: A Gaussian-3 and Gaussian-3X Study

The Gaussian-3 (G3) and Gaussian-3X (G3X) models of theory have been used to calculate the thermochemical data for chlorine fluorides ClFn, n = 1−7, as well as for their singly charged cations and anions. The quantities calculated include the heats of formation (ΔHf) and bond dissociation energies (DEs) of all of the species, as well as the ionization energies (IEs) and electron affinities (EAs) of the neutrals. By comparing the well-established experimental data of ClF and ClF3 with the G3 and G3X results, it is found that the G3X method yields more accurate ΔHf values. In addition, the G3 values on the IEs and EAs for ClF and ClF3 are similar to the corresponding G3X results. On the basis of these findings, the G3X results are used to assess the sometimes conflicting experimental data. The fair to excellent agreement between the known experimental values and the G3X results also lends support to our predictions for the missing experimental thermochemical data of chlorine fluorides and their ions. Furthe...

Heats of formation for the boron hydrides: a Gaussian-3 study

Applying the Gaussian-3 (G3) model, using both the isodesmic and atomization schemes, the heats of formation ( ΔH f) at 0 and 298 K are calculated for 19 boron hydrides with 1–10 boron atoms. Upon examining the results, it is found that essentially all of the isodesmic ΔH f0 and ΔH f298 values are well within ±10 kJ mol −1 of the experimental data, the only exceptions being those for B 10H 14. Hence, the isodesmic G3 results for those boron hydrides (studied in this work) with no experimental data should be reliable estimates. On the other hand, the atomization scheme leads to much more inferior results, even though, as shown in previous work, the same method yields very good results for the (CH) 6 isomers. A possible source for this deficiency of the G3 method has been suggested.

The synthesis of short- and medium-chain-length poly(hydroxyalkanoate) mixtures from glucose- or alkanoic acid-grown Pseudomonas oleovorans

NRRL B-778 accumulated mixtures of poly-3-hydroxybutyrate (PHB) and medium-chain-length poly(hydroxyalkanoates) (mcl-PHAs) when grown on glucose, octanoic acid or oleic acid, whereas growth on nonanoic acid or undecanoic acid resulted in copolymers of poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHB-co-HV). Acetone fractionation verified the presence of PHB/mcl-PHA mixtures. The acetone-insoluble (AIS) fractions of the polymers derived from glucose (PHA-glucose), octanoic acid (PHA-octanoic) and oleic acid (PHA-oleic) were exclusively PHB while the acetone-soluble (AS) fractions contained mcl-PHA composed of differing ratios of 3-hydroxy-acid monomer units, which ranged in chain length from 6 to 14 carbon atoms. In contrast, both the AIS and AS fractions from the polymers derived from nonanoic acid (PHA-nonanoic) and undecanoic acid (PHA-undecanoic) were composed of comparable ratios of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV). The unfractionated PHA-glucose, PHA-octanoic and PHA-oleic polymers had melting temperatures (Tm) between 177 and 179°C, enthalpies of fusion (ΔHf) of 20 cal/g and glass transition temperatures (Tg) of 3–4°C. This was due to the large PHB content in the polymer mixtures. On the other hand, the PHA-nonanoic and PHA-undecanoic polymers had thermal properties that supported their copolymer nature. In both cases, the Tm values were 161°C, ΔHf values were 7cal/g and Tg values were −3°C. Journal of Industrial Microbiology & Biotechnology (2002) 28, 147–153 DOI: 10.1038/sj/jim/7000231

B3LYP/6-31G∗, RHF/6-31G∗ and MM3 heats of formation of disaccharide analogs

ΔHf values were calculated with the bond- and group-enthalpy method for analogs of 13 disaccharides to learn the relative stabilities for various configurations about the carbon atoms connected to the linkage oxygen atom. The analogs were based on tetrahydropyran and, in the case of the sucrose analogs, tetrahydrofuran. The molecular mechanics program MM3 calculates these values as an option. The method was also used with RHF/6-31G∗ and B3LYP/6-31G∗ quantum mechanics (QM) theory. ΔHf values had ranges of 15–17kcal (all energies herein are molar) by the different methods, with the analogs of the three trehaloses and sucrose having the lowest values. ΔHf values for the isomeric analogs of non-reducing sugars with two anomeric centers are about −150kcal, about 8kcal lower than for the analogs of the reducing dimers (nigerose, maltose, laminarabiose, cellobiose and galabiose (α-d-galactosyl-d-galactose)) by all three types of calculation. The analogs of the di-axial, axial–equatorial and di-equatorial, non-glycosidic pseudo-disaccharides had ΔHf values within 1.0kcal by each of the three methods. They were about 8kcal higher than for the molecules that contain one glycosidic sequence. The relative ΔHf values by QM were within 1.0kcal of their corresponding relative electronic energies, except for the methylated sucrose analog, for which the discrepancy was about 1.85kcal at both QM levels. Compared to our B3LYP results, the RHF-based calculations overestimated the stability of all molecules by about 2.85kcal. The MM3 values were close to the B3LYP numbers, with the largest discrepancy for the cellobiose analog, 1.76kcal. The stabilization embodied in the group enthalpy increment for anomeric centers is another manifestation of the anomeric effect.

Calculated heats of formation of simple phosphinidenes (phosphanylidenes, R–P)

Ab initio MO and DFT calculations up to the coupled-cluster theory level, CCSD(T) with large 6-311++G(3df,2p) and cc-pVTZ basis sets, have been applied to determine the heats of formation of a series of simple phosphinidenes (R–P) that exhibit a triplet ground state. Three different exchange and isogyric reactions of the type RP+HX→HP+RX have been used as working reactions. A set of consistent values for ΔHf,298° (R–P) are evaluated as follows (values in kcal/mol): H3CP, 47.6; H2NP, 48.6; HOP, −0.5; FP, −12.5 (exp: −12.4); H3SiP, 61.9; H2PP, 58.0; HSP, 47.9; ClP, 30.0 (exp: 30.8); BrP, 40.0 (exp: 38.9) with an estimated error bar of ±2kcal/mol. DFT/B3LYP calculations have been found to reproduce these values with acceptable accuracy. Empirical corrections for the DFT/B3LYP calculations were estimated and heats of formation for a series of other phosphinidenes were also evaluated using this correction: H2FCP, 10.2; HF2CP, −43.6; F3CP, −96.0; H2ClCP, 46.6; H2BrCP, 56.8; OCHP, 34.6; SCHP, 81.2; HNCHP, 77.3; CH3CH2P, 45.4; H2CCHP, 70.4; HCC–P, 100.5. Except for the diatomic species, no experimental values for RP are available yet.

Synthesis, Structure, Strain Energy, and Excess Strain of a Phospha[3]triangulane

(7-Phenyl-7-phosphadispiro[2.0.2.1]heptane)pentacarbonyltungsten (8), a phospha[3]triangulane, was synthesized from bicyclopropylidene. Its single-crystal X-ray structure determination is reported. Comparison of the crystal structure data with those of the related phosphaspiropentane 7 and phosphirane 6 complexes suggests that the phosphirane ring tightens when the number of spiro atoms is increased. This is supported by the B3LYP and MP2/6-31G* computed geometries of the uncomplexed parent systems. Ab initio calculated heats of formation and strain energies (SE) are reported for the parent phosphirane 11, phosphaspiropentane 12, and phospha[3]triangulane 13 using both G2MP2 theory and ring separation reactions. Our best estimates for the ΔHf of 11, 12, and 13 are 18.3, 48.4, and 78.2 kcal/mol, respectively, with corresponding SE values of 21.3, 54.7, and 87.9 kcal/mol. For comparison, the slightly modified G2MP2 method was also applied to cyclopropane 1, spiropentane 2, and [3]triangulane 3 to give respective ΔHf values of 12.6, 44.3, and 75.3 kcal/mol, with corresponding SEs of 28.0, 64.6, and 100.5 kcal/mol, all of which are in excellent agreement with reported experimental data. These strain energies suggest that the excess strain per spiro atom is 5.3 kcal/mol for phospha[n]triangulanes, which is smaller than the 8.6 kcal/mol determined from the heat of combustion measurements for the [n]triangulanes.

Ab Initio Calculation of the Heats of Formation of Nitrosamides: Comparison with Nitramides

Ab initio calculations of the structures and energies of the mono-, di-, and trinitrosamides are compared to mono-, di-, and trinitramides. Highly accurate standard heats of formation of this series of nitramides and nitrosamides are calculated using isodesmic reactions and high-level theories. The nitrosamides have heats of formation at 0 K that are 20−40 kcal/mol higher than the analogous nitramides. The N−N bond strengths are comparable whether a NO or NO2 group is bonded to the central nitrogen atom. The data suggest that converting the nitro groups to nitroso groups will result in compounds that have lower weight, shorter N−N bond lengths, and higher energy content, at the expense of lower oxygen balance. This suggests that nitrosamides are potentially useful high-energy materials. A revised ΔHf° for NH2NO2 is proposed to be 3.8 kcal/mol at 0 K, in close agreement with previous theory and very different from experiment. Recommended values for ΔHf° are 34 and 71 kcal/mol for di- and trinitramide, resp...

Thermodynamic Properties ΔHf°298, S°298, and Cp(T) for 2-Fluoro-2-Methylpropane, ΔHf°298 of Fluorinated Ethanes, and Group Additivity for Fluoroalkanes

G2(MP2) composite calculations are performed to obtain thermodynamic properties (ΔHf°298, S°298 and Cp(T)'s of 2-fluoro-2-methylpropane. ΔHf°298 is calculated from the G2(MP2) calculated enthalpy of reaction (ΔHrxn°298) and use of isodesmic reactions. Standard entropy (S°298 in cal/(mol·K)) and heat capacities (Cp(T)'s in cal/(mol·K)) are calculated using the rigid-rotor−harmonic-oscillator approximation with direct integration over energy levels of the intramolecular rotation potential energy curve. These thermodynamic properties are used to estimate data for the C/C3/F group. Enthalpies of formation (ΔHf°298 in kcal/mol) for 1,2-difluoroethane (−102.7), 1,1,2-trifluoroethane (−156.9), 1,1,2,2- and 1,1,1,2-tetrafluoroethane (209.6 and 213.3), and pentafluoroethane (−264.1), are calculated using total energies obtained from G2(MP2) composite ab initio methods. Isodesmic reactions with existing literature values of ΔHf°298 for ethane, 1-fluoroethane, 1,1-difluoroethane and 1,1,1-trifluoroethane are used. Fluorine/fluorine interaction terms, F/F, 2F/F, 3F/F, 2F/2F, and 3F/2F, where “/” indicates interaction for alkane compounds, for ΔHf°298 are reevaluated based on ΔHf°298 of the above five fluoroethanes. Thermodynamic properties (ΔHf°298, S°298 and Cp(T)'s (300 ≤ T/K ≤ 1500)) for fluorinated carbon groups, C/C3/F, C/C2/F/H, C/C2/F2, are calculated using data from ab initio methods and existing literature data. Fluorine−methyl (alkyl) group additivity corrections for gauche interactions are also evaluated.

Theoretical Study of the Heats of Formation of Small Silicon-Containing Compounds

Heats of formation for nine small silicon-containing molecules were obtained from large basis set ab initio calculations using coupled cluster theory with a perturbative treatment of triple excitations. After adjusting the atomization energies for the finite basis set truncation error, core/valence correlation, scalar relativistic, higher order correlation, and atomic spin−orbit effects, the theoretical and experimental 0 K values of ΔHf values were in good agreement. Using 106.6 kcal/mol as the heat of formation of silicon, we obtain ΔHf values of SiH = 87.7 ± 0.4 vs 89.5 ± 0.7 (expt); SiH2(1A1) = 64.1 ± 0.4 vs 65.5 ± 0.7 (expt); SiH2(3B1) = 85.4 ± 0.4 vs 86.5 ± 0.7 (expt); SiH3 = 47.3 ± 0.5 vs 47.7 ± 1.2 (expt); SiH4 = 8.7 ± 0.6 vs 9.5 ± 0.5 (expt); Si2 = 138.8 ± 0.4 vs 139.2 (expt); Si2H6 = 19.7 ± 0.5 vs 20.9 ± 0.3 (expt); SiF = −14.8 ± 0.4 vs −5.2 ± 3 (expt); SiF2 = −151.7 ± 0.5 vs −140.3 ± 3 (expt); and SiF4 = −384.5 ± 0.9 vs −384.9 ± 0.2 (expt). Based on the present work, we suggest a number of revisions in the interpretation of the experimental data. Although a revision in ΔHf°(Si) to 107.4 ± 0.6 kcal/mol at 0 K leads to improved agreement between theory and experiment for the SixHy compounds, it worsens agreement for SiF4. Given the remaining uncertainties in the theoretical approach, more definitive conclusions do not appear to be warranted.

G2 Theory and Experiment in Concert: Enthalpy of Formation of CH3O−CO+ and Its Isomers Revisited

Ab initio molecular orbital calculations at the Gaussian-2 level of theory on a set of isodesmic, atomization, and substitution type reactions have been used to deduce the enthalpy of formation of the methoxycarbonyl ion as ΔHf298[CH3O−CO+] = 130 ± 2 kcal/mol. From the G2 computed ionization energy (IEa = 7.32 eV) and ΔHf298 (−40 kcal/mol) of the parent radical CH3O−C•O, we arrive at 129 kcal/mol for its ionic counterpart. Combining these theoretical findings with a reevaluation of existing experimental data (appearance energy measurements) yields 129 ± 2 kcal/mol as our recommended value for ΔHf298[CH3O−CO+], a large upward revision of the current literature value of 120 kcal/mol. By use of the new value as the anchor point, G2 derived ΔHf298 values for the isomers HOCH2−CO+, +CH2−H···OCO, +CH(OH)-C(O)H, +CH2−O−C(O)H, and +CH2O−C−OH have been calculated as 147, 131, 157, 144, 144, 140, and 177 kcal/mol, respectively.

Konfigurations- und konformationsisomere paratrope, rotationsdynamische Tetraepoxy[32]annulene(6.2.6.2) und diatrope Tetraoxa[30]porphyrin(6.2.6.2)-Dikationen: Nachweis eines Tetraepoxy[31]annulen(6.2.6.2)-Radikalkations

Configurational and Conformational Isomeric Paratopic, Rotational Dynamics Tetraepoxy[30]annulenes(6.2.6.2) and Diatropic Tetraoxa[30]porphyrin(6.2.6.2) Dications: Detection of a Tetraepoxy[31]annulene(6.2.6.2)Radical Cation The synthesis of tetraepoxy[32]annulenes(6.2.6.2) (4) by a cyclizing twofold Wittig reaction of (E,E,E)-5,5′-(hexa-1,3,5-triene-1,6-diyl)bis[furan-2-carbaldehyde] (6) and the corresponding bis-phosphonium salt 7 is described (Scheme 1). Contrary to the configuration of the educts, the obtained annulenes 4a and 4b are (Z,E,E,E,Z,E,E,E)- and (E,Z,E,E,E,Z,E,E)-configurated, respectively. The 1H-NMR spectra establish the paratropic, antiaromatic character of 4. The annulenes 4 are highly dynamic systems, the (E)-ethenediyl bridges rotate around the adjacent σ-bonds, these rotations are frozen at −80°. The McMurry condensation of dialdehyde 6 yields the (E,E,Z,E,E,E,Z)-4,5-dihydrotetraepoxy[32]annulene(6.2.6.2) (13a), where the configuration of the dialdehyde 6 – beside the hydrogenated double bond – is retained. As result of an intramolecular McMurry reaction of 6, (Z,E,Z,Z)-dioxa[16]annulene(6.2) 14 is formed. By oxidation of the [32]annulenes(6.2.6.2) 4a and 4b, a mixture of the four stereoisomeric tetraoxa[30]porphyrin(6.2.6.2) dications 5a/5a′/5b/5c is obtained; the configuration of the isomers is determined by COSY, NOESY, and NOE experiments. The Δδ values (26.81, 25.83, and 21.11 ppm) underline the diatropic, aromatic character of the dications 5, the Soret bands are shifted bathochromically to 550 nm, and the Q-bands are in the NIR region (896 – 1039 nm). The dihydroannulene 13a is dehydrogenated by p-chloroanil (tetrachloro-1,4-benzoquinone) to give the annulenes 4a and 4b, its oxidation with DDQ (=4,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile) results in the same mixture of dications 5. Entirely different results are obtained by reaction of the dihydroannulene 13a with DDQ. Here, the (E,E,E,Z,E,E,E,Z) tetraoxa[30]porphyrin(6.2.6.2) dication 5c – formed only in traces from 4a/4b – is the main product. Beside 5c, a by-product (3%) can be isolated, which turns out (ESR, conductivity) to be the (E,E,E,Z,E,E,E,Z)-tetraoxa[31]porphyrin(6.2.6.2) radical cation 16, obviously the intermediate in the oxidation sequence of the annulene to the dication. This result leads to the conclusion that the reaction of the dihydro compound 13a with p-chloroanil and DDQ follows different reaction mechanisms. For all isolated stereoisomeric tetraepoxy annulenes and tetraoxaporphyrin dications, the ΔHf values are calculated by the semiempiric AM1 method. The results are in agreement with the experimental observations. All data confirm the antiaromaticity of the tetraepoxy[32]annulenes(6.2.6.2) 4 and the aromaticity of the tetraoxa[30]porphyrin(6.2.6.2) dications.

The recombination of hydrogen atoms with nitric oxide at high temperatures

The rate constant for the H+NO+N2 reaction (R1,N2) has been determined in the temperature range 1000–1170K from flow-reactor experiments on the CO/O2/H2O/N2 system perturbed with different amounts of NO. The initiation temperature of this system is highly sensitive to reaction R1, which is the rate-controlling step in the nitric oxide catalyzed removal of hydrogen atoms. Based on the flow-reactor results and the limited amount of data reported in literature, a rate constant for the H+NO+N2 reaction of 4.0×1020T−1.75 cm6/(mol2s) was determined. This value is in good agreement with the recent result of Allen and Dryer at 1000K but significantly lower at high temperatures than the recommendation of Tsang and Herron. With the recently determined value of ΔHf,298 (HNO) of 26.0 kcal/mol, which is 2 kcal/mol higher than previous estimates, our results correspond to a rate constant of 1.7×1019T−1.5 exp(−23,400/T) cm3/(mol s) for the HNO+N2 dissociation reaction in the 1000–2500 K range. The sharp drop-off in the rate constant for H+NO+M at high temperatures suggested by the flow-reactor results are supported by reinterpretation of data reported in literature on H2/O2/N2 flames doped with NO. Theoretical considerations suggest that the effect can be attributed to weak-collision effects.

Heat of Fusion and Surface Tension of Solids Confined in Porous Materials Derived from a Combined Use of NMR and Calorimetry

Assuming the heat of fusion (Δhf) of a material confined in a porous material to be approximated by a function Δhf = Δh0 (1 + a0(103/T) + a1(103/T)2), where T is the absolute temperature, a theoretical model is derived that enables the coefficients ai to be determined from a combined use of NMR and calorimetric measurements. The model has been applied on solid ice confined in cement pastes resulting in a0 = −0.136 K and a1 = −0.00413 K2 in the temperature range 273 K > T > 210 K. Δh0 was determined from the known value of Δhf of bulk water at 273 K, giving Δh0 = 749 J/g. Likewise, assuming the surface tension (γ) of the ice−water interface to be approximated by a corresponding second-order polynomial in 1/T, i.e., γ = γ0 (1 + b0(103/T) + b1(103/T)2), the coefficients bi were determined from the Gibbs−Thomson equation: ΔT = Kf(γ/ρΔhf)(1/R), where Kf is a constant, ρ the density, and ΔT the lowering of the melting point of ice confined in pores with radius R. The model fit revealed a best fit to a linear f...