The heat of fusion of 66 nylon

The heat of fusion ΔH_f of 66 nylon has been determined by use of the Clapeyron equation. Measurements of ΔH_f and the unit-cell parameters on molding pellets show that this material contains the α₂ crystal phase, which is less dense than the α₁ phase obtained by crystallization from solution. The value of ΔH_f-45–46 cal/g, is in good agreement with earlier reports.     

Structures and Energetics of NI3 and N2I4

We have applied new methods for performing coupled-cluster calculations to small molecules containing iodine atoms; specifically, NI₃ and N₂I₄. Because NI₃ is known to be very reactive, attempts to measure its thermodynamic properties have been challenging at best. To date, N₂I₄ has not been isolated, and our results suggest that its isolation will be just as challenging. We find that the ΔH_f(NI₃)=+307.7 kJ mol⁻¹ and ΔH_f(N₂I₄)=+551.6 kJ mol⁻¹, confirming that they are unstable with respect to their decomposition products N₂ and I₂.     

Collisionally activated dissociation of 2,4,6-triphenylpyridinium cations

Thresholds for the appearance of fragment ions allowed the estimation of threshold fragmentation energies (TFE) for the collisionally activated dissociation (CAD) in the gas phase of laser-desorbed pyridine-ring substituted N-benzylpyridinium cations to form pyridine and a carbocation. p-Methylbenzylpyridinium cation underwent an alternative CAD into pyridinium cation and the p-quinodimethane. The TFE are discussed in comparison with the energy differences (ΔΔH_f = ΔH_f(Py) + ΔH_f(R⁺) ? ΔH_f(Py ⁺R) calculated by the AM1 method to provide strong evidence for benzyl to tropylium cation rearrangement in an ion-molecule pair.     

Spectroscopic studies and determination of the formation constants and thermodynamic parameters for a new water-soluble Co(II) Schiff-base complex and some imidazole derivatives

A new water-soluble Co(II) Schiff-base complex, sodium[{N,N′-bis(5-sulfosalicylidene)-1,8-diamino-3,6-dioxaoctan}cobalt] dihydrate, abbreviated as Na₂[Co(II)L], was synthesized and characterized. The formation constants and thermodynamic parameters for the interaction of this complex with imidazole (Im) and 1-methylimidazole (MeIm) were determined spectroscopically in aqueous solution, ethanol/water (10/90), and methanol/water (10/90) under physiological conditions (pH?=?7), constant ionic strength (I?=?0.1?mol?dm^?3 KNO₃), and various temperatures ranging from 294 to 310?K. Our spectroscopic and thermodynamic results show that this adduct formation is endothermic and the positive values of ΔS _f° make ΔG _f° negative. The trend in variation of ΔH _f° and ΔS _f° for Im is in the order water?>?methanol?>?ethanol, but for MeIm it is in the opposite order which is related to the hydrogen bonding between solvents and these donors. Formation constants between MeIm and Na₂[Co(II)L] in these three solvents are larger than for Im which depends on the electron donation of methyl on MeIm.     

Non-newtonian viscosity of polypeptides in the helix–coil transition region

The non-Newtonian intrinsic viscosities [η] of poly(γ-methyl L-glutamate) were measured in the helix–coil transition region under various conditions in this work. The helix content f_H, which represents the degree of conformational transition, was obtained by using a polarimeter. Our experimental results show that the non-Newtonian behavior of the polypeptide is markedly affected by its conformation; i.e., the non-Newtonian effect becomes larger as f_H increases. The effect of external pressure ΔP on [η] was studied carefully; [η] increases with f_H when ΔP < 1.5 psi, but it decreases when ΔP > 1.5 psi and f_H > 0.8. The reason for this result is considered in the text.     

Group equivalents for converting ab initio energies to enthalpies of formation

Group equivalents which are useful for converting energies derived from ab initio calculations into enthalpies of formation have been obtained. They allow ΔH_f to be estimated from 6-31G* energies with an uncertainty on the order of ±2 kcal/mol.     

Prediction of hydrocarbon enthalpies of formation by various thermochemical schemes

The correlation consistent composite approach (ccCA) has been used to compute the enthalpies of formation (ΔH_f′s) for 60 closed‐shell, neutral hydrocarbon molecules selected from an established set (Cioslowski et al., J. Chem. Phys. 2000 , 113, 9377). This set of thermodynamic values includes ΔH_f's for hydrocarbons that span a range of molecular sizes, degrees of aromaticity, and geometrical configurations, and, as such, provides a rigorous assessment of ccCA's applicability to a variety of hydrocarbons. The ΔH_f's were calculated via atomization energies, isodesmic reactions, and hypohomodesmotic reactions. In addition, for 12 of the aromatic molecules in the set that are larger than benzene, the energies of ring‐conserved isodesmic reactions were used to calculate the ΔH_f′s. Using an atomization energy approach to determine the ΔH_f′s, the lowest mean absolute deviation (MAD) from experiment achieved by ccCA for the 60 hydrocarbons was 1.10 kcal mol^?1. The use of the mixed Gaussian/inverse exponential complete basis set extrapolation scheme (ccCA‐P) in conjunction with hypohomodesmotic reaction energies resulted in a MAD of 0.87 kcal mol^?1. This value is compared with MADs of 1.17, 1.18, and 1.28 kcal mol^?1 obtained via the Gaussian‐4 (G4), Gaussian‐3 (G3), and Gaussian‐3(MP2) [G3(MP2)] methods, respectively (using the hypohomodesmotic reactions). © 2012 Wiley Periodicals, Inc.     

Axial force fluctuations in long-chain molecules

The fluctuations Δf in the axial force f acting on a long-chain molecule whose end-to-end displacement r is fixed are computed for a freely jointed model in which the covalent bonds are represented by stiff linear springs. It is found that the relative fluctuation Δf/f remains large regardless of the length of the chain. A time-dependent computer simulation of the model serves both to verify the theoretical calculation of Delta;f and to provide physical insight. The result helps to clarify the difference in behavior of the strain ensemble (r is fixed and f fluctuates) and the stress ensemble (f is fixed and r fluctuates) for which, in the absence of excluded volume, the strain ensemble predicts that f = 0 for r = 0, while for the stress ensemble with f = 0 it is found that 〈r²〉₀ > 0.     

A Computational Study of 3‐D Helium Clusters

Physical and thermodynamic properties have been calculated and analyzed for the best and optimized geometries of the 3‐D clusters with N = 3 to N = 10 atoms and unit cells of three types of crystalline systems using ab initio RHF/6–31G^** method. Dependence of the lattice binding energy on the cluster parameter, R, has been studied. Similar behavior observed for the binding energies for all clusters shows that probabilities of their existence in the condensed phase are more or less the same. In the next step, thermodynamic properties have been calculated and analyzed for He₂₇ 3‐D helium clusters with simple cubic, body centered cubic (bcc), trigonal and hexagonal (hcp) configurations. The results show that the hexagonal cluster is more favored over other clusters. It is found that these clusters are electronically stable over a limited range of the values for the lattice parameter. Δ_fH is constant in this stability region and thus the Δ_fG exactly follows the variations of TΔ_fS. Surface effects have been investigated by comparing the square and hexagonal He₉ 2‐D lattices with the cubic and hexagonal He₂₇ 3‐D lattices, respectively. The lattice parameters, densities and molar volumes calculated for the clusters with hcp and bcc configurations have satisfactory agreement with the available experimental values. Properties of the He₁₃, He₃₄ and He₁₀₄ hcp clusters have also been calculated and analyzed.     

Molecular symmetry depresses the entropy of fusion of organic molecules with regard to their expected values when comparing structural isomers

Using published data, the symmetry effect on the entropy of fusion, ΔS_f, of rigid organic molecules has been investigated by comparing structural isomers. Because ΔS_f is dependent on the enthalpy of fusion, ΔH_f, [A.S. Gilbert, Thermochim. Acta 339 (1999) 131-142] this dependence must be allowed for as ΔH_f values can vary quite widely. When this is done, it is found that more symmetric isomers usually have lower than expected values of ΔS_f, in relation to ΔH_f, than their less symmetric counterparts. This is the reason that more symmetric isomers generally melt at higher temperatures, a fact noted by many workers over the years.     

Keto-to-enol isomerization in the molecular ion of dimethylmethylphosphonate

A mechanism for keto-to-enol isomerization in dimethylmethylphosphonate (DMMP) has been proposed based on deuterium-labeling studies, a model compound and thermodynamic data. An electron ionization study of H/D exchange occurring in CD₃-labeled DMMP suggests that rapid keto-to-enol isomerization occurs in the ion source and a reaction mechanism based on sequential 1,4-H migrations rather than by the direct 1,3-H transfer or by sequential 1,2-H migrations is proposed. The examination of the mass-analyzed ion kinetic energy/collision-induced dissociation spectrum of the methylphosphonic acid molecular ion suggests that keto-to-enol isomerization does not occur for this species and that 1,2- and 1,3-H migrations are not favored. Available thermodynamic data were employed to construct a potential energy surface for keto-to-enol isomerization of the DMMP molecular ion. The thermodynamic data show that the energy barrier to isomerization is below the internal energy required for decomposition of the DMMP molecular ion. Additionally, the ΔH_f° for the intermediate and enolic isbmers are shown to be significantly less than the ΔH_f° for the keto form of the DMMP molecular ion.     

Bond dissociation energies and radical heats of formation in CH3Cl,CH2Cl2, CH3Br,CH2Br2, CH2FCl,and CHFCl2

An analysis of thermochemical and kinetic data on the bromination of the halomethanes CH_4–nX_n (X = F, Cl, Br; n = 1–3), the two chlorofluoromethanes, CH₂FCl and CHFCl₂, and CH₄, shows that the recently reported heats of formation of the radicals CH₂Cl, CHCl₂, CHBr₂, and CFCl₂, and the C? H bond dissociation energies in the matching halomethanes are not compatible with the activation energies for the corresponding reverse reactions. From the observed trends in CH₄ and the other halomethanes, the following revised ΔH°_f,298 (R) values have been derived: ΔH°_f(CH₂Cl) = 29.1 ± 1.0, ΔH°_f(CHCl₂) = 23.5 ± 1.2, ΔH_f(CH₂Br) = 40.4 ± 1.0, ΔH°_f(CHBr₂) = 45.0 ± 2.2, and ΔH°_f(CFCl₂) = ?21.3 ± 2.4 kcal mol^?1. The previously unavailable radical heat of formation, ΔH°_f(CHFCl) = ?14.5 ± 2.4 kcal mol^?1 has also been deduced. These values are used with the heats of formation of the parent compounds from the literature to evaluate C? H and C? X bond dissociation energies in CH₃Cl, CH₂Cl₂, CH₃Br, CH₂Br₂, CH₂FCl, and CHFCl₂.     

Birefringence of amorphous polyoxides: Stress-optical behavior of poly(3-methyltetrahydrofuran)

The stress-optical behavior of an unswollen elastomeric network of poly(3-methyltetrahydrofuran) was measured for elongation ratios a in the range 1.182–1.549, at several temperatures between 20 and 60°C. No evidence of strain-induced crystallization was found; moreover, the dependence of birefringence Δn on true stress f/A was linear in the interval of α investigated. Values of Δa ranged from 2.4 to 2.8 in units of 10^?24 cm³, in the temperature range studied, with a temperature coefficient 3.1 × 10^?3 K^?1. Theoretical calculations carried out with the rotational-isomeric-state model gave values of Δa noticeably smaller than the experimental results; however, a small increase in the backbone valence angles θ improved the theoretical result of Δa without worsening that of the dipole ratio. Analysis of the Δa results seems to corroborate the conclusion obtained through the study of dipole moments concerning the preference for nucleophilic attack on the less hindered α carbon in the monomer. Theoretical and experimental values of the temperature coefficient of Δa were in clear disagreement; a qualitative explanation for this discrepancy is discussed.     

Evaluation of MINDO/3 calculated structures. III. Saturated acyclic compounds with chlorine,nitrogen, oxygen,or sulfur

Fifty-one structures have been calculated by the MINDO /3 method to evaluate the errors introduced by branching and by the presence of the heteroatom. The structures are evaluated by comparisons which reveal that the calculated ΔH_f values reflect a bias because of the presence of the heteroatom. With two carbons or more in a chain attached to a heteroatom group, a linear relationship exists which makes possible the calculation of reasonably accurate ΔH_f values for unbranched alcohols, primary or secondary amines, ethers, thioethers, thiols, and alkyl chlorides. Branching errors do not seem to be linearly related among the systems. Some errors in calculations of geometries are also discussed.     

Kinetics and equilibria in the system Br + t-BuO2H ⇆ HBr + t-BuO2. OH bond dissociation energy in t-BuO2-H

The kinetics and equilibria in the system Br + t-BuO₂H ? HBr + t-BuO₂· have been measured in the range of 300–350 K using the very low pressure reactor (VLPR) technique. Using an estimated entropy change in reaction (1) ΔS₁ = 3.0 ± 0.4 cal/mol·K together with the measured ΔG₁, we find ΔH₁ = 1.9 ± 0.2 kcal/mol and DHº (t-BuO₂-H) = 89.4 ± 0.2 kcal/mol ΔH_f·(tBuO₂·) = 20.7 kcal/mol and DH^º (t-Bu-O₂) = 29.1 kcal/mol. The latter values make use of recent values of ΔH_f·(t-Bu) = 8.4 ± 0.5 kcal/mol and the known thermochemistry of the other species. The activation energy E₁ is found to be 3.3 ± 0.6 kcal/mol, about 1 kcal lower than the value found for Br attack on H₂O₂. It suggests a bond 1 kcal stronger in H₂O₂ than in tBuO₂H.     

Determination of constant‐volume combustion energy for the complexes of zinc nitrate with three amino acids

Five solid complexes of zinc with L‐α‐methionine, L‐α‐phenylalanine and L‐α‐histidine were prepared. The constant‐volume combustion energies of the complexes, ΔE_c (coordination), were determined by a precise rotating bomb calorimeter at 298.15 K. They were ‐ 2969.03 ± 0.34, ‐2929.46 ± 1.59, ‐9597.13 ± 6.12, ‐4378.98 ± 3.27 and ‐14047 ± 6.75 kJ/mol, respectively. Their standard enthalpies of combustion, ΔH^θ_m,c(coordination, s, 298.15 K), and standard enthalpies of formation, ΔH^θ_m,f (coordination, s, 298.15 K), were calculated. They were ‐2959.73 ± 0.34, ‐2923.88 ± 1.59, ‐9649.18 ± 6.12, ‐4373.40 ± 3.27, ‐14048.53 ± 6.75 kj/mol and ‐1180.94 ± 0.92, ‐1401.26 ± 1.77, ‐2501.69 ± 6.50, ‐1381.47 ± 3.49, ‐1950.19 ± 7.65 kJ/mol, respectively.     

Synthesis,Crystal Structure and Thermochemical Properties of a One‐Dimensional Chain Complex [Cd(succ)PIP]n

A new complex, [Cd(succ)PIP]_n (PIP=2‐phenyl‐imidazo[4,5‐f]1,10‐phenanthroline, H₂‐succ=succinate), was synthesized and characterized by X‐ray crystallography, elemental analysis, and TG‐DTG. The results show that the complex crystallizes in an orthorhombic space group Pcca; a=14.065(2) Å, b=9.901(8) Å, c=28.933(2) Å and Z=8. The structure of the complex is one‐dimensional chain [Cd(succ)PIP]_n, and each Cd²⁺ is five‐coordinated by two chelating nitrogen atoms from one PIP ligand, three oxygen atoms from three different succ dianionic ligands to form a distorted trigonal‐bipyramida geometry. The constant‐volume combustion energy of the complex, Δ_cU, was determined by an intelligent micro‐rotating‐bomb calorimeter (IMRBC‐type I) at 298.15 K. Then the standard molar enthalpy of combustion, Δ_cH_m^?, and the standard molar enthalpy of formation, Δ_fH_m^? have been calculated.     

Quantitative Line-Shape Analysis of Temperature- and Concentration-Dependent 13C-NMR Spectra of 6Li- and 13C-Labelled Organolithium Compounds. Kinetic and Thermodynamic Data for Exchange Processes in Dibromomethyllithium, (Phenylthio)methyllithium,and Butyllithium

Using the ‘permutation of indices’ method proposed by Kaplan and Fraenkel, we could formulate the density-matrix equations required to fit the temperature-dependent ¹³C-NMR spectra observed with the title compounds. For ⁶Li¹³CHBr₂ ( 1 ) and ⁶Li¹³CH₂SC₆H₅ ( 2 ) an exchange mechanism is proposed by which monomers interchange C- and Li-atoms via a non-observed dimeric intermediate; the activation parameters of these intermolecular dynamic processes have been found to be ΔH^≠ = 10.2 kcal/mol, ΔS^≠ = 13.7 cal/mol·K for 1 and ΔH^≠ = 11.1 kcal/mol, ΔS^≠ = 20.6 cal/mol·K for 2 ((D₈)THF as solvent). In the case of (⁶Li)butyllithium ( 3 ), the observed low-temperature spectra indicate that dimeric ( 3b ) and tetrameric ( 3a ) species are in dynamic equilibrium interchanging the C₃HCH₂ groups (and THF molecules) bonded to the ⁶Li-atoms. The relative concentrations of the dimer and of the tetramer have been determined by peak integration or by line-shape fitting; the ‘pseudo’- equilibrium constant, defined by K′_eq = [ 3b ]²/[ 3a ], was found to be 2.6·10^?2 mol/1 (at ?88°) and corresponds to ΔG_R (?88°) = 2 ΔG°_f( 3b ) – ΔG°_f( 3a ) = 1.34 kcal/mol. The activation parameters of the dynamic process responsible for the exchange were estimated as ΔH^≠ = 3.78 kcal/mol and ΔS^≠ = ?31.3 cal/mol·K. Tentative interpretation of the thermodynamic and kinetic parameters is given.     

Stress-optical behavior of cycloaliphatic polyesters

Stress-strain-birefringence measurements were carried out on elastomeric networks of poly(oxymethylene-1,4-cis-cyclohexylenemethyleneoxysebacoyl) at several temperatures between 5 and 80°C. The dependence of both the birefringence Δn and the true stress f/A on temperature was found to be linear for T > 30°C; for T < 30°C an anomalous increase in the birefringence and a sharp decrease in the stress was observed. This behavior suggests that crystallinity is developed in the strained networks at low temperatures, and the crystallites are oriented in the direction of the elongation. Values of the optical configuration parameter Δa ranged from 9.15 to 8.28 in units of 10²⁴ cm³ in the temperature range studied. The value at 40°C of 10²⁴Δa, obtained from experiments performed on swollen networks, amounted to 7.47 cm³. These results suggest that intermolecular interactions enhance the birefringence of the strained networks. The quantities Δa and d In Δa/dT were calculated by using the valence optical scheme. Although the calculations reproduce the temperature coefficient fairly well, the theoretical values of Δa are smaller than the experimental ones. The agreement between theory and experiment is better assuming that the CH₂CH₂? COOCH₂ segment is freely rotating.     

Single pulse shock tube study on the thermal stability of ketones

5-Methyl-hexanone-2, 3-methyl-pentanone-2, and hexanone-2 have been decomposed in comparative rate single pulse shock tube experiments. The mechanism of decomposition involves the breaking of carbon-carbon bonds as well as molecular processes involving 6-center complexes. The following rate expressions at 1100 K have been obtained: These results lead to ΔH_f(CH₃?O) = ? 13.8 kJ and ΔH_f(CH₃COCH_2·) = ? 12.6 kJ at 300 K. They are compared with existing literature values and some generalizations are made with regard to the stability of carbonyl compounds.