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.     

Kinetics of the droplet formation at the early and intermediate stages of the spinodal decomposition in homopolymer blends

The kinetics of the droplet formation during the spinodal decomposition (SD) of the homopolymer blends has been studied by numerical integration of the Cahn‐Hilliard‐Cook equation. We have found that the droplet formation and growth occurs when the minority phase volume fraction, f_m , approaches the percolation threshold value, f_thr = 0.3 ± 0.01. The time for the formation of the disperse droplet morphology (coarsening time) depends only on the equilibrium minority phase volume fraction, f_m . f_m approaches its equilibrium value logarithmically at the late SD stages, and, therefore, the coarsening time decreases exponentially as the average volume fraction or the quench depth decrease. Since the temporal evolution of the total interfacial area does not depend on the quench conditions and blend morphology, the average droplet size and the droplet number density is determined by the coarsening time. Within the time scale studied, the droplet number density decreases with time as t ^{–0.63±0.03}; the average mean curvature decreases as t ^{–0.35±0.05}; the average Gaussian curvature decreases as t ^{–0.42±0.03}, and the average droplet compactness ˜V/S^3/2 where S is the surface area and V is the volume) approaches a spherical limit logarithmically with time. The droplets with larger area have lower compactness and in the low compactness limit their area is a parabolic function of compactness. The size and shape distribution functions have been also investigated.     

A reassessment of some restricted Hartree–Fock limit molecular energies and an investigation of the applicability of Ermler and Kern's procedure for their estimation

New estimates of Hartree–Fock limit energies (E_RHF) for selected AH and AH_n hydrides, diatomic and linear polyatomic molecules have been made utilizing E_SCF values recently reported in the literature for HF, N₂, CO, NH₃, and CH₄ which are very close to the respective limits. These new values have been used to investigate the applicability of Ermler and Kern's procedure for estimating E_RHF: i.e., a factor f is first evaluated from data for reference molecules, where f = E_RHF/E_SCF, which is then used with E_SCF values for other molecules to obtain their E_RHF values. f has been evaluated for three groups of reference molecules? HF, H₂O, NH₃, CH₄, N₂, and CO; CH₄, C₂H₂, C₂H₄, and C₂H₆; and C₂H₂, HCN, and N₂? utilizing E_SCF data in the literature for many Gaussian-type orbital (GTO) basis sets together with some new values calculated at the (9,5,1) to (13,8,2) levels. Trends in the variation of f within each group of reference molecules from one basis set to another, and the trends in f from one group of reference molecules to another, are discussed in detail. To minimize the influence of these effects in an E_RHF estimate it is recommended that the f value should be derived from reference molecules which possess a similar combination of structural features, i.e., bonded hydrogen, single, double, or triple bonds, and the number of lone-pair electrons. Further calculations show that an f value based on data for closed-shell molecules is not applicable to open-shell species.     

Gauge parameters for extrapolation of oscillator strengths ofE1 transitions along the boron isoelectronic sequence

The oscillator strengthsf forE1 transitions along an isoelectronic sequence can be written asf=aK ²+bK+c whereK is a gauge parameter representing the gauge condition of the electromagnetic field. The coefficientsa,b, andc are functions of length (f _l) and velocity (f _v) values of the oscillator strengths at the Hartree-Fock level. We have shown by making a perturbation expansion of oscillator strengthsf,f _l andf _v that the gauge parameterK is independent of the nuclear charge. This property has been exploited to extrapolatef values along the isoelectronic sequence of Boron for some representativeE1 transitions within then=2 complex. We obtain good agreement between the extrapolated results with the configuration interaction results.     

Enthalpies of the formation of aliphatic carbonyl-containing radicals

A collection of data on enthalpies of the formation(Δ_f H ^o) of aliphatic carbonyl-containing radicals is analyzed and expanded. The Δ_f H ^o values for 29 carbonyl-containing radicals are determined for the first time, and are strongly revised for 17 carbonyl-containing radicals using the literature data on the dissociation energies of the bonds in molecules. The data is analyzed on the basis of the structureproperty (enthalpy of formation) relation within the additive-group approach, with the determination and specification of the parameters. It is concluded that the Δ_f H ^o values of carbonyl-containing radicals calculated from the obtained parameters (a total of 96 compounds was considered) agree well with the experimental data.     

Changes in the Transfer Function of a flow microcalorimeter

Flow microcalorimeters are used to determine thermodynamic properties of liquid mixtures, the accuracy of these measures depends on the right calibration of the instrument. In this work the system is identified by means of the transfer functions of the two poles, it is proven that the first time constant and the sensitivity change with the value of rc _p f of the injected liquids (r - density, c _p - heat capacity, f - injection flow), and that the sensitivities obtained in the electrical and chemical calibrations are different for the same value of rc _p f because the dissipation in each case does not occur in the same place. As a summary of the calibration carried out, it is proposed a sensitivity value of 313±4 mV W^-1 for rc _p f<15 mW K^-1 that permits to make thermal measures with an uncertainty of 3%. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dihydroxyacetone (DHA) monomer complexes with CaBr2 and CdCl2

Two hydrated complexes of monomeric dihydroxyacetone (DHA; the simplest ketose), viz. the calcium bromide complex bis(μ‐dihydroxyacetone)bis[tetraaquacalcium(II)] tetrabromide (isomorphous with the chloride compound reported previously), [Ca₂(C₃H₆O₃)₂(H₂O)₈]Br₄, (2e), and the cadmium chloride complex poly[[bis(μ‐dihydroxyacetone)bis[bis(dihydroxyacetone)cadmium(II)]] [diaquatetradeca‐μ‐chlorido‐dichloridohexacadmium(II)] tetrahydrate], {[Cd₂(C₃H₆O₃)₆][Cd₆Cl₁₆(H₂O)₂]·4H₂O}_n, (2f), are described. The Ca²⁺ or Cd²⁺ ions are bridged by the carbonyl O atoms from two DHA molecules to form centrosymmetric dimers, with Ca...Ca distances of 4.334 (2) and 4.300 (2) Å in (2e), and a Cd...Cd distance of 4.195 (1) Å in (2f). Almost identical in shape, the eight‐coordinate polyhedra of the Ca²⁺ and Cd²⁺ ions are composed of 2n O atoms from n DHA molecules [n = 2 in (2e) and n = 4 in (2f)] and are completed by four water molecules in (2e). DHA molecules chelate the cations via both the hydroxyl and carbonyl groups and exist in an extended conformation, with both hydroxyl groups being synperiplanar to the carbonyl O atom. The crystal structures are stabilized by similar extensive O—H...X (X = Cl or Br) and O—H...O hydrogen‐bond networks involving all hydroxyl groups and the water molecules.     

Influence of the rotational mobility of probe molecules on their orientation factors

The influence of a small rotational mobility on the measured orientation factors f₂ and f₄ (connected with the second and fourth moments of the orientation distribution function), of uniaxial probe molecules in uniaxial polymer systems is calculated with special regard to the fluorescence polarization method. A mean angle of rotation of about 10° within the time scale of the experiment, i.e., the fluorescence lifetime, affects f₄ considerably, while it has practically no influence on f₂. On the other hand, the knowledge of the true f₄ of the sample and some assumptions on the mobility—orientation correlation make it possible to evaluate the mean rotational angle θ from the measured f₄. Moreover, the measured f₂ value can be precisely corrected. The effect of the assumptions on the reliability of f₂ and θ is analyzed. Experimental results on two partially mobile probe molecules in drawn polyethylene are discussed.     

Determination of enthalpies of formation of organic free radicals from bond dissociation energies 2. Halosubstituted radicals

The enthalpies of formation (ΔH _f ^o) for 23 halosubstituted radicals were determined from the published data on bond dissociation energies. The ΔH _f ^o values of the corresponding molecules necessary for the calculation of ΔH _f ^o of the radicals were taken from handbooks or calculated by the additive-group method. The conjugation energies of the radicals are calculated, and the effect of substituents at the π-system on these values was shown. Errors of determination of the ΔH _f ^o values of the radicals were estimated. For Part 1, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 643–646, April, 1998.     

On the conjoint gradient correction to the Hartree—Fock kinetic and exchange energy density functionals

The kinetic and the exchange energy functionals are expressed in the form T[ρ] = C_TF∫ drρ^5/3(r)f_t(s) and K[ρ] = C_D∫ drρ^4/3(r)f_K(s), where C_TF = (3/10)(3π²)^2/3 and C_D = −(3/4)(3/π)^4/3 are the Thomas-Fermi and the Dirac coefficients, respectively, and s = |∇ρ(r)|/C_sρ^4/3(r), with C_s = 2(3π²)^1/3. These expressions are used to perform a comparison of f_T(s) and f_K(s) in terms of their generalized gradient expansion approximations. It is shown that f_κ(s) and is congruent to f_T(s) in the range characteristic of the interior regions of atoms and many solids and that the second-order gradient expansion of the kinetic energy provides a rather reasonable approximation to the generalized gradient expansion approximation of both the kinetic and the exchange energy functionals. © 1996 John Wiley & Sons, Inc.     

Acquisition times,relaxation rates and solvent and temperature effects as sensitivity parameters for quadrupolar nuclei

The relationship between the signal-to-noise ration and T₂ and, hence, the line width of a resonance for quadrupolar nuclei, with particular reference to ¹⁷O at natural abundance, is discussed. Emphasis is placed on the importance of choosing appropriate FID acquisition times related to the T₂ values or resonance widths of the quadrupolar nuclei. When this is feasible (i.e. when the principal resonances in a spectrum have similar widths) the signal-to-noise ratio attainable against a flat base line in a given time of spectral accumulation is independent of resonance line width. Although resolution improvements can be made by using appropriate solvents or elevated sample temperatures, the sensitivity has been to be strongly dependent on the acquisition parameters used.     

Real‐time homonuclear broadband and band‐selective decoupled pure‐shift ROESY

Unambiguous spectral assignments in ¹H solution‐state NMR are central, for accurate structural elucidation of complex molecules, which is often hampered by signal overlap, primarily because of scalar coupling multiplets, even at typical high magnetic fields. The recent advances in homodecoupling methods have shown powerful means of achieving high resolution pure‐shift ¹H spectra in 1D and also in 2D J‐correlated experiments, by effectively collapsing the multiplet structures. The present work extends these decoupling strategies to through‐space correlation experiments as well and describes two new pure‐shift ROESY pulse schemes with homodecoupling during acquisition, viz., homodecoupled broadband (HOBB)‐ROESY and homodecoupled band‐selective (HOBS)‐ROESY. Furthermore, the ROESY blocks suppress the undesired interferences of TOCSY cross peaks and other offsets. Despite the reduced signal sensitivity and prolonged experimental times, the HOBB‐ROESY is particularly useful for molecules that exhibit an extensive scalar coupling network spread over the entire ¹H chemical shift range, such as natural/synthetic organic molecules. On the other hand, the HOBS‐ROESY is useful for molecules that exhibit well‐separated chemical shift regions such as peptides (NH, Hα and side‐chain protons). The HOBS‐ROESY sensitivities are comparable with the conventional ROESY, thereby saves the experimental time significantly. The power of these pure‐shift ROESY sequences is demonstrated for two different organic molecules, wherein complex conventional ROE cross peaks are greatly simplified with high resolution and sensitivity. The enhanced resolution allows deriving possibly more numbers of ROEs with better accuracy, thereby facilitating superior means of structural characterization of medium‐size molecules. Copyright © 2014 John Wiley & Sons, Ltd.     

The construction of graph models for calculations of the properties of substitution isomers of basis structures on the basis of additivity of energy contributions

A method for the construction of additive models for calculations of the properties of substitution isomers of basis structures is described for the example of a series of X-substituted methylsilanes CH_{3 − k} X _k -SiH_{3 − l} X _l (where X = CH₃, F, Cl, …, k, l = 0, 1, 2, 3). The method is based on similarity of subgraphs in graphs of several molecules and the arrangement of polygonal numbers (triangular, tetrahedral) of the Pascal triangle. Parameters taking into account multiple nonvalence interactions (-C-Si<, >C-Si<, …) through two atoms along the molecular chain of an X-substituted methylsilane (X = CH₃) were for the first time explicitly included in the calculation scheme. Taking these interactions into account allows us to completely differentiate all the structural isomers of certain molecules and obtain numerical parameter values for predicting properties P under consideration in various approximations. Numerical calculations of Δ_f H _{g,298 K}^o were performed for 16 alkylsilanes (as X-substituted methylsilanes), including 7 compounds not studied experimentally.     

Stability of isomerous chelates in M(II)-thiocarbamoylmethanamide-ethandial systems according to the DFT B3LYP method (M = Mn, Fe, Co, Ni, Cu, Zn)

Calculations of the optimal geometry and standard thermodynamic parameters of Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) isomerous macrotricyclic complexes with MN₂O₂, MN₂S₂, and MN₄ chelate bonds, which can in principle appear as a result of template processes between gelatine-immobilized hexacyanoferrates (II) of corresponding M(II) metal ions, thiocarbamoylmethaneamide (thiooxamide) H₂N-C(=S)-C(=O)-NH₂, and ethanedial HC(=O)-CH(=O), were performed according to the B3LYP hybrid density functional method using a 6-31G(d) basis set with the Gaussian 98 program. It was found that of all of the considered M(II), the most stable are complexes with MN₄ chelate bonds, where the values of a standard enthalpy Δ_f H ₂₉₈^o and a standard Gibbs energy, Δ_f G ^o for all complexes studied are positive.     

Synthesis,characterization and oxygen‐sensing properties of a novel luminescent Cu(I) complex

A novel luminescent copper(I) complex with formula [Cu(PPh₃)₂(PIP)]BF₄ (PPh₃ = triphenyl phosphine, PIP = 2‐phenyl‐1H‐imidazo[4,5‐f][1,10]phenanthroline) has been synthesized and characterized by ¹H NMR, IR, elemental analysis and X‐ray crystal structure analysis. In solid state, it displays broad band emission upon excitation at λ = 420 nm with the emission maximum locates at 551 nm. Its excited‐state lifetime is in the microsecond time scale (3.02 µs); as a result, its emission intensity is sensitive to oxygen concentration and shows oxygen‐sensing properties after being encapsulated into mesoporous silica MCM‐41. For the system with 60 mg/g loading level, a sensitivity (I₀/I) of 4.35, a fluorescence quenching time (t_Q) of 5 s and a recovery time (t_R) of 36 s were achieved. Even after aging for 5 months, the sensitivities of the three loading level systems can be retained, ignoring the measurement error, which indicates that they possess long‐term stability. Copyright © 2009 John Wiley & Sons, Ltd.     

On the Existence of the Trimer of Gallium Trichloride in the Gaseous Phase

The dependence of the gallium trichloride saturated and unsaturated vapor pressures on temperature was studied by the static method using a quartz membrane zero‐manometer and taking into account the volume of its working chamber and substance mass. Conclusions about the presence of a distinguishable amount of trimeric molecules along with dimeric and momomeric molecules in the vapor were drawn on the basis of the obtained data. The following rough thermodynamic characteristics of a gaseous trimer of gallium trichloride were calculated: Δ_fH° (Ga₃Cl₉, gas, 298 K) = –1466 kJ · mol^–1. S°(Ga₃Cl₉, gas, 298 K) = 654 J · mol^–1 · K^–1. These data were used to elucidate the composition of the gaseous phase at a total pressure of 1 atm in the temperature range of 400–750 K. The suggested existence of trimeric molecules was not contradicted by vibrational spectroscopic analysis of gallium trichloride saturated vapor.     

Compressibility effects in packed and open tubular gas and supercritical fluid chromatography

The influence of the pressure drop on the efficiency and speed of analysis in packed and open tubular supercritical fluid chromatography (SFC) is described: methods previously developed to describe the effects of mobile phase compressibility on the performance of open tubular columns in SFC have been extended to packed columns. The Horvath and Lin equation has been used to elucidate the influence of variations in velocity, diffusivity, and capacity factor along the column on the overall efficiency of packed column SFC. In packed columns, in contrast with the situation in open tubular columns, because the increase in velocity is no longer compensated by an increase in diffusion coefficients, the increase in both linear velocity and capacity factor which result from a significant pressure drop cause the plate height to increase along the column. The effect of fluid decompression along the length of the column on the speed of analysis in SFC has been studied and numerical expressions derived which enable calculation of compressibility correction factors for the plate height. Both the f₁ and f₂ correction factors remain very close to unity for acceptable pressure drops, which means that the pressure drop has virtually no effect on the number of plates generated per unit time for an unretained component. For retained species, the decompression of the mobile phase across the column causes the capacity factor to increase and hence leads to increased analysis times.     

Developing nonuniform sampling strategies to improve sensitivity and resolution in 1,1-ADEQUATE experiments

Nonuniform sampling (NUS) strategies are developed for acquiring highly resolved 1,1-ADEQUATE spectra, in both conventional and homodecoupled (HD) variants with improved sensitivity. Specifically, the quantile-directed and Poisson gap methods were critically compared for distributing the samples nonuniformly, and the quantile schedules were further optimized for weighting. Both maximum entropy and iterative soft thresholding spectral estimation algorithms were evaluated. All NUS approaches were robust when the degree of data reduction is moderate, on the order of a 50% reduction of sampling points. Further sampling reduction by NUS is facilitated by using weighted schedules designed by the quantile method, which also suppresses sampling noise well. Seed independence and the ability to specify the sample weighting in quantile scheduling are important in optimizing NUS for 1,1-ADEQUATE data acquisition. Using NUS yields an improvement in sensitivity, while also making longer evolution times accessible that would be difficult or impractical to attain by uniform sampling. Theoretical predictions for the sensitivity enhancements in these experiments are in the range of 5–20%; NUS is shown to disambiguate weak signals, reveal some ⁿJ_CC correlations obscured by noise, and improve signal strength relative to uniform sampling in the same experimental time. This work presents sample schedule development for applying NUS to challenging experiments. The schedules developed here are made available for general use and should facilitate the broader utilization of ADEQUATE experiments (including 1,1-, 1,n-, and HD- variants) for challenging structure elucidation problems.     

Concentrated solutions of polydisperse rodlike polymers in the isotropic and lyotropic liquid-crystalline phases. II. Model simulation of the dielectric relaxation behavior of poly(n-alkylisocyanates) in toluene and comparison with experimental results

A simulation has been made of the dielectric relaxation behavior of poly(n-hexylisocyanate) in solution covering the isotropic, biphasic, and anisotropic ranges. The simulation incorporates the Flory-Abe statistical mechanical theory for the phase behavior of rodlike macromolecules in solution and the Warchol, Vaughan, Wang, and Pecora theory for the dynamics of a rodlike molecule in a virtual cone prescribed by the neighboring molecules. It is shown that asymmetric Gaussian, Gaussian, or Poisson distributions of molecular weight do not lead to dielectric behavior of the type observed experimentally by Moscicki, Williams, and Aharoni but addition of a high-molecular-weight “tail” to such distributions and taking account of the dependence of relaxation time on molecular length gives a simulation of the dielectric increment Δε, the loss maximum ε, and frequency of maximum loss f_m, which vary with polymer concentration in a manner entirely consistent with the experimental data.