Gas-phase Standard Enthalpies Of Formation Research Articles

Determination and Analysis of Thermodynamic Properties of Methyl Methylanthranilate Isomers.

The enthalpies of formation in the gaseous phase of methyl 3-methylanthranilate and methyl 5-methylanthranilate were determined from experimental measurements of the corresponding standard energies of combustion, obtained from combustion calorimetry, and the standard enthalpies of vaporization and sublimation, obtained from Calvet microcalorimetry and Knudsen mass-loss effusion. A computational study, using the G3(MP2)//B3LYP composite method, has also been performed for the calculation of the gas-phase standard enthalpies of formation of those two molecules at T = 298.15 K, as well as for the remaining isomers, methyl 4-methylanthranilate and methyl 6-methylanthranilate. The results have been used to evaluate and analyze the energetic effect of the methyl substituent in different positions of the ring.

Calculation of Gas-phase Standard Formation Enthalpy via Ring-Preserved Connectivity-Based Hierarchy and Automatic Bond Separation Reaction Platform

Bond separation reaction (BSR) is an effective scheme to derive reliable thermochemical properties by systematic error cancellation, while some challenges exist before its wide applications. The traditional BSR schemes are less feasible to cyclic molecules, and automatic BSR programs are not yet fully developed, unbeneficial to maintain the reliability and efficiency of BSR. Herein, we propose an improved scheme of BSR, the Ring-Preserved Connectivity-Based Hierarchy (RCBH) scheme, to enhance the predicted accuracy of gas-phase standard enthalpy of formation (EOF) by quantum chemical calculations. The accuracy and time–cost of RCBH scheme are tested at various DFT levels, involving 12 functionals and 6 basis sets, with samples covering 250 cyclic organic molecules. The superiority of RCBH in calculating EOF is verified, with M062X/6-31 + G(2df,p) recommended as the most cost-effective DFT level, which can approach the accuracy of G4 with only 4% CPU-time of G4. The EOFs calculated by M062X/6-31 + G(2df,p) with RCBH correction are highly consistent with the experimental measurements, with a MAE of 1.1 kcal/mol. We also develop an automated BSR software package, Automatic Bond Separation Reaction Platform (ABSRP), feasible for all categories of organic molecules. The RCBH scheme and ABSRP are expected to be used in calculating other thermochemical properties of extensive categories of neutral organic molecules, with high accuracy while low computational cost.

High-Level Ab Initio Predictions of Thermochemical Properties of Organosilicon Species: Critical Evaluation of Experimental Data and a Reliable Benchmark Database for Extending Group Additivity Approaches

A high-level composite quantum chemical method, W1X-1, is used herein to calculate the gas-phase standard enthalpy of formation, entropy, and heat capacity of 159 organosilicon compounds. The results set a new benchmark in the field that allows, for the first time, an in-depth assessment of existing experimental data on standard enthalpies of formation, enabling the identification of important trends and possible outliers. The calculated thermochemical data are used to determine Benson group additivity contributions for 60 Benson groups and group pairs involving silicon. These values allow fast and accurate estimation of thermochemical parameters of organosilicon compounds of varying complexity, and the data acquired are used to assess the reliability of experimental work of Voronkov et al. that has been repeatedly criticized by Becerra and Walsh. Recent results from other computational investigations in the field are also carefully discussed through the prism of reported advancements.

Thermochemistry, Bond Energies and Internal Rotor Potentials of Acetic Acid Hydrazide, Acetamide, N-Methyl Acetamide (NMA) and Radicals

Structures, thermochemical properties, bond energies, and internal rotation potentials of acetic acid hydrazide (CH3CONHNH2), acetamide (CH3CONH2), and N-methyl acetamide (CH3CONHCH3), and their radicals corresponding to the loss of hydrogen atom, have been studied. Gas-phase standard enthalpies of formation and bond energies were calculated using the DFT methods B3LYP/6-31G(d,p), B3LYP/6-31G(2d,2p) and the composite CBS-QB3 methods employing a series of work reactions further to improve the accuracy of the ΔHf°(298 K). Molecular structures, vibration frequencies, and internal rotor potentials were calculated at the DFT level. The parent molecules’ standard formation enthalpies of CH3–C=ONHNH2, CH3–C=ONH2, and CH3–C=ONHCH3 were evaluated as −27.08, −57.40, and −56.48 kcal mol−1, respectively, from the CBS–QB3 calculations. Structures, internal rotor potentials, and C–H and N–H bond dissociation energies are reported. The DFT and the CBS-QB3 enthalpy values show close agreement, and this accord is attributed to the use of isodesmic work reactions for the analysis. The agreement also suggests this combination of the B3LYP/work reaction approach is acceptable for larger molecules. Internal rotor potentials for the amides are high, ranging from 16 to 22 kcal mol−1.

Assessment of property estimation methods for the thermodynamics of carbon dioxide-based products

Carbon dioxide can be used as feedstock to produce chemicals. It represents a stimulating defiance to manufacture novel cost competitive materials with less environmental impact, besides to investigate new opportunities for catalysts and industrial chemistry. The contribution of carbon dioxide conversion goes beyond lowering global warming, by reducing fossil resource depletion or even yielding more benign production pathways. Albeit promising, the literature data regarding the quantity of energy needed to convert carbon dioxide into chemicals is limited and narrowed to the most studied processes and products. In order to understand and model the formation of species using carbon dioxide as raw material, some basic thermodynamic data are needed. The development of detailed reaction schemes in the field is also scarce. To enhance and further complete the database of the products obtained from carbon dioxide, this study investigates different procedures to estimate the basic thermodynamic properties of the reactants and products of these reactions. To date various methods have been developed and introduced to determine the gas-phase standard enthalpies of formation and Gibbs energy. Among them, group additivity and semi-empirical methods are widely employed due to their accuracy and effort time for implementation compared to more rigorous methods. Semi empirical quantum-chemistry methods were compared with group additivity methods. Available literature data were used to select the best method for property estimation of the whole set of species, whereby produced from carbon dioxide. The products from carbon dioxide were categorized in sixteen chemical classes, the reaction enthalpy for the direct route to manufacture the products were assessed and indicate a large difference among the classes. The results of this investigation show that semi empirical quantum-chemistry methods revealed to be more accurate for the studied species; additionally, the method demonstrates robustness in estimating the properties. Together, these results provide important insights into the thermodynamics of carbon dioxide related products.

Gas-phase standard enthalpy of formation of 3,5-dimethyl-4-nitroisoxazole determined by semi-micro-combustion calorimetry and thermal analysis

The present paper reports the determination of the standard (p° = 0.1 MPa) molar enthalpy of formation in gas phase at T = 298.15 K of 3,5-dimethyl-4-nitroisoxazole, which was derived from the enthalpy of combustion in solid state and the sublimation enthalpy. The combustion enthalpy was derived from the respective combustion energy value, which was obtained from combustion calorimetry using a static bomb combustion calorimeter in oxygen atmosphere at T = 298.15 K. The sublimation enthalpy was obtained at the same temperature by using thermogravimetry experiments in isothermal regime. The purity and the constant pressure heat capacity of the compound were determined by using differential scanning calorimetry, priori to carry out the combustion experiments. The value obtained of formation enthalpy of compound in gas phase presents high precision and low uncertainty.

Thermodynamic properties of ε-caprolactam and ε-caprothiolactam

This paper is concerned with experimental and computational thermochemical studies of ε-caprolactam and ε-caprothiolactam, molecules whose flexible ring structure provides several conformational forms.The gas-phase standard enthalpy of formation at the reference temperature of 298.15 K of the two title compounds have been determined from the enthalpy of formation in the crystalline phase, derived from bomb combustion calorimetry, and from the enthalpy of sublimation, derived from Calvet microcalorimetry and Knudsen effusion techniques.The equilibrium geometries and the thermodynamic properties of three minimum energy conformers obtained with the composite G3(MP2)//B3LYP approach were used to determine the conformational composition of the two lactam derivatives, by means of Boltzmann’s distribution. The computed gas-phase standard molar enthalpies of formation of ε-caprolactam and ε-caprothiolactam have been determined using hypothetical gas-phase reactions, taking into account the conformational compositions of each one of the species. This parameter together with the corresponding experimental one are compared.

Experimental enthalpies of formation and sublimation of urea compounds: An accuracy assessment

The thermochemical network of 91 isodesmic and other balanced reactions was used to adjust simultaneously the enthalpies of formation of 14 urea compounds. The experimental enthalpies of formation of these compounds were treated as unknowns in the least-squares minimization of deviations between the theoretically predicted enthalpies of reaction and experimental values. A result of a least-squares fit confirms the accuracy of experimental enthalpies of formation of 8 ureas, whereas the experimental measurements for 6 urea compounds require re-examination. The gas-phase standard enthalpies of formation of 8 ureas were used as a consistent basis to obtain values for other 15 ureas through isodesmic reactions. To estimate the enthalpies of formation in crystalline state, the sublimation enthalpies were predicted using the molecular electrostatic potential model.

Energetic vs structural effects of aminoalkyl substituents in the morpholine

Abstract Experimental and computational studies were performed in this work with the aim of evaluating and understanding the energetic effect inherent to the substitution of the hydrogen of the morpholine amino group by aminoalkyl substituents: N -(2-aminoethyl)morpholine and N -(3-aminopropyl)morpholine. The standard enthalpies of vaporization and the standard energies of combustion of the two morpholine derivatives obtained, respectively, from Calvet microcalorimetry and combustion calorimetry measurements, are reported. These data were used to derive the standard enthalpies of formation of the morpholine derivatives, in the liquid and gaseous phases, at T = 298.15 K. The computational study involved the energetic analysis of the most stable conformers on the potential energy surfaces and the determination of their gas-phase standard enthalpies of formation at the reference temperature of 298.15 K. All the computational calculations were performed using the G3(MP2)//B3LYP composite method. The combination of experimental and computational data determined in this work for morpholine derivatives, together with other available in the literature for related molecules, enabled the analyses of the energetic effects associated with the substitution of the hydrogen of the morpholine amino group by substituents aminoalkyl and alkyl, as well as the establishment of incremental schemes for the determination of the gas-phase enthalpies of formation.

Gas-phase standard enthalpies of formation of urea-derived compounds: A quantum-chemical study

Gas-phase standard enthalpies of formation of selected ureas and s-triazines were calculated at the CCSD(F12∗)(T)/cc-pVTZ-F12//ωB97X-D/cc-pVTZ level of theory by employing isodesmic reactions. The following values were obtained (T=298.15K, units: kJmol−1, estimated confidence interval 95%): urea, −231.9±2.8; biuret, −430.0±4.5; triuret, −620.3±6.3; cyanuric acid, −451.6±6.3; ammelide, −303.9±6.4; ammeline, −106.5±6.2; melamine, 70.1±7.0. The standard enthalpies of formation of methanimine and methylamine, which were required for the isodesmic reactions, were calculated from atomization reactions by using several variants of the HEAT approach. The following results were considered most reliable (T=298.15K, units: kJmol−1, estimated confidence interval 95%): methanimine, 89.0±1.0; methylamine, −20.7±1.0.

Thermodynamic Parameters Including Acid Dissociation Constants for Bromochlorophenols (BCPs)

This contribution reports standard gas-phase enthalpies of formation (ΔfH°298), entropies (S°298), and heat capacities (Cp°(T)) for all plausible 64 bromochlorophenols (BCPs) at the M062X meta hybrid level using a polarized basis set of 6-311+G(d,p). Isodesmic work reactions served to calculate the standard enthalpies of formation for all bromochlorophenol molecules and several bromochlorophenoxy radicals. Standard entropies and heat capacities comprise correction terms due to the treatment of O–H bonds as hindered rotors. Values of the bond dissociation enthalpies (BDHs) of O–H bonds, calculated for a selected series of bromochlorophenols, vary slightly with the change in the pattern and degree of halogenation of the phenyl ring. A thermodynamic cycle facilitated the estimation of pKa values, based on the calculated solvation and gas-phase deprotonation energies. We estimated the solvation energies of 19 out of 64 BCPs and their respective anions based on the integral equation formalism polarizable conti...

Active Thermochemical Tables: Sequential Bond Dissociation Enthalpies of Methane, Ethane, and Methanol and the Related Thermochemistry.

Active Thermochemical Tables (ATcT) thermochemistry for the sequential bond dissociations of methane, ethane, and methanol systems were obtained by analyzing and solving a very large thermochemical network (TN). Values for all possible C-H, C-C, C-O, and O-H bond dissociation enthalpies at 298.15 K (BDE298) and bond dissociation energies at 0 K (D0) are presented. The corresponding ATcT standard gas-phase enthalpies of formation of the resulting CHn, n = 4-0 species (methane, methyl, methylene, methylidyne, and carbon atom), C2Hn, n = 6-0 species (ethane, ethyl, ethylene, ethylidene, vinyl, ethylidyne, acetylene, vinylidene, ethynyl, and ethynylene), and COHn, n = 4-0 species (methanol, hydroxymethyl, methoxy, formaldehyde, hydroxymethylene, formyl, isoformyl, and carbon monoxide) are also presented. The ATcT thermochemistry of carbon dioxide, water, hydroxyl, and carbon, oxygen, and hydrogen atoms is also included, together with the sequential BDEs of CO2 and H2O. The provenances of the ATcT enthalpies of formation, which are quite distributed and involve a large number of relevant determinations, are analyzed by variance decomposition and discussed in terms of principal contributions. The underlying reasons for periodic appearances of remarkably low and/or unusually high BDEs, alternating along the dissociation sequences, are analyzed and quantitatively rationalized. The present ATcT results are the most accurate thermochemical values currently available for these species.

Thermochemical insights on the conformational energetics of azepan and azepan-1-ylacetonitrile.

This paper is concerned with computational and experimental thermochemical studies of azepan and azepan-1-ylacetonitrile, molecules whose flexible ring structure provides several conformational forms with low energy barriers among them. The computational study describes the energetic analysis of the six most stable conformers on the potential energy surfaces and the determination of their gas-phase standard enthalpy of formation at the reference temperature of 298.15 K. The same gas-phase enthalpic parameters are also derived from the enthalpies of formation in the liquid phase and the enthalpies of vaporization, at T = 298.15 K, determined experimentally using the combustion calorimetry and the Calvet microcalorimetry techniques, respectively. The experimental data reported in this work for the two titled compounds together with other available in the literature for related molecules enabled the establishment of an increments scheme, providing a reliable approach on the prevision of gas-phase enthalpy of formation of cyclic/acyclic hydrocarbons and amines. Complementary, natural bond orbital (NBO) calculations were also performed, allowing an advance on the analysis of the structural and reactivity behavior of these type of compounds.

A theoretical determination of the composition of by-products obtained in the production of butylated hydroxytoluene from 2,6-di-tert-butylphenol

The hydrogenolysis of N,N-dimethyl(3,5-di-tert-butyl-4-hydroxybenzyl)amine, which leads to butylated hydroxytoluene (BHT) as the main product, has been investigated by the density functional theory (DFT) method with the BPE functional and the SVP basis set. A set of valid reaction paths was determined from both thermodynamic and kinetic points of view. Possible products of the reaction were identified, it was shown that the most probable by-product is 4,4′-ethylenebis(2,6-di-tert-butylphenol). The standard gas-phase enthalpies of formation of some possible products of the reaction have been predicted by using the BPE/SVP approach, coupled with isodesmic reactions.

Bond dissociation enthalpies in chlorinated benzenes and phenols and enthalpies of formation of their free radicals: A Gaussian-4 prediction

The bond dissociation enthalpies (BDEs) in chlorinated benzenes and phenols and the standard gas-phase enthalpies of formation of chlorinated phenyl and phenoxy radicals are predicted by using Gaussian-4 (G4) and Gaussian-3X (G3X) model chemistries. The predicted G4 BDEs are systematically smaller than the G3X ones, with difference as much as ∼15 kJ/mol for the C6Cl5-Cl bond, and the G4 enthalpies of formation of the free radicals are systematically smaller than the G3X ones. The discrepancies increase gradually with the degree of chlorination; whereas for the closed-shell species, G4 and G3X enthalpies of formation agree closely within 2 kJ/mol. The difference between G4 and G3X arises mainly from the noncanceling high-level correction terms in G4. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 62–69, 2011

Heat Capacity, Enthalpy of Formation, and Entropy of Methyl Carbamate

The heat capacity (Cp) of methyl carbamate (MC) was measured with an adiabatic calorimeter. The standard enthalpy of formation (ΔfH0(cr, 298.15 K)) and the standard entropy (S0(cr, 298.15 K)) for MC were determined. Thus, Cp(T) values of MC were obtained for the crystal and liquid forms in the temperature range of 298.15−433.10 K. A quadratic trinomial was used to correlate the data of Cp(T). The enthalpy of fusion of MC was measured using differential scanning calorimetry (DSC), and its sublimation enthalpy was estimated. The gas-phase standard enthalpy of formation (ΔfH0(g, 298.15 K)) of MC was calculated via the method of group contribution developed by Benson. Based on the above data, the values of ΔfH0(cr, 298.15 K) and S0(cr, 298.15 K) of MC were estimated to be −513.57 kJ/mol and 77.1 J mol−1 K−1, respectively. A thermochemical cycle including a reversible reaction was designed to examine the reliability of the estimated values by measuring the equilibrium constant of the reaction. The results show...

Prediction of gas-phase thermodynamic properties for polychlorinated naphthalenes using G3X model chemistry and density functional theory

The standard gas-phase enthalpies of formation of polychlorinated naphthalenes (PCNs) have been predicted using G3X model chemistry, density functional theory (DFT), and second-order Muller-Plesset (MP2) theory. Two isodesmic reactions are used for better prediction of formation enthalpies. The first (IR1) employs chlorobenzene as a reference species and the second (IR2) employs polychlorinated benzenes as reference species. Among congeners, PCNs with simultaneous Cl-substitutions at positions 1 and 8 or 4 and 5 are the least stable, where the strong repulsion between Cl-atoms leads to non-planar structures for a few PCNs. The potential energy curves for ring-wagging motions in 1,8- or 4,5-PCNs are also extremely flat in the vicinity of equilibrium conformations, leading to extremely low harmonic frequencies for the ring-wagging modes. The contributions of these ring-wagging modes to entropy, heat capacity, and thermal corrections have been calculated using the numerically evaluated energy levels. The PCN isomer patterns are discussed based on the calculated Gibbs free energies.

Calorimetric and computational thermochemical study of difluorophenol isomers

The standard gas-phase enthalpies of formation of difluoro-substituted phenols have been determined experimentally, and have also been predicted by means of computational and empirical estimative methods. Combustion calorimetric studies were used to determine the standard molar enthalpies of formation of the six difluorophenol isomers, whereas their standard molar enthalpies of sublimation or vaporization, at T = 298.15 K was measured by the Calvet drop calorimetric technique. Difluorophenol isomer Δ f H m ∘ ( cr,l ) /(kJ · mol −1) Δ cr,l g H m ∘ /(kJ · mol −1) 2,3-Difluorophenol (cr) −529.4 ± 1.8 68.2 ± 1.5 2,4-Difluorophenol (l) −529.1 ± 1.8 58.8 ± 0.9 2,5-Difluorophenol (cr) −545.2 ± 1.8 68.0 ± 1.4 2,6-Difluorophenol (cr) −548.5 ± 1.7 77.8 ± 2.0 3,4-Difluorophenol (cr) −535.3 ± 1.8 72.9 ± 1.5 3,5-Difluorophenol (cr) −555.8 ± 1.8 72.8 ± 1.5 From the values obtained for Δ f H m ∘ (cr, l) and Δ cr,l g H m ∘ ( 298.15 K ) , the standard molar enthalpies of formation in the gas-phase, at T = 298.15 K, were derived for the six isomers. Moreover, the standard gas-phase enthalpies of formation of difluorophenols have been predicted based on two different methodologies: one using the empirical scheme developed by Cox and the other one based on density functional theory calculations performed at the B3LYP/6-311++G(d,p) level of theory coupled with suitable homodesmic reactions and were compared to the experimental values.

The Enthalpies of Formation for Polychlorinated Dibenzofurans with Use of G3XMP2 Model Chemistry and Density Functional Theory

The standard gas-phase enthalpies of formation of polychlorinated dibenzofurans (PCDFs) have been predicted by using G3XMP2 model chemistry, density functional theory (DFT), and second-order Muller-Plesset (MP2) theory, coupled with isodesmic reactions. The results show a large difference between G3XMP2 and DFT methods with 6-31G(2df,p) and 6-311++G(3df,3pd) basis sets, while MP2/G3MP2Large calculations agree closely with G3XMP2. Two isodesmic reaction schemes are used for better prediction of formation enthalpies. The first (IR1) employs monochlorobenzene as a reference species and the second (IR2) employs polychlorinated benzenes as reference species. The relative stability of PCDFs is rationalized by positional interactions. While the Cl-substitution at position 1/9 leads to the most stable isomers, the simultaneous substitutions at positions 1 and 9 result in a strong repulsion between Cl atoms. Failure of DFT-B3LYP is due to the overestimation of o-ClCl repulsion. For 1,9-PCDFs, the torsion motions of the benzene rings have extremely low harmonic vibrational frequencies. Their contributions to entropy, heat capacity, and thermal corrections have been calculated by using the numerically evaluated energy levels. The PCDF isomer patterns are also discussed based on the calculated thermodynamic parameters.

A Gaussian-3X Prediction on the Enthalpies of Formation of Chlorinated Phenols and Dibenzo-p-dioxins

The standard gas-phase enthalpies of formation of chlorinated benzenes, phenols and dibenzo-p-dioxins have been predicted using G3X and/or G3XMP2 model chemistries coupled with isodesmic reactions and compared to the previous theoretical and experimental values. A set of values for chlorinated benzenes are first suggested based on experimental measurements and the closed agreed G3X calculations with different isodesmic reactions. The results on polychlorinated dibenzo-p-dioxins (PCDDs) show a large difference between G3XMP2 and previous experimental measurements and predictions using group additivity methods, semiempirical quantum chemistry, and DFT calculations, especially for highly chlorinated species. Using the well-balanced isodesmic reactions (IR3 and IR5), the discrepancies between G3XMP2 and DFT predictions on PCDDs can be reduced to within 16 kJ/mol. The relative stability of PCDD isomers can be rationalized by the positional interactions, and the overestimation by DFT with less balanced isodesmic reactions is due to the overestimation of the ortho-Cl-Cl repulsive interactions when comparing with G3XMP2. Our calculations suggest further experimental measurements, especially on highly chlorinated phenols and PCDDs.