Group Additivity Values Research Articles

Thermochemistry and Bond Dissociation Energies of Ketones

Ketones are a major class of organic chemicals and solvents, which contribute to hydrocarbon sources in the atmosphere, and are important intermediates in the oxidation and combustion of hydrocarbons and biofuels. Their stability, thermochemical properties, and chemical kinetics are important to understanding their reaction paths and their role as intermediates in combustion processes and in atmospheric chemistry. In this study, enthalpies (ΔH°(f 298)), entropies (S°(T)), heat capacities (C(p)°(T)), and internal rotor potentials are reported for 2-butanone, 3-pentanone, 2-pentanone, 3-methyl-2-butanone, and 2-methyl-3-pentanone, and their radicals corresponding to loss of hydrogen atoms. A detailed evaluation of the carbon-hydrogen bond dissociation energies (C-H BDEs) is also performed for the parent ketones for the first time. Standard enthalpies of formation and bond energies are calculated at the B3LYP/6-31G(d,p), B3LYP/6-311G(2d,2p), CBS-QB3, and G3MP2B3 levels of theory using isodesmic reactions to minimize calculation errors. Structures, moments of inertia, vibrational frequencies, and internal rotor potentials are calculated at the B3LYP/6-31G(d,p) density functional level and are used to determine the entropies and heat capacities. The recommended ideal gas-phase ΔH°(f 298), from the average of the CBS-QB3 and G3MP2B3 levels of theory, as well as the calculated values for entropy and heat capacity are shown to compare well with the available experimental data for the parent ketones. Bond energies for primary, secondary, and tertiary radicals are determined; here, we find the C-H BDEs on carbons in the α position to the ketone group decrease significantly with increasing substitution on these α carbons. Group additivity and hydrogen-bond increment values for these ketone radicals are also determined.

Statistical thermodynamics of 1-butanol, 2-methyl-1-propanol, and butanal

The purpose of the present investigation is to calculate partition functions and thermodynamic quantities, viz., entropy, enthalpy, heat capacity, and Gibbs free energies, for 1-butanol, 2-methyl-1-propanol, and butanal in the vapor phase. We employed the multi-structural (MS) anharmonicity method and electronic structure calculations including both explicitly correlated coupled cluster theory and density functional theory. The calculations are performed using all structures for each molecule and employing both the local harmonic approximation (MS-LH) and the inclusion of torsional anharmonicity (MS-T). The results obtained from the MS-T calculations are in excellent agreement with experimental data taken from the Thermodynamics Research Center data series and the CRC Handbook of Chemistry and Physics, where available. They are also compared with Benson's empirical group additivity values, where available; in most cases, the present results are more accurate than the group additivity values. In other cases, where experimental data (but not group additivity values) are available, we also obtain good agreement with experiment. This validates the accuracy of the electronic structure calculations when combined with the MS-T method for estimating the thermodynamic properties of systems with multiple torsions, and it increases our confidence in the predictions made with this method for molecules and temperatures where experimental or empirical data are not available.

Kinetics of α hydrogen abstractions from thiols, sulfides and thiocarbonyl compounds

Hydrogen abstraction reactions involving organosulfur compounds play an important role in many industrial, biological and atmospheric processes. Despite their chemical relevance, little is known about their kinetics. In this work a group additivity model is developed that allows predicting the Arrhenius parameters for abstraction reactions of α hydrogen atoms from thiols, alkyl sulfides, alkyl disulfides and thiocarbonyl compounds by carbon-centered radicals at temperatures ranging from 300 to 1500 K. Rate coefficients for 102 hydrogen abstractions were obtained using conventional transition state theory within the high-pressure limit. Electronic barriers were calculated using the CBS-QB3 method and the rate coefficients were corrected for tunneling and hindered rotation about the transitional bond. Group additivity values for 46 groups are determined. To account for resonance and hyperconjugative stabilization in the transition state, 8 resonance corrections were fitted to a set of 32 reactions. The developed group additivity scheme was validated using a test set containing an additional 30 reactions. The group additivity scheme succeeds in reproducing the rate coefficients on average within a factor of 2.4 at 300 K and 1.4 at 1000 K. Mean absolute deviations of the Arrhenius parameters amount to, respectively, 2.5 kJ mol(-1) for E(a) and 0.13 for log A, both at 300 and 1000 K. This work hence illustrates that the recently developed group additivity methods for Arrhenius parameters extrapolate successfully to hetero-element containing compounds.

Group Additivity Values for Estimating the Enthalpy of Formation of Organic Compounds: An Update and Reappraisal. 1. C, H, and O

This study examines critically the present state and utility of the Benson additivity schemes for estimating the enthalpy of formation of organic compounds. Old and new group additivity values (GAV) for a wide variety of compounds containing C, H and O are described and are revised where appropriate. When new terms are proposed, or old ones significantly altered, the rationale for so doing is provided. Corrections for such items as cis-isomer effects, gauche interactions, ring strain energies, double-bond position, conjugation effects, steric hindrance in aromatic molecules, etc. are included and discussed. Also provided are the thermochemical consequences of functional group replacements, in which one group in a molecule is substituted by another, thus providing quick short cuts to estimating new Δ(f)H(0) values. Results derived from the new additivity terms are consistent with those produced by computational chemistry methods in general use.

Modeling the Gas‐Phase Thermochemistry of Organosulfur Compounds

Key to understanding the involvement of organosulfur compounds in a variety of radical chemistries, such as atmospheric chemistry, polymerization, pyrolysis, and so forth, is knowledge of their thermochemical properties. For organosulfur compounds and radicals, thermochemical data are, however, much less well documented than for hydrocarbons. The traditional recourse to the Benson group additivity method offers no solace since only a very limited number of group additivity values (GAVs) is available. In this work, CBS-QB3 calculations augmented with 1D hindered rotor corrections for 122 organosulfur compounds and 45 organosulfur radicals were used to derive 93 Benson group additivity values, 18 ring-strain corrections, 2 non-nearest-neighbor interactions, and 3 resonance corrections for standard enthalpies of formation, standard molar entropies, and heat capacities for organosulfur compounds and organosulfur radicals. The reported GAVs are consistent with previously reported GAVs for hydrocarbons and hydrocarbon radicals and include 77 contributions, among which 26 radical contributions, which, to the best of our knowledge, have not been reported before. The GAVs allow one to estimate the standard enthalpies of formation at 298 K, the standard entropies at 298 K, and standard heat capacities in the temperature range 300-1500 K for a large set of organosulfur compounds, that is, thiols, thioketons, polysulfides, alkylsulfides, thials, dithioates, and cyclic sulfur compounds. For a validation set of 26 organosulfur compounds, the mean absolute deviation between experimental and group additively modeled enthalpies of formation amounts to 1.9 kJ mol(-1). For an additional set of 14 organosulfur compounds, it was shown that the mean absolute deviations between calculated and group additively modeled standard entropies and heat capacities are restricted to 4 and 2 J mol(-1) K(-1), respectively. As an alternative to Benson GAVs, 26 new hydrogen-bond increments are reported, which can also be useful for the prediction of radical thermochemistry.

ChemInform Abstract: The Calculation of Thermodynamic Properties of Molecules

AbstractReview: 83 refs.

Enthalpies of formation of β-alanine, sarcosine, and 4-aminobutanoic acid from quantum chemical calculations

The standard molar enthalpies of formation of β-alanine, sarcosine, and 4-aminobutanoic acid in the gaseous phase at T = 298.15 K were calculated by the high-level Gaussian-4 method. The agreement with available experimental data is good for β-alanine and 4-aminobutanoic acid, while a significant discrepancy between theoretical and experimental enthalpy of formation values for sarcosine suggests the inaccuracy of experimental data for this compound. These results were also supported by isodesmic reaction calculations. The recommended values of Δ f H m ∘ (β-alanine, g, 298.15 K) = (−419.7 ± 4.0) kJ · mol −1, Δ f H m ∘ (sarcosine, g, 298.15 K) = (−389.0 ± 4.0) kJ · mol −1, and Δ f H m ∘ (4-aminobutanoic acid, g, 298.15 K) = (−436.7 ± 5.0) kJ · mol −1 together with those obtained earlier for glycine and alanine, enable one to determine the group additivity values (GAVs) for some groups with nitrogen bound to carbon. These GAVs were used to estimate the enthalpies of formation of 5-aminopentanoic acid, 6-aminohexanoic acid, and 8-aminooctanoic acid. For the two first species, the experimental and calculated values agree well. A large deviation observed for 8-aminooctanoic acid suggests that experimental Δ f H m ∘ ( cr ) value for this compound is not reliable.

Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse β‐Scission Reactions: Modeling of Activation Energies and Pre‐Exponential Factors

The group additivity method for Arrhenius parameters is applied to hydrogen addition to alkenes and alkynes and the reverse beta-scission reactions, an important family of reactions in thermal processes based on radical chemistry. A consistent set of group additive values for 33 groups is derived to calculate the activation energy and pre-exponential factor for a broad range of hydrogen addition reactions. The group additive values are determined from CBS-QB3 ab-initio-calculated rate coefficients. A mean factor of deviation of only two between CBS-QB3 and experimental rate coefficients for seven reactions in the range 300-1000 K is found. Tunneling coefficients for these reactions were found to be significant below 400 K and a correlation accounting for tunneling is presented. Application of the obtained group additive values to predict the kinetics for a set of 11 additions and beta-scissions yields rate coefficients within a factor of 3.5 of the CBS-QB3 results except for two beta-scissions with severe steric effects. The mean factor of deviation with respect to experimental rate coefficients of 2.0 shows that the group additive method with tunneling corrections can accurately predict the kinetics and is at least as accurate as the most commonly used density functional methods. The constructed group additive model can hence be applied to predict the kinetics of hydrogen radical additions for a broad range of unsaturated compounds.

The calculation of thermodynamic properties of molecules

Thermodynamic data are key in the understanding and design of chemical processes. Next to the experimental evaluation of such data, computational methods are valuable and sometimes indispensable tools in obtaining heats of formation and Gibbs free energies. The major toolboxes to obtain such quantities by computation are quantum mechanical methods and group contribution methods. Although a lot of progress was made over the last decade, for the majority of chemical species we are still quite a bit away from what is often referred to as chemical accuracy, i.e.'1 kcal mol(-1)'. Currently, for larger molecules the combination of group contribution methods with group additive values that are determined with the best available computational ab initio methods seems to be a viable alternative to obtain thermodynamic properties near chemical accuracy. New developments and full use of existing tools may lead to further improvements (critical review, 83 references).

Modeling the influence of resonance stabilization on the kinetics of hydrogen abstractions

Resonance stabilization of the transition state is one of the key factors in modeling the kinetics of hydrogen abstraction reactions between hydrocarbons. A group additive model is developed which allows the prediction of rate coefficients for bimolecular hydrogen abstraction reactions over a broad range of hydrocarbons and hydrocarbon radicals between 300 and 1300 K. Group additive values for 50 groups are determined from rate coefficients determined using the high level CBS-QB3 ab initio method, corrected for tunneling and the hindered internal rotation around the transitional bond. Resonance and hyperconjugative stabilization of the transition state is accounted for by introducing 4 corrections based on the structure of the reactive moiety of the transition state. The corrections, fitted to a set of 28 reactions, are temperature-independent and reduce the mean absolute deviation on E(a) to 0.7 kJ mol(-1) and to 0.05 for log A. Tunneling contributions are accounted for by using a fourth order polynomial in the activation energy. Final validation for 19 reactions yields a mean factor of deviation between group additive prediction and ab initio calculation of 2.4 at 300 K and 1.8 at 1000 K. In comparison with 6 experimental rate coefficients (600-719 K), the mean factor of deviation is less than 3.

Thermochemistry of Radicals and Molecules Relevant to Atmospheric Chemistry: Determination of Group Additivity Values using G3//B3LYP Theory

Thermochemistry of radicals is not as extensively tabulated as that of stable molecular species, even when group additivity schemes are applied. When these radicals contain oxygen or nitrogen atoms, the availability of radical groups is even more limited. Many oxygen- and nitrogen-containing radicals and molecules are present in the atmosphere, and thermochemistry is a valuable component of the development of atmospheric models with predictive capabilities. This paper presents quantum chemical calculations using G3//B3LYP that have been performed to obtain heats of formation, entropies, and heat capacities as a function of the temperature of radicals and molecules from which group additivity values were obtained. Isodesmic and homodesmotic reactions were used to obtain improved estimates of the heats of formation. Thermodynamic property estimates were corrected to account for internal rotations. A total of 323 molecules were studied from which a total of 122 different groups, 21 gauche and cis corrections, and 5 secondary corrections were regressed.

First Principles Based Group Additive Values for the Gas Phase Standard Entropy and Heat Capacity of Hydrocarbons and Hydrocarbon Radicals

In this work a complete and consistent set of 95 Benson group additive values (GAVs) for standard entropies S(o) and heat capacities C(p)(o) of hydrocarbons and hydrocarbon radicals is presented. These GAVs include 46 groups, among which 25 radical groups, which, to the best of our knowledge, have not been reported before. The GAVs have been determined from a set of B3LYP/6-311G(d,p) ideal gas statistical thermodynamics values for 265 species, consistently with previously reported GAVs for standard enthalpies of formation. One-dimensional hindered rotor corrections for all internal rotations are included. The computational methodology has been compared to experimental entropies (298 K) for 39 species, with a mean absolute deviation (MAD) between experiment and calculation of 1.2 J mol(-1) K(-1), and to 46 experimental heat capacities (298 K) with a resulting MAD = 1.8 J mol(-1) K(-1). The constructed database allowed evaluation of corrections on S(o) and C(p)(o) for non-nearest-neighbor effects, which have not been determined previously. The group additive model predicts the S(o) and C(p)(o) within approximately 5 J mol(-1) K(-1) of the ab initio values for 11 of the 14 molecules of the test set, corresponding to an acceptable maximal deviation of a factor of 1.6 on the equilibrium coefficient. The obtained GAVs can be applied for the prediction of S(o) and C(p)(o) for a wide range of hydrocarbons and hydrocarbon radicals. The constructed database also allowed determination of a large set of hydrogen bond increments, which can be useful for the prediction of radical thermochemistry.

Organic Carbonates: Experiment and ab Initio Calculations for Prediction of Thermochemical Properties

This work has been undertaken in order to obtain data on thermodynamic properties of organic carbonates and to revise the group-additivity values necessary for predicting their standard enthalpies of formation and enthalpies of vaporization. The standard molar enthalpies of formation of dibenzyl carbonate, tert-butyl phenyl carbonate, and diphenyl carbonate were measured using combustion calorimetry. Molar enthalpies of vaporization of these compounds were obtained from the temperature dependence of the vapor pressure measured by the transpiration method. Molar enthalpy of sublimation of diphenyl carbonate was measured in the same way. Ab initio calculations of molar enthalpies of formation of organic carbonates have been performed using the G3MP2 method, and results are in excellent agreement with the available experiment. Then the group-contribution method has been developed to predict values of the enthalpies of formation and enthalpies of vaporization of organic carbonates.

Thermochemical Properties and Group Values for Nitrogen-Containing Molecules

Gas-phase thermochemical group additivity values were derived from CBS-QB3 computational chemistry calculations for 105 noncyclic C/H/O/N molecules. The molecules contain nitrile, nitro, nitroso, nitrite, nitrate, amine, imino, and azo functional groups. The enthalpy of formation, entropy, and heat capacity values for 49 atom-centered groups were derived. The effect of hindered internal rotations was included via rotor potential energy scans and solution of the one-dimensional Schrodinger equation. The average 95% confidence intervals across all derived groups are 1.4 kcal mol(-1) for the enthalpy, 1.3 cal mol(-1) K(-1) for the entropy, and 1.0 cal/mol K for the heat capacity. The presented group values will be useful when employing automatic reaction mechanism generation tools to examine the role of fuel-bound or molecular nitrogen in energy-related or atmospheric processes.

A statistical‐mechanical analysis of group additivity. Calculation of thermochemical values from frequency distributions

AbstractMolecular vibrational frequencies of homologous series plotted as cumulative frequency distributions are very similar, and the fine structure of the distributions exhibit identical features. An obvious explanation is that the changes in the molecular frequency distributions (MFDs) from one homologue to the next is independent of the chain length and of the functional groups in the molecule. Since group additivity is valid for the chosen homologous series there is here an explanation for the linearity of thermochemical values expressed by group additivity. For these properties the following hypothesis is proposed: group additivity is observed when the MFD is a sum of group frequency distributions (GFDs). This leads to additivity for the zero‐point vibrational energy which is confirmed by analysis of the frequencies of 126 organic molecules from 11 homologous series. The frequency distribution for a methylene group is estimated from that of octadecane. From this GFD combined with 11 different MFD it is possible to calculate model frequencies for the homologous series which are in good agreement with frequencies from ab initio calculations. For three thermochemical parameters (the logarithm of the vibrational partition function, the vibrational excitation energy and the vibrational heat capacity), the combination of the estimated methylene GFD with 11 different MFDs lead to group additivity values for a methylene group which are identical over a wide temperature range. The derivation of Benson additivity for thermochemical functions from frequency distributions is at step towards a better understanding of Benson additivity. Copyright © 2008 John Wiley & Sons, Ltd.

Carbon‐Centered Radical Addition and β‐Scission Reactions: Modeling of Activation Energies and Pre‐exponential Factors

A consistent set of group additive values DeltaGAV degrees for 46 groups is derived, allowing the calculation of rate coefficients for hydrocarbon radical additions and beta-scission reactions. A database of 51 rate coefficients based on CBS-QB3 calculations with corrections for hindered internal rotation was used as training set. The results of this computational method agree well with experimentally observed rate coefficients with a mean factor of deviation of 3, as benchmarked on a set of nine reactions. The temperature dependence on the resulting DeltaGAV degrees s in the broad range of 300-1300 K is limited to +/-4.5 kJ mol(-1) on activation energies and to +/-0.4 on logA (A: pre-exponential factor) for 90 % of the groups. Validation of the DeltaGAV degrees s was performed for a test set of 13 reactions. In the absence of severe steric hindrance and resonance effects in the transition state, the rate coefficients predicted by group additivity are within a factor of 3 of the CBS-QB3 ab initio rate coefficients for more than 90 % of the reactions in the test set. It can thus be expected that in most cases the GA method performs even better than standard DFT calculations for which a deviation factor of 10 is generally considered to be acceptable.

Thermochemistry of Chlorobenzenes and Chlorophenols: Ambient Temperature Vapor Pressures and Enthalpies of Phase Transitions

This work has been undertaken in order to obtain additional data on vapor pressures of chlorobenzene derivatives and to develop the group-additivity values necessary for predicting their vaporization enthalpies at the reference temperature T = 298.15 K. Molar enthalpies of sublimation and of vaporization of hexachlorobenzene and of mono-, di-, tri-, and pentachlorophenol were obtained from the temperature dependence of the vapor pressure measured by the transpiration method. Thermochemical investigations of chlorobenzenes and chlorophenols available in the literature were collected and combined with our own experimental results to obtain their reliable standard molar enthalpies of vaporizaton at T = 298.15 K. The COSMO-RS procedure has been used for a priori prediction of the vapor pressures and vaporization enthalpies of the whole data set of chlorobenzenes and chlorophenols. The new results help to resolve uncertainties in the available thermochemical data on chlorobenzenes and chlorophenols studied.

Enthalpies of Formation, Bond Dissociation Energies, and Molecular Structures of the n-Aldehydes (Acetaldehyde, Propanal, Butanal, Pentanal, Hexanal, and Heptanal) and Their Radicals

Aldehydes are important intermediates and products in a variety of combustion and gas-phase oxidation processes, such as in low-temperature combustion, in the atmosphere, and in interstellar media. Despite their importance, the enthalpies of formation and bond dissociation energies (BDEs) for the aldehydes are not accurately known. We have determined enthalpies of formation for acetaldehyde, propanal, and butanal from thermodynamic cycles, using experimentally measured reaction and formation enthalpies. All enthalpy values used for reference molecules and reactions were first verified to be accurate to within around 1 kcal mol-1 using high-level ab initio calculations. Enthalpies of formation were found to be -39.72 +/- 0.16 kcal mol-1 for acetaldehyde, -45.18 +/- 1.1 kcal mol-1 for propanal, and -49.27 +/- 0.16 kcal mol-1 for butanal. Enthalpies of formation for these three aldehydes, as well as for pentanal, hexanal, and heptanal, were calculated using the G3, G3B3, and CBS-APNO theoretical methods, in conjunction with bond-isodesmic work reactions. On the basis of the results of our thermodynamic cycles, theoretical calculations using isodesmic work reactions, and existing experimental measurements, we suggest that the best available formation enthalpies for the aldehydes acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal are -39.72, -45.18, -50.0, -54.61, -59.37, and -64.2 kcal mol-1, respectively. Our calculations also identify that the literature enthalpy of formation of crotonaldehyde is in error by as much as 1 kcal mol-1, and we suggest a value of -25.1 kcal mol-1, which we calculate using isodesmic work reactions. Bond energies for each of the bonds in the aldehydes up to pentanal were calculated at the CBS-APNO level. Analysis of the BDEs reveals the R-CH(2)CH=O to be the weakest bond in all aldehydes larger than acetaldehyde, due to formation of the resonantly stabilized vinoxy radical (vinyloxy radical/formyl methyl radical). It is proposed that the vinoxy radical as well as the more commonly considered formyl and acetyl radicals are important products of aldehyde combustion and oxidation, and the reaction pathways of the vinoxy, formyl, and acetyl radicals are discussed. Group additivity values for the carbon-oxygen-hydrogen groups common to the aldehydes are also determined. Internal rotor profiles and electrostatic potential surfaces are used to study the dipole induced dipole-dipole interaction in the synperiplanar conformation of propanal. It is proposed that the loss of this dipole-dipole interaction in RC(.-)HCH(2)CH=O radicals causes a ca. 1-2 kcal mol-1 decrease in the aldehyde C-H and C-C bond energies corresponding to RC(.-)HCH(2)CH=O radical formation.

Computational Study of the Thermochemistry of Organophosphorus(III) Compounds

The enthalpies of formation of organophosphorus(III) compounds have been calculated at the G3X, G3X(MP2), and B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels of theory using the atomization energy procedure and the method of isodesmic reactions. The Delta f H298 degree values for 50 relatively large molecules with up to 10 non-hydrogen atoms, such as P(CH3)3, P(C2H5)3, P(OCH3)3, n-C4H9OPCl2, [(CH3)2N]2PCl, (C2H5)2NPCl2, and [(CH3)2N]2PCN, have been calculated directly from the G3X atomization energies. A good agreement between the known experimental values and G3X results for 14 compounds provides support to our predictions for remaining species whose experimental enthalpies of formation are unknown or known with relatively large uncertainties. On the basis of our calculations and sometimes conflicting experimental data a set of internally consistent enthalpies of formation has been recommended for organophosphorus(III) compounds. Our computational results call into question the experimental enthalpies of formation of P(C2H5)3 and P(n-C4H9)3. From comparison with most reliable experimental data, the accuracy of the theoretical enthalpies of formation is estimated as ranging from 5 to 10 kJ/mol. The recommended Delta f H298 degree values were used to derive the group additivity values (GAVs) for 45 groups involving the phosphorus(III) atom. These GAVs significantly extend the applicability of Benson's group additivity method and may be used to estimate the enthalpies of formation of larger organophosphorus(III) compounds, where high level quantum chemical calculations are impracticable.

Re-tooling Benson's Group Additivity Method for Estimation of the Enthalpy of Formation of Free Radicals: C/H and C/H/O Groups

Enthalpy of formation of free radicals is often required for the discrimination of reaction mechanisms (for complex processes such as combustion, hydrocarbon oxidation, or decomposition of peroxides and hydroperoxides) involving both molecular species and free radicals. Despite the expanding availability of the experimental measurements, the database for the enthalpy of formation of radicals is still not comprehensive. One of the simplest, efficient, and reliable methods for estimating the quantity for organic radicals from their molecular structure is the group-additivity method due to Benson and co-workers. Perusal of the literature shows that the group-additivity values (GAV) for many radical groups have remained either undetermined or obtained by assignment to the corresponding molecular groups without justification. In this paper, we devise simple methodologies to evaluate GAVs for a number of new oxygen-containing radical groups and re-estimate several alkyl and oxygen-containing radical groups using experimental data on the radical enthalpy of formation. The validity of these GAVs has been established by comparing the predicted enthalpies with experimental data. Finally, some of these updated GAVs were used in estimating the enthalpy change associated with the typical elementary steps in a peroxide decomposition reaction.