Isodesmic Bond Separation Reactions Research Articles

Quantum-Chemical Ab Initio Calculations on Ala-(C5H5Al) and Galabenzene (C5H5Ga)

1 Quantum-chemical ab initio and time-dependent density functional theory (TD-DFT) calculations employing various basis sets were used to elucidate the spatial as well as the electronic structure of C5H5Al () and C5H5Ga (2) (ala- and galabenzene). The lowest closed shell singlet states of both compounds were found to have a non-planar structure of C S symmetry with C-X-C bond angles of about 116° (MP2/6-311++G**) and 125° (CCSD/aug-cc-pVDZ). At approximately 103°, the corresponding angles of the lowest triplets are significantly smaller. The lowest triplet state of alabenzene is also non-planar (C S) at the MP2 level while optimization with the CCSD and the CASPT2 method resulted in planar structures with C 2v symmetry. The corresponding state of galabenzene has C 2v symmetry at all levels of optimization. The relative stability of the lowest closed shell singlet and the lowest triplet (ΔE(T 1-S 0)) state is small and its sign even strongly method-dependent. However, according to the highest levels of theory applied in this study the singlet states of both molecules are slightly lower in energy than the corresponding triplets with singlet/triplet gaps between about 0.5 and 5.8 kcal/mol in favour of the singlet states. Most of the applied methods give a slightly smaller splitting for ala- than for galabenzene. Independent of the applied method (TD-DFT/CAM-B3LYP/6-311++G(3df,3pd)//MP2/6- 311++G** or SAC-CI/6-31++G(3df,3pd)//MP2/6-311++G**), the general shape of the calculated UV/VIS spectral curves are quite similar for the lowest singlet states of ala- and galabenzene, and the same applies to the spectra of the normal modes. The calculated UV/VIS spectra of C5H5Al and C5H5Ga are featured by long wavelength bands of moderate intensity around 900 nm at the TD-DFT and between 1300 and 1500 nm at the SAC-CI level. According to both methods these bands are predominantly due to HOMO(π)→LUMO(σ*) transitions. The results of isodesmic bond separation reactions for the singlet states indicate some degree of stabilization due to delocalization in both of the title compounds. With our best values between 29 and 32 kcal/mol this stabilization appears to be only slightly less than the previously reported value for borabenzene (∼38 kcal/mol).

ChemInform Abstract: Homodesmotic Reactions for Thermochemistry

AbstractReview: 105 refs.

Knowledge of a Molecule: An Experimental and Theoretical Study of the Structure and Enthalpy of Formation of Tetrahydro-2H-1,3-oxazine-2-thione

In a continuation of our investigations of the thermochemistry of heterocyclic ring systems and sulfur-containing compounds, the standard molar enthalpy of formation of tetrahydro-2H-1,3-oxazine-2-thione (CAS no. 17374-18-4) at T = 298 K has been evaluated both experimentally and computationally. Combined enthalpies of combustion and sublimation, measured by rotary bomb combustion calorimetry and the Knudsen effusion technique, yielded a gas-phase enthalpy of formation of −(104.0 ± 5.5) kJ·mol–1. The G3 value of −104.6 kJ·mol–1 calculated using an isodesmic bond separation reaction is in excellent agreement with the experimental result. The variation in enthalpy of formation with ring size is discussed, and the G3 enthalpy of formation of tetrahydro-2H-1,3-thiazine-2-thione (72.4 kJ·mol–1) was also computed for this purpose.

Homodesmotic reactions for thermochemistry

AbstractAchieving highly accurate thermochemical predictions from relatively low levels of electronic structure theory has long been a goal of computational chemistry. One route to such a goal is to exploit the systematic cancellation of errors. This approach was spearheaded by the Pople group in the 1970s with the introduction of isodesmic bond‐separation reactions, and then extended by other groups to homodesmotic (HD) and hyperhomodesmotic reactions over the ensuing years. Unfortunately, the propagation of multiple, nonequivalent definitions of HD reactions, accompanied by a panoply of related reaction classes, has lead to a great deal of confusion. We review recent efforts to introduce a refined hierarchy of HD reactions for use in thermochemical predictions and to quantify the degree of error cancellation that can be achieved. Examples of the use of reactions from the HD hierarchy from the literature are presented, as are current limitations of this HD hierarchy for thermochemistry. Although the use of such error‐balanced reactions are no longer mandatory for high‐accuracy thermochemical predictions of small molecules, they still offer significant advantages in this context, and offer one route to accurate thermochemical predictions of larger molecules. © 2011 John Wiley & Sons, Ltd.This article is categorized under: Structure and Mechanism > Molecular Structures

Theoretical Calculation of Jet Fuel Thermochemistry. 1. Tetrahydrodicylopentadiene (JP10) Thermochemistry Using the CBS-QB3 and G3(MP2)//B3LYP Methods

High-level ab initio calculations have been performed on the exo and endo isomers of gas-phase tetrahydrodicyclopentadiene (THDCPD), a principal component of the jet fuel JP10, using the Gaussian G(x) and G(x)(MP(x)) composite methods, as well as the CBS-QB3 method, and using a variety of isodesmic and homodesmotic reaction schemes. The impetus for this work is to help resolve large discrepancies existing between literature measurements of the formation enthalpy Delta(f)H degrees (298) for exo-THDCPD. We find that use of the isodesmic bond separation reaction C(10)H(16) + 14CH(4) --> 12C(2)H(6) yields results for the exo isomer (JP10) in between the two experimentally accepted values, for the composite methods G3(MP2), G3(MP2)//B3LYP, and CBS-QB3. Application of this same isodesmic bond separation scheme to gas-phase adamantane yields a value for Delta(f)H degrees (298) within 5 kJ/mol of experiment. Isodesmic bond separation calculations for the endo isomer give a heat of formation in excellent agreement with the experimental measurement. Combining our calculated values for the gas-phase heat of formation with recent measurements of the heat of vaporization yields recommended values for Delta(f)H degrees (298)liq of -126.4 and -114.7 kJ/mol for the exo and endo isomers, respectively.

How Strained are Carbomeric-Cycloalkanes?

The ring strain energies of carbomeric-cycloalkanes (molecules with one or more acetylene spacer units placed into carbon single bonds) are assessed using a series of isodesmic, homodesmotic, and hyperhomodesmotic chemical equations. Isodesmic bond separation reactions and other equations derived from the explicitly defined hierarchy of homodesmotic equations are insufficient for accurately determining these values, since not all perturbing effects (i.e., conjugation and hyperconjugation) are fully balanced. A set of homodesmotic reactions is proposed, which succeeds in balancing all stereoelectronic effects present within the carbomeric rings, allowing for a direct assessment of the strain energies. Values calculated from chemical equations are validated using an increment/additivity approach. The ring strain energy decreases as acetylene units are added, manifesting from the net stabilization gained by opening the C-CH(2)-C angle around the methylene groups and the destabilization arising from bending the C-C identical withC angles of the spacer groups. This destabilization vanishes with increasing parent ring size (i.e., the angle distortion is less in the carbomeric-cyclobutanes than in the carbomeric-cyclopropanes), leading to strain energies near zero for carbo(n)-cyclopentanes and carbo(n)-cyclohexanes.

A hierarchy of homodesmotic reactions for thermochemistry.

Chemical equations that balance bond types and atom hybridization to different degrees are often used in computational thermochemistry, for example, to increase accuracy when lower levels of theory are employed. We expose the widespread confusion over such classes of equations and demonstrate that the two most widely used definitions of "homodesmotic" reactions are not equivalent. New definitions are introduced, and a consistent hierarchy of reaction classes (RC1-RC5) for hydrocarbons is constructed: isogyric (RC1) superset of isodesmic (RC2) superset of hypohomodesmotic (RC3) superset of homodesmotic (RC4) superset of hyperhomodesmotic (RC5). Each of these successively conserves larger molecular fragments. The concept of isodesmic bond separation reactions is generalized to all classes in this hierarchy, providing a unique sectioning of a given molecule for each reaction type. Several ab initio and density functional methods are applied to the bond separation reactions of 38 hydrocarbons containing five or six carbon atoms. RC4 and RC5 reactions provide bond separation enthalpies with errors consistently less than 0.4 kcal mol(-1) across a wide range of theoretical levels, performing significantly better than the other reaction types and far superior to atomization routes. Our recommended bond separation reactions are demonstrated by determining the enthalpies of formation (at 298 K) of 1,3,5-hexatriyne (163.7 +/- 0.4 kcal mol(-1)), 1,3,5,7-octatetrayne (217.5 +/- 0.6 kcal mol(-1)), the larger polyynes C(10)H(2) through C(26)H(2), and an infinite acetylenic carbon chain.

Synthesis and thermochemical properties of NF 2-containing energetic salts

Ammonium, 1,5-diamino-4-methyl-tetrazolium and 4-amino-1-methyl-triazolium salts of 5-difluoroaminodifluoromethyl-tetrazolate (TA-CF 2NF 2) were prepared by metathesis reactions of silver 5-difluoroaminodifluoromethyl-tetrazolate and the corresponding iodides. All are thermally stable to ∼150 °C. The ammonium salt has a density of 1.88 g cm −3. The combination of the CBS-4 method and isodesmic bond separation reactions was found to be an economical and reliable method to estimate heats of formation for polyfluorinated molecules. The standard heats of formation ( Δ f H 298 ° ) of ammonium 5-difluoroaminodifluoromethyl-tetrazolate was calculated to be −53.13 kcal mol −1 using the CBS-4 method. While its detonation pressures ( P) and velocities ( D) were estimated using Cheetah 4.0: P = 28.78 GPa; D = 8490 m s −1; detonation properties for 1,5-diamino-4-methyl-tetrazolium salts of 5-difluoroaminomethyltetrazolate (TA-CH 2NF 2), 5-difluoroaminotetrazolate (TA-NF 2) and 5-difluoroaminodinitromethyl-tetrazolate (TA-C(NO 2) 2NF 2) are also compared based on predicted densities and computed heats of formation.

Experimental and computational thermochemical study of 3-hydroxypropanenitrile

This paper reports the values of the standard ( p ∘ = 0.1 MPa) molar enthalpy of formation in the gas phase, at T = 298.15 K, for 3-hydroxypropanenitrile derived from the respective enthalpy of combustion in oxygen, measured by static bomb combustion calorimetry and the standard molar enthalpy of vaporization, at T = 298.15 K, determined using the transference (transpiration) method in a saturated N 2 stream, the value of the gas-phase enthalpy of formation being Δ f H m ∘ ( g ) = - ( 91.2 ± 1.4 ) kJ · mol - 1 . Furthermore, we have performed high-level ab initio molecular orbital calculations at the G3 level. Four different conformations corresponding to local minima of the potential energy surface have been found for 3-hydroxypropanenitrile. The optimised molecular and electronic structure of the most stable conformer of 3-hydroxypropanenitrile shows a gauche arrangement with a weak intramolecular hydrogen bond from OH hydrogen to the π-bond charge cloud of the nitrile group. The calculated values for the enthalpy of formation of the most stable conformer using atomization and isodesmic bond separation reactions, −94.7 and −95.0 kJ · mol −1, respectively, are in very good agreement with the experimental value.

Thermochemistry of Furancarboxylic Acids

The relative stabilities of 2- and 3-furancarboxylic acids have been studied, using an experimental thermochemical investigation and high-level ab initio calculations. The enthalpies of combustion and sublimation of 2- and 3-furancarboxylic acids were measured and the gas-phase enthalpies of formation at 298.15 K were determined. Standard ab initio molecular orbital calculations at the G2(MP2) and G2 levels were performed, and a theoretical study on the molecular and electronic structures of the studied compounds has been conducted. Calculated enthalpies of formation, using atomization and isodesmic bond separation reactions, are compared with the experimental data. Experimental and theoretical results show that 3-furancarboxylic acid is thermodynamically more stable than the 2-isomer. A comparison of the substituent effect of the carboxylic group in benzene, thiophene, and furan rings has been made.

Experimental and Computational Thermochemistry of 2- and 3-Thiophenecarboxylic Acids

The enthalpies of combustion and sublimation of 2- and 3-thiophenecarboxylic acids were measured and the gas-phase enthalpies of formation at 298.15 K were determined. Standard ab initio molecular orbital calculations at the G2(MP2) and G2 levels were performed, and a theoretical study on molecular and electronic structures of the studied compounds has been carried out. Calculated enthalpies of formation using atomization and isodesmic bond separation reactions are compared with the experimental data. From experimental and theoretical results it seems that 3-thiophenecarboxylic acid is slightly more stable than the 2-isomer. A comparison of substituent effect of the carboxylic group in benzene and thiophene rings has been made.

Assessment of complete basis set methods for calculation of enthalpies of formation

Three complete basis set models of Petersson et al. [J. Chem. Phys. 104, 2598 (1996)], CBS-Q, CBS-q, and CBS-4, have been assessed on the G2 neutral test set of 148 molecules [J. Chem. Phys. 106, 1063 (1997)]. The average absolute deviations with experiment of the calculated enthalpies of formation from the three CBS methods are 1.57 kcal/mol (CBS-Q), 2.13 kcal/mol (CBS-q), and 3.06 kcal/mol (CBS-4). The maximum deviations of the methods are 11.2, 10.3, and 14.4 kcal/mol. respectively. The most accurate method, CBS-Q, has an average absolute deviation similar to that of G2 theory. The three CBS methods have also been assessed on a 40 molecule set using isodesmic bond separation reactions to calculate enthalpies of formation. There is a significant improvement in the accuracy of the enthalpies compared to those calculated using atomization energies, although not as much as for G2 theory. In a test on naphthalene, enthalpies calculated using the CBS methods have large deviations. The CBS-Q method has a deviation of 28.7 kcal/mol and, surprisingly, the deviation increases to 34.3 kcal/mol when isodesmic bond separation reaction energies are used.

Accurate density functional thermochemistry for larger molecules

Density functional methods are combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. Seven different density functionals are assessed for the evaluation of heats of formation, Delta H 0 (298 K), for a test set of 40 molecules composed of H, C, O and N. The use of bond separation energies results in a dramatic improvement in the accuracy of all the density functionals. The B3-LYP functional has the smallest mean absolute deviation from experiment (1.5 kcal mol/f).

Accurate density functional thermochemistry for larger molecules

Density functional methods are combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. Seven different density functionals are assessed for the evaluation of heats of formation, Δ H f 0 (298 K), for a test set of 40 molecules composed of H, C, O and N. The use of bond separation energies results in a dramatic improvement in the accuracy of all the density functionals. The B3-LYP functional has the smallest mean absolute deviation from experiment (1·5 kcal mol f -1).

Accurate thermochemistry for larger molecules: Gaussian-2 theory with bond separation energies

Gaussian-2 (G2) theory is combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. For a test set of 40 molecules composed of H, C, O, and N, our method yields enthalpies of formation, ΔHf0(298 K), with a mean absolute deviation from experiment of only 0.5 kcal/mol. This is an improvement of a factor of three over the deviation of 1.5 kcal/mol seen in standard G2 theory.

Theoretical evidence for P—S and P—Npπ—pπ bonding within the heterocyclopentadienyl framework

SCF and Møller–Plesset calculations have been carried out for systems containing P—S and P—N pπ—pπ bonds: namely, the 1,3,2-dithiaphospholium cation, the 1,3,2-diazaphospholium cation, and the 1,3,2-azathiaphospholium cation. The computed geometries are reported, along with Mulliken atomic charges and selected vibrational frequencies. ΔH0values for heterolytic P—H bond dissociation and for isodesmic bond separation reactions provide information on the energetics and stabilization of the P—S and P—N pπ—pπ bonds. Keywords: SCF and MP2 calculations, third period pπ—pπ bonding, bond separation reactions.