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Enthalpies of formation, ΔfH298°, in both the gas and condensed phase, and enthalpies of sublimation or vaporization have been estimated for hydrazine, NH2NH2, and its 36 various derivatives using quantum chemical calculations. The composite G4 method has been used along with isodesmic reaction schemes to derive a set of self-consistent high-accuracy gas-phase enthalpies of formation. To estimate the enthalpies of sublimation and vaporization with reasonable accuracy (5-20 kJ/mol), the method of molecular electrostatic potential (MEP) has been used. The value of ΔfH298°(NH2NH2,g) = 97.0 ± 3.0 kJ/mol was determined from 75 isogyric reactions involving about 50 reference species; for most of these species, the accurate ΔfH298°(g) values are available in Active Thermochemical Tables (ATcT). The calculated value is in excellent agreement with the reported results of the most accurate models based on coupled cluster theory (97.3 kJ/mol, the average of six calculations). Thus, the difference between the values predicted by high-level theoretical calculations and the experimental value of ΔfH298°(NH2NH2,g) = 95.55 ± 0.19 kJ/mol recommended in the ATcT and other comprehensive reference sources is sufficiently large and requires further investigation. Different hydrazine derivatives have been also considered in this work. For some of them, both the enthalpy of formation in the condensed phase and the enthalpy of sublimation or vaporization are available; for other compounds, experimental data for only one of these properties exist. Evidence of accuracy of experimental data for the first group of compounds was provided by the agreement with theoretical ΔfH298°(g) value. The unknown property for the second group of compounds was predicted using the MEP model. This paper presents a systematic comparison of experimentally determined enthalpies of formation and enthalpies of sublimation or vaporization with the results of calculations. Because of relatively large uncertainty in the estimated enthalpies of sublimation, it was not always possible to evaluate the accuracy of the experimental values; however, this model allowed us to detect large errors in the experimental data, as in the case of 5,5'-hydrazinebistetrazole. The enthalpies of formation and enthalpies of sublimation or vaporization have been predicted for the first time for ten hydrazine derivatives with no experimental data. A recommended set of self-consistent experimental and calculated gas-phase enthalpies of formation of hydrazine derivatives can be used as reference ΔfH298°(g) values to predict the enthalpies of formation of various hydrazines by means of isodesmic reactions. [ABSTRACT FROM AUTHOR]

Gas-phase O-H homolytic bond dissociation enthalpy in phenol, DH298°(C6H5O-H), is still disputed, despite a large number of experimental and computational studies. In estimating this value, the experimental enthalpy of formation of phenol, ΔfH298°(C6H5OH, g) = -96.4 ± 0.6 kJ/mol (Cox, J. D. Pure Appl. Chem. 1961, 2, 125-128), is often used assuming high accuracy of the experimental value. In the present work a substantially less negative value of ΔfH298°(C6H5OH, g) = -91.8 ± 2.5 kJ/mol was calculated combining G4 theory with an isodesmic reaction approach. A benchmark quality of this result was achieved by using a large number of reliable reference species in isodesmic reaction calculations. Among these are the most accurate ΔfH298° values currently available from the Active Thermochemical Tables (ATcT) for 36 species (neutral molecules, radicals, and ions), anisole with recently reassessed enthalpy of formation, and 13 substituted phenols. The internal consistency of the calculated ΔfH298°(C6H5OH, g) value with the experimental enthalpies of formation of more than 50 reference species suggests that the reported experimental enthalpy of formation of phenol is in error. Taking into account that the enthalpy of formation of phenol has not been investigated experimentally since 1961, the new measurements would be extremely valuable. Using the accurate enthalpies of formation of C6H5OH and C6H5O• calculated in the present work, we obtained DH298°(C6H5O-H) = 369.6 ± 4.0 kJ/mol. This value is in satisfactory agreement with that determined from the most precise experimental measurement. [ABSTRACT FROM AUTHOR]

Accurate gas-phase enthalpies offormation (ΔfH298°) of 20 common α-aminoacids, seven uncommon aminoacids, and three small peptides were calculated by combining G4 theorycalculations with an isodesmic reaction approach. The internal consistencyover a set of ΔfH298°(g) values was achievedby sequential adjustment of their values through the isodesmic reactions.Four amino acids, alanine, β-alanine, sarcosine, and glycine,with reliable internally self-consistent experimental data, were chosenas the key reference compounds. These amino acids together with about100 compounds with reliable experimental data (their accuracy wassupported by G4 calculations) were used to estimate the enthalpiesof formation of remaining amino acids. All of the amino acids withthe previously established enthalpies of formation were later usedas the reference species in the isodesmic reactions for the otheramino acids. A systematic comparison was made of 14 experimentallydetermined enthalpies of formation with the results of calculations.The experimental enthalpies of formation for 10 amino acids were reproducedwith good accuracy, but the experimental and calculated values for4 compounds differed by 11–21 kJ/mol. For these species, thetheoretical ΔfH298°(g) values were suggestedas more reliable than the experimental values. On the basis of theoreticalresults, the recommended values for the gas-phase enthalpies of formationwere also provided for amino acids for which the experimental ΔfH298°(g) were not available. The enthalpiesof sublimation were evaluated for all compounds by taking into accountthe literature data on the solid-phase enthalpies of formation andthe ΔfH298°(g) values recommended in our work.A special attention was paid to the accurate prediction of enthalpiesof formation of amino acids from the atomization reactions. The problemsassociated with conformational flexibility of these compounds andharmonic treatment of low frequency torsional modes were discussed.The surprisingly good agreement between the ΔfH298°(g) values calculated from the atomization and isodesmic reactionsis largely the result of a fortuitous mutual compensation of variouscorrections used in the atomization reaction procedure. [ABSTRACT FROM AUTHOR]

Accurategas-phase enthalpies of formation (ΔfH298°)of 29 azides are recommended by combining G4 theory calculations withan isodesmic reaction approach. The internal consistency over a setof ΔfH298°values was achieved by sequentialadjustment of their values through the isodesmic reactions. The HN3was chosen as a key reference compound. Of the experimentaldata available for 16 compounds, our predictive values agree wellwith 9 of them, while the deviations from 25 to 55 kJ/mol are observedfor 7 compounds; possible systematic errors in the experimental datafor phenyl azide, 2-azidoethanol, azidocyclopentane, azidocyclohexane,3-azido-3-ethylpentane, 2-azido-2-phenylpropane, and 1-azidoadamantaneare discussed. The recommended enthalpies of formation of organicazides were used as reference values to estimate the enthalpy of formationof four nitrogen-rich carbon nitrides. The calculations do not supportthe high value of the solid-state enthalpy of formation of TAAT (4,4′,6,6′-tetra(azido)azo-1,3,5-triazine);its value is estimated to be 300–400 kJ/mol lower than thatmeasured experimentally. [ABSTRACT FROM AUTHOR]