Accurate gas-phase enthalpies of formation (ΔfH298°) of 20 common α-amino acids, seven uncommon amino acids, and three small peptides were calculated by combining G4 theory calculations with an isodesmic reaction approach. The internal consistency over a set of ΔfH298°(g) values was achieved by 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 chosen as the key reference compounds. These amino acids together with about 100 compounds with reliable experimental data (their accuracy was supported by G4 calculations) were used to estimate the enthalpies of formation of remaining amino acids. All of the amino acids with the previously established enthalpies of formation were later used as the reference species in the isodesmic reactions for the other amino acids. A systematic comparison was made of 14 experimentally determined enthalpies of formation with the results of calculations. The experimental enthalpies of formation for 10 amino acids were reproduced with good accuracy, but the experimental and calculated values for 4 compounds differed by 11–21 kJ/mol. For these species, the theoretical ΔfH298°(g) values were suggested as more reliable than the experimental values. On the basis of theoretical results, the recommended values for the gas-phase enthalpies of formation were also provided for amino acids for which the experimental ΔfH298°(g) were not available. The enthalpies of sublimation were evaluated for all compounds by taking into account the literature data on the solid-phase enthalpies of formation and the ΔfH298°(g) values recommended in our work. A special attention was paid to the accurate prediction of enthalpies of formation of amino acids from the atomization reactions. The problems associated with conformational flexibility of these compounds and harmonic treatment of low frequency torsional modes were discussed. The surprisingly good agreement between the ΔfH298°(g) values calculated from the atomization and isodesmic reactions is largely the result of a fortuitous mutual compensation of various corrections used in the atomization reaction procedure.

Accurate gas-phase enthalpies of formation (Δf H 298 °) of 29 azides are recommended by combining G4 theory calculations with an isodesmic reaction approach. The internal consistency over a set of Δf H 298 ° values was achieved by sequential adjustment of their values through the isodesmic reactions. The HN3 was chosen as a key reference compound. Of the experimental data available for 16 compounds, our predictive values agree well with 9 of them, while the deviations from 25 to 55 kJ/mol are observed for 7 compounds; possible systematic errors in the experimental data for phenyl azide, 2-azidoethanol, azidocyclopentane, azidocyclohexane, 3-azido-3-ethylpentane, 2-azido-2-phenylpropane, and 1-azidoadamantane are discussed. The recommended enthalpies of formation of organic azides were used as reference values to estimate the enthalpy of formation of four nitrogen-rich carbon nitrides. The calculations do not support the 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 that measured experimentally.

The gas-phase enthalpies of formation (Δ f H 298 o ) of 57 aliphatic nitro compounds and nitramines (mono-and polynitro compounds including cyclic compounds and well-known explosives such as hexogen, octogen, and CL-20) were calculated using the Gaussian-4 (G4) theory applied to the atomization and isodesmic reaction energies. The Δ f H 298 o (g) values calculated from the atomization reactions were underestimated by an average of 10 kJ·mol −1 , and they could not be used for the assessment of inaccuracies in the experimental enthalpies of formation. Much better agreement between theory and experiment was obtained using the isodesmic reaction procedure. Several isodesmic reactions were investigated for each compound. Evidence of high accuracy of most experimental data was provided by the agreement with theoretical results. The differences between the calculated and the experimental enthalpies of formation in the range from (8 to 53) kJ·mol −1 were assigned to possible errors in the experimental values for 17 compounds. The theoretical Δ f H 298 o (g) values were recommended for these compounds as being more reliable than the experimental values. As a result, a reference data set of internally consistent gas-phase enthalpies of formation of nitro compounds and nitramines was provided. Both experimental and calculated values are included in this data set. The enthalpies of sublimation or vaporization were evaluated for some compounds by taking into account the literature data on the condensed phase enthalpies of formation and the Δ f H 298 o (g) values recommended in our work. Thus, a set of self-consistent values of the enthalpy of formation in both condensed and gaseous phases and the enthalpy of sublimation or vaporization is presented for most of nitro compounds studied.

The molecular structure and conformation of nitrobenzene has been reinvestigated by gas-phase electron diffraction (GED), combined analysis of GED and microwave (MW) spectroscopic data, and quantum chemical calculations. The equilibrium r e structure of nitrobenzene was determined by a joint analysis of the GED data and rotational constants taken from the literature. The necessary anharmonic vibrational corrections to the internuclear distances (r e -r a ) and to rotational constants (B e (i) -B 0 (i) ) were calculated from the B3LYP/ccpVTZ quadratic and cubic force fields. A combined analysis of GED and MW data led to following structural parameters (r e ) of planar nitrobenzene (the total estimated uncertainties are in parentheses): r(C-C) av = 1.391 (3) Å , r(C-N) = 1.468(4) Å , r(N-O) = 1.223(2) Å , r(C-H) av = 1.071(3) Å , \C2-C1-C6 = 123.5(6)°, \C1-C2-C3 = 117.8(3)°, \C2-C3-C4 = 120.3(3)°, \C3-C4-C5 = 120.5(6)°, \C-C-N = 118.2(3)°, \C-N-O = 117.9(2)°, \O-N-O = 124.2(4)°, \(C-C-H) av = 120.6(20)°. These structural parameters reproduce the experimental B 0 (i) values within 0.05 MHz. The experimental results are in good agreement with the theoretical calculations.The barrier height to internal rotation of nitro group, 4.1±1.0 kcal/mol, was estimated from the GED analysis using a dynamic model. The equilibrium structure was also calculated using the experimental rotational constants for nitrobenzene isotopomers and theoretical rotation-vibration interaction constants.

The available structural parameters, fundamental frequencies, and relative energies of different stable conformers, if any, for cyclopropane, cyclopropene, cyclobutane, cyclobutene, 1,3-cyclobutadiene, cyclopentane, cyclopentene, 1,3-cyclopentadiene, cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, cycloheptane, cycloheptene, 1,3-cycloheptadiene, 1,3,5-cycloheptatriene, cyclooctane, cyclooctene, 1,3-cyclooctadiene, 1,5-cyclooctadiene, and 1,3,5,7-cyclooctatetraene were critically evaluated and the recommended values selected. Molecular constants for some molecules were estimated as the experimental values for these compounds are not available. This information was utilized to calculate the ideal gas thermodynamic properties C○p, S○, −(G○−H○0)/T, H○−H○0, and log Kf from 100 to 1500 K. The thermal functions were obtained using the rigid-rotor harmonic-oscillator approximation. The contributions derived for the inversion motion of cyclobutane and cyclopentene were obtained from energy levels calculated with the potential functions. For cyclopentane the pseudorotational contributions to thermal functions were calculated by assuming the pseudorotation as the free rotation of the molecule. The calculated values of the thermal functions are compared with those reported in other work. Agreement with experimental data, where such are available, is satisfactory within the experimental uncertainties.