Gas-phase enthalpies of formation of 122 relatively large organic molecules with up to 15 non-hydrogen atoms have been calculated at the Gaussian-4 (G4

As far as fundamental knowledge is concerned, the methyl derivatives of uracil can be considered as the simplest objects for studying the structural effects due to the substitution in the pyrimidyne nucleobases. From this point of view, 1-methyluracil is of special importance in biochemistry because uracil attaches ribose in ribonucleic acid (RNA) just precisely at the N1 atom. The semi-experimental equilibrium structure (r(e)(se)) of 1-methyluracil has been determined for the first time by the gas electron diffraction (GED) method taking into account rovibrational corrections to the thermal-average internuclear distances calculated with harmonic and anharmonic (cubic) MP2/cc-pVTZ force constants with consideration of the methyl torsion as a large-amplitude motion. For the first time, the structure of the molecule has been optimized by the very time-consuming coupled-cluster method with single and double excitations and perturbative treatment of connected triples using the correlation-consistent polarized weighted core-valence triple-ζ basis set with all electrons being correlated (CCSD(T)(all)/cc-pwCVTZ) and extrapolated to the complete basis set (CBS) with the help of the MP2 calculations. Small differences between similar bond lengths of equilibrium configurations were assumed in the GED analysis at the CCSD(T)(all)/CBS values. A remarkable agreement between the semi-experimental and computed equilibrium structures points out the high accuracy of both the GED determination and the coupled-cluster computations. The effect of methylation on the structure of uracil has been analyzed.

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The accuracy of experimental data on enthalpies of formation and vaporization of aliphatic amines (primary, secondary, and tertiary amines, di-, tri-, and tetramines, and cycloalkylamines) was assessed by theoretical calculations. The gas-phase enthalpies of formation were calculated using the Gaussian 4 (G4) method combined with isodesmic reactions. Three amines, CH 3 NH 2 , CH 3 NHCH 3 , and (CH 3 ) 3 N, were used as the main reference species in the isodesmic reactions; their experimental enthalpies of formation were recently revisited in the Active Thermochemical Tables. The enthalpies of vaporization were estimated by three group additivity and molecular electrostatic potential models and then were used to calculate the enthalpies of formation in the liquid state. When the deviations of experimental values from those provided by the theoretical methods were analyzed, a set of accurate and consistent values of enthalpy of formation of aliphatic amines was recommended for use in thermochemical calculations and as benchmarks for theoretical chemistry.