Heats of Formation from G2, G2(MP2), and G2(MP2,SVP) Total Energies

Abstract

The translation of G2, G2(MP2), and G2(MP2,SVP) total energies to heats of formation via atomization and formation reaction procedures is evaluated. While both approaches give similar results, the former is shown to perform somewhat better, especially for organic molecules. The recommended general procedure for obtaining heats of formation at 0 K from G2-type total energies is to use G2-type atomization energies in conjunction with experimental gas-phase heats of formation for the constituent atoms. The corrections required to give heats of formation at finite temperatures can be obtained using scaled theoretically derived vibrational frequencies for the species under consideration together with temperature correction terms for the constituent elements based on experimental data. While the harmonic oscillator model is a good approximation for most vibrational modes, internal rotations with relatively low frequencies are more accurately treated within the free rotor approximation.

Protonation Thermochemistry of Gaseous 2,2'-, 4,4'- and 2,4'-Bipyridines and 1,10-phenanthroline

“…The corrections required to obtain the corresponding 298 K heats of formation combine (i) the theoretical contribution obtained from calculated vibrational frequencies for the species of interest and, (ii) temperature correction terms for the elements based on experimental data. 31 This procedure is expected to provide heats of formation within less than 5 kJ/mol when using G3 or G4 methods. The usual pictures used to explain conformational stabilities in bipyridine systems involve a compromise between several factors: in one hand, -conjugation between the two aromatic rings and stabilizing interaction between ortho-H and ortho'-N atoms which favor a planar arrangement of the bipyridine atoms, and, in the other hand, repulsion between ortho and ortho' hydrogen and(or) nitrogen atoms, which obviously destabilizes planar conformations.…”

Gas phase enthalpies of formation for aminonitroacetylene, aminonitromethane, and diaminodinitromethane: A Gaussian-4 (G4) theoretical study

“…b atomization energy approach as described in ref. [8,9]. kJ/mol for 1 by Golovin and Takhistov [10] appears to be in error when compared to the current G4 calculations and prior G2 and G3 estimates.…”

“…[7] and ref. [8,9] to provide additional ∆ f H • (g) estimates for these compounds. Three-dimensional visualizations of the G4 optimized geometries are shown in Figure 2, and full G09 archive entries (including geometry coordinates) are provided in the Supporting Information.…”

A G4/W1BD theoretical study into the gas phase enthalpies of formation for potential high energy materials

“…Although known compounds such as 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), pentaerythritol tetranitrate (PETN), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 2,4,6,8,10,4,6,8,10,, and octanitrocubane have received favorable applications in the high energy materials (HEMs) field, there is a continuing search for new molecules that can improve on their properties [1]. Advancements in computational power and improved theoretical chemistry software now facilitates the extensive modeling of HEMs either prior to, or in conjunction with, synthetic efforts and subsequent lab/field testing.…”