A three-parametric modification equation and the least-squares approach are adopted to calibrating hybrid density-functional theory energies of C(1)-C(10) straight-chain aldehydes, alcohols, and alkoxides to accurate enthalpies of formation DeltaH(f) and Gibbs free energies of formation DeltaG(f), respectively. All calculated energies of the C-H-O composite compounds were obtained based on B3LYP6-311++G(3df,2pd) single-point energies and the related thermal corrections of B3LYP6-31G(d,p) optimized geometries. This investigation revealed that all compounds had 0.05% average absolute relative error (ARE) for the atomization energies, with mean value of absolute error (MAE) of just 2.1 kJ/mol (0.5 kcal/mol) for the DeltaH(f) and 2.4 kJ/mol (0.6 kcal/mol) for the DeltaG(f) of formation.
A test set of 65 hydrocarbons was examined to elucidate theoretically their thermodynamic properties by performing the density-functional theory (DFT) and ab initio calculations. All the calculated data were modified using a three-parameter calibration equation and the least-squares approach, to determine accurately enthalpies of formation (DeltaH(f)), entropies (S), and heat capacities (C(p)). Calculation results demonstrated that the atomization energies of all compounds exhibited an average absolute relative error ranging between 0.11- 0.13%, and an DeltaH(f) of formation with a mean absolute absolute error (M.|A.E.|) ranging from only 5.7-6.8 kJ/mol (1.3-1.6 kcal/mol) (i.e., those results correlated with those of Dr. Herndon's 1.1 kcal/mol). Additionally, the entropy ranged from 3.5-4.2 J/mol K (0.8-1.0 cal/mol K) M.|A.E.|; a heat capacity between 2.3-2.9 J/mol K (0.5-0.7 cal/mol K) M.|A.E.| was obtained as well.
Density functional theory (DFT) calculations are made and least squares calibration performed for various halohydrocabons, which were 27 straight-chain alkyl halides, 20 branch-chain alkyl halides and 19 aromatic halides, to determine their enthalpies of formation (DH f ). The mean absolute error (M. |A.E.|) in DH f across 66 molecular computations was only 7.8 kJ/mol (1.9 kcal/mol). Grouping the molecules by their structural characteristics improved M. |A.E.| of DH f by 0.2-2.2 kJ/mol over that obtained using corresponding modified data for the same 66 unclassified molecules.
This study investigated the RDX (1,3,5-Trinitro-1,3,5-triazine) molecule to elucidate its possible decomposition species and the corresponding energies by performing the density-functional theory (DFT) calculations. Reasonable decomposition mechanisms are proposed based on the bond energy calculated using the differential overlap (INDO) program, which yields the weakest bonding site for reference and determines the site of easy cleavage. Computational results indicate that the activation energy of direct cis-form HONO elimination is lower than that of direct trans-form HONO elimination and that of a two-stage elimination of two forms of HONO (N-N bond fission combined with C-H bond breaking) in the initial decomposition step, which are 213.9 kJ/mol and 93.8-101.8 kJ/mol, respectively. Two possible pathways are proposed; (1) N-N bond homolytic cleavage followed by elimination of cis-form HONO, and (2) N-N bond homolytic cleavage followed by elimination of trans-form HONO.
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