Calculating heats of formation

Unlike the other stmctural isomers of C2H N, /V-methylenemethylamine (33,34), ethylideneimine (35), and vinylamine [593-67-9] (36,37) and the analogous phosphoms compound, phosphirane (38), ethyleneimine is stable at room temperature provided CO2 is excluded from the air (39). Unexpectedly, ethyleneimine has the highest calculated relative heat of formation of the C2H N isomers (40). Relative calculated heats of formation are ethylideneimine. 

Oxidative degradation of [B qH q] and [B22H22] C to boric acid is extremely difficult and requires Kjeldahl digestion or neutral permanganate. The heat of reaction obtained from the permanganate degradation leads to a calculated heat of formation for [B qH q] (aq) of 92.5 21.1 kJ/mol (22.1 5.0 kcal/mol) (99). The oxidative coupling of both [B qH q] and has been studied ia some detail (100). 

Although we are dealing with work described in the literature by means of a posteriori molecular calculation it is useful to see the relative calculated heats of formation of the isomeric propellanes and dispirans and note particularly the right-most columns so as to be in a position to gauge these against the experimental results or vice versa. 

Fig. 6), which can be used as a bridgehead scaffold for the all-carbon cage molecule Q-C6o (42), an isomer of Ih-buckminsterfullerene that is 767 kcal-mol 1 higher in energy by the calculated heats of formation (RHF/3-21G).110,291 A similar macrocyde (45, C6oH6) of easier synthetic accessibility is based on the 1,3,5-triethynylbenzene moiety. 

Quantum-chemical calculations (AMI, PM3) have been carried out in order to investigate the thermodynamic behavior of the possible equilibrium between variously substituted 6-azidotetrazolo[l,5-A pyridazine 11 and the bis-tetrazole 12 <2005JST65> (Scheme 2). From the calculated heat of formation, the authors concluded that this value is consistently lower for the azide tautomers 11 than for the corresponding tetrazoles 12 on average by 20kcalmol 1 and, thus, the azide isomers- in full accordance with the experimental observations - are more stable than the ring-closed fused tetrazoles. 

The calculated heat of formation of singlet carbene agrees well with experiment. That of triplet carbene is not known. The ionization potential of singlet carbene, determined spectroscopically 51 (10.26 eV) is much less than the electron impact value S3 (11.9 eV) if the latter refers to triplet carbene, as seems likely, the singlet-triplet separation is 1.64 eV or 37.8 kcal/mole, which is quite close to our estimate (28.5 kcal/mole). 

Table 14. Smaller complexes1) contributions to the calculated heats of formation for some ion-H20...

Fig. 11. Plot of the calculated heat of formation (Hi kcal/mole at 25°) vs. the reaction coordinate ri for addition of triplet (Ti) carbene to ethylene and for rearrangement of the resulting biradical...

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

DCB radical anions. Stabilities of the intermediates were estimated by calculated heats of formation. Employing these, the theoretical results were in good agreement with the experiment (Scheme 15) [21]. 

Octanitrocubane (ONC) has a density of 1.979 g/cm, a calculated heat of formation of 594 kJ/mol, and a decomposition temperature above 200 °C. The explosive performance of octanitrocubane (41) from theoretical calculations is predicted to be exdemely high. The most recent theoretical estimate "" of VOD is 9900 m/s, making this compound one of the most powerful explosives synthesized to date. Surprisingly, the density of heptanitrocubane... 

A conformational analysis of thiacycloalkanes including 2-thiacyclobu-tane, utilizing molecular mechanics for the calculation of the geometries and energies, was made by Allinger and Hickey. The calculated C—C, C—S, C—S—C, C—C—C, and C—C—S bond parameters for the thietane structure, in comparison to those that were derived from electron diffraction, NMR, and microwave measurement, are reported. The experimental and calculated heat of formation of the thiacyclobutane are 14.49 and 14.58 kcal/mol, respectively. 

Calculating Heats of Formation from Bond Separation Reactions... 

A bond separation reaction is uniquely defined. Therefore, a bond separation energy is a molecular property . Given that a bond separation reaction leads to products, the heats of formation of which are either known experimentally or can be determined from calculations, combining a calculated bond separation energy with experimental (or calculated) heats of formation, gives rise to a unique value for the heat of formation. For example, a heat of formation for methylhydrazine may be obtained from the thermochemical cycle. 

Here, AErx is the calculated energy of the bond separation reaction of methylhydrazine and AHf(NH3), AHf(CH3NH2) and AHf(NH2NH2) are experimental (or calculated) heats of formation. 

The second source of uncertainty can add a few kcal mol to the imcer-tainty estimates and will scale with the munber of bonds in the molecule. The use of different reference values would shift our calculated heats of formation as a group, with the consequence that calculated bond dissociation enthalpies and reaction enthalpies are affected less than the heats of formation. Overall, we believe that the uncertainties in the BAC-MP4 heats of formation lie in the 2-5 kcalmol" range. 

Group Values for Calculating Heats of Formation of Trans Azoalkanes... 

Orbital mapping analysis of thermal isomerization of aza- and diaza-bicyclo[2.2.0]hexa-dienes has been carried out (80MI51205). Molecular orbital calculations (MNDO) of a series of bisdehydropyridines showed that the 2-azabicyclo[2.2.0]hexa-l,3,5-triene (125) is the least stable form having a calculated heat of formation of 728.4 kJ mol-1 (77CC539). 

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