Isomers stability prediction

We have approached these multi-faceted systems by looking in particular at two local molecular properties the electrostatic potential, P(r) and Vs(r). and the local ionization energy, /s(r). In terms of these, we have addressed hydrogen bonding, lone pair-lone pair repulsion, conformer and isomer stability, acidity/basicity and local polarizability. We have sought to show how theoretical and computational analyses can complement experimental studies in characterizing and predicting molecular behavior. ... 

FSSF is isomeric with SSF (3). Qualitative evidence has been presented (1 ) which indicates that SSFg is the more stable isomer. From our estimated free energy data, we predict a Gibbs energy change for the isomerization reaction FSSF(g) = SSF (g) of -15.4 kcal mol at 298.15 K, indicating that SSF is substantially more stable than FSSF. This conflicts with relative stability predictions based on results from two Independent MO studies (5, 6). These semiempirical (CNDO/2) calculations indicate that the instability of FSSF relative to SSF amounts to only about 2.0 kcal mol". If we assume that these MO calculations are correct, we calculate that Dq (FS-SF) > Dq (S=SF2) by roughly 7.0 kcal mol". This seems very unlikely in view of what is known (1 ) about the... 

There are enthalpy of formation data for the 2,4- and 4,6-dinitro-l,3-benzenediols. From the archival enthalpy of formation of solid m-dinitrobenzene and 5 (OH/H), the enthalpy of formation of either dinitrobenzenediol isomer is predicted to be —434.2 kJ moH. Compared to this estimated value, the 2,4-isomer is ca 11 kJ moH destabilized and the 4,6-isomer is ca 9 kJ mol stabilized. Each of these compounds has a pair of meta hydroxy groups, a pair of meta nitro groups and two pairs of ortho hydroxy/nitro groups. The 2,4-isomer has an additional ortho hydroxy/nitro interaction. 

The angular overlap model has been used to determine the relative stability of the systems, cis- and rrans-MA2B4, isomers of MA3B3, axially and equatorially substituted square-pyramidal and trigonal-bipyramidal MAB4 and square-planar MA2B2- The most stable isomer was predicted in each case, based on d-orbital stability. ... 

Neither the cyclic nor the acyclic isomers of NgH have been observed experimentally only organic derivatives of cyclopentazadiene, c-NgH, are known, but their thermal stability is low. The acyclic isomers of NgH are supposed to consist of a double-bonded N2 unit with a hydrogen and an N3 substituent. Each of the (Z) and (E) isomers forms an s-cis or s-trans conformer depending on the spatial arrangement of the approximately linear N3 unit with respect to the N-N double bond. An ab initio SCF MO calculation at the STO-3G level showed that cyclopentazadiene is the most stable isomer of NgH, followed by the (E) s-trans isomer of acyclic NgH and the (Z) s-trans, the (E) s-cis, and the (Z) s-cis isomers. The decomposition to HN3 and N2 is exothermal for all isomers of NgH. A mutual conversion between the (Z) and (E) isomers is predicted not to occur [1]. 

The elaborate treatment for the H2O system is only possible beeause of its small size. For larger systems, less rigorous methods must be employed. Let us as a more realistic example consider a determination of the relative stability of the C4H6 isomers shown in Figure 11.15. There are experimental values for the first eight structures, which allows an evaluation of the performance of different methods. This in turn enables an estimate of how much trust should be put on the predicted values for 9, 10 and 11. 

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