Isodesmic calculations

Classically, cation (3) might be anticipated to be considerably stabilized by the neighboring oxygen atom and double bond on the other hand, its 47r-electron system would suggest anti-aromaticity. The isodesmic calculations showed it to be about 200 kJ mol-1 less stable than the model cation (19b) and about 40 kJ mol-1 less stable than the allyl cation. Thus it appears that it would have an anti-aromatic electronic structure, if it was formed (80TL1807). 

Term (1) is the gas-phase isodesmically calculated deprotonation free energy of the big acid CH2FCOOH it is to be calculated from terms (2) and (3). 

Figure 1.4 Stability of some carbenes, as determined by isodesmic calculations for the reaction of each carbene with methane to generate NHCH2 and dihydrocarbene [13].

Note that, for themiochemical purposes, there is no requirement that we can actually cany out the reaction. Systematic computational errors will, in some measure, cancel between the right and left sides of isodesmic reactions (10-20), giving an estimate of the GAMESS calculations at the STO-3G level lead... 

Table I.IS gives total bonding energies in kilocalories per mole for some simple molecules. The B3iyP results are comparable in accuracy to G1 and G2 results. Another comparison was done with a series of cyclic hydrocarbons as the test case. The calculations were done using an isodesmic reaction scheme. The results are given in Table 1.19. Density functional calculations have also been successfully extended to functionalized molecules. ...

Interesting stabilization and structural trends have been noted using MP2/6-31G calculations on the effect of substituents on imines. The data below give AE for the isodesmic reaction and show that stabilization tends to increase with Xbe 1 gi (>up electronegativity of the substituent. The X—N=CH2 bond angle decreases with Xbe-... 

Calculations are frequently done on the basis of isodesmic reactions in order to provide for maximum cancellation of errors in the total energies (see Section 1.3). The "experimental AH of the process can be obtained from the tabulated AHf values of the reactants and products. Table 4.2 compares the errors in AHf for some isodesmic reactions with those for the corresponding atomization reactions for G2 calculations on some... 

Calculations Using Atomization versus Isodesmic Reactions" ... 

Table 8.5 lists the enthalpies for a series of isodesmic reactions involving conversion of carbonyl derivatives to the methyl ketones. The A// of the reactions is given both from thermodynamic data and as calculated at the MP3/6—3H--I-G level. These... 

The isodesmic reaction approach (see Section 4.1) has also been applied to calculation of the resonance stabilization of benzene. 

Use the calculated energies for the molecules shown below to calculate isodesmic reaction energies for the equation ... 

The effect of substituents has been probed by MO calculations at the STO-3G level. An isodesmic reaction corresponding to transfer of a proton from a substituted <7-complex to an unsubstituted one will indicate the stabilizing or destabilizing effect of the substituent. The results are given in Table 10.1. 

Energy changes are for the isodesmic reaction shown in Scheme 13.33 h Calculated from the ab initio energies of R-[16]-OH and R-[17].33... 

The calculated values of AH for the isodesmic reactions shown in Scheme 13 (Table 4) suggest that the effects of a-carbonyl and a-thiocarbonyl substituents are controlled largely by the balance between two opposing interactions at the alkenes R-[17] ... 

The data in Table 3 also show that the N-H BDE of aniline (9b) and the C-H BDE of P-picoline (9c) are quite similar and are calculated to differ by only 0.1 kcal/mol at the BVWN5/AUG-cc-pVTZ level of theory. This is also true for cases other than 9b versus 9c as shown by the calculated enthalpies in Table 4.77 The isodesmic reaction in Table 4 gives the difference between the N-H BDEs of RNH2 and the C-H BDEs of comparable R CH3 species. For R=R =Ph, the calculated difference in BDEs between aniline (9b) and toluene is only 0.3 kcal/mol and for R=R =H, the N-H BDE in NH3 is computed to be only 1.4 kcal/mol larger than the C-H BDE in CH4. The latter energy difference is close to the experimental value of 3.7 kcal/mol.76 Thus, the results in Table 4 show that the N-H BDEs of primary amines are, in general, nearly... 

The most common evaluation of aromaticity via energetic criteria is done using calculations either a type of isodesmic reaction (34) or comparison of two isomers that differ only through the aromaticity of one (3). We were interested in the possibility of evaluating stability experimentally and the electrochemical formation of dications such as 8 was attractive. In this approach, the redox potential for formation of the dication would be compared to the redox potential for formation of dications which could not be antiaromatic. If 8 was antiaromatic, its redox potential should be larger and more positive than that of the reference system. This approach was applied to the evaluation of the antiaromaticity of 9... 

Isodesmic reactions 2 and 3 were calculated as a measure of the relative ease of formation of the aza-epoxides, in order to obtain some insight as to the effect of nitrogen atom on metabolic activation of aza-PAHs. It was presumed that formation of the epoxides is not as relevant as carbocation formation in determining the relative reactivities of Phe, BhQ and BfQ, because almost no energy differences were observed for the isodesmic reactions involving these compounds. 

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