Isogyric

Previous attempts to calculate bond energies in tin compounds employed levels of theory that were inadequate to provide accurate results. As discussed above, accurate bond energies require the use of either composite ah initio methods or methods employing a high level of electron correlation coupled with isogyric reactions to minimize basis set truncation and other systematic errors. Consequently, the results reported by Basch [46,96], which use a number of imcorrected ah initio methods or with very simple corrections (i.e., across-the-board energy corrections by finite amounts), are unhkely to be particularly accurate. 

The necessary experimental heats of formation are known to exquisite accuracy (or defined as zero, in the case of H2), and the calculations will be trivial for such small molecules, but accurately accounting for the enormous differences in the natures of the bonds on the two sides of Eq. (10.40) will require levels of electronic-structure theory nearly as high as those that would be necessary for a direct or parametric computation on 6-methylquinoline alone. The one virtue of Eq. (10.40), which is an example of a bond separation reaction , is that the total amount of unpaired electron spin on the two sides of the reaction is the same (in this case, zero) such a reaction is called isogyric . Note that atomization processes are... 

The CH bond dissociation enthalpy DH of 1 is 106.3 kcal mol 1, which is 11.2 kcal mol 1 larger than that of the secondary CH bond of propane (95.1 kcal mol 1)8911. At the UMP2/6-31G(d) level, a dissociation energy DE of 107.7 kcal mol"1 is calculated, which can be improved by using isogyric reactions (the number of unpaired electron spins is preserved) such as equation 21 ... 

An indirect means of dealing with the correlation problem when the molecule is too large to be treated by advanced methods such as the G2 is to include it in a reaction (often hypothetical) in which the reactants and products are similar in terms of one or more of several electronic and structural factors, e.g. number of electron pairs, types of bonds, atom environments, etc. [15,20,21], It is hoped that in computing AH(298 K) for such a process, the errors for the reactants and products will largely cancel. The desired AHf can then be obtained if the AHf of all of the other species are known. There are several categories of such reactions isogyric ones conserve the number of electron pairs, isodesmic maintain unchanged... 

Modified Neglect of Diatomic Overlap Parametric Method Number 3 (MNDO-PM3) 3.10.6 The MNDO/d Method 88 89 5.5 5.6 5.4.5 Correlation Consistent Basis Sets Extrapolation Procedures Isogyric and Isodesmic Reactions 162 164 169... 

Figure 5.4 CH4 + H An example of an isogyric reaction For systems involving elements from the third row or higher in the periodic table there is a large number of core electrons which in general are unimportant in a chemical sense.

The calculation of the dissociation energy is much simpler for the a n stale because the dissociation reaction 10 is isogyric. Our theoretical Dq value for the a Il state of ArF is 16.0 kcal/mol. 

Isogyre The black or gray areas of an interference figure (q.v.). They change position as the microscope stage is rotated. 

Consider now the reaction in Figure 5.4. The difference between the reactions in Figures 5.3 and 5.4 is that the latter has the same number of electron pairs on both sides such reactions are called isogyric. The task of calculating all the correlation energy of a C-H bond is replaced by calculating the difference in correlation energy between a C-H and H-H bond. The latter wiU benefit from cancellation of errors, and therefore... 

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