Isogyric reactions

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 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 ... 

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.

Several theoretical studies were performed on the FO2 radical. The best and more recent one is that by Francisco et al (56). References to previous work on the subject are given there. The calculation of AH o for the direct reaction proved to be exceedingly difficult and inaccurate. Therefore, the authors studied also some other isodesmic and/or isogyric reactions, namely FO2 + HO —> HO2 + FO... 

Reactions in which the numbers of paired electrons are not conserved are classified as nonisogyric reactions. Examples include homolytic bond dissociations and transition state calculations. In contrast, isogyric reactions conserve the number of paired electrons, such as in heterolytic bond dissociations and some isomerizations. Last, isodesmic reactions are bond conserving and not only maintain the division of electrons into shared and unshared pairs, but also maintain the number and type of bonds between heavy atoms (nonhydrogen). 

Errors of about 30-60 kJmol have been noted for isogyric reactions [102], p. 13. 

Recent theoretical results by Nicolaides and Radom , who employed isodesmic and isogyric reactions for an accurate determination of the heats of formation of both ions at the G2 level of theory, suggest that the silabenzyl cation 85 is more stable than the silatropylium ion 86 by approximately 40 kcalmol For the corresponding [C7,H7]+ surface, they found a reverse ordering, here the tropylium ion is 29 kcalmol more stable than the benzyl cation. These findings presumably reflect the well-documented inability of silicon to form 7r-bonds with significant strength. 

Several classes of reactions have been defined that display some degree of similarity in reactants and products. These include isogyric, isodesmic, and homodesmotic reactions. In isogyric reactions (Pople et al. 1983), the numbers of pairs of electrons in reactants and products is the same (and hence so is the number of unpaired electrons). For example, the following are isogyric processes ... 

Field (CASSCF) Second-order Perturbation Theory (CAS-PT2) Configuration Interaction Core-Valence Correlation Effects Coupled-cluster Theory Experimental Data Evaluation and Quality Control G2 Theory Heats of Formation Isoelectronic Isogyric Reactions M0ller-Plesset Perturbation Theory Numerical Hartree-Fock Methods for Molecules r 12-Dependent Wavefunctions Relativistic Theory and Applications Spectroscopy Computational Methods Spin Contamination Transition Metals Applications,... 

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