Electronic structure enthalpy predictions

As discussed in Section 3, it is usually more challenging to compute accurately the energies than the geometries of RIs. In contrast, although geometries of most RIs are hard to obtain experimentally, it is often possible to measure the enthalpy of activation for the formation and/or disappearance of an RI accurately. This experimental value can then be compared with the values predicted by different levels of electronic structure calculations, in order to test the ability of a particular level of theory to mirror accurately the experimental energetics. 

Perhaps more valuable over time than the quantitative predictions of spectra, structural parameters, and relative enthalpies and entropies of RIs, which can be obtained from electronic structure calculations, are the qualitative models of the electronic structures and reactivities of RIs that emerge from the computational results. Any model, to be successful, must do two things. 

Molecular orbital theory predicts that CO has a very similar electronic structure to N2, with which it is isoelectronic that is, CO and N2 have the same number of electrons (14). CO and N2 have different bond lengths and molar bond enthalpies (113 pm and 110 pm, and 1 076kJmol 1 and 945 kJ mol-1, respectively) because the orbitals are not exactly the same in the two molecules. The values are, however, much closer to each other than they are to bond lengths and enthalpies of molecules with different bond orders. 

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