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Frontier molecular orbital theory has proved to be an immensely useful concept. The concept of HOMO-LUMO interactions are frequently employed in detailing the mechanistic descriptions of reactions in organic chemistry. It seems to me that there are numerous examples of HOMO-LUMO interactions seem to be responsible for kinetic phenomena.

For example, secondary orbital interactions allow the preference for the endo product in the Diels-Alder reaction under kinetic control.

Nucleophilicity, which is often linked to the energy of the HOMO of the nucleophile, is also often regarded as a kinetic phenomenon. In fact, Lewis basicity itself may even be seen as a kinetic concept. For example, we make the assertion that phosphine is a better nucleophile (and also better Lewis base) compared to ammonia because $\ce {P}$ is less electronegative than $\ce {N}$ and thus, the HOMO of $\ce {PH3}$ is thus higher in energy, compared to that of $\ce {NH3}$.

Should we thus classify frontier orbital interactions as a kinetic concept?

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  • $\begingroup$ This apply in thermochemistry as well once you are under kinetic control. $\endgroup$ – Alchimista Dec 27 '18 at 9:10

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