Quantitative determination of acid-base interaction strength

2 Quantitative determination of acid-base interaction strength 

Hudson and Klopman proposed an equation to deseribe the effeet of orbital perturbation of two moleeules on ehemieal reaetivity. Their hypothesis is that the initial perturbation determines the eourse of a reaetion or an interaetion. They applied quantum-meehanieal method to treat the eneounter of two interaeting systems as reaetivity. Their equation for interaetion energy ean be simplified by ineluding only two terms [10.2.2] the Coulombie interaetion and the frontier orbital interaetion between HOMO and LUMO. 

2 Hard-Soft Acid-Base (HSAB) principie 

The energy gap between HOMO and LUMO has been equated to the absolute hardness of the HSAB prineiple. This prineiple deseribes some basie rules about kineties and equilibrium of the aeid-base interaetions in solutions. The HSAB prineiple will be deseribed as it has evolved in reeent years on the basis of the density-funetional theory. For organie interaetions, the following statements were proposed  

The principle states that hard acid prefers to interact with a hard base, and vice versa, a soft acid with a soft base. A hard-hard interaction is fast because of large Coulombic attraction, a soft-soft interaction is fast because of large orbital overlap between HOMO and LUMO. However the problem is, what is the physical meaning of hardness  

Qn total charges of nucleophile N Qe total charges of electrophile E Cm coefficient of the atomic orbital N 

Ehomo energy level of the HOMO orbital Elumo energy level of the LUMO orbital 

---