LUMO lowest unoccupied substituent effects

Correlation of the effect of substituents on the rates of reactions with early transition states often is best accomplished in terms of perturbational molecular orbital theory, and polar effects can play a major role for such reactions [100, 101]. Essentially this theory states that energy differences between the highest occupied molecular orbital (HOMO) of one reactant and the lowest unoccupied molecular orbital (LUMO) of the other reactant are decisive in determining the reaction rate the smaller the difference in energy, the faster the predicted rate of reaction [102,103]. Since the HOMO of a free radical is the SOMO, the energy difference between the SOMO and the alkene HOMO and/or LUMO is of considerable importance in determining the rates of radical additions to alkenes [84],... 

The reduction of 2-methylcyclohexanone and 2-isopropylcyclohexanone by LiAlH was subjected to DFT analysis (B3LYP/6-31G(d,p)) to optimize the TSS. Four TSSs were located for each ketone for the axial and equatorial attacks by LiAlH4. Electronic potential maps were used to investigate the electronic effect of the substituents on TSS stabilization. The lowest unoccupied molecular orbital (LUMO) maps and natural bond orbital (NBO) analysis helped in elucidating the uneven distribution of molecular orbital around the carbonyl tt-plane, and the preference for the hydride attack in terms of tensional and electronic properties. ... 

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