Direct calculation of delocalization and resonance energies using truncated LMOs

Abstract

The small “tails” of energy-localized SCF LMOs have been truncated to give TLMOs which are then symmetrically orthogonalized to obtain OTLMOs with a variational energy E(OTLMO). The “delocalization energy” DE= E(SCF)- E(OTLMO) is found to be reasonable when characteristic LMO hyperconjugation-delocalization corrections, HDCs, and geometry-change corrections, GCCs, are considered. The “resonance energy” is defined to be RE= DE-GCC- ▪HDC i , e.g., for benzene − RE≅90−23−3(8)=43 kcal mol −1 is estimated for the π-electrons, which is close to earlier estimates. We also estimate RE(CO 2)= −149 kcal mol −1, and RE(H 2CCCH 2)=−6 kcal mol −1. Through using isodesmic reactions, errors due to electron correlation and incompleteness of basis sets are hopefully largely eliminated. The HDCs are also reasonable. We treat the delocalization energies of C 6H 6, CO 2, H 2CCCH 2, C 2H 4, C 2H 6, H 2CO and CH 4.