Comparison for pyrrole and pyrazole of orbital energies and population analyses from ab-initio SCF, CNDO/2, INDO, extended hückel, and ARCANA calculations

Abstract

The results of our recent ab-initio SCF calculations on pyrrole and pyrazole using large Gaussian basis sets are compared to those using less rigorous methods: CNDO/2, INDO, PCILO extended Huckel, and ARCANA (a type of iterative extended Huckel calculation). The ab-initio orbital energies are higher in energy than all those resulting from less rigorous methods. For the two highest occupied orbitals, both of π type, (pyrrole— 1a2 and 2b1; pyrazole—3a″ and 2a″) there was excellent agreement between the measured photoelectron spectroscopic ionization potentials and the Koopman's theorem values from the ab-initio results. Thus the orbital energies calculated by the less rigorous methods all lie too low in energy. The CNDO/2 and INDO orbital energies also span too large a width for the valence band compared to the ab-initio results. The gross atomic populations from the CNDO/2, INDO and PCILO methods show discrepancies from the ab-initio results for both the heteroatoms and the hydrogens. Total overlap populations calculated from the ab-initio SCF wave functions show large negative overlap populations between nonbonded ring atoms. Using the off-diagonal density matrix elements from CNDO/2 and INDO wave functions as if they were true off-diagonal elements from non-ZDO calculations, scaling them (shown previously to lead to reasonable values for TOPS between bonded atoms) led to TOPS between nonbonded ring atoms that were essentially zero. Wiberg bond indices, by definition only capable of giving positive numbers, also cannot reproduce these negative overlap populations. The extended Huckel and ARCANA methods, which are non-ZDO methods, both gave negative tops between nonbonded ring atoms, although not as large as those which resulted from ab-initio calculations.