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. INW. PCILO extended Hiickel, and ARCANA (a type of iterative extended Hiickel 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 n type, (pyrrole-la, and 2b, ; 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 CNW/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 C N D O /~, INDO and PCILO methods show discrepancies from the ab-initio resultsfor 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 C N D O /~ and INDO wave functions as if they were true off-diagonal elements from n o n -z m 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 Hiickel and ARCANA methods, which are n o n -z w methods, both gave negative TOPS between nonbonded ring atoms, although not as large as those which resulted from ab-initio calculations.

Quantum chemical, other theoretical and physicochemical studies on narcotics and narcotic antagonists to understand their mechanism of action

“…The hydrogens bear a good part of the excess positive charge. In accord with our earlier findings about the nonagreement of C N D O /~ electron densities with those from good ab initio LCAO-MO-SCF calculations [34], we consider of primary significance the fact that comparison of our CNDO/2 electron densities on the N of the free base narcotic (or narcotic antagonist) show them to differ only by a small fraction ( -0.2) of an electronic charge from the CND0/2 calculated electron density in the respective protonated form. We would attach only secondary significance to the magnitudes (and even sometimes the signs) of the C N D O /~ calculated charges.…”

“…it was obvious that these were not optimal physical structures.) (Comparison of gross atomic populations corresponding to electron densities calculated by the C N D O /~ and INDO methods with those resulting from ab initio calculations on pyrrole and pyrazole [33] showed that there were notable discrep ancies in the resulting molecular charge distributions calculated by C N D O /~ and INDO methods [34]. However, calculated differences in closely related congeners hopefully should reflect realistically the differences in molecular charge distributions.)…”