PROTON AFFINITIES OF MOLECULES CONTAINING NITROGEN AND OXYGEN - COMPARING ABINITIO AND SEMIEMPIRICAL RESULTS TO EXPERIMENTS

We report a comparison of theoretical and experimental proton affinities at nitrogen and oxygen sites within a series of small molecules. The calculated proton affinities are determined using the semiempirical methods AM1, MNDO, and PM3; the ab initio Hartree-Fock method at the following basis levels: 3-21G//3-21G, 3-21+G//3-21G, 6-31G*//6-31G*, and 6-31+G(d,p)//6-31G*; and Moller-Plesset perturbation calculations: MP2/6-31G*//6-31G*, MP3/6-31G*//6-31G*, MP2/6-31G+(d,p)//6-31G*, MP3/6-31G+(d,p)//6-31G*, and MP4(SDTQ)/6-31G+G(d,p)//6-31G*. The semiempirical methods have more nonsystematic scatter from the experimental values, compared to even the minimal 3-21G level ab initio calculations. The thermodynamically corrected 6-31G*//6-31G* proton affinities provide acceptable results compared to experiment, and we see no significant improvement over 6-31G*//6-31G* in the proton affinities with any of the higher-level calculations.