Proton affinities of molecules containing nitrogen and oxygen: comparing ab initio 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 AM 1, MNDO , and PM 3; 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 Møller–Plesset perturbation calculations: MP 2/6-31G *//6-31G *, MP 3/6-31G *//6-31G *, MP 2/6-31G +(d, p)//6-31G *, MP 3/6-31G +(d, p)//6-31G *, and MP 4(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. © 1992 John Wiley & Sons, Inc.